- Assertion testing
- Asynchronous context tracking
- Async hooks
- Buffer
- C++ addons
- C/C++ addons with Node-API
- C++ embedder API
- Child processes
- Cluster
- Command-line options
- Console
- Corepack
- Crypto
- Debugger
- Deprecated APIs
- Diagnostics Channel
- DNS
- Domain
- Errors
- Events
- File system
- Globals
- HTTP
- HTTP/2
- HTTPS
- Inspector
- Internationalization
- Modules: CommonJS modules
- Modules: ECMAScript modules
- Modules:
node:module
API - Modules: Packages
- Net
- OS
- Path
- Performance hooks
- Permissions
- Process
- Punycode
- Query strings
- Readline
- REPL
- Report
- Single executable applications
- Stream
- String decoder
- Test runner
- Timers
- TLS/SSL
- Trace events
- TTY
- UDP/datagram
- URL
- Utilities
- V8
- VM
- WASI
- Web Crypto API
- Web Streams API
- Worker threads
- Zlib
Node.js v20.0.0-test09a4bb152f documentation
- Node.js v20.0.0-test09a4bb152f
- ► Other versions
- ► Options
Table of contents
- About this documentation
- Usage and example
- Assert
- Strict assertion mode
- Legacy assertion mode
- Class: assert.AssertionError
- Class:
assert.CallTracker
assert(value[, message])
assert.deepEqual(actual, expected[, message])
assert.deepStrictEqual(actual, expected[, message])
assert.doesNotMatch(string, regexp[, message])
assert.doesNotReject(asyncFn[, error][, message])
assert.doesNotThrow(fn[, error][, message])
assert.equal(actual, expected[, message])
assert.fail([message])
assert.fail(actual, expected[, message[, operator[, stackStartFn]]])
assert.ifError(value)
assert.match(string, regexp[, message])
assert.notDeepEqual(actual, expected[, message])
assert.notDeepStrictEqual(actual, expected[, message])
assert.notEqual(actual, expected[, message])
assert.notStrictEqual(actual, expected[, message])
assert.ok(value[, message])
assert.rejects(asyncFn[, error][, message])
assert.strictEqual(actual, expected[, message])
assert.throws(fn[, error][, message])
- Asynchronous context tracking
- Introduction
- Class:
AsyncLocalStorage
new AsyncLocalStorage()
- Static method:
AsyncLocalStorage.bind(fn)
- Static method:
AsyncLocalStorage.snapshot()
asyncLocalStorage.disable()
asyncLocalStorage.getStore()
asyncLocalStorage.enterWith(store)
asyncLocalStorage.run(store, callback[, ...args])
asyncLocalStorage.exit(callback[, ...args])
- Usage with
async/await
- Troubleshooting: Context loss
- Class:
AsyncResource
new AsyncResource(type[, options])
- Static method:
AsyncResource.bind(fn[, type[, thisArg]])
asyncResource.bind(fn[, thisArg])
asyncResource.runInAsyncScope(fn[, thisArg, ...args])
asyncResource.emitDestroy()
asyncResource.asyncId()
asyncResource.triggerAsyncId()
- Using
AsyncResource
for aWorker
thread pool - Integrating
AsyncResource
withEventEmitter
- Async hooks
- Terminology
- Overview
async_hooks.createHook(callbacks)
- Class:
AsyncHook
- Promise execution tracking
- JavaScript embedder API
- Class:
AsyncLocalStorage
- Buffer
- Buffers and character encodings
- Buffers and TypedArrays
- Buffers and iteration
- Class:
Blob
- Class:
Buffer
- Static method:
Buffer.alloc(size[, fill[, encoding]])
- Static method:
Buffer.allocUnsafe(size)
- Static method:
Buffer.allocUnsafeSlow(size)
- Static method:
Buffer.byteLength(string[, encoding])
- Static method:
Buffer.compare(buf1, buf2)
- Static method:
Buffer.concat(list[, totalLength])
- Static method:
Buffer.copyBytesFrom(view[, offset[, length]])
- Static method:
Buffer.from(array)
- Static method:
Buffer.from(arrayBuffer[, byteOffset[, length]])
- Static method:
Buffer.from(buffer)
- Static method:
Buffer.from(object[, offsetOrEncoding[, length]])
- Static method:
Buffer.from(string[, encoding])
- Static method:
Buffer.isBuffer(obj)
- Static method:
Buffer.isEncoding(encoding)
- Class property:
Buffer.poolSize
buf[index]
buf.buffer
buf.byteOffset
buf.compare(target[, targetStart[, targetEnd[, sourceStart[, sourceEnd]]]])
buf.copy(target[, targetStart[, sourceStart[, sourceEnd]]])
buf.entries()
buf.equals(otherBuffer)
buf.fill(value[, offset[, end]][, encoding])
buf.includes(value[, byteOffset][, encoding])
buf.indexOf(value[, byteOffset][, encoding])
buf.keys()
buf.lastIndexOf(value[, byteOffset][, encoding])
buf.length
buf.parent
buf.readBigInt64BE([offset])
buf.readBigInt64LE([offset])
buf.readBigUInt64BE([offset])
buf.readBigUInt64LE([offset])
buf.readDoubleBE([offset])
buf.readDoubleLE([offset])
buf.readFloatBE([offset])
buf.readFloatLE([offset])
buf.readInt8([offset])
buf.readInt16BE([offset])
buf.readInt16LE([offset])
buf.readInt32BE([offset])
buf.readInt32LE([offset])
buf.readIntBE(offset, byteLength)
buf.readIntLE(offset, byteLength)
buf.readUInt8([offset])
buf.readUInt16BE([offset])
buf.readUInt16LE([offset])
buf.readUInt32BE([offset])
buf.readUInt32LE([offset])
buf.readUIntBE(offset, byteLength)
buf.readUIntLE(offset, byteLength)
buf.subarray([start[, end]])
buf.slice([start[, end]])
buf.swap16()
buf.swap32()
buf.swap64()
buf.toJSON()
buf.toString([encoding[, start[, end]]])
buf.values()
buf.write(string[, offset[, length]][, encoding])
buf.writeBigInt64BE(value[, offset])
buf.writeBigInt64LE(value[, offset])
buf.writeBigUInt64BE(value[, offset])
buf.writeBigUInt64LE(value[, offset])
buf.writeDoubleBE(value[, offset])
buf.writeDoubleLE(value[, offset])
buf.writeFloatBE(value[, offset])
buf.writeFloatLE(value[, offset])
buf.writeInt8(value[, offset])
buf.writeInt16BE(value[, offset])
buf.writeInt16LE(value[, offset])
buf.writeInt32BE(value[, offset])
buf.writeInt32LE(value[, offset])
buf.writeIntBE(value, offset, byteLength)
buf.writeIntLE(value, offset, byteLength)
buf.writeUInt8(value[, offset])
buf.writeUInt16BE(value[, offset])
buf.writeUInt16LE(value[, offset])
buf.writeUInt32BE(value[, offset])
buf.writeUInt32LE(value[, offset])
buf.writeUIntBE(value, offset, byteLength)
buf.writeUIntLE(value, offset, byteLength)
new Buffer(array)
new Buffer(arrayBuffer[, byteOffset[, length]])
new Buffer(buffer)
new Buffer(size)
new Buffer(string[, encoding])
- Static method:
- Class:
File
node:buffer
module APIsBuffer.from()
,Buffer.alloc()
, andBuffer.allocUnsafe()
- C++ addons
- Node-API
- Implications of ABI stability
- Building
- Usage
- Node-API version matrix
- Environment life cycle APIs
- Basic Node-API data types
- Error handling
- Object lifetime management
- Module registration
- Working with JavaScript values
- Enum types
- Object creation functions
napi_create_array
napi_create_array_with_length
napi_create_arraybuffer
napi_create_buffer
napi_create_buffer_copy
napi_create_date
napi_create_external
napi_create_external_arraybuffer
napi_create_external_buffer
napi_create_object
napi_create_symbol
node_api_symbol_for
napi_create_typedarray
napi_create_dataview
- Functions to convert from C types to Node-API
- Functions to convert from Node-API to C types
napi_get_array_length
napi_get_arraybuffer_info
napi_get_buffer_info
napi_get_prototype
napi_get_typedarray_info
napi_get_dataview_info
napi_get_date_value
napi_get_value_bool
napi_get_value_double
napi_get_value_bigint_int64
napi_get_value_bigint_uint64
napi_get_value_bigint_words
napi_get_value_external
napi_get_value_int32
napi_get_value_int64
napi_get_value_string_latin1
napi_get_value_string_utf8
napi_get_value_string_utf16
napi_get_value_uint32
- Functions to get global instances
- Working with JavaScript values and abstract operations
- Working with JavaScript properties
- Structures
- Functions
napi_get_property_names
napi_get_all_property_names
napi_set_property
napi_get_property
napi_has_property
napi_delete_property
napi_has_own_property
napi_set_named_property
napi_get_named_property
napi_has_named_property
napi_set_element
napi_get_element
napi_has_element
napi_delete_element
napi_define_properties
napi_object_freeze
napi_object_seal
- Working with JavaScript functions
- Object wrap
- Simple asynchronous operations
- Custom asynchronous operations
- Version management
- Memory management
- Promises
- Script execution
- libuv event loop
- Asynchronous thread-safe function calls
- Calling a thread-safe function
- Reference counting of thread-safe functions
- Deciding whether to keep the process running
napi_create_threadsafe_function
napi_get_threadsafe_function_context
napi_call_threadsafe_function
napi_acquire_threadsafe_function
napi_release_threadsafe_function
napi_ref_threadsafe_function
napi_unref_threadsafe_function
- Miscellaneous utilities
- C++ embedder API
- Child process
- Asynchronous process creation
- Synchronous process creation
- Class:
ChildProcess
- Event:
'close'
- Event:
'disconnect'
- Event:
'error'
- Event:
'exit'
- Event:
'message'
- Event:
'spawn'
subprocess.channel
subprocess.connected
subprocess.disconnect()
subprocess.exitCode
subprocess.kill([signal])
subprocess.killed
subprocess.pid
subprocess.ref()
subprocess.send(message[, sendHandle[, options]][, callback])
subprocess.signalCode
subprocess.spawnargs
subprocess.spawnfile
subprocess.stderr
subprocess.stdin
subprocess.stdio
subprocess.stdout
subprocess.unref()
- Event:
maxBuffer
and Unicode- Shell requirements
- Default Windows shell
- Advanced serialization
- Cluster
- How it works
- Class:
Worker
- Event:
'disconnect'
- Event:
'exit'
- Event:
'fork'
- Event:
'listening'
- Event:
'message'
- Event:
'online'
- Event:
'setup'
cluster.disconnect([callback])
cluster.fork([env])
cluster.isMaster
cluster.isPrimary
cluster.isWorker
cluster.schedulingPolicy
cluster.settings
cluster.setupMaster([settings])
cluster.setupPrimary([settings])
cluster.worker
cluster.workers
- Command-line API
- Synopsis
- Program entry point
- Options
-
--
--abort-on-uncaught-exception
--allow-child-process
--allow-fs-read
--allow-fs-write
--allow-worker
--build-snapshot
--completion-bash
-C condition
,--conditions=condition
--cpu-prof
--cpu-prof-dir
--cpu-prof-interval
--cpu-prof-name
--diagnostic-dir=directory
--disable-proto=mode
--disallow-code-generation-from-strings
--dns-result-order=order
--enable-fips
--enable-network-family-autoselection
--enable-source-maps
--experimental-import-meta-resolve
--experimental-loader=module
--experimental-network-imports
--experimental-permission
--experimental-policy
--no-experimental-fetch
--no-experimental-global-webcrypto
--no-experimental-global-customevent
--no-experimental-repl-await
--experimental-shadow-realm
--experimental-test-coverage
--experimental-vm-modules
--experimental-wasi-unstable-preview1
--experimental-wasm-modules
--force-context-aware
--force-fips
--frozen-intrinsics
--force-node-api-uncaught-exceptions-policy
--heapsnapshot-near-heap-limit=max_count
--heapsnapshot-signal=signal
--heap-prof
--heap-prof-dir
--heap-prof-interval
--heap-prof-name
--icu-data-dir=file
--import=module
--input-type=type
--inspect-brk[=[host:]port]
--inspect-port=[host:]port
--inspect[=[host:]port]
--inspect-publish-uid=stderr,http
--insecure-http-parser
--jitless
--max-http-header-size=size
--napi-modules
--no-addons
--no-deprecation
--no-extra-info-on-fatal-exception
--no-force-async-hooks-checks
--no-global-search-paths
--no-warnings
--node-memory-debug
--openssl-config=file
--openssl-shared-config
--openssl-legacy-provider
--pending-deprecation
--policy-integrity=sri
--preserve-symlinks
--preserve-symlinks-main
--prof
--prof-process
--redirect-warnings=file
--report-compact
--report-dir=directory
,report-directory=directory
--report-filename=filename
--report-on-fatalerror
--report-on-signal
--report-signal=signal
--report-uncaught-exception
--secure-heap=n
--secure-heap-min=n
--snapshot-blob=path
--test
--test-name-pattern
--test-reporter
--test-reporter-destination
--test-only
--throw-deprecation
--title=title
--tls-cipher-list=list
--tls-keylog=file
--tls-max-v1.2
--tls-max-v1.3
--tls-min-v1.0
--tls-min-v1.1
--tls-min-v1.2
--tls-min-v1.3
--trace-atomics-wait
--trace-deprecation
--trace-event-categories
--trace-event-file-pattern
--trace-events-enabled
--trace-exit
--trace-sigint
--trace-sync-io
--trace-tls
--trace-uncaught
--trace-warnings
--track-heap-objects
--unhandled-rejections=mode
--use-bundled-ca
,--use-openssl-ca
--use-largepages=mode
--v8-options
--v8-pool-size=num
--watch
--watch-path
--watch-preserve-output
--zero-fill-buffers
-c
,--check
-e
,--eval "script"
-h
,--help
-i
,--interactive
-p
,--print "script"
-r
,--require module
-v
,--version
- Environment variables
FORCE_COLOR=[1, 2, 3]
NODE_DEBUG=module[,…]
NODE_DEBUG_NATIVE=module[,…]
NODE_DISABLE_COLORS=1
NODE_EXTRA_CA_CERTS=file
NODE_ICU_DATA=file
NODE_NO_WARNINGS=1
NODE_OPTIONS=options...
NODE_PATH=path[:…]
NODE_PENDING_DEPRECATION=1
NODE_PENDING_PIPE_INSTANCES=instances
NODE_PRESERVE_SYMLINKS=1
NODE_REDIRECT_WARNINGS=file
NODE_REPL_HISTORY=file
NODE_REPL_EXTERNAL_MODULE=file
NODE_SKIP_PLATFORM_CHECK=value
NODE_TEST_CONTEXT=value
NODE_TLS_REJECT_UNAUTHORIZED=value
NODE_V8_COVERAGE=dir
NO_COLOR=<any>
OPENSSL_CONF=file
SSL_CERT_DIR=dir
SSL_CERT_FILE=file
TZ
UV_THREADPOOL_SIZE=size
- Useful V8 options
- Console
- Class:
Console
new Console(stdout[, stderr][, ignoreErrors])
new Console(options)
console.assert(value[, ...message])
console.clear()
console.count([label])
console.countReset([label])
console.debug(data[, ...args])
console.dir(obj[, options])
console.dirxml(...data)
console.error([data][, ...args])
console.group([...label])
console.groupCollapsed()
console.groupEnd()
console.info([data][, ...args])
console.log([data][, ...args])
console.table(tabularData[, properties])
console.time([label])
console.timeEnd([label])
console.timeLog([label][, ...data])
console.trace([message][, ...args])
console.warn([data][, ...args])
- Inspector only methods
- Class:
- Corepack
- Crypto
- Determining if crypto support is unavailable
- Class:
Certificate
- Class:
Cipher
- Class:
Decipher
- Class:
DiffieHellman
diffieHellman.computeSecret(otherPublicKey[, inputEncoding][, outputEncoding])
diffieHellman.generateKeys([encoding])
diffieHellman.getGenerator([encoding])
diffieHellman.getPrime([encoding])
diffieHellman.getPrivateKey([encoding])
diffieHellman.getPublicKey([encoding])
diffieHellman.setPrivateKey(privateKey[, encoding])
diffieHellman.setPublicKey(publicKey[, encoding])
diffieHellman.verifyError
- Class:
DiffieHellmanGroup
- Class:
ECDH
- Static method:
ECDH.convertKey(key, curve[, inputEncoding[, outputEncoding[, format]]])
ecdh.computeSecret(otherPublicKey[, inputEncoding][, outputEncoding])
ecdh.generateKeys([encoding[, format]])
ecdh.getPrivateKey([encoding])
ecdh.getPublicKey([encoding][, format])
ecdh.setPrivateKey(privateKey[, encoding])
ecdh.setPublicKey(publicKey[, encoding])
- Static method:
- Class:
Hash
- Class:
Hmac
- Class:
KeyObject
- Class:
Sign
- Class:
Verify
- Class:
X509Certificate
new X509Certificate(buffer)
x509.ca
x509.checkEmail(email[, options])
x509.checkHost(name[, options])
x509.checkIP(ip)
x509.checkIssued(otherCert)
x509.checkPrivateKey(privateKey)
x509.fingerprint
x509.fingerprint256
x509.fingerprint512
x509.infoAccess
x509.issuer
x509.issuerCertificate
x509.keyUsage
x509.publicKey
x509.raw
x509.serialNumber
x509.subject
x509.subjectAltName
x509.toJSON()
x509.toLegacyObject()
x509.toString()
x509.validFrom
x509.validTo
x509.verify(publicKey)
node:crypto
module methods and propertiescrypto.constants
crypto.fips
crypto.checkPrime(candidate[, options], callback)
crypto.checkPrimeSync(candidate[, options])
crypto.createCipher(algorithm, password[, options])
crypto.createCipheriv(algorithm, key, iv[, options])
crypto.createDecipher(algorithm, password[, options])
crypto.createDecipheriv(algorithm, key, iv[, options])
crypto.createDiffieHellman(prime[, primeEncoding][, generator][, generatorEncoding])
crypto.createDiffieHellman(primeLength[, generator])
crypto.createDiffieHellmanGroup(name)
crypto.createECDH(curveName)
crypto.createHash(algorithm[, options])
crypto.createHmac(algorithm, key[, options])
crypto.createPrivateKey(key)
crypto.createPublicKey(key)
crypto.createSecretKey(key[, encoding])
crypto.createSign(algorithm[, options])
crypto.createVerify(algorithm[, options])
crypto.diffieHellman(options)
crypto.generateKey(type, options, callback)
crypto.generateKeyPair(type, options, callback)
crypto.generateKeyPairSync(type, options)
crypto.generateKeySync(type, options)
crypto.generatePrime(size[, options[, callback]])
crypto.generatePrimeSync(size[, options])
crypto.getCipherInfo(nameOrNid[, options])
crypto.getCiphers()
crypto.getCurves()
crypto.getDiffieHellman(groupName)
crypto.getFips()
crypto.getHashes()
crypto.getRandomValues(typedArray)
crypto.hkdf(digest, ikm, salt, info, keylen, callback)
crypto.hkdfSync(digest, ikm, salt, info, keylen)
crypto.pbkdf2(password, salt, iterations, keylen, digest, callback)
crypto.pbkdf2Sync(password, salt, iterations, keylen, digest)
crypto.privateDecrypt(privateKey, buffer)
crypto.privateEncrypt(privateKey, buffer)
crypto.publicDecrypt(key, buffer)
crypto.publicEncrypt(key, buffer)
crypto.randomBytes(size[, callback])
crypto.randomFillSync(buffer[, offset][, size])
crypto.randomFill(buffer[, offset][, size], callback)
crypto.randomInt([min, ]max[, callback])
crypto.randomUUID([options])
crypto.scrypt(password, salt, keylen[, options], callback)
crypto.scryptSync(password, salt, keylen[, options])
crypto.secureHeapUsed()
crypto.setEngine(engine[, flags])
crypto.setFips(bool)
crypto.sign(algorithm, data, key[, callback])
crypto.subtle
crypto.timingSafeEqual(a, b)
crypto.verify(algorithm, data, key, signature[, callback])
crypto.webcrypto
- Notes
- Crypto constants
- Debugger
- Deprecated APIs
- Revoking deprecations
- List of deprecated APIs
- DEP0001:
http.OutgoingMessage.prototype.flush
- DEP0002:
require('_linklist')
- DEP0003:
_writableState.buffer
- DEP0004:
CryptoStream.prototype.readyState
- DEP0005:
Buffer()
constructor - DEP0006:
child_process
options.customFds
- DEP0007: Replace
cluster
worker.suicide
withworker.exitedAfterDisconnect
- DEP0008:
require('node:constants')
- DEP0009:
crypto.pbkdf2
without digest - DEP0010:
crypto.createCredentials
- DEP0011:
crypto.Credentials
- DEP0012:
Domain.dispose
- DEP0013:
fs
asynchronous function without callback - DEP0014:
fs.read
legacy String interface - DEP0015:
fs.readSync
legacy String interface - DEP0016:
GLOBAL
/root
- DEP0017:
Intl.v8BreakIterator
- DEP0018: Unhandled promise rejections
- DEP0019:
require('.')
resolved outside directory - DEP0020:
Server.connections
- DEP0021:
Server.listenFD
- DEP0022:
os.tmpDir()
- DEP0023:
os.getNetworkInterfaces()
- DEP0024:
REPLServer.prototype.convertToContext()
- DEP0025:
require('node:sys')
- DEP0026:
util.print()
- DEP0027:
util.puts()
- DEP0028:
util.debug()
- DEP0029:
util.error()
- DEP0030:
SlowBuffer
- DEP0031:
ecdh.setPublicKey()
- DEP0032:
node:domain
module - DEP0033:
EventEmitter.listenerCount()
- DEP0034:
fs.exists(path, callback)
- DEP0035:
fs.lchmod(path, mode, callback)
- DEP0036:
fs.lchmodSync(path, mode)
- DEP0037:
fs.lchown(path, uid, gid, callback)
- DEP0038:
fs.lchownSync(path, uid, gid)
- DEP0039:
require.extensions
- DEP0040:
node:punycode
module - DEP0041:
NODE_REPL_HISTORY_FILE
environment variable - DEP0042:
tls.CryptoStream
- DEP0043:
tls.SecurePair
- DEP0044:
util.isArray()
- DEP0045:
util.isBoolean()
- DEP0046:
util.isBuffer()
- DEP0047:
util.isDate()
- DEP0048:
util.isError()
- DEP0049:
util.isFunction()
- DEP0050:
util.isNull()
- DEP0051:
util.isNullOrUndefined()
- DEP0052:
util.isNumber()
- DEP0053:
util.isObject()
- DEP0054:
util.isPrimitive()
- DEP0055:
util.isRegExp()
- DEP0056:
util.isString()
- DEP0057:
util.isSymbol()
- DEP0058:
util.isUndefined()
- DEP0059:
util.log()
- DEP0060:
util._extend()
- DEP0061:
fs.SyncWriteStream
- DEP0062:
node --debug
- DEP0063:
ServerResponse.prototype.writeHeader()
- DEP0064:
tls.createSecurePair()
- DEP0065:
repl.REPL_MODE_MAGIC
andNODE_REPL_MODE=magic
- DEP0066:
OutgoingMessage.prototype._headers, OutgoingMessage.prototype._headerNames
- DEP0067:
OutgoingMessage.prototype._renderHeaders
- DEP0068:
node debug
- DEP0069:
vm.runInDebugContext(string)
- DEP0070:
async_hooks.currentId()
- DEP0071:
async_hooks.triggerId()
- DEP0072:
async_hooks.AsyncResource.triggerId()
- DEP0073: Several internal properties of
net.Server
- DEP0074:
REPLServer.bufferedCommand
- DEP0075:
REPLServer.parseREPLKeyword()
- DEP0076:
tls.parseCertString()
- DEP0077:
Module._debug()
- DEP0078:
REPLServer.turnOffEditorMode()
- DEP0079: Custom inspection function on objects via
.inspect()
- DEP0080:
path._makeLong()
- DEP0081:
fs.truncate()
using a file descriptor - DEP0082:
REPLServer.prototype.memory()
- DEP0083: Disabling ECDH by setting
ecdhCurve
tofalse
- DEP0084: requiring bundled internal dependencies
- DEP0085: AsyncHooks sensitive API
- DEP0086: Remove
runInAsyncIdScope
- DEP0089:
require('node:assert')
- DEP0090: Invalid GCM authentication tag lengths
- DEP0091:
crypto.DEFAULT_ENCODING
- DEP0092: Top-level
this
bound tomodule.exports
- DEP0093:
crypto.fips
is deprecated and replaced - DEP0094: Using
assert.fail()
with more than one argument - DEP0095:
timers.enroll()
- DEP0096:
timers.unenroll()
- DEP0097:
MakeCallback
withdomain
property - DEP0098: AsyncHooks embedder
AsyncResource.emitBefore
andAsyncResource.emitAfter
APIs - DEP0099: Async context-unaware
node::MakeCallback
C++ APIs - DEP0100:
process.assert()
- DEP0101:
--with-lttng
- DEP0102: Using
noAssert
inBuffer#(read|write)
operations - DEP0103:
process.binding('util').is[...]
typechecks - DEP0104:
process.env
string coercion - DEP0105:
decipher.finaltol
- DEP0106:
crypto.createCipher
andcrypto.createDecipher
- DEP0107:
tls.convertNPNProtocols()
- DEP0108:
zlib.bytesRead
- DEP0109:
http
,https
, andtls
support for invalid URLs - DEP0110:
vm.Script
cached data - DEP0111:
process.binding()
- DEP0112:
dgram
private APIs - DEP0113:
Cipher.setAuthTag()
,Decipher.getAuthTag()
- DEP0114:
crypto._toBuf()
- DEP0115:
crypto.prng()
,crypto.pseudoRandomBytes()
,crypto.rng()
- DEP0116: Legacy URL API
- DEP0117: Native crypto handles
- DEP0118:
dns.lookup()
support for a falsy host name - DEP0119:
process.binding('uv').errname()
private API - DEP0120: Windows Performance Counter support
- DEP0121:
net._setSimultaneousAccepts()
- DEP0122:
tls
Server.prototype.setOptions()
- DEP0123: setting the TLS ServerName to an IP address
- DEP0124: using
REPLServer.rli
- DEP0125:
require('node:_stream_wrap')
- DEP0126:
timers.active()
- DEP0127:
timers._unrefActive()
- DEP0128: modules with an invalid
main
entry and anindex.js
file - DEP0129:
ChildProcess._channel
- DEP0130:
Module.createRequireFromPath()
- DEP0131: Legacy HTTP parser
- DEP0132:
worker.terminate()
with callback - DEP0133:
http
connection
- DEP0134:
process._tickCallback
- DEP0135:
WriteStream.open()
andReadStream.open()
are internal - DEP0136:
http
finished
- DEP0137: Closing fs.FileHandle on garbage collection
- DEP0138:
process.mainModule
- DEP0139:
process.umask()
with no arguments - DEP0140: Use
request.destroy()
instead ofrequest.abort()
- DEP0141:
repl.inputStream
andrepl.outputStream
- DEP0142:
repl._builtinLibs
- DEP0143:
Transform._transformState
- DEP0144:
module.parent
- DEP0145:
socket.bufferSize
- DEP0146:
new crypto.Certificate()
- DEP0147:
fs.rmdir(path, { recursive: true })
- DEP0148: Folder mappings in
"exports"
(trailing"/"
) - DEP0149:
http.IncomingMessage#connection
- DEP0150: Changing the value of
process.config
- DEP0151: Main index lookup and extension searching
- DEP0152: Extension PerformanceEntry properties
- DEP0153:
dns.lookup
anddnsPromises.lookup
options type coercion - DEP0154: RSA-PSS generate key pair options
- DEP0155: Trailing slashes in pattern specifier resolutions
- DEP0156:
.aborted
property and'abort'
,'aborted'
event inhttp
- DEP0157: Thenable support in streams
- DEP0158:
buffer.slice(start, end)
- DEP0159:
ERR_INVALID_CALLBACK
- DEP0160:
process.on('multipleResolves', handler)
- DEP0161:
process._getActiveRequests()
andprocess._getActiveHandles()
- DEP0162:
fs.write()
,fs.writeFileSync()
coercion to string - DEP0163:
channel.subscribe(onMessage)
,channel.unsubscribe(onMessage)
- DEP0164:
process.exit(code)
,process.exitCode
coercion to integer - DEP0165:
--trace-atomics-wait
- DEP0166: Double slashes in imports and exports targets
- DEP0167: Weak
DiffieHellmanGroup
instances (modp1
,modp2
,modp5
) - DEP0168: Unhandled exception in Node-API callbacks
- DEP0169: Insecure url.parse()
- DEP0170: Invalid port when using
url.parse()
- DEP0171: Setters for
http.IncomingMessage
headers and trailers - DEP0172: The
asyncResource
property ofAsyncResource
bound functions
- DEP0001:
- Diagnostics Channel
- DNS
- Class:
dns.Resolver
dns.getServers()
dns.lookup(hostname[, options], callback)
dns.lookupService(address, port, callback)
dns.resolve(hostname[, rrtype], callback)
dns.resolve4(hostname[, options], callback)
dns.resolve6(hostname[, options], callback)
dns.resolveAny(hostname, callback)
dns.resolveCname(hostname, callback)
dns.resolveCaa(hostname, callback)
dns.resolveMx(hostname, callback)
dns.resolveNaptr(hostname, callback)
dns.resolveNs(hostname, callback)
dns.resolvePtr(hostname, callback)
dns.resolveSoa(hostname, callback)
dns.resolveSrv(hostname, callback)
dns.resolveTxt(hostname, callback)
dns.reverse(ip, callback)
dns.setDefaultResultOrder(order)
dns.setServers(servers)
- DNS promises API
- Class:
dnsPromises.Resolver
resolver.cancel()
dnsPromises.getServers()
dnsPromises.lookup(hostname[, options])
dnsPromises.lookupService(address, port)
dnsPromises.resolve(hostname[, rrtype])
dnsPromises.resolve4(hostname[, options])
dnsPromises.resolve6(hostname[, options])
dnsPromises.resolveAny(hostname)
dnsPromises.resolveCaa(hostname)
dnsPromises.resolveCname(hostname)
dnsPromises.resolveMx(hostname)
dnsPromises.resolveNaptr(hostname)
dnsPromises.resolveNs(hostname)
dnsPromises.resolvePtr(hostname)
dnsPromises.resolveSoa(hostname)
dnsPromises.resolveSrv(hostname)
dnsPromises.resolveTxt(hostname)
dnsPromises.reverse(ip)
dnsPromises.setDefaultResultOrder(order)
dnsPromises.setServers(servers)
- Class:
- Error codes
- Implementation considerations
- Class:
- Domain
- Errors
- Error propagation and interception
- Class:
Error
- Class:
AssertionError
- Class:
RangeError
- Class:
ReferenceError
- Class:
SyntaxError
- Class:
SystemError
- Class:
TypeError
- Exceptions vs. errors
- OpenSSL errors
- Node.js error codes
ABORT_ERR
ERR_ACCESS_DENIED
ERR_AMBIGUOUS_ARGUMENT
ERR_ARG_NOT_ITERABLE
ERR_ASSERTION
ERR_ASYNC_CALLBACK
ERR_ASYNC_TYPE
ERR_BROTLI_COMPRESSION_FAILED
ERR_BROTLI_INVALID_PARAM
ERR_BUFFER_CONTEXT_NOT_AVAILABLE
ERR_BUFFER_OUT_OF_BOUNDS
ERR_BUFFER_TOO_LARGE
ERR_CANNOT_WATCH_SIGINT
ERR_CHILD_CLOSED_BEFORE_REPLY
ERR_CHILD_PROCESS_IPC_REQUIRED
ERR_CHILD_PROCESS_STDIO_MAXBUFFER
ERR_CLOSED_MESSAGE_PORT
ERR_CONSOLE_WRITABLE_STREAM
ERR_CONSTRUCT_CALL_INVALID
ERR_CONSTRUCT_CALL_REQUIRED
ERR_CONTEXT_NOT_INITIALIZED
ERR_CRYPTO_CUSTOM_ENGINE_NOT_SUPPORTED
ERR_CRYPTO_ECDH_INVALID_FORMAT
ERR_CRYPTO_ECDH_INVALID_PUBLIC_KEY
ERR_CRYPTO_ENGINE_UNKNOWN
ERR_CRYPTO_FIPS_FORCED
ERR_CRYPTO_FIPS_UNAVAILABLE
ERR_CRYPTO_HASH_FINALIZED
ERR_CRYPTO_HASH_UPDATE_FAILED
ERR_CRYPTO_INCOMPATIBLE_KEY
ERR_CRYPTO_INCOMPATIBLE_KEY_OPTIONS
ERR_CRYPTO_INITIALIZATION_FAILED
ERR_CRYPTO_INVALID_AUTH_TAG
ERR_CRYPTO_INVALID_COUNTER
ERR_CRYPTO_INVALID_CURVE
ERR_CRYPTO_INVALID_DIGEST
ERR_CRYPTO_INVALID_IV
ERR_CRYPTO_INVALID_JWK
ERR_CRYPTO_INVALID_KEY_OBJECT_TYPE
ERR_CRYPTO_INVALID_KEYLEN
ERR_CRYPTO_INVALID_KEYPAIR
ERR_CRYPTO_INVALID_KEYTYPE
ERR_CRYPTO_INVALID_MESSAGELEN
ERR_CRYPTO_INVALID_SCRYPT_PARAMS
ERR_CRYPTO_INVALID_STATE
ERR_CRYPTO_INVALID_TAG_LENGTH
ERR_CRYPTO_JOB_INIT_FAILED
ERR_CRYPTO_JWK_UNSUPPORTED_CURVE
ERR_CRYPTO_JWK_UNSUPPORTED_KEY_TYPE
ERR_CRYPTO_OPERATION_FAILED
ERR_CRYPTO_PBKDF2_ERROR
ERR_CRYPTO_SCRYPT_INVALID_PARAMETER
ERR_CRYPTO_SCRYPT_NOT_SUPPORTED
ERR_CRYPTO_SIGN_KEY_REQUIRED
ERR_CRYPTO_TIMING_SAFE_EQUAL_LENGTH
ERR_CRYPTO_UNKNOWN_CIPHER
ERR_CRYPTO_UNKNOWN_DH_GROUP
ERR_CRYPTO_UNSUPPORTED_OPERATION
ERR_DEBUGGER_ERROR
ERR_DEBUGGER_STARTUP_ERROR
ERR_DLOPEN_DISABLED
ERR_DLOPEN_FAILED
ERR_DIR_CLOSED
ERR_DIR_CONCURRENT_OPERATION
ERR_DNS_SET_SERVERS_FAILED
ERR_DOMAIN_CALLBACK_NOT_AVAILABLE
ERR_DOMAIN_CANNOT_SET_UNCAUGHT_EXCEPTION_CAPTURE
ERR_DUPLICATE_STARTUP_SNAPSHOT_MAIN_FUNCTION
ERR_ENCODING_INVALID_ENCODED_DATA
ERR_ENCODING_NOT_SUPPORTED
ERR_EVAL_ESM_CANNOT_PRINT
ERR_EVENT_RECURSION
ERR_EXECUTION_ENVIRONMENT_NOT_AVAILABLE
ERR_FALSY_VALUE_REJECTION
ERR_FEATURE_UNAVAILABLE_ON_PLATFORM
ERR_FS_CP_DIR_TO_NON_DIR
ERR_FS_CP_EEXIST
ERR_FS_CP_EINVAL
ERR_HTTP_CONTENT_LENGTH_MISMATCH
ERR_FS_CP_FIFO_PIPE
ERR_FS_CP_NON_DIR_TO_DIR
ERR_FS_CP_SOCKET
ERR_FS_CP_SYMLINK_TO_SUBDIRECTORY
ERR_FS_CP_UNKNOWN
ERR_FS_EISDIR
ERR_FS_FILE_TOO_LARGE
ERR_FS_INVALID_SYMLINK_TYPE
ERR_HTTP_HEADERS_SENT
ERR_HTTP_INVALID_HEADER_VALUE
ERR_HTTP_INVALID_STATUS_CODE
ERR_HTTP_REQUEST_TIMEOUT
ERR_HTTP_SOCKET_ENCODING
ERR_HTTP_TRAILER_INVALID
ERR_HTTP2_ALTSVC_INVALID_ORIGIN
ERR_HTTP2_ALTSVC_LENGTH
ERR_HTTP2_CONNECT_AUTHORITY
ERR_HTTP2_CONNECT_PATH
ERR_HTTP2_CONNECT_SCHEME
ERR_HTTP2_ERROR
ERR_HTTP2_GOAWAY_SESSION
ERR_HTTP2_HEADER_SINGLE_VALUE
ERR_HTTP2_HEADERS_AFTER_RESPOND
ERR_HTTP2_HEADERS_SENT
ERR_HTTP2_INFO_STATUS_NOT_ALLOWED
ERR_HTTP2_INVALID_CONNECTION_HEADERS
ERR_HTTP2_INVALID_HEADER_VALUE
ERR_HTTP2_INVALID_INFO_STATUS
ERR_HTTP2_INVALID_ORIGIN
ERR_HTTP2_INVALID_PACKED_SETTINGS_LENGTH
ERR_HTTP2_INVALID_PSEUDOHEADER
ERR_HTTP2_INVALID_SESSION
ERR_HTTP2_INVALID_SETTING_VALUE
ERR_HTTP2_INVALID_STREAM
ERR_HTTP2_MAX_PENDING_SETTINGS_ACK
ERR_HTTP2_NESTED_PUSH
ERR_HTTP2_NO_MEM
ERR_HTTP2_NO_SOCKET_MANIPULATION
ERR_HTTP2_ORIGIN_LENGTH
ERR_HTTP2_OUT_OF_STREAMS
ERR_HTTP2_PAYLOAD_FORBIDDEN
ERR_HTTP2_PING_CANCEL
ERR_HTTP2_PING_LENGTH
ERR_HTTP2_PSEUDOHEADER_NOT_ALLOWED
ERR_HTTP2_PUSH_DISABLED
ERR_HTTP2_SEND_FILE
ERR_HTTP2_SEND_FILE_NOSEEK
ERR_HTTP2_SESSION_ERROR
ERR_HTTP2_SETTINGS_CANCEL
ERR_HTTP2_SOCKET_BOUND
ERR_HTTP2_SOCKET_UNBOUND
ERR_HTTP2_STATUS_101
ERR_HTTP2_STATUS_INVALID
ERR_HTTP2_STREAM_CANCEL
ERR_HTTP2_STREAM_ERROR
ERR_HTTP2_STREAM_SELF_DEPENDENCY
ERR_HTTP2_TOO_MANY_INVALID_FRAMES
ERR_HTTP2_TRAILERS_ALREADY_SENT
ERR_HTTP2_TRAILERS_NOT_READY
ERR_HTTP2_UNSUPPORTED_PROTOCOL
ERR_ILLEGAL_CONSTRUCTOR
ERR_IMPORT_ASSERTION_TYPE_FAILED
ERR_IMPORT_ASSERTION_TYPE_MISSING
ERR_IMPORT_ASSERTION_TYPE_UNSUPPORTED
ERR_INCOMPATIBLE_OPTION_PAIR
ERR_INPUT_TYPE_NOT_ALLOWED
ERR_INSPECTOR_ALREADY_ACTIVATED
ERR_INSPECTOR_ALREADY_CONNECTED
ERR_INSPECTOR_CLOSED
ERR_INSPECTOR_COMMAND
ERR_INSPECTOR_NOT_ACTIVE
ERR_INSPECTOR_NOT_AVAILABLE
ERR_INSPECTOR_NOT_CONNECTED
ERR_INSPECTOR_NOT_WORKER
ERR_INTERNAL_ASSERTION
ERR_INVALID_ADDRESS_FAMILY
ERR_INVALID_ARG_TYPE
ERR_INVALID_ARG_VALUE
ERR_INVALID_ASYNC_ID
ERR_INVALID_BUFFER_SIZE
ERR_INVALID_CHAR
ERR_INVALID_CURSOR_POS
ERR_INVALID_FD
ERR_INVALID_FD_TYPE
ERR_INVALID_FILE_URL_HOST
ERR_INVALID_FILE_URL_PATH
ERR_INVALID_HANDLE_TYPE
ERR_INVALID_HTTP_TOKEN
ERR_INVALID_IP_ADDRESS
ERR_INVALID_MIME_SYNTAX
ERR_INVALID_MODULE
ERR_INVALID_MODULE_SPECIFIER
ERR_INVALID_OBJECT_DEFINE_PROPERTY
ERR_INVALID_PACKAGE_CONFIG
ERR_INVALID_PACKAGE_TARGET
ERR_INVALID_PERFORMANCE_MARK
ERR_INVALID_PROTOCOL
ERR_INVALID_REPL_EVAL_CONFIG
ERR_INVALID_REPL_INPUT
ERR_INVALID_RETURN_PROPERTY
ERR_INVALID_RETURN_PROPERTY_VALUE
ERR_INVALID_RETURN_VALUE
ERR_INVALID_STATE
ERR_INVALID_SYNC_FORK_INPUT
ERR_INVALID_THIS
ERR_INVALID_TRANSFER_OBJECT
ERR_INVALID_TUPLE
ERR_INVALID_URI
ERR_INVALID_URL
ERR_INVALID_URL_SCHEME
ERR_IPC_CHANNEL_CLOSED
ERR_IPC_DISCONNECTED
ERR_IPC_ONE_PIPE
ERR_IPC_SYNC_FORK
ERR_LOADER_CHAIN_INCOMPLETE
ERR_MANIFEST_ASSERT_INTEGRITY
ERR_MANIFEST_DEPENDENCY_MISSING
ERR_MANIFEST_INTEGRITY_MISMATCH
ERR_MANIFEST_INVALID_RESOURCE_FIELD
ERR_MANIFEST_INVALID_SPECIFIER
ERR_MANIFEST_PARSE_POLICY
ERR_MANIFEST_TDZ
ERR_MANIFEST_UNKNOWN_ONERROR
ERR_MEMORY_ALLOCATION_FAILED
ERR_MESSAGE_TARGET_CONTEXT_UNAVAILABLE
ERR_METHOD_NOT_IMPLEMENTED
ERR_MISSING_ARGS
ERR_MISSING_OPTION
ERR_MISSING_PASSPHRASE
ERR_MISSING_PLATFORM_FOR_WORKER
ERR_MISSING_TRANSFERABLE_IN_TRANSFER_LIST
ERR_MODULE_NOT_FOUND
ERR_MULTIPLE_CALLBACK
ERR_NAPI_CONS_FUNCTION
ERR_NAPI_INVALID_DATAVIEW_ARGS
ERR_NAPI_INVALID_TYPEDARRAY_ALIGNMENT
ERR_NAPI_INVALID_TYPEDARRAY_LENGTH
ERR_NAPI_TSFN_CALL_JS
ERR_NAPI_TSFN_GET_UNDEFINED
ERR_NAPI_TSFN_START_IDLE_LOOP
ERR_NAPI_TSFN_STOP_IDLE_LOOP
ERR_NOT_BUILDING_SNAPSHOT
ERR_NO_CRYPTO
ERR_NO_ICU
ERR_NON_CONTEXT_AWARE_DISABLED
ERR_OUT_OF_RANGE
ERR_PACKAGE_IMPORT_NOT_DEFINED
ERR_PACKAGE_PATH_NOT_EXPORTED
ERR_PARSE_ARGS_INVALID_OPTION_VALUE
ERR_PARSE_ARGS_UNEXPECTED_POSITIONAL
ERR_PARSE_ARGS_UNKNOWN_OPTION
ERR_PERFORMANCE_INVALID_TIMESTAMP
ERR_PERFORMANCE_MEASURE_INVALID_OPTIONS
ERR_PROTO_ACCESS
ERR_REQUIRE_ESM
ERR_SCRIPT_EXECUTION_INTERRUPTED
ERR_SCRIPT_EXECUTION_TIMEOUT
ERR_SERVER_ALREADY_LISTEN
ERR_SERVER_NOT_RUNNING
ERR_SOCKET_ALREADY_BOUND
ERR_SOCKET_BAD_BUFFER_SIZE
ERR_SOCKET_BAD_PORT
ERR_SOCKET_BAD_TYPE
ERR_SOCKET_BUFFER_SIZE
ERR_SOCKET_CLOSED
ERR_SOCKET_CLOSED_BEFORE_CONNECTION
ERR_SOCKET_DGRAM_IS_CONNECTED
ERR_SOCKET_DGRAM_NOT_CONNECTED
ERR_SOCKET_DGRAM_NOT_RUNNING
ERR_SRI_PARSE
ERR_STREAM_ALREADY_FINISHED
ERR_STREAM_CANNOT_PIPE
ERR_STREAM_DESTROYED
ERR_STREAM_NULL_VALUES
ERR_STREAM_PREMATURE_CLOSE
ERR_STREAM_PUSH_AFTER_EOF
ERR_STREAM_UNSHIFT_AFTER_END_EVENT
ERR_STREAM_WRAP
ERR_STREAM_WRITE_AFTER_END
ERR_STRING_TOO_LONG
ERR_SYNTHETIC
ERR_SYSTEM_ERROR
ERR_TAP_LEXER_ERROR
ERR_TAP_PARSER_ERROR
ERR_TAP_VALIDATION_ERROR
ERR_TEST_FAILURE
ERR_TLS_CERT_ALTNAME_FORMAT
ERR_TLS_CERT_ALTNAME_INVALID
ERR_TLS_DH_PARAM_SIZE
ERR_TLS_HANDSHAKE_TIMEOUT
ERR_TLS_INVALID_CONTEXT
ERR_TLS_INVALID_PROTOCOL_METHOD
ERR_TLS_INVALID_PROTOCOL_VERSION
ERR_TLS_INVALID_STATE
ERR_TLS_PROTOCOL_VERSION_CONFLICT
ERR_TLS_PSK_SET_IDENTIY_HINT_FAILED
ERR_TLS_RENEGOTIATION_DISABLED
ERR_TLS_REQUIRED_SERVER_NAME
ERR_TLS_SESSION_ATTACK
ERR_TLS_SNI_FROM_SERVER
ERR_TRACE_EVENTS_CATEGORY_REQUIRED
ERR_TRACE_EVENTS_UNAVAILABLE
ERR_TRANSFORM_ALREADY_TRANSFORMING
ERR_TRANSFORM_WITH_LENGTH_0
ERR_TTY_INIT_FAILED
ERR_UNAVAILABLE_DURING_EXIT
ERR_UNCAUGHT_EXCEPTION_CAPTURE_ALREADY_SET
ERR_UNESCAPED_CHARACTERS
ERR_UNHANDLED_ERROR
ERR_UNKNOWN_BUILTIN_MODULE
ERR_UNKNOWN_CREDENTIAL
ERR_UNKNOWN_ENCODING
ERR_UNKNOWN_FILE_EXTENSION
ERR_UNKNOWN_MODULE_FORMAT
ERR_UNKNOWN_SIGNAL
ERR_UNSUPPORTED_DIR_IMPORT
ERR_UNSUPPORTED_ESM_URL_SCHEME
ERR_USE_AFTER_CLOSE
ERR_VALID_PERFORMANCE_ENTRY_TYPE
ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING
ERR_VM_MODULE_ALREADY_LINKED
ERR_VM_MODULE_CACHED_DATA_REJECTED
ERR_VM_MODULE_CANNOT_CREATE_CACHED_DATA
ERR_VM_MODULE_DIFFERENT_CONTEXT
ERR_VM_MODULE_LINK_FAILURE
ERR_VM_MODULE_NOT_MODULE
ERR_VM_MODULE_STATUS
ERR_WASI_ALREADY_STARTED
ERR_WASI_NOT_STARTED
ERR_WEBASSEMBLY_RESPONSE
ERR_WORKER_INIT_FAILED
ERR_WORKER_INVALID_EXEC_ARGV
ERR_WORKER_NOT_RUNNING
ERR_WORKER_OUT_OF_MEMORY
ERR_WORKER_PATH
ERR_WORKER_UNSERIALIZABLE_ERROR
ERR_WORKER_UNSUPPORTED_OPERATION
ERR_ZLIB_INITIALIZATION_FAILED
HPE_HEADER_OVERFLOW
HPE_UNEXPECTED_CONTENT_LENGTH
MODULE_NOT_FOUND
- Legacy Node.js error codes
ERR_CANNOT_TRANSFER_OBJECT
ERR_CRYPTO_HASH_DIGEST_NO_UTF16
ERR_HTTP2_FRAME_ERROR
ERR_HTTP2_HEADERS_OBJECT
ERR_HTTP2_HEADER_REQUIRED
ERR_HTTP2_INFO_HEADERS_AFTER_RESPOND
ERR_HTTP2_STREAM_CLOSED
ERR_HTTP_INVALID_CHAR
ERR_INDEX_OUT_OF_RANGE
ERR_INVALID_OPT_VALUE
ERR_INVALID_OPT_VALUE_ENCODING
ERR_MISSING_MESSAGE_PORT_IN_TRANSFER_LIST
ERR_NAPI_CONS_PROTOTYPE_OBJECT
ERR_NETWORK_IMPORT_BAD_RESPONSE
ERR_NETWORK_IMPORT_DISALLOWED
ERR_NO_LONGER_SUPPORTED
ERR_OPERATION_FAILED
ERR_OUTOFMEMORY
ERR_PARSE_HISTORY_DATA
ERR_SOCKET_CANNOT_SEND
ERR_STDERR_CLOSE
ERR_STDOUT_CLOSE
ERR_STREAM_READ_NOT_IMPLEMENTED
ERR_TLS_RENEGOTIATION_FAILED
ERR_TRANSFERRING_EXTERNALIZED_SHAREDARRAYBUFFER
ERR_UNKNOWN_STDIN_TYPE
ERR_UNKNOWN_STREAM_TYPE
ERR_V8BREAKITERATOR
ERR_VALUE_OUT_OF_RANGE
ERR_VM_MODULE_NOT_LINKED
ERR_VM_MODULE_LINKING_ERRORED
ERR_WORKER_UNSUPPORTED_EXTENSION
ERR_ZLIB_BINDING_CLOSED
ERR_CPU_USAGE
- Events
- Passing arguments and
this
to listeners - Asynchronous vs. synchronous
- Handling events only once
- Error events
- Capture rejections of promises
- Class:
EventEmitter
- Event:
'newListener'
- Event:
'removeListener'
emitter.addListener(eventName, listener)
emitter.emit(eventName[, ...args])
emitter.eventNames()
emitter.getMaxListeners()
emitter.listenerCount(eventName[, listener])
emitter.listeners(eventName)
emitter.off(eventName, listener)
emitter.on(eventName, listener)
emitter.once(eventName, listener)
emitter.prependListener(eventName, listener)
emitter.prependOnceListener(eventName, listener)
emitter.removeAllListeners([eventName])
emitter.removeListener(eventName, listener)
emitter.setMaxListeners(n)
emitter.rawListeners(eventName)
emitter[Symbol.for('nodejs.rejection')](err, eventName[, ...args])
- Event:
events.defaultMaxListeners
events.errorMonitor
events.getEventListeners(emitterOrTarget, eventName)
events.getMaxListeners(emitterOrTarget)
events.once(emitter, name[, options])
events.captureRejections
events.captureRejectionSymbol
events.listenerCount(emitter, eventName)
events.on(emitter, eventName[, options])
events.setMaxListeners(n[, ...eventTargets])
- Class:
events.EventEmitterAsyncResource extends EventEmitter
EventTarget
andEvent
API- Node.js
EventTarget
vs. DOMEventTarget
NodeEventTarget
vs.EventEmitter
- Event listener
EventTarget
error handling- Class:
Event
event.bubbles
event.cancelBubble
event.cancelable
event.composed
event.composedPath()
event.currentTarget
event.defaultPrevented
event.eventPhase
event.initEvent(type[, bubbles[, cancelable]])
event.isTrusted
event.preventDefault()
event.returnValue
event.srcElement
event.stopImmediatePropagation()
event.stopPropagation()
event.target
event.timeStamp
event.type
- Class:
EventTarget
- Class:
CustomEvent
- Class:
NodeEventTarget
nodeEventTarget.addListener(type, listener)
nodeEventTarget.emit(type, arg)
nodeEventTarget.eventNames()
nodeEventTarget.listenerCount(type)
nodeEventTarget.setMaxListeners(n)
nodeEventTarget.getMaxListeners()
nodeEventTarget.off(type, listener[, options])
nodeEventTarget.on(type, listener)
nodeEventTarget.once(type, listener)
nodeEventTarget.removeAllListeners([type])
nodeEventTarget.removeListener(type, listener[, options])
- Node.js
- Passing arguments and
- File system
- Promise example
- Callback example
- Synchronous example
- Promises API
- Class:
FileHandle
- Event:
'close'
filehandle.appendFile(data[, options])
filehandle.chmod(mode)
filehandle.chown(uid, gid)
filehandle.close()
filehandle.createReadStream([options])
filehandle.createWriteStream([options])
filehandle.datasync()
filehandle.fd
filehandle.read(buffer, offset, length, position)
filehandle.read([options])
filehandle.read(buffer[, options])
filehandle.readableWebStream()
filehandle.readFile(options)
filehandle.readLines([options])
filehandle.readv(buffers[, position])
filehandle.stat([options])
filehandle.sync()
filehandle.truncate(len)
filehandle.utimes(atime, mtime)
filehandle.write(buffer, offset[, length[, position]])
filehandle.write(buffer[, options])
filehandle.write(string[, position[, encoding]])
filehandle.writeFile(data, options)
filehandle.writev(buffers[, position])
- Event:
fsPromises.access(path[, mode])
fsPromises.appendFile(path, data[, options])
fsPromises.chmod(path, mode)
fsPromises.chown(path, uid, gid)
fsPromises.copyFile(src, dest[, mode])
fsPromises.cp(src, dest[, options])
fsPromises.lchmod(path, mode)
fsPromises.lchown(path, uid, gid)
fsPromises.lutimes(path, atime, mtime)
fsPromises.link(existingPath, newPath)
fsPromises.lstat(path[, options])
fsPromises.mkdir(path[, options])
fsPromises.mkdtemp(prefix[, options])
fsPromises.open(path, flags[, mode])
fsPromises.opendir(path[, options])
fsPromises.readdir(path[, options])
fsPromises.readFile(path[, options])
fsPromises.readlink(path[, options])
fsPromises.realpath(path[, options])
fsPromises.rename(oldPath, newPath)
fsPromises.rmdir(path[, options])
fsPromises.rm(path[, options])
fsPromises.stat(path[, options])
fsPromises.statfs(path[, options])
fsPromises.symlink(target, path[, type])
fsPromises.truncate(path[, len])
fsPromises.unlink(path)
fsPromises.utimes(path, atime, mtime)
fsPromises.watch(filename[, options])
fsPromises.writeFile(file, data[, options])
fsPromises.constants
- Class:
- Callback API
fs.access(path[, mode], callback)
fs.appendFile(path, data[, options], callback)
fs.chmod(path, mode, callback)
fs.chown(path, uid, gid, callback)
fs.close(fd[, callback])
fs.copyFile(src, dest[, mode], callback)
fs.cp(src, dest[, options], callback)
fs.createReadStream(path[, options])
fs.createWriteStream(path[, options])
fs.exists(path, callback)
fs.fchmod(fd, mode, callback)
fs.fchown(fd, uid, gid, callback)
fs.fdatasync(fd, callback)
fs.fstat(fd[, options], callback)
fs.fsync(fd, callback)
fs.ftruncate(fd[, len], callback)
fs.futimes(fd, atime, mtime, callback)
fs.lchmod(path, mode, callback)
fs.lchown(path, uid, gid, callback)
fs.lutimes(path, atime, mtime, callback)
fs.link(existingPath, newPath, callback)
fs.lstat(path[, options], callback)
fs.mkdir(path[, options], callback)
fs.mkdtemp(prefix[, options], callback)
fs.open(path[, flags[, mode]], callback)
fs.openAsBlob(path[, options])
fs.opendir(path[, options], callback)
fs.read(fd, buffer, offset, length, position, callback)
fs.read(fd[, options], callback)
fs.read(fd, buffer[, options], callback)
fs.readdir(path[, options], callback)
fs.readFile(path[, options], callback)
fs.readlink(path[, options], callback)
fs.readv(fd, buffers[, position], callback)
fs.realpath(path[, options], callback)
fs.realpath.native(path[, options], callback)
fs.rename(oldPath, newPath, callback)
fs.rmdir(path[, options], callback)
fs.rm(path[, options], callback)
fs.stat(path[, options], callback)
fs.statfs(path[, options], callback)
fs.symlink(target, path[, type], callback)
fs.truncate(path[, len], callback)
fs.unlink(path, callback)
fs.unwatchFile(filename[, listener])
fs.utimes(path, atime, mtime, callback)
fs.watch(filename[, options][, listener])
fs.watchFile(filename[, options], listener)
fs.write(fd, buffer, offset[, length[, position]], callback)
fs.write(fd, buffer[, options], callback)
fs.write(fd, string[, position[, encoding]], callback)
fs.writeFile(file, data[, options], callback)
fs.writev(fd, buffers[, position], callback)
- Synchronous API
fs.accessSync(path[, mode])
fs.appendFileSync(path, data[, options])
fs.chmodSync(path, mode)
fs.chownSync(path, uid, gid)
fs.closeSync(fd)
fs.copyFileSync(src, dest[, mode])
fs.cpSync(src, dest[, options])
fs.existsSync(path)
fs.fchmodSync(fd, mode)
fs.fchownSync(fd, uid, gid)
fs.fdatasyncSync(fd)
fs.fstatSync(fd[, options])
fs.fsyncSync(fd)
fs.ftruncateSync(fd[, len])
fs.futimesSync(fd, atime, mtime)
fs.lchmodSync(path, mode)
fs.lchownSync(path, uid, gid)
fs.lutimesSync(path, atime, mtime)
fs.linkSync(existingPath, newPath)
fs.lstatSync(path[, options])
fs.mkdirSync(path[, options])
fs.mkdtempSync(prefix[, options])
fs.opendirSync(path[, options])
fs.openSync(path[, flags[, mode]])
fs.readdirSync(path[, options])
fs.readFileSync(path[, options])
fs.readlinkSync(path[, options])
fs.readSync(fd, buffer, offset, length[, position])
fs.readSync(fd, buffer[, options])
fs.readvSync(fd, buffers[, position])
fs.realpathSync(path[, options])
fs.realpathSync.native(path[, options])
fs.renameSync(oldPath, newPath)
fs.rmdirSync(path[, options])
fs.rmSync(path[, options])
fs.statSync(path[, options])
fs.statfsSync(path[, options])
fs.symlinkSync(target, path[, type])
fs.truncateSync(path[, len])
fs.unlinkSync(path)
fs.utimesSync(path, atime, mtime)
fs.writeFileSync(file, data[, options])
fs.writeSync(fd, buffer, offset[, length[, position]])
fs.writeSync(fd, buffer[, options])
fs.writeSync(fd, string[, position[, encoding]])
fs.writevSync(fd, buffers[, position])
- Common Objects
- Class:
fs.Dir
- Class:
fs.Dirent
- Class:
fs.FSWatcher
- Class:
fs.StatWatcher
- Class:
fs.ReadStream
- Class:
fs.Stats
stats.isBlockDevice()
stats.isCharacterDevice()
stats.isDirectory()
stats.isFIFO()
stats.isFile()
stats.isSocket()
stats.isSymbolicLink()
stats.dev
stats.ino
stats.mode
stats.nlink
stats.uid
stats.gid
stats.rdev
stats.size
stats.blksize
stats.blocks
stats.atimeMs
stats.mtimeMs
stats.ctimeMs
stats.birthtimeMs
stats.atimeNs
stats.mtimeNs
stats.ctimeNs
stats.birthtimeNs
stats.atime
stats.mtime
stats.ctime
stats.birthtime
- Stat time values
- Class:
fs.StatFs
- Class:
fs.WriteStream
fs.constants
- Class:
- Notes
- Global objects
- Class:
AbortController
- Class:
Blob
- Class:
Buffer
- Class:
ByteLengthQueuingStrategy
__dirname
__filename
atob(data)
BroadcastChannel
btoa(data)
clearImmediate(immediateObject)
clearInterval(intervalObject)
clearTimeout(timeoutObject)
- Class:
CompressionStream
console
- Class:
CountQueuingStrategy
Crypto
crypto
CryptoKey
CustomEvent
- Class:
DecompressionStream
Event
EventTarget
exports
fetch
- Class:
File
- Class
FormData
global
- Class
Headers
MessageChannel
MessageEvent
MessagePort
module
PerformanceEntry
PerformanceMark
PerformanceMeasure
PerformanceObserver
PerformanceObserverEntryList
PerformanceResourceTiming
performance
process
queueMicrotask(callback)
- Class:
ReadableByteStreamController
- Class:
ReadableStream
- Class:
ReadableStreamBYOBReader
- Class:
ReadableStreamBYOBRequest
- Class:
ReadableStreamDefaultController
- Class:
ReadableStreamDefaultReader
require()
Response
Request
setImmediate(callback[, ...args])
setInterval(callback, delay[, ...args])
setTimeout(callback, delay[, ...args])
structuredClone(value[, options])
SubtleCrypto
DOMException
TextDecoder
- Class:
TextDecoderStream
TextEncoder
- Class:
TextEncoderStream
- Class:
TransformStream
- Class:
TransformStreamDefaultController
URL
URLSearchParams
WebAssembly
- Class:
WritableStream
- Class:
WritableStreamDefaultController
- Class:
WritableStreamDefaultWriter
- Class:
- HTTP
- Class:
http.Agent
- Class:
http.ClientRequest
- Event:
'abort'
- Event:
'close'
- Event:
'connect'
- Event:
'continue'
- Event:
'finish'
- Event:
'information'
- Event:
'response'
- Event:
'socket'
- Event:
'timeout'
- Event:
'upgrade'
request.abort()
request.aborted
request.connection
request.cork()
request.end([data[, encoding]][, callback])
request.destroy([error])
request.finished
request.flushHeaders()
request.getHeader(name)
request.getHeaderNames()
request.getHeaders()
request.getRawHeaderNames()
request.hasHeader(name)
request.maxHeadersCount
request.path
request.method
request.host
request.protocol
request.removeHeader(name)
request.reusedSocket
request.setHeader(name, value)
request.setNoDelay([noDelay])
request.setSocketKeepAlive([enable][, initialDelay])
request.setTimeout(timeout[, callback])
request.socket
request.uncork()
request.writableEnded
request.writableFinished
request.write(chunk[, encoding][, callback])
- Event:
- Class:
http.Server
- Event:
'checkContinue'
- Event:
'checkExpectation'
- Event:
'clientError'
- Event:
'close'
- Event:
'connect'
- Event:
'connection'
- Event:
'dropRequest'
- Event:
'request'
- Event:
'upgrade'
server.close([callback])
server.closeAllConnections()
server.closeIdleConnections()
server.headersTimeout
server.listen()
server.listening
server.maxHeadersCount
server.requestTimeout
server.setTimeout([msecs][, callback])
server.maxRequestsPerSocket
server.timeout
server.keepAliveTimeout
- Event:
- Class:
http.ServerResponse
- Event:
'close'
- Event:
'finish'
response.addTrailers(headers)
response.connection
response.cork()
response.end([data[, encoding]][, callback])
response.finished
response.flushHeaders()
response.getHeader(name)
response.getHeaderNames()
response.getHeaders()
response.hasHeader(name)
response.headersSent
response.removeHeader(name)
response.req
response.sendDate
response.setHeader(name, value)
response.setTimeout(msecs[, callback])
response.socket
response.statusCode
response.statusMessage
response.strictContentLength
response.uncork()
response.writableEnded
response.writableFinished
response.write(chunk[, encoding][, callback])
response.writeContinue()
response.writeEarlyHints(hints[, callback])
response.writeHead(statusCode[, statusMessage][, headers])
response.writeProcessing()
- Event:
- Class:
http.IncomingMessage
- Event:
'aborted'
- Event:
'close'
message.aborted
message.complete
message.connection
message.destroy([error])
message.headers
message.headersDistinct
message.httpVersion
message.method
message.rawHeaders
message.rawTrailers
message.setTimeout(msecs[, callback])
message.socket
message.statusCode
message.statusMessage
message.trailers
message.trailersDistinct
message.url
- Event:
- Class:
http.OutgoingMessage
- Event:
'drain'
- Event:
'finish'
- Event:
'prefinish'
outgoingMessage.addTrailers(headers)
outgoingMessage.appendHeader(name, value)
outgoingMessage.connection
outgoingMessage.cork()
outgoingMessage.destroy([error])
outgoingMessage.end(chunk[, encoding][, callback])
outgoingMessage.flushHeaders()
outgoingMessage.getHeader(name)
outgoingMessage.getHeaderNames()
outgoingMessage.getHeaders()
outgoingMessage.hasHeader(name)
outgoingMessage.headersSent
outgoingMessage.pipe()
outgoingMessage.removeHeader(name)
outgoingMessage.setHeader(name, value)
outgoingMessage.setHeaders(headers)
outgoingMessage.setTimeout(msesc[, callback])
outgoingMessage.socket
outgoingMessage.uncork()
outgoingMessage.writableCorked
outgoingMessage.writableEnded
outgoingMessage.writableFinished
outgoingMessage.writableHighWaterMark
outgoingMessage.writableLength
outgoingMessage.writableObjectMode
outgoingMessage.write(chunk[, encoding][, callback])
- Event:
http.METHODS
http.STATUS_CODES
http.createServer([options][, requestListener])
http.get(options[, callback])
http.get(url[, options][, callback])
http.globalAgent
http.maxHeaderSize
http.request(options[, callback])
http.request(url[, options][, callback])
http.validateHeaderName(name[, label])
http.validateHeaderValue(name, value)
http.setMaxIdleHTTPParsers(max)
- Class:
- HTTP/2
- Determining if crypto support is unavailable
- Core API
- Server-side example
- Client-side example
- Class:
Http2Session
Http2Session
and sockets- Event:
'close'
- Event:
'connect'
- Event:
'error'
- Event:
'frameError'
- Event:
'goaway'
- Event:
'localSettings'
- Event:
'ping'
- Event:
'remoteSettings'
- Event:
'stream'
- Event:
'timeout'
http2session.alpnProtocol
http2session.close([callback])
http2session.closed
http2session.connecting
http2session.destroy([error][, code])
http2session.destroyed
http2session.encrypted
http2session.goaway([code[, lastStreamID[, opaqueData]]])
http2session.localSettings
http2session.originSet
http2session.pendingSettingsAck
http2session.ping([payload, ]callback)
http2session.ref()
http2session.remoteSettings
http2session.setLocalWindowSize(windowSize)
http2session.setTimeout(msecs, callback)
http2session.socket
http2session.state
http2session.settings([settings][, callback])
http2session.type
http2session.unref()
- Class:
ServerHttp2Session
- Class:
ClientHttp2Session
- Class:
Http2Stream
Http2Stream
Lifecycle- Event:
'aborted'
- Event:
'close'
- Event:
'error'
- Event:
'frameError'
- Event:
'ready'
- Event:
'timeout'
- Event:
'trailers'
- Event:
'wantTrailers'
http2stream.aborted
http2stream.bufferSize
http2stream.close(code[, callback])
http2stream.closed
http2stream.destroyed
http2stream.endAfterHeaders
http2stream.id
http2stream.pending
http2stream.priority(options)
http2stream.rstCode
http2stream.sentHeaders
http2stream.sentInfoHeaders
http2stream.sentTrailers
http2stream.session
http2stream.setTimeout(msecs, callback)
http2stream.state
http2stream.sendTrailers(headers)
- Class:
ClientHttp2Stream
- Class:
ServerHttp2Stream
- Class:
Http2Server
- Class:
Http2SecureServer
http2.createServer([options][, onRequestHandler])
http2.createSecureServer(options[, onRequestHandler])
http2.connect(authority[, options][, listener])
http2.constants
http2.getDefaultSettings()
http2.getPackedSettings([settings])
http2.getUnpackedSettings(buf)
http2.sensitiveHeaders
- Headers object
- Settings object
- Error handling
- Invalid character handling in header names and values
- Push streams on the client
- Supporting the
CONNECT
method - The extended
CONNECT
protocol
- Compatibility API
- ALPN negotiation
- Class:
http2.Http2ServerRequest
- Event:
'aborted'
- Event:
'close'
request.aborted
request.authority
request.complete
request.connection
request.destroy([error])
request.headers
request.httpVersion
request.method
request.rawHeaders
request.rawTrailers
request.scheme
request.setTimeout(msecs, callback)
request.socket
request.stream
request.trailers
request.url
- Event:
- Class:
http2.Http2ServerResponse
- Event:
'close'
- Event:
'finish'
response.addTrailers(headers)
response.connection
response.createPushResponse(headers, callback)
response.end([data[, encoding]][, callback])
response.finished
response.getHeader(name)
response.getHeaderNames()
response.getHeaders()
response.hasHeader(name)
response.headersSent
response.removeHeader(name)
response.req
response.sendDate
response.setHeader(name, value)
response.setTimeout(msecs[, callback])
response.socket
response.statusCode
response.statusMessage
response.stream
response.writableEnded
response.write(chunk[, encoding][, callback])
response.writeContinue()
response.writeEarlyHints(hints)
response.writeHead(statusCode[, statusMessage][, headers])
- Event:
- Collecting HTTP/2 performance metrics
- Note on
:authority
andhost
- HTTPS
- Inspector
- Internationalization support
- Modules: CommonJS modules
- Enabling
- Accessing the main module
- Package manager tips
- The
.mjs
extension - All together
- Caching
- Core modules
- Cycles
- File modules
- Folders as modules
- Loading from
node_modules
folders - Loading from the global folders
- The module wrapper
- The module scope
- The
module
object - The
Module
object - Source map v3 support
- Modules: ECMAScript modules
- Modules:
node:module
API - Modules: Packages
- Net
- IPC support
- Class:
net.BlockList
- Class:
net.SocketAddress
- Class:
net.Server
- Class:
net.Socket
new net.Socket([options])
- Event:
'close'
- Event:
'connect'
- Event:
'data'
- Event:
'drain'
- Event:
'end'
- Event:
'error'
- Event:
'lookup'
- Event:
'ready'
- Event:
'timeout'
socket.address()
socket.autoSelectFamilyAttemptedAddresses
socket.bufferSize
socket.bytesRead
socket.bytesWritten
socket.connect()
socket.connecting
socket.destroy([error])
socket.destroyed
socket.destroySoon()
socket.end([data[, encoding]][, callback])
socket.localAddress
socket.localPort
socket.localFamily
socket.pause()
socket.pending
socket.ref()
socket.remoteAddress
socket.remoteFamily
socket.remotePort
socket.resetAndDestroy()
socket.resume()
socket.setEncoding([encoding])
socket.setKeepAlive([enable][, initialDelay])
socket.setNoDelay([noDelay])
socket.setTimeout(timeout[, callback])
socket.timeout
socket.unref()
socket.write(data[, encoding][, callback])
socket.readyState
net.connect()
net.createConnection()
net.createServer([options][, connectionListener])
net.getDefaultAutoSelectFamily()
net.setDefaultAutoSelectFamily(value)
net.getDefaultAutoSelectFamilyAttemptTimeout()
net.setDefaultAutoSelectFamilyAttemptTimeout(value)
net.isIP(input)
net.isIPv4(input)
net.isIPv6(input)
- OS
os.EOL
os.availableParallelism()
os.arch()
os.constants
os.cpus()
os.devNull
os.endianness()
os.freemem()
os.getPriority([pid])
os.homedir()
os.hostname()
os.loadavg()
os.machine()
os.networkInterfaces()
os.platform()
os.release()
os.setPriority([pid, ]priority)
os.tmpdir()
os.totalmem()
os.type()
os.uptime()
os.userInfo([options])
os.version()
- OS constants
- Path
- Windows vs. POSIX
path.basename(path[, suffix])
path.delimiter
path.dirname(path)
path.extname(path)
path.format(pathObject)
path.isAbsolute(path)
path.join([...paths])
path.normalize(path)
path.parse(path)
path.posix
path.relative(from, to)
path.resolve([...paths])
path.sep
path.toNamespacedPath(path)
path.win32
- Performance measurement APIs
perf_hooks.performance
performance.clearMarks([name])
performance.clearMeasures([name])
performance.clearResourceTimings([name])
performance.eventLoopUtilization([utilization1[, utilization2]])
performance.getEntries()
performance.getEntriesByName(name[, type])
performance.getEntriesByType(type)
performance.mark(name[, options])
performance.markResourceTiming(timingInfo, requestedUrl, initiatorType, global, cacheMode)
performance.measure(name[, startMarkOrOptions[, endMark]])
performance.nodeTiming
performance.now()
performance.setResourceTimingBufferSize(maxSize)
performance.timeOrigin
performance.timerify(fn[, options])
performance.toJSON()
- Class:
PerformanceEntry
- Class:
PerformanceMark
- Class:
PerformanceMeasure
- Class:
PerformanceNodeEntry
- Class:
PerformanceNodeTiming
- Class:
PerformanceResourceTiming
performanceResourceTiming.workerStart
performanceResourceTiming.redirectStart
performanceResourceTiming.redirectEnd
performanceResourceTiming.fetchStart
performanceResourceTiming.domainLookupStart
performanceResourceTiming.domainLookupEnd
performanceResourceTiming.connectStart
performanceResourceTiming.connectEnd
performanceResourceTiming.secureConnectionStart
performanceResourceTiming.requestStart
performanceResourceTiming.responseEnd
performanceResourceTiming.transferSize
performanceResourceTiming.encodedBodySize
performanceResourceTiming.decodedBodySize
performanceResourceTiming.toJSON()
- Class:
PerformanceObserver
- Class:
PerformanceObserverEntryList
perf_hooks.createHistogram([options])
perf_hooks.monitorEventLoopDelay([options])
- Class:
Histogram
histogram.count
histogram.countBigInt
histogram.exceeds
histogram.exceedsBigInt
histogram.max
histogram.maxBigInt
histogram.mean
histogram.min
histogram.minBigInt
histogram.percentile(percentile)
histogram.percentileBigInt(percentile)
histogram.percentiles
histogram.percentilesBigInt
histogram.reset()
histogram.stddev
- Class:
IntervalHistogram extends Histogram
- Class:
RecordableHistogram extends Histogram
- Examples
- Permissions
- Process
- Process events
process.abort()
process.allowedNodeEnvironmentFlags
process.arch
process.argv
process.argv0
process.channel
process.chdir(directory)
process.config
process.connected
process.constrainedMemory()
process.cpuUsage([previousValue])
process.cwd()
process.debugPort
process.disconnect()
process.dlopen(module, filename[, flags])
process.emitWarning(warning[, options])
process.emitWarning(warning[, type[, code]][, ctor])
process.env
process.execArgv
process.execPath
process.exit([code])
process.exitCode
process.getActiveResourcesInfo()
process.getegid()
process.geteuid()
process.getgid()
process.getgroups()
process.getuid()
process.hasUncaughtExceptionCaptureCallback()
process.hrtime([time])
process.hrtime.bigint()
process.initgroups(user, extraGroup)
process.kill(pid[, signal])
process.mainModule
process.memoryUsage()
process.memoryUsage.rss()
process.nextTick(callback[, ...args])
process.noDeprecation
process.permission
process.pid
process.platform
process.ppid
process.release
process.report
process.resourceUsage()
process.send(message[, sendHandle[, options]][, callback])
process.setegid(id)
process.seteuid(id)
process.setgid(id)
process.setgroups(groups)
process.setuid(id)
process.setSourceMapsEnabled(val)
process.setUncaughtExceptionCaptureCallback(fn)
process.stderr
process.stdin
process.stdout
process.throwDeprecation
process.title
process.traceDeprecation
process.umask()
process.umask(mask)
process.uptime()
process.version
process.versions
- Exit codes
- Punycode
- Query string
- Readline
- Class:
InterfaceConstructor
- Event:
'close'
- Event:
'line'
- Event:
'history'
- Event:
'pause'
- Event:
'resume'
- Event:
'SIGCONT'
- Event:
'SIGINT'
- Event:
'SIGTSTP'
rl.close()
rl.pause()
rl.prompt([preserveCursor])
rl.resume()
rl.setPrompt(prompt)
rl.getPrompt()
rl.write(data[, key])
rl[Symbol.asyncIterator]()
rl.line
rl.cursor
rl.getCursorPos()
- Event:
- Promises API
- Callback API
readline.emitKeypressEvents(stream[, interface])
- Example: Tiny CLI
- Example: Read file stream line-by-Line
- TTY keybindings
- Class:
- REPL
- Diagnostic report
- Single executable applications
- Stream
- Organization of this document
- Types of streams
- API for stream consumers
- Writable streams
- Class:
stream.Writable
- Event:
'close'
- Event:
'drain'
- Event:
'error'
- Event:
'finish'
- Event:
'pipe'
- Event:
'unpipe'
writable.cork()
writable.destroy([error])
writable.closed
writable.destroyed
writable.end([chunk[, encoding]][, callback])
writable.setDefaultEncoding(encoding)
writable.uncork()
writable.writable
writable.writableAborted
writable.writableEnded
writable.writableCorked
writable.errored
writable.writableFinished
writable.writableHighWaterMark
writable.writableLength
writable.writableNeedDrain
writable.writableObjectMode
writable.write(chunk[, encoding][, callback])
- Event:
- Class:
- Readable streams
- Two reading modes
- Three states
- Choose one API style
- Class:
stream.Readable
- Event:
'close'
- Event:
'data'
- Event:
'end'
- Event:
'error'
- Event:
'pause'
- Event:
'readable'
- Event:
'resume'
readable.destroy([error])
readable.closed
readable.destroyed
readable.isPaused()
readable.pause()
readable.pipe(destination[, options])
readable.read([size])
readable.readable
readable.readableAborted
readable.readableDidRead
readable.readableEncoding
readable.readableEnded
readable.errored
readable.readableFlowing
readable.readableHighWaterMark
readable.readableLength
readable.readableObjectMode
readable.resume()
readable.setEncoding(encoding)
readable.unpipe([destination])
readable.unshift(chunk[, encoding])
readable.wrap(stream)
readable[Symbol.asyncIterator]()
readable.compose(stream[, options])
readable.iterator([options])
readable.map(fn[, options])
readable.filter(fn[, options])
readable.forEach(fn[, options])
readable.toArray([options])
readable.some(fn[, options])
readable.find(fn[, options])
readable.every(fn[, options])
readable.flatMap(fn[, options])
readable.drop(limit[, options])
readable.take(limit[, options])
readable.asIndexedPairs([options])
readable.reduce(fn[, initial[, options]])
- Event:
- Duplex and transform streams
stream.finished(stream[, options], callback)
stream.pipeline(source[, ...transforms], destination, callback)
stream.pipeline(streams, callback)
stream.compose(...streams)
stream.Readable.from(iterable[, options])
stream.Readable.fromWeb(readableStream[, options])
stream.Readable.isDisturbed(stream)
stream.isErrored(stream)
stream.isReadable(stream)
stream.Readable.toWeb(streamReadable[, options])
stream.Writable.fromWeb(writableStream[, options])
stream.Writable.toWeb(streamWritable)
stream.Duplex.from(src)
stream.Duplex.fromWeb(pair[, options])
stream.Duplex.toWeb(streamDuplex)
stream.addAbortSignal(signal, stream)
- Writable streams
- API for stream implementers
- Additional notes
- String decoder
- Test runner
- Subtests
- Skipping tests
describe
/it
syntaxonly
tests- Filtering tests by name
- Extraneous asynchronous activity
- Watch mode
- Running tests from the command line
- Collecting code coverage
- Mocking
- Test reporters
run([options])
test([name][, options][, fn])
describe([name][, options][, fn])
describe.skip([name][, options][, fn])
describe.todo([name][, options][, fn])
describe.only([name][, options][, fn])
it([name][, options][, fn])
it.skip([name][, options][, fn])
it.todo([name][, options][, fn])
it.only([name][, options][, fn])
before([fn][, options])
after([fn][, options])
beforeEach([fn][, options])
afterEach([fn][, options])
- Class:
MockFunctionContext
- Class:
MockTracker
- Class:
TestsStream
- Class:
TestContext
- Class:
SuiteContext
- Timers
- TLS (SSL)
- Determining if crypto support is unavailable
- TLS/SSL concepts
- Modifying the default TLS cipher suite
- X509 certificate error codes
- Class:
tls.CryptoStream
- Class:
tls.SecurePair
- Class:
tls.Server
- Event:
'connection'
- Event:
'keylog'
- Event:
'newSession'
- Event:
'OCSPRequest'
- Event:
'resumeSession'
- Event:
'secureConnection'
- Event:
'tlsClientError'
server.addContext(hostname, context)
server.address()
server.close([callback])
server.getTicketKeys()
server.listen()
server.setSecureContext(options)
server.setTicketKeys(keys)
- Event:
- Class:
tls.TLSSocket
new tls.TLSSocket(socket[, options])
- Event:
'keylog'
- Event:
'OCSPResponse'
- Event:
'secureConnect'
- Event:
'session'
tlsSocket.address()
tlsSocket.authorizationError
tlsSocket.authorized
tlsSocket.disableRenegotiation()
tlsSocket.enableTrace()
tlsSocket.encrypted
tlsSocket.exportKeyingMaterial(length, label[, context])
tlsSocket.getCertificate()
tlsSocket.getCipher()
tlsSocket.getEphemeralKeyInfo()
tlsSocket.getFinished()
tlsSocket.getPeerCertificate([detailed])
tlsSocket.getPeerFinished()
tlsSocket.getPeerX509Certificate()
tlsSocket.getProtocol()
tlsSocket.getSession()
tlsSocket.getSharedSigalgs()
tlsSocket.getTLSTicket()
tlsSocket.getX509Certificate()
tlsSocket.isSessionReused()
tlsSocket.localAddress
tlsSocket.localPort
tlsSocket.remoteAddress
tlsSocket.remoteFamily
tlsSocket.remotePort
tlsSocket.renegotiate(options, callback)
tlsSocket.setMaxSendFragment(size)
tls.checkServerIdentity(hostname, cert)
tls.connect(options[, callback])
tls.connect(path[, options][, callback])
tls.connect(port[, host][, options][, callback])
tls.createSecureContext([options])
tls.createSecurePair([context][, isServer][, requestCert][, rejectUnauthorized][, options])
tls.createServer([options][, secureConnectionListener])
tls.getCiphers()
tls.rootCertificates
tls.DEFAULT_ECDH_CURVE
tls.DEFAULT_MAX_VERSION
tls.DEFAULT_MIN_VERSION
tls.DEFAULT_CIPHERS
- Trace events
- TTY
- Class:
tty.ReadStream
- Class:
tty.WriteStream
- Event:
'resize'
writeStream.clearLine(dir[, callback])
writeStream.clearScreenDown([callback])
writeStream.columns
writeStream.cursorTo(x[, y][, callback])
writeStream.getColorDepth([env])
writeStream.getWindowSize()
writeStream.hasColors([count][, env])
writeStream.isTTY
writeStream.moveCursor(dx, dy[, callback])
writeStream.rows
- Event:
tty.isatty(fd)
- Class:
- UDP/datagram sockets
- Class:
dgram.Socket
- Event:
'close'
- Event:
'connect'
- Event:
'error'
- Event:
'listening'
- Event:
'message'
socket.addMembership(multicastAddress[, multicastInterface])
socket.addSourceSpecificMembership(sourceAddress, groupAddress[, multicastInterface])
socket.address()
socket.bind([port][, address][, callback])
socket.bind(options[, callback])
socket.close([callback])
socket.connect(port[, address][, callback])
socket.disconnect()
socket.dropMembership(multicastAddress[, multicastInterface])
socket.dropSourceSpecificMembership(sourceAddress, groupAddress[, multicastInterface])
socket.getRecvBufferSize()
socket.getSendBufferSize()
socket.getSendQueueSize()
socket.getSendQueueCount()
socket.ref()
socket.remoteAddress()
socket.send(msg[, offset, length][, port][, address][, callback])
socket.setBroadcast(flag)
socket.setMulticastInterface(multicastInterface)
socket.setMulticastLoopback(flag)
socket.setMulticastTTL(ttl)
socket.setRecvBufferSize(size)
socket.setSendBufferSize(size)
socket.setTTL(ttl)
socket.unref()
- Event:
node:dgram
module functions
- Class:
- URL
- URL strings and URL objects
- The WHATWG URL API
- Class:
URL
- Class:
URLSearchParams
new URLSearchParams()
new URLSearchParams(string)
new URLSearchParams(obj)
new URLSearchParams(iterable)
urlSearchParams.append(name, value)
urlSearchParams.delete(name)
urlSearchParams.entries()
urlSearchParams.forEach(fn[, thisArg])
urlSearchParams.get(name)
urlSearchParams.getAll(name)
urlSearchParams.has(name)
urlSearchParams.keys()
urlSearchParams.set(name, value)
urlSearchParams.size
urlSearchParams.sort()
urlSearchParams.toString()
urlSearchParams.values()
urlSearchParams[Symbol.iterator]()
url.domainToASCII(domain)
url.domainToUnicode(domain)
url.fileURLToPath(url)
url.format(URL[, options])
url.pathToFileURL(path)
url.urlToHttpOptions(url)
- Class:
- Legacy URL API
- Percent-encoding in URLs
- Util
util.callbackify(original)
util.debuglog(section[, callback])
util.debug(section)
util.deprecate(fn, msg[, code])
util.format(format[, ...args])
util.formatWithOptions(inspectOptions, format[, ...args])
util.getSystemErrorName(err)
util.getSystemErrorMap()
util.inherits(constructor, superConstructor)
util.inspect(object[, options])
util.inspect(object[, showHidden[, depth[, colors]]])
util.isDeepStrictEqual(val1, val2)
- Class:
util.MIMEType
util.parseArgs([config])
util.promisify(original)
util.stripVTControlCharacters(str)
- Class:
util.TextDecoder
- Class:
util.TextEncoder
util.toUSVString(string)
util.transferableAbortController()
util.transferableAbortSignal(signal)
util.aborted(signal, resource)
util.types
util.types.isAnyArrayBuffer(value)
util.types.isArrayBufferView(value)
util.types.isArgumentsObject(value)
util.types.isArrayBuffer(value)
util.types.isAsyncFunction(value)
util.types.isBigInt64Array(value)
util.types.isBigUint64Array(value)
util.types.isBooleanObject(value)
util.types.isBoxedPrimitive(value)
util.types.isCryptoKey(value)
util.types.isDataView(value)
util.types.isDate(value)
util.types.isExternal(value)
util.types.isFloat32Array(value)
util.types.isFloat64Array(value)
util.types.isGeneratorFunction(value)
util.types.isGeneratorObject(value)
util.types.isInt8Array(value)
util.types.isInt16Array(value)
util.types.isInt32Array(value)
util.types.isKeyObject(value)
util.types.isMap(value)
util.types.isMapIterator(value)
util.types.isModuleNamespaceObject(value)
util.types.isNativeError(value)
util.types.isNumberObject(value)
util.types.isPromise(value)
util.types.isProxy(value)
util.types.isRegExp(value)
util.types.isSet(value)
util.types.isSetIterator(value)
util.types.isSharedArrayBuffer(value)
util.types.isStringObject(value)
util.types.isSymbolObject(value)
util.types.isTypedArray(value)
util.types.isUint8Array(value)
util.types.isUint8ClampedArray(value)
util.types.isUint16Array(value)
util.types.isUint32Array(value)
util.types.isWeakMap(value)
util.types.isWeakSet(value)
util.types.isWebAssemblyCompiledModule(value)
- Deprecated APIs
util._extend(target, source)
util.isArray(object)
util.isBoolean(object)
util.isBuffer(object)
util.isDate(object)
util.isError(object)
util.isFunction(object)
util.isNull(object)
util.isNullOrUndefined(object)
util.isNumber(object)
util.isObject(object)
util.isPrimitive(object)
util.isRegExp(object)
util.isString(object)
util.isSymbol(object)
util.isUndefined(object)
util.log(string)
- V8
v8.cachedDataVersionTag()
v8.getHeapCodeStatistics()
v8.getHeapSnapshot([options])
v8.getHeapSpaceStatistics()
v8.getHeapStatistics()
v8.setFlagsFromString(flags)
v8.stopCoverage()
v8.takeCoverage()
v8.writeHeapSnapshot([filename[,options]])
v8.setHeapSnapshotNearHeapLimit(limit)
- Serialization API
v8.serialize(value)
v8.deserialize(buffer)
- Class:
v8.Serializer
new Serializer()
serializer.writeHeader()
serializer.writeValue(value)
serializer.releaseBuffer()
serializer.transferArrayBuffer(id, arrayBuffer)
serializer.writeUint32(value)
serializer.writeUint64(hi, lo)
serializer.writeDouble(value)
serializer.writeRawBytes(buffer)
serializer._writeHostObject(object)
serializer._getDataCloneError(message)
serializer._getSharedArrayBufferId(sharedArrayBuffer)
serializer._setTreatArrayBufferViewsAsHostObjects(flag)
- Class:
v8.Deserializer
new Deserializer(buffer)
deserializer.readHeader()
deserializer.readValue()
deserializer.transferArrayBuffer(id, arrayBuffer)
deserializer.getWireFormatVersion()
deserializer.readUint32()
deserializer.readUint64()
deserializer.readDouble()
deserializer.readRawBytes(length)
deserializer._readHostObject()
- Class:
v8.DefaultSerializer
- Class:
v8.DefaultDeserializer
- Promise hooks
- Startup Snapshot API
- Class:
v8.GCProfiler
- VM (executing JavaScript)
- Class:
vm.Script
- Class:
vm.Module
- Class:
vm.SourceTextModule
- Class:
vm.SyntheticModule
vm.compileFunction(code[, params[, options]])
vm.createContext([contextObject[, options]])
vm.isContext(object)
vm.measureMemory([options])
vm.runInContext(code, contextifiedObject[, options])
vm.runInNewContext(code[, contextObject[, options]])
vm.runInThisContext(code[, options])
- Example: Running an HTTP server within a VM
- What does it mean to "contextify" an object?
- Timeout interactions with asynchronous tasks and Promises
- Class:
- WebAssembly System Interface (WASI)
- Web Crypto API
- Examples
- Algorithm matrix
- Class:
Crypto
- Class:
CryptoKey
- Class:
CryptoKeyPair
- Class:
SubtleCrypto
subtle.decrypt(algorithm, key, data)
subtle.deriveBits(algorithm, baseKey, length)
subtle.deriveKey(algorithm, baseKey, derivedKeyAlgorithm, extractable, keyUsages)
subtle.digest(algorithm, data)
subtle.encrypt(algorithm, key, data)
subtle.exportKey(format, key)
subtle.generateKey(algorithm, extractable, keyUsages)
subtle.importKey(format, keyData, algorithm, extractable, keyUsages)
subtle.sign(algorithm, key, data)
subtle.unwrapKey(format, wrappedKey, unwrappingKey, unwrapAlgo, unwrappedKeyAlgo, extractable, keyUsages)
subtle.verify(algorithm, key, signature, data)
subtle.wrapKey(format, key, wrappingKey, wrapAlgo)
- Algorithm parameters
- Class:
AlgorithmIdentifier
- Class:
AesCbcParams
- Class:
AesCtrParams
- Class:
AesGcmParams
- Class:
AesKeyGenParams
- Class:
EcdhKeyDeriveParams
- Class:
EcdsaParams
- Class:
EcKeyGenParams
- Class:
EcKeyImportParams
- Class:
Ed448Params
- Class:
HkdfParams
- Class:
HmacImportParams
- Class:
HmacKeyGenParams
- Class:
Pbkdf2Params
- Class:
RsaHashedImportParams
- Class:
RsaHashedKeyGenParams
- Class:
RsaOaepParams
- Class:
RsaPssParams
- Class:
- Web Streams API
- Overview
- API
- Class:
ReadableStream
new ReadableStream([underlyingSource [, strategy]])
readableStream.locked
readableStream.cancel([reason])
readableStream.getReader([options])
readableStream.pipeThrough(transform[, options])
readableStream.pipeTo(destination[, options])
readableStream.tee()
readableStream.values([options])
- Async Iteration
- Transferring with
postMessage()
- Class:
ReadableStreamDefaultReader
- Class:
ReadableStreamBYOBReader
- Class:
ReadableStreamDefaultController
- Class:
ReadableByteStreamController
- Class:
ReadableStreamBYOBRequest
- Class:
WritableStream
- Class:
WritableStreamDefaultWriter
new WritableStreamDefaultWriter(stream)
writableStreamDefaultWriter.abort([reason])
writableStreamDefaultWriter.close()
writableStreamDefaultWriter.closed
writableStreamDefaultWriter.desiredSize
writableStreamDefaultWriter.ready
writableStreamDefaultWriter.releaseLock()
writableStreamDefaultWriter.write([chunk])
- Class:
WritableStreamDefaultController
- Class:
TransformStream
- Class:
TransformStreamDefaultController
- Class:
ByteLengthQueuingStrategy
- Class:
CountQueuingStrategy
- Class:
TextEncoderStream
- Class:
TextDecoderStream
- Class:
CompressionStream
- Class:
DecompressionStream
- Utility Consumers
- Class:
- Worker threads
worker.getEnvironmentData(key)
worker.isMainThread
worker.markAsUntransferable(object)
worker.moveMessagePortToContext(port, contextifiedSandbox)
worker.parentPort
worker.receiveMessageOnPort(port)
worker.resourceLimits
worker.SHARE_ENV
worker.setEnvironmentData(key[, value])
worker.threadId
worker.workerData
- Class:
BroadcastChannel extends EventTarget
- Class:
MessageChannel
- Class:
MessagePort
- Class:
Worker
new Worker(filename[, options])
- Event:
'error'
- Event:
'exit'
- Event:
'message'
- Event:
'messageerror'
- Event:
'online'
worker.getHeapSnapshot([options])
worker.performance
worker.postMessage(value[, transferList])
worker.ref()
worker.resourceLimits
worker.stderr
worker.stdin
worker.stdout
worker.terminate()
worker.threadId
worker.unref()
- Notes
- Zlib
- Threadpool usage and performance considerations
- Compressing HTTP requests and responses
- Memory usage tuning
- Flushing
- Constants
- Class:
Options
- Class:
BrotliOptions
- Class:
zlib.BrotliCompress
- Class:
zlib.BrotliDecompress
- Class:
zlib.Deflate
- Class:
zlib.DeflateRaw
- Class:
zlib.Gunzip
- Class:
zlib.Gzip
- Class:
zlib.Inflate
- Class:
zlib.InflateRaw
- Class:
zlib.Unzip
- Class:
zlib.ZlibBase
zlib.constants
zlib.createBrotliCompress([options])
zlib.createBrotliDecompress([options])
zlib.createDeflate([options])
zlib.createDeflateRaw([options])
zlib.createGunzip([options])
zlib.createGzip([options])
zlib.createInflate([options])
zlib.createInflateRaw([options])
zlib.createUnzip([options])
- Convenience methods
zlib.brotliCompress(buffer[, options], callback)
zlib.brotliCompressSync(buffer[, options])
zlib.brotliDecompress(buffer[, options], callback)
zlib.brotliDecompressSync(buffer[, options])
zlib.deflate(buffer[, options], callback)
zlib.deflateSync(buffer[, options])
zlib.deflateRaw(buffer[, options], callback)
zlib.deflateRawSync(buffer[, options])
zlib.gunzip(buffer[, options], callback)
zlib.gunzipSync(buffer[, options])
zlib.gzip(buffer[, options], callback)
zlib.gzipSync(buffer[, options])
zlib.inflate(buffer[, options], callback)
zlib.inflateSync(buffer[, options])
zlib.inflateRaw(buffer[, options], callback)
zlib.inflateRawSync(buffer[, options])
zlib.unzip(buffer[, options], callback)
zlib.unzipSync(buffer[, options])
About this documentation#
Welcome to the official API reference documentation for Node.js!
Node.js is a JavaScript runtime built on the V8 JavaScript engine.
Contributing#
Report errors in this documentation in the issue tracker. See the contributing guide for directions on how to submit pull requests.
Stability index#
Throughout the documentation are indications of a section's stability. Some APIs are so proven and so relied upon that they are unlikely to ever change at all. Others are brand new and experimental, or known to be hazardous.
The stability indices are as follows:
Experimental features are subdivided into stages:
- 1.0 - Early development. Experimental features at this stage are unfinished and subject to substantial change.
- 1.1 - Active development. Experimental features at this stage are nearing minimum viability.
- 1.2 - Release candidate. Experimental features at this stage are hopefully ready to become stable. No further breaking changes are anticipated but may still occur in response to user feedback. We encourage user testing and feedback so that we can know that this feature is ready to be marked as stable.
Features are marked as legacy rather than being deprecated if their use does no harm, and they are widely relied upon within the npm ecosystem. Bugs found in legacy features are unlikely to be fixed.
Use caution when making use of Experimental features, particularly when authoring libraries. Users may not be aware that experimental features are being used. Bugs or behavior changes may surprise users when Experimental API modifications occur. To avoid surprises, use of an Experimental feature may need a command-line flag. Experimental features may also emit a warning.
Stability overview#
JSON output#
Every .html
document has a corresponding .json
document. This is for IDEs
and other utilities that consume the documentation.
System calls and man pages#
Node.js functions which wrap a system call will document that. The docs link to the corresponding man pages which describe how the system call works.
Most Unix system calls have Windows analogues. Still, behavior differences may be unavoidable.
Usage and example#
Usage#
node [options] [V8 options] [script.js | -e "script" | - ] [arguments]
Please see the Command-line options document for more information.
Example#
An example of a web server written with Node.js which responds with
'Hello, World!'
:
Commands in this document start with $
or >
to replicate how they would
appear in a user's terminal. Do not include the $
and >
characters. They are
there to show the start of each command.
Lines that don't start with $
or >
character show the output of the previous
command.
First, make sure to have downloaded and installed Node.js. See Installing Node.js via package manager for further install information.
Now, create an empty project folder called projects
, then navigate into it.
Linux and Mac:
$ mkdir ~/projects
$ cd ~/projects
Windows CMD:
> mkdir %USERPROFILE%\projects
> cd %USERPROFILE%\projects
Windows PowerShell:
> mkdir $env:USERPROFILE\projects
> cd $env:USERPROFILE\projects
Next, create a new source file in the projects
folder and call it hello-world.js
.
Open hello-world.js
in any preferred text editor and
paste in the following content:
const http = require('node:http');
const hostname = '127.0.0.1';
const port = 3000;
const server = http.createServer((req, res) => {
res.statusCode = 200;
res.setHeader('Content-Type', 'text/plain');
res.end('Hello, World!\n');
});
server.listen(port, hostname, () => {
console.log(`Server running at http://${hostname}:${port}/`);
});
Save the file, go back to the terminal window, and enter the following command:
$ node hello-world.js
Output like this should appear in the terminal:
Server running at http://127.0.0.1:3000/
Now, open any preferred web browser and visit http://127.0.0.1:3000
.
If the browser displays the string Hello, World!
, that indicates
the server is working.
Assert#
Source Code: lib/assert.js
The node:assert
module provides a set of assertion functions for verifying
invariants.
Strict assertion mode#
In strict assertion mode, non-strict methods behave like their corresponding
strict methods. For example, assert.deepEqual()
will behave like
assert.deepStrictEqual()
.
In strict assertion mode, error messages for objects display a diff. In legacy assertion mode, error messages for objects display the objects, often truncated.
To use strict assertion mode:
import { strict as assert } from 'node:assert';
const assert = require('node:assert').strict;
import assert from 'node:assert/strict';
const assert = require('node:assert/strict');
Example error diff:
import { strict as assert } from 'node:assert';
assert.deepEqual([[[1, 2, 3]], 4, 5], [[[1, 2, '3']], 4, 5]);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected ... Lines skipped
//
// [
// [
// ...
// 2,
// + 3
// - '3'
// ],
// ...
// 5
// ]
const assert = require('node:assert/strict');
assert.deepEqual([[[1, 2, 3]], 4, 5], [[[1, 2, '3']], 4, 5]);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected ... Lines skipped
//
// [
// [
// ...
// 2,
// + 3
// - '3'
// ],
// ...
// 5
// ]
To deactivate the colors, use the NO_COLOR
or NODE_DISABLE_COLORS
environment variables. This will also deactivate the colors in the REPL. For
more on color support in terminal environments, read the tty
getColorDepth()
documentation.
Legacy assertion mode#
Legacy assertion mode uses the ==
operator in:
To use legacy assertion mode:
import assert from 'node:assert';
const assert = require('node:assert');
Legacy assertion mode may have surprising results, especially when using
assert.deepEqual()
:
// WARNING: This does not throw an AssertionError in legacy assertion mode!
assert.deepEqual(/a/gi, new Date());
Class: assert.AssertionError[src]#
- Extends: <errors.Error>
Indicates the failure of an assertion. All errors thrown by the node:assert
module will be instances of the AssertionError
class.
new assert.AssertionError(options)
#
options
<Object>message
<string> If provided, the error message is set to this value.actual
<any> Theactual
property on the error instance.expected
<any> Theexpected
property on the error instance.operator
<string> Theoperator
property on the error instance.stackStartFn
<Function> If provided, the generated stack trace omits frames before this function.
A subclass of Error
that indicates the failure of an assertion.
All instances contain the built-in Error
properties (message
and name
)
and:
actual
<any> Set to theactual
argument for methods such asassert.strictEqual()
.expected
<any> Set to theexpected
value for methods such asassert.strictEqual()
.generatedMessage
<boolean> Indicates if the message was auto-generated (true
) or not.code
<string> Value is alwaysERR_ASSERTION
to show that the error is an assertion error.operator
<string> Set to the passed in operator value.
import assert from 'node:assert';
// Generate an AssertionError to compare the error message later:
const { message } = new assert.AssertionError({
actual: 1,
expected: 2,
operator: 'strictEqual',
});
// Verify error output:
try {
assert.strictEqual(1, 2);
} catch (err) {
assert(err instanceof assert.AssertionError);
assert.strictEqual(err.message, message);
assert.strictEqual(err.name, 'AssertionError');
assert.strictEqual(err.actual, 1);
assert.strictEqual(err.expected, 2);
assert.strictEqual(err.code, 'ERR_ASSERTION');
assert.strictEqual(err.operator, 'strictEqual');
assert.strictEqual(err.generatedMessage, true);
}
const assert = require('node:assert');
// Generate an AssertionError to compare the error message later:
const { message } = new assert.AssertionError({
actual: 1,
expected: 2,
operator: 'strictEqual',
});
// Verify error output:
try {
assert.strictEqual(1, 2);
} catch (err) {
assert(err instanceof assert.AssertionError);
assert.strictEqual(err.message, message);
assert.strictEqual(err.name, 'AssertionError');
assert.strictEqual(err.actual, 1);
assert.strictEqual(err.expected, 2);
assert.strictEqual(err.code, 'ERR_ASSERTION');
assert.strictEqual(err.operator, 'strictEqual');
assert.strictEqual(err.generatedMessage, true);
}
Class: assert.CallTracker
#
This feature is currently experimental and behavior might still change.
new assert.CallTracker()
#
Creates a new CallTracker
object which can be used to track if functions
were called a specific number of times. The tracker.verify()
must be called
for the verification to take place. The usual pattern would be to call it in a
process.on('exit')
handler.
import assert from 'node:assert';
import process from 'node:process';
const tracker = new assert.CallTracker();
function func() {}
// callsfunc() must be called exactly 1 time before tracker.verify().
const callsfunc = tracker.calls(func, 1);
callsfunc();
// Calls tracker.verify() and verifies if all tracker.calls() functions have
// been called exact times.
process.on('exit', () => {
tracker.verify();
});
const assert = require('node:assert');
const tracker = new assert.CallTracker();
function func() {}
// callsfunc() must be called exactly 1 time before tracker.verify().
const callsfunc = tracker.calls(func, 1);
callsfunc();
// Calls tracker.verify() and verifies if all tracker.calls() functions have
// been called exact times.
process.on('exit', () => {
tracker.verify();
});
tracker.calls([fn][, exact])
#
fn
<Function> Default: A no-op function.exact
<number> Default:1
.- Returns: <Function> that wraps
fn
.
The wrapper function is expected to be called exactly exact
times. If the
function has not been called exactly exact
times when
tracker.verify()
is called, then tracker.verify()
will throw an
error.
import assert from 'node:assert';
// Creates call tracker.
const tracker = new assert.CallTracker();
function func() {}
// Returns a function that wraps func() that must be called exact times
// before tracker.verify().
const callsfunc = tracker.calls(func);
const assert = require('node:assert');
// Creates call tracker.
const tracker = new assert.CallTracker();
function func() {}
// Returns a function that wraps func() that must be called exact times
// before tracker.verify().
const callsfunc = tracker.calls(func);
tracker.getCalls(fn)
#
-
fn
<Function>. -
Returns: <Array> with all the calls to a tracked function.
-
Object <Object>
import assert from 'node:assert';
const tracker = new assert.CallTracker();
function func() {}
const callsfunc = tracker.calls(func);
callsfunc(1, 2, 3);
assert.deepStrictEqual(tracker.getCalls(callsfunc),
[{ thisArg: this, arguments: [1, 2, 3 ] }]);
const assert = require('node:assert');
// Creates call tracker.
const tracker = new assert.CallTracker();
function func() {}
const callsfunc = tracker.calls(func);
callsfunc(1, 2, 3);
assert.deepStrictEqual(tracker.getCalls(callsfunc),
[{ thisArg: this, arguments: [1, 2, 3 ] }]);
tracker.report()
#
- Returns: <Array> of objects containing information about the wrapper functions
returned by
tracker.calls()
. - Object <Object>
The arrays contains information about the expected and actual number of calls of the functions that have not been called the expected number of times.
import assert from 'node:assert';
// Creates call tracker.
const tracker = new assert.CallTracker();
function func() {}
// Returns a function that wraps func() that must be called exact times
// before tracker.verify().
const callsfunc = tracker.calls(func, 2);
// Returns an array containing information on callsfunc()
tracker.report();
// [
// {
// message: 'Expected the func function to be executed 2 time(s) but was
// executed 0 time(s).',
// actual: 0,
// expected: 2,
// operator: 'func',
// stack: stack trace
// }
// ]
const assert = require('node:assert');
// Creates call tracker.
const tracker = new assert.CallTracker();
function func() {}
// Returns a function that wraps func() that must be called exact times
// before tracker.verify().
const callsfunc = tracker.calls(func, 2);
// Returns an array containing information on callsfunc()
tracker.report();
// [
// {
// message: 'Expected the func function to be executed 2 time(s) but was
// executed 0 time(s).',
// actual: 0,
// expected: 2,
// operator: 'func',
// stack: stack trace
// }
// ]
tracker.reset([fn])
#
fn
<Function> a tracked function to reset.
reset calls of the call tracker. if a tracked function is passed as an argument, the calls will be reset for it. if no arguments are passed, all tracked functions will be reset
import assert from 'node:assert';
const tracker = new assert.CallTracker();
function func() {}
const callsfunc = tracker.calls(func);
callsfunc();
// Tracker was called once
tracker.getCalls(callsfunc).length === 1;
tracker.reset(callsfunc);
tracker.getCalls(callsfunc).length === 0;
const assert = require('node:assert');
function func() {}
const callsfunc = tracker.calls(func);
callsfunc();
// Tracker was called once
tracker.getCalls(callsfunc).length === 1;
tracker.reset(callsfunc);
tracker.getCalls(callsfunc).length === 0;
tracker.verify()
#
Iterates through the list of functions passed to
tracker.calls()
and will throw an error for functions that
have not been called the expected number of times.
import assert from 'node:assert';
// Creates call tracker.
const tracker = new assert.CallTracker();
function func() {}
// Returns a function that wraps func() that must be called exact times
// before tracker.verify().
const callsfunc = tracker.calls(func, 2);
callsfunc();
// Will throw an error since callsfunc() was only called once.
tracker.verify();
const assert = require('node:assert');
// Creates call tracker.
const tracker = new assert.CallTracker();
function func() {}
// Returns a function that wraps func() that must be called exact times
// before tracker.verify().
const callsfunc = tracker.calls(func, 2);
callsfunc();
// Will throw an error since callsfunc() was only called once.
tracker.verify();
assert(value[, message])
#
An alias of assert.ok()
.
assert.deepEqual(actual, expected[, message])
#
Strict assertion mode
An alias of assert.deepStrictEqual()
.
Legacy assertion mode
assert.deepStrictEqual()
instead.Tests for deep equality between the actual
and expected
parameters. Consider
using assert.deepStrictEqual()
instead. assert.deepEqual()
can have
surprising results.
Deep equality means that the enumerable "own" properties of child objects are also recursively evaluated by the following rules.
Comparison details#
- Primitive values are compared with the
==
operator, with the exception ofNaN
. It is treated as being identical in case both sides areNaN
. - Type tags of objects should be the same.
- Only enumerable "own" properties are considered.
Error
names and messages are always compared, even if these are not enumerable properties.- Object wrappers are compared both as objects and unwrapped values.
Object
properties are compared unordered.Map
keys andSet
items are compared unordered.- Recursion stops when both sides differ or both sides encounter a circular reference.
- Implementation does not test the
[[Prototype]]
of objects. Symbol
properties are not compared.WeakMap
andWeakSet
comparison does not rely on their values.RegExp
lastIndex, flags, and source are always compared, even if these are not enumerable properties.
The following example does not throw an AssertionError
because the
primitives are compared using the ==
operator.
import assert from 'node:assert';
// WARNING: This does not throw an AssertionError!
assert.deepEqual('+00000000', false);
const assert = require('node:assert');
// WARNING: This does not throw an AssertionError!
assert.deepEqual('+00000000', false);
"Deep" equality means that the enumerable "own" properties of child objects are evaluated also:
import assert from 'node:assert';
const obj1 = {
a: {
b: 1,
},
};
const obj2 = {
a: {
b: 2,
},
};
const obj3 = {
a: {
b: 1,
},
};
const obj4 = { __proto__: obj1 };
assert.deepEqual(obj1, obj1);
// OK
// Values of b are different:
assert.deepEqual(obj1, obj2);
// AssertionError: { a: { b: 1 } } deepEqual { a: { b: 2 } }
assert.deepEqual(obj1, obj3);
// OK
// Prototypes are ignored:
assert.deepEqual(obj1, obj4);
// AssertionError: { a: { b: 1 } } deepEqual {}
const assert = require('node:assert');
const obj1 = {
a: {
b: 1,
},
};
const obj2 = {
a: {
b: 2,
},
};
const obj3 = {
a: {
b: 1,
},
};
const obj4 = { __proto__: obj1 };
assert.deepEqual(obj1, obj1);
// OK
// Values of b are different:
assert.deepEqual(obj1, obj2);
// AssertionError: { a: { b: 1 } } deepEqual { a: { b: 2 } }
assert.deepEqual(obj1, obj3);
// OK
// Prototypes are ignored:
assert.deepEqual(obj1, obj4);
// AssertionError: { a: { b: 1 } } deepEqual {}
If the values are not equal, an AssertionError
is thrown with a message
property set equal to the value of the message
parameter. If the message
parameter is undefined, a default error message is assigned. If the message
parameter is an instance of an Error
then it will be thrown instead of the
AssertionError
.
assert.deepStrictEqual(actual, expected[, message])
#
Tests for deep equality between the actual
and expected
parameters.
"Deep" equality means that the enumerable "own" properties of child objects
are recursively evaluated also by the following rules.
Comparison details#
- Primitive values are compared using
Object.is()
. - Type tags of objects should be the same.
[[Prototype]]
of objects are compared using the===
operator.- Only enumerable "own" properties are considered.
Error
names and messages are always compared, even if these are not enumerable properties.- Enumerable own
Symbol
properties are compared as well. - Object wrappers are compared both as objects and unwrapped values.
Object
properties are compared unordered.Map
keys andSet
items are compared unordered.- Recursion stops when both sides differ or both sides encounter a circular reference.
WeakMap
andWeakSet
comparison does not rely on their values. See below for further details.RegExp
lastIndex, flags, and source are always compared, even if these are not enumerable properties.
import assert from 'node:assert/strict';
// This fails because 1 !== '1'.
assert.deepStrictEqual({ a: 1 }, { a: '1' });
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// {
// + a: 1
// - a: '1'
// }
// The following objects don't have own properties
const date = new Date();
const object = {};
const fakeDate = {};
Object.setPrototypeOf(fakeDate, Date.prototype);
// Different [[Prototype]]:
assert.deepStrictEqual(object, fakeDate);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + {}
// - Date {}
// Different type tags:
assert.deepStrictEqual(date, fakeDate);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + 2018-04-26T00:49:08.604Z
// - Date {}
assert.deepStrictEqual(NaN, NaN);
// OK because Object.is(NaN, NaN) is true.
// Different unwrapped numbers:
assert.deepStrictEqual(new Number(1), new Number(2));
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + [Number: 1]
// - [Number: 2]
assert.deepStrictEqual(new String('foo'), Object('foo'));
// OK because the object and the string are identical when unwrapped.
assert.deepStrictEqual(-0, -0);
// OK
// Different zeros:
assert.deepStrictEqual(0, -0);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + 0
// - -0
const symbol1 = Symbol();
const symbol2 = Symbol();
assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol1]: 1 });
// OK, because it is the same symbol on both objects.
assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol2]: 1 });
// AssertionError [ERR_ASSERTION]: Inputs identical but not reference equal:
//
// {
// [Symbol()]: 1
// }
const weakMap1 = new WeakMap();
const weakMap2 = new WeakMap([[{}, {}]]);
const weakMap3 = new WeakMap();
weakMap3.unequal = true;
assert.deepStrictEqual(weakMap1, weakMap2);
// OK, because it is impossible to compare the entries
// Fails because weakMap3 has a property that weakMap1 does not contain:
assert.deepStrictEqual(weakMap1, weakMap3);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// WeakMap {
// + [items unknown]
// - [items unknown],
// - unequal: true
// }
const assert = require('node:assert/strict');
// This fails because 1 !== '1'.
assert.deepStrictEqual({ a: 1 }, { a: '1' });
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// {
// + a: 1
// - a: '1'
// }
// The following objects don't have own properties
const date = new Date();
const object = {};
const fakeDate = {};
Object.setPrototypeOf(fakeDate, Date.prototype);
// Different [[Prototype]]:
assert.deepStrictEqual(object, fakeDate);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + {}
// - Date {}
// Different type tags:
assert.deepStrictEqual(date, fakeDate);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + 2018-04-26T00:49:08.604Z
// - Date {}
assert.deepStrictEqual(NaN, NaN);
// OK because Object.is(NaN, NaN) is true.
// Different unwrapped numbers:
assert.deepStrictEqual(new Number(1), new Number(2));
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + [Number: 1]
// - [Number: 2]
assert.deepStrictEqual(new String('foo'), Object('foo'));
// OK because the object and the string are identical when unwrapped.
assert.deepStrictEqual(-0, -0);
// OK
// Different zeros:
assert.deepStrictEqual(0, -0);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + 0
// - -0
const symbol1 = Symbol();
const symbol2 = Symbol();
assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol1]: 1 });
// OK, because it is the same symbol on both objects.
assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol2]: 1 });
// AssertionError [ERR_ASSERTION]: Inputs identical but not reference equal:
//
// {
// [Symbol()]: 1
// }
const weakMap1 = new WeakMap();
const weakMap2 = new WeakMap([[{}, {}]]);
const weakMap3 = new WeakMap();
weakMap3.unequal = true;
assert.deepStrictEqual(weakMap1, weakMap2);
// OK, because it is impossible to compare the entries
// Fails because weakMap3 has a property that weakMap1 does not contain:
assert.deepStrictEqual(weakMap1, weakMap3);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// WeakMap {
// + [items unknown]
// - [items unknown],
// - unequal: true
// }
If the values are not equal, an AssertionError
is thrown with a message
property set equal to the value of the message
parameter. If the message
parameter is undefined, a default error message is assigned. If the message
parameter is an instance of an Error
then it will be thrown instead of the
AssertionError
.
assert.doesNotMatch(string, regexp[, message])
#
Expects the string
input not to match the regular expression.
import assert from 'node:assert/strict';
assert.doesNotMatch('I will fail', /fail/);
// AssertionError [ERR_ASSERTION]: The input was expected to not match the ...
assert.doesNotMatch(123, /pass/);
// AssertionError [ERR_ASSERTION]: The "string" argument must be of type string.
assert.doesNotMatch('I will pass', /different/);
// OK
const assert = require('node:assert/strict');
assert.doesNotMatch('I will fail', /fail/);
// AssertionError [ERR_ASSERTION]: The input was expected to not match the ...
assert.doesNotMatch(123, /pass/);
// AssertionError [ERR_ASSERTION]: The "string" argument must be of type string.
assert.doesNotMatch('I will pass', /different/);
// OK
If the values do match, or if the string
argument is of another type than
string
, an AssertionError
is thrown with a message
property set equal
to the value of the message
parameter. If the message
parameter is
undefined, a default error message is assigned. If the message
parameter is an
instance of an Error
then it will be thrown instead of the
AssertionError
.
assert.doesNotReject(asyncFn[, error][, message])
#
asyncFn
<Function> | <Promise>error
<RegExp> | <Function>message
<string>
Awaits the asyncFn
promise or, if asyncFn
is a function, immediately
calls the function and awaits the returned promise to complete. It will then
check that the promise is not rejected.
If asyncFn
is a function and it throws an error synchronously,
assert.doesNotReject()
will return a rejected Promise
with that error. If
the function does not return a promise, assert.doesNotReject()
will return a
rejected Promise
with an ERR_INVALID_RETURN_VALUE
error. In both cases
the error handler is skipped.
Using assert.doesNotReject()
is actually not useful because there is little
benefit in catching a rejection and then rejecting it again. Instead, consider
adding a comment next to the specific code path that should not reject and keep
error messages as expressive as possible.
If specified, error
can be a Class
, RegExp
, or a validation
function. See assert.throws()
for more details.
Besides the async nature to await the completion behaves identically to
assert.doesNotThrow()
.
import assert from 'node:assert/strict';
await assert.doesNotReject(
async () => {
throw new TypeError('Wrong value');
},
SyntaxError,
);
const assert = require('node:assert/strict');
(async () => {
await assert.doesNotReject(
async () => {
throw new TypeError('Wrong value');
},
SyntaxError,
);
})();
import assert from 'node:assert/strict';
assert.doesNotReject(Promise.reject(new TypeError('Wrong value')))
.then(() => {
// ...
});
const assert = require('node:assert/strict');
assert.doesNotReject(Promise.reject(new TypeError('Wrong value')))
.then(() => {
// ...
});
assert.doesNotThrow(fn[, error][, message])
#
fn
<Function>error
<RegExp> | <Function>message
<string>
Asserts that the function fn
does not throw an error.
Using assert.doesNotThrow()
is actually not useful because there
is no benefit in catching an error and then rethrowing it. Instead, consider
adding a comment next to the specific code path that should not throw and keep
error messages as expressive as possible.
When assert.doesNotThrow()
is called, it will immediately call the fn
function.
If an error is thrown and it is the same type as that specified by the error
parameter, then an AssertionError
is thrown. If the error is of a
different type, or if the error
parameter is undefined, the error is
propagated back to the caller.
If specified, error
can be a Class
, RegExp
, or a validation
function. See assert.throws()
for more details.
The following, for instance, will throw the TypeError
because there is no
matching error type in the assertion:
import assert from 'node:assert/strict';
assert.doesNotThrow(
() => {
throw new TypeError('Wrong value');
},
SyntaxError,
);
const assert = require('node:assert/strict');
assert.doesNotThrow(
() => {
throw new TypeError('Wrong value');
},
SyntaxError,
);
However, the following will result in an AssertionError
with the message
'Got unwanted exception...':
import assert from 'node:assert/strict';
assert.doesNotThrow(
() => {
throw new TypeError('Wrong value');
},
TypeError,
);
const assert = require('node:assert/strict');
assert.doesNotThrow(
() => {
throw new TypeError('Wrong value');
},
TypeError,
);
If an AssertionError
is thrown and a value is provided for the message
parameter, the value of message
will be appended to the AssertionError
message:
import assert from 'node:assert/strict';
assert.doesNotThrow(
() => {
throw new TypeError('Wrong value');
},
/Wrong value/,
'Whoops',
);
// Throws: AssertionError: Got unwanted exception: Whoops
const assert = require('node:assert/strict');
assert.doesNotThrow(
() => {
throw new TypeError('Wrong value');
},
/Wrong value/,
'Whoops',
);
// Throws: AssertionError: Got unwanted exception: Whoops
assert.equal(actual, expected[, message])
#
Strict assertion mode
An alias of assert.strictEqual()
.
Legacy assertion mode
assert.strictEqual()
instead.Tests shallow, coercive equality between the actual
and expected
parameters
using the ==
operator. NaN
is specially handled
and treated as being identical if both sides are NaN
.
import assert from 'node:assert';
assert.equal(1, 1);
// OK, 1 == 1
assert.equal(1, '1');
// OK, 1 == '1'
assert.equal(NaN, NaN);
// OK
assert.equal(1, 2);
// AssertionError: 1 == 2
assert.equal({ a: { b: 1 } }, { a: { b: 1 } });
// AssertionError: { a: { b: 1 } } == { a: { b: 1 } }
const assert = require('node:assert');
assert.equal(1, 1);
// OK, 1 == 1
assert.equal(1, '1');
// OK, 1 == '1'
assert.equal(NaN, NaN);
// OK
assert.equal(1, 2);
// AssertionError: 1 == 2
assert.equal({ a: { b: 1 } }, { a: { b: 1 } });
// AssertionError: { a: { b: 1 } } == { a: { b: 1 } }
If the values are not equal, an AssertionError
is thrown with a message
property set equal to the value of the message
parameter. If the message
parameter is undefined, a default error message is assigned. If the message
parameter is an instance of an Error
then it will be thrown instead of the
AssertionError
.
assert.fail([message])
#
Throws an AssertionError
with the provided error message or a default
error message. If the message
parameter is an instance of an Error
then
it will be thrown instead of the AssertionError
.
import assert from 'node:assert/strict';
assert.fail();
// AssertionError [ERR_ASSERTION]: Failed
assert.fail('boom');
// AssertionError [ERR_ASSERTION]: boom
assert.fail(new TypeError('need array'));
// TypeError: need array
const assert = require('node:assert/strict');
assert.fail();
// AssertionError [ERR_ASSERTION]: Failed
assert.fail('boom');
// AssertionError [ERR_ASSERTION]: boom
assert.fail(new TypeError('need array'));
// TypeError: need array
Using assert.fail()
with more than two arguments is possible but deprecated.
See below for further details.
assert.fail(actual, expected[, message[, operator[, stackStartFn]]])
#
assert.fail([message])
or other assert
functions instead.actual
<any>expected
<any>message
<string> | <Error>operator
<string> Default:'!='
stackStartFn
<Function> Default:assert.fail
If message
is falsy, the error message is set as the values of actual
and
expected
separated by the provided operator
. If just the two actual
and
expected
arguments are provided, operator
will default to '!='
. If
message
is provided as third argument it will be used as the error message and
the other arguments will be stored as properties on the thrown object. If
stackStartFn
is provided, all stack frames above that function will be
removed from stacktrace (see Error.captureStackTrace
). If no arguments are
given, the default message Failed
will be used.
import assert from 'node:assert/strict';
assert.fail('a', 'b');
// AssertionError [ERR_ASSERTION]: 'a' != 'b'
assert.fail(1, 2, undefined, '>');
// AssertionError [ERR_ASSERTION]: 1 > 2
assert.fail(1, 2, 'fail');
// AssertionError [ERR_ASSERTION]: fail
assert.fail(1, 2, 'whoops', '>');
// AssertionError [ERR_ASSERTION]: whoops
assert.fail(1, 2, new TypeError('need array'));
// TypeError: need array
const assert = require('node:assert/strict');
assert.fail('a', 'b');
// AssertionError [ERR_ASSERTION]: 'a' != 'b'
assert.fail(1, 2, undefined, '>');
// AssertionError [ERR_ASSERTION]: 1 > 2
assert.fail(1, 2, 'fail');
// AssertionError [ERR_ASSERTION]: fail
assert.fail(1, 2, 'whoops', '>');
// AssertionError [ERR_ASSERTION]: whoops
assert.fail(1, 2, new TypeError('need array'));
// TypeError: need array
In the last three cases actual
, expected
, and operator
have no
influence on the error message.
Example use of stackStartFn
for truncating the exception's stacktrace:
import assert from 'node:assert/strict';
function suppressFrame() {
assert.fail('a', 'b', undefined, '!==', suppressFrame);
}
suppressFrame();
// AssertionError [ERR_ASSERTION]: 'a' !== 'b'
// at repl:1:1
// at ContextifyScript.Script.runInThisContext (vm.js:44:33)
// ...
const assert = require('node:assert/strict');
function suppressFrame() {
assert.fail('a', 'b', undefined, '!==', suppressFrame);
}
suppressFrame();
// AssertionError [ERR_ASSERTION]: 'a' !== 'b'
// at repl:1:1
// at ContextifyScript.Script.runInThisContext (vm.js:44:33)
// ...
assert.ifError(value)
#
value
<any>
Throws value
if value
is not undefined
or null
. This is useful when
testing the error
argument in callbacks. The stack trace contains all frames
from the error passed to ifError()
including the potential new frames for
ifError()
itself.
import assert from 'node:assert/strict';
assert.ifError(null);
// OK
assert.ifError(0);
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 0
assert.ifError('error');
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 'error'
assert.ifError(new Error());
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: Error
// Create some random error frames.
let err;
(function errorFrame() {
err = new Error('test error');
})();
(function ifErrorFrame() {
assert.ifError(err);
})();
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: test error
// at ifErrorFrame
// at errorFrame
const assert = require('node:assert/strict');
assert.ifError(null);
// OK
assert.ifError(0);
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 0
assert.ifError('error');
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 'error'
assert.ifError(new Error());
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: Error
// Create some random error frames.
let err;
(function errorFrame() {
err = new Error('test error');
})();
(function ifErrorFrame() {
assert.ifError(err);
})();
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: test error
// at ifErrorFrame
// at errorFrame
assert.match(string, regexp[, message])
#
Expects the string
input to match the regular expression.
import assert from 'node:assert/strict';
assert.match('I will fail', /pass/);
// AssertionError [ERR_ASSERTION]: The input did not match the regular ...
assert.match(123, /pass/);
// AssertionError [ERR_ASSERTION]: The "string" argument must be of type string.
assert.match('I will pass', /pass/);
// OK
const assert = require('node:assert/strict');
assert.match('I will fail', /pass/);
// AssertionError [ERR_ASSERTION]: The input did not match the regular ...
assert.match(123, /pass/);
// AssertionError [ERR_ASSERTION]: The "string" argument must be of type string.
assert.match('I will pass', /pass/);
// OK
If the values do not match, or if the string
argument is of another type than
string
, an AssertionError
is thrown with a message
property set equal
to the value of the message
parameter. If the message
parameter is
undefined, a default error message is assigned. If the message
parameter is an
instance of an Error
then it will be thrown instead of the
AssertionError
.
assert.notDeepEqual(actual, expected[, message])
#
Strict assertion mode
An alias of assert.notDeepStrictEqual()
.
Legacy assertion mode
assert.notDeepStrictEqual()
instead.Tests for any deep inequality. Opposite of assert.deepEqual()
.
import assert from 'node:assert';
const obj1 = {
a: {
b: 1,
},
};
const obj2 = {
a: {
b: 2,
},
};
const obj3 = {
a: {
b: 1,
},
};
const obj4 = { __proto__: obj1 };
assert.notDeepEqual(obj1, obj1);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }
assert.notDeepEqual(obj1, obj2);
// OK
assert.notDeepEqual(obj1, obj3);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }
assert.notDeepEqual(obj1, obj4);
// OK
const assert = require('node:assert');
const obj1 = {
a: {
b: 1,
},
};
const obj2 = {
a: {
b: 2,
},
};
const obj3 = {
a: {
b: 1,
},
};
const obj4 = { __proto__: obj1 };
assert.notDeepEqual(obj1, obj1);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }
assert.notDeepEqual(obj1, obj2);
// OK
assert.notDeepEqual(obj1, obj3);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }
assert.notDeepEqual(obj1, obj4);
// OK
If the values are deeply equal, an AssertionError
is thrown with a
message
property set equal to the value of the message
parameter. If the
message
parameter is undefined, a default error message is assigned. If the
message
parameter is an instance of an Error
then it will be thrown
instead of the AssertionError
.
assert.notDeepStrictEqual(actual, expected[, message])
#
Tests for deep strict inequality. Opposite of assert.deepStrictEqual()
.
import assert from 'node:assert/strict';
assert.notDeepStrictEqual({ a: 1 }, { a: '1' });
// OK
const assert = require('node:assert/strict');
assert.notDeepStrictEqual({ a: 1 }, { a: '1' });
// OK
If the values are deeply and strictly equal, an AssertionError
is thrown
with a message
property set equal to the value of the message
parameter. If
the message
parameter is undefined, a default error message is assigned. If
the message
parameter is an instance of an Error
then it will be thrown
instead of the AssertionError
.
assert.notEqual(actual, expected[, message])
#
Strict assertion mode
An alias of assert.notStrictEqual()
.
Legacy assertion mode
assert.notStrictEqual()
instead.Tests shallow, coercive inequality with the !=
operator. NaN
is
specially handled and treated as being identical if both sides are NaN
.
import assert from 'node:assert';
assert.notEqual(1, 2);
// OK
assert.notEqual(1, 1);
// AssertionError: 1 != 1
assert.notEqual(1, '1');
// AssertionError: 1 != '1'
const assert = require('node:assert');
assert.notEqual(1, 2);
// OK
assert.notEqual(1, 1);
// AssertionError: 1 != 1
assert.notEqual(1, '1');
// AssertionError: 1 != '1'
If the values are equal, an AssertionError
is thrown with a message
property set equal to the value of the message
parameter. If the message
parameter is undefined, a default error message is assigned. If the message
parameter is an instance of an Error
then it will be thrown instead of the
AssertionError
.
assert.notStrictEqual(actual, expected[, message])
#
Tests strict inequality between the actual
and expected
parameters as
determined by Object.is()
.
import assert from 'node:assert/strict';
assert.notStrictEqual(1, 2);
// OK
assert.notStrictEqual(1, 1);
// AssertionError [ERR_ASSERTION]: Expected "actual" to be strictly unequal to:
//
// 1
assert.notStrictEqual(1, '1');
// OK
const assert = require('node:assert/strict');
assert.notStrictEqual(1, 2);
// OK
assert.notStrictEqual(1, 1);
// AssertionError [ERR_ASSERTION]: Expected "actual" to be strictly unequal to:
//
// 1
assert.notStrictEqual(1, '1');
// OK
If the values are strictly equal, an AssertionError
is thrown with a
message
property set equal to the value of the message
parameter. If the
message
parameter is undefined, a default error message is assigned. If the
message
parameter is an instance of an Error
then it will be thrown
instead of the AssertionError
.
assert.ok(value[, message])
#
Tests if value
is truthy. It is equivalent to
assert.equal(!!value, true, message)
.
If value
is not truthy, an AssertionError
is thrown with a message
property set equal to the value of the message
parameter. If the message
parameter is undefined
, a default error message is assigned. If the message
parameter is an instance of an Error
then it will be thrown instead of the
AssertionError
.
If no arguments are passed in at all message
will be set to the string:
'No value argument passed to `assert.ok()`'
.
Be aware that in the repl
the error message will be different to the one
thrown in a file! See below for further details.
import assert from 'node:assert/strict';
assert.ok(true);
// OK
assert.ok(1);
// OK
assert.ok();
// AssertionError: No value argument passed to `assert.ok()`
assert.ok(false, 'it\'s false');
// AssertionError: it's false
// In the repl:
assert.ok(typeof 123 === 'string');
// AssertionError: false == true
// In a file (e.g. test.js):
assert.ok(typeof 123 === 'string');
// AssertionError: The expression evaluated to a falsy value:
//
// assert.ok(typeof 123 === 'string')
assert.ok(false);
// AssertionError: The expression evaluated to a falsy value:
//
// assert.ok(false)
assert.ok(0);
// AssertionError: The expression evaluated to a falsy value:
//
// assert.ok(0)
const assert = require('node:assert/strict');
assert.ok(true);
// OK
assert.ok(1);
// OK
assert.ok();
// AssertionError: No value argument passed to `assert.ok()`
assert.ok(false, 'it\'s false');
// AssertionError: it's false
// In the repl:
assert.ok(typeof 123 === 'string');
// AssertionError: false == true
// In a file (e.g. test.js):
assert.ok(typeof 123 === 'string');
// AssertionError: The expression evaluated to a falsy value:
//
// assert.ok(typeof 123 === 'string')
assert.ok(false);
// AssertionError: The expression evaluated to a falsy value:
//
// assert.ok(false)
assert.ok(0);
// AssertionError: The expression evaluated to a falsy value:
//
// assert.ok(0)
import assert from 'node:assert/strict';
// Using `assert()` works the same:
assert(0);
// AssertionError: The expression evaluated to a falsy value:
//
// assert(0)
const assert = require('node:assert');
// Using `assert()` works the same:
assert(0);
// AssertionError: The expression evaluated to a falsy value:
//
// assert(0)
assert.rejects(asyncFn[, error][, message])
#
asyncFn
<Function> | <Promise>error
<RegExp> | <Function> | <Object> | <Error>message
<string>
Awaits the asyncFn
promise or, if asyncFn
is a function, immediately
calls the function and awaits the returned promise to complete. It will then
check that the promise is rejected.
If asyncFn
is a function and it throws an error synchronously,
assert.rejects()
will return a rejected Promise
with that error. If the
function does not return a promise, assert.rejects()
will return a rejected
Promise
with an ERR_INVALID_RETURN_VALUE
error. In both cases the error
handler is skipped.
Besides the async nature to await the completion behaves identically to
assert.throws()
.
If specified, error
can be a Class
, RegExp
, a validation function,
an object where each property will be tested for, or an instance of error where
each property will be tested for including the non-enumerable message
and
name
properties.
If specified, message
will be the message provided by the AssertionError
if the asyncFn
fails to reject.
import assert from 'node:assert/strict';
await assert.rejects(
async () => {
throw new TypeError('Wrong value');
},
{
name: 'TypeError',
message: 'Wrong value',
},
);
const assert = require('node:assert/strict');
(async () => {
await assert.rejects(
async () => {
throw new TypeError('Wrong value');
},
{
name: 'TypeError',
message: 'Wrong value',
},
);
})();
import assert from 'node:assert/strict';
await assert.rejects(
async () => {
throw new TypeError('Wrong value');
},
(err) => {
assert.strictEqual(err.name, 'TypeError');
assert.strictEqual(err.message, 'Wrong value');
return true;
},
);
const assert = require('node:assert/strict');
(async () => {
await assert.rejects(
async () => {
throw new TypeError('Wrong value');
},
(err) => {
assert.strictEqual(err.name, 'TypeError');
assert.strictEqual(err.message, 'Wrong value');
return true;
},
);
})();
import assert from 'node:assert/strict';
assert.rejects(
Promise.reject(new Error('Wrong value')),
Error,
).then(() => {
// ...
});
const assert = require('node:assert/strict');
assert.rejects(
Promise.reject(new Error('Wrong value')),
Error,
).then(() => {
// ...
});
error
cannot be a string. If a string is provided as the second
argument, then error
is assumed to be omitted and the string will be used for
message
instead. This can lead to easy-to-miss mistakes. Please read the
example in assert.throws()
carefully if using a string as the second
argument gets considered.
assert.strictEqual(actual, expected[, message])
#
Tests strict equality between the actual
and expected
parameters as
determined by Object.is()
.
import assert from 'node:assert/strict';
assert.strictEqual(1, 2);
// AssertionError [ERR_ASSERTION]: Expected inputs to be strictly equal:
//
// 1 !== 2
assert.strictEqual(1, 1);
// OK
assert.strictEqual('Hello foobar', 'Hello World!');
// AssertionError [ERR_ASSERTION]: Expected inputs to be strictly equal:
// + actual - expected
//
// + 'Hello foobar'
// - 'Hello World!'
// ^
const apples = 1;
const oranges = 2;
assert.strictEqual(apples, oranges, `apples ${apples} !== oranges ${oranges}`);
// AssertionError [ERR_ASSERTION]: apples 1 !== oranges 2
assert.strictEqual(1, '1', new TypeError('Inputs are not identical'));
// TypeError: Inputs are not identical
const assert = require('node:assert/strict');
assert.strictEqual(1, 2);
// AssertionError [ERR_ASSERTION]: Expected inputs to be strictly equal:
//
// 1 !== 2
assert.strictEqual(1, 1);
// OK
assert.strictEqual('Hello foobar', 'Hello World!');
// AssertionError [ERR_ASSERTION]: Expected inputs to be strictly equal:
// + actual - expected
//
// + 'Hello foobar'
// - 'Hello World!'
// ^
const apples = 1;
const oranges = 2;
assert.strictEqual(apples, oranges, `apples ${apples} !== oranges ${oranges}`);
// AssertionError [ERR_ASSERTION]: apples 1 !== oranges 2
assert.strictEqual(1, '1', new TypeError('Inputs are not identical'));
// TypeError: Inputs are not identical
If the values are not strictly equal, an AssertionError
is thrown with a
message
property set equal to the value of the message
parameter. If the
message
parameter is undefined, a default error message is assigned. If the
message
parameter is an instance of an Error
then it will be thrown
instead of the AssertionError
.
assert.throws(fn[, error][, message])
#
fn
<Function>error
<RegExp> | <Function> | <Object> | <Error>message
<string>
Expects the function fn
to throw an error.
If specified, error
can be a Class
, RegExp
, a validation function,
a validation object where each property will be tested for strict deep equality,
or an instance of error where each property will be tested for strict deep
equality including the non-enumerable message
and name
properties. When
using an object, it is also possible to use a regular expression, when
validating against a string property. See below for examples.
If specified, message
will be appended to the message provided by the
AssertionError
if the fn
call fails to throw or in case the error validation
fails.
Custom validation object/error instance:
import assert from 'node:assert/strict';
const err = new TypeError('Wrong value');
err.code = 404;
err.foo = 'bar';
err.info = {
nested: true,
baz: 'text',
};
err.reg = /abc/i;
assert.throws(
() => {
throw err;
},
{
name: 'TypeError',
message: 'Wrong value',
info: {
nested: true,
baz: 'text',
},
// Only properties on the validation object will be tested for.
// Using nested objects requires all properties to be present. Otherwise
// the validation is going to fail.
},
);
// Using regular expressions to validate error properties:
assert.throws(
() => {
throw err;
},
{
// The `name` and `message` properties are strings and using regular
// expressions on those will match against the string. If they fail, an
// error is thrown.
name: /^TypeError$/,
message: /Wrong/,
foo: 'bar',
info: {
nested: true,
// It is not possible to use regular expressions for nested properties!
baz: 'text',
},
// The `reg` property contains a regular expression and only if the
// validation object contains an identical regular expression, it is going
// to pass.
reg: /abc/i,
},
);
// Fails due to the different `message` and `name` properties:
assert.throws(
() => {
const otherErr = new Error('Not found');
// Copy all enumerable properties from `err` to `otherErr`.
for (const [key, value] of Object.entries(err)) {
otherErr[key] = value;
}
throw otherErr;
},
// The error's `message` and `name` properties will also be checked when using
// an error as validation object.
err,
);
const assert = require('node:assert/strict');
const err = new TypeError('Wrong value');
err.code = 404;
err.foo = 'bar';
err.info = {
nested: true,
baz: 'text',
};
err.reg = /abc/i;
assert.throws(
() => {
throw err;
},
{
name: 'TypeError',
message: 'Wrong value',
info: {
nested: true,
baz: 'text',
},
// Only properties on the validation object will be tested for.
// Using nested objects requires all properties to be present. Otherwise
// the validation is going to fail.
},
);
// Using regular expressions to validate error properties:
assert.throws(
() => {
throw err;
},
{
// The `name` and `message` properties are strings and using regular
// expressions on those will match against the string. If they fail, an
// error is thrown.
name: /^TypeError$/,
message: /Wrong/,
foo: 'bar',
info: {
nested: true,
// It is not possible to use regular expressions for nested properties!
baz: 'text',
},
// The `reg` property contains a regular expression and only if the
// validation object contains an identical regular expression, it is going
// to pass.
reg: /abc/i,
},
);
// Fails due to the different `message` and `name` properties:
assert.throws(
() => {
const otherErr = new Error('Not found');
// Copy all enumerable properties from `err` to `otherErr`.
for (const [key, value] of Object.entries(err)) {
otherErr[key] = value;
}
throw otherErr;
},
// The error's `message` and `name` properties will also be checked when using
// an error as validation object.
err,
);
Validate instanceof using constructor:
import assert from 'node:assert/strict';
assert.throws(
() => {
throw new Error('Wrong value');
},
Error,
);
const assert = require('node:assert/strict');
assert.throws(
() => {
throw new Error('Wrong value');
},
Error,
);
Validate error message using RegExp
:
Using a regular expression runs .toString
on the error object, and will
therefore also include the error name.
import assert from 'node:assert/strict';
assert.throws(
() => {
throw new Error('Wrong value');
},
/^Error: Wrong value$/,
);
const assert = require('node:assert/strict');
assert.throws(
() => {
throw new Error('Wrong value');
},
/^Error: Wrong value$/,
);
Custom error validation:
The function must return true
to indicate all internal validations passed.
It will otherwise fail with an AssertionError
.
import assert from 'node:assert/strict';
assert.throws(
() => {
throw new Error('Wrong value');
},
(err) => {
assert(err instanceof Error);
assert(/value/.test(err));
// Avoid returning anything from validation functions besides `true`.
// Otherwise, it's not clear what part of the validation failed. Instead,
// throw an error about the specific validation that failed (as done in this
// example) and add as much helpful debugging information to that error as
// possible.
return true;
},
'unexpected error',
);
const assert = require('node:assert/strict');
assert.throws(
() => {
throw new Error('Wrong value');
},
(err) => {
assert(err instanceof Error);
assert(/value/.test(err));
// Avoid returning anything from validation functions besides `true`.
// Otherwise, it's not clear what part of the validation failed. Instead,
// throw an error about the specific validation that failed (as done in this
// example) and add as much helpful debugging information to that error as
// possible.
return true;
},
'unexpected error',
);
error
cannot be a string. If a string is provided as the second
argument, then error
is assumed to be omitted and the string will be used for
message
instead. This can lead to easy-to-miss mistakes. Using the same
message as the thrown error message is going to result in an
ERR_AMBIGUOUS_ARGUMENT
error. Please read the example below carefully if using
a string as the second argument gets considered:
import assert from 'node:assert/strict';
function throwingFirst() {
throw new Error('First');
}
function throwingSecond() {
throw new Error('Second');
}
function notThrowing() {}
// The second argument is a string and the input function threw an Error.
// The first case will not throw as it does not match for the error message
// thrown by the input function!
assert.throws(throwingFirst, 'Second');
// In the next example the message has no benefit over the message from the
// error and since it is not clear if the user intended to actually match
// against the error message, Node.js throws an `ERR_AMBIGUOUS_ARGUMENT` error.
assert.throws(throwingSecond, 'Second');
// TypeError [ERR_AMBIGUOUS_ARGUMENT]
// The string is only used (as message) in case the function does not throw:
assert.throws(notThrowing, 'Second');
// AssertionError [ERR_ASSERTION]: Missing expected exception: Second
// If it was intended to match for the error message do this instead:
// It does not throw because the error messages match.
assert.throws(throwingSecond, /Second$/);
// If the error message does not match, an AssertionError is thrown.
assert.throws(throwingFirst, /Second$/);
// AssertionError [ERR_ASSERTION]
const assert = require('node:assert/strict');
function throwingFirst() {
throw new Error('First');
}
function throwingSecond() {
throw new Error('Second');
}
function notThrowing() {}
// The second argument is a string and the input function threw an Error.
// The first case will not throw as it does not match for the error message
// thrown by the input function!
assert.throws(throwingFirst, 'Second');
// In the next example the message has no benefit over the message from the
// error and since it is not clear if the user intended to actually match
// against the error message, Node.js throws an `ERR_AMBIGUOUS_ARGUMENT` error.
assert.throws(throwingSecond, 'Second');
// TypeError [ERR_AMBIGUOUS_ARGUMENT]
// The string is only used (as message) in case the function does not throw:
assert.throws(notThrowing, 'Second');
// AssertionError [ERR_ASSERTION]: Missing expected exception: Second
// If it was intended to match for the error message do this instead:
// It does not throw because the error messages match.
assert.throws(throwingSecond, /Second$/);
// If the error message does not match, an AssertionError is thrown.
assert.throws(throwingFirst, /Second$/);
// AssertionError [ERR_ASSERTION]
Due to the confusing error-prone notation, avoid a string as the second argument.
Asynchronous context tracking#
Source Code: lib/async_hooks.js
Introduction#
These classes are used to associate state and propagate it throughout callbacks and promise chains. They allow storing data throughout the lifetime of a web request or any other asynchronous duration. It is similar to thread-local storage in other languages.
The AsyncLocalStorage
and AsyncResource
classes are part of the
node:async_hooks
module:
import { AsyncLocalStorage, AsyncResource } from 'node:async_hooks';
const { AsyncLocalStorage, AsyncResource } = require('node:async_hooks');
Class: AsyncLocalStorage
#
This class creates stores that stay coherent through asynchronous operations.
While you can create your own implementation on top of the node:async_hooks
module, AsyncLocalStorage
should be preferred as it is a performant and memory
safe implementation that involves significant optimizations that are non-obvious
to implement.
The following example uses AsyncLocalStorage
to build a simple logger
that assigns IDs to incoming HTTP requests and includes them in messages
logged within each request.
import http from 'node:http';
import { AsyncLocalStorage } from 'node:async_hooks';
const asyncLocalStorage = new AsyncLocalStorage();
function logWithId(msg) {
const id = asyncLocalStorage.getStore();
console.log(`${id !== undefined ? id : '-'}:`, msg);
}
let idSeq = 0;
http.createServer((req, res) => {
asyncLocalStorage.run(idSeq++, () => {
logWithId('start');
// Imagine any chain of async operations here
setImmediate(() => {
logWithId('finish');
res.end();
});
});
}).listen(8080);
http.get('http://localhost:8080');
http.get('http://localhost:8080');
// Prints:
// 0: start
// 1: start
// 0: finish
// 1: finish
const http = require('node:http');
const { AsyncLocalStorage } = require('node:async_hooks');
const asyncLocalStorage = new AsyncLocalStorage();
function logWithId(msg) {
const id = asyncLocalStorage.getStore();
console.log(`${id !== undefined ? id : '-'}:`, msg);
}
let idSeq = 0;
http.createServer((req, res) => {
asyncLocalStorage.run(idSeq++, () => {
logWithId('start');
// Imagine any chain of async operations here
setImmediate(() => {
logWithId('finish');
res.end();
});
});
}).listen(8080);
http.get('http://localhost:8080');
http.get('http://localhost:8080');
// Prints:
// 0: start
// 1: start
// 0: finish
// 1: finish
Each instance of AsyncLocalStorage
maintains an independent storage context.
Multiple instances can safely exist simultaneously without risk of interfering
with each other's data.
new AsyncLocalStorage()
#
Creates a new instance of AsyncLocalStorage
. Store is only provided within a
run()
call or after an enterWith()
call.
Static method: AsyncLocalStorage.bind(fn)
#
fn
<Function> The function to bind to the current execution context.- Returns: <Function> A new function that calls
fn
within the captured execution context.
Binds the given function to the current execution context.
Static method: AsyncLocalStorage.snapshot()
#
- Returns: <Function> A new function with the signature
(fn: (...args) : R, ...args) : R
.
Captures the current execution context and returns a function that accepts a function as an argument. Whenever the returned function is called, it calls the function passed to it within the captured context.
const asyncLocalStorage = new AsyncLocalStorage();
const runInAsyncScope = asyncLocalStorage.run(123, () => AsyncLocalStorage.snapshot());
const result = asyncLocalStorage.run(321, () => runInAsyncScope(() => asyncLocalStorage.getStore()));
console.log(result); // returns 123
AsyncLocalStorage.snapshot() can replace the use of AsyncResource for simple async context tracking purposes, for example:
class Foo {
#runInAsyncScope = AsyncLocalStorage.snapshot();
get() { return this.#runInAsyncScope(() => asyncLocalStorage.getStore()); }
}
const foo = asyncLocalStorage.run(123, () => new Foo());
console.log(asyncLocalStorage.run(321, () => foo.get())); // returns 123
asyncLocalStorage.disable()
#
Disables the instance of AsyncLocalStorage
. All subsequent calls
to asyncLocalStorage.getStore()
will return undefined
until
asyncLocalStorage.run()
or asyncLocalStorage.enterWith()
is called again.
When calling asyncLocalStorage.disable()
, all current contexts linked to the
instance will be exited.
Calling asyncLocalStorage.disable()
is required before the
asyncLocalStorage
can be garbage collected. This does not apply to stores
provided by the asyncLocalStorage
, as those objects are garbage collected
along with the corresponding async resources.
Use this method when the asyncLocalStorage
is not in use anymore
in the current process.
asyncLocalStorage.getStore()
#
- Returns: <any>
Returns the current store.
If called outside of an asynchronous context initialized by
calling asyncLocalStorage.run()
or asyncLocalStorage.enterWith()
, it
returns undefined
.
asyncLocalStorage.enterWith(store)
#
store
<any>
Transitions into the context for the remainder of the current synchronous execution and then persists the store through any following asynchronous calls.
Example:
const store = { id: 1 };
// Replaces previous store with the given store object
asyncLocalStorage.enterWith(store);
asyncLocalStorage.getStore(); // Returns the store object
someAsyncOperation(() => {
asyncLocalStorage.getStore(); // Returns the same object
});
This transition will continue for the entire synchronous execution.
This means that if, for example, the context is entered within an event
handler subsequent event handlers will also run within that context unless
specifically bound to another context with an AsyncResource
. That is why
run()
should be preferred over enterWith()
unless there are strong reasons
to use the latter method.
const store = { id: 1 };
emitter.on('my-event', () => {
asyncLocalStorage.enterWith(store);
});
emitter.on('my-event', () => {
asyncLocalStorage.getStore(); // Returns the same object
});
asyncLocalStorage.getStore(); // Returns undefined
emitter.emit('my-event');
asyncLocalStorage.getStore(); // Returns the same object
asyncLocalStorage.run(store, callback[, ...args])
#
store
<any>callback
<Function>...args
<any>
Runs a function synchronously within a context and returns its return value. The store is not accessible outside of the callback function. The store is accessible to any asynchronous operations created within the callback.
The optional args
are passed to the callback function.
If the callback function throws an error, the error is thrown by run()
too.
The stacktrace is not impacted by this call and the context is exited.
Example:
const store = { id: 2 };
try {
asyncLocalStorage.run(store, () => {
asyncLocalStorage.getStore(); // Returns the store object
setTimeout(() => {
asyncLocalStorage.getStore(); // Returns the store object
}, 200);
throw new Error();
});
} catch (e) {
asyncLocalStorage.getStore(); // Returns undefined
// The error will be caught here
}
asyncLocalStorage.exit(callback[, ...args])
#
callback
<Function>...args
<any>
Runs a function synchronously outside of a context and returns its
return value. The store is not accessible within the callback function or
the asynchronous operations created within the callback. Any getStore()
call done within the callback function will always return undefined
.
The optional args
are passed to the callback function.
If the callback function throws an error, the error is thrown by exit()
too.
The stacktrace is not impacted by this call and the context is re-entered.
Example:
// Within a call to run
try {
asyncLocalStorage.getStore(); // Returns the store object or value
asyncLocalStorage.exit(() => {
asyncLocalStorage.getStore(); // Returns undefined
throw new Error();
});
} catch (e) {
asyncLocalStorage.getStore(); // Returns the same object or value
// The error will be caught here
}
Usage with async/await
#
If, within an async function, only one await
call is to run within a context,
the following pattern should be used:
async function fn() {
await asyncLocalStorage.run(new Map(), () => {
asyncLocalStorage.getStore().set('key', value);
return foo(); // The return value of foo will be awaited
});
}
In this example, the store is only available in the callback function and the
functions called by foo
. Outside of run
, calling getStore
will return
undefined
.
Troubleshooting: Context loss#
In most cases, AsyncLocalStorage
works without issues. In rare situations, the
current store is lost in one of the asynchronous operations.
If your code is callback-based, it is enough to promisify it with
util.promisify()
so it starts working with native promises.
If you need to use a callback-based API or your code assumes
a custom thenable implementation, use the AsyncResource
class
to associate the asynchronous operation with the correct execution context.
Find the function call responsible for the context loss by logging the content
of asyncLocalStorage.getStore()
after the calls you suspect are responsible
for the loss. When the code logs undefined
, the last callback called is
probably responsible for the context loss.
Class: AsyncResource
#
The class AsyncResource
is designed to be extended by the embedder's async
resources. Using this, users can easily trigger the lifetime events of their
own resources.
The init
hook will trigger when an AsyncResource
is instantiated.
The following is an overview of the AsyncResource
API.
import { AsyncResource, executionAsyncId } from 'node:async_hooks';
// AsyncResource() is meant to be extended. Instantiating a
// new AsyncResource() also triggers init. If triggerAsyncId is omitted then
// async_hook.executionAsyncId() is used.
const asyncResource = new AsyncResource(
type, { triggerAsyncId: executionAsyncId(), requireManualDestroy: false },
);
// Run a function in the execution context of the resource. This will
// * establish the context of the resource
// * trigger the AsyncHooks before callbacks
// * call the provided function `fn` with the supplied arguments
// * trigger the AsyncHooks after callbacks
// * restore the original execution context
asyncResource.runInAsyncScope(fn, thisArg, ...args);
// Call AsyncHooks destroy callbacks.
asyncResource.emitDestroy();
// Return the unique ID assigned to the AsyncResource instance.
asyncResource.asyncId();
// Return the trigger ID for the AsyncResource instance.
asyncResource.triggerAsyncId();
const { AsyncResource, executionAsyncId } = require('node:async_hooks');
// AsyncResource() is meant to be extended. Instantiating a
// new AsyncResource() also triggers init. If triggerAsyncId is omitted then
// async_hook.executionAsyncId() is used.
const asyncResource = new AsyncResource(
type, { triggerAsyncId: executionAsyncId(), requireManualDestroy: false },
);
// Run a function in the execution context of the resource. This will
// * establish the context of the resource
// * trigger the AsyncHooks before callbacks
// * call the provided function `fn` with the supplied arguments
// * trigger the AsyncHooks after callbacks
// * restore the original execution context
asyncResource.runInAsyncScope(fn, thisArg, ...args);
// Call AsyncHooks destroy callbacks.
asyncResource.emitDestroy();
// Return the unique ID assigned to the AsyncResource instance.
asyncResource.asyncId();
// Return the trigger ID for the AsyncResource instance.
asyncResource.triggerAsyncId();
new AsyncResource(type[, options])
#
type
<string> The type of async event.options
<Object>triggerAsyncId
<number> The ID of the execution context that created this async event. Default:executionAsyncId()
.requireManualDestroy
<boolean> If set totrue
, disablesemitDestroy
when the object is garbage collected. This usually does not need to be set (even ifemitDestroy
is called manually), unless the resource'sasyncId
is retrieved and the sensitive API'semitDestroy
is called with it. When set tofalse
, theemitDestroy
call on garbage collection will only take place if there is at least one activedestroy
hook. Default:false
.
Example usage:
class DBQuery extends AsyncResource {
constructor(db) {
super('DBQuery');
this.db = db;
}
getInfo(query, callback) {
this.db.get(query, (err, data) => {
this.runInAsyncScope(callback, null, err, data);
});
}
close() {
this.db = null;
this.emitDestroy();
}
}
Static method: AsyncResource.bind(fn[, type[, thisArg]])
#
fn
<Function> The function to bind to the current execution context.type
<string> An optional name to associate with the underlyingAsyncResource
.thisArg
<any>
Binds the given function to the current execution context.
asyncResource.bind(fn[, thisArg])
#
fn
<Function> The function to bind to the currentAsyncResource
.thisArg
<any>
Binds the given function to execute to this AsyncResource
's scope.
asyncResource.runInAsyncScope(fn[, thisArg, ...args])
#
fn
<Function> The function to call in the execution context of this async resource.thisArg
<any> The receiver to be used for the function call....args
<any> Optional arguments to pass to the function.
Call the provided function with the provided arguments in the execution context of the async resource. This will establish the context, trigger the AsyncHooks before callbacks, call the function, trigger the AsyncHooks after callbacks, and then restore the original execution context.
asyncResource.emitDestroy()
#
- Returns: <AsyncResource> A reference to
asyncResource
.
Call all destroy
hooks. This should only ever be called once. An error will
be thrown if it is called more than once. This must be manually called. If
the resource is left to be collected by the GC then the destroy
hooks will
never be called.
asyncResource.asyncId()
#
- Returns: <number> The unique
asyncId
assigned to the resource.
asyncResource.triggerAsyncId()
#
- Returns: <number> The same
triggerAsyncId
that is passed to theAsyncResource
constructor.
Using AsyncResource
for a Worker
thread pool#
The following example shows how to use the AsyncResource
class to properly
provide async tracking for a Worker
pool. Other resource pools, such as
database connection pools, can follow a similar model.
Assuming that the task is adding two numbers, using a file named
task_processor.js
with the following content:
import { parentPort } from 'node:worker_threads';
parentPort.on('message', (task) => {
parentPort.postMessage(task.a + task.b);
});
const { parentPort } = require('node:worker_threads');
parentPort.on('message', (task) => {
parentPort.postMessage(task.a + task.b);
});
a Worker pool around it could use the following structure:
import { AsyncResource } from 'node:async_hooks';
import { EventEmitter } from 'node:events';
import path from 'node:path';
import { Worker } from 'node:worker_threads';
const kTaskInfo = Symbol('kTaskInfo');
const kWorkerFreedEvent = Symbol('kWorkerFreedEvent');
class WorkerPoolTaskInfo extends AsyncResource {
constructor(callback) {
super('WorkerPoolTaskInfo');
this.callback = callback;
}
done(err, result) {
this.runInAsyncScope(this.callback, null, err, result);
this.emitDestroy(); // `TaskInfo`s are used only once.
}
}
export default class WorkerPool extends EventEmitter {
constructor(numThreads) {
super();
this.numThreads = numThreads;
this.workers = [];
this.freeWorkers = [];
this.tasks = [];
for (let i = 0; i < numThreads; i++)
this.addNewWorker();
// Any time the kWorkerFreedEvent is emitted, dispatch
// the next task pending in the queue, if any.
this.on(kWorkerFreedEvent, () => {
if (this.tasks.length > 0) {
const { task, callback } = this.tasks.shift();
this.runTask(task, callback);
}
});
}
addNewWorker() {
const worker = new Worker(new URL('task_processor.js', import.meta.url));
worker.on('message', (result) => {
// In case of success: Call the callback that was passed to `runTask`,
// remove the `TaskInfo` associated with the Worker, and mark it as free
// again.
worker[kTaskInfo].done(null, result);
worker[kTaskInfo] = null;
this.freeWorkers.push(worker);
this.emit(kWorkerFreedEvent);
});
worker.on('error', (err) => {
// In case of an uncaught exception: Call the callback that was passed to
// `runTask` with the error.
if (worker[kTaskInfo])
worker[kTaskInfo].done(err, null);
else
this.emit('error', err);
// Remove the worker from the list and start a new Worker to replace the
// current one.
this.workers.splice(this.workers.indexOf(worker), 1);
this.addNewWorker();
});
this.workers.push(worker);
this.freeWorkers.push(worker);
this.emit(kWorkerFreedEvent);
}
runTask(task, callback) {
if (this.freeWorkers.length === 0) {
// No free threads, wait until a worker thread becomes free.
this.tasks.push({ task, callback });
return;
}
const worker = this.freeWorkers.pop();
worker[kTaskInfo] = new WorkerPoolTaskInfo(callback);
worker.postMessage(task);
}
close() {
for (const worker of this.workers) worker.terminate();
}
}
const { AsyncResource } = require('node:async_hooks');
const { EventEmitter } = require('node:events');
const path = require('node:path');
const { Worker } = require('node:worker_threads');
const kTaskInfo = Symbol('kTaskInfo');
const kWorkerFreedEvent = Symbol('kWorkerFreedEvent');
class WorkerPoolTaskInfo extends AsyncResource {
constructor(callback) {
super('WorkerPoolTaskInfo');
this.callback = callback;
}
done(err, result) {
this.runInAsyncScope(this.callback, null, err, result);
this.emitDestroy(); // `TaskInfo`s are used only once.
}
}
class WorkerPool extends EventEmitter {
constructor(numThreads) {
super();
this.numThreads = numThreads;
this.workers = [];
this.freeWorkers = [];
this.tasks = [];
for (let i = 0; i < numThreads; i++)
this.addNewWorker();
// Any time the kWorkerFreedEvent is emitted, dispatch
// the next task pending in the queue, if any.
this.on(kWorkerFreedEvent, () => {
if (this.tasks.length > 0) {
const { task, callback } = this.tasks.shift();
this.runTask(task, callback);
}
});
}
addNewWorker() {
const worker = new Worker(path.resolve(__dirname, 'task_processor.js'));
worker.on('message', (result) => {
// In case of success: Call the callback that was passed to `runTask`,
// remove the `TaskInfo` associated with the Worker, and mark it as free
// again.
worker[kTaskInfo].done(null, result);
worker[kTaskInfo] = null;
this.freeWorkers.push(worker);
this.emit(kWorkerFreedEvent);
});
worker.on('error', (err) => {
// In case of an uncaught exception: Call the callback that was passed to
// `runTask` with the error.
if (worker[kTaskInfo])
worker[kTaskInfo].done(err, null);
else
this.emit('error', err);
// Remove the worker from the list and start a new Worker to replace the
// current one.
this.workers.splice(this.workers.indexOf(worker), 1);
this.addNewWorker();
});
this.workers.push(worker);
this.freeWorkers.push(worker);
this.emit(kWorkerFreedEvent);
}
runTask(task, callback) {
if (this.freeWorkers.length === 0) {
// No free threads, wait until a worker thread becomes free.
this.tasks.push({ task, callback });
return;
}
const worker = this.freeWorkers.pop();
worker[kTaskInfo] = new WorkerPoolTaskInfo(callback);
worker.postMessage(task);
}
close() {
for (const worker of this.workers) worker.terminate();
}
}
module.exports = WorkerPool;
Without the explicit tracking added by the WorkerPoolTaskInfo
objects,
it would appear that the callbacks are associated with the individual Worker
objects. However, the creation of the Worker
s is not associated with the
creation of the tasks and does not provide information about when tasks
were scheduled.
This pool could be used as follows:
import WorkerPool from './worker_pool.js';
import os from 'node:os';
const pool = new WorkerPool(os.availableParallelism());
let finished = 0;
for (let i = 0; i < 10; i++) {
pool.runTask({ a: 42, b: 100 }, (err, result) => {
console.log(i, err, result);
if (++finished === 10)
pool.close();
});
}
const WorkerPool = require('./worker_pool.js');
const os = require('node:os');
const pool = new WorkerPool(os.availableParallelism());
let finished = 0;
for (let i = 0; i < 10; i++) {
pool.runTask({ a: 42, b: 100 }, (err, result) => {
console.log(i, err, result);
if (++finished === 10)
pool.close();
});
}
Integrating AsyncResource
with EventEmitter
#
Event listeners triggered by an EventEmitter
may be run in a different
execution context than the one that was active when eventEmitter.on()
was
called.
The following example shows how to use the AsyncResource
class to properly
associate an event listener with the correct execution context. The same
approach can be applied to a Stream
or a similar event-driven class.
import { createServer } from 'node:http';
import { AsyncResource, executionAsyncId } from 'node:async_hooks';
const server = createServer((req, res) => {
req.on('close', AsyncResource.bind(() => {
// Execution context is bound to the current outer scope.
}));
req.on('close', () => {
// Execution context is bound to the scope that caused 'close' to emit.
});
res.end();
}).listen(3000);
const { createServer } = require('node:http');
const { AsyncResource, executionAsyncId } = require('node:async_hooks');
const server = createServer((req, res) => {
req.on('close', AsyncResource.bind(() => {
// Execution context is bound to the current outer scope.
}));
req.on('close', () => {
// Execution context is bound to the scope that caused 'close' to emit.
});
res.end();
}).listen(3000);
Async hooks#
createHook
, AsyncHook
, and
executionAsyncResource
APIs as they have usability issues, safety risks,
and performance implications. Async context tracking use cases are better
served by the stable AsyncLocalStorage
API. If you have a use case for
createHook
, AsyncHook
, or executionAsyncResource
beyond the context
tracking need solved by AsyncLocalStorage
or diagnostics data currently
provided by Diagnostics Channel, please open an issue at
https://github.com/nodejs/node/issues describing your use case so we can
create a more purpose-focused API.Source Code: lib/async_hooks.js
We strongly discourage the use of the async_hooks
API.
Other APIs that can cover most of its use cases include:
AsyncLocalStorage
tracks async contextprocess.getActiveResourcesInfo()
tracks active resources
The node:async_hooks
module provides an API to track asynchronous resources.
It can be accessed using:
import async_hooks from 'node:async_hooks';
const async_hooks = require('node:async_hooks');
Terminology#
An asynchronous resource represents an object with an associated callback.
This callback may be called multiple times, such as the 'connection'
event in net.createServer()
, or just a single time like in fs.open()
.
A resource can also be closed before the callback is called. AsyncHook
does
not explicitly distinguish between these different cases but will represent them
as the abstract concept that is a resource.
If Worker
s are used, each thread has an independent async_hooks
interface, and each thread will use a new set of async IDs.
Overview#
Following is a simple overview of the public API.
import async_hooks from 'node:async_hooks';
// Return the ID of the current execution context.
const eid = async_hooks.executionAsyncId();
// Return the ID of the handle responsible for triggering the callback of the
// current execution scope to call.
const tid = async_hooks.triggerAsyncId();
// Create a new AsyncHook instance. All of these callbacks are optional.
const asyncHook =
async_hooks.createHook({ init, before, after, destroy, promiseResolve });
// Allow callbacks of this AsyncHook instance to call. This is not an implicit
// action after running the constructor, and must be explicitly run to begin
// executing callbacks.
asyncHook.enable();
// Disable listening for new asynchronous events.
asyncHook.disable();
//
// The following are the callbacks that can be passed to createHook().
//
// init() is called during object construction. The resource may not have
// completed construction when this callback runs. Therefore, all fields of the
// resource referenced by "asyncId" may not have been populated.
function init(asyncId, type, triggerAsyncId, resource) { }
// before() is called just before the resource's callback is called. It can be
// called 0-N times for handles (such as TCPWrap), and will be called exactly 1
// time for requests (such as FSReqCallback).
function before(asyncId) { }
// after() is called just after the resource's callback has finished.
function after(asyncId) { }
// destroy() is called when the resource is destroyed.
function destroy(asyncId) { }
// promiseResolve() is called only for promise resources, when the
// resolve() function passed to the Promise constructor is invoked
// (either directly or through other means of resolving a promise).
function promiseResolve(asyncId) { }
const async_hooks = require('node:async_hooks');
// Return the ID of the current execution context.
const eid = async_hooks.executionAsyncId();
// Return the ID of the handle responsible for triggering the callback of the
// current execution scope to call.
const tid = async_hooks.triggerAsyncId();
// Create a new AsyncHook instance. All of these callbacks are optional.
const asyncHook =
async_hooks.createHook({ init, before, after, destroy, promiseResolve });
// Allow callbacks of this AsyncHook instance to call. This is not an implicit
// action after running the constructor, and must be explicitly run to begin
// executing callbacks.
asyncHook.enable();
// Disable listening for new asynchronous events.
asyncHook.disable();
//
// The following are the callbacks that can be passed to createHook().
//
// init() is called during object construction. The resource may not have
// completed construction when this callback runs. Therefore, all fields of the
// resource referenced by "asyncId" may not have been populated.
function init(asyncId, type, triggerAsyncId, resource) { }
// before() is called just before the resource's callback is called. It can be
// called 0-N times for handles (such as TCPWrap), and will be called exactly 1
// time for requests (such as FSReqCallback).
function before(asyncId) { }
// after() is called just after the resource's callback has finished.
function after(asyncId) { }
// destroy() is called when the resource is destroyed.
function destroy(asyncId) { }
// promiseResolve() is called only for promise resources, when the
// resolve() function passed to the Promise constructor is invoked
// (either directly or through other means of resolving a promise).
function promiseResolve(asyncId) { }
async_hooks.createHook(callbacks)
#
callbacks
<Object> The Hook Callbacks to registerinit
<Function> Theinit
callback.before
<Function> Thebefore
callback.after
<Function> Theafter
callback.destroy
<Function> Thedestroy
callback.promiseResolve
<Function> ThepromiseResolve
callback.
- Returns: <AsyncHook> Instance used for disabling and enabling hooks
Registers functions to be called for different lifetime events of each async operation.
The callbacks init()
/before()
/after()
/destroy()
are called for the
respective asynchronous event during a resource's lifetime.
All callbacks are optional. For example, if only resource cleanup needs to
be tracked, then only the destroy
callback needs to be passed. The
specifics of all functions that can be passed to callbacks
is in the
Hook Callbacks section.
import { createHook } from 'node:async_hooks';
const asyncHook = createHook({
init(asyncId, type, triggerAsyncId, resource) { },
destroy(asyncId) { },
});
const async_hooks = require('node:async_hooks');
const asyncHook = async_hooks.createHook({
init(asyncId, type, triggerAsyncId, resource) { },
destroy(asyncId) { },
});
The callbacks will be inherited via the prototype chain:
class MyAsyncCallbacks {
init(asyncId, type, triggerAsyncId, resource) { }
destroy(asyncId) {}
}
class MyAddedCallbacks extends MyAsyncCallbacks {
before(asyncId) { }
after(asyncId) { }
}
const asyncHook = async_hooks.createHook(new MyAddedCallbacks());
Because promises are asynchronous resources whose lifecycle is tracked
via the async hooks mechanism, the init()
, before()
, after()
, and
destroy()
callbacks must not be async functions that return promises.
Error handling#
If any AsyncHook
callbacks throw, the application will print the stack trace
and exit. The exit path does follow that of an uncaught exception, but
all 'uncaughtException'
listeners are removed, thus forcing the process to
exit. The 'exit'
callbacks will still be called unless the application is run
with --abort-on-uncaught-exception
, in which case a stack trace will be
printed and the application exits, leaving a core file.
The reason for this error handling behavior is that these callbacks are running at potentially volatile points in an object's lifetime, for example during class construction and destruction. Because of this, it is deemed necessary to bring down the process quickly in order to prevent an unintentional abort in the future. This is subject to change in the future if a comprehensive analysis is performed to ensure an exception can follow the normal control flow without unintentional side effects.
Printing in AsyncHook
callbacks#
Because printing to the console is an asynchronous operation, console.log()
will cause AsyncHook
callbacks to be called. Using console.log()
or
similar asynchronous operations inside an AsyncHook
callback function will
cause an infinite recursion. An easy solution to this when debugging is to use a
synchronous logging operation such as fs.writeFileSync(file, msg, flag)
.
This will print to the file and will not invoke AsyncHook
recursively because
it is synchronous.
import { writeFileSync } from 'node:fs';
import { format } from 'node:util';
function debug(...args) {
// Use a function like this one when debugging inside an AsyncHook callback
writeFileSync('log.out', `${format(...args)}\n`, { flag: 'a' });
}
const fs = require('node:fs');
const util = require('node:util');
function debug(...args) {
// Use a function like this one when debugging inside an AsyncHook callback
fs.writeFileSync('log.out', `${util.format(...args)}\n`, { flag: 'a' });
}
If an asynchronous operation is needed for logging, it is possible to keep
track of what caused the asynchronous operation using the information
provided by AsyncHook
itself. The logging should then be skipped when
it was the logging itself that caused the AsyncHook
callback to be called. By
doing this, the otherwise infinite recursion is broken.
Class: AsyncHook
#
The class AsyncHook
exposes an interface for tracking lifetime events
of asynchronous operations.
asyncHook.enable()
#
- Returns: <AsyncHook> A reference to
asyncHook
.
Enable the callbacks for a given AsyncHook
instance. If no callbacks are
provided, enabling is a no-op.
The AsyncHook
instance is disabled by default. If the AsyncHook
instance
should be enabled immediately after creation, the following pattern can be used.
import { createHook } from 'node:async_hooks';
const hook = createHook(callbacks).enable();
const async_hooks = require('node:async_hooks');
const hook = async_hooks.createHook(callbacks).enable();
asyncHook.disable()
#
- Returns: <AsyncHook> A reference to
asyncHook
.
Disable the callbacks for a given AsyncHook
instance from the global pool of
AsyncHook
callbacks to be executed. Once a hook has been disabled it will not
be called again until enabled.
For API consistency disable()
also returns the AsyncHook
instance.
Hook callbacks#
Key events in the lifetime of asynchronous events have been categorized into four areas: instantiation, before/after the callback is called, and when the instance is destroyed.
init(asyncId, type, triggerAsyncId, resource)
#
asyncId
<number> A unique ID for the async resource.type
<string> The type of the async resource.triggerAsyncId
<number> The unique ID of the async resource in whose execution context this async resource was created.resource
<Object> Reference to the resource representing the async operation, needs to be released during destroy.
Called when a class is constructed that has the possibility to emit an
asynchronous event. This does not mean the instance must call
before
/after
before destroy
is called, only that the possibility
exists.
This behavior can be observed by doing something like opening a resource then closing it before the resource can be used. The following snippet demonstrates this.
import { createServer } from 'node:net';
createServer().listen(function() { this.close(); });
// OR
clearTimeout(setTimeout(() => {}, 10));
require('node:net').createServer().listen(function() { this.close(); });
// OR
clearTimeout(setTimeout(() => {}, 10));
Every new resource is assigned an ID that is unique within the scope of the current Node.js instance.
type
#
The type
is a string identifying the type of resource that caused
init
to be called. Generally, it will correspond to the name of the
resource's constructor.
The type
of resources created by Node.js itself can change in any Node.js
release. Valid values include TLSWRAP
,
TCPWRAP
, TCPSERVERWRAP
, GETADDRINFOREQWRAP
, FSREQCALLBACK
,
Microtask
, and Timeout
. Inspect the source code of the Node.js version used
to get the full list.
Furthermore users of AsyncResource
create async resources independent
of Node.js itself.
There is also the PROMISE
resource type, which is used to track Promise
instances and asynchronous work scheduled by them.
Users are able to define their own type
when using the public embedder API.
It is possible to have type name collisions. Embedders are encouraged to use unique prefixes, such as the npm package name, to prevent collisions when listening to the hooks.
triggerAsyncId
#
triggerAsyncId
is the asyncId
of the resource that caused (or "triggered")
the new resource to initialize and that caused init
to call. This is different
from async_hooks.executionAsyncId()
that only shows when a resource was
created, while triggerAsyncId
shows why a resource was created.
The following is a simple demonstration of triggerAsyncId
:
import { createHook, executionAsyncId } from 'node:async_hooks';
import { stdout } from 'node:process';
import net from 'node:net';
import fs from 'node:fs';
createHook({
init(asyncId, type, triggerAsyncId) {
const eid = executionAsyncId();
fs.writeSync(
stdout.fd,
`${type}(${asyncId}): trigger: ${triggerAsyncId} execution: ${eid}\n`);
},
}).enable();
net.createServer((conn) => {}).listen(8080);
const { createHook, executionAsyncId } = require('node:async_hooks');
const { stdout } = require('node:process');
const net = require('node:net');
const fs = require('node:fs');
createHook({
init(asyncId, type, triggerAsyncId) {
const eid = executionAsyncId();
fs.writeSync(
stdout.fd,
`${type}(${asyncId}): trigger: ${triggerAsyncId} execution: ${eid}\n`);
},
}).enable();
net.createServer((conn) => {}).listen(8080);
Output when hitting the server with nc localhost 8080
:
TCPSERVERWRAP(5): trigger: 1 execution: 1
TCPWRAP(7): trigger: 5 execution: 0
The TCPSERVERWRAP
is the server which receives the connections.
The TCPWRAP
is the new connection from the client. When a new
connection is made, the TCPWrap
instance is immediately constructed. This
happens outside of any JavaScript stack. (An executionAsyncId()
of 0
means
that it is being executed from C++ with no JavaScript stack above it.) With only
that information, it would be impossible to link resources together in
terms of what caused them to be created, so triggerAsyncId
is given the task
of propagating what resource is responsible for the new resource's existence.
resource
#
resource
is an object that represents the actual async resource that has
been initialized. The API to access the object may be specified by the
creator of the resource. Resources created by Node.js itself are internal
and may change at any time. Therefore no API is specified for these.
In some cases the resource object is reused for performance reasons, it is
thus not safe to use it as a key in a WeakMap
or add properties to it.
Asynchronous context example#
The context tracking use case is covered by the stable API AsyncLocalStorage
.
This example only illustrates async hooks operation but AsyncLocalStorage
fits better to this use case.
The following is an example with additional information about the calls to
init
between the before
and after
calls, specifically what the
callback to listen()
will look like. The output formatting is slightly more
elaborate to make calling context easier to see.
const async_hooks = require('node:async_hooks');
const fs = require('node:fs');
const net = require('node:net');
const { fd } = process.stdout;
let indent = 0;
async_hooks.createHook({
init(asyncId, type, triggerAsyncId) {
const eid = async_hooks.executionAsyncId();
const indentStr = ' '.repeat(indent);
fs.writeSync(
fd,
`${indentStr}${type}(${asyncId}):` +
` trigger: ${triggerAsyncId} execution: ${eid}\n`);
},
before(asyncId) {
const indentStr = ' '.repeat(indent);
fs.writeSync(fd, `${indentStr}before: ${asyncId}\n`);
indent += 2;
},
after(asyncId) {
indent -= 2;
const indentStr = ' '.repeat(indent);
fs.writeSync(fd, `${indentStr}after: ${asyncId}\n`);
},
destroy(asyncId) {
const indentStr = ' '.repeat(indent);
fs.writeSync(fd, `${indentStr}destroy: ${asyncId}\n`);
},
}).enable();
net.createServer(() => {}).listen(8080, () => {
// Let's wait 10ms before logging the server started.
setTimeout(() => {
console.log('>>>', async_hooks.executionAsyncId());
}, 10);
});
Output from only starting the server:
TCPSERVERWRAP(5): trigger: 1 execution: 1
TickObject(6): trigger: 5 execution: 1
before: 6
Timeout(7): trigger: 6 execution: 6
after: 6
destroy: 6
before: 7
>>> 7
TickObject(8): trigger: 7 execution: 7
after: 7
before: 8
after: 8
As illustrated in the example, executionAsyncId()
and execution
each specify
the value of the current execution context; which is delineated by calls to
before
and after
.
Only using execution
to graph resource allocation results in the following:
root(1)
^
|
TickObject(6)
^
|
Timeout(7)
The TCPSERVERWRAP
is not part of this graph, even though it was the reason for
console.log()
being called. This is because binding to a port without a host
name is a synchronous operation, but to maintain a completely asynchronous
API the user's callback is placed in a process.nextTick()
. Which is why
TickObject
is present in the output and is a 'parent' for .listen()
callback.
The graph only shows when a resource was created, not why, so to track
the why use triggerAsyncId
. Which can be represented with the following
graph:
bootstrap(1)
|
˅
TCPSERVERWRAP(5)
|
˅
TickObject(6)
|
˅
Timeout(7)
before(asyncId)
#
asyncId
<number>
When an asynchronous operation is initiated (such as a TCP server receiving a
new connection) or completes (such as writing data to disk) a callback is
called to notify the user. The before
callback is called just before said
callback is executed. asyncId
is the unique identifier assigned to the
resource about to execute the callback.
The before
callback will be called 0 to N times. The before
callback
will typically be called 0 times if the asynchronous operation was cancelled
or, for example, if no connections are received by a TCP server. Persistent
asynchronous resources like a TCP server will typically call the before
callback multiple times, while other operations like fs.open()
will call
it only once.
after(asyncId)
#
asyncId
<number>
Called immediately after the callback specified in before
is completed.
If an uncaught exception occurs during execution of the callback, then after
will run after the 'uncaughtException'
event is emitted or a domain
's
handler runs.
destroy(asyncId)
#
asyncId
<number>
Called after the resource corresponding to asyncId
is destroyed. It is also
called asynchronously from the embedder API emitDestroy()
.
Some resources depend on garbage collection for cleanup, so if a reference is
made to the resource
object passed to init
it is possible that destroy
will never be called, causing a memory leak in the application. If the resource
does not depend on garbage collection, then this will not be an issue.
Using the destroy hook results in additional overhead because it enables
tracking of Promise
instances via the garbage collector.
promiseResolve(asyncId)
#
asyncId
<number>
Called when the resolve
function passed to the Promise
constructor is
invoked (either directly or through other means of resolving a promise).
resolve()
does not do any observable synchronous work.
The Promise
is not necessarily fulfilled or rejected at this point if the
Promise
was resolved by assuming the state of another Promise
.
new Promise((resolve) => resolve(true)).then((a) => {});
calls the following callbacks:
init for PROMISE with id 5, trigger id: 1
promise resolve 5 # corresponds to resolve(true)
init for PROMISE with id 6, trigger id: 5 # the Promise returned by then()
before 6 # the then() callback is entered
promise resolve 6 # the then() callback resolves the promise by returning
after 6
async_hooks.executionAsyncResource()
#
- Returns: <Object> The resource representing the current execution. Useful to store data within the resource.
Resource objects returned by executionAsyncResource()
are most often internal
Node.js handle objects with undocumented APIs. Using any functions or properties
on the object is likely to crash your application and should be avoided.
Using executionAsyncResource()
in the top-level execution context will
return an empty object as there is no handle or request object to use,
but having an object representing the top-level can be helpful.
import { open } from 'node:fs';
import { executionAsyncId, executionAsyncResource } from 'node:async_hooks';
console.log(executionAsyncId(), executionAsyncResource()); // 1 {}
open(new URL(import.meta.url), 'r', (err, fd) => {
console.log(executionAsyncId(), executionAsyncResource()); // 7 FSReqWrap
});
const { open } = require('node:fs');
const { executionAsyncId, executionAsyncResource } = require('node:async_hooks');
console.log(executionAsyncId(), executionAsyncResource()); // 1 {}
open(__filename, 'r', (err, fd) => {
console.log(executionAsyncId(), executionAsyncResource()); // 7 FSReqWrap
});
This can be used to implement continuation local storage without the
use of a tracking Map
to store the metadata:
import { createServer } from 'node:http';
import {
executionAsyncId,
executionAsyncResource,
createHook,
} from 'async_hooks';
const sym = Symbol('state'); // Private symbol to avoid pollution
createHook({
init(asyncId, type, triggerAsyncId, resource) {
const cr = executionAsyncResource();
if (cr) {
resource[sym] = cr[sym];
}
},
}).enable();
const server = createServer((req, res) => {
executionAsyncResource()[sym] = { state: req.url };
setTimeout(function() {
res.end(JSON.stringify(executionAsyncResource()[sym]));
}, 100);
}).listen(3000);
const { createServer } = require('node:http');
const {
executionAsyncId,
executionAsyncResource,
createHook,
} = require('node:async_hooks');
const sym = Symbol('state'); // Private symbol to avoid pollution
createHook({
init(asyncId, type, triggerAsyncId, resource) {
const cr = executionAsyncResource();
if (cr) {
resource[sym] = cr[sym];
}
},
}).enable();
const server = createServer((req, res) => {
executionAsyncResource()[sym] = { state: req.url };
setTimeout(function() {
res.end(JSON.stringify(executionAsyncResource()[sym]));
}, 100);
}).listen(3000);
async_hooks.executionAsyncId()
#
- Returns: <number> The
asyncId
of the current execution context. Useful to track when something calls.
import { executionAsyncId } from 'node:async_hooks';
import fs from 'node:fs';
console.log(executionAsyncId()); // 1 - bootstrap
fs.open(path, 'r', (err, fd) => {
console.log(executionAsyncId()); // 6 - open()
});
const async_hooks = require('node:async_hooks');
const fs = require('node:fs');
console.log(async_hooks.executionAsyncId()); // 1 - bootstrap
fs.open(path, 'r', (err, fd) => {
console.log(async_hooks.executionAsyncId()); // 6 - open()
});
The ID returned from executionAsyncId()
is related to execution timing, not
causality (which is covered by triggerAsyncId()
):
const server = net.createServer((conn) => {
// Returns the ID of the server, not of the new connection, because the
// callback runs in the execution scope of the server's MakeCallback().
async_hooks.executionAsyncId();
}).listen(port, () => {
// Returns the ID of a TickObject (process.nextTick()) because all
// callbacks passed to .listen() are wrapped in a nextTick().
async_hooks.executionAsyncId();
});
Promise contexts may not get precise executionAsyncIds
by default.
See the section on promise execution tracking.
async_hooks.triggerAsyncId()
#
- Returns: <number> The ID of the resource responsible for calling the callback that is currently being executed.
const server = net.createServer((conn) => {
// The resource that caused (or triggered) this callback to be called
// was that of the new connection. Thus the return value of triggerAsyncId()
// is the asyncId of "conn".
async_hooks.triggerAsyncId();
}).listen(port, () => {
// Even though all callbacks passed to .listen() are wrapped in a nextTick()
// the callback itself exists because the call to the server's .listen()
// was made. So the return value would be the ID of the server.
async_hooks.triggerAsyncId();
});
Promise contexts may not get valid triggerAsyncId
s by default. See
the section on promise execution tracking.
async_hooks.asyncWrapProviders
#
- Returns: A map of provider types to the corresponding numeric id.
This map contains all the event types that might be emitted by the
async_hooks.init()
event.
This feature suppresses the deprecated usage of process.binding('async_wrap').Providers
.
See: DEP0111
Promise execution tracking#
By default, promise executions are not assigned asyncId
s due to the relatively
expensive nature of the promise introspection API provided by
V8. This means that programs using promises or async
/await
will not get
correct execution and trigger ids for promise callback contexts by default.
import { executionAsyncId, triggerAsyncId } from 'node:async_hooks';
Promise.resolve(1729).then(() => {
console.log(`eid ${executionAsyncId()} tid ${triggerAsyncId()}`);
});
// produces:
// eid 1 tid 0
const { executionAsyncId, triggerAsyncId } = require('node:async_hooks');
Promise.resolve(1729).then(() => {
console.log(`eid ${executionAsyncId()} tid ${triggerAsyncId()}`);
});
// produces:
// eid 1 tid 0
Observe that the then()
callback claims to have executed in the context of the
outer scope even though there was an asynchronous hop involved. Also,
the triggerAsyncId
value is 0
, which means that we are missing context about
the resource that caused (triggered) the then()
callback to be executed.
Installing async hooks via async_hooks.createHook
enables promise execution
tracking:
import { createHook, executionAsyncId, triggerAsyncId } from 'node:async_hooks';
createHook({ init() {} }).enable(); // forces PromiseHooks to be enabled.
Promise.resolve(1729).then(() => {
console.log(`eid ${executionAsyncId()} tid ${triggerAsyncId()}`);
});
// produces:
// eid 7 tid 6
const { createHook, executionAsyncId, triggerAsyncId } = require('node:async_hooks');
createHook({ init() {} }).enable(); // forces PromiseHooks to be enabled.
Promise.resolve(1729).then(() => {
console.log(`eid ${executionAsyncId()} tid ${triggerAsyncId()}`);
});
// produces:
// eid 7 tid 6
In this example, adding any actual hook function enabled the tracking of
promises. There are two promises in the example above; the promise created by
Promise.resolve()
and the promise returned by the call to then()
. In the
example above, the first promise got the asyncId
6
and the latter got
asyncId
7
. During the execution of the then()
callback, we are executing
in the context of promise with asyncId
7
. This promise was triggered by
async resource 6
.
Another subtlety with promises is that before
and after
callbacks are run
only on chained promises. That means promises not created by then()
/catch()
will not have the before
and after
callbacks fired on them. For more details
see the details of the V8 PromiseHooks API.
JavaScript embedder API#
Library developers that handle their own asynchronous resources performing tasks
like I/O, connection pooling, or managing callback queues may use the
AsyncResource
JavaScript API so that all the appropriate callbacks are called.
Class: AsyncResource
#
The documentation for this class has moved AsyncResource
.
Class: AsyncLocalStorage
#
The documentation for this class has moved AsyncLocalStorage
.
Buffer#
Source Code: lib/buffer.js
Buffer
objects are used to represent a fixed-length sequence of bytes. Many
Node.js APIs support Buffer
s.
The Buffer
class is a subclass of JavaScript's Uint8Array
class and
extends it with methods that cover additional use cases. Node.js APIs accept
plain Uint8Array
s wherever Buffer
s are supported as well.
While the Buffer
class is available within the global scope, it is still
recommended to explicitly reference it via an import or require statement.
import { Buffer } from 'node:buffer';
// Creates a zero-filled Buffer of length 10.
const buf1 = Buffer.alloc(10);
// Creates a Buffer of length 10,
// filled with bytes which all have the value `1`.
const buf2 = Buffer.alloc(10, 1);
// Creates an uninitialized buffer of length 10.
// This is faster than calling Buffer.alloc() but the returned
// Buffer instance might contain old data that needs to be
// overwritten using fill(), write(), or other functions that fill the Buffer's
// contents.
const buf3 = Buffer.allocUnsafe(10);
// Creates a Buffer containing the bytes [1, 2, 3].
const buf4 = Buffer.from([1, 2, 3]);
// Creates a Buffer containing the bytes [1, 1, 1, 1] – the entries
// are all truncated using `(value & 255)` to fit into the range 0–255.
const buf5 = Buffer.from([257, 257.5, -255, '1']);
// Creates a Buffer containing the UTF-8-encoded bytes for the string 'tést':
// [0x74, 0xc3, 0xa9, 0x73, 0x74] (in hexadecimal notation)
// [116, 195, 169, 115, 116] (in decimal notation)
const buf6 = Buffer.from('tést');
// Creates a Buffer containing the Latin-1 bytes [0x74, 0xe9, 0x73, 0x74].
const buf7 = Buffer.from('tést', 'latin1');
const { Buffer } = require('node:buffer');
// Creates a zero-filled Buffer of length 10.
const buf1 = Buffer.alloc(10);
// Creates a Buffer of length 10,
// filled with bytes which all have the value `1`.
const buf2 = Buffer.alloc(10, 1);
// Creates an uninitialized buffer of length 10.
// This is faster than calling Buffer.alloc() but the returned
// Buffer instance might contain old data that needs to be
// overwritten using fill(), write(), or other functions that fill the Buffer's
// contents.
const buf3 = Buffer.allocUnsafe(10);
// Creates a Buffer containing the bytes [1, 2, 3].
const buf4 = Buffer.from([1, 2, 3]);
// Creates a Buffer containing the bytes [1, 1, 1, 1] – the entries
// are all truncated using `(value & 255)` to fit into the range 0–255.
const buf5 = Buffer.from([257, 257.5, -255, '1']);
// Creates a Buffer containing the UTF-8-encoded bytes for the string 'tést':
// [0x74, 0xc3, 0xa9, 0x73, 0x74] (in hexadecimal notation)
// [116, 195, 169, 115, 116] (in decimal notation)
const buf6 = Buffer.from('tést');
// Creates a Buffer containing the Latin-1 bytes [0x74, 0xe9, 0x73, 0x74].
const buf7 = Buffer.from('tést', 'latin1');
Buffers and character encodings#
When converting between Buffer
s and strings, a character encoding may be
specified. If no character encoding is specified, UTF-8 will be used as the
default.
import { Buffer } from 'node:buffer';
const buf = Buffer.from('hello world', 'utf8');
console.log(buf.toString('hex'));
// Prints: 68656c6c6f20776f726c64
console.log(buf.toString('base64'));
// Prints: aGVsbG8gd29ybGQ=
console.log(Buffer.from('fhqwhgads', 'utf8'));
// Prints: <Buffer 66 68 71 77 68 67 61 64 73>
console.log(Buffer.from('fhqwhgads', 'utf16le'));
// Prints: <Buffer 66 00 68 00 71 00 77 00 68 00 67 00 61 00 64 00 73 00>
const { Buffer } = require('node:buffer');
const buf = Buffer.from('hello world', 'utf8');
console.log(buf.toString('hex'));
// Prints: 68656c6c6f20776f726c64
console.log(buf.toString('base64'));
// Prints: aGVsbG8gd29ybGQ=
console.log(Buffer.from('fhqwhgads', 'utf8'));
// Prints: <Buffer 66 68 71 77 68 67 61 64 73>
console.log(Buffer.from('fhqwhgads', 'utf16le'));
// Prints: <Buffer 66 00 68 00 71 00 77 00 68 00 67 00 61 00 64 00 73 00>
Node.js buffers accept all case variations of encoding strings that they
receive. For example, UTF-8 can be specified as 'utf8'
, 'UTF8'
, or 'uTf8'
.
The character encodings currently supported by Node.js are the following:
-
'utf8'
(alias:'utf-8'
): Multi-byte encoded Unicode characters. Many web pages and other document formats use UTF-8. This is the default character encoding. When decoding aBuffer
into a string that does not exclusively contain valid UTF-8 data, the Unicode replacement characterU+FFFD
� will be used to represent those errors. -
'utf16le'
(alias:'utf-16le'
): Multi-byte encoded Unicode characters. Unlike'utf8'
, each character in the string will be encoded using either 2 or 4 bytes. Node.js only supports the little-endian variant of UTF-16. -
'latin1'
: Latin-1 stands for ISO-8859-1. This character encoding only supports the Unicode characters fromU+0000
toU+00FF
. Each character is encoded using a single byte. Characters that do not fit into that range are truncated and will be mapped to characters in that range.
Converting a Buffer
into a string using one of the above is referred to as
decoding, and converting a string into a Buffer
is referred to as encoding.
Node.js also supports the following binary-to-text encodings. For
binary-to-text encodings, the naming convention is reversed: Converting a
Buffer
into a string is typically referred to as encoding, and converting a
string into a Buffer
as decoding.
-
'base64'
: Base64 encoding. When creating aBuffer
from a string, this encoding will also correctly accept "URL and Filename Safe Alphabet" as specified in RFC 4648, Section 5. Whitespace characters such as spaces, tabs, and new lines contained within the base64-encoded string are ignored. -
'base64url'
: base64url encoding as specified in RFC 4648, Section 5. When creating aBuffer
from a string, this encoding will also correctly accept regular base64-encoded strings. When encoding aBuffer
to a string, this encoding will omit padding. -
'hex'
: Encode each byte as two hexadecimal characters. Data truncation may occur when decoding strings that do not exclusively consist of an even number of hexadecimal characters. See below for an example.
The following legacy character encodings are also supported:
-
'ascii'
: For 7-bit ASCII data only. When encoding a string into aBuffer
, this is equivalent to using'latin1'
. When decoding aBuffer
into a string, using this encoding will additionally unset the highest bit of each byte before decoding as'latin1'
. Generally, there should be no reason to use this encoding, as'utf8'
(or, if the data is known to always be ASCII-only,'latin1'
) will be a better choice when encoding or decoding ASCII-only text. It is only provided for legacy compatibility. -
'binary'
: Alias for'latin1'
. See binary strings for more background on this topic. The name of this encoding can be very misleading, as all of the encodings listed here convert between strings and binary data. For converting between strings andBuffer
s, typically'utf8'
is the right choice. -
'ucs2'
,'ucs-2'
: Aliases of'utf16le'
. UCS-2 used to refer to a variant of UTF-16 that did not support characters that had code points larger than U+FFFF. In Node.js, these code points are always supported.
import { Buffer } from 'node:buffer';
Buffer.from('1ag123', 'hex');
// Prints <Buffer 1a>, data truncated when first non-hexadecimal value
// ('g') encountered.
Buffer.from('1a7', 'hex');
// Prints <Buffer 1a>, data truncated when data ends in single digit ('7').
Buffer.from('1634', 'hex');
// Prints <Buffer 16 34>, all data represented.
const { Buffer } = require('node:buffer');
Buffer.from('1ag123', 'hex');
// Prints <Buffer 1a>, data truncated when first non-hexadecimal value
// ('g') encountered.
Buffer.from('1a7', 'hex');
// Prints <Buffer 1a>, data truncated when data ends in single digit ('7').
Buffer.from('1634', 'hex');
// Prints <Buffer 16 34>, all data represented.
Modern Web browsers follow the WHATWG Encoding Standard which aliases
both 'latin1'
and 'ISO-8859-1'
to 'win-1252'
. This means that while doing
something like http.get()
, if the returned charset is one of those listed in
the WHATWG specification it is possible that the server actually returned
'win-1252'
-encoded data, and using 'latin1'
encoding may incorrectly decode
the characters.
Buffers and TypedArrays#
Buffer
instances are also JavaScript Uint8Array
and TypedArray
instances. All TypedArray
methods are available on Buffer
s. There are,
however, subtle incompatibilities between the Buffer
API and the
TypedArray
API.
In particular:
- While
TypedArray.prototype.slice()
creates a copy of part of theTypedArray
,Buffer.prototype.slice()
creates a view over the existingBuffer
without copying. This behavior can be surprising, and only exists for legacy compatibility.TypedArray.prototype.subarray()
can be used to achieve the behavior ofBuffer.prototype.slice()
on bothBuffer
s and otherTypedArray
s and should be preferred. buf.toString()
is incompatible with itsTypedArray
equivalent.- A number of methods, e.g.
buf.indexOf()
, support additional arguments.
There are two ways to create new TypedArray
instances from a Buffer
:
- Passing a
Buffer
to aTypedArray
constructor will copy theBuffer
s contents, interpreted as an array of integers, and not as a byte sequence of the target type.
import { Buffer } from 'node:buffer';
const buf = Buffer.from([1, 2, 3, 4]);
const uint32array = new Uint32Array(buf);
console.log(uint32array);
// Prints: Uint32Array(4) [ 1, 2, 3, 4 ]
const { Buffer } = require('node:buffer');
const buf = Buffer.from([1, 2, 3, 4]);
const uint32array = new Uint32Array(buf);
console.log(uint32array);
// Prints: Uint32Array(4) [ 1, 2, 3, 4 ]
- Passing the
Buffer
s underlyingArrayBuffer
will create aTypedArray
that shares its memory with theBuffer
.
import { Buffer } from 'node:buffer';
const buf = Buffer.from('hello', 'utf16le');
const uint16array = new Uint16Array(
buf.buffer,
buf.byteOffset,
buf.length / Uint16Array.BYTES_PER_ELEMENT);
console.log(uint16array);
// Prints: Uint16Array(5) [ 104, 101, 108, 108, 111 ]
const { Buffer } = require('node:buffer');
const buf = Buffer.from('hello', 'utf16le');
const uint16array = new Uint16Array(
buf.buffer,
buf.byteOffset,
buf.length / Uint16Array.BYTES_PER_ELEMENT);
console.log(uint16array);
// Prints: Uint16Array(5) [ 104, 101, 108, 108, 111 ]
It is possible to create a new Buffer
that shares the same allocated
memory as a TypedArray
instance by using the TypedArray
object's
.buffer
property in the same way. Buffer.from()
behaves like new Uint8Array()
in this context.
import { Buffer } from 'node:buffer';
const arr = new Uint16Array(2);
arr[0] = 5000;
arr[1] = 4000;
// Copies the contents of `arr`.
const buf1 = Buffer.from(arr);
// Shares memory with `arr`.
const buf2 = Buffer.from(arr.buffer);
console.log(buf1);
// Prints: <Buffer 88 a0>
console.log(buf2);
// Prints: <Buffer 88 13 a0 0f>
arr[1] = 6000;
console.log(buf1);
// Prints: <Buffer 88 a0>
console.log(buf2);
// Prints: <Buffer 88 13 70 17>
const { Buffer } = require('node:buffer');
const arr = new Uint16Array(2);
arr[0] = 5000;
arr[1] = 4000;
// Copies the contents of `arr`.
const buf1 = Buffer.from(arr);
// Shares memory with `arr`.
const buf2 = Buffer.from(arr.buffer);
console.log(buf1);
// Prints: <Buffer 88 a0>
console.log(buf2);
// Prints: <Buffer 88 13 a0 0f>
arr[1] = 6000;
console.log(buf1);
// Prints: <Buffer 88 a0>
console.log(buf2);
// Prints: <Buffer 88 13 70 17>
When creating a Buffer
using a TypedArray
's .buffer
, it is
possible to use only a portion of the underlying ArrayBuffer
by passing in
byteOffset
and length
parameters.
import { Buffer } from 'node:buffer';
const arr = new Uint16Array(20);
const buf = Buffer.from(arr.buffer, 0, 16);
console.log(buf.length);
// Prints: 16
const { Buffer } = require('node:buffer');
const arr = new Uint16Array(20);
const buf = Buffer.from(arr.buffer, 0, 16);
console.log(buf.length);
// Prints: 16
The Buffer.from()
and TypedArray.from()
have different signatures and
implementations. Specifically, the TypedArray
variants accept a second
argument that is a mapping function that is invoked on every element of the
typed array:
TypedArray.from(source[, mapFn[, thisArg]])
The Buffer.from()
method, however, does not support the use of a mapping
function:
Buffers and iteration#
Buffer
instances can be iterated over using for..of
syntax:
import { Buffer } from 'node:buffer';
const buf = Buffer.from([1, 2, 3]);
for (const b of buf) {
console.log(b);
}
// Prints:
// 1
// 2
// 3
const { Buffer } = require('node:buffer');
const buf = Buffer.from([1, 2, 3]);
for (const b of buf) {
console.log(b);
}
// Prints:
// 1
// 2
// 3
Additionally, the buf.values()
, buf.keys()
, and
buf.entries()
methods can be used to create iterators.
Class: Blob
#
A Blob
encapsulates immutable, raw data that can be safely shared across
multiple worker threads.
new buffer.Blob([sources[, options]])
#
sources
<string[]> | <ArrayBuffer[]> | <TypedArray[]> | <DataView[]> | <Blob[]> An array of string, <ArrayBuffer>, <TypedArray>, <DataView>, or <Blob> objects, or any mix of such objects, that will be stored within theBlob
.options
<Object>endings
<string> One of either'transparent'
or'native'
. When set to'native'
, line endings in string source parts will be converted to the platform native line-ending as specified byrequire('node:os').EOL
.type
<string> The Blob content-type. The intent is fortype
to convey the MIME media type of the data, however no validation of the type format is performed.
Creates a new Blob
object containing a concatenation of the given sources.
<ArrayBuffer>, <TypedArray>, <DataView>, and <Buffer> sources are copied into the 'Blob' and can therefore be safely modified after the 'Blob' is created.
String sources are encoded as UTF-8 byte sequences and copied into the Blob. Unmatched surrogate pairs within each string part will be replaced by Unicode U+FFFD replacement characters.
blob.arrayBuffer()
#
- Returns: <Promise>
Returns a promise that fulfills with an <ArrayBuffer> containing a copy of
the Blob
data.
blob.size
#
The total size of the Blob
in bytes.
blob.slice([start[, end[, type]]])
#
start
<number> The starting index.end
<number> The ending index.type
<string> The content-type for the newBlob
Creates and returns a new Blob
containing a subset of this Blob
objects
data. The original Blob
is not altered.
blob.stream()
#
- Returns: <ReadableStream>
Returns a new ReadableStream
that allows the content of the Blob
to be read.
blob.text()
#
- Returns: <Promise>
Returns a promise that fulfills with the contents of the Blob
decoded as a
UTF-8 string.
blob.type
#
- Type: <string>
The content-type of the Blob
.
Blob
objects and MessageChannel
#
Once a <Blob> object is created, it can be sent via MessagePort
to multiple
destinations without transferring or immediately copying the data. The data
contained by the Blob
is copied only when the arrayBuffer()
or text()
methods are called.
import { Blob, Buffer } from 'node:buffer';
import { setTimeout as delay } from 'node:timers/promises';
const blob = new Blob(['hello there']);
const mc1 = new MessageChannel();
const mc2 = new MessageChannel();
mc1.port1.onmessage = async ({ data }) => {
console.log(await data.arrayBuffer());
mc1.port1.close();
};
mc2.port1.onmessage = async ({ data }) => {
await delay(1000);
console.log(await data.arrayBuffer());
mc2.port1.close();
};
mc1.port2.postMessage(blob);
mc2.port2.postMessage(blob);
// The Blob is still usable after posting.
blob.text().then(console.log);
const { Blob, Buffer } = require('node:buffer');
const { setTimeout: delay } = require('node:timers/promises');
const blob = new Blob(['hello there']);
const mc1 = new MessageChannel();
const mc2 = new MessageChannel();
mc1.port1.onmessage = async ({ data }) => {
console.log(await data.arrayBuffer());
mc1.port1.close();
};
mc2.port1.onmessage = async ({ data }) => {
await delay(1000);
console.log(await data.arrayBuffer());
mc2.port1.close();
};
mc1.port2.postMessage(blob);
mc2.port2.postMessage(blob);
// The Blob is still usable after posting.
blob.text().then(console.log);
Class: Buffer
#
The Buffer
class is a global type for dealing with binary data directly.
It can be constructed in a variety of ways.
Static method: Buffer.alloc(size[, fill[, encoding]])
#
size
<integer> The desired length of the newBuffer
.fill
<string> | <Buffer> | <Uint8Array> | <integer> A value to pre-fill the newBuffer
with. Default:0
.encoding
<string> Iffill
is a string, this is its encoding. Default:'utf8'
.
Allocates a new Buffer
of size
bytes. If fill
is undefined
, the
Buffer
will be zero-filled.
import { Buffer } from 'node:buffer';
const buf = Buffer.alloc(5);
console.log(buf);
// Prints: <Buffer 00 00 00 00 00>
const { Buffer } = require('node:buffer');
const buf = Buffer.alloc(5);
console.log(buf);
// Prints: <Buffer 00 00 00 00 00>
If size
is larger than
buffer.constants.MAX_LENGTH
or smaller than 0, ERR_INVALID_ARG_VALUE
is thrown.
If fill
is specified, the allocated Buffer
will be initialized by calling
buf.fill(fill)
.
import { Buffer } from 'node:buffer';
const buf = Buffer.alloc(5, 'a');
console.log(buf);
// Prints: <Buffer 61 61 61 61 61>
const { Buffer } = require('node:buffer');
const buf = Buffer.alloc(5, 'a');
console.log(buf);
// Prints: <Buffer 61 61 61 61 61>
If both fill
and encoding
are specified, the allocated Buffer
will be
initialized by calling buf.fill(fill, encoding)
.
import { Buffer } from 'node:buffer';
const buf = Buffer.alloc(11, 'aGVsbG8gd29ybGQ=', 'base64');
console.log(buf);
// Prints: <Buffer 68 65 6c 6c 6f 20 77 6f 72 6c 64>
const { Buffer } = require('node:buffer');
const buf = Buffer.alloc(11, 'aGVsbG8gd29ybGQ=', 'base64');
console.log(buf);
// Prints: <Buffer 68 65 6c 6c 6f 20 77 6f 72 6c 64>
Calling Buffer.alloc()
can be measurably slower than the alternative
Buffer.allocUnsafe()
but ensures that the newly created Buffer
instance
contents will never contain sensitive data from previous allocations, including
data that might not have been allocated for Buffer
s.
A TypeError
will be thrown if size
is not a number.
Static method: Buffer.allocUnsafe(size)
#
size
<integer> The desired length of the newBuffer
.
Allocates a new Buffer
of size
bytes. If size
is larger than
buffer.constants.MAX_LENGTH
or smaller than 0, ERR_INVALID_ARG_VALUE
is thrown.
The underlying memory for Buffer
instances created in this way is not
initialized. The contents of the newly created Buffer
are unknown and
may contain sensitive data. Use Buffer.alloc()
instead to initialize
Buffer
instances with zeroes.
import { Buffer } from 'node:buffer';
const buf = Buffer.allocUnsafe(10);
console.log(buf);
// Prints (contents may vary): <Buffer a0 8b 28 3f 01 00 00 00 50 32>
buf.fill(0);
console.log(buf);
// Prints: <Buffer 00 00 00 00 00 00 00 00 00 00>
const { Buffer } = require('node:buffer');
const buf = Buffer.allocUnsafe(10);
console.log(buf);
// Prints (contents may vary): <Buffer a0 8b 28 3f 01 00 00 00 50 32>
buf.fill(0);
console.log(buf);
// Prints: <Buffer 00 00 00 00 00 00 00 00 00 00>
A TypeError
will be thrown if size
is not a number.
The Buffer
module pre-allocates an internal Buffer
instance of
size Buffer.poolSize
that is used as a pool for the fast allocation of new
Buffer
instances created using Buffer.allocUnsafe()
,
Buffer.from(array)
, Buffer.concat()
, and the deprecated
new Buffer(size)
constructor only when size
is less than or equal
to Buffer.poolSize >> 1
(floor of Buffer.poolSize
divided by two).
Use of this pre-allocated internal memory pool is a key difference between
calling Buffer.alloc(size, fill)
vs. Buffer.allocUnsafe(size).fill(fill)
.
Specifically, Buffer.alloc(size, fill)
will never use the internal Buffer
pool, while Buffer.allocUnsafe(size).fill(fill)
will use the internal
Buffer
pool if size
is less than or equal to half Buffer.poolSize
. The
difference is subtle but can be important when an application requires the
additional performance that Buffer.allocUnsafe()
provides.
Static method: Buffer.allocUnsafeSlow(size)
#
size
<integer> The desired length of the newBuffer
.
Allocates a new Buffer
of size
bytes. If size
is larger than
buffer.constants.MAX_LENGTH
or smaller than 0, ERR_INVALID_ARG_VALUE
is thrown. A zero-length Buffer
is created if size
is 0.
The underlying memory for Buffer
instances created in this way is not
initialized. The contents of the newly created Buffer
are unknown and
may contain sensitive data. Use buf.fill(0)
to initialize
such Buffer
instances with zeroes.
When using Buffer.allocUnsafe()
to allocate new Buffer
instances,
allocations under 4 KiB are sliced from a single pre-allocated Buffer
. This
allows applications to avoid the garbage collection overhead of creating many
individually allocated Buffer
instances. This approach improves both
performance and memory usage by eliminating the need to track and clean up as
many individual ArrayBuffer
objects.
However, in the case where a developer may need to retain a small chunk of
memory from a pool for an indeterminate amount of time, it may be appropriate
to create an un-pooled Buffer
instance using Buffer.allocUnsafeSlow()
and
then copying out the relevant bits.
import { Buffer } from 'node:buffer';
// Need to keep around a few small chunks of memory.
const store = [];
socket.on('readable', () => {
let data;
while (null !== (data = readable.read())) {
// Allocate for retained data.
const sb = Buffer.allocUnsafeSlow(10);
// Copy the data into the new allocation.
data.copy(sb, 0, 0, 10);
store.push(sb);
}
});
const { Buffer } = require('node:buffer');
// Need to keep around a few small chunks of memory.
const store = [];
socket.on('readable', () => {
let data;
while (null !== (data = readable.read())) {
// Allocate for retained data.
const sb = Buffer.allocUnsafeSlow(10);
// Copy the data into the new allocation.
data.copy(sb, 0, 0, 10);
store.push(sb);
}
});
A TypeError
will be thrown if size
is not a number.
Static method: Buffer.byteLength(string[, encoding])
#
string
<string> | <Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <SharedArrayBuffer> A value to calculate the length of.encoding
<string> Ifstring
is a string, this is its encoding. Default:'utf8'
.- Returns: <integer> The number of bytes contained within
string
.
Returns the byte length of a string when encoded using encoding
.
This is not the same as String.prototype.length
, which does not account
for the encoding that is used to convert the string into bytes.
For 'base64'
, 'base64url'
, and 'hex'
, this function assumes valid input.
For strings that contain non-base64/hex-encoded data (e.g. whitespace), the
return value might be greater than the length of a Buffer
created from the
string.
import { Buffer } from 'node:buffer';
const str = '\u00bd + \u00bc = \u00be';
console.log(`${str}: ${str.length} characters, ` +
`${Buffer.byteLength(str, 'utf8')} bytes`);
// Prints: ½ + ¼ = ¾: 9 characters, 12 bytes
const { Buffer } = require('node:buffer');
const str = '\u00bd + \u00bc = \u00be';
console.log(`${str}: ${str.length} characters, ` +
`${Buffer.byteLength(str, 'utf8')} bytes`);
// Prints: ½ + ¼ = ¾: 9 characters, 12 bytes
When string
is a Buffer
/DataView
/TypedArray
/ArrayBuffer
/
SharedArrayBuffer
, the byte length as reported by .byteLength
is returned.
Static method: Buffer.compare(buf1, buf2)
#
buf1
<Buffer> | <Uint8Array>buf2
<Buffer> | <Uint8Array>- Returns: <integer> Either
-1
,0
, or1
, depending on the result of the comparison. Seebuf.compare()
for details.
Compares buf1
to buf2
, typically for the purpose of sorting arrays of
Buffer
instances. This is equivalent to calling
buf1.compare(buf2)
.
import { Buffer } from 'node:buffer';
const buf1 = Buffer.from('1234');
const buf2 = Buffer.from('0123');
const arr = [buf1, buf2];
console.log(arr.sort(Buffer.compare));
// Prints: [ <Buffer 30 31 32 33>, <Buffer 31 32 33 34> ]
// (This result is equal to: [buf2, buf1].)
const { Buffer } = require('node:buffer');
const buf1 = Buffer.from('1234');
const buf2 = Buffer.from('0123');
const arr = [buf1, buf2];
console.log(arr.sort(Buffer.compare));
// Prints: [ <Buffer 30 31 32 33>, <Buffer 31 32 33 34> ]
// (This result is equal to: [buf2, buf1].)
Static method: Buffer.concat(list[, totalLength])
#
list
<Buffer[]> | <Uint8Array[]> List ofBuffer
orUint8Array
instances to concatenate.totalLength
<integer> Total length of theBuffer
instances inlist
when concatenated.- Returns: <Buffer>
Returns a new Buffer
which is the result of concatenating all the Buffer
instances in the list
together.
If the list has no items, or if the totalLength
is 0, then a new zero-length
Buffer
is returned.
If totalLength
is not provided, it is calculated from the Buffer
instances
in list
by adding their lengths.
If totalLength
is provided, it is coerced to an unsigned integer. If the
combined length of the Buffer
s in list
exceeds totalLength
, the result is
truncated to totalLength
.
import { Buffer } from 'node:buffer';
// Create a single `Buffer` from a list of three `Buffer` instances.
const buf1 = Buffer.alloc(10);
const buf2 = Buffer.alloc(14);
const buf3 = Buffer.alloc(18);
const totalLength = buf1.length + buf2.length + buf3.length;
console.log(totalLength);
// Prints: 42
const bufA = Buffer.concat([buf1, buf2, buf3], totalLength);
console.log(bufA);
// Prints: <Buffer 00 00 00 00 ...>
console.log(bufA.length);
// Prints: 42
const { Buffer } = require('node:buffer');
// Create a single `Buffer` from a list of three `Buffer` instances.
const buf1 = Buffer.alloc(10);
const buf2 = Buffer.alloc(14);
const buf3 = Buffer.alloc(18);
const totalLength = buf1.length + buf2.length + buf3.length;
console.log(totalLength);
// Prints: 42
const bufA = Buffer.concat([buf1, buf2, buf3], totalLength);
console.log(bufA);
// Prints: <Buffer 00 00 00 00 ...>
console.log(bufA.length);
// Prints: 42
Buffer.concat()
may also use the internal Buffer
pool like
Buffer.allocUnsafe()
does.
Static method: Buffer.copyBytesFrom(view[, offset[, length]])
#
view
<TypedArray> The <TypedArray> to copy.offset
<integer> The starting offset withinview
. Default::0
.length
<integer> The number of elements fromview
to copy. Default:view.length - offset
.
Copies the underlying memory of view
into a new Buffer
.
const u16 = new Uint16Array([0, 0xffff]);
const buf = Buffer.copyBytesFrom(u16, 0, 1);
u16[1] = 0;
console.log(buf.length); // 2
console.log(buf[0]); // 255
console.log(buf[1]); // 255
Static method: Buffer.from(array)
#
array
<integer[]>
Allocates a new Buffer
using an array
of bytes in the range 0
– 255
.
Array entries outside that range will be truncated to fit into it.
import { Buffer } from 'node:buffer';
// Creates a new Buffer containing the UTF-8 bytes of the string 'buffer'.
const buf = Buffer.from([0x62, 0x75, 0x66, 0x66, 0x65, 0x72]);
const { Buffer } = require('node:buffer');
// Creates a new Buffer containing the UTF-8 bytes of the string 'buffer'.
const buf = Buffer.from([0x62, 0x75, 0x66, 0x66, 0x65, 0x72]);
A TypeError
will be thrown if array
is not an Array
or another type
appropriate for Buffer.from()
variants.
Buffer.from(array)
and Buffer.from(string)
may also use the internal
Buffer
pool like Buffer.allocUnsafe()
does.
Static method: Buffer.from(arrayBuffer[, byteOffset[, length]])
#
arrayBuffer
<ArrayBuffer> | <SharedArrayBuffer> AnArrayBuffer
,SharedArrayBuffer
, for example the.buffer
property of aTypedArray
.byteOffset
<integer> Index of first byte to expose. Default:0
.length
<integer> Number of bytes to expose. Default:arrayBuffer.byteLength - byteOffset
.
This creates a view of the ArrayBuffer
without copying the underlying
memory. For example, when passed a reference to the .buffer
property of a
TypedArray
instance, the newly created Buffer
will share the same
allocated memory as the TypedArray
's underlying ArrayBuffer
.
import { Buffer } from 'node:buffer';
const arr = new Uint16Array(2);
arr[0] = 5000;
arr[1] = 4000;
// Shares memory with `arr`.
const buf = Buffer.from(arr.buffer);
console.log(buf);
// Prints: <Buffer 88 13 a0 0f>
// Changing the original Uint16Array changes the Buffer also.
arr[1] = 6000;
console.log(buf);
// Prints: <Buffer 88 13 70 17>
const { Buffer } = require('node:buffer');
const arr = new Uint16Array(2);
arr[0] = 5000;
arr[1] = 4000;
// Shares memory with `arr`.
const buf = Buffer.from(arr.buffer);
console.log(buf);
// Prints: <Buffer 88 13 a0 0f>
// Changing the original Uint16Array changes the Buffer also.
arr[1] = 6000;
console.log(buf);
// Prints: <Buffer 88 13 70 17>
The optional byteOffset
and length
arguments specify a memory range within
the arrayBuffer
that will be shared by the Buffer
.
import { Buffer } from 'node:buffer';
const ab = new ArrayBuffer(10);
const buf = Buffer.from(ab, 0, 2);
console.log(buf.length);
// Prints: 2
const { Buffer } = require('node:buffer');
const ab = new ArrayBuffer(10);
const buf = Buffer.from(ab, 0, 2);
console.log(buf.length);
// Prints: 2
A TypeError
will be thrown if arrayBuffer
is not an ArrayBuffer
or a
SharedArrayBuffer
or another type appropriate for Buffer.from()
variants.
It is important to remember that a backing ArrayBuffer
can cover a range
of memory that extends beyond the bounds of a TypedArray
view. A new
Buffer
created using the buffer
property of a TypedArray
may extend
beyond the range of the TypedArray
:
import { Buffer } from 'node:buffer';
const arrA = Uint8Array.from([0x63, 0x64, 0x65, 0x66]); // 4 elements
const arrB = new Uint8Array(arrA.buffer, 1, 2); // 2 elements
console.log(arrA.buffer === arrB.buffer); // true
const buf = Buffer.from(arrB.buffer);
console.log(buf);
// Prints: <Buffer 63 64 65 66>
const { Buffer } = require('node:buffer');
const arrA = Uint8Array.from([0x63, 0x64, 0x65, 0x66]); // 4 elements
const arrB = new Uint8Array(arrA.buffer, 1, 2); // 2 elements
console.log(arrA.buffer === arrB.buffer); // true
const buf = Buffer.from(arrB.buffer);
console.log(buf);
// Prints: <Buffer 63 64 65 66>
Static method: Buffer.from(buffer)
#
buffer
<Buffer> | <Uint8Array> An existingBuffer
orUint8Array
from which to copy data.
Copies the passed buffer
data onto a new Buffer
instance.
import { Buffer } from 'node:buffer';
const buf1 = Buffer.from('buffer');
const buf2 = Buffer.from(buf1);
buf1[0] = 0x61;
console.log(buf1.toString());
// Prints: auffer
console.log(buf2.toString());
// Prints: buffer
const { Buffer } = require('node:buffer');
const buf1 = Buffer.from('buffer');
const buf2 = Buffer.from(buf1);
buf1[0] = 0x61;
console.log(buf1.toString());
// Prints: auffer
console.log(buf2.toString());
// Prints: buffer
A TypeError
will be thrown if buffer
is not a Buffer
or another type
appropriate for Buffer.from()
variants.
Static method: Buffer.from(object[, offsetOrEncoding[, length]])
#
object
<Object> An object supportingSymbol.toPrimitive
orvalueOf()
.offsetOrEncoding
<integer> | <string> A byte-offset or encoding.length
<integer> A length.
For objects whose valueOf()
function returns a value not strictly equal to
object
, returns Buffer.from(object.valueOf(), offsetOrEncoding, length)
.
import { Buffer } from 'node:buffer';
const buf = Buffer.from(new String('this is a test'));
// Prints: <Buffer 74 68 69 73 20 69 73 20 61 20 74 65 73 74>
const { Buffer } = require('node:buffer');
const buf = Buffer.from(new String('this is a test'));
// Prints: <Buffer 74 68 69 73 20 69 73 20 61 20 74 65 73 74>
For objects that support Symbol.toPrimitive
, returns
Buffer.from(object[Symbol.toPrimitive]('string'), offsetOrEncoding)
.
import { Buffer } from 'node:buffer';
class Foo {
[Symbol.toPrimitive]() {
return 'this is a test';
}
}
const buf = Buffer.from(new Foo(), 'utf8');
// Prints: <Buffer 74 68 69 73 20 69 73 20 61 20 74 65 73 74>
const { Buffer } = require('node:buffer');
class Foo {
[Symbol.toPrimitive]() {
return 'this is a test';
}
}
const buf = Buffer.from(new Foo(), 'utf8');
// Prints: <Buffer 74 68 69 73 20 69 73 20 61 20 74 65 73 74>
A TypeError
will be thrown if object
does not have the mentioned methods or
is not of another type appropriate for Buffer.from()
variants.
Static method: Buffer.from(string[, encoding])
#
Creates a new Buffer
containing string
. The encoding
parameter identifies
the character encoding to be used when converting string
into bytes.
import { Buffer } from 'node:buffer';
const buf1 = Buffer.from('this is a tést');
const buf2 = Buffer.from('7468697320697320612074c3a97374', 'hex');
console.log(buf1.toString());
// Prints: this is a tést
console.log(buf2.toString());
// Prints: this is a tést
console.log(buf1.toString('latin1'));
// Prints: this is a tést
const { Buffer } = require('node:buffer');
const buf1 = Buffer.from('this is a tést');
const buf2 = Buffer.from('7468697320697320612074c3a97374', 'hex');
console.log(buf1.toString());
// Prints: this is a tést
console.log(buf2.toString());
// Prints: this is a tést
console.log(buf1.toString('latin1'));
// Prints: this is a tést
A TypeError
will be thrown if string
is not a string or another type
appropriate for Buffer.from()
variants.
Static method: Buffer.isBuffer(obj)
#
Returns true
if obj
is a Buffer
, false
otherwise.
import { Buffer } from 'node:buffer';
Buffer.isBuffer(Buffer.alloc(10)); // true
Buffer.isBuffer(Buffer.from('foo')); // true
Buffer.isBuffer('a string'); // false
Buffer.isBuffer([]); // false
Buffer.isBuffer(new Uint8Array(1024)); // false
const { Buffer } = require('node:buffer');
Buffer.isBuffer(Buffer.alloc(10)); // true
Buffer.isBuffer(Buffer.from('foo')); // true
Buffer.isBuffer('a string'); // false
Buffer.isBuffer([]); // false
Buffer.isBuffer(new Uint8Array(1024)); // false
Static method: Buffer.isEncoding(encoding)
#
Returns true
if encoding
is the name of a supported character encoding,
or false
otherwise.
import { Buffer } from 'node:buffer';
console.log(Buffer.isEncoding('utf8'));
// Prints: true
console.log(Buffer.isEncoding('hex'));
// Prints: true
console.log(Buffer.isEncoding('utf/8'));
// Prints: false
console.log(Buffer.isEncoding(''));
// Prints: false
const { Buffer } = require('node:buffer');
console.log(Buffer.isEncoding('utf8'));
// Prints: true
console.log(Buffer.isEncoding('hex'));
// Prints: true
console.log(Buffer.isEncoding('utf/8'));
// Prints: false
console.log(Buffer.isEncoding(''));
// Prints: false
Class property: Buffer.poolSize
#
- <integer> Default:
8192
This is the size (in bytes) of pre-allocated internal Buffer
instances used
for pooling. This value may be modified.
buf[index]
#
index
<integer>
The index operator [index]
can be used to get and set the octet at position
index
in buf
. The values refer to individual bytes, so the legal value
range is between 0x00
and 0xFF
(hex) or 0
and 255
(decimal).
This operator is inherited from Uint8Array
, so its behavior on out-of-bounds
access is the same as Uint8Array
. In other words, buf[index]
returns
undefined
when index
is negative or greater or equal to buf.length
, and
buf[index] = value
does not modify the buffer if index
is negative or
>= buf.length
.
import { Buffer } from 'node:buffer';
// Copy an ASCII string into a `Buffer` one byte at a time.
// (This only works for ASCII-only strings. In general, one should use
// `Buffer.from()` to perform this conversion.)
const str = 'Node.js';
const buf = Buffer.allocUnsafe(str.length);
for (let i = 0; i < str.length; i++) {
buf[i] = str.charCodeAt(i);
}
console.log(buf.toString('utf8'));
// Prints: Node.js
const { Buffer } = require('node:buffer');
// Copy an ASCII string into a `Buffer` one byte at a time.
// (This only works for ASCII-only strings. In general, one should use
// `Buffer.from()` to perform this conversion.)
const str = 'Node.js';
const buf = Buffer.allocUnsafe(str.length);
for (let i = 0; i < str.length; i++) {
buf[i] = str.charCodeAt(i);
}
console.log(buf.toString('utf8'));
// Prints: Node.js
buf.buffer
#
- <ArrayBuffer> The underlying
ArrayBuffer
object based on which thisBuffer
object is created.
This ArrayBuffer
is not guaranteed to correspond exactly to the original
Buffer
. See the notes on buf.byteOffset
for details.
import { Buffer } from 'node:buffer';
const arrayBuffer = new ArrayBuffer(16);
const buffer = Buffer.from(arrayBuffer);
console.log(buffer.buffer === arrayBuffer);
// Prints: true
const { Buffer } = require('node:buffer');
const arrayBuffer = new ArrayBuffer(16);
const buffer = Buffer.from(arrayBuffer);
console.log(buffer.buffer === arrayBuffer);
// Prints: true
buf.byteOffset
#
- <integer> The
byteOffset
of theBuffer
s underlyingArrayBuffer
object.
When setting byteOffset
in Buffer.from(ArrayBuffer, byteOffset, length)
,
or sometimes when allocating a Buffer
smaller than Buffer.poolSize
, the
buffer does not start from a zero offset on the underlying ArrayBuffer
.
This can cause problems when accessing the underlying ArrayBuffer
directly
using buf.buffer
, as other parts of the ArrayBuffer
may be unrelated
to the Buffer
object itself.
A common issue when creating a TypedArray
object that shares its memory with
a Buffer
is that in this case one needs to specify the byteOffset
correctly:
import { Buffer } from 'node:buffer';
// Create a buffer smaller than `Buffer.poolSize`.
const nodeBuffer = Buffer.from([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
// When casting the Node.js Buffer to an Int8Array, use the byteOffset
// to refer only to the part of `nodeBuffer.buffer` that contains the memory
// for `nodeBuffer`.
new Int8Array(nodeBuffer.buffer, nodeBuffer.byteOffset, nodeBuffer.length);
const { Buffer } = require('node:buffer');
// Create a buffer smaller than `Buffer.poolSize`.
const nodeBuffer = Buffer.from([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
// When casting the Node.js Buffer to an Int8Array, use the byteOffset
// to refer only to the part of `nodeBuffer.buffer` that contains the memory
// for `nodeBuffer`.
new Int8Array(nodeBuffer.buffer, nodeBuffer.byteOffset, nodeBuffer.length);
buf.compare(target[, targetStart[, targetEnd[, sourceStart[, sourceEnd]]]])
#
target
<Buffer> | <Uint8Array> ABuffer
orUint8Array
with which to comparebuf
.targetStart
<integer> The offset withintarget
at which to begin comparison. Default:0
.targetEnd
<integer> The offset withintarget
at which to end comparison (not inclusive). Default:target.length
.sourceStart
<integer> The offset withinbuf
at which to begin comparison. Default:0
.sourceEnd
<integer> The offset withinbuf
at which to end comparison (not inclusive). Default:buf.length
.- Returns: <integer>
Compares buf
with target
and returns a number indicating whether buf
comes before, after, or is the same as target
in sort order.
Comparison is based on the actual sequence of bytes in each Buffer
.
0
is returned iftarget
is the same asbuf
1
is returned iftarget
should come beforebuf
when sorted.-1
is returned iftarget
should come afterbuf
when sorted.
import { Buffer } from 'node:buffer';
const buf1 = Buffer.from('ABC');
const buf2 = Buffer.from('BCD');
const buf3 = Buffer.from('ABCD');
console.log(buf1.compare(buf1));
// Prints: 0
console.log(buf1.compare(buf2));
// Prints: -1
console.log(buf1.compare(buf3));
// Prints: -1
console.log(buf2.compare(buf1));
// Prints: 1
console.log(buf2.compare(buf3));
// Prints: 1
console.log([buf1, buf2, buf3].sort(Buffer.compare));
// Prints: [ <Buffer 41 42 43>, <Buffer 41 42 43 44>, <Buffer 42 43 44> ]
// (This result is equal to: [buf1, buf3, buf2].)
const { Buffer } = require('node:buffer');
const buf1 = Buffer.from('ABC');
const buf2 = Buffer.from('BCD');
const buf3 = Buffer.from('ABCD');
console.log(buf1.compare(buf1));
// Prints: 0
console.log(buf1.compare(buf2));
// Prints: -1
console.log(buf1.compare(buf3));
// Prints: -1
console.log(buf2.compare(buf1));
// Prints: 1
console.log(buf2.compare(buf3));
// Prints: 1
console.log([buf1, buf2, buf3].sort(Buffer.compare));
// Prints: [ <Buffer 41 42 43>, <Buffer 41 42 43 44>, <Buffer 42 43 44> ]
// (This result is equal to: [buf1, buf3, buf2].)
The optional targetStart
, targetEnd
, sourceStart
, and sourceEnd
arguments can be used to limit the comparison to specific ranges within target
and buf
respectively.
import { Buffer } from 'node:buffer';
const buf1 = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8, 9]);
const buf2 = Buffer.from([5, 6, 7, 8, 9, 1, 2, 3, 4]);
console.log(buf1.compare(buf2, 5, 9, 0, 4));
// Prints: 0
console.log(buf1.compare(buf2, 0, 6, 4));
// Prints: -1
console.log(buf1.compare(buf2, 5, 6, 5));
// Prints: 1
const { Buffer } = require('node:buffer');
const buf1 = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8, 9]);
const buf2 = Buffer.from([5, 6, 7, 8, 9, 1, 2, 3, 4]);
console.log(buf1.compare(buf2, 5, 9, 0, 4));
// Prints: 0
console.log(buf1.compare(buf2, 0, 6, 4));
// Prints: -1
console.log(buf1.compare(buf2, 5, 6, 5));
// Prints: 1
ERR_OUT_OF_RANGE
is thrown if targetStart < 0
, sourceStart < 0
,
targetEnd > target.byteLength
, or sourceEnd > source.byteLength
.
buf.copy(target[, targetStart[, sourceStart[, sourceEnd]]])
#
target
<Buffer> | <Uint8Array> ABuffer
orUint8Array
to copy into.targetStart
<integer> The offset withintarget
at which to begin writing. Default:0
.sourceStart
<integer> The offset withinbuf
from which to begin copying. Default:0
.sourceEnd
<integer> The offset withinbuf
at which to stop copying (not inclusive). Default:buf.length
.- Returns: <integer> The number of bytes copied.
Copies data from a region of buf
to a region in target
, even if the target
memory region overlaps with buf
.
TypedArray.prototype.set()
performs the same operation, and is available
for all TypedArrays, including Node.js Buffer
s, although it takes
different function arguments.
import { Buffer } from 'node:buffer';
// Create two `Buffer` instances.
const buf1 = Buffer.allocUnsafe(26);
const buf2 = Buffer.allocUnsafe(26).fill('!');
for (let i = 0; i < 26; i++) {
// 97 is the decimal ASCII value for 'a'.
buf1[i] = i + 97;
}
// Copy `buf1` bytes 16 through 19 into `buf2` starting at byte 8 of `buf2`.
buf1.copy(buf2, 8, 16, 20);
// This is equivalent to:
// buf2.set(buf1.subarray(16, 20), 8);
console.log(buf2.toString('ascii', 0, 25));
// Prints: !!!!!!!!qrst!!!!!!!!!!!!!
const { Buffer } = require('node:buffer');
// Create two `Buffer` instances.
const buf1 = Buffer.allocUnsafe(26);
const buf2 = Buffer.allocUnsafe(26).fill('!');
for (let i = 0; i < 26; i++) {
// 97 is the decimal ASCII value for 'a'.
buf1[i] = i + 97;
}
// Copy `buf1` bytes 16 through 19 into `buf2` starting at byte 8 of `buf2`.
buf1.copy(buf2, 8, 16, 20);
// This is equivalent to:
// buf2.set(buf1.subarray(16, 20), 8);
console.log(buf2.toString('ascii', 0, 25));
// Prints: !!!!!!!!qrst!!!!!!!!!!!!!
import { Buffer } from 'node:buffer';
// Create a `Buffer` and copy data from one region to an overlapping region
// within the same `Buffer`.
const buf = Buffer.allocUnsafe(26);
for (let i = 0; i < 26; i++) {
// 97 is the decimal ASCII value for 'a'.
buf[i] = i + 97;
}
buf.copy(buf, 0, 4, 10);
console.log(buf.toString());
// Prints: efghijghijklmnopqrstuvwxyz
const { Buffer } = require('node:buffer');
// Create a `Buffer` and copy data from one region to an overlapping region
// within the same `Buffer`.
const buf = Buffer.allocUnsafe(26);
for (let i = 0; i < 26; i++) {
// 97 is the decimal ASCII value for 'a'.
buf[i] = i + 97;
}
buf.copy(buf, 0, 4, 10);
console.log(buf.toString());
// Prints: efghijghijklmnopqrstuvwxyz
buf.entries()
#
- Returns: <Iterator>
Creates and returns an iterator of [index, byte]
pairs from the contents
of buf
.
import { Buffer } from 'node:buffer';
// Log the entire contents of a `Buffer`.
const buf = Buffer.from('buffer');
for (const pair of buf.entries()) {
console.log(pair);
}
// Prints:
// [0, 98]
// [1, 117]
// [2, 102]
// [3, 102]
// [4, 101]
// [5, 114]
const { Buffer } = require('node:buffer');
// Log the entire contents of a `Buffer`.
const buf = Buffer.from('buffer');
for (const pair of buf.entries()) {
console.log(pair);
}
// Prints:
// [0, 98]
// [1, 117]
// [2, 102]
// [3, 102]
// [4, 101]
// [5, 114]
buf.equals(otherBuffer)
#
otherBuffer
<Buffer> | <Uint8Array> ABuffer
orUint8Array
with which to comparebuf
.- Returns: <boolean>
Returns true
if both buf
and otherBuffer
have exactly the same bytes,
false
otherwise. Equivalent to
buf.compare(otherBuffer) === 0
.
import { Buffer } from 'node:buffer';
const buf1 = Buffer.from('ABC');
const buf2 = Buffer.from('414243', 'hex');
const buf3 = Buffer.from('ABCD');
console.log(buf1.equals(buf2));
// Prints: true
console.log(buf1.equals(buf3));
// Prints: false
const { Buffer } = require('node:buffer');
const buf1 = Buffer.from('ABC');
const buf2 = Buffer.from('414243', 'hex');
const buf3 = Buffer.from('ABCD');
console.log(buf1.equals(buf2));
// Prints: true
console.log(buf1.equals(buf3));
// Prints: false
buf.fill(value[, offset[, end]][, encoding])
#
value
<string> | <Buffer> | <Uint8Array> | <integer> The value with which to fillbuf
. Empty value (string, Uint8Array, Buffer) is coerced to0
.offset
<integer> Number of bytes to skip before starting to fillbuf
. Default:0
.end
<integer> Where to stop fillingbuf
(not inclusive). Default:buf.length
.encoding
<string> The encoding forvalue
ifvalue
is a string. Default:'utf8'
.- Returns: <Buffer> A reference to
buf
.
Fills buf
with the specified value
. If the offset
and end
are not given,
the entire buf
will be filled:
import { Buffer } from 'node:buffer';
// Fill a `Buffer` with the ASCII character 'h'.
const b = Buffer.allocUnsafe(50).fill('h');
console.log(b.toString());
// Prints: hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh
// Fill a buffer with empty string
const c = Buffer.allocUnsafe(5).fill('');
console.log(c.fill(''));
// Prints: <Buffer 00 00 00 00 00>
const { Buffer } = require('node:buffer');
// Fill a `Buffer` with the ASCII character 'h'.
const b = Buffer.allocUnsafe(50).fill('h');
console.log(b.toString());
// Prints: hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh
// Fill a buffer with empty string
const c = Buffer.allocUnsafe(5).fill('');
console.log(c.fill(''));
// Prints: <Buffer 00 00 00 00 00>
value
is coerced to a uint32
value if it is not a string, Buffer
, or
integer. If the resulting integer is greater than 255
(decimal), buf
will be
filled with value & 255
.
If the final write of a fill()
operation falls on a multi-byte character,
then only the bytes of that character that fit into buf
are written:
import { Buffer } from 'node:buffer';
// Fill a `Buffer` with character that takes up two bytes in UTF-8.
console.log(Buffer.allocUnsafe(5).fill('\u0222'));
// Prints: <Buffer c8 a2 c8 a2 c8>
const { Buffer } = require('node:buffer');
// Fill a `Buffer` with character that takes up two bytes in UTF-8.
console.log(Buffer.allocUnsafe(5).fill('\u0222'));
// Prints: <Buffer c8 a2 c8 a2 c8>
If value
contains invalid characters, it is truncated; if no valid
fill data remains, an exception is thrown:
import { Buffer } from 'node:buffer';
const buf = Buffer.allocUnsafe(5);
console.log(buf.fill('a'));
// Prints: <Buffer 61 61 61 61 61>
console.log(buf.fill('aazz', 'hex'));
// Prints: <Buffer aa aa aa aa aa>
console.log(buf.fill('zz', 'hex'));
// Throws an exception.
const { Buffer } = require('node:buffer');
const buf = Buffer.allocUnsafe(5);
console.log(buf.fill('a'));
// Prints: <Buffer 61 61 61 61 61>
console.log(buf.fill('aazz', 'hex'));
// Prints: <Buffer aa aa aa aa aa>
console.log(buf.fill('zz', 'hex'));
// Throws an exception.
buf.includes(value[, byteOffset][, encoding])
#
value
<string> | <Buffer> | <Uint8Array> | <integer> What to search for.byteOffset
<integer> Where to begin searching inbuf
. If negative, then offset is calculated from the end ofbuf
. Default:0
.encoding
<string> Ifvalue
is a string, this is its encoding. Default:'utf8'
.- Returns: <boolean>
true
ifvalue
was found inbuf
,false
otherwise.
Equivalent to buf.indexOf() !== -1
.
import { Buffer } from 'node:buffer';
const buf = Buffer.from('this is a buffer');
console.log(buf.includes('this'));
// Prints: true
console.log(buf.includes('is'));
// Prints: true
console.log(buf.includes(Buffer.from('a buffer')));
// Prints: true
console.log(buf.includes(97));
// Prints: true (97 is the decimal ASCII value for 'a')
console.log(buf.includes(Buffer.from('a buffer example')));
// Prints: false
console.log(buf.includes(Buffer.from('a buffer example').slice(0, 8)));
// Prints: true
console.log(buf.includes('this', 4));
// Prints: false
const { Buffer } = require('node:buffer');
const buf = Buffer.from('this is a buffer');
console.log(buf.includes('this'));
// Prints: true
console.log(buf.includes('is'));
// Prints: true
console.log(buf.includes(Buffer.from('a buffer')));
// Prints: true
console.log(buf.includes(97));
// Prints: true (97 is the decimal ASCII value for 'a')
console.log(buf.includes(Buffer.from('a buffer example')));
// Prints: false
console.log(buf.includes(Buffer.from('a buffer example').slice(0, 8)));
// Prints: true
console.log(buf.includes('this', 4));
// Prints: false
buf.indexOf(value[, byteOffset][, encoding])
#
value
<string> | <Buffer> | <Uint8Array> | <integer> What to search for.byteOffset
<integer> Where to begin searching inbuf
. If negative, then offset is calculated from the end ofbuf
. Default:0
.encoding
<string> Ifvalue
is a string, this is the encoding used to determine the binary representation of the string that will be searched for inbuf
. Default:'utf8'
.- Returns: <integer> The index of the first occurrence of
value
inbuf
, or-1
ifbuf
does not containvalue
.
If value
is:
- a string,
value
is interpreted according to the character encoding inencoding
. - a
Buffer
orUint8Array
,value
will be used in its entirety. To compare a partialBuffer
, usebuf.subarray
. - a number,
value
will be interpreted as an unsigned 8-bit integer value between0
and255
.
import { Buffer } from 'node:buffer';
const buf = Buffer.from('this is a buffer');
console.log(buf.indexOf('this'));
// Prints: 0
console.log(buf.indexOf('is'));
// Prints: 2
console.log(buf.indexOf(Buffer.from('a buffer')));
// Prints: 8
console.log(buf.indexOf(97));
// Prints: 8 (97 is the decimal ASCII value for 'a')
console.log(buf.indexOf(Buffer.from('a buffer example')));
// Prints: -1
console.log(buf.indexOf(Buffer.from('a buffer example').slice(0, 8)));
// Prints: 8
const utf16Buffer = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'utf16le');
console.log(utf16Buffer.indexOf('\u03a3', 0, 'utf16le'));
// Prints: 4
console.log(utf16Buffer.indexOf('\u03a3', -4, 'utf16le'));
// Prints: 6
const { Buffer } = require('node:buffer');
const buf = Buffer.from('this is a buffer');
console.log(buf.indexOf('this'));
// Prints: 0
console.log(buf.indexOf('is'));
// Prints: 2
console.log(buf.indexOf(Buffer.from('a buffer')));
// Prints: 8
console.log(buf.indexOf(97));
// Prints: 8 (97 is the decimal ASCII value for 'a')
console.log(buf.indexOf(Buffer.from('a buffer example')));
// Prints: -1
console.log(buf.indexOf(Buffer.from('a buffer example').slice(0, 8)));
// Prints: 8
const utf16Buffer = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'utf16le');
console.log(utf16Buffer.indexOf('\u03a3', 0, 'utf16le'));
// Prints: 4
console.log(utf16Buffer.indexOf('\u03a3', -4, 'utf16le'));
// Prints: 6
If value
is not a string, number, or Buffer
, this method will throw a
TypeError
. If value
is a number, it will be coerced to a valid byte value,
an integer between 0 and 255.
If byteOffset
is not a number, it will be coerced to a number. If the result
of coercion is NaN
or 0
, then the entire buffer will be searched. This
behavior matches String.prototype.indexOf()
.
import { Buffer } from 'node:buffer';
const b = Buffer.from('abcdef');
// Passing a value that's a number, but not a valid byte.
// Prints: 2, equivalent to searching for 99 or 'c'.
console.log(b.indexOf(99.9));
console.log(b.indexOf(256 + 99));
// Passing a byteOffset that coerces to NaN or 0.
// Prints: 1, searching the whole buffer.
console.log(b.indexOf('b', undefined));
console.log(b.indexOf('b', {}));
console.log(b.indexOf('b', null));
console.log(b.indexOf('b', []));
const { Buffer } = require('node:buffer');
const b = Buffer.from('abcdef');
// Passing a value that's a number, but not a valid byte.
// Prints: 2, equivalent to searching for 99 or 'c'.
console.log(b.indexOf(99.9));
console.log(b.indexOf(256 + 99));
// Passing a byteOffset that coerces to NaN or 0.
// Prints: 1, searching the whole buffer.
console.log(b.indexOf('b', undefined));
console.log(b.indexOf('b', {}));
console.log(b.indexOf('b', null));
console.log(b.indexOf('b', []));
If value
is an empty string or empty Buffer
and byteOffset
is less
than buf.length
, byteOffset
will be returned. If value
is empty and
byteOffset
is at least buf.length
, buf.length
will be returned.
buf.keys()
#
- Returns: <Iterator>
Creates and returns an iterator of buf
keys (indices).
import { Buffer } from 'node:buffer';
const buf = Buffer.from('buffer');
for (const key of buf.keys()) {
console.log(key);
}
// Prints:
// 0
// 1
// 2
// 3
// 4
// 5
const { Buffer } = require('node:buffer');
const buf = Buffer.from('buffer');
for (const key of buf.keys()) {
console.log(key);
}
// Prints:
// 0
// 1
// 2
// 3
// 4
// 5
buf.lastIndexOf(value[, byteOffset][, encoding])
#
value
<string> | <Buffer> | <Uint8Array> | <integer> What to search for.byteOffset
<integer> Where to begin searching inbuf
. If negative, then offset is calculated from the end ofbuf
. Default:buf.length - 1
.encoding
<string> Ifvalue
is a string, this is the encoding used to determine the binary representation of the string that will be searched for inbuf
. Default:'utf8'
.- Returns: <integer> The index of the last occurrence of
value
inbuf
, or-1
ifbuf
does not containvalue
.
Identical to buf.indexOf()
, except the last occurrence of value
is found
rather than the first occurrence.
import { Buffer } from 'node:buffer';
const buf = Buffer.from('this buffer is a buffer');
console.log(buf.lastIndexOf('this'));
// Prints: 0
console.log(buf.lastIndexOf('buffer'));
// Prints: 17
console.log(buf.lastIndexOf(Buffer.from('buffer')));
// Prints: 17
console.log(buf.lastIndexOf(97));
// Prints: 15 (97 is the decimal ASCII value for 'a')
console.log(buf.lastIndexOf(Buffer.from('yolo')));
// Prints: -1
console.log(buf.lastIndexOf('buffer', 5));
// Prints: 5
console.log(buf.lastIndexOf('buffer', 4));
// Prints: -1
const utf16Buffer = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'utf16le');
console.log(utf16Buffer.lastIndexOf('\u03a3', undefined, 'utf16le'));
// Prints: 6
console.log(utf16Buffer.lastIndexOf('\u03a3', -5, 'utf16le'));
// Prints: 4
const { Buffer } = require('node:buffer');
const buf = Buffer.from('this buffer is a buffer');
console.log(buf.lastIndexOf('this'));
// Prints: 0
console.log(buf.lastIndexOf('buffer'));
// Prints: 17
console.log(buf.lastIndexOf(Buffer.from('buffer')));
// Prints: 17
console.log(buf.lastIndexOf(97));
// Prints: 15 (97 is the decimal ASCII value for 'a')
console.log(buf.lastIndexOf(Buffer.from('yolo')));
// Prints: -1
console.log(buf.lastIndexOf('buffer', 5));
// Prints: 5
console.log(buf.lastIndexOf('buffer', 4));
// Prints: -1
const utf16Buffer = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'utf16le');
console.log(utf16Buffer.lastIndexOf('\u03a3', undefined, 'utf16le'));
// Prints: 6
console.log(utf16Buffer.lastIndexOf('\u03a3', -5, 'utf16le'));
// Prints: 4
If value
is not a string, number, or Buffer
, this method will throw a
TypeError
. If value
is a number, it will be coerced to a valid byte value,
an integer between 0 and 255.
If byteOffset
is not a number, it will be coerced to a number. Any arguments
that coerce to NaN
, like {}
or undefined
, will search the whole buffer.
This behavior matches String.prototype.lastIndexOf()
.
import { Buffer } from 'node:buffer';
const b = Buffer.from('abcdef');
// Passing a value that's a number, but not a valid byte.
// Prints: 2, equivalent to searching for 99 or 'c'.
console.log(b.lastIndexOf(99.9));
console.log(b.lastIndexOf(256 + 99));
// Passing a byteOffset that coerces to NaN.
// Prints: 1, searching the whole buffer.
console.log(b.lastIndexOf('b', undefined));
console.log(b.lastIndexOf('b', {}));
// Passing a byteOffset that coerces to 0.
// Prints: -1, equivalent to passing 0.
console.log(b.lastIndexOf('b', null));
console.log(b.lastIndexOf('b', []));
const { Buffer } = require('node:buffer');
const b = Buffer.from('abcdef');
// Passing a value that's a number, but not a valid byte.
// Prints: 2, equivalent to searching for 99 or 'c'.
console.log(b.lastIndexOf(99.9));
console.log(b.lastIndexOf(256 + 99));
// Passing a byteOffset that coerces to NaN.
// Prints: 1, searching the whole buffer.
console.log(b.lastIndexOf('b', undefined));
console.log(b.lastIndexOf('b', {}));
// Passing a byteOffset that coerces to 0.
// Prints: -1, equivalent to passing 0.
console.log(b.lastIndexOf('b', null));
console.log(b.lastIndexOf('b', []));
If value
is an empty string or empty Buffer
, byteOffset
will be returned.
buf.length
#
Returns the number of bytes in buf
.
import { Buffer } from 'node:buffer';
// Create a `Buffer` and write a shorter string to it using UTF-8.
const buf = Buffer.alloc(1234);
console.log(buf.length);
// Prints: 1234
buf.write('some string', 0, 'utf8');
console.log(buf.length);
// Prints: 1234
const { Buffer } = require('node:buffer');
// Create a `Buffer` and write a shorter string to it using UTF-8.
const buf = Buffer.alloc(1234);
console.log(buf.length);
// Prints: 1234
buf.write('some string', 0, 'utf8');
console.log(buf.length);
// Prints: 1234
buf.parent
#
buf.buffer
instead.The buf.parent
property is a deprecated alias for buf.buffer
.
buf.readBigInt64BE([offset])
#
offset
<integer> Number of bytes to skip before starting to read. Must satisfy:0 <= offset <= buf.length - 8
. Default:0
.- Returns: <bigint>
Reads a signed, big-endian 64-bit integer from buf
at the specified offset
.
Integers read from a Buffer
are interpreted as two's complement signed
values.
buf.readBigInt64LE([offset])
#
offset
<integer> Number of bytes to skip before starting to read. Must satisfy:0 <= offset <= buf.length - 8
. Default:0
.- Returns: <bigint>
Reads a signed, little-endian 64-bit integer from buf
at the specified
offset
.
Integers read from a Buffer
are interpreted as two's complement signed
values.
buf.readBigUInt64BE([offset])
#
offset
<integer> Number of bytes to skip before starting to read. Must satisfy:0 <= offset <= buf.length - 8
. Default:0
.- Returns: <bigint>
Reads an unsigned, big-endian 64-bit integer from buf
at the specified
offset
.
This function is also available under the readBigUint64BE
alias.
import { Buffer } from 'node:buffer';
const buf = Buffer.from([0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0xff, 0xff]);
console.log(buf.readBigUInt64BE(0));
// Prints: 4294967295n
const { Buffer } = require('node:buffer');
const buf = Buffer.from([0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0xff, 0xff]);
console.log(buf.readBigUInt64BE(0));
// Prints: 4294967295n
buf.readBigUInt64LE([offset])
#
offset
<integer> Number of bytes to skip before starting to read. Must satisfy:0 <= offset <= buf.length - 8
. Default:0
.- Returns: <bigint>
Reads an unsigned, little-endian 64-bit integer from buf
at the specified
offset
.
This function is also available under the readBigUint64LE
alias.
import { Buffer } from 'node:buffer';
const buf = Buffer.from([0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0xff, 0xff]);
console.log(buf.readBigUInt64LE(0));
// Prints: 18446744069414584320n
const { Buffer } = require('node:buffer');
const buf = Buffer.from([0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0xff, 0xff]);
console.log(buf.readBigUInt64LE(0));
// Prints: 18446744069414584320n
buf.readDoubleBE([offset])
#
offset
<integer> Number of bytes to skip before starting to read. Must satisfy0 <= offset <= buf.length - 8
. Default:0
.- Returns: <number>
Reads a 64-bit, big-endian double from buf
at the specified offset
.
import { Buffer } from 'node:buffer';
const buf = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8]);
console.log(buf.readDoubleBE(0));
// Prints: 8.20788039913184e-304
const { Buffer } = require('node:buffer');
const buf = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8]);
console.log(buf.readDoubleBE(0));
// Prints: 8.20788039913184e-304
buf.readDoubleLE([offset])
#
offset
<integer> Number of bytes to skip before starting to read. Must satisfy0 <= offset <= buf.length - 8
. Default:0
.- Returns: <number>
Reads a 64-bit, little-endian double from buf
at the specified offset
.
import { Buffer } from 'node:buffer';
const buf = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8]);
console.log(buf.readDoubleLE(0));
// Prints: 5.447603722011605e-270
console.log(buf.readDoubleLE(1));
// Throws ERR_OUT_OF_RANGE.
const { Buffer } = require('node:buffer');
const buf = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8]);
console.log(buf.readDoubleLE(0));
// Prints: 5.447603722011605e-270
console.log(buf.readDoubleLE(1));
// Throws ERR_OUT_OF_RANGE.
buf.readFloatBE([offset])
#
offset
<integer> Number of bytes to skip before starting to read. Must satisfy0 <= offset <= buf.length - 4
. Default:0
.- Returns: <number>
Reads a 32-bit, big-endian float from buf
at the specified offset
.
import { Buffer } from 'node:buffer';
const buf = Buffer.from([1, 2, 3, 4]);
console.log(buf.readFloatBE(0));
// Prints: 2.387939260590663e-38
const { Buffer } = require('node:buffer');
const buf = Buffer.from([1, 2, 3, 4]);
console.log(buf.readFloatBE(0));
// Prints: 2.387939260590663e-38
buf.readFloatLE([offset])
#
offset
<integer> Number of bytes to skip before starting to read. Must satisfy0 <= offset <= buf.length - 4
. Default:0
.- Returns: <number>
Reads a 32-bit, little-endian float from buf
at the specified offset
.
import { Buffer } from 'node:buffer';
const buf = Buffer.from([1, 2, 3, 4]);
console.log(buf.readFloatLE(0));
// Prints: 1.539989614439558e-36
console.log(buf.readFloatLE(1));
// Throws ERR_OUT_OF_RANGE.
const { Buffer } = require('node:buffer');
const buf = Buffer.from([1, 2, 3, 4]);
console.log(buf.readFloatLE(0));
// Prints: 1.539989614439558e-36
console.log(buf.readFloatLE(1));
// Throws ERR_OUT_OF_RANGE.
buf.readInt8([offset])
#
offset
<integer> Number of bytes to skip before starting to read. Must satisfy0 <= offset <= buf.length - 1
. Default:0
.- Returns: <integer>
Reads a signed 8-bit integer from buf
at the specified offset
.
Integers read from a Buffer
are interpreted as two's complement signed values.
import { Buffer } from 'node:buffer';
const buf = Buffer.from([-1, 5]);
console.log(buf.readInt8(0));
// Prints: -1
console.log(buf.readInt8(1));
// Prints: 5
console.log(buf.readInt8(2));
// Throws ERR_OUT_OF_RANGE.
const { Buffer } = require('node:buffer');
const buf = Buffer.from([-1, 5]);
console.log(buf.readInt8(0));
// Prints: -1
console.log(buf.readInt8(1));
// Prints: 5
console.log(buf.readInt8(2));
// Throws ERR_OUT_OF_RANGE.
buf.readInt16BE([offset])
#
offset
<integer> Number of bytes to skip before starting to read. Must satisfy0 <= offset <= buf.length - 2
. Default:0
.- Returns: <integer>
Reads a signed, big-endian 16-bit integer from buf
at the specified offset
.
Integers read from a Buffer
are interpreted as two's complement signed values.
import { Buffer } from 'node:buffer';
const buf = Buffer.from([0, 5]);
console.log(buf.readInt16BE(0));
// Prints: 5
const { Buffer } = require('node:buffer');
const buf = Buffer.from([0, 5]);
console.log(buf.readInt16BE(0));
// Prints: 5
buf.readInt16LE([offset])
#
offset
<integer> Number of bytes to skip before starting to read. Must satisfy0 <= offset <= buf.length - 2
. Default:0
.- Returns: <integer>
Reads a signed, little-endian 16-bit integer from buf
at the specified
offset
.
Integers read from a Buffer
are interpreted as two's complement signed values.
import { Buffer } from 'node:buffer';
const buf = Buffer.from([0, 5]);
console.log(buf.readInt16LE(0));
// Prints: 1280
console.log(buf.readInt16LE(1));
// Throws ERR_OUT_OF_RANGE.
const { Buffer } = require('node:buffer');
const buf = Buffer.from([0, 5]);
console.log(buf.readInt16LE(0));
// Prints: 1280
console.log(buf.readInt16LE(1));
// Throws ERR_OUT_OF_RANGE.
buf.readInt32BE([offset])
#
offset
<integer> Number of bytes to skip before starting to read. Must satisfy0 <= offset <= buf.length - 4
. Default:0
.- Returns: <integer>
Reads a signed, big-endian 32-bit integer from buf
at the specified offset
.
Integers read from a Buffer
are interpreted as two's complement signed values.
import { Buffer } from 'node:buffer';
const buf = Buffer.from([0, 0, 0, 5]);
console.log(buf.readInt32BE(0));
// Prints: 5
const { Buffer } = require('node:buffer');
const buf = Buffer.from([0, 0, 0, 5]);
console.log(buf.readInt32BE(0));
// Prints: 5
buf.readInt32LE([offset])
#
offset
<integer> Number of bytes to skip before starting to read. Must satisfy0 <= offset <= buf.length - 4
. Default:0
.- Returns: <integer>
Reads a signed, little-endian 32-bit integer from buf
at the specified
offset
.
Integers read from a Buffer
are interpreted as two's complement signed values.
import { Buffer } from 'node:buffer';
const buf = Buffer.from([0, 0, 0, 5]);
console.log(buf.readInt32LE(0));
// Prints: 83886080
console.log(buf.readInt32LE(1));
// Throws ERR_OUT_OF_RANGE.
const { Buffer } = require('node:buffer');
const buf = Buffer.from([0, 0, 0, 5]);
console.log(buf.readInt32LE(0));
// Prints: 83886080
console.log(buf.readInt32LE(1));
// Throws ERR_OUT_OF_RANGE.
buf.readIntBE(offset, byteLength)
#
offset
<integer> Number of bytes to skip before starting to read. Must satisfy0 <= offset <= buf.length - byteLength
.byteLength
<integer> Number of bytes to read. Must satisfy0 < byteLength <= 6
.- Returns: <integer>
Reads byteLength
number of bytes from buf
at the specified offset
and interprets the result as a big-endian, two's complement signed value
supporting up to 48 bits of accuracy.
import { Buffer } from 'node:buffer';
const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);
console.log(buf.readIntBE(0, 6).toString(16));
// Prints: 1234567890ab
console.log(buf.readIntBE(1, 6).toString(16));
// Throws ERR_OUT_OF_RANGE.
console.log(buf.readIntBE(1, 0).toString(16));
// Throws ERR_OUT_OF_RANGE.
const { Buffer } = require('node:buffer');
const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);
console.log(buf.readIntBE(0, 6).toString(16));
// Prints: 1234567890ab
console.log(buf.readIntBE(1, 6).toString(16));
// Throws ERR_OUT_OF_RANGE.
console.log(buf.readIntBE(1, 0).toString(16));
// Throws ERR_OUT_OF_RANGE.
buf.readIntLE(offset, byteLength)
#
offset
<integer> Number of bytes to skip before starting to read. Must satisfy0 <= offset <= buf.length - byteLength
.byteLength
<integer> Number of bytes to read. Must satisfy0 < byteLength <= 6
.- Returns: <integer>
Reads byteLength
number of bytes from buf
at the specified offset
and interprets the result as a little-endian, two's complement signed value
supporting up to 48 bits of accuracy.
import { Buffer } from 'node:buffer';
const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);
console.log(buf.readIntLE(0, 6).toString(16));
// Prints: -546f87a9cbee
const { Buffer } = require('node:buffer');
const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);
console.log(buf.readIntLE(0, 6).toString(16));
// Prints: -546f87a9cbee
buf.readUInt8([offset])
#
offset
<integer> Number of bytes to skip before starting to read. Must satisfy0 <= offset <= buf.length - 1
. Default:0
.- Returns: <integer>
Reads an unsigned 8-bit integer from buf
at the specified offset
.
This function is also available under the readUint8
alias.
import { Buffer } from 'node:buffer';
const buf = Buffer.from([1, -2]);
console.log(buf.readUInt8(0));
// Prints: 1
console.log(buf.readUInt8(1));
// Prints: 254
console.log(buf.readUInt8(2));
// Throws ERR_OUT_OF_RANGE.
const { Buffer } = require('node:buffer');
const buf = Buffer.from([1, -2]);
console.log(buf.readUInt8(0));
// Prints: 1
console.log(buf.readUInt8(1));
// Prints: 254
console.log(buf.readUInt8(2));
// Throws ERR_OUT_OF_RANGE.
buf.readUInt16BE([offset])
#
offset
<integer> Number of bytes to skip before starting to read. Must satisfy0 <= offset <= buf.length - 2
. Default:0
.- Returns: <integer>
Reads an unsigned, big-endian 16-bit integer from buf
at the specified
offset
.
This function is also available under the readUint16BE
alias.
import { Buffer } from 'node:buffer';
const buf = Buffer.from([0x12, 0x34, 0x56]);
console.log(buf.readUInt16BE(0).toString(16));
// Prints: 1234
console.log(buf.readUInt16BE(1).toString(16));
// Prints: 3456
const { Buffer } = require('node:buffer');
const buf = Buffer.from([0x12, 0x34, 0x56]);
console.log(buf.readUInt16BE(0).toString(16));
// Prints: 1234
console.log(buf.readUInt16BE(1).toString(16));
// Prints: 3456
buf.readUInt16LE([offset])
#
offset
<integer> Number of bytes to skip before starting to read. Must satisfy0 <= offset <= buf.length - 2
. Default:0
.- Returns: <integer>
Reads an unsigned, little-endian 16-bit integer from buf
at the specified
offset
.
This function is also available under the readUint16LE
alias.
import { Buffer } from 'node:buffer';
const buf = Buffer.from([0x12, 0x34, 0x56]);
console.log(buf.readUInt16LE(0).toString(16));
// Prints: 3412
console.log(buf.readUInt16LE(1).toString(16));
// Prints: 5634
console.log(buf.readUInt16LE(2).toString(16));
// Throws ERR_OUT_OF_RANGE.
const { Buffer } = require('node:buffer');
const buf = Buffer.from([0x12, 0x34, 0x56]);
console.log(buf.readUInt16LE(0).toString(16));
// Prints: 3412
console.log(buf.readUInt16LE(1).toString(16));
// Prints: 5634
console.log(buf.readUInt16LE(2).toString(16));
// Throws ERR_OUT_OF_RANGE.
buf.readUInt32BE([offset])
#
offset
<integer> Number of bytes to skip before starting to read. Must satisfy0 <= offset <= buf.length - 4
. Default:0
.- Returns: <integer>
Reads an unsigned, big-endian 32-bit integer from buf
at the specified
offset
.
This function is also available under the readUint32BE
alias.
import { Buffer } from 'node:buffer';
const buf = Buffer.from([0x12, 0x34, 0x56, 0x78]);
console.log(buf.readUInt32BE(0).toString(16));
// Prints: 12345678
const { Buffer } = require('node:buffer');
const buf = Buffer.from([0x12, 0x34, 0x56, 0x78]);
console.log(buf.readUInt32BE(0).toString(16));
// Prints: 12345678
buf.readUInt32LE([offset])
#
offset
<integer> Number of bytes to skip before starting to read. Must satisfy0 <= offset <= buf.length - 4
. Default:0
.- Returns: <integer>
Reads an unsigned, little-endian 32-bit integer from buf
at the specified
offset
.
This function is also available under the readUint32LE
alias.
import { Buffer } from 'node:buffer';
const buf = Buffer.from([0x12, 0x34, 0x56, 0x78]);
console.log(buf.readUInt32LE(0).toString(16));
// Prints: 78563412
console.log(buf.readUInt32LE(1).toString(16));
// Throws ERR_OUT_OF_RANGE.
const { Buffer } = require('node:buffer');
const buf = Buffer.from([0x12, 0x34, 0x56, 0x78]);
console.log(buf.readUInt32LE(0).toString(16));
// Prints: 78563412
console.log(buf.readUInt32LE(1).toString(16));
// Throws ERR_OUT_OF_RANGE.
buf.readUIntBE(offset, byteLength)
#
offset
<integer> Number of bytes to skip before starting to read. Must satisfy0 <= offset <= buf.length - byteLength
.byteLength
<integer> Number of bytes to read. Must satisfy0 < byteLength <= 6
.- Returns: <integer>
Reads byteLength
number of bytes from buf
at the specified offset
and interprets the result as an unsigned big-endian integer supporting
up to 48 bits of accuracy.
This function is also available under the readUintBE
alias.
import { Buffer } from 'node:buffer';
const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);
console.log(buf.readUIntBE(0, 6).toString(16));
// Prints: 1234567890ab
console.log(buf.readUIntBE(1, 6).toString(16));
// Throws ERR_OUT_OF_RANGE.
const { Buffer } = require('node:buffer');
const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);
console.log(buf.readUIntBE(0, 6).toString(16));
// Prints: 1234567890ab
console.log(buf.readUIntBE(1, 6).toString(16));
// Throws ERR_OUT_OF_RANGE.
buf.readUIntLE(offset, byteLength)
#
offset
<integer> Number of bytes to skip before starting to read. Must satisfy0 <= offset <= buf.length - byteLength
.byteLength
<integer> Number of bytes to read. Must satisfy0 < byteLength <= 6
.- Returns: <integer>
Reads byteLength
number of bytes from buf
at the specified offset
and interprets the result as an unsigned, little-endian integer supporting
up to 48 bits of accuracy.
This function is also available under the readUintLE
alias.
import { Buffer } from 'node:buffer';
const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);
console.log(buf.readUIntLE(0, 6).toString(16));
// Prints: ab9078563412
const { Buffer } = require('node:buffer');
const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);
console.log(buf.readUIntLE(0, 6).toString(16));
// Prints: ab9078563412
buf.subarray([start[, end]])
#
start
<integer> Where the newBuffer
will start. Default:0
.end
<integer> Where the newBuffer
will end (not inclusive). Default:buf.length
.- Returns: <Buffer>
Returns a new Buffer
that references the same memory as the original, but
offset and cropped by the start
and end
indices.
Specifying end
greater than buf.length
will return the same result as
that of end
equal to buf.length
.
This method is inherited from TypedArray.prototype.subarray()
.
Modifying the new Buffer
slice will modify the memory in the original Buffer
because the allocated memory of the two objects overlap.
import { Buffer } from 'node:buffer';
// Create a `Buffer` with the ASCII alphabet, take a slice, and modify one byte
// from the original `Buffer`.
const buf1 = Buffer.allocUnsafe(26);
for (let i = 0; i < 26; i++) {
// 97 is the decimal ASCII value for 'a'.
buf1[i] = i + 97;
}
const buf2 = buf1.subarray(0, 3);
console.log(buf2.toString('ascii', 0, buf2.length));
// Prints: abc
buf1[0] = 33;
console.log(buf2.toString('ascii', 0, buf2.length));
// Prints: !bc
const { Buffer } = require('node:buffer');
// Create a `Buffer` with the ASCII alphabet, take a slice, and modify one byte
// from the original `Buffer`.
const buf1 = Buffer.allocUnsafe(26);
for (let i = 0; i < 26; i++) {
// 97 is the decimal ASCII value for 'a'.
buf1[i] = i + 97;
}
const buf2 = buf1.subarray(0, 3);
console.log(buf2.toString('ascii', 0, buf2.length));
// Prints: abc
buf1[0] = 33;
console.log(buf2.toString('ascii', 0, buf2.length));
// Prints: !bc
Specifying negative indexes causes the slice to be generated relative to the
end of buf
rather than the beginning.
import { Buffer } from 'node:buffer';
const buf = Buffer.from('buffer');
console.log(buf.subarray(-6, -1).toString());
// Prints: buffe
// (Equivalent to buf.subarray(0, 5).)
console.log(buf.subarray(-6, -2).toString());
// Prints: buff
// (Equivalent to buf.subarray(0, 4).)
console.log(buf.subarray(-5, -2).toString());
// Prints: uff
// (Equivalent to buf.subarray(1, 4).)
const { Buffer } = require('node:buffer');
const buf = Buffer.from('buffer');
console.log(buf.subarray(-6, -1).toString());
// Prints: buffe
// (Equivalent to buf.subarray(0, 5).)
console.log(buf.subarray(-6, -2).toString());
// Prints: buff
// (Equivalent to buf.subarray(0, 4).)
console.log(buf.subarray(-5, -2).toString());
// Prints: uff
// (Equivalent to buf.subarray(1, 4).)
buf.slice([start[, end]])
#
start
<integer> Where the newBuffer
will start. Default:0
.end
<integer> Where the newBuffer
will end (not inclusive). Default:buf.length
.- Returns: <Buffer>
buf.subarray
instead.Returns a new Buffer
that references the same memory as the original, but
offset and cropped by the start
and end
indices.
This method is not compatible with the Uint8Array.prototype.slice()
,
which is a superclass of Buffer
. To copy the slice, use
Uint8Array.prototype.slice()
.
import { Buffer } from 'node:buffer';
const buf = Buffer.from('buffer');
const copiedBuf = Uint8Array.prototype.slice.call(buf);
copiedBuf[0]++;
console.log(copiedBuf.toString());
// Prints: cuffer
console.log(buf.toString());
// Prints: buffer
// With buf.slice(), the original buffer is modified.
const notReallyCopiedBuf = buf.slice();
notReallyCopiedBuf[0]++;
console.log(notReallyCopiedBuf.toString());
// Prints: cuffer
console.log(buf.toString());
// Also prints: cuffer (!)
const { Buffer } = require('node:buffer');
const buf = Buffer.from('buffer');
const copiedBuf = Uint8Array.prototype.slice.call(buf);
copiedBuf[0]++;
console.log(copiedBuf.toString());
// Prints: cuffer
console.log(buf.toString());
// Prints: buffer
// With buf.slice(), the original buffer is modified.
const notReallyCopiedBuf = buf.slice();
notReallyCopiedBuf[0]++;
console.log(notReallyCopiedBuf.toString());
// Prints: cuffer
console.log(buf.toString());
// Also prints: cuffer (!)
buf.swap16()
#
- Returns: <Buffer> A reference to
buf
.
Interprets buf
as an array of unsigned 16-bit integers and swaps the
byte order in-place. Throws ERR_INVALID_BUFFER_SIZE
if buf.length
is not a multiple of 2.
import { Buffer } from 'node:buffer';
const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);
console.log(buf1);
// Prints: <Buffer 01 02 03 04 05 06 07 08>
buf1.swap16();
console.log(buf1);
// Prints: <Buffer 02 01 04 03 06 05 08 07>
const buf2 = Buffer.from([0x1, 0x2, 0x3]);
buf2.swap16();
// Throws ERR_INVALID_BUFFER_SIZE.
const { Buffer } = require('node:buffer');
const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);
console.log(buf1);
// Prints: <Buffer 01 02 03 04 05 06 07 08>
buf1.swap16();
console.log(buf1);
// Prints: <Buffer 02 01 04 03 06 05 08 07>
const buf2 = Buffer.from([0x1, 0x2, 0x3]);
buf2.swap16();
// Throws ERR_INVALID_BUFFER_SIZE.
One convenient use of buf.swap16()
is to perform a fast in-place conversion
between UTF-16 little-endian and UTF-16 big-endian:
import { Buffer } from 'node:buffer';
const buf = Buffer.from('This is little-endian UTF-16', 'utf16le');
buf.swap16(); // Convert to big-endian UTF-16 text.
const { Buffer } = require('node:buffer');
const buf = Buffer.from('This is little-endian UTF-16', 'utf16le');
buf.swap16(); // Convert to big-endian UTF-16 text.
buf.swap32()
#
- Returns: <Buffer> A reference to
buf
.
Interprets buf
as an array of unsigned 32-bit integers and swaps the
byte order in-place. Throws ERR_INVALID_BUFFER_SIZE
if buf.length
is not a multiple of 4.
import { Buffer } from 'node:buffer';
const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);
console.log(buf1);
// Prints: <Buffer 01 02 03 04 05 06 07 08>
buf1.swap32();
console.log(buf1);
// Prints: <Buffer 04 03 02 01 08 07 06 05>
const buf2 = Buffer.from([0x1, 0x2, 0x3]);
buf2.swap32();
// Throws ERR_INVALID_BUFFER_SIZE.
const { Buffer } = require('node:buffer');
const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);
console.log(buf1);
// Prints: <Buffer 01 02 03 04 05 06 07 08>
buf1.swap32();
console.log(buf1);
// Prints: <Buffer 04 03 02 01 08 07 06 05>
const buf2 = Buffer.from([0x1, 0x2, 0x3]);
buf2.swap32();
// Throws ERR_INVALID_BUFFER_SIZE.
buf.swap64()
#
- Returns: <Buffer> A reference to
buf
.
Interprets buf
as an array of 64-bit numbers and swaps byte order in-place.
Throws ERR_INVALID_BUFFER_SIZE
if buf.length
is not a multiple of 8.
import { Buffer } from 'node:buffer';
const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);
console.log(buf1);
// Prints: <Buffer 01 02 03 04 05 06 07 08>
buf1.swap64();
console.log(buf1);
// Prints: <Buffer 08 07 06 05 04 03 02 01>
const buf2 = Buffer.from([0x1, 0x2, 0x3]);
buf2.swap64();
// Throws ERR_INVALID_BUFFER_SIZE.
const { Buffer } = require('node:buffer');
const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);
console.log(buf1);
// Prints: <Buffer 01 02 03 04 05 06 07 08>
buf1.swap64();
console.log(buf1);
// Prints: <Buffer 08 07 06 05 04 03 02 01>
const buf2 = Buffer.from([0x1, 0x2, 0x3]);
buf2.swap64();
// Throws ERR_INVALID_BUFFER_SIZE.
buf.toJSON()
#
- Returns: <Object>
Returns a JSON representation of buf
. JSON.stringify()
implicitly calls
this function when stringifying a Buffer
instance.
Buffer.from()
accepts objects in the format returned from this method.
In particular, Buffer.from(buf.toJSON())
works like Buffer.from(buf)
.
import { Buffer } from 'node:buffer';
const buf = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5]);
const json = JSON.stringify(buf);
console.log(json);
// Prints: {"type":"Buffer","data":[1,2,3,4,5]}
const copy = JSON.parse(json, (key, value) => {
return value && value.type === 'Buffer' ?
Buffer.from(value) :
value;
});
console.log(copy);
// Prints: <Buffer 01 02 03 04 05>
const { Buffer } = require('node:buffer');
const buf = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5]);
const json = JSON.stringify(buf);
console.log(json);
// Prints: {"type":"Buffer","data":[1,2,3,4,5]}
const copy = JSON.parse(json, (key, value) => {
return value && value.type === 'Buffer' ?
Buffer.from(value) :
value;
});
console.log(copy);
// Prints: <Buffer 01 02 03 04 05>
buf.toString([encoding[, start[, end]]])
#
encoding
<string> The character encoding to use. Default:'utf8'
.start
<integer> The byte offset to start decoding at. Default:0
.end
<integer> The byte offset to stop decoding at (not inclusive). Default:buf.length
.- Returns: <string>
Decodes buf
to a string according to the specified character encoding in
encoding
. start
and end
may be passed to decode only a subset of buf
.
If encoding
is 'utf8'
and a byte sequence in the input is not valid UTF-8,
then each invalid byte is replaced with the replacement character U+FFFD
.
The maximum length of a string instance (in UTF-16 code units) is available
as buffer.constants.MAX_STRING_LENGTH
.
import { Buffer } from 'node:buffer';
const buf1 = Buffer.allocUnsafe(26);
for (let i = 0; i < 26; i++) {
// 97 is the decimal ASCII value for 'a'.
buf1[i] = i + 97;
}
console.log(buf1.toString('utf8'));
// Prints: abcdefghijklmnopqrstuvwxyz
console.log(buf1.toString('utf8', 0, 5));
// Prints: abcde
const buf2 = Buffer.from('tést');
console.log(buf2.toString('hex'));
// Prints: 74c3a97374
console.log(buf2.toString('utf8', 0, 3));
// Prints: té
console.log(buf2.toString(undefined, 0, 3));
// Prints: té
const { Buffer } = require('node:buffer');
const buf1 = Buffer.allocUnsafe(26);
for (let i = 0; i < 26; i++) {
// 97 is the decimal ASCII value for 'a'.
buf1[i] = i + 97;
}
console.log(buf1.toString('utf8'));
// Prints: abcdefghijklmnopqrstuvwxyz
console.log(buf1.toString('utf8', 0, 5));
// Prints: abcde
const buf2 = Buffer.from('tést');
console.log(buf2.toString('hex'));
// Prints: 74c3a97374
console.log(buf2.toString('utf8', 0, 3));
// Prints: té
console.log(buf2.toString(undefined, 0, 3));
// Prints: té
buf.values()
#
- Returns: <Iterator>
Creates and returns an iterator for buf
values (bytes). This function is
called automatically when a Buffer
is used in a for..of
statement.
import { Buffer } from 'node:buffer';
const buf = Buffer.from('buffer');
for (const value of buf.values()) {
console.log(value);
}
// Prints:
// 98
// 117
// 102
// 102
// 101
// 114
for (const value of buf) {
console.log(value);
}
// Prints:
// 98
// 117
// 102
// 102
// 101
// 114
const { Buffer } = require('node:buffer');
const buf = Buffer.from('buffer');
for (const value of buf.values()) {
console.log(value);
}
// Prints:
// 98
// 117
// 102
// 102
// 101
// 114
for (const value of buf) {
console.log(value);
}
// Prints:
// 98
// 117
// 102
// 102
// 101
// 114
buf.write(string[, offset[, length]][, encoding])
#
string
<string> String to write tobuf
.offset
<integer> Number of bytes to skip before starting to writestring
. Default:0
.length
<integer> Maximum number of bytes to write (written bytes will not exceedbuf.length - offset
). Default:buf.length - offset
.encoding
<string> The character encoding ofstring
. Default:'utf8'
.- Returns: <integer> Number of bytes written.
Writes string
to buf
at offset
according to the character encoding in
encoding
. The length
parameter is the number of bytes to write. If buf
did
not contain enough space to fit the entire string, only part of string
will be
written. However, partially encoded characters will not be written.
import { Buffer } from 'node:buffer';
const buf = Buffer.alloc(256);
const len = buf.write('\u00bd + \u00bc = \u00be', 0);
console.log(`${len} bytes: ${buf.toString('utf8', 0, len)}`);
// Prints: 12 bytes: ½ + ¼ = ¾
const buffer = Buffer.alloc(10);
const length = buffer.write('abcd', 8);
console.log(`${length} bytes: ${buffer.toString('utf8', 8, 10)}`);
// Prints: 2 bytes : ab
const { Buffer } = require('node:buffer');
const buf = Buffer.alloc(256);
const len = buf.write('\u00bd + \u00bc = \u00be', 0);
console.log(`${len} bytes: ${buf.toString('utf8', 0, len)}`);
// Prints: 12 bytes: ½ + ¼ = ¾
const buffer = Buffer.alloc(10);
const length = buffer.write('abcd', 8);
console.log(`${length} bytes: ${buffer.toString('utf8', 8, 10)}`);
// Prints: 2 bytes : ab
buf.writeBigInt64BE(value[, offset])
#
value
<bigint> Number to be written tobuf
.offset
<integer> Number of bytes to skip before starting to write. Must satisfy:0 <= offset <= buf.length - 8
. Default:0
.- Returns: <integer>
offset
plus the number of bytes written.
Writes value
to buf
at the specified offset
as big-endian.
value
is interpreted and written as a two's complement signed integer.
import { Buffer } from 'node:buffer';
const buf = Buffer.allocUnsafe(8);
buf.writeBigInt64BE(0x0102030405060708n, 0);
console.log(buf);
// Prints: <Buffer 01 02 03 04 05 06 07 08>
const { Buffer } = require('node:buffer');
const buf = Buffer.allocUnsafe(8);
buf.writeBigInt64BE(0x0102030405060708n, 0);
console.log(buf);
// Prints: <Buffer 01 02 03 04 05 06 07 08>
buf.writeBigInt64LE(value[, offset])
#
value
<bigint> Number to be written tobuf
.offset
<integer> Number of bytes to skip before starting to write. Must satisfy:0 <= offset <= buf.length - 8
. Default:0
.- Returns: <integer>
offset
plus the number of bytes written.
Writes value
to buf
at the specified offset
as little-endian.
value
is interpreted and written as a two's complement signed integer.
import { Buffer } from 'node:buffer';
const buf = Buffer.allocUnsafe(8);
buf.writeBigInt64LE(0x0102030405060708n, 0);
console.log(buf);
// Prints: <Buffer 08 07 06 05 04 03 02 01>
const { Buffer } = require('node:buffer');
const buf = Buffer.allocUnsafe(8);
buf.writeBigInt64LE(0x0102030405060708n, 0);
console.log(buf);
// Prints: <Buffer 08 07 06 05 04 03 02 01>
buf.writeBigUInt64BE(value[, offset])
#
value
<bigint> Number to be written tobuf
.offset
<integer> Number of bytes to skip before starting to write. Must satisfy:0 <= offset <= buf.length - 8
. Default:0
.- Returns: <integer>
offset
plus the number of bytes written.
Writes value
to buf
at the specified offset
as big-endian.
This function is also available under the writeBigUint64BE
alias.
import { Buffer } from 'node:buffer';
const buf = Buffer.allocUnsafe(8);
buf.writeBigUInt64BE(0xdecafafecacefaden, 0);
console.log(buf);
// Prints: <Buffer de ca fa fe ca ce fa de>
const { Buffer } = require('node:buffer');
const buf = Buffer.allocUnsafe(8);
buf.writeBigUInt64BE(0xdecafafecacefaden, 0);
console.log(buf);
// Prints: <Buffer de ca fa fe ca ce fa de>
buf.writeBigUInt64LE(value[, offset])
#
value
<bigint> Number to be written tobuf
.offset
<integer> Number of bytes to skip before starting to write. Must satisfy:0 <= offset <= buf.length - 8
. Default:0
.- Returns: <integer>
offset
plus the number of bytes written.
Writes value
to buf
at the specified offset
as little-endian
import { Buffer } from 'node:buffer';
const buf = Buffer.allocUnsafe(8);
buf.writeBigUInt64LE(0xdecafafecacefaden, 0);
console.log(buf);
// Prints: <Buffer de fa ce ca fe fa ca de>
const { Buffer } = require('node:buffer');
const buf = Buffer.allocUnsafe(8);
buf.writeBigUInt64LE(0xdecafafecacefaden, 0);
console.log(buf);
// Prints: <Buffer de fa ce ca fe fa ca de>
This function is also available under the writeBigUint64LE
alias.
buf.writeDoubleBE(value[, offset])
#
value
<number> Number to be written tobuf
.offset
<integer> Number of bytes to skip before starting to write. Must satisfy0 <= offset <= buf.length - 8
. Default:0
.- Returns: <integer>
offset
plus the number of bytes written.
Writes value
to buf
at the specified offset
as big-endian. The value
must be a JavaScript number. Behavior is undefined when value
is anything
other than a JavaScript number.
import { Buffer } from 'node:buffer';
const buf = Buffer.allocUnsafe(8);
buf.writeDoubleBE(123.456, 0);
console.log(buf);
// Prints: <Buffer 40 5e dd 2f 1a 9f be 77>
const { Buffer } = require('node:buffer');
const buf = Buffer.allocUnsafe(8);
buf.writeDoubleBE(123.456, 0);
console.log(buf);
// Prints: <Buffer 40 5e dd 2f 1a 9f be 77>
buf.writeDoubleLE(value[, offset])
#
value
<number> Number to be written tobuf
.offset
<integer> Number of bytes to skip before starting to write. Must satisfy0 <= offset <= buf.length - 8
. Default:0
.- Returns: <integer>
offset
plus the number of bytes written.
Writes value
to buf
at the specified offset
as little-endian. The value
must be a JavaScript number. Behavior is undefined when value
is anything
other than a JavaScript number.
import { Buffer } from 'node:buffer';
const buf = Buffer.allocUnsafe(8);
buf.writeDoubleLE(123.456, 0);
console.log(buf);
// Prints: <Buffer 77 be 9f 1a 2f dd 5e 40>
const { Buffer } = require('node:buffer');
const buf = Buffer.allocUnsafe(8);
buf.writeDoubleLE(123.456, 0);
console.log(buf);
// Prints: <Buffer 77 be 9f 1a 2f dd 5e 40>
buf.writeFloatBE(value[, offset])
#
value
<number> Number to be written tobuf
.offset
<integer> Number of bytes to skip before starting to write. Must satisfy0 <= offset <= buf.length - 4
. Default:0
.- Returns: <integer>
offset
plus the number of bytes written.
Writes value
to buf
at the specified offset
as big-endian. Behavior is
undefined when value
is anything other than a JavaScript number.
import { Buffer } from 'node:buffer';
const buf = Buffer.allocUnsafe(4);
buf.writeFloatBE(0xcafebabe, 0);
console.log(buf);
// Prints: <Buffer 4f 4a fe bb>
const { Buffer } = require('node:buffer');
const buf = Buffer.allocUnsafe(4);
buf.writeFloatBE(0xcafebabe, 0);
console.log(buf);
// Prints: <Buffer 4f 4a fe bb>
buf.writeFloatLE(value[, offset])
#
value
<number> Number to be written tobuf
.offset
<integer> Number of bytes to skip before starting to write. Must satisfy0 <= offset <= buf.length - 4
. Default:0
.- Returns: <integer>
offset
plus the number of bytes written.
Writes value
to buf
at the specified offset
as little-endian. Behavior is
undefined when value
is anything other than a JavaScript number.
import { Buffer } from 'node:buffer';
const buf = Buffer.allocUnsafe(4);
buf.writeFloatLE(0xcafebabe, 0);
console.log(buf);
// Prints: <Buffer bb fe 4a 4f>
const { Buffer } = require('node:buffer');
const buf = Buffer.allocUnsafe(4);
buf.writeFloatLE(0xcafebabe, 0);
console.log(buf);
// Prints: <Buffer bb fe 4a 4f>
buf.writeInt8(value[, offset])
#
value
<integer> Number to be written tobuf
.offset
<integer> Number of bytes to skip before starting to write. Must satisfy0 <= offset <= buf.length - 1
. Default:0
.- Returns: <integer>
offset
plus the number of bytes written.
Writes value
to buf
at the specified offset
. value
must be a valid
signed 8-bit integer. Behavior is undefined when value
is anything other than
a signed 8-bit integer.
value
is interpreted and written as a two's complement signed integer.
import { Buffer } from 'node:buffer';
const buf = Buffer.allocUnsafe(2);
buf.writeInt8(2, 0);
buf.writeInt8(-2, 1);
console.log(buf);
// Prints: <Buffer 02 fe>
const { Buffer } = require('node:buffer');
const buf = Buffer.allocUnsafe(2);
buf.writeInt8(2, 0);
buf.writeInt8(-2, 1);
console.log(buf);
// Prints: <Buffer 02 fe>
buf.writeInt16BE(value[, offset])
#
value
<integer> Number to be written tobuf
.offset
<integer> Number of bytes to skip before starting to write. Must satisfy0 <= offset <= buf.length - 2
. Default:0
.- Returns: <integer>
offset
plus the number of bytes written.
Writes value
to buf
at the specified offset
as big-endian. The value
must be a valid signed 16-bit integer. Behavior is undefined when value
is
anything other than a signed 16-bit integer.
The value
is interpreted and written as a two's complement signed integer.
import { Buffer } from 'node:buffer';
const buf = Buffer.allocUnsafe(2);
buf.writeInt16BE(0x0102, 0);
console.log(buf);
// Prints: <Buffer 01 02>
const { Buffer } = require('node:buffer');
const buf = Buffer.allocUnsafe(2);
buf.writeInt16BE(0x0102, 0);
console.log(buf);
// Prints: <Buffer 01 02>
buf.writeInt16LE(value[, offset])
#
value
<integer> Number to be written tobuf
.offset
<integer> Number of bytes to skip before starting to write. Must satisfy0 <= offset <= buf.length - 2
. Default:0
.- Returns: <integer>
offset
plus the number of bytes written.
Writes value
to buf
at the specified offset
as little-endian. The value
must be a valid signed 16-bit integer. Behavior is undefined when value
is
anything other than a signed 16-bit integer.
The value
is interpreted and written as a two's complement signed integer.
import { Buffer } from 'node:buffer';
const buf = Buffer.allocUnsafe(2);
buf.writeInt16LE(0x0304, 0);
console.log(buf);
// Prints: <Buffer 04 03>
const { Buffer } = require('node:buffer');
const buf = Buffer.allocUnsafe(2);
buf.writeInt16LE(0x0304, 0);
console.log(buf);
// Prints: <Buffer 04 03>
buf.writeInt32BE(value[, offset])
#
value
<integer> Number to be written tobuf
.offset
<integer> Number of bytes to skip before starting to write. Must satisfy0 <= offset <= buf.length - 4
. Default:0
.- Returns: <integer>
offset
plus the number of bytes written.
Writes value
to buf
at the specified offset
as big-endian. The value
must be a valid signed 32-bit integer. Behavior is undefined when value
is
anything other than a signed 32-bit integer.
The value
is interpreted and written as a two's complement signed integer.
import { Buffer } from 'node:buffer';
const buf = Buffer.allocUnsafe(4);
buf.writeInt32BE(0x01020304, 0);
console.log(buf);
// Prints: <Buffer 01 02 03 04>
const { Buffer } = require('node:buffer');
const buf = Buffer.allocUnsafe(4);
buf.writeInt32BE(0x01020304, 0);
console.log(buf);
// Prints: <Buffer 01 02 03 04>
buf.writeInt32LE(value[, offset])
#
value
<integer> Number to be written tobuf
.offset
<integer> Number of bytes to skip before starting to write. Must satisfy0 <= offset <= buf.length - 4
. Default:0
.- Returns: <integer>
offset
plus the number of bytes written.
Writes value
to buf
at the specified offset
as little-endian. The value
must be a valid signed 32-bit integer. Behavior is undefined when value
is
anything other than a signed 32-bit integer.
The value
is interpreted and written as a two's complement signed integer.
import { Buffer } from 'node:buffer';
const buf = Buffer.allocUnsafe(4);
buf.writeInt32LE(0x05060708, 0);
console.log(buf);
// Prints: <Buffer 08 07 06 05>
const { Buffer } = require('node:buffer');
const buf = Buffer.allocUnsafe(4);
buf.writeInt32LE(0x05060708, 0);
console.log(buf);
// Prints: <Buffer 08 07 06 05>
buf.writeIntBE(value, offset, byteLength)
#
value
<integer> Number to be written tobuf
.offset
<integer> Number of bytes to skip before starting to write. Must satisfy0 <= offset <= buf.length - byteLength
.byteLength
<integer> Number of bytes to write. Must satisfy0 < byteLength <= 6
.- Returns: <integer>
offset
plus the number of bytes written.
Writes byteLength
bytes of value
to buf
at the specified offset
as big-endian. Supports up to 48 bits of accuracy. Behavior is undefined when
value
is anything other than a signed integer.
import { Buffer } from 'node:buffer';
const buf = Buffer.allocUnsafe(6);
buf.writeIntBE(0x1234567890ab, 0, 6);
console.log(buf);
// Prints: <Buffer 12 34 56 78 90 ab>
const { Buffer } = require('node:buffer');
const buf = Buffer.allocUnsafe(6);
buf.writeIntBE(0x1234567890ab, 0, 6);
console.log(buf);
// Prints: <Buffer 12 34 56 78 90 ab>
buf.writeIntLE(value, offset, byteLength)
#
value
<integer> Number to be written tobuf
.offset
<integer> Number of bytes to skip before starting to write. Must satisfy0 <= offset <= buf.length - byteLength
.byteLength
<integer> Number of bytes to write. Must satisfy0 < byteLength <= 6
.- Returns: <integer>
offset
plus the number of bytes written.
Writes byteLength
bytes of value
to buf
at the specified offset
as little-endian. Supports up to 48 bits of accuracy. Behavior is undefined
when value
is anything other than a signed integer.
import { Buffer } from 'node:buffer';
const buf = Buffer.allocUnsafe(6);
buf.writeIntLE(0x1234567890ab, 0, 6);
console.log(buf);
// Prints: <Buffer ab 90 78 56 34 12>
const { Buffer } = require('node:buffer');
const buf = Buffer.allocUnsafe(6);
buf.writeIntLE(0x1234567890ab, 0, 6);
console.log(buf);
// Prints: <Buffer ab 90 78 56 34 12>
buf.writeUInt8(value[, offset])
#
value
<integer> Number to be written tobuf
.offset
<integer> Number of bytes to skip before starting to write. Must satisfy0 <= offset <= buf.length - 1
. Default:0
.- Returns: <integer>
offset
plus the number of bytes written.
Writes value
to buf
at the specified offset
. value
must be a
valid unsigned 8-bit integer. Behavior is undefined when value
is anything
other than an unsigned 8-bit integer.
This function is also available under the writeUint8
alias.
import { Buffer } from 'node:buffer';
const buf = Buffer.allocUnsafe(4);
buf.writeUInt8(0x3, 0);
buf.writeUInt8(0x4, 1);
buf.writeUInt8(0x23, 2);
buf.writeUInt8(0x42, 3);
console.log(buf);
// Prints: <Buffer 03 04 23 42>
const { Buffer } = require('node:buffer');
const buf = Buffer.allocUnsafe(4);
buf.writeUInt8(0x3, 0);
buf.writeUInt8(0x4, 1);
buf.writeUInt8(0x23, 2);
buf.writeUInt8(0x42, 3);
console.log(buf);
// Prints: <Buffer 03 04 23 42>
buf.writeUInt16BE(value[, offset])
#
value
<integer> Number to be written tobuf
.offset
<integer> Number of bytes to skip before starting to write. Must satisfy0 <= offset <= buf.length - 2
. Default:0
.- Returns: <integer>
offset
plus the number of bytes written.
Writes value
to buf
at the specified offset
as big-endian. The value
must be a valid unsigned 16-bit integer. Behavior is undefined when value
is anything other than an unsigned 16-bit integer.
This function is also available under the writeUint16BE
alias.
import { Buffer } from 'node:buffer';
const buf = Buffer.allocUnsafe(4);
buf.writeUInt16BE(0xdead, 0);
buf.writeUInt16BE(0xbeef, 2);
console.log(buf);
// Prints: <Buffer de ad be ef>
const { Buffer } = require('node:buffer');
const buf = Buffer.allocUnsafe(4);
buf.writeUInt16BE(0xdead, 0);
buf.writeUInt16BE(0xbeef, 2);
console.log(buf);
// Prints: <Buffer de ad be ef>
buf.writeUInt16LE(value[, offset])
#
value
<integer> Number to be written tobuf
.offset
<integer> Number of bytes to skip before starting to write. Must satisfy0 <= offset <= buf.length - 2
. Default:0
.- Returns: <integer>
offset
plus the number of bytes written.
Writes value
to buf
at the specified offset
as little-endian. The value
must be a valid unsigned 16-bit integer. Behavior is undefined when value
is
anything other than an unsigned 16-bit integer.
This function is also available under the writeUint16LE
alias.
import { Buffer } from 'node:buffer';
const buf = Buffer.allocUnsafe(4);
buf.writeUInt16LE(0xdead, 0);
buf.writeUInt16LE(0xbeef, 2);
console.log(buf);
// Prints: <Buffer ad de ef be>
const { Buffer } = require('node:buffer');
const buf = Buffer.allocUnsafe(4);
buf.writeUInt16LE(0xdead, 0);
buf.writeUInt16LE(0xbeef, 2);
console.log(buf);
// Prints: <Buffer ad de ef be>
buf.writeUInt32BE(value[, offset])
#
value
<integer> Number to be written tobuf
.offset
<integer> Number of bytes to skip before starting to write. Must satisfy0 <= offset <= buf.length - 4
. Default:0
.- Returns: <integer>
offset
plus the number of bytes written.
Writes value
to buf
at the specified offset
as big-endian. The value
must be a valid unsigned 32-bit integer. Behavior is undefined when value
is anything other than an unsigned 32-bit integer.
This function is also available under the writeUint32BE
alias.
import { Buffer } from 'node:buffer';
const buf = Buffer.allocUnsafe(4);
buf.writeUInt32BE(0xfeedface, 0);
console.log(buf);
// Prints: <Buffer fe ed fa ce>
const { Buffer } = require('node:buffer');
const buf = Buffer.allocUnsafe(4);
buf.writeUInt32BE(0xfeedface, 0);
console.log(buf);
// Prints: <Buffer fe ed fa ce>
buf.writeUInt32LE(value[, offset])
#
value
<integer> Number to be written tobuf
.offset
<integer> Number of bytes to skip before starting to write. Must satisfy0 <= offset <= buf.length - 4
. Default:0
.- Returns: <integer>
offset
plus the number of bytes written.
Writes value
to buf
at the specified offset
as little-endian. The value
must be a valid unsigned 32-bit integer. Behavior is undefined when value
is
anything other than an unsigned 32-bit integer.
This function is also available under the writeUint32LE
alias.
import { Buffer } from 'node:buffer';
const buf = Buffer.allocUnsafe(4);
buf.writeUInt32LE(0xfeedface, 0);
console.log(buf);
// Prints: <Buffer ce fa ed fe>
const { Buffer } = require('node:buffer');
const buf = Buffer.allocUnsafe(4);
buf.writeUInt32LE(0xfeedface, 0);
console.log(buf);
// Prints: <Buffer ce fa ed fe>
buf.writeUIntBE(value, offset, byteLength)
#
value
<integer> Number to be written tobuf
.offset
<integer> Number of bytes to skip before starting to write. Must satisfy0 <= offset <= buf.length - byteLength
.byteLength
<integer> Number of bytes to write. Must satisfy0 < byteLength <= 6
.- Returns: <integer>
offset
plus the number of bytes written.
Writes byteLength
bytes of value
to buf
at the specified offset
as big-endian. Supports up to 48 bits of accuracy. Behavior is undefined
when value
is anything other than an unsigned integer.
This function is also available under the writeUintBE
alias.
import { Buffer } from 'node:buffer';
const buf = Buffer.allocUnsafe(6);
buf.writeUIntBE(0x1234567890ab, 0, 6);
console.log(buf);
// Prints: <Buffer 12 34 56 78 90 ab>
const { Buffer } = require('node:buffer');
const buf = Buffer.allocUnsafe(6);
buf.writeUIntBE(0x1234567890ab, 0, 6);
console.log(buf);
// Prints: <Buffer 12 34 56 78 90 ab>
buf.writeUIntLE(value, offset, byteLength)
#
value
<integer> Number to be written tobuf
.offset
<integer> Number of bytes to skip before starting to write. Must satisfy0 <= offset <= buf.length - byteLength
.byteLength
<integer> Number of bytes to write. Must satisfy0 < byteLength <= 6
.- Returns: <integer>
offset
plus the number of bytes written.
Writes byteLength
bytes of value
to buf
at the specified offset
as little-endian. Supports up to 48 bits of accuracy. Behavior is undefined
when value
is anything other than an unsigned integer.
This function is also available under the writeUintLE
alias.
import { Buffer } from 'node:buffer';
const buf = Buffer.allocUnsafe(6);
buf.writeUIntLE(0x1234567890ab, 0, 6);
console.log(buf);
// Prints: <Buffer ab 90 78 56 34 12>
const { Buffer } = require('node:buffer');
const buf = Buffer.allocUnsafe(6);
buf.writeUIntLE(0x1234567890ab, 0, 6);
console.log(buf);
// Prints: <Buffer ab 90 78 56 34 12>
new Buffer(array)
#
Buffer.from(array)
instead.array
<integer[]> An array of bytes to copy from.
See Buffer.from(array)
.
new Buffer(arrayBuffer[, byteOffset[, length]])
#
Buffer.from(arrayBuffer[, byteOffset[, length]])
instead.arrayBuffer
<ArrayBuffer> | <SharedArrayBuffer> AnArrayBuffer
,SharedArrayBuffer
or the.buffer
property of aTypedArray
.byteOffset
<integer> Index of first byte to expose. Default:0
.length
<integer> Number of bytes to expose. Default:arrayBuffer.byteLength - byteOffset
.
See
Buffer.from(arrayBuffer[, byteOffset[, length]])
.
new Buffer(buffer)
#
Buffer.from(buffer)
instead.buffer
<Buffer> | <Uint8Array> An existingBuffer
orUint8Array
from which to copy data.
See Buffer.from(buffer)
.
new Buffer(size)
#
size
<integer> The desired length of the newBuffer
.
See Buffer.alloc()
and Buffer.allocUnsafe()
. This variant of the
constructor is equivalent to Buffer.alloc()
.
new Buffer(string[, encoding])
#
Buffer.from(string[, encoding])
instead.Class: File
#
- Extends: <Blob>
A File
provides information about files.
new buffer.File(sources, fileName[, options])
#
sources
<string[]> | <ArrayBuffer[]> | <TypedArray[]> | <DataView[]> | <Blob[]> | <File[]> An array of string, <ArrayBuffer>, <TypedArray>, <DataView>, <File>, or <Blob> objects, or any mix of such objects, that will be stored within theFile
.fileName
<string> The name of the file.options
<Object>endings
<string> One of either'transparent'
or'native'
. When set to'native'
, line endings in string source parts will be converted to the platform native line-ending as specified byrequire('node:os').EOL
.type
<string> The File content-type.lastModified
<number> The last modified date of the file. Default:Date.now()
.
file.name
#
- Type: <string>
The name of the File
.
file.lastModified
#
- Type: <number>
The last modified date of the File
.
node:buffer
module APIs#
While, the Buffer
object is available as a global, there are additional
Buffer
-related APIs that are available only via the node:buffer
module
accessed using require('node:buffer')
.
buffer.atob(data)
#
Buffer.from(data, 'base64')
instead.data
<any> The Base64-encoded input string.
Decodes a string of Base64-encoded data into bytes, and encodes those bytes into a string using Latin-1 (ISO-8859-1).
The data
may be any JavaScript-value that can be coerced into a string.
This function is only provided for compatibility with legacy web platform APIs
and should never be used in new code, because they use strings to represent
binary data and predate the introduction of typed arrays in JavaScript.
For code running using Node.js APIs, converting between base64-encoded strings
and binary data should be performed using Buffer.from(str, 'base64')
and
buf.toString('base64')
.
buffer.btoa(data)
#
buf.toString('base64')
instead.data
<any> An ASCII (Latin1) string.
Decodes a string into bytes using Latin-1 (ISO-8859), and encodes those bytes into a string using Base64.
The data
may be any JavaScript-value that can be coerced into a string.
This function is only provided for compatibility with legacy web platform APIs
and should never be used in new code, because they use strings to represent
binary data and predate the introduction of typed arrays in JavaScript.
For code running using Node.js APIs, converting between base64-encoded strings
and binary data should be performed using Buffer.from(str, 'base64')
and
buf.toString('base64')
.
buffer.isAscii(input)
#
- input <Buffer> | <ArrayBuffer> | <TypedArray> The input to validate.
- Returns: <boolean>
This function returns true
if input
contains only valid ASCII-encoded data,
including the case in which input
is empty.
Throws if the input
is a detached array buffer.
buffer.isUtf8(input)
#
- input <Buffer> | <ArrayBuffer> | <TypedArray> The input to validate.
- Returns: <boolean>
This function returns true
if input
contains only valid UTF-8-encoded data,
including the case in which input
is empty.
Throws if the input
is a detached array buffer.
buffer.INSPECT_MAX_BYTES
#
- <integer> Default:
50
Returns the maximum number of bytes that will be returned when
buf.inspect()
is called. This can be overridden by user modules. See
util.inspect()
for more details on buf.inspect()
behavior.
buffer.kMaxLength
#
- <integer> The largest size allowed for a single
Buffer
instance.
An alias for buffer.constants.MAX_LENGTH
.
buffer.kStringMaxLength
#
- <integer> The largest length allowed for a single
string
instance.
An alias for buffer.constants.MAX_STRING_LENGTH
.
buffer.resolveObjectURL(id)
#
id
<string> A'blob:nodedata:...
URL string returned by a prior call toURL.createObjectURL()
.- Returns: <Blob>
Resolves a 'blob:nodedata:...'
an associated <Blob> object registered using
a prior call to URL.createObjectURL()
.
buffer.transcode(source, fromEnc, toEnc)
#
source
<Buffer> | <Uint8Array> ABuffer
orUint8Array
instance.fromEnc
<string> The current encoding.toEnc
<string> To target encoding.- Returns: <Buffer>
Re-encodes the given Buffer
or Uint8Array
instance from one character
encoding to another. Returns a new Buffer
instance.
Throws if the fromEnc
or toEnc
specify invalid character encodings or if
conversion from fromEnc
to toEnc
is not permitted.
Encodings supported by buffer.transcode()
are: 'ascii'
, 'utf8'
,
'utf16le'
, 'ucs2'
, 'latin1'
, and 'binary'
.
The transcoding process will use substitution characters if a given byte sequence cannot be adequately represented in the target encoding. For instance:
import { Buffer, transcode } from 'node:buffer';
const newBuf = transcode(Buffer.from('€'), 'utf8', 'ascii');
console.log(newBuf.toString('ascii'));
// Prints: '?'
const { Buffer, transcode } = require('node:buffer');
const newBuf = transcode(Buffer.from('€'), 'utf8', 'ascii');
console.log(newBuf.toString('ascii'));
// Prints: '?'
Because the Euro (€
) sign is not representable in US-ASCII, it is replaced
with ?
in the transcoded Buffer
.
Class: SlowBuffer
#
Buffer.allocUnsafeSlow()
instead.See Buffer.allocUnsafeSlow()
. This was never a class in the sense that
the constructor always returned a Buffer
instance, rather than a SlowBuffer
instance.
new SlowBuffer(size)
#
Buffer.allocUnsafeSlow()
instead.size
<integer> The desired length of the newSlowBuffer
.
Buffer constants#
buffer.constants.MAX_LENGTH
#
- <integer> The largest size allowed for a single
Buffer
instance.
On 32-bit architectures, this value currently is 230 - 1 (about 1 GiB).
On 64-bit architectures, this value currently is 232 (about 4 GiB).
It reflects v8::TypedArray::kMaxLength
under the hood.
This value is also available as buffer.kMaxLength
.
buffer.constants.MAX_STRING_LENGTH
#
- <integer> The largest length allowed for a single
string
instance.
Represents the largest length
that a string
primitive can have, counted
in UTF-16 code units.
This value may depend on the JS engine that is being used.
Buffer.from()
, Buffer.alloc()
, and Buffer.allocUnsafe()
#
In versions of Node.js prior to 6.0.0, Buffer
instances were created using the
Buffer
constructor function, which allocates the returned Buffer
differently based on what arguments are provided:
- Passing a number as the first argument to
Buffer()
(e.g.new Buffer(10)
) allocates a newBuffer
object of the specified size. Prior to Node.js 8.0.0, the memory allocated for suchBuffer
instances is not initialized and can contain sensitive data. SuchBuffer
instances must be subsequently initialized by using eitherbuf.fill(0)
or by writing to the entireBuffer
before reading data from theBuffer
. While this behavior is intentional to improve performance, development experience has demonstrated that a more explicit distinction is required between creating a fast-but-uninitializedBuffer
versus creating a slower-but-saferBuffer
. Since Node.js 8.0.0,Buffer(num)
andnew Buffer(num)
return aBuffer
with initialized memory. - Passing a string, array, or
Buffer
as the first argument copies the passed object's data into theBuffer
. - Passing an
ArrayBuffer
or aSharedArrayBuffer
returns aBuffer
that shares allocated memory with the given array buffer.
Because the behavior of new Buffer()
is different depending on the type of the
first argument, security and reliability issues can be inadvertently introduced
into applications when argument validation or Buffer
initialization is not
performed.
For example, if an attacker can cause an application to receive a number where
a string is expected, the application may call new Buffer(100)
instead of new Buffer("100")
, leading it to allocate a 100 byte buffer instead
of allocating a 3 byte buffer with content "100"
. This is commonly possible
using JSON API calls. Since JSON distinguishes between numeric and string types,
it allows injection of numbers where a naively written application that does not
validate its input sufficiently might expect to always receive a string.
Before Node.js 8.0.0, the 100 byte buffer might contain
arbitrary pre-existing in-memory data, so may be used to expose in-memory
secrets to a remote attacker. Since Node.js 8.0.0, exposure of memory cannot
occur because the data is zero-filled. However, other attacks are still
possible, such as causing very large buffers to be allocated by the server,
leading to performance degradation or crashing on memory exhaustion.
To make the creation of Buffer
instances more reliable and less error-prone,
the various forms of the new Buffer()
constructor have been deprecated
and replaced by separate Buffer.from()
, Buffer.alloc()
, and
Buffer.allocUnsafe()
methods.
Developers should migrate all existing uses of the new Buffer()
constructors
to one of these new APIs.
Buffer.from(array)
returns a newBuffer
that contains a copy of the provided octets.Buffer.from(arrayBuffer[, byteOffset[, length]])
returns a newBuffer
that shares the same allocated memory as the givenArrayBuffer
.Buffer.from(buffer)
returns a newBuffer
that contains a copy of the contents of the givenBuffer
.Buffer.from(string[, encoding])
returns a newBuffer
that contains a copy of the provided string.Buffer.alloc(size[, fill[, encoding]])
returns a new initializedBuffer
of the specified size. This method is slower thanBuffer.allocUnsafe(size)
but guarantees that newly createdBuffer
instances never contain old data that is potentially sensitive. ATypeError
will be thrown ifsize
is not a number.Buffer.allocUnsafe(size)
andBuffer.allocUnsafeSlow(size)
each return a new uninitializedBuffer
of the specifiedsize
. Because theBuffer
is uninitialized, the allocated segment of memory might contain old data that is potentially sensitive.
Buffer
instances returned by Buffer.allocUnsafe()
and
Buffer.from(array)
may be allocated off a shared internal memory pool
if size
is less than or equal to half Buffer.poolSize
. Instances
returned by Buffer.allocUnsafeSlow()
never use the shared internal
memory pool.
The --zero-fill-buffers
command-line option#
Node.js can be started using the --zero-fill-buffers
command-line option to
cause all newly-allocated Buffer
instances to be zero-filled upon creation by
default. Without the option, buffers created with Buffer.allocUnsafe()
,
Buffer.allocUnsafeSlow()
, and new SlowBuffer(size)
are not zero-filled.
Use of this flag can have a measurable negative impact on performance. Use the
--zero-fill-buffers
option only when necessary to enforce that newly allocated
Buffer
instances cannot contain old data that is potentially sensitive.
$ node --zero-fill-buffers
> Buffer.allocUnsafe(5);
<Buffer 00 00 00 00 00>
What makes Buffer.allocUnsafe()
and Buffer.allocUnsafeSlow()
"unsafe"?#
When calling Buffer.allocUnsafe()
and Buffer.allocUnsafeSlow()
, the
segment of allocated memory is uninitialized (it is not zeroed-out). While
this design makes the allocation of memory quite fast, the allocated segment of
memory might contain old data that is potentially sensitive. Using a Buffer
created by Buffer.allocUnsafe()
without completely overwriting the
memory can allow this old data to be leaked when the Buffer
memory is read.
While there are clear performance advantages to using
Buffer.allocUnsafe()
, extra care must be taken in order to avoid
introducing security vulnerabilities into an application.
C++ addons#
Addons are dynamically-linked shared objects written in C++. The
require()
function can load addons as ordinary Node.js modules.
Addons provide an interface between JavaScript and C/C++ libraries.
There are three options for implementing addons: Node-API, nan, or direct use of internal V8, libuv, and Node.js libraries. Unless there is a need for direct access to functionality which is not exposed by Node-API, use Node-API. Refer to C/C++ addons with Node-API for more information on Node-API.
When not using Node-API, implementing addons is complicated, involving knowledge of several components and APIs:
-
V8: the C++ library Node.js uses to provide the JavaScript implementation. V8 provides the mechanisms for creating objects, calling functions, etc. V8's API is documented mostly in the
v8.h
header file (deps/v8/include/v8.h
in the Node.js source tree), which is also available online. -
libuv: The C library that implements the Node.js event loop, its worker threads and all of the asynchronous behaviors of the platform. It also serves as a cross-platform abstraction library, giving easy, POSIX-like access across all major operating systems to many common system tasks, such as interacting with the file system, sockets, timers, and system events. libuv also provides a threading abstraction similar to POSIX threads for more sophisticated asynchronous addons that need to move beyond the standard event loop. Addon authors should avoid blocking the event loop with I/O or other time-intensive tasks by offloading work via libuv to non-blocking system operations, worker threads, or a custom use of libuv threads.
-
Internal Node.js libraries. Node.js itself exports C++ APIs that addons can use, the most important of which is the
node::ObjectWrap
class. -
Node.js includes other statically linked libraries including OpenSSL. These other libraries are located in the
deps/
directory in the Node.js source tree. Only the libuv, OpenSSL, V8, and zlib symbols are purposefully re-exported by Node.js and may be used to various extents by addons. See Linking to libraries included with Node.js for additional information.
All of the following examples are available for download and may be used as the starting-point for an addon.
Hello world#
This "Hello world" example is a simple addon, written in C++, that is the equivalent of the following JavaScript code:
module.exports.hello = () => 'world';
First, create the file hello.cc
:
// hello.cc
#include <node.h>
namespace demo {
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::String;
using v8::Value;
void Method(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
args.GetReturnValue().Set(String::NewFromUtf8(
isolate, "world").ToLocalChecked());
}
void Initialize(Local<Object> exports) {
NODE_SET_METHOD(exports, "hello", Method);
}
NODE_MODULE(NODE_GYP_MODULE_NAME, Initialize)
} // namespace demo
All Node.js addons must export an initialization function following the pattern:
void Initialize(Local<Object> exports);
NODE_MODULE(NODE_GYP_MODULE_NAME, Initialize)
There is no semi-colon after NODE_MODULE
as it's not a function (see
node.h
).
The module_name
must match the filename of the final binary (excluding
the .node
suffix).
In the hello.cc
example, then, the initialization function is Initialize
and the addon module name is addon
.
When building addons with node-gyp
, using the macro NODE_GYP_MODULE_NAME
as
the first parameter of NODE_MODULE()
will ensure that the name of the final
binary will be passed to NODE_MODULE()
.
Addons defined with NODE_MODULE()
can not be loaded in multiple contexts or
multiple threads at the same time.
Context-aware addons#
There are environments in which Node.js addons may need to be loaded multiple
times in multiple contexts. For example, the Electron runtime runs multiple
instances of Node.js in a single process. Each instance will have its own
require()
cache, and thus each instance will need a native addon to behave
correctly when loaded via require()
. This means that the addon
must support multiple initializations.
A context-aware addon can be constructed by using the macro
NODE_MODULE_INITIALIZER
, which expands to the name of a function which Node.js
will expect to find when it loads an addon. An addon can thus be initialized as
in the following example:
using namespace v8;
extern "C" NODE_MODULE_EXPORT void
NODE_MODULE_INITIALIZER(Local<Object> exports,
Local<Value> module,
Local<Context> context) {
/* Perform addon initialization steps here. */
}
Another option is to use the macro NODE_MODULE_INIT()
, which will also
construct a context-aware addon. Unlike NODE_MODULE()
, which is used to
construct an addon around a given addon initializer function,
NODE_MODULE_INIT()
serves as the declaration of such an initializer to be
followed by a function body.
The following three variables may be used inside the function body following an
invocation of NODE_MODULE_INIT()
:
Local<Object> exports
,Local<Value> module
, andLocal<Context> context
The choice to build a context-aware addon carries with it the responsibility of carefully managing global static data. Since the addon may be loaded multiple times, potentially even from different threads, any global static data stored in the addon must be properly protected, and must not contain any persistent references to JavaScript objects. The reason for this is that JavaScript objects are only valid in one context, and will likely cause a crash when accessed from the wrong context or from a different thread than the one on which they were created.
The context-aware addon can be structured to avoid global static data by performing the following steps:
- Define a class which will hold per-addon-instance data and which has a static
member of the form
static void DeleteInstance(void* data) { // Cast `data` to an instance of the class and delete it. }
- Heap-allocate an instance of this class in the addon initializer. This can be
accomplished using the
new
keyword. - Call
node::AddEnvironmentCleanupHook()
, passing it the above-created instance and a pointer toDeleteInstance()
. This will ensure the instance is deleted when the environment is torn down. - Store the instance of the class in a
v8::External
, and - Pass the
v8::External
to all methods exposed to JavaScript by passing it tov8::FunctionTemplate::New()
orv8::Function::New()
which creates the native-backed JavaScript functions. The third parameter ofv8::FunctionTemplate::New()
orv8::Function::New()
accepts thev8::External
and makes it available in the native callback using thev8::FunctionCallbackInfo::Data()
method.
This will ensure that the per-addon-instance data reaches each binding that can be called from JavaScript. The per-addon-instance data must also be passed into any asynchronous callbacks the addon may create.
The following example illustrates the implementation of a context-aware addon:
#include <node.h>
using namespace v8;
class AddonData {
public:
explicit AddonData(Isolate* isolate):
call_count(0) {
// Ensure this per-addon-instance data is deleted at environment cleanup.
node::AddEnvironmentCleanupHook(isolate, DeleteInstance, this);
}
// Per-addon data.
int call_count;
static void DeleteInstance(void* data) {
delete static_cast<AddonData*>(data);
}
};
static void Method(const v8::FunctionCallbackInfo<v8::Value>& info) {
// Retrieve the per-addon-instance data.
AddonData* data =
reinterpret_cast<AddonData*>(info.Data().As<External>()->Value());
data->call_count++;
info.GetReturnValue().Set((double)data->call_count);
}
// Initialize this addon to be context-aware.
NODE_MODULE_INIT(/* exports, module, context */) {
Isolate* isolate = context->GetIsolate();
// Create a new instance of `AddonData` for this instance of the addon and
// tie its life cycle to that of the Node.js environment.
AddonData* data = new AddonData(isolate);
// Wrap the data in a `v8::External` so we can pass it to the method we
// expose.
Local<External> external = External::New(isolate, data);
// Expose the method `Method` to JavaScript, and make sure it receives the
// per-addon-instance data we created above by passing `external` as the
// third parameter to the `FunctionTemplate` constructor.
exports->Set(context,
String::NewFromUtf8(isolate, "method").ToLocalChecked(),
FunctionTemplate::New(isolate, Method, external)
->GetFunction(context).ToLocalChecked()).FromJust();
}
Worker support#
In order to be loaded from multiple Node.js environments, such as a main thread and a Worker thread, an add-on needs to either:
- Be an Node-API addon, or
- Be declared as context-aware using
NODE_MODULE_INIT()
as described above
In order to support Worker
threads, addons need to clean up any resources
they may have allocated when such a thread exists. This can be achieved through
the usage of the AddEnvironmentCleanupHook()
function:
void AddEnvironmentCleanupHook(v8::Isolate* isolate,
void (*fun)(void* arg),
void* arg);
This function adds a hook that will run before a given Node.js instance shuts
down. If necessary, such hooks can be removed before they are run using
RemoveEnvironmentCleanupHook()
, which has the same signature. Callbacks are
run in last-in first-out order.
If necessary, there is an additional pair of AddEnvironmentCleanupHook()
and RemoveEnvironmentCleanupHook()
overloads, where the cleanup hook takes a
callback function. This can be used for shutting down asynchronous resources,
such as any libuv handles registered by the addon.
The following addon.cc
uses AddEnvironmentCleanupHook
:
// addon.cc
#include <node.h>
#include <assert.h>
#include <stdlib.h>
using node::AddEnvironmentCleanupHook;
using v8::HandleScope;
using v8::Isolate;
using v8::Local;
using v8::Object;
// Note: In a real-world application, do not rely on static/global data.
static char cookie[] = "yum yum";
static int cleanup_cb1_called = 0;
static int cleanup_cb2_called = 0;
static void cleanup_cb1(void* arg) {
Isolate* isolate = static_cast<Isolate*>(arg);
HandleScope scope(isolate);
Local<Object> obj = Object::New(isolate);
assert(!obj.IsEmpty()); // assert VM is still alive
assert(obj->IsObject());
cleanup_cb1_called++;
}
static void cleanup_cb2(void* arg) {
assert(arg == static_cast<void*>(cookie));
cleanup_cb2_called++;
}
static void sanity_check(void*) {
assert(cleanup_cb1_called == 1);
assert(cleanup_cb2_called == 1);
}
// Initialize this addon to be context-aware.
NODE_MODULE_INIT(/* exports, module, context */) {
Isolate* isolate = context->GetIsolate();
AddEnvironmentCleanupHook(isolate, sanity_check, nullptr);
AddEnvironmentCleanupHook(isolate, cleanup_cb2, cookie);
AddEnvironmentCleanupHook(isolate, cleanup_cb1, isolate);
}
Test in JavaScript by running:
// test.js
require('./build/Release/addon');
Building#
Once the source code has been written, it must be compiled into the binary
addon.node
file. To do so, create a file called binding.gyp
in the
top-level of the project describing the build configuration of the module
using a JSON-like format. This file is used by node-gyp, a tool written
specifically to compile Node.js addons.
{
"targets": [
{
"target_name": "addon",
"sources": [ "hello.cc" ]
}
]
}
A version of the node-gyp
utility is bundled and distributed with
Node.js as part of npm
. This version is not made directly available for
developers to use and is intended only to support the ability to use the
npm install
command to compile and install addons. Developers who wish to
use node-gyp
directly can install it using the command
npm install -g node-gyp
. See the node-gyp
installation instructions for
more information, including platform-specific requirements.
Once the binding.gyp
file has been created, use node-gyp configure
to
generate the appropriate project build files for the current platform. This
will generate either a Makefile
(on Unix platforms) or a vcxproj
file
(on Windows) in the build/
directory.
Next, invoke the node-gyp build
command to generate the compiled addon.node
file. This will be put into the build/Release/
directory.
When using npm install
to install a Node.js addon, npm uses its own bundled
version of node-gyp
to perform this same set of actions, generating a
compiled version of the addon for the user's platform on demand.
Once built, the binary addon can be used from within Node.js by pointing
require()
to the built addon.node
module:
// hello.js
const addon = require('./build/Release/addon');
console.log(addon.hello());
// Prints: 'world'
Because the exact path to the compiled addon binary can vary depending on how
it is compiled (i.e. sometimes it may be in ./build/Debug/
), addons can use
the bindings package to load the compiled module.
While the bindings
package implementation is more sophisticated in how it
locates addon modules, it is essentially using a try…catch
pattern similar to:
try {
return require('./build/Release/addon.node');
} catch (err) {
return require('./build/Debug/addon.node');
}
Linking to libraries included with Node.js#
Node.js uses statically linked libraries such as V8, libuv, and OpenSSL. All
addons are required to link to V8 and may link to any of the other dependencies
as well. Typically, this is as simple as including the appropriate
#include <...>
statements (e.g. #include <v8.h>
) and node-gyp
will locate
the appropriate headers automatically. However, there are a few caveats to be
aware of:
-
When
node-gyp
runs, it will detect the specific release version of Node.js and download either the full source tarball or just the headers. If the full source is downloaded, addons will have complete access to the full set of Node.js dependencies. However, if only the Node.js headers are downloaded, then only the symbols exported by Node.js will be available. -
node-gyp
can be run using the--nodedir
flag pointing at a local Node.js source image. Using this option, the addon will have access to the full set of dependencies.
Loading addons using require()
#
The filename extension of the compiled addon binary is .node
(as opposed
to .dll
or .so
). The require()
function is written to look for
files with the .node
file extension and initialize those as dynamically-linked
libraries.
When calling require()
, the .node
extension can usually be
omitted and Node.js will still find and initialize the addon. One caveat,
however, is that Node.js will first attempt to locate and load modules or
JavaScript files that happen to share the same base name. For instance, if
there is a file addon.js
in the same directory as the binary addon.node
,
then require('addon')
will give precedence to the addon.js
file
and load it instead.
Native abstractions for Node.js#
Each of the examples illustrated in this document directly use the Node.js and V8 APIs for implementing addons. The V8 API can, and has, changed dramatically from one V8 release to the next (and one major Node.js release to the next). With each change, addons may need to be updated and recompiled in order to continue functioning. The Node.js release schedule is designed to minimize the frequency and impact of such changes but there is little that Node.js can do to ensure stability of the V8 APIs.
The Native Abstractions for Node.js (or nan
) provide a set of tools that
addon developers are recommended to use to keep compatibility between past and
future releases of V8 and Node.js. See the nan
examples for an
illustration of how it can be used.
Node-API#
Node-API is an API for building native addons. It is independent from the underlying JavaScript runtime (e.g. V8) and is maintained as part of Node.js itself. This API will be Application Binary Interface (ABI) stable across versions of Node.js. It is intended to insulate addons from changes in the underlying JavaScript engine and allow modules compiled for one version to run on later versions of Node.js without recompilation. Addons are built/packaged with the same approach/tools outlined in this document (node-gyp, etc.). The only difference is the set of APIs that are used by the native code. Instead of using the V8 or Native Abstractions for Node.js APIs, the functions available in the Node-API are used.
Creating and maintaining an addon that benefits from the ABI stability provided by Node-API carries with it certain implementation considerations.
To use Node-API in the above "Hello world" example, replace the content of
hello.cc
with the following. All other instructions remain the same.
// hello.cc using Node-API
#include <node_api.h>
namespace demo {
napi_value Method(napi_env env, napi_callback_info args) {
napi_value greeting;
napi_status status;
status = napi_create_string_utf8(env, "world", NAPI_AUTO_LENGTH, &greeting);
if (status != napi_ok) return nullptr;
return greeting;
}
napi_value init(napi_env env, napi_value exports) {
napi_status status;
napi_value fn;
status = napi_create_function(env, nullptr, 0, Method, nullptr, &fn);
if (status != napi_ok) return nullptr;
status = napi_set_named_property(env, exports, "hello", fn);
if (status != napi_ok) return nullptr;
return exports;
}
NAPI_MODULE(NODE_GYP_MODULE_NAME, init)
} // namespace demo
The functions available and how to use them are documented in C/C++ addons with Node-API.
Addon examples#
Following are some example addons intended to help developers get started. The examples use the V8 APIs. Refer to the online V8 reference for help with the various V8 calls, and V8's Embedder's Guide for an explanation of several concepts used such as handles, scopes, function templates, etc.
Each of these examples using the following binding.gyp
file:
{
"targets": [
{
"target_name": "addon",
"sources": [ "addon.cc" ]
}
]
}
In cases where there is more than one .cc
file, simply add the additional
filename to the sources
array:
"sources": ["addon.cc", "myexample.cc"]
Once the binding.gyp
file is ready, the example addons can be configured and
built using node-gyp
:
$ node-gyp configure build
Function arguments#
Addons will typically expose objects and functions that can be accessed from JavaScript running within Node.js. When functions are invoked from JavaScript, the input arguments and return value must be mapped to and from the C/C++ code.
The following example illustrates how to read function arguments passed from JavaScript and how to return a result:
// addon.cc
#include <node.h>
namespace demo {
using v8::Exception;
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Number;
using v8::Object;
using v8::String;
using v8::Value;
// This is the implementation of the "add" method
// Input arguments are passed using the
// const FunctionCallbackInfo<Value>& args struct
void Add(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
// Check the number of arguments passed.
if (args.Length() < 2) {
// Throw an Error that is passed back to JavaScript
isolate->ThrowException(Exception::TypeError(
String::NewFromUtf8(isolate,
"Wrong number of arguments").ToLocalChecked()));
return;
}
// Check the argument types
if (!args[0]->IsNumber() || !args[1]->IsNumber()) {
isolate->ThrowException(Exception::TypeError(
String::NewFromUtf8(isolate,
"Wrong arguments").ToLocalChecked()));
return;
}
// Perform the operation
double value =
args[0].As<Number>()->Value() + args[1].As<Number>()->Value();
Local<Number> num = Number::New(isolate, value);
// Set the return value (using the passed in
// FunctionCallbackInfo<Value>&)
args.GetReturnValue().Set(num);
}
void Init(Local<Object> exports) {
NODE_SET_METHOD(exports, "add", Add);
}
NODE_MODULE(NODE_GYP_MODULE_NAME, Init)
} // namespace demo
Once compiled, the example addon can be required and used from within Node.js:
// test.js
const addon = require('./build/Release/addon');
console.log('This should be eight:', addon.add(3, 5));
Callbacks#
It is common practice within addons to pass JavaScript functions to a C++ function and execute them from there. The following example illustrates how to invoke such callbacks:
// addon.cc
#include <node.h>
namespace demo {
using v8::Context;
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Null;
using v8::Object;
using v8::String;
using v8::Value;
void RunCallback(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
Local<Context> context = isolate->GetCurrentContext();
Local<Function> cb = Local<Function>::Cast(args[0]);
const unsigned argc = 1;
Local<Value> argv[argc] = {
String::NewFromUtf8(isolate,
"hello world").ToLocalChecked() };
cb->Call(context, Null(isolate), argc, argv).ToLocalChecked();
}
void Init(Local<Object> exports, Local<Object> module) {
NODE_SET_METHOD(module, "exports", RunCallback);
}
NODE_MODULE(NODE_GYP_MODULE_NAME, Init)
} // namespace demo
This example uses a two-argument form of Init()
that receives the full
module
object as the second argument. This allows the addon to completely
overwrite exports
with a single function instead of adding the function as a
property of exports
.
To test it, run the following JavaScript:
// test.js
const addon = require('./build/Release/addon');
addon((msg) => {
console.log(msg);
// Prints: 'hello world'
});
In this example, the callback function is invoked synchronously.
Object factory#
Addons can create and return new objects from within a C++ function as
illustrated in the following example. An object is created and returned with a
property msg
that echoes the string passed to createObject()
:
// addon.cc
#include <node.h>
namespace demo {
using v8::Context;
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::String;
using v8::Value;
void CreateObject(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
Local<Context> context = isolate->GetCurrentContext();
Local<Object> obj = Object::New(isolate);
obj->Set(context,
String::NewFromUtf8(isolate,
"msg").ToLocalChecked(),
args[0]->ToString(context).ToLocalChecked())
.FromJust();
args.GetReturnValue().Set(obj);
}
void Init(Local<Object> exports, Local<Object> module) {
NODE_SET_METHOD(module, "exports", CreateObject);
}
NODE_MODULE(NODE_GYP_MODULE_NAME, Init)
} // namespace demo
To test it in JavaScript:
// test.js
const addon = require('./build/Release/addon');
const obj1 = addon('hello');
const obj2 = addon('world');
console.log(obj1.msg, obj2.msg);
// Prints: 'hello world'
Function factory#
Another common scenario is creating JavaScript functions that wrap C++ functions and returning those back to JavaScript:
// addon.cc
#include <node.h>
namespace demo {
using v8::Context;
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::FunctionTemplate;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::String;
using v8::Value;
void MyFunction(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
args.GetReturnValue().Set(String::NewFromUtf8(
isolate, "hello world").ToLocalChecked());
}
void CreateFunction(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
Local<Context> context = isolate->GetCurrentContext();
Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, MyFunction);
Local<Function> fn = tpl->GetFunction(context).ToLocalChecked();
// omit this to make it anonymous
fn->SetName(String::NewFromUtf8(
isolate, "theFunction").ToLocalChecked());
args.GetReturnValue().Set(fn);
}
void Init(Local<Object> exports, Local<Object> module) {
NODE_SET_METHOD(module, "exports", CreateFunction);
}
NODE_MODULE(NODE_GYP_MODULE_NAME, Init)
} // namespace demo
To test:
// test.js
const addon = require('./build/Release/addon');
const fn = addon();
console.log(fn());
// Prints: 'hello world'
Wrapping C++ objects#
It is also possible to wrap C++ objects/classes in a way that allows new
instances to be created using the JavaScript new
operator:
// addon.cc
#include <node.h>
#include "myobject.h"
namespace demo {
using v8::Local;
using v8::Object;
void InitAll(Local<Object> exports) {
MyObject::Init(exports);
}
NODE_MODULE(NODE_GYP_MODULE_NAME, InitAll)
} // namespace demo
Then, in myobject.h
, the wrapper class inherits from node::ObjectWrap
:
// myobject.h
#ifndef MYOBJECT_H
#define MYOBJECT_H
#include <node.h>
#include <node_object_wrap.h>
namespace demo {
class MyObject : public node::ObjectWrap {
public:
static void Init(v8::Local<v8::Object> exports);
private:
explicit MyObject(double value = 0);
~MyObject();
static void New(const v8::FunctionCallbackInfo<v8::Value>& args);
static void PlusOne(const v8::FunctionCallbackInfo<v8::Value>& args);
double value_;
};
} // namespace demo
#endif
In myobject.cc
, implement the various methods that are to be exposed.
Below, the method plusOne()
is exposed by adding it to the constructor's
prototype:
// myobject.cc
#include "myobject.h"
namespace demo {
using v8::Context;
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::FunctionTemplate;
using v8::Isolate;
using v8::Local;
using v8::Number;
using v8::Object;
using v8::ObjectTemplate;
using v8::String;
using v8::Value;
MyObject::MyObject(double value) : value_(value) {
}
MyObject::~MyObject() {
}
void MyObject::Init(Local<Object> exports) {
Isolate* isolate = exports->GetIsolate();
Local<Context> context = isolate->GetCurrentContext();
Local<ObjectTemplate> addon_data_tpl = ObjectTemplate::New(isolate);
addon_data_tpl->SetInternalFieldCount(1); // 1 field for the MyObject::New()
Local<Object> addon_data =
addon_data_tpl->NewInstance(context).ToLocalChecked();
// Prepare constructor template
Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New, addon_data);
tpl->SetClassName(String::NewFromUtf8(isolate, "MyObject").ToLocalChecked());
tpl->InstanceTemplate()->SetInternalFieldCount(1);
// Prototype
NODE_SET_PROTOTYPE_METHOD(tpl, "plusOne", PlusOne);
Local<Function> constructor = tpl->GetFunction(context).ToLocalChecked();
addon_data->SetInternalField(0, constructor);
exports->Set(context, String::NewFromUtf8(
isolate, "MyObject").ToLocalChecked(),
constructor).FromJust();
}
void MyObject::New(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
Local<Context> context = isolate->GetCurrentContext();
if (args.IsConstructCall()) {
// Invoked as constructor: `new MyObject(...)`
double value = args[0]->IsUndefined() ?
0 : args[0]->NumberValue(context).FromMaybe(0);
MyObject* obj = new MyObject(value);
obj->Wrap(args.This());
args.GetReturnValue().Set(args.This());
} else {
// Invoked as plain function `MyObject(...)`, turn into construct call.
const int argc = 1;
Local<Value> argv[argc] = { args[0] };
Local<Function> cons =
args.Data().As<Object>()->GetInternalField(0).As<Function>();
Local<Object> result =
cons->NewInstance(context, argc, argv).ToLocalChecked();
args.GetReturnValue().Set(result);
}
}
void MyObject::PlusOne(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
MyObject* obj = ObjectWrap::Unwrap<MyObject>(args.Holder());
obj->value_ += 1;
args.GetReturnValue().Set(Number::New(isolate, obj->value_));
}
} // namespace demo
To build this example, the myobject.cc
file must be added to the
binding.gyp
:
{
"targets": [
{
"target_name": "addon",
"sources": [
"addon.cc",
"myobject.cc"
]
}
]
}
Test it with:
// test.js
const addon = require('./build/Release/addon');
const obj = new addon.MyObject(10);
console.log(obj.plusOne());
// Prints: 11
console.log(obj.plusOne());
// Prints: 12
console.log(obj.plusOne());
// Prints: 13
The destructor for a wrapper object will run when the object is garbage-collected. For destructor testing, there are command-line flags that can be used to make it possible to force garbage collection. These flags are provided by the underlying V8 JavaScript engine. They are subject to change or removal at any time. They are not documented by Node.js or V8, and they should never be used outside of testing.
During shutdown of the process or worker threads destructors are not called by the JS engine. Therefore it's the responsibility of the user to track these objects and ensure proper destruction to avoid resource leaks.
Factory of wrapped objects#
Alternatively, it is possible to use a factory pattern to avoid explicitly
creating object instances using the JavaScript new
operator:
const obj = addon.createObject();
// instead of:
// const obj = new addon.Object();
First, the createObject()
method is implemented in addon.cc
:
// addon.cc
#include <node.h>
#include "myobject.h"
namespace demo {
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::String;
using v8::Value;
void CreateObject(const FunctionCallbackInfo<Value>& args) {
MyObject::NewInstance(args);
}
void InitAll(Local<Object> exports, Local<Object> module) {
MyObject::Init(exports->GetIsolate());
NODE_SET_METHOD(module, "exports", CreateObject);
}
NODE_MODULE(NODE_GYP_MODULE_NAME, InitAll)
} // namespace demo
In myobject.h
, the static method NewInstance()
is added to handle
instantiating the object. This method takes the place of using new
in
JavaScript:
// myobject.h
#ifndef MYOBJECT_H
#define MYOBJECT_H
#include <node.h>
#include <node_object_wrap.h>
namespace demo {
class MyObject : public node::ObjectWrap {
public:
static void Init(v8::Isolate* isolate);
static void NewInstance(const v8::FunctionCallbackInfo<v8::Value>& args);
private:
explicit MyObject(double value = 0);
~MyObject();
static void New(const v8::FunctionCallbackInfo<v8::Value>& args);
static void PlusOne(const v8::FunctionCallbackInfo<v8::Value>& args);
static v8::Global<v8::Function> constructor;
double value_;
};
} // namespace demo
#endif
The implementation in myobject.cc
is similar to the previous example:
// myobject.cc
#include <node.h>
#include "myobject.h"
namespace demo {
using node::AddEnvironmentCleanupHook;
using v8::Context;
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::FunctionTemplate;
using v8::Global;
using v8::Isolate;
using v8::Local;
using v8::Number;
using v8::Object;
using v8::String;
using v8::Value;
// Warning! This is not thread-safe, this addon cannot be used for worker
// threads.
Global<Function> MyObject::constructor;
MyObject::MyObject(double value) : value_(value) {
}
MyObject::~MyObject() {
}
void MyObject::Init(Isolate* isolate) {
// Prepare constructor template
Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New);
tpl->SetClassName(String::NewFromUtf8(isolate, "MyObject").ToLocalChecked());
tpl->InstanceTemplate()->SetInternalFieldCount(1);
// Prototype
NODE_SET_PROTOTYPE_METHOD(tpl, "plusOne", PlusOne);
Local<Context> context = isolate->GetCurrentContext();
constructor.Reset(isolate, tpl->GetFunction(context).ToLocalChecked());
AddEnvironmentCleanupHook(isolate, [](void*) {
constructor.Reset();
}, nullptr);
}
void MyObject::New(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
Local<Context> context = isolate->GetCurrentContext();
if (args.IsConstructCall()) {
// Invoked as constructor: `new MyObject(...)`
double value = args[0]->IsUndefined() ?
0 : args[0]->NumberValue(context).FromMaybe(0);
MyObject* obj = new MyObject(value);
obj->Wrap(args.This());
args.GetReturnValue().Set(args.This());
} else {
// Invoked as plain function `MyObject(...)`, turn into construct call.
const int argc = 1;
Local<Value> argv[argc] = { args[0] };
Local<Function> cons = Local<Function>::New(isolate, constructor);
Local<Object> instance =
cons->NewInstance(context, argc, argv).ToLocalChecked();
args.GetReturnValue().Set(instance);
}
}
void MyObject::NewInstance(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
const unsigned argc = 1;
Local<Value> argv[argc] = { args[0] };
Local<Function> cons = Local<Function>::New(isolate, constructor);
Local<Context> context = isolate->GetCurrentContext();
Local<Object> instance =
cons->NewInstance(context, argc, argv).ToLocalChecked();
args.GetReturnValue().Set(instance);
}
void MyObject::PlusOne(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
MyObject* obj = ObjectWrap::Unwrap<MyObject>(args.Holder());
obj->value_ += 1;
args.GetReturnValue().Set(Number::New(isolate, obj->value_));
}
} // namespace demo
Once again, to build this example, the myobject.cc
file must be added to the
binding.gyp
:
{
"targets": [
{
"target_name": "addon",
"sources": [
"addon.cc",
"myobject.cc"
]
}
]
}
Test it with:
// test.js
const createObject = require('./build/Release/addon');
const obj = createObject(10);
console.log(obj.plusOne());
// Prints: 11
console.log(obj.plusOne());
// Prints: 12
console.log(obj.plusOne());
// Prints: 13
const obj2 = createObject(20);
console.log(obj2.plusOne());
// Prints: 21
console.log(obj2.plusOne());
// Prints: 22
console.log(obj2.plusOne());
// Prints: 23
Passing wrapped objects around#
In addition to wrapping and returning C++ objects, it is possible to pass
wrapped objects around by unwrapping them with the Node.js helper function
node::ObjectWrap::Unwrap
. The following examples shows a function add()
that can take two MyObject
objects as input arguments:
// addon.cc
#include <node.h>
#include <node_object_wrap.h>
#include "myobject.h"
namespace demo {
using v8::Context;
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Number;
using v8::Object;
using v8::String;
using v8::Value;
void CreateObject(const FunctionCallbackInfo<Value>& args) {
MyObject::NewInstance(args);
}
void Add(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
Local<Context> context = isolate->GetCurrentContext();
MyObject* obj1 = node::ObjectWrap::Unwrap<MyObject>(
args[0]->ToObject(context).ToLocalChecked());
MyObject* obj2 = node::ObjectWrap::Unwrap<MyObject>(
args[1]->ToObject(context).ToLocalChecked());
double sum = obj1->value() + obj2->value();
args.GetReturnValue().Set(Number::New(isolate, sum));
}
void InitAll(Local<Object> exports) {
MyObject::Init(exports->GetIsolate());
NODE_SET_METHOD(exports, "createObject", CreateObject);
NODE_SET_METHOD(exports, "add", Add);
}
NODE_MODULE(NODE_GYP_MODULE_NAME, InitAll)
} // namespace demo
In myobject.h
, a new public method is added to allow access to private values
after unwrapping the object.
// myobject.h
#ifndef MYOBJECT_H
#define MYOBJECT_H
#include <node.h>
#include <node_object_wrap.h>
namespace demo {
class MyObject : public node::ObjectWrap {
public:
static void Init(v8::Isolate* isolate);
static void NewInstance(const v8::FunctionCallbackInfo<v8::Value>& args);
inline double value() const { return value_; }
private:
explicit MyObject(double value = 0);
~MyObject();
static void New(const v8::FunctionCallbackInfo<v8::Value>& args);
static v8::Global<v8::Function> constructor;
double value_;
};
} // namespace demo
#endif
The implementation of myobject.cc
is similar to before:
// myobject.cc
#include <node.h>
#include "myobject.h"
namespace demo {
using node::AddEnvironmentCleanupHook;
using v8::Context;
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::FunctionTemplate;
using v8::Global;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::String;
using v8::Value;
// Warning! This is not thread-safe, this addon cannot be used for worker
// threads.
Global<Function> MyObject::constructor;
MyObject::MyObject(double value) : value_(value) {
}
MyObject::~MyObject() {
}
void MyObject::Init(Isolate* isolate) {
// Prepare constructor template
Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New);
tpl->SetClassName(String::NewFromUtf8(isolate, "MyObject").ToLocalChecked());
tpl->InstanceTemplate()->SetInternalFieldCount(1);
Local<Context> context = isolate->GetCurrentContext();
constructor.Reset(isolate, tpl->GetFunction(context).ToLocalChecked());
AddEnvironmentCleanupHook(isolate, [](void*) {
constructor.Reset();
}, nullptr);
}
void MyObject::New(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
Local<Context> context = isolate->GetCurrentContext();
if (args.IsConstructCall()) {
// Invoked as constructor: `new MyObject(...)`
double value = args[0]->IsUndefined() ?
0 : args[0]->NumberValue(context).FromMaybe(0);
MyObject* obj = new MyObject(value);
obj->Wrap(args.This());
args.GetReturnValue().Set(args.This());
} else {
// Invoked as plain function `MyObject(...)`, turn into construct call.
const int argc = 1;
Local<Value> argv[argc] = { args[0] };
Local<Function> cons = Local<Function>::New(isolate, constructor);
Local<Object> instance =
cons->NewInstance(context, argc, argv).ToLocalChecked();
args.GetReturnValue().Set(instance);
}
}
void MyObject::NewInstance(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
const unsigned argc = 1;
Local<Value> argv[argc] = { args[0] };
Local<Function> cons = Local<Function>::New(isolate, constructor);
Local<Context> context = isolate->GetCurrentContext();
Local<Object> instance =
cons->NewInstance(context, argc, argv).ToLocalChecked();
args.GetReturnValue().Set(instance);
}
} // namespace demo
Test it with:
// test.js
const addon = require('./build/Release/addon');
const obj1 = addon.createObject(10);
const obj2 = addon.createObject(20);
const result = addon.add(obj1, obj2);
console.log(result);
// Prints: 30
Node-API#
Node-API (formerly N-API) is an API for building native Addons. It is independent from the underlying JavaScript runtime (for example, V8) and is maintained as part of Node.js itself. This API will be Application Binary Interface (ABI) stable across versions of Node.js. It is intended to insulate addons from changes in the underlying JavaScript engine and allow modules compiled for one major version to run on later major versions of Node.js without recompilation. The ABI Stability guide provides a more in-depth explanation.
Addons are built/packaged with the same approach/tools outlined in the section titled C++ Addons. The only difference is the set of APIs that are used by the native code. Instead of using the V8 or Native Abstractions for Node.js APIs, the functions available in Node-API are used.
APIs exposed by Node-API are generally used to create and manipulate JavaScript values. Concepts and operations generally map to ideas specified in the ECMA-262 Language Specification. The APIs have the following properties:
- All Node-API calls return a status code of type
napi_status
. This status indicates whether the API call succeeded or failed. - The API's return value is passed via an out parameter.
- All JavaScript values are abstracted behind an opaque type named
napi_value
. - In case of an error status code, additional information can be obtained
using
napi_get_last_error_info
. More information can be found in the error handling section Error handling.
Node-API is a C API that ensures ABI stability across Node.js versions
and different compiler levels. A C++ API can be easier to use.
To support using C++, the project maintains a
C++ wrapper module called node-addon-api
.
This wrapper provides an inlineable C++ API. Binaries built
with node-addon-api
will depend on the symbols for the Node-API C-based
functions exported by Node.js. node-addon-api
is a more
efficient way to write code that calls Node-API. Take, for example, the
following node-addon-api
code. The first section shows the
node-addon-api
code and the second section shows what actually gets
used in the addon.
Object obj = Object::New(env);
obj["foo"] = String::New(env, "bar");
napi_status status;
napi_value object, string;
status = napi_create_object(env, &object);
if (status != napi_ok) {
napi_throw_error(env, ...);
return;
}
status = napi_create_string_utf8(env, "bar", NAPI_AUTO_LENGTH, &string);
if (status != napi_ok) {
napi_throw_error(env, ...);
return;
}
status = napi_set_named_property(env, object, "foo", string);
if (status != napi_ok) {
napi_throw_error(env, ...);
return;
}
The end result is that the addon only uses the exported C APIs. As a result, it still gets the benefits of the ABI stability provided by the C API.
When using node-addon-api
instead of the C APIs, start with the API docs
for node-addon-api
.
The Node-API Resource offers
an excellent orientation and tips for developers just getting started with
Node-API and node-addon-api
. Additional media resources can be found on the
Node-API Media page.
Implications of ABI stability#
Although Node-API provides an ABI stability guarantee, other parts of Node.js do not, and any external libraries used from the addon may not. In particular, none of the following APIs provide an ABI stability guarantee across major versions:
-
the Node.js C++ APIs available via any of
#include <node.h> #include <node_buffer.h> #include <node_version.h> #include <node_object_wrap.h>
-
the libuv APIs which are also included with Node.js and available via
#include <uv.h>
-
the V8 API available via
#include <v8.h>
Thus, for an addon to remain ABI-compatible across Node.js major versions, it must use Node-API exclusively by restricting itself to using
#include <node_api.h>
and by checking, for all external libraries that it uses, that the external library makes ABI stability guarantees similar to Node-API.
Building#
Unlike modules written in JavaScript, developing and deploying Node.js native addons using Node-API requires an additional set of tools. Besides the basic tools required to develop for Node.js, the native addon developer requires a toolchain that can compile C and C++ code into a binary. In addition, depending upon how the native addon is deployed, the user of the native addon will also need to have a C/C++ toolchain installed.
For Linux developers, the necessary C/C++ toolchain packages are readily available. GCC is widely used in the Node.js community to build and test across a variety of platforms. For many developers, the LLVM compiler infrastructure is also a good choice.
For Mac developers, Xcode offers all the required compiler tools. However, it is not necessary to install the entire Xcode IDE. The following command installs the necessary toolchain:
xcode-select --install
For Windows developers, Visual Studio offers all the required compiler tools. However, it is not necessary to install the entire Visual Studio IDE. The following command installs the necessary toolchain:
npm install --global windows-build-tools
The sections below describe the additional tools available for developing and deploying Node.js native addons.
Build tools#
Both the tools listed here require that users of the native addon have a C/C++ toolchain installed in order to successfully install the native addon.
node-gyp#
node-gyp is a build system based on the gyp-next fork of Google's GYP tool and comes bundled with npm. GYP, and therefore node-gyp, requires that Python be installed.
Historically, node-gyp has been the tool of choice for building native addons. It has widespread adoption and documentation. However, some developers have run into limitations in node-gyp.
CMake.js#
CMake.js is an alternative build system based on CMake.
CMake.js is a good choice for projects that already use CMake or for developers affected by limitations in node-gyp.
Uploading precompiled binaries#
The three tools listed here permit native addon developers and maintainers to create and upload binaries to public or private servers. These tools are typically integrated with CI/CD build systems like Travis CI and AppVeyor to build and upload binaries for a variety of platforms and architectures. These binaries are then available for download by users who do not need to have a C/C++ toolchain installed.
node-pre-gyp#
node-pre-gyp is a tool based on node-gyp that adds the ability to upload binaries to a server of the developer's choice. node-pre-gyp has particularly good support for uploading binaries to Amazon S3.
prebuild#
prebuild is a tool that supports builds using either node-gyp or CMake.js. Unlike node-pre-gyp which supports a variety of servers, prebuild uploads binaries only to GitHub releases. prebuild is a good choice for GitHub projects using CMake.js.
prebuildify#
prebuildify is a tool based on node-gyp. The advantage of prebuildify is that the built binaries are bundled with the native addon when it's uploaded to npm. The binaries are downloaded from npm and are immediately available to the module user when the native addon is installed.
Usage#
In order to use the Node-API functions, include the file node_api.h
which
is located in the src directory in the node development tree:
#include <node_api.h>
This will opt into the default NAPI_VERSION
for the given release of Node.js.
In order to ensure compatibility with specific versions of Node-API, the version
can be specified explicitly when including the header:
#define NAPI_VERSION 3
#include <node_api.h>
This restricts the Node-API surface to just the functionality that was available in the specified (and earlier) versions.
Some of the Node-API surface is experimental and requires explicit opt-in:
#define NAPI_EXPERIMENTAL
#include <node_api.h>
In this case the entire API surface, including any experimental APIs, will be available to the module code.
Node-API version matrix#
Node-API versions are additive and versioned independently from Node.js. Version 4 is an extension to version 3 in that it has all of the APIs from version 3 with some additions. This means that it is not necessary to recompile for new versions of Node.js which are listed as supporting a later version.
1 | 2 | 3 | |
---|---|---|---|
v6.x | v6.14.2* | ||
v8.x | v8.6.0** | v8.10.0* | v8.11.2 |
v9.x | v9.0.0* | v9.3.0* | v9.11.0* |
≥ v10.x | all releases | all releases | all releases |
4 | 5 | 6 | 7 | 8 | |
---|---|---|---|---|---|
v10.x | v10.16.0 | v10.17.0 | v10.20.0 | v10.23.0 | |
v11.x | v11.8.0 | ||||
v12.x | v12.0.0 | v12.11.0 | v12.17.0 | v12.19.0 | v12.22.0 |
v13.x | v13.0.0 | v13.0.0 | |||
v14.x | v14.0.0 | v14.0.0 | v14.0.0 | v14.12.0 | v14.17.0 |
v15.x | v15.0.0 | v15.0.0 | v15.0.0 | v15.0.0 | v15.12.0 |
v16.x | v16.0.0 | v16.0.0 | v16.0.0 | v16.0.0 | v16.0.0 |
* Node-API was experimental.
** Node.js 8.0.0 included Node-API as experimental. It was released as Node-API version 1 but continued to evolve until Node.js 8.6.0. The API is different in versions prior to Node.js 8.6.0. We recommend Node-API version 3 or later.
Each API documented for Node-API will have a header named added in:
, and APIs
which are stable will have the additional header Node-API version:
.
APIs are directly usable when using a Node.js version which supports
the Node-API version shown in Node-API version:
or higher.
When using a Node.js version that does not support the
Node-API version:
listed or if there is no Node-API version:
listed,
then the API will only be available if
#define NAPI_EXPERIMENTAL
precedes the inclusion of node_api.h
or js_native_api.h
. If an API appears not to be available on
a version of Node.js which is later than the one shown in added in:
then
this is most likely the reason for the apparent absence.
The Node-APIs associated strictly with accessing ECMAScript features from native
code can be found separately in js_native_api.h
and js_native_api_types.h
.
The APIs defined in these headers are included in node_api.h
and
node_api_types.h
. The headers are structured in this way in order to allow
implementations of Node-API outside of Node.js. For those implementations the
Node.js specific APIs may not be applicable.
The Node.js-specific parts of an addon can be separated from the code that
exposes the actual functionality to the JavaScript environment so that the
latter may be used with multiple implementations of Node-API. In the example
below, addon.c
and addon.h
refer only to js_native_api.h
. This ensures
that addon.c
can be reused to compile against either the Node.js
implementation of Node-API or any implementation of Node-API outside of Node.js.
addon_node.c
is a separate file that contains the Node.js specific entry point
to the addon and which instantiates the addon by calling into addon.c
when the
addon is loaded into a Node.js environment.
// addon.h
#ifndef _ADDON_H_
#define _ADDON_H_
#include <js_native_api.h>
napi_value create_addon(napi_env env);
#endif // _ADDON_H_
// addon.c
#include "addon.h"
#define NAPI_CALL(env, call) \
do { \
napi_status status = (call); \
if (status != napi_ok) { \
const napi_extended_error_info* error_info = NULL; \
napi_get_last_error_info((env), &error_info); \
const char* err_message = error_info->error_message; \
bool is_pending; \
napi_is_exception_pending((env), &is_pending); \
if (!is_pending) { \
const char* message = (err_message == NULL) \
? "empty error message" \
: err_message; \
napi_throw_error((env), NULL, message); \
return NULL; \
} \
} \
} while(0)
static napi_value
DoSomethingUseful(napi_env env, napi_callback_info info) {
// Do something useful.
return NULL;
}
napi_value create_addon(napi_env env) {
napi_value result;
NAPI_CALL(env, napi_create_object(env, &result));
napi_value exported_function;
NAPI_CALL(env, napi_create_function(env,
"doSomethingUseful",
NAPI_AUTO_LENGTH,
DoSomethingUseful,
NULL,
&exported_function));
NAPI_CALL(env, napi_set_named_property(env,
result,
"doSomethingUseful",
exported_function));
return result;
}
// addon_node.c
#include <node_api.h>
#include "addon.h"
NAPI_MODULE_INIT() {
// This function body is expected to return a `napi_value`.
// The variables `napi_env env` and `napi_value exports` may be used within
// the body, as they are provided by the definition of `NAPI_MODULE_INIT()`.
return create_addon(env);
}
Environment life cycle APIs#
Section 8.7 of the ECMAScript Language Specification defines the concept of an "Agent" as a self-contained environment in which JavaScript code runs. Multiple such Agents may be started and terminated either concurrently or in sequence by the process.
A Node.js environment corresponds to an ECMAScript Agent. In the main process, an environment is created at startup, and additional environments can be created on separate threads to serve as worker threads. When Node.js is embedded in another application, the main thread of the application may also construct and destroy a Node.js environment multiple times during the life cycle of the application process such that each Node.js environment created by the application may, in turn, during its life cycle create and destroy additional environments as worker threads.
From the perspective of a native addon this means that the bindings it provides may be called multiple times, from multiple contexts, and even concurrently from multiple threads.
Native addons may need to allocate global state which they use during their life cycle of an Node.js environment such that the state can be unique to each instance of the addon.
To this end, Node-API provides a way to associate data such that its life cycle is tied to the life cycle of a Node.js environment.
napi_set_instance_data
#
napi_status napi_set_instance_data(napi_env env,
void* data,
napi_finalize finalize_cb,
void* finalize_hint);
[in] env
: The environment that the Node-API call is invoked under.[in] data
: The data item to make available to bindings of this instance.[in] finalize_cb
: The function to call when the environment is being torn down. The function receivesdata
so that it might free it.napi_finalize
provides more details.[in] finalize_hint
: Optional hint to pass to the finalize callback during collection.
Returns napi_ok
if the API succeeded.
This API associates data
with the currently running Node.js environment. data
can later be retrieved using napi_get_instance_data()
. Any existing data
associated with the currently running Node.js environment which was set by means
of a previous call to napi_set_instance_data()
will be overwritten. If a
finalize_cb
was provided by the previous call, it will not be called.
napi_get_instance_data
#
napi_status napi_get_instance_data(napi_env env,
void** data);
[in] env
: The environment that the Node-API call is invoked under.[out] data
: The data item that was previously associated with the currently running Node.js environment by a call tonapi_set_instance_data()
.
Returns napi_ok
if the API succeeded.
This API retrieves data that was previously associated with the currently
running Node.js environment via napi_set_instance_data()
. If no data is set,
the call will succeed and data
will be set to NULL
.
Basic Node-API data types#
Node-API exposes the following fundamental datatypes as abstractions that are consumed by the various APIs. These APIs should be treated as opaque, introspectable only with other Node-API calls.
napi_status
#
Integral status code indicating the success or failure of a Node-API call. Currently, the following status codes are supported.
typedef enum {
napi_ok,
napi_invalid_arg,
napi_object_expected,
napi_string_expected,
napi_name_expected,
napi_function_expected,
napi_number_expected,
napi_boolean_expected,
napi_array_expected,
napi_generic_failure,
napi_pending_exception,
napi_cancelled,
napi_escape_called_twice,
napi_handle_scope_mismatch,
napi_callback_scope_mismatch,
napi_queue_full,
napi_closing,
napi_bigint_expected,
napi_date_expected,
napi_arraybuffer_expected,
napi_detachable_arraybuffer_expected,
napi_would_deadlock, /* unused */
napi_no_external_buffers_allowed
} napi_status;
If additional information is required upon an API returning a failed status,
it can be obtained by calling napi_get_last_error_info
.
napi_extended_error_info
#
typedef struct {
const char* error_message;
void* engine_reserved;
uint32_t engine_error_code;
napi_status error_code;
} napi_extended_error_info;
error_message
: UTF8-encoded string containing a VM-neutral description of the error.engine_reserved
: Reserved for VM-specific error details. This is currently not implemented for any VM.engine_error_code
: VM-specific error code. This is currently not implemented for any VM.error_code
: The Node-API status code that originated with the last error.
See the Error handling section for additional information.
napi_env
#
napi_env
is used to represent a context that the underlying Node-API
implementation can use to persist VM-specific state. This structure is passed
to native functions when they're invoked, and it must be passed back when
making Node-API calls. Specifically, the same napi_env
that was passed in when
the initial native function was called must be passed to any subsequent
nested Node-API calls. Caching the napi_env
for the purpose of general reuse,
and passing the napi_env
between instances of the same addon running on
different Worker
threads is not allowed. The napi_env
becomes invalid
when an instance of a native addon is unloaded. Notification of this event is
delivered through the callbacks given to napi_add_env_cleanup_hook
and
napi_set_instance_data
.
napi_value
#
This is an opaque pointer that is used to represent a JavaScript value.
napi_threadsafe_function
#
This is an opaque pointer that represents a JavaScript function which can be
called asynchronously from multiple threads via
napi_call_threadsafe_function()
.
napi_threadsafe_function_release_mode
#
A value to be given to napi_release_threadsafe_function()
to indicate whether
the thread-safe function is to be closed immediately (napi_tsfn_abort
) or
merely released (napi_tsfn_release
) and thus available for subsequent use via
napi_acquire_threadsafe_function()
and napi_call_threadsafe_function()
.
typedef enum {
napi_tsfn_release,
napi_tsfn_abort
} napi_threadsafe_function_release_mode;
napi_threadsafe_function_call_mode
#
A value to be given to napi_call_threadsafe_function()
to indicate whether
the call should block whenever the queue associated with the thread-safe
function is full.
typedef enum {
napi_tsfn_nonblocking,
napi_tsfn_blocking
} napi_threadsafe_function_call_mode;
Node-API memory management types#
napi_handle_scope
#
This is an abstraction used to control and modify the lifetime of objects created within a particular scope. In general, Node-API values are created within the context of a handle scope. When a native method is called from JavaScript, a default handle scope will exist. If the user does not explicitly create a new handle scope, Node-API values will be created in the default handle scope. For any invocations of code outside the execution of a native method (for instance, during a libuv callback invocation), the module is required to create a scope before invoking any functions that can result in the creation of JavaScript values.
Handle scopes are created using napi_open_handle_scope
and are destroyed
using napi_close_handle_scope
. Closing the scope can indicate to the GC
that all napi_value
s created during the lifetime of the handle scope are no
longer referenced from the current stack frame.
For more details, review the Object lifetime management.
napi_escapable_handle_scope
#
Escapable handle scopes are a special type of handle scope to return values created within a particular handle scope to a parent scope.
napi_ref
#
This is the abstraction to use to reference a napi_value
. This allows for
users to manage the lifetimes of JavaScript values, including defining their
minimum lifetimes explicitly.
For more details, review the Object lifetime management.
napi_type_tag
#
A 128-bit value stored as two unsigned 64-bit integers. It serves as a UUID
with which JavaScript objects or externals can be "tagged" in order to
ensure that they are of a certain type. This is a stronger check than
napi_instanceof
, because the latter can report a false positive if the
object's prototype has been manipulated. Type-tagging is most useful in
conjunction with napi_wrap
because it ensures that the pointer retrieved
from a wrapped object can be safely cast to the native type corresponding to the
type tag that had been previously applied to the JavaScript object.
typedef struct {
uint64_t lower;
uint64_t upper;
} napi_type_tag;
napi_async_cleanup_hook_handle
#
An opaque value returned by napi_add_async_cleanup_hook
. It must be passed
to napi_remove_async_cleanup_hook
when the chain of asynchronous cleanup
events completes.
Node-API callback types#
napi_callback_info
#
Opaque datatype that is passed to a callback function. It can be used for getting additional information about the context in which the callback was invoked.
napi_callback
#
Function pointer type for user-provided native functions which are to be exposed to JavaScript via Node-API. Callback functions should satisfy the following signature:
typedef napi_value (*napi_callback)(napi_env, napi_callback_info);
Unless for reasons discussed in Object Lifetime Management, creating a
handle and/or callback scope inside a napi_callback
is not necessary.
napi_finalize
#
Function pointer type for add-on provided functions that allow the user to be
notified when externally-owned data is ready to be cleaned up because the
object with which it was associated with, has been garbage-collected. The user
must provide a function satisfying the following signature which would get
called upon the object's collection. Currently, napi_finalize
can be used for
finding out when objects that have external data are collected.
typedef void (*napi_finalize)(napi_env env,
void* finalize_data,
void* finalize_hint);
Unless for reasons discussed in Object Lifetime Management, creating a handle and/or callback scope inside the function body is not necessary.
napi_async_execute_callback
#
Function pointer used with functions that support asynchronous operations. Callback functions must satisfy the following signature:
typedef void (*napi_async_execute_callback)(napi_env env, void* data);
Implementations of this function must avoid making Node-API calls that execute
JavaScript or interact with JavaScript objects. Node-API calls should be in the
napi_async_complete_callback
instead. Do not use the napi_env
parameter as
it will likely result in execution of JavaScript.
napi_async_complete_callback
#
Function pointer used with functions that support asynchronous operations. Callback functions must satisfy the following signature:
typedef void (*napi_async_complete_callback)(napi_env env,
napi_status status,
void* data);
Unless for reasons discussed in Object Lifetime Management, creating a handle and/or callback scope inside the function body is not necessary.
napi_threadsafe_function_call_js
#
Function pointer used with asynchronous thread-safe function calls. The callback
will be called on the main thread. Its purpose is to use a data item arriving
via the queue from one of the secondary threads to construct the parameters
necessary for a call into JavaScript, usually via napi_call_function
, and then
make the call into JavaScript.
The data arriving from the secondary thread via the queue is given in the data
parameter and the JavaScript function to call is given in the js_callback
parameter.
Node-API sets up the environment prior to calling this callback, so it is
sufficient to call the JavaScript function via napi_call_function
rather than
via napi_make_callback
.
Callback functions must satisfy the following signature:
typedef void (*napi_threadsafe_function_call_js)(napi_env env,
napi_value js_callback,
void* context,
void* data);
[in] env
: The environment to use for API calls, orNULL
if the thread-safe function is being torn down anddata
may need to be freed.[in] js_callback
: The JavaScript function to call, orNULL
if the thread-safe function is being torn down anddata
may need to be freed. It may also beNULL
if the thread-safe function was created withoutjs_callback
.[in] context
: The optional data with which the thread-safe function was created.[in] data
: Data created by the secondary thread. It is the responsibility of the callback to convert this native data to JavaScript values (with Node-API functions) that can be passed as parameters whenjs_callback
is invoked. This pointer is managed entirely by the threads and this callback. Thus this callback should free the data.
Unless for reasons discussed in Object Lifetime Management, creating a handle and/or callback scope inside the function body is not necessary.
napi_cleanup_hook
#
Function pointer used with napi_add_env_cleanup_hook
. It will be called
when the environment is being torn down.
Callback functions must satisfy the following signature:
typedef void (*napi_cleanup_hook)(void* data);
[in] data
: The data that was passed tonapi_add_env_cleanup_hook
.
napi_async_cleanup_hook
#
Function pointer used with napi_add_async_cleanup_hook
. It will be called
when the environment is being torn down.
Callback functions must satisfy the following signature:
typedef void (*napi_async_cleanup_hook)(napi_async_cleanup_hook_handle handle,
void* data);
[in] handle
: The handle that must be passed tonapi_remove_async_cleanup_hook
after completion of the asynchronous cleanup.[in] data
: The data that was passed tonapi_add_async_cleanup_hook
.
The body of the function should initiate the asynchronous cleanup actions at the
end of which handle
must be passed in a call to
napi_remove_async_cleanup_hook
.
Error handling#
Node-API uses both return values and JavaScript exceptions for error handling. The following sections explain the approach for each case.
Return values#
All of the Node-API functions share the same error handling pattern. The
return type of all API functions is napi_status
.
The return value will be napi_ok
if the request was successful and
no uncaught JavaScript exception was thrown. If an error occurred AND
an exception was thrown, the napi_status
value for the error
will be returned. If an exception was thrown, and no error occurred,
napi_pending_exception
will be returned.
In cases where a return value other than napi_ok
or
napi_pending_exception
is returned, napi_is_exception_pending
must be called to check if an exception is pending.
See the section on exceptions for more details.
The full set of possible napi_status
values is defined
in napi_api_types.h
.
The napi_status
return value provides a VM-independent representation of
the error which occurred. In some cases it is useful to be able to get
more detailed information, including a string representing the error as well as
VM (engine)-specific information.
In order to retrieve this information napi_get_last_error_info
is provided which returns a napi_extended_error_info
structure.
The format of the napi_extended_error_info
structure is as follows:
typedef struct napi_extended_error_info {
const char* error_message;
void* engine_reserved;
uint32_t engine_error_code;
napi_status error_code;
};
error_message
: Textual representation of the error that occurred.engine_reserved
: Opaque handle reserved for engine use only.engine_error_code
: VM specific error code.error_code
: Node-API status code for the last error.
napi_get_last_error_info
returns the information for the last
Node-API call that was made.
Do not rely on the content or format of any of the extended information as it is not subject to SemVer and may change at any time. It is intended only for logging purposes.
napi_get_last_error_info
#
napi_status
napi_get_last_error_info(napi_env env,
const napi_extended_error_info** result);
[in] env
: The environment that the API is invoked under.[out] result
: Thenapi_extended_error_info
structure with more information about the error.
Returns napi_ok
if the API succeeded.
This API retrieves a napi_extended_error_info
structure with information
about the last error that occurred.
The content of the napi_extended_error_info
returned is only valid up until
a Node-API function is called on the same env
. This includes a call to
napi_is_exception_pending
so it may often be necessary to make a copy
of the information so that it can be used later. The pointer returned
in error_message
points to a statically-defined string so it is safe to use
that pointer if you have copied it out of the error_message
field (which will
be overwritten) before another Node-API function was called.
Do not rely on the content or format of any of the extended information as it is not subject to SemVer and may change at any time. It is intended only for logging purposes.
This API can be called even if there is a pending JavaScript exception.
Exceptions#
Any Node-API function call may result in a pending JavaScript exception. This is the case for any of the API functions, even those that may not cause the execution of JavaScript.
If the napi_status
returned by a function is napi_ok
then no
exception is pending and no additional action is required. If the
napi_status
returned is anything other than napi_ok
or
napi_pending_exception
, in order to try to recover and continue
instead of simply returning immediately, napi_is_exception_pending
must be called in order to determine if an exception is pending or not.
In many cases when a Node-API function is called and an exception is
already pending, the function will return immediately with a
napi_status
of napi_pending_exception
. However, this is not the case
for all functions. Node-API allows a subset of the functions to be
called to allow for some minimal cleanup before returning to JavaScript.
In that case, napi_status
will reflect the status for the function. It
will not reflect previous pending exceptions. To avoid confusion, check
the error status after every function call.
When an exception is pending one of two approaches can be employed.
The first approach is to do any appropriate cleanup and then return so that
execution will return to JavaScript. As part of the transition back to
JavaScript, the exception will be thrown at the point in the JavaScript
code where the native method was invoked. The behavior of most Node-API calls
is unspecified while an exception is pending, and many will simply return
napi_pending_exception
, so do as little as possible and then return to
JavaScript where the exception can be handled.
The second approach is to try to handle the exception. There will be cases
where the native code can catch the exception, take the appropriate action,
and then continue. This is only recommended in specific cases
where it is known that the exception can be safely handled. In these
cases napi_get_and_clear_last_exception
can be used to get and
clear the exception. On success, result will contain the handle to
the last JavaScript Object
thrown. If it is determined, after
retrieving the exception, the exception cannot be handled after all
it can be re-thrown it with napi_throw
where error is the
JavaScript value to be thrown.
The following utility functions are also available in case native code
needs to throw an exception or determine if a napi_value
is an instance
of a JavaScript Error
object: napi_throw_error
,
napi_throw_type_error
, napi_throw_range_error
, node_api_throw_syntax_error
and napi_is_error
.
The following utility functions are also available in case native
code needs to create an Error
object: napi_create_error
,
napi_create_type_error
, napi_create_range_error
and node_api_create_syntax_error
,
where result is the napi_value
that refers to the newly created
JavaScript Error
object.
The Node.js project is adding error codes to all of the errors
generated internally. The goal is for applications to use these
error codes for all error checking. The associated error messages
will remain, but will only be meant to be used for logging and
display with the expectation that the message can change without
SemVer applying. In order to support this model with Node-API, both
in internal functionality and for module specific functionality
(as its good practice), the throw_
and create_
functions
take an optional code parameter which is the string for the code
to be added to the error object. If the optional parameter is NULL
then no code will be associated with the error. If a code is provided,
the name associated with the error is also updated to be:
originalName [code]
where originalName
is the original name associated with the error
and code
is the code that was provided. For example, if the code
is 'ERR_ERROR_1'
and a TypeError
is being created the name will be:
TypeError [ERR_ERROR_1]
napi_throw
#
NAPI_EXTERN napi_status napi_throw(napi_env env, napi_value error);
[in] env
: The environment that the API is invoked under.[in] error
: The JavaScript value to be thrown.
Returns napi_ok
if the API succeeded.
This API throws the JavaScript value provided.
napi_throw_error
#
NAPI_EXTERN napi_status napi_throw_error(napi_env env,
const char* code,
const char* msg);
[in] env
: The environment that the API is invoked under.[in] code
: Optional error code to be set on the error.[in] msg
: C string representing the text to be associated with the error.
Returns napi_ok
if the API succeeded.
This API throws a JavaScript Error
with the text provided.
napi_throw_type_error
#
NAPI_EXTERN napi_status napi_throw_type_error(napi_env env,
const char* code,
const char* msg);
[in] env
: The environment that the API is invoked under.[in] code
: Optional error code to be set on the error.[in] msg
: C string representing the text to be associated with the error.
Returns napi_ok
if the API succeeded.
This API throws a JavaScript TypeError
with the text provided.
napi_throw_range_error
#
NAPI_EXTERN napi_status napi_throw_range_error(napi_env env,
const char* code,
const char* msg);
[in] env
: The environment that the API is invoked under.[in] code
: Optional error code to be set on the error.[in] msg
: C string representing the text to be associated with the error.
Returns napi_ok
if the API succeeded.
This API throws a JavaScript RangeError
with the text provided.
node_api_throw_syntax_error
#
NAPI_EXTERN napi_status node_api_throw_syntax_error(napi_env env,
const char* code,
const char* msg);
[in] env
: The environment that the API is invoked under.[in] code
: Optional error code to be set on the error.[in] msg
: C string representing the text to be associated with the error.
Returns napi_ok
if the API succeeded.
This API throws a JavaScript SyntaxError
with the text provided.
napi_is_error
#
NAPI_EXTERN napi_status napi_is_error(napi_env env,
napi_value value,
bool* result);
[in] env
: The environment that the API is invoked under.[in] value
: Thenapi_value
to be checked.[out] result
: Boolean value that is set to true ifnapi_value
represents an error, false otherwise.
Returns napi_ok
if the API succeeded.
This API queries a napi_value
to check if it represents an error object.
napi_create_error
#
NAPI_EXTERN napi_status napi_create_error(napi_env env,
napi_value code,
napi_value msg,
napi_value* result);
[in] env
: The environment that the API is invoked under.[in] code
: Optionalnapi_value
with the string for the error code to be associated with the error.[in] msg
:napi_value
that references a JavaScriptstring
to be used as the message for theError
.[out] result
:napi_value
representing the error created.
Returns napi_ok
if the API succeeded.
This API returns a JavaScript Error
with the text provided.
napi_create_type_error
#
NAPI_EXTERN napi_status napi_create_type_error(napi_env env,
napi_value code,
napi_value msg,
napi_value* result);
[in] env
: The environment that the API is invoked under.[in] code
: Optionalnapi_value
with the string for the error code to be associated with the error.[in] msg
:napi_value
that references a JavaScriptstring
to be used as the message for theError
.[out] result
:napi_value
representing the error created.
Returns napi_ok
if the API succeeded.
This API returns a JavaScript TypeError
with the text provided.
napi_create_range_error
#
NAPI_EXTERN napi_status napi_create_range_error(napi_env env,
napi_value code,
napi_value msg,
napi_value* result);
[in] env
: The environment that the API is invoked under.[in] code
: Optionalnapi_value
with the string for the error code to be associated with the error.[in] msg
:napi_value
that references a JavaScriptstring
to be used as the message for theError
.[out] result
:napi_value
representing the error created.
Returns napi_ok
if the API succeeded.
This API returns a JavaScript RangeError
with the text provided.
node_api_create_syntax_error
#
NAPI_EXTERN napi_status node_api_create_syntax_error(napi_env env,
napi_value code,
napi_value msg,
napi_value* result);
[in] env
: The environment that the API is invoked under.[in] code
: Optionalnapi_value
with the string for the error code to be associated with the error.[in] msg
:napi_value
that references a JavaScriptstring
to be used as the message for theError
.[out] result
:napi_value
representing the error created.
Returns napi_ok
if the API succeeded.
This API returns a JavaScript SyntaxError
with the text provided.
napi_get_and_clear_last_exception
#
napi_status napi_get_and_clear_last_exception(napi_env env,
napi_value* result);
[in] env
: The environment that the API is invoked under.[out] result
: The exception if one is pending,NULL
otherwise.
Returns napi_ok
if the API succeeded.
This API can be called even if there is a pending JavaScript exception.
napi_is_exception_pending
#
napi_status napi_is_exception_pending(napi_env env, bool* result);
[in] env
: The environment that the API is invoked under.[out] result
: Boolean value that is set to true if an exception is pending.
Returns napi_ok
if the API succeeded.
This API can be called even if there is a pending JavaScript exception.
napi_fatal_exception
#
napi_status napi_fatal_exception(napi_env env, napi_value err);
[in] env
: The environment that the API is invoked under.[in] err
: The error that is passed to'uncaughtException'
.
Trigger an 'uncaughtException'
in JavaScript. Useful if an async
callback throws an exception with no way to recover.
Fatal errors#
In the event of an unrecoverable error in a native addon, a fatal error can be thrown to immediately terminate the process.
napi_fatal_error
#
NAPI_NO_RETURN void napi_fatal_error(const char* location,
size_t location_len,
const char* message,
size_t message_len);
[in] location
: Optional location at which the error occurred.[in] location_len
: The length of the location in bytes, orNAPI_AUTO_LENGTH
if it is null-terminated.[in] message
: The message associated with the error.[in] message_len
: The length of the message in bytes, orNAPI_AUTO_LENGTH
if it is null-terminated.
The function call does not return, the process will be terminated.
This API can be called even if there is a pending JavaScript exception.
Object lifetime management#
As Node-API calls are made, handles to objects in the heap for the underlying
VM may be returned as napi_values
. These handles must hold the
objects 'live' until they are no longer required by the native code,
otherwise the objects could be collected before the native code was
finished using them.
As object handles are returned they are associated with a 'scope'. The lifespan for the default scope is tied to the lifespan of the native method call. The result is that, by default, handles remain valid and the objects associated with these handles will be held live for the lifespan of the native method call.
In many cases, however, it is necessary that the handles remain valid for either a shorter or longer lifespan than that of the native method. The sections which follow describe the Node-API functions that can be used to change the handle lifespan from the default.
Making handle lifespan shorter than that of the native method#
It is often necessary to make the lifespan of handles shorter than the lifespan of a native method. For example, consider a native method that has a loop which iterates through the elements in a large array:
for (int i = 0; i < 1000000; i++) {
napi_value result;
napi_status status = napi_get_element(env, object, i, &result);
if (status != napi_ok) {
break;
}
// do something with element
}
This would result in a large number of handles being created, consuming substantial resources. In addition, even though the native code could only use the most recent handle, all of the associated objects would also be kept alive since they all share the same scope.
To handle this case, Node-API provides the ability to establish a new 'scope' to
which newly created handles will be associated. Once those handles
are no longer required, the scope can be 'closed' and any handles associated
with the scope are invalidated. The methods available to open/close scopes are
napi_open_handle_scope
and napi_close_handle_scope
.
Node-API only supports a single nested hierarchy of scopes. There is only one active scope at any time, and all new handles will be associated with that scope while it is active. Scopes must be closed in the reverse order from which they are opened. In addition, all scopes created within a native method must be closed before returning from that method.
Taking the earlier example, adding calls to napi_open_handle_scope
and
napi_close_handle_scope
would ensure that at most a single handle
is valid throughout the execution of the loop:
for (int i = 0; i < 1000000; i++) {
napi_handle_scope scope;
napi_status status = napi_open_handle_scope(env, &scope);
if (status != napi_ok) {
break;
}
napi_value result;
status = napi_get_element(env, object, i, &result);
if (status != napi_ok) {
break;
}
// do something with element
status = napi_close_handle_scope(env, scope);
if (status != napi_ok) {
break;
}
}
When nesting scopes, there are cases where a handle from an inner scope needs to live beyond the lifespan of that scope. Node-API supports an 'escapable scope' in order to support this case. An escapable scope allows one handle to be 'promoted' so that it 'escapes' the current scope and the lifespan of the handle changes from the current scope to that of the outer scope.
The methods available to open/close escapable scopes are
napi_open_escapable_handle_scope
and
napi_close_escapable_handle_scope
.
The request to promote a handle is made through napi_escape_handle
which
can only be called once.
napi_open_handle_scope
#
NAPI_EXTERN napi_status napi_open_handle_scope(napi_env env,
napi_handle_scope* result);
[in] env
: The environment that the API is invoked under.[out] result
:napi_value
representing the new scope.
Returns napi_ok
if the API succeeded.
This API opens a new scope.
napi_close_handle_scope
#
NAPI_EXTERN napi_status napi_close_handle_scope(napi_env env,
napi_handle_scope scope);
[in] env
: The environment that the API is invoked under.[in] scope
:napi_value
representing the scope to be closed.
Returns napi_ok
if the API succeeded.
This API closes the scope passed in. Scopes must be closed in the reverse order from which they were created.
This API can be called even if there is a pending JavaScript exception.
napi_open_escapable_handle_scope
#
NAPI_EXTERN napi_status
napi_open_escapable_handle_scope(napi_env env,
napi_handle_scope* result);
[in] env
: The environment that the API is invoked under.[out] result
:napi_value
representing the new scope.
Returns napi_ok
if the API succeeded.
This API opens a new scope from which one object can be promoted to the outer scope.
napi_close_escapable_handle_scope
#
NAPI_EXTERN napi_status
napi_close_escapable_handle_scope(napi_env env,
napi_handle_scope scope);
[in] env
: The environment that the API is invoked under.[in] scope
:napi_value
representing the scope to be closed.
Returns napi_ok
if the API succeeded.
This API closes the scope passed in. Scopes must be closed in the reverse order from which they were created.
This API can be called even if there is a pending JavaScript exception.
napi_escape_handle
#
napi_status napi_escape_handle(napi_env env,
napi_escapable_handle_scope scope,
napi_value escapee,
napi_value* result);
[in] env
: The environment that the API is invoked under.[in] scope
:napi_value
representing the current scope.[in] escapee
:napi_value
representing the JavaScriptObject
to be escaped.[out] result
:napi_value
representing the handle to the escapedObject
in the outer scope.
Returns napi_ok
if the API succeeded.
This API promotes the handle to the JavaScript object so that it is valid for the lifetime of the outer scope. It can only be called once per scope. If it is called more than once an error will be returned.
This API can be called even if there is a pending JavaScript exception.
References to objects with a lifespan longer than that of the native method#
In some cases an addon will need to be able to create and reference objects
with a lifespan longer than that of a single native method invocation. For
example, to create a constructor and later use that constructor
in a request to creates instances, it must be possible to reference
the constructor object across many different instance creation requests. This
would not be possible with a normal handle returned as a napi_value
as
described in the earlier section. The lifespan of a normal handle is
managed by scopes and all scopes must be closed before the end of a native
method.
Node-API provides methods to create persistent references to an object. Each persistent reference has an associated count with a value of 0 or higher. The count determines if the reference will keep the corresponding object live. References with a count of 0 do not prevent the object from being collected and are often called 'weak' references. Any count greater than 0 will prevent the object from being collected.
References can be created with an initial reference count. The count can
then be modified through napi_reference_ref
and
napi_reference_unref
. If an object is collected while the count
for a reference is 0, all subsequent calls to
get the object associated with the reference napi_get_reference_value
will return NULL
for the returned napi_value
. An attempt to call
napi_reference_ref
for a reference whose object has been collected
results in an error.
References must be deleted once they are no longer required by the addon. When a reference is deleted, it will no longer prevent the corresponding object from being collected. Failure to delete a persistent reference results in a 'memory leak' with both the native memory for the persistent reference and the corresponding object on the heap being retained forever.
There can be multiple persistent references created which refer to the same
object, each of which will either keep the object live or not based on its
individual count. Multiple persistent references to the same object
can result in unexpectedly keeping alive native memory. The native structures
for a persistent reference must be kept alive until finalizers for the
referenced object are executed. If a new persistent reference is created
for the same object, the finalizers for that object will not be
run and the native memory pointed by the earlier persistent reference
will not be freed. This can be avoided by calling
napi_delete_reference
in addition to napi_reference_unref
when possible.
napi_create_reference
#
NAPI_EXTERN napi_status napi_create_reference(napi_env env,
napi_value value,
uint32_t initial_refcount,
napi_ref* result);
[in] env
: The environment that the API is invoked under.[in] value
:napi_value
representing theObject
to which we want a reference.[in] initial_refcount
: Initial reference count for the new reference.[out] result
:napi_ref
pointing to the new reference.
Returns napi_ok
if the API succeeded.
This API creates a new reference with the specified reference count
to the Object
passed in.
napi_delete_reference
#
NAPI_EXTERN napi_status napi_delete_reference(napi_env env, napi_ref ref);
[in] env
: The environment that the API is invoked under.[in] ref
:napi_ref
to be deleted.
Returns napi_ok
if the API succeeded.
This API deletes the reference passed in.
This API can be called even if there is a pending JavaScript exception.
napi_reference_ref
#
NAPI_EXTERN napi_status napi_reference_ref(napi_env env,
napi_ref ref,
uint32_t* result);
[in] env
: The environment that the API is invoked under.[in] ref
:napi_ref
for which the reference count will be incremented.[out] result
: The new reference count.
Returns napi_ok
if the API succeeded.
This API increments the reference count for the reference passed in and returns the resulting reference count.
napi_reference_unref
#
NAPI_EXTERN napi_status napi_reference_unref(napi_env env,
napi_ref ref,
uint32_t* result);
[in] env
: The environment that the API is invoked under.[in] ref
:napi_ref
for which the reference count will be decremented.[out] result
: The new reference count.
Returns napi_ok
if the API succeeded.
This API decrements the reference count for the reference passed in and returns the resulting reference count.
napi_get_reference_value
#
NAPI_EXTERN napi_status napi_get_reference_value(napi_env env,
napi_ref ref,
napi_value* result);
the napi_value passed
in or out of these methods is a handle to the
object to which the reference is related.
[in] env
: The environment that the API is invoked under.[in] ref
:napi_ref
for which we requesting the correspondingObject
.[out] result
: Thenapi_value
for theObject
referenced by thenapi_ref
.
Returns napi_ok
if the API succeeded.
If still valid, this API returns the napi_value
representing the
JavaScript Object
associated with the napi_ref
. Otherwise, result
will be NULL
.
Cleanup on exit of the current Node.js environment#
While a Node.js process typically releases all its resources when exiting, embedders of Node.js, or future Worker support, may require addons to register clean-up hooks that will be run once the current Node.js environment exits.
Node-API provides functions for registering and un-registering such callbacks. When those callbacks are run, all resources that are being held by the addon should be freed up.
napi_add_env_cleanup_hook
#
NODE_EXTERN napi_status napi_add_env_cleanup_hook(napi_env env,
napi_cleanup_hook fun,
void* arg);
Registers fun
as a function to be run with the arg
parameter once the
current Node.js environment exits.
A function can safely be specified multiple times with different
arg
values. In that case, it will be called multiple times as well.
Providing the same fun
and arg
values multiple times is not allowed
and will lead the process to abort.
The hooks will be called in reverse order, i.e. the most recently added one will be called first.
Removing this hook can be done by using napi_remove_env_cleanup_hook
.
Typically, that happens when the resource for which this hook was added
is being torn down anyway.
For asynchronous cleanup, napi_add_async_cleanup_hook
is available.
napi_remove_env_cleanup_hook
#
NAPI_EXTERN napi_status napi_remove_env_cleanup_hook(napi_env env,
void (*fun)(void* arg),
void* arg);
Unregisters fun
as a function to be run with the arg
parameter once the
current Node.js environment exits. Both the argument and the function value
need to be exact matches.
The function must have originally been registered
with napi_add_env_cleanup_hook
, otherwise the process will abort.
napi_add_async_cleanup_hook
#
NAPI_EXTERN napi_status napi_add_async_cleanup_hook(
napi_env env,
napi_async_cleanup_hook hook,
void* arg,
napi_async_cleanup_hook_handle* remove_handle);
[in] env
: The environment that the API is invoked under.[in] hook
: The function pointer to call at environment teardown.[in] arg
: The pointer to pass tohook
when it gets called.[out] remove_handle
: Optional handle that refers to the asynchronous cleanup hook.
Registers hook
, which is a function of type napi_async_cleanup_hook
, as
a function to be run with the remove_handle
and arg
parameters once the
current Node.js environment exits.
Unlike napi_add_env_cleanup_hook
, the hook is allowed to be asynchronous.
Otherwise, behavior generally matches that of napi_add_env_cleanup_hook
.
If remove_handle
is not NULL
, an opaque value will be stored in it
that must later be passed to napi_remove_async_cleanup_hook
,
regardless of whether the hook has already been invoked.
Typically, that happens when the resource for which this hook was added
is being torn down anyway.
napi_remove_async_cleanup_hook
#
NAPI_EXTERN napi_status napi_remove_async_cleanup_hook(
napi_async_cleanup_hook_handle remove_handle);
[in] remove_handle
: The handle to an asynchronous cleanup hook that was created withnapi_add_async_cleanup_hook
.
Unregisters the cleanup hook corresponding to remove_handle
. This will prevent
the hook from being executed, unless it has already started executing.
This must be called on any napi_async_cleanup_hook_handle
value obtained
from napi_add_async_cleanup_hook
.
Finalization on the exit of the Node.js environment#
The Node.js environment may be torn down at an arbitrary time as soon as
possible with JavaScript execution disallowed, like on the request of
worker.terminate()
. When the environment is being torn down, the
registered napi_finalize
callbacks of JavaScript objects, Thread-safe
functions and environment instance data are invoked immediately and
independently.
The invocation of napi_finalize
callbacks are scheduled after the manually
registered cleanup hooks. In order to ensure a proper order of addon
finalization during environment shutdown to avoid use-after-free in the
napi_finalize
callback, addons should register a cleanup hook with
napi_add_env_cleanup_hook
and napi_add_async_cleanup_hook
to manually
release the allocated resource in a proper order.
Module registration#
Node-API modules are registered in a manner similar to other modules
except that instead of using the NODE_MODULE
macro the following
is used:
NAPI_MODULE(NODE_GYP_MODULE_NAME, Init)
The next difference is the signature for the Init
method. For a Node-API
module it is as follows:
napi_value Init(napi_env env, napi_value exports);
The return value from Init
is treated as the exports
object for the module.
The Init
method is passed an empty object via the exports
parameter as a
convenience. If Init
returns NULL
, the parameter passed as exports
is
exported by the module. Node-API modules cannot modify the module
object but
can specify anything as the exports
property of the module.
To add the method hello
as a function so that it can be called as a method
provided by the addon:
napi_value Init(napi_env env, napi_value exports) {
napi_status status;
napi_property_descriptor desc = {
"hello",
NULL,
Method,
NULL,
NULL,
NULL,
napi_writable | napi_enumerable | napi_configurable,
NULL
};
status = napi_define_properties(env, exports, 1, &desc);
if (status != napi_ok) return NULL;
return exports;
}
To set a function to be returned by the require()
for the addon:
napi_value Init(napi_env env, napi_value exports) {
napi_value method;
napi_status status;
status = napi_create_function(env, "exports", NAPI_AUTO_LENGTH, Method, NULL, &method);
if (status != napi_ok) return NULL;
return method;
}
To define a class so that new instances can be created (often used with Object wrap):
// NOTE: partial example, not all referenced code is included
napi_value Init(napi_env env, napi_value exports) {
napi_status status;
napi_property_descriptor properties[] = {
{ "value", NULL, NULL, GetValue, SetValue, NULL, napi_writable | napi_configurable, NULL },
DECLARE_NAPI_METHOD("plusOne", PlusOne),
DECLARE_NAPI_METHOD("multiply", Multiply),
};
napi_value cons;
status =
napi_define_class(env, "MyObject", New, NULL, 3, properties, &cons);
if (status != napi_ok) return NULL;
status = napi_create_reference(env, cons, 1, &constructor);
if (status != napi_ok) return NULL;
status = napi_set_named_property(env, exports, "MyObject", cons);
if (status != napi_ok) return NULL;
return exports;
}
You can also use the NAPI_MODULE_INIT
macro, which acts as a shorthand
for NAPI_MODULE
and defining an Init
function:
NAPI_MODULE_INIT() {
napi_value answer;
napi_status result;
status = napi_create_int64(env, 42, &answer);
if (status != napi_ok) return NULL;
status = napi_set_named_property(env, exports, "answer", answer);
if (status != napi_ok) return NULL;
return exports;
}
All Node-API addons are context-aware, meaning they may be loaded multiple times. There are a few design considerations when declaring such a module. The documentation on context-aware addons provides more details.
The variables env
and exports
will be available inside the function body
following the macro invocation.
For more details on setting properties on objects, see the section on Working with JavaScript properties.
For more details on building addon modules in general, refer to the existing API.
Working with JavaScript values#
Node-API exposes a set of APIs to create all types of JavaScript values. Some of these types are documented under Section 6 of the ECMAScript Language Specification.
Fundamentally, these APIs are used to do one of the following:
- Create a new JavaScript object
- Convert from a primitive C type to a Node-API value
- Convert from Node-API value to a primitive C type
- Get global instances including
undefined
andnull
Node-API values are represented by the type napi_value
.
Any Node-API call that requires a JavaScript value takes in a napi_value
.
In some cases, the API does check the type of the napi_value
up-front.
However, for better performance, it's better for the caller to make sure that
the napi_value
in question is of the JavaScript type expected by the API.
Enum types#
napi_key_collection_mode
#
typedef enum {
napi_key_include_prototypes,
napi_key_own_only
} napi_key_collection_mode;
Describes the Keys/Properties
filter enums:
napi_key_collection_mode
limits the range of collected properties.
napi_key_own_only
limits the collected properties to the given
object only. napi_key_include_prototypes
will include all keys
of the objects's prototype chain as well.
napi_key_filter
#
typedef enum {
napi_key_all_properties = 0,
napi_key_writable = 1,
napi_key_enumerable = 1 << 1,
napi_key_configurable = 1 << 2,
napi_key_skip_strings = 1 << 3,
napi_key_skip_symbols = 1 << 4
} napi_key_filter;
Property filter bits. They can be or'ed to build a composite filter.
napi_key_conversion
#
typedef enum {
napi_key_keep_numbers,
napi_key_numbers_to_strings
} napi_key_conversion;
napi_key_numbers_to_strings
will convert integer indices to
strings. napi_key_keep_numbers
will return numbers for integer
indices.
napi_valuetype
#
typedef enum {
// ES6 types (corresponds to typeof)
napi_undefined,
napi_null,
napi_boolean,
napi_number,
napi_string,
napi_symbol,
napi_object,
napi_function,
napi_external,
napi_bigint,
} napi_valuetype;
Describes the type of a napi_value
. This generally corresponds to the types
described in Section 6.1 of the ECMAScript Language Specification.
In addition to types in that section, napi_valuetype
can also represent
Function
s and Object
s with external data.
A JavaScript value of type napi_external
appears in JavaScript as a plain
object such that no properties can be set on it, and no prototype.
napi_typedarray_type
#
typedef enum {
napi_int8_array,
napi_uint8_array,
napi_uint8_clamped_array,
napi_int16_array,
napi_uint16_array,
napi_int32_array,
napi_uint32_array,
napi_float32_array,
napi_float64_array,
napi_bigint64_array,
napi_biguint64_array,
} napi_typedarray_type;
This represents the underlying binary scalar datatype of the TypedArray
.
Elements of this enum correspond to
Section 22.2 of the ECMAScript Language Specification.
Object creation functions#
napi_create_array
#
napi_status napi_create_array(napi_env env, napi_value* result)
[in] env
: The environment that the Node-API call is invoked under.[out] result
: Anapi_value
representing a JavaScriptArray
.
Returns napi_ok
if the API succeeded.
This API returns a Node-API value corresponding to a JavaScript Array
type.
JavaScript arrays are described in
Section 22.1 of the ECMAScript Language Specification.
napi_create_array_with_length
#
napi_status napi_create_array_with_length(napi_env env,
size_t length,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] length
: The initial length of theArray
.[out] result
: Anapi_value
representing a JavaScriptArray
.
Returns napi_ok
if the API succeeded.
This API returns a Node-API value corresponding to a JavaScript Array
type.
The Array
's length property is set to the passed-in length parameter.
However, the underlying buffer is not guaranteed to be pre-allocated by the VM
when the array is created. That behavior is left to the underlying VM
implementation. If the buffer must be a contiguous block of memory that can be
directly read and/or written via C, consider using
napi_create_external_arraybuffer
.
JavaScript arrays are described in Section 22.1 of the ECMAScript Language Specification.
napi_create_arraybuffer
#
napi_status napi_create_arraybuffer(napi_env env,
size_t byte_length,
void** data,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] length
: The length in bytes of the array buffer to create.[out] data
: Pointer to the underlying byte buffer of theArrayBuffer
.data
can optionally be ignored by passingNULL
.[out] result
: Anapi_value
representing a JavaScriptArrayBuffer
.
Returns napi_ok
if the API succeeded.
This API returns a Node-API value corresponding to a JavaScript ArrayBuffer
.
ArrayBuffer
s are used to represent fixed-length binary data buffers. They are
normally used as a backing-buffer for TypedArray
objects.
The ArrayBuffer
allocated will have an underlying byte buffer whose size is
determined by the length
parameter that's passed in.
The underlying buffer is optionally returned back to the caller in case the
caller wants to directly manipulate the buffer. This buffer can only be
written to directly from native code. To write to this buffer from JavaScript,
a typed array or DataView
object would need to be created.
JavaScript ArrayBuffer
objects are described in
Section 24.1 of the ECMAScript Language Specification.
napi_create_buffer
#
napi_status napi_create_buffer(napi_env env,
size_t size,
void** data,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] size
: Size in bytes of the underlying buffer.[out] data
: Raw pointer to the underlying buffer.data
can optionally be ignored by passingNULL
.[out] result
: Anapi_value
representing anode::Buffer
.
Returns napi_ok
if the API succeeded.
This API allocates a node::Buffer
object. While this is still a
fully-supported data structure, in most cases using a TypedArray
will suffice.
napi_create_buffer_copy
#
napi_status napi_create_buffer_copy(napi_env env,
size_t length,
const void* data,
void** result_data,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] size
: Size in bytes of the input buffer (should be the same as the size of the new buffer).[in] data
: Raw pointer to the underlying buffer to copy from.[out] result_data
: Pointer to the newBuffer
's underlying data buffer.result_data
can optionally be ignored by passingNULL
.[out] result
: Anapi_value
representing anode::Buffer
.
Returns napi_ok
if the API succeeded.
This API allocates a node::Buffer
object and initializes it with data copied
from the passed-in buffer. While this is still a fully-supported data
structure, in most cases using a TypedArray
will suffice.
napi_create_date
#
napi_status napi_create_date(napi_env env,
double time,
napi_value* result);
[in] env
: The environment that the API is invoked under.[in] time
: ECMAScript time value in milliseconds since 01 January, 1970 UTC.[out] result
: Anapi_value
representing a JavaScriptDate
.
Returns napi_ok
if the API succeeded.
This API does not observe leap seconds; they are ignored, as ECMAScript aligns with POSIX time specification.
This API allocates a JavaScript Date
object.
JavaScript Date
objects are described in
Section 20.3 of the ECMAScript Language Specification.
napi_create_external
#
napi_status napi_create_external(napi_env env,
void* data,
napi_finalize finalize_cb,
void* finalize_hint,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] data
: Raw pointer to the external data.[in] finalize_cb
: Optional callback to call when the external value is being collected.napi_finalize
provides more details.[in] finalize_hint
: Optional hint to pass to the finalize callback during collection.[out] result
: Anapi_value
representing an external value.
Returns napi_ok
if the API succeeded.
This API allocates a JavaScript value with external data attached to it. This
is used to pass external data through JavaScript code, so it can be retrieved
later by native code using napi_get_value_external
.
The API adds a napi_finalize
callback which will be called when the JavaScript
object just created has been garbage collected.
The created value is not an object, and therefore does not support additional
properties. It is considered a distinct value type: calling napi_typeof()
with
an external value yields napi_external
.
napi_create_external_arraybuffer
#
napi_status
napi_create_external_arraybuffer(napi_env env,
void* external_data,
size_t byte_length,
napi_finalize finalize_cb,
void* finalize_hint,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] external_data
: Pointer to the underlying byte buffer of theArrayBuffer
.[in] byte_length
: The length in bytes of the underlying buffer.[in] finalize_cb
: Optional callback to call when theArrayBuffer
is being collected.napi_finalize
provides more details.[in] finalize_hint
: Optional hint to pass to the finalize callback during collection.[out] result
: Anapi_value
representing a JavaScriptArrayBuffer
.
Returns napi_ok
if the API succeeded.
Some runtimes other than Node.js have dropped support for external buffers.
On runtimes other than Node.js this method may return
napi_no_external_buffers_allowed
to indicate that external
buffers are not supported. One such runtime is Electron as
described in this issue
electron/issues/35801.
In order to maintain broadest compatibility with all runtimes
you may define NODE_API_NO_EXTERNAL_BUFFERS_ALLOWED
in your addon before
includes for the node-api headers. Doing so will hide the 2 functions
that create external buffers. This will ensure a compilation error
occurs if you accidentally use one of these methods.
This API returns a Node-API value corresponding to a JavaScript ArrayBuffer
.
The underlying byte buffer of the ArrayBuffer
is externally allocated and
managed. The caller must ensure that the byte buffer remains valid until the
finalize callback is called.
The API adds a napi_finalize
callback which will be called when the JavaScript
object just created has been garbage collected.
JavaScript ArrayBuffer
s are described in
Section 24.1 of the ECMAScript Language Specification.
napi_create_external_buffer
#
napi_status napi_create_external_buffer(napi_env env,
size_t length,
void* data,
napi_finalize finalize_cb,
void* finalize_hint,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] length
: Size in bytes of the input buffer (should be the same as the size of the new buffer).[in] data
: Raw pointer to the underlying buffer to expose to JavaScript.[in] finalize_cb
: Optional callback to call when theArrayBuffer
is being collected.napi_finalize
provides more details.[in] finalize_hint
: Optional hint to pass to the finalize callback during collection.[out] result
: Anapi_value
representing anode::Buffer
.
Returns napi_ok
if the API succeeded.
Some runtimes other than Node.js have dropped support for external buffers.
On runtimes other than Node.js this method may return
napi_no_external_buffers_allowed
to indicate that external
buffers are not supported. One such runtime is Electron as
described in this issue
electron/issues/35801.
In order to maintain broadest compatibility with all runtimes
you may define NODE_API_NO_EXTERNAL_BUFFERS_ALLOWED
in your addon before
includes for the node-api headers. Doing so will hide the 2 functions
that create external buffers. This will ensure a compilation error
occurs if you accidentally use one of these methods.
This API allocates a node::Buffer
object and initializes it with data
backed by the passed in buffer. While this is still a fully-supported data
structure, in most cases using a TypedArray
will suffice.
The API adds a napi_finalize
callback which will be called when the JavaScript
object just created has been garbage collected.
For Node.js >=4 Buffers
are Uint8Array
s.
napi_create_object
#
napi_status napi_create_object(napi_env env, napi_value* result)
[in] env
: The environment that the API is invoked under.[out] result
: Anapi_value
representing a JavaScriptObject
.
Returns napi_ok
if the API succeeded.
This API allocates a default JavaScript Object
.
It is the equivalent of doing new Object()
in JavaScript.
The JavaScript Object
type is described in Section 6.1.7 of the
ECMAScript Language Specification.
napi_create_symbol
#
napi_status napi_create_symbol(napi_env env,
napi_value description,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] description
: Optionalnapi_value
which refers to a JavaScriptstring
to be set as the description for the symbol.[out] result
: Anapi_value
representing a JavaScriptsymbol
.
Returns napi_ok
if the API succeeded.
This API creates a JavaScript symbol
value from a UTF8-encoded C string.
The JavaScript symbol
type is described in Section 19.4
of the ECMAScript Language Specification.
node_api_symbol_for
#
napi_status node_api_symbol_for(napi_env env,
const char* utf8description,
size_t length,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] utf8description
: UTF-8 C string representing the text to be used as the description for the symbol.[in] length
: The length of the description string in bytes, orNAPI_AUTO_LENGTH
if it is null-terminated.[out] result
: Anapi_value
representing a JavaScriptsymbol
.
Returns napi_ok
if the API succeeded.
This API searches in the global registry for an existing symbol with the given description. If the symbol already exists it will be returned, otherwise a new symbol will be created in the registry.
The JavaScript symbol
type is described in Section 19.4 of the ECMAScript
Language Specification.
napi_create_typedarray
#
napi_status napi_create_typedarray(napi_env env,
napi_typedarray_type type,
size_t length,
napi_value arraybuffer,
size_t byte_offset,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] type
: Scalar datatype of the elements within theTypedArray
.[in] length
: Number of elements in theTypedArray
.[in] arraybuffer
:ArrayBuffer
underlying the typed array.[in] byte_offset
: The byte offset within theArrayBuffer
from which to start projecting theTypedArray
.[out] result
: Anapi_value
representing a JavaScriptTypedArray
.
Returns napi_ok
if the API succeeded.
This API creates a JavaScript TypedArray
object over an existing
ArrayBuffer
. TypedArray
objects provide an array-like view over an
underlying data buffer where each element has the same underlying binary scalar
datatype.
It's required that (length * size_of_element) + byte_offset
should
be <= the size in bytes of the array passed in. If not, a RangeError
exception
is raised.
JavaScript TypedArray
objects are described in
Section 22.2 of the ECMAScript Language Specification.
napi_create_dataview
#
napi_status napi_create_dataview(napi_env env,
size_t byte_length,
napi_value arraybuffer,
size_t byte_offset,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] length
: Number of elements in theDataView
.[in] arraybuffer
:ArrayBuffer
underlying theDataView
.[in] byte_offset
: The byte offset within theArrayBuffer
from which to start projecting theDataView
.[out] result
: Anapi_value
representing a JavaScriptDataView
.
Returns napi_ok
if the API succeeded.
This API creates a JavaScript DataView
object over an existing ArrayBuffer
.
DataView
objects provide an array-like view over an underlying data buffer,
but one which allows items of different size and type in the ArrayBuffer
.
It is required that byte_length + byte_offset
is less than or equal to the
size in bytes of the array passed in. If not, a RangeError
exception is
raised.
JavaScript DataView
objects are described in
Section 24.3 of the ECMAScript Language Specification.
Functions to convert from C types to Node-API#
napi_create_int32
#
napi_status napi_create_int32(napi_env env, int32_t value, napi_value* result)
[in] env
: The environment that the API is invoked under.[in] value
: Integer value to be represented in JavaScript.[out] result
: Anapi_value
representing a JavaScriptnumber
.
Returns napi_ok
if the API succeeded.
This API is used to convert from the C int32_t
type to the JavaScript
number
type.
The JavaScript number
type is described in
Section 6.1.6 of the ECMAScript Language Specification.
napi_create_uint32
#
napi_status napi_create_uint32(napi_env env, uint32_t value, napi_value* result)
[in] env
: The environment that the API is invoked under.[in] value
: Unsigned integer value to be represented in JavaScript.[out] result
: Anapi_value
representing a JavaScriptnumber
.
Returns napi_ok
if the API succeeded.
This API is used to convert from the C uint32_t
type to the JavaScript
number
type.
The JavaScript number
type is described in
Section 6.1.6 of the ECMAScript Language Specification.
napi_create_int64
#
napi_status napi_create_int64(napi_env env, int64_t value, napi_value* result)
[in] env
: The environment that the API is invoked under.[in] value
: Integer value to be represented in JavaScript.[out] result
: Anapi_value
representing a JavaScriptnumber
.
Returns napi_ok
if the API succeeded.
This API is used to convert from the C int64_t
type to the JavaScript
number
type.
The JavaScript number
type is described in Section 6.1.6
of the ECMAScript Language Specification. Note the complete range of int64_t
cannot be represented with full precision in JavaScript. Integer values
outside the range of Number.MIN_SAFE_INTEGER
-(2**53 - 1)
-
Number.MAX_SAFE_INTEGER
(2**53 - 1)
will lose precision.
napi_create_double
#
napi_status napi_create_double(napi_env env, double value, napi_value* result)
[in] env
: The environment that the API is invoked under.[in] value
: Double-precision value to be represented in JavaScript.[out] result
: Anapi_value
representing a JavaScriptnumber
.
Returns napi_ok
if the API succeeded.
This API is used to convert from the C double
type to the JavaScript
number
type.
The JavaScript number
type is described in
Section 6.1.6 of the ECMAScript Language Specification.
napi_create_bigint_int64
#
napi_status napi_create_bigint_int64(napi_env env,
int64_t value,
napi_value* result);
[in] env
: The environment that the API is invoked under.[in] value
: Integer value to be represented in JavaScript.[out] result
: Anapi_value
representing a JavaScriptBigInt
.
Returns napi_ok
if the API succeeded.
This API converts the C int64_t
type to the JavaScript BigInt
type.
napi_create_bigint_uint64
#
napi_status napi_create_bigint_uint64(napi_env env,
uint64_t value,
napi_value* result);
[in] env
: The environment that the API is invoked under.[in] value
: Unsigned integer value to be represented in JavaScript.[out] result
: Anapi_value
representing a JavaScriptBigInt
.
Returns napi_ok
if the API succeeded.
This API converts the C uint64_t
type to the JavaScript BigInt
type.
napi_create_bigint_words
#
napi_status napi_create_bigint_words(napi_env env,
int sign_bit,
size_t word_count,
const uint64_t* words,
napi_value* result);
[in] env
: The environment that the API is invoked under.[in] sign_bit
: Determines if the resultingBigInt
will be positive or negative.[in] word_count
: The length of thewords
array.[in] words
: An array ofuint64_t
little-endian 64-bit words.[out] result
: Anapi_value
representing a JavaScriptBigInt
.
Returns napi_ok
if the API succeeded.
This API converts an array of unsigned 64-bit words into a single BigInt
value.
The resulting BigInt
is calculated as: (–1)sign_bit
(words[0]
× (264)0 + words[1]
× (264)1 + …)
napi_create_string_latin1
#
napi_status napi_create_string_latin1(napi_env env,
const char* str,
size_t length,
napi_value* result);
[in] env
: The environment that the API is invoked under.[in] str
: Character buffer representing an ISO-8859-1-encoded string.[in] length
: The length of the string in bytes, orNAPI_AUTO_LENGTH
if it is null-terminated.[out] result
: Anapi_value
representing a JavaScriptstring
.
Returns napi_ok
if the API succeeded.
This API creates a JavaScript string
value from an ISO-8859-1-encoded C
string. The native string is copied.
The JavaScript string
type is described in
Section 6.1.4 of the ECMAScript Language Specification.
napi_create_string_utf16
#
napi_status napi_create_string_utf16(napi_env env,
const char16_t* str,
size_t length,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] str
: Character buffer representing a UTF16-LE-encoded string.[in] length
: The length of the string in two-byte code units, orNAPI_AUTO_LENGTH
if it is null-terminated.[out] result
: Anapi_value
representing a JavaScriptstring
.
Returns napi_ok
if the API succeeded.
This API creates a JavaScript string
value from a UTF16-LE-encoded C string.
The native string is copied.
The JavaScript string
type is described in
Section 6.1.4 of the ECMAScript Language Specification.
napi_create_string_utf8
#
napi_status napi_create_string_utf8(napi_env env,
const char* str,
size_t length,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] str
: Character buffer representing a UTF8-encoded string.[in] length
: The length of the string in bytes, orNAPI_AUTO_LENGTH
if it is null-terminated.[out] result
: Anapi_value
representing a JavaScriptstring
.
Returns napi_ok
if the API succeeded.
This API creates a JavaScript string
value from a UTF8-encoded C string.
The native string is copied.
The JavaScript string
type is described in
Section 6.1.4 of the ECMAScript Language Specification.
Functions to convert from Node-API to C types#
napi_get_array_length
#
napi_status napi_get_array_length(napi_env env,
napi_value value,
uint32_t* result)
[in] env
: The environment that the API is invoked under.[in] value
:napi_value
representing the JavaScriptArray
whose length is being queried.[out] result
:uint32
representing length of the array.
Returns napi_ok
if the API succeeded.
This API returns the length of an array.
Array
length is described in Section 22.1.4.1 of the ECMAScript Language
Specification.
napi_get_arraybuffer_info
#
napi_status napi_get_arraybuffer_info(napi_env env,
napi_value arraybuffer,
void** data,
size_t* byte_length)
[in] env
: The environment that the API is invoked under.[in] arraybuffer
:napi_value
representing theArrayBuffer
being queried.[out] data
: The underlying data buffer of theArrayBuffer
. If byte_length is0
, this may beNULL
or any other pointer value.[out] byte_length
: Length in bytes of the underlying data buffer.
Returns napi_ok
if the API succeeded.
This API is used to retrieve the underlying data buffer of an ArrayBuffer
and
its length.
WARNING: Use caution while using this API. The lifetime of the underlying data
buffer is managed by the ArrayBuffer
even after it's returned. A
possible safe way to use this API is in conjunction with
napi_create_reference
, which can be used to guarantee control over the
lifetime of the ArrayBuffer
. It's also safe to use the returned data buffer
within the same callback as long as there are no calls to other APIs that might
trigger a GC.
napi_get_buffer_info
#
napi_status napi_get_buffer_info(napi_env env,
napi_value value,
void** data,
size_t* length)
[in] env
: The environment that the API is invoked under.[in] value
:napi_value
representing thenode::Buffer
being queried.[out] data
: The underlying data buffer of thenode::Buffer
. If length is0
, this may beNULL
or any other pointer value.[out] length
: Length in bytes of the underlying data buffer.
Returns napi_ok
if the API succeeded.
This API is used to retrieve the underlying data buffer of a node::Buffer
and its length.
Warning: Use caution while using this API since the underlying data buffer's lifetime is not guaranteed if it's managed by the VM.
napi_get_prototype
#
napi_status napi_get_prototype(napi_env env,
napi_value object,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] object
:napi_value
representing JavaScriptObject
whose prototype to return. This returns the equivalent ofObject.getPrototypeOf
(which is not the same as the function'sprototype
property).[out] result
:napi_value
representing prototype of the given object.
Returns napi_ok
if the API succeeded.
napi_get_typedarray_info
#
napi_status napi_get_typedarray_info(napi_env env,
napi_value typedarray,
napi_typedarray_type* type,
size_t* length,
void** data,
napi_value* arraybuffer,
size_t* byte_offset)
[in] env
: The environment that the API is invoked under.[in] typedarray
:napi_value
representing theTypedArray
whose properties to query.[out] type
: Scalar datatype of the elements within theTypedArray
.[out] length
: The number of elements in theTypedArray
.[out] data
: The data buffer underlying theTypedArray
adjusted by thebyte_offset
value so that it points to the first element in theTypedArray
. If the length of the array is0
, this may beNULL
or any other pointer value.[out] arraybuffer
: TheArrayBuffer
underlying theTypedArray
.[out] byte_offset
: The byte offset within the underlying native array at which the first element of the arrays is located. The value for the data parameter has already been adjusted so that data points to the first element in the array. Therefore, the first byte of the native array would be atdata - byte_offset
.
Returns napi_ok
if the API succeeded.
This API returns various properties of a typed array.
Any of the out parameters may be NULL
if that property is unneeded.
Warning: Use caution while using this API since the underlying data buffer is managed by the VM.
napi_get_dataview_info
#
napi_status napi_get_dataview_info(napi_env env,
napi_value dataview,
size_t* byte_length,
void** data,
napi_value* arraybuffer,
size_t* byte_offset)
[in] env
: The environment that the API is invoked under.[in] dataview
:napi_value
representing theDataView
whose properties to query.[out] byte_length
: Number of bytes in theDataView
.[out] data
: The data buffer underlying theDataView
. If byte_length is0
, this may beNULL
or any other pointer value.[out] arraybuffer
:ArrayBuffer
underlying theDataView
.[out] byte_offset
: The byte offset within the data buffer from which to start projecting theDataView
.
Returns napi_ok
if the API succeeded.
Any of the out parameters may be NULL
if that property is unneeded.
This API returns various properties of a DataView
.
napi_get_date_value
#
napi_status napi_get_date_value(napi_env env,
napi_value value,
double* result)
[in] env
: The environment that the API is invoked under.[in] value
:napi_value
representing a JavaScriptDate
.[out] result
: Time value as adouble
represented as milliseconds since midnight at the beginning of 01 January, 1970 UTC.
This API does not observe leap seconds; they are ignored, as ECMAScript aligns with POSIX time specification.
Returns napi_ok
if the API succeeded. If a non-date napi_value
is passed
in it returns napi_date_expected
.
This API returns the C double primitive of time value for the given JavaScript
Date
.
napi_get_value_bool
#
napi_status napi_get_value_bool(napi_env env, napi_value value, bool* result)
[in] env
: The environment that the API is invoked under.[in] value
:napi_value
representing JavaScriptBoolean
.[out] result
: C boolean primitive equivalent of the given JavaScriptBoolean
.
Returns napi_ok
if the API succeeded. If a non-boolean napi_value
is
passed in it returns napi_boolean_expected
.
This API returns the C boolean primitive equivalent of the given JavaScript
Boolean
.
napi_get_value_double
#
napi_status napi_get_value_double(napi_env env,
napi_value value,
double* result)
[in] env
: The environment that the API is invoked under.[in] value
:napi_value
representing JavaScriptnumber
.[out] result
: C double primitive equivalent of the given JavaScriptnumber
.
Returns napi_ok
if the API succeeded. If a non-number napi_value
is passed
in it returns napi_number_expected
.
This API returns the C double primitive equivalent of the given JavaScript
number
.
napi_get_value_bigint_int64
#
napi_status napi_get_value_bigint_int64(napi_env env,
napi_value value,
int64_t* result,
bool* lossless);
[in] env
: The environment that the API is invoked under[in] value
:napi_value
representing JavaScriptBigInt
.[out] result
: Cint64_t
primitive equivalent of the given JavaScriptBigInt
.[out] lossless
: Indicates whether theBigInt
value was converted losslessly.
Returns napi_ok
if the API succeeded. If a non-BigInt
is passed in it
returns napi_bigint_expected
.
This API returns the C int64_t
primitive equivalent of the given JavaScript
BigInt
. If needed it will truncate the value, setting lossless
to false
.
napi_get_value_bigint_uint64
#
napi_status napi_get_value_bigint_uint64(napi_env env,
napi_value value,
uint64_t* result,
bool* lossless);
[in] env
: The environment that the API is invoked under.[in] value
:napi_value
representing JavaScriptBigInt
.[out] result
: Cuint64_t
primitive equivalent of the given JavaScriptBigInt
.[out] lossless
: Indicates whether theBigInt
value was converted losslessly.
Returns napi_ok
if the API succeeded. If a non-BigInt
is passed in it
returns napi_bigint_expected
.
This API returns the C uint64_t
primitive equivalent of the given JavaScript
BigInt
. If needed it will truncate the value, setting lossless
to false
.
napi_get_value_bigint_words
#
napi_status napi_get_value_bigint_words(napi_env env,
napi_value value,
int* sign_bit,
size_t* word_count,
uint64_t* words);
[in] env
: The environment that the API is invoked under.[in] value
:napi_value
representing JavaScriptBigInt
.[out] sign_bit
: Integer representing if the JavaScriptBigInt
is positive or negative.[in/out] word_count
: Must be initialized to the length of thewords
array. Upon return, it will be set to the actual number of words that would be needed to store thisBigInt
.[out] words
: Pointer to a pre-allocated 64-bit word array.
Returns napi_ok
if the API succeeded.
This API converts a single BigInt
value into a sign bit, 64-bit little-endian
array, and the number of elements in the array. sign_bit
and words
may be
both set to NULL
, in order to get only word_count
.
napi_get_value_external
#
napi_status napi_get_value_external(napi_env env,
napi_value value,
void** result)
[in] env
: The environment that the API is invoked under.[in] value
:napi_value
representing JavaScript external value.[out] result
: Pointer to the data wrapped by the JavaScript external value.
Returns napi_ok
if the API succeeded. If a non-external napi_value
is
passed in it returns napi_invalid_arg
.
This API retrieves the external data pointer that was previously passed to
napi_create_external()
.
napi_get_value_int32
#
napi_status napi_get_value_int32(napi_env env,
napi_value value,
int32_t* result)
[in] env
: The environment that the API is invoked under.[in] value
:napi_value
representing JavaScriptnumber
.[out] result
: Cint32
primitive equivalent of the given JavaScriptnumber
.
Returns napi_ok
if the API succeeded. If a non-number napi_value
is passed in napi_number_expected
.
This API returns the C int32
primitive equivalent
of the given JavaScript number
.
If the number exceeds the range of the 32 bit integer, then the result is truncated to the equivalent of the bottom 32 bits. This can result in a large positive number becoming a negative number if the value is > 231 - 1.
Non-finite number values (NaN
, +Infinity
, or -Infinity
) set the
result to zero.
napi_get_value_int64
#
napi_status napi_get_value_int64(napi_env env,
napi_value value,
int64_t* result)
[in] env
: The environment that the API is invoked under.[in] value
:napi_value
representing JavaScriptnumber
.[out] result
: Cint64
primitive equivalent of the given JavaScriptnumber
.
Returns napi_ok
if the API succeeded. If a non-number napi_value
is passed in it returns napi_number_expected
.
This API returns the C int64
primitive equivalent of the given JavaScript
number
.
number
values outside the range of Number.MIN_SAFE_INTEGER
-(2**53 - 1)
- Number.MAX_SAFE_INTEGER
(2**53 - 1)
will lose
precision.
Non-finite number values (NaN
, +Infinity
, or -Infinity
) set the
result to zero.
napi_get_value_string_latin1
#
napi_status napi_get_value_string_latin1(napi_env env,
napi_value value,
char* buf,
size_t bufsize,
size_t* result)
[in] env
: The environment that the API is invoked under.[in] value
:napi_value
representing JavaScript string.[in] buf
: Buffer to write the ISO-8859-1-encoded string into. IfNULL
is passed in, the length of the string in bytes and excluding the null terminator is returned inresult
.[in] bufsize
: Size of the destination buffer. When this value is insufficient, the returned string is truncated and null-terminated.[out] result
: Number of bytes copied into the buffer, excluding the null terminator.
Returns napi_ok
if the API succeeded. If a non-string
napi_value
is passed in it returns napi_string_expected
.
This API returns the ISO-8859-1-encoded string corresponding the value passed in.
napi_get_value_string_utf8
#
napi_status napi_get_value_string_utf8(napi_env env,
napi_value value,
char* buf,
size_t bufsize,
size_t* result)
[in] env
: The environment that the API is invoked under.[in] value
:napi_value
representing JavaScript string.[in] buf
: Buffer to write the UTF8-encoded string into. IfNULL
is passed in, the length of the string in bytes and excluding the null terminator is returned inresult
.[in] bufsize
: Size of the destination buffer. When this value is insufficient, the returned string is truncated and null-terminated.[out] result
: Number of bytes copied into the buffer, excluding the null terminator.
Returns napi_ok
if the API succeeded. If a non-string
napi_value
is passed in it returns napi_string_expected
.
This API returns the UTF8-encoded string corresponding the value passed in.
napi_get_value_string_utf16
#
napi_status napi_get_value_string_utf16(napi_env env,
napi_value value,
char16_t* buf,
size_t bufsize,
size_t* result)
[in] env
: The environment that the API is invoked under.[in] value
:napi_value
representing JavaScript string.[in] buf
: Buffer to write the UTF16-LE-encoded string into. IfNULL
is passed in, the length of the string in 2-byte code units and excluding the null terminator is returned.[in] bufsize
: Size of the destination buffer. When this value is insufficient, the returned string is truncated and null-terminated.[out] result
: Number of 2-byte code units copied into the buffer, excluding the null terminator.
Returns napi_ok
if the API succeeded. If a non-string
napi_value
is passed in it returns napi_string_expected
.
This API returns the UTF16-encoded string corresponding the value passed in.
napi_get_value_uint32
#
napi_status napi_get_value_uint32(napi_env env,
napi_value value,
uint32_t* result)
[in] env
: The environment that the API is invoked under.[in] value
:napi_value
representing JavaScriptnumber
.[out] result
: C primitive equivalent of the givennapi_value
as auint32_t
.
Returns napi_ok
if the API succeeded. If a non-number napi_value
is passed in it returns napi_number_expected
.
This API returns the C primitive equivalent of the given napi_value
as a
uint32_t
.
Functions to get global instances#
napi_get_boolean
#
napi_status napi_get_boolean(napi_env env, bool value, napi_value* result)
[in] env
: The environment that the API is invoked under.[in] value
: The value of the boolean to retrieve.[out] result
:napi_value
representing JavaScriptBoolean
singleton to retrieve.
Returns napi_ok
if the API succeeded.
This API is used to return the JavaScript singleton object that is used to represent the given boolean value.
napi_get_global
#
napi_status napi_get_global(napi_env env, napi_value* result)
[in] env
: The environment that the API is invoked under.[out] result
:napi_value
representing JavaScriptglobal
object.
Returns napi_ok
if the API succeeded.
This API returns the global
object.
napi_get_null
#
napi_status napi_get_null(napi_env env, napi_value* result)
[in] env
: The environment that the API is invoked under.[out] result
:napi_value
representing JavaScriptnull
object.
Returns napi_ok
if the API succeeded.
This API returns the null
object.
napi_get_undefined
#
napi_status napi_get_undefined(napi_env env, napi_value* result)
[in] env
: The environment that the API is invoked under.[out] result
:napi_value
representing JavaScript Undefined value.
Returns napi_ok
if the API succeeded.
This API returns the Undefined object.
Working with JavaScript values and abstract operations#
Node-API exposes a set of APIs to perform some abstract operations on JavaScript values. Some of these operations are documented under Section 7 of the ECMAScript Language Specification.
These APIs support doing one of the following:
- Coerce JavaScript values to specific JavaScript types (such as
number
orstring
). - Check the type of a JavaScript value.
- Check for equality between two JavaScript values.
napi_coerce_to_bool
#
napi_status napi_coerce_to_bool(napi_env env,
napi_value value,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] value
: The JavaScript value to coerce.[out] result
:napi_value
representing the coerced JavaScriptBoolean
.
Returns napi_ok
if the API succeeded.
This API implements the abstract operation ToBoolean()
as defined in
Section 7.1.2 of the ECMAScript Language Specification.
napi_coerce_to_number
#
napi_status napi_coerce_to_number(napi_env env,
napi_value value,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] value
: The JavaScript value to coerce.[out] result
:napi_value
representing the coerced JavaScriptnumber
.
Returns napi_ok
if the API succeeded.
This API implements the abstract operation ToNumber()
as defined in
Section 7.1.3 of the ECMAScript Language Specification.
This function potentially runs JS code if the passed-in value is an
object.
napi_coerce_to_object
#
napi_status napi_coerce_to_object(napi_env env,
napi_value value,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] value
: The JavaScript value to coerce.[out] result
:napi_value
representing the coerced JavaScriptObject
.
Returns napi_ok
if the API succeeded.
This API implements the abstract operation ToObject()
as defined in
Section 7.1.13 of the ECMAScript Language Specification.
napi_coerce_to_string
#
napi_status napi_coerce_to_string(napi_env env,
napi_value value,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] value
: The JavaScript value to coerce.[out] result
:napi_value
representing the coerced JavaScriptstring
.
Returns napi_ok
if the API succeeded.
This API implements the abstract operation ToString()
as defined in
Section 7.1.13 of the ECMAScript Language Specification.
This function potentially runs JS code if the passed-in value is an
object.
napi_typeof
#
napi_status napi_typeof(napi_env env, napi_value value, napi_valuetype* result)
[in] env
: The environment that the API is invoked under.[in] value
: The JavaScript value whose type to query.[out] result
: The type of the JavaScript value.
Returns napi_ok
if the API succeeded.
napi_invalid_arg
if the type ofvalue
is not a known ECMAScript type andvalue
is not an External value.
This API represents behavior similar to invoking the typeof
Operator on
the object as defined in Section 12.5.5 of the ECMAScript Language
Specification. However, there are some differences:
- It has support for detecting an External value.
- It detects
null
as a separate type, while ECMAScripttypeof
would detectobject
.
If value
has a type that is invalid, an error is returned.
napi_instanceof
#
napi_status napi_instanceof(napi_env env,
napi_value object,
napi_value constructor,
bool* result)
[in] env
: The environment that the API is invoked under.[in] object
: The JavaScript value to check.[in] constructor
: The JavaScript function object of the constructor function to check against.[out] result
: Boolean that is set to true ifobject instanceof constructor
is true.
Returns napi_ok
if the API succeeded.
This API represents invoking the instanceof
Operator on the object as
defined in Section 12.10.4 of the ECMAScript Language Specification.
napi_is_array
#
napi_status napi_is_array(napi_env env, napi_value value, bool* result)
[in] env
: The environment that the API is invoked under.[in] value
: The JavaScript value to check.[out] result
: Whether the given object is an array.
Returns napi_ok
if the API succeeded.
This API represents invoking the IsArray
operation on the object
as defined in Section 7.2.2 of the ECMAScript Language Specification.
napi_is_arraybuffer
#
napi_status napi_is_arraybuffer(napi_env env, napi_value value, bool* result)
[in] env
: The environment that the API is invoked under.[in] value
: The JavaScript value to check.[out] result
: Whether the given object is anArrayBuffer
.
Returns napi_ok
if the API succeeded.
This API checks if the Object
passed in is an array buffer.
napi_is_buffer
#
napi_status napi_is_buffer(napi_env env, napi_value value, bool* result)
[in] env
: The environment that the API is invoked under.[in] value
: The JavaScript value to check.[out] result
: Whether the givennapi_value
represents anode::Buffer
object.
Returns napi_ok
if the API succeeded.
This API checks if the Object
passed in is a buffer.
napi_is_date
#
napi_status napi_is_date(napi_env env, napi_value value, bool* result)
[in] env
: The environment that the API is invoked under.[in] value
: The JavaScript value to check.[out] result
: Whether the givennapi_value
represents a JavaScriptDate
object.
Returns napi_ok
if the API succeeded.
This API checks if the Object
passed in is a date.
napi_is_error
#
napi_status napi_is_error(napi_env env, napi_value value, bool* result)
[in] env
: The environment that the API is invoked under.[in] value
: The JavaScript value to check.[out] result
: Whether the givennapi_value
represents anError
object.
Returns napi_ok
if the API succeeded.
This API checks if the Object
passed in is an Error
.
napi_is_typedarray
#
napi_status napi_is_typedarray(napi_env env, napi_value value, bool* result)
[in] env
: The environment that the API is invoked under.[in] value
: The JavaScript value to check.[out] result
: Whether the givennapi_value
represents aTypedArray
.
Returns napi_ok
if the API succeeded.
This API checks if the Object
passed in is a typed array.
napi_is_dataview
#
napi_status napi_is_dataview(napi_env env, napi_value value, bool* result)
[in] env
: The environment that the API is invoked under.[in] value
: The JavaScript value to check.[out] result
: Whether the givennapi_value
represents aDataView
.
Returns napi_ok
if the API succeeded.
This API checks if the Object
passed in is a DataView
.
napi_strict_equals
#
napi_status napi_strict_equals(napi_env env,
napi_value lhs,
napi_value rhs,
bool* result)
[in] env
: The environment that the API is invoked under.[in] lhs
: The JavaScript value to check.[in] rhs
: The JavaScript value to check against.[out] result
: Whether the twonapi_value
objects are equal.
Returns napi_ok
if the API succeeded.
This API represents the invocation of the Strict Equality algorithm as defined in Section 7.2.14 of the ECMAScript Language Specification.
napi_detach_arraybuffer
#
napi_status napi_detach_arraybuffer(napi_env env,
napi_value arraybuffer)
[in] env
: The environment that the API is invoked under.[in] arraybuffer
: The JavaScriptArrayBuffer
to be detached.
Returns napi_ok
if the API succeeded. If a non-detachable ArrayBuffer
is
passed in it returns napi_detachable_arraybuffer_expected
.
Generally, an ArrayBuffer
is non-detachable if it has been detached before.
The engine may impose additional conditions on whether an ArrayBuffer
is
detachable. For example, V8 requires that the ArrayBuffer
be external,
that is, created with napi_create_external_arraybuffer
.
This API represents the invocation of the ArrayBuffer
detach operation as
defined in Section 24.1.1.3 of the ECMAScript Language Specification.
napi_is_detached_arraybuffer
#
napi_status napi_is_detached_arraybuffer(napi_env env,
napi_value arraybuffer,
bool* result)
[in] env
: The environment that the API is invoked under.[in] arraybuffer
: The JavaScriptArrayBuffer
to be checked.[out] result
: Whether thearraybuffer
is detached.
Returns napi_ok
if the API succeeded.
The ArrayBuffer
is considered detached if its internal data is null
.
This API represents the invocation of the ArrayBuffer
IsDetachedBuffer
operation as defined in Section 24.1.1.2 of the ECMAScript Language
Specification.
Working with JavaScript properties#
Node-API exposes a set of APIs to get and set properties on JavaScript objects. Some of these types are documented under Section 7 of the ECMAScript Language Specification.
Properties in JavaScript are represented as a tuple of a key and a value. Fundamentally, all property keys in Node-API can be represented in one of the following forms:
- Named: a simple UTF8-encoded string
- Integer-Indexed: an index value represented by
uint32_t
- JavaScript value: these are represented in Node-API by
napi_value
. This can be anapi_value
representing astring
,number
, orsymbol
.
Node-API values are represented by the type napi_value
.
Any Node-API call that requires a JavaScript value takes in a napi_value
.
However, it's the caller's responsibility to make sure that the
napi_value
in question is of the JavaScript type expected by the API.
The APIs documented in this section provide a simple interface to
get and set properties on arbitrary JavaScript objects represented by
napi_value
.
For instance, consider the following JavaScript code snippet:
const obj = {};
obj.myProp = 123;
The equivalent can be done using Node-API values with the following snippet:
napi_status status = napi_generic_failure;
// const obj = {}
napi_value obj, value;
status = napi_create_object(env, &obj);
if (status != napi_ok) return status;
// Create a napi_value for 123
status = napi_create_int32(env, 123, &value);
if (status != napi_ok) return status;
// obj.myProp = 123
status = napi_set_named_property(env, obj, "myProp", value);
if (status != napi_ok) return status;
Indexed properties can be set in a similar manner. Consider the following JavaScript snippet:
const arr = [];
arr[123] = 'hello';
The equivalent can be done using Node-API values with the following snippet:
napi_status status = napi_generic_failure;
// const arr = [];
napi_value arr, value;
status = napi_create_array(env, &arr);
if (status != napi_ok) return status;
// Create a napi_value for 'hello'
status = napi_create_string_utf8(env, "hello", NAPI_AUTO_LENGTH, &value);
if (status != napi_ok) return status;
// arr[123] = 'hello';
status = napi_set_element(env, arr, 123, value);
if (status != napi_ok) return status;
Properties can be retrieved using the APIs described in this section. Consider the following JavaScript snippet:
const arr = [];
const value = arr[123];
The following is the approximate equivalent of the Node-API counterpart:
napi_status status = napi_generic_failure;
// const arr = []
napi_value arr, value;
status = napi_create_array(env, &arr);
if (status != napi_ok) return status;
// const value = arr[123]
status = napi_get_element(env, arr, 123, &value);
if (status != napi_ok) return status;
Finally, multiple properties can also be defined on an object for performance reasons. Consider the following JavaScript:
const obj = {};
Object.defineProperties(obj, {
'foo': { value: 123, writable: true, configurable: true, enumerable: true },
'bar': { value: 456, writable: true, configurable: true, enumerable: true },
});
The following is the approximate equivalent of the Node-API counterpart:
napi_status status = napi_status_generic_failure;
// const obj = {};
napi_value obj;
status = napi_create_object(env, &obj);
if (status != napi_ok) return status;
// Create napi_values for 123 and 456
napi_value fooValue, barValue;
status = napi_create_int32(env, 123, &fooValue);
if (status != napi_ok) return status;
status = napi_create_int32(env, 456, &barValue);
if (status != napi_ok) return status;
// Set the properties
napi_property_descriptor descriptors[] = {
{ "foo", NULL, NULL, NULL, NULL, fooValue, napi_writable | napi_configurable, NULL },
{ "bar", NULL, NULL, NULL, NULL, barValue, napi_writable | napi_configurable, NULL }
}
status = napi_define_properties(env,
obj,
sizeof(descriptors) / sizeof(descriptors[0]),
descriptors);
if (status != napi_ok) return status;
Structures#
napi_property_attributes
#
typedef enum {
napi_default = 0,
napi_writable = 1 << 0,
napi_enumerable = 1 << 1,
napi_configurable = 1 << 2,
// Used with napi_define_class to distinguish static properties
// from instance properties. Ignored by napi_define_properties.
napi_static = 1 << 10,
// Default for class methods.
napi_default_method = napi_writable | napi_configurable,
// Default for object properties, like in JS obj[prop].
napi_default_jsproperty = napi_writable |
napi_enumerable |
napi_configurable,
} napi_property_attributes;
napi_property_attributes
are flags used to control the behavior of properties
set on a JavaScript object. Other than napi_static
they correspond to the
attributes listed in Section 6.1.7.1
of the ECMAScript Language Specification.
They can be one or more of the following bitflags:
napi_default
: No explicit attributes are set on the property. By default, a property is read only, not enumerable and not configurable.napi_writable
: The property is writable.napi_enumerable
: The property is enumerable.napi_configurable
: The property is configurable as defined in Section 6.1.7.1 of the ECMAScript Language Specification.napi_static
: The property will be defined as a static property on a class as opposed to an instance property, which is the default. This is used only bynapi_define_class
. It is ignored bynapi_define_properties
.napi_default_method
: Like a method in a JS class, the property is configurable and writeable, but not enumerable.napi_default_jsproperty
: Like a property set via assignment in JavaScript, the property is writable, enumerable, and configurable.
napi_property_descriptor
#
typedef struct {
// One of utf8name or name should be NULL.
const char* utf8name;
napi_value name;
napi_callback method;
napi_callback getter;
napi_callback setter;
napi_value value;
napi_property_attributes attributes;
void* data;
} napi_property_descriptor;
utf8name
: Optional string describing the key for the property, encoded as UTF8. One ofutf8name
orname
must be provided for the property.name
: Optionalnapi_value
that points to a JavaScript string or symbol to be used as the key for the property. One ofutf8name
orname
must be provided for the property.value
: The value that's retrieved by a get access of the property if the property is a data property. If this is passed in, setgetter
,setter
,method
anddata
toNULL
(since these members won't be used).getter
: A function to call when a get access of the property is performed. If this is passed in, setvalue
andmethod
toNULL
(since these members won't be used). The given function is called implicitly by the runtime when the property is accessed from JavaScript code (or if a get on the property is performed using a Node-API call).napi_callback
provides more details.setter
: A function to call when a set access of the property is performed. If this is passed in, setvalue
andmethod
toNULL
(since these members won't be used). The given function is called implicitly by the runtime when the property is set from JavaScript code (or if a set on the property is performed using a Node-API call).napi_callback
provides more details.method
: Set this to make the property descriptor object'svalue
property to be a JavaScript function represented bymethod
. If this is passed in, setvalue
,getter
andsetter
toNULL
(since these members won't be used).napi_callback
provides more details.attributes
: The attributes associated with the particular property. Seenapi_property_attributes
.data
: The callback data passed intomethod
,getter
andsetter
if this function is invoked.
Functions#
napi_get_property_names
#
napi_status napi_get_property_names(napi_env env,
napi_value object,
napi_value* result);
[in] env
: The environment that the Node-API call is invoked under.[in] object
: The object from which to retrieve the properties.[out] result
: Anapi_value
representing an array of JavaScript values that represent the property names of the object. The API can be used to iterate overresult
usingnapi_get_array_length
andnapi_get_element
.
Returns napi_ok
if the API succeeded.
This API returns the names of the enumerable properties of object
as an array
of strings. The properties of object
whose key is a symbol will not be
included.
napi_get_all_property_names
#
napi_get_all_property_names(napi_env env,
napi_value object,
napi_key_collection_mode key_mode,
napi_key_filter key_filter,
napi_key_conversion key_conversion,
napi_value* result);
[in] env
: The environment that the Node-API call is invoked under.[in] object
: The object from which to retrieve the properties.[in] key_mode
: Whether to retrieve prototype properties as well.[in] key_filter
: Which properties to retrieve (enumerable/readable/writable).[in] key_conversion
: Whether to convert numbered property keys to strings.[out] result
: Anapi_value
representing an array of JavaScript values that represent the property names of the object.napi_get_array_length
andnapi_get_element
can be used to iterate overresult
.
Returns napi_ok
if the API succeeded.
This API returns an array containing the names of the available properties of this object.
napi_set_property
#
napi_status napi_set_property(napi_env env,
napi_value object,
napi_value key,
napi_value value);
[in] env
: The environment that the Node-API call is invoked under.[in] object
: The object on which to set the property.[in] key
: The name of the property to set.[in] value
: The property value.
Returns napi_ok
if the API succeeded.
This API set a property on the Object
passed in.
napi_get_property
#
napi_status napi_get_property(napi_env env,
napi_value object,
napi_value key,
napi_value* result);
[in] env
: The environment that the Node-API call is invoked under.[in] object
: The object from which to retrieve the property.[in] key
: The name of the property to retrieve.[out] result
: The value of the property.
Returns napi_ok
if the API succeeded.
This API gets the requested property from the Object
passed in.
napi_has_property
#
napi_status napi_has_property(napi_env env,
napi_value object,
napi_value key,
bool* result);
[in] env
: The environment that the Node-API call is invoked under.[in] object
: The object to query.[in] key
: The name of the property whose existence to check.[out] result
: Whether the property exists on the object or not.
Returns napi_ok
if the API succeeded.
This API checks if the Object
passed in has the named property.
napi_delete_property
#
napi_status napi_delete_property(napi_env env,
napi_value object,
napi_value key,
bool* result);
[in] env
: The environment that the Node-API call is invoked under.[in] object
: The object to query.[in] key
: The name of the property to delete.[out] result
: Whether the property deletion succeeded or not.result
can optionally be ignored by passingNULL
.
Returns napi_ok
if the API succeeded.
This API attempts to delete the key
own property from object
.
napi_has_own_property
#
napi_status napi_has_own_property(napi_env env,
napi_value object,
napi_value key,
bool* result);
[in] env
: The environment that the Node-API call is invoked under.[in] object
: The object to query.[in] key
: The name of the own property whose existence to check.[out] result
: Whether the own property exists on the object or not.
Returns napi_ok
if the API succeeded.
This API checks if the Object
passed in has the named own property. key
must
be a string
or a symbol
, or an error will be thrown. Node-API will not
perform any conversion between data types.
napi_set_named_property
#
napi_status napi_set_named_property(napi_env env,
napi_value object,
const char* utf8Name,
napi_value value);
[in] env
: The environment that the Node-API call is invoked under.[in] object
: The object on which to set the property.[in] utf8Name
: The name of the property to set.[in] value
: The property value.
Returns napi_ok
if the API succeeded.
This method is equivalent to calling napi_set_property
with a napi_value
created from the string passed in as utf8Name
.
napi_get_named_property
#
napi_status napi_get_named_property(napi_env env,
napi_value object,
const char* utf8Name,
napi_value* result);
[in] env
: The environment that the Node-API call is invoked under.[in] object
: The object from which to retrieve the property.[in] utf8Name
: The name of the property to get.[out] result
: The value of the property.
Returns napi_ok
if the API succeeded.
This method is equivalent to calling napi_get_property
with a napi_value
created from the string passed in as utf8Name
.
napi_has_named_property
#
napi_status napi_has_named_property(napi_env env,
napi_value object,
const char* utf8Name,
bool* result);
[in] env
: The environment that the Node-API call is invoked under.[in] object
: The object to query.[in] utf8Name
: The name of the property whose existence to check.[out] result
: Whether the property exists on the object or not.
Returns napi_ok
if the API succeeded.
This method is equivalent to calling napi_has_property
with a napi_value
created from the string passed in as utf8Name
.
napi_set_element
#
napi_status napi_set_element(napi_env env,
napi_value object,
uint32_t index,
napi_value value);
[in] env
: The environment that the Node-API call is invoked under.[in] object
: The object from which to set the properties.[in] index
: The index of the property to set.[in] value
: The property value.
Returns napi_ok
if the API succeeded.
This API sets an element on the Object
passed in.
napi_get_element
#
napi_status napi_get_element(napi_env env,
napi_value object,
uint32_t index,
napi_value* result);
[in] env
: The environment that the Node-API call is invoked under.[in] object
: The object from which to retrieve the property.[in] index
: The index of the property to get.[out] result
: The value of the property.
Returns napi_ok
if the API succeeded.
This API gets the element at the requested index.
napi_has_element
#
napi_status napi_has_element(napi_env env,
napi_value object,
uint32_t index,
bool* result);
[in] env
: The environment that the Node-API call is invoked under.[in] object
: The object to query.[in] index
: The index of the property whose existence to check.[out] result
: Whether the property exists on the object or not.
Returns napi_ok
if the API succeeded.
This API returns if the Object
passed in has an element at the
requested index.
napi_delete_element
#
napi_status napi_delete_element(napi_env env,
napi_value object,
uint32_t index,
bool* result);
[in] env
: The environment that the Node-API call is invoked under.[in] object
: The object to query.[in] index
: The index of the property to delete.[out] result
: Whether the element deletion succeeded or not.result
can optionally be ignored by passingNULL
.
Returns napi_ok
if the API succeeded.
This API attempts to delete the specified index
from object
.
napi_define_properties
#
napi_status napi_define_properties(napi_env env,
napi_value object,
size_t property_count,
const napi_property_descriptor* properties);
[in] env
: The environment that the Node-API call is invoked under.[in] object
: The object from which to retrieve the properties.[in] property_count
: The number of elements in theproperties
array.[in] properties
: The array of property descriptors.
Returns napi_ok
if the API succeeded.
This method allows the efficient definition of multiple properties on a given
object. The properties are defined using property descriptors (see
napi_property_descriptor
). Given an array of such property descriptors,
this API will set the properties on the object one at a time, as defined by
DefineOwnProperty()
(described in Section 9.1.6 of the ECMA-262
specification).
napi_object_freeze
#
napi_status napi_object_freeze(napi_env env,
napi_value object);
[in] env
: The environment that the Node-API call is invoked under.[in] object
: The object to freeze.
Returns napi_ok
if the API succeeded.
This method freezes a given object. This prevents new properties from being added to it, existing properties from being removed, prevents changing the enumerability, configurability, or writability of existing properties, and prevents the values of existing properties from being changed. It also prevents the object's prototype from being changed. This is described in Section 19.1.2.6 of the ECMA-262 specification.
napi_object_seal
#
napi_status napi_object_seal(napi_env env,
napi_value object);
[in] env
: The environment that the Node-API call is invoked under.[in] object
: The object to seal.
Returns napi_ok
if the API succeeded.
This method seals a given object. This prevents new properties from being added to it, as well as marking all existing properties as non-configurable. This is described in Section 19.1.2.20 of the ECMA-262 specification.
Working with JavaScript functions#
Node-API provides a set of APIs that allow JavaScript code to
call back into native code. Node-APIs that support calling back
into native code take in a callback functions represented by
the napi_callback
type. When the JavaScript VM calls back to
native code, the napi_callback
function provided is invoked. The APIs
documented in this section allow the callback function to do the
following:
- Get information about the context in which the callback was invoked.
- Get the arguments passed into the callback.
- Return a
napi_value
back from the callback.
Additionally, Node-API provides a set of functions which allow calling JavaScript functions from native code. One can either call a function like a regular JavaScript function call, or as a constructor function.
Any non-NULL
data which is passed to this API via the data
field of the
napi_property_descriptor
items can be associated with object
and freed
whenever object
is garbage-collected by passing both object
and the data to
napi_add_finalizer
.
napi_call_function
#
NAPI_EXTERN napi_status napi_call_function(napi_env env,
napi_value recv,
napi_value func,
size_t argc,
const napi_value* argv,
napi_value* result);
[in] env
: The environment that the API is invoked under.[in] recv
: Thethis
value passed to the called function.[in] func
:napi_value
representing the JavaScript function to be invoked.[in] argc
: The count of elements in theargv
array.[in] argv
: Array ofnapi_values
representing JavaScript values passed in as arguments to the function.[out] result
:napi_value
representing the JavaScript object returned.
Returns napi_ok
if the API succeeded.
This method allows a JavaScript function object to be called from a native
add-on. This is the primary mechanism of calling back from the add-on's
native code into JavaScript. For the special case of calling into JavaScript
after an async operation, see napi_make_callback
.
A sample use case might look as follows. Consider the following JavaScript snippet:
function AddTwo(num) {
return num + 2;
}
global.AddTwo = AddTwo;
Then, the above function can be invoked from a native add-on using the following code:
// Get the function named "AddTwo" on the global object
napi_value global, add_two, arg;
napi_status status = napi_get_global(env, &global);
if (status != napi_ok) return;
status = napi_get_named_property(env, global, "AddTwo", &add_two);
if (status != napi_ok) return;
// const arg = 1337
status = napi_create_int32(env, 1337, &arg);
if (status != napi_ok) return;
napi_value* argv = &arg;
size_t argc = 1;
// AddTwo(arg);
napi_value return_val;
status = napi_call_function(env, global, add_two, argc, argv, &return_val);
if (status != napi_ok) return;
// Convert the result back to a native type
int32_t result;
status = napi_get_value_int32(env, return_val, &result);
if (status != napi_ok) return;
napi_create_function
#
napi_status napi_create_function(napi_env env,
const char* utf8name,
size_t length,
napi_callback cb,
void* data,
napi_value* result);
[in] env
: The environment that the API is invoked under.[in] utf8Name
: Optional name of the function encoded as UTF8. This is visible within JavaScript as the new function object'sname
property.[in] length
: The length of theutf8name
in bytes, orNAPI_AUTO_LENGTH
if it is null-terminated.[in] cb
: The native function which should be called when this function object is invoked.napi_callback
provides more details.[in] data
: User-provided data context. This will be passed back into the function when invoked later.[out] result
:napi_value
representing the JavaScript function object for the newly created function.
Returns napi_ok
if the API succeeded.
This API allows an add-on author to create a function object in native code. This is the primary mechanism to allow calling into the add-on's native code from JavaScript.
The newly created function is not automatically visible from script after this call. Instead, a property must be explicitly set on any object that is visible to JavaScript, in order for the function to be accessible from script.
In order to expose a function as part of the add-on's module exports, set the newly created function on the exports object. A sample module might look as follows:
napi_value SayHello(napi_env env, napi_callback_info info) {
printf("Hello\n");
return NULL;
}
napi_value Init(napi_env env, napi_value exports) {
napi_status status;
napi_value fn;
status = napi_create_function(env, NULL, 0, SayHello, NULL, &fn);
if (status != napi_ok) return NULL;
status = napi_set_named_property(env, exports, "sayHello", fn);
if (status != napi_ok) return NULL;
return exports;
}
NAPI_MODULE(NODE_GYP_MODULE_NAME, Init)
Given the above code, the add-on can be used from JavaScript as follows:
const myaddon = require('./addon');
myaddon.sayHello();
The string passed to require()
is the name of the target in binding.gyp
responsible for creating the .node
file.
Any non-NULL
data which is passed to this API via the data
parameter can
be associated with the resulting JavaScript function (which is returned in the
result
parameter) and freed whenever the function is garbage-collected by
passing both the JavaScript function and the data to napi_add_finalizer
.
JavaScript Function
s are described in Section 19.2 of the ECMAScript
Language Specification.
napi_get_cb_info
#
napi_status napi_get_cb_info(napi_env env,
napi_callback_info cbinfo,
size_t* argc,
napi_value* argv,
napi_value* thisArg,
void** data)
[in] env
: The environment that the API is invoked under.[in] cbinfo
: The callback info passed into the callback function.[in-out] argc
: Specifies the length of the providedargv
array and receives the actual count of arguments.argc
can optionally be ignored by passingNULL
.[out] argv
: C array ofnapi_value
s to which the arguments will be copied. If there are more arguments than the provided count, only the requested number of arguments are copied. If there are fewer arguments provided than claimed, the rest ofargv
is filled withnapi_value
values that representundefined
.argv
can optionally be ignored by passingNULL
.[out] thisArg
: Receives the JavaScriptthis
argument for the call.thisArg
can optionally be ignored by passingNULL
.[out] data
: Receives the data pointer for the callback.data
can optionally be ignored by passingNULL
.
Returns napi_ok
if the API succeeded.
This method is used within a callback function to retrieve details about the
call like the arguments and the this
pointer from a given callback info.
napi_get_new_target
#
napi_status napi_get_new_target(napi_env env,
napi_callback_info cbinfo,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] cbinfo
: The callback info passed into the callback function.[out] result
: Thenew.target
of the constructor call.
Returns napi_ok
if the API succeeded.
This API returns the new.target
of the constructor call. If the current
callback is not a constructor call, the result is NULL
.
napi_new_instance
#
napi_status napi_new_instance(napi_env env,
napi_value cons,
size_t argc,
napi_value* argv,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] cons
:napi_value
representing the JavaScript function to be invoked as a constructor.[in] argc
: The count of elements in theargv
array.[in] argv
: Array of JavaScript values asnapi_value
representing the arguments to the constructor. Ifargc
is zero this parameter may be omitted by passing inNULL
.[out] result
:napi_value
representing the JavaScript object returned, which in this case is the constructed object.
This method is used to instantiate a new JavaScript value using a given
napi_value
that represents the constructor for the object. For example,
consider the following snippet:
function MyObject(param) {
this.param = param;
}
const arg = 'hello';
const value = new MyObject(arg);
The following can be approximated in Node-API using the following snippet:
// Get the constructor function MyObject
napi_value global, constructor, arg, value;
napi_status status = napi_get_global(env, &global);
if (status != napi_ok) return;
status = napi_get_named_property(env, global, "MyObject", &constructor);
if (status != napi_ok) return;
// const arg = "hello"
status = napi_create_string_utf8(env, "hello", NAPI_AUTO_LENGTH, &arg);
if (status != napi_ok) return;
napi_value* argv = &arg;
size_t argc = 1;
// const value = new MyObject(arg)
status = napi_new_instance(env, constructor, argc, argv, &value);
Returns napi_ok
if the API succeeded.
Object wrap#
Node-API offers a way to "wrap" C++ classes and instances so that the class constructor and methods can be called from JavaScript.
- The
napi_define_class
API defines a JavaScript class with constructor, static properties and methods, and instance properties and methods that correspond to the C++ class. - When JavaScript code invokes the constructor, the constructor callback
uses
napi_wrap
to wrap a new C++ instance in a JavaScript object, then returns the wrapper object. - When JavaScript code invokes a method or property accessor on the class,
the corresponding
napi_callback
C++ function is invoked. For an instance callback,napi_unwrap
obtains the C++ instance that is the target of the call.
For wrapped objects it may be difficult to distinguish between a function
called on a class prototype and a function called on an instance of a class.
A common pattern used to address this problem is to save a persistent
reference to the class constructor for later instanceof
checks.
napi_value MyClass_constructor = NULL;
status = napi_get_reference_value(env, MyClass::es_constructor, &MyClass_constructor);
assert(napi_ok == status);
bool is_instance = false;
status = napi_instanceof(env, es_this, MyClass_constructor, &is_instance);
assert(napi_ok == status);
if (is_instance) {
// napi_unwrap() ...
} else {
// otherwise...
}
The reference must be freed once it is no longer needed.
There are occasions where napi_instanceof()
is insufficient for ensuring that
a JavaScript object is a wrapper for a certain native type. This is the case
especially when wrapped JavaScript objects are passed back into the addon via
static methods rather than as the this
value of prototype methods. In such
cases there is a chance that they may be unwrapped incorrectly.
const myAddon = require('./build/Release/my_addon.node');
// `openDatabase()` returns a JavaScript object that wraps a native database
// handle.
const dbHandle = myAddon.openDatabase();
// `query()` returns a JavaScript object that wraps a native query handle.
const queryHandle = myAddon.query(dbHandle, 'Gimme ALL the things!');
// There is an accidental error in the line below. The first parameter to
// `myAddon.queryHasRecords()` should be the database handle (`dbHandle`), not
// the query handle (`query`), so the correct condition for the while-loop
// should be
//
// myAddon.queryHasRecords(dbHandle, queryHandle)
//
while (myAddon.queryHasRecords(queryHandle, dbHandle)) {
// retrieve records
}
In the above example myAddon.queryHasRecords()
is a method that accepts two
arguments. The first is a database handle and the second is a query handle.
Internally, it unwraps the first argument and casts the resulting pointer to a
native database handle. It then unwraps the second argument and casts the
resulting pointer to a query handle. If the arguments are passed in the wrong
order, the casts will work, however, there is a good chance that the underlying
database operation will fail, or will even cause an invalid memory access.
To ensure that the pointer retrieved from the first argument is indeed a pointer
to a database handle and, similarly, that the pointer retrieved from the second
argument is indeed a pointer to a query handle, the implementation of
queryHasRecords()
has to perform a type validation. Retaining the JavaScript
class constructor from which the database handle was instantiated and the
constructor from which the query handle was instantiated in napi_ref
s can
help, because napi_instanceof()
can then be used to ensure that the instances
passed into queryHashRecords()
are indeed of the correct type.
Unfortunately, napi_instanceof()
does not protect against prototype
manipulation. For example, the prototype of the database handle instance can be
set to the prototype of the constructor for query handle instances. In this
case, the database handle instance can appear as a query handle instance, and it
will pass the napi_instanceof()
test for a query handle instance, while still
containing a pointer to a database handle.
To this end, Node-API provides type-tagging capabilities.
A type tag is a 128-bit integer unique to the addon. Node-API provides the
napi_type_tag
structure for storing a type tag. When such a value is passed
along with a JavaScript object or external stored in a napi_value
to
napi_type_tag_object()
, the JavaScript object will be "marked" with the
type tag. The "mark" is invisible on the JavaScript side. When a JavaScript
object arrives into a native binding, napi_check_object_type_tag()
can be used
along with the original type tag to determine whether the JavaScript object was
previously "marked" with the type tag. This creates a type-checking capability
of a higher fidelity than napi_instanceof()
can provide, because such type-
tagging survives prototype manipulation and addon unloading/reloading.
Continuing the above example, the following skeleton addon implementation
illustrates the use of napi_type_tag_object()
and
napi_check_object_type_tag()
.
// This value is the type tag for a database handle. The command
//
// uuidgen | sed -r -e 's/-//g' -e 's/(.{16})(.*)/0x\1, 0x\2/'
//
// can be used to obtain the two values with which to initialize the structure.
static const napi_type_tag DatabaseHandleTypeTag = {
0x1edf75a38336451d, 0xa5ed9ce2e4c00c38
};
// This value is the type tag for a query handle.
static const napi_type_tag QueryHandleTypeTag = {
0x9c73317f9fad44a3, 0x93c3920bf3b0ad6a
};
static napi_value
openDatabase(napi_env env, napi_callback_info info) {
napi_status status;
napi_value result;
// Perform the underlying action which results in a database handle.
DatabaseHandle* dbHandle = open_database();
// Create a new, empty JS object.
status = napi_create_object(env, &result);
if (status != napi_ok) return NULL;
// Tag the object to indicate that it holds a pointer to a `DatabaseHandle`.
status = napi_type_tag_object(env, result, &DatabaseHandleTypeTag);
if (status != napi_ok) return NULL;
// Store the pointer to the `DatabaseHandle` structure inside the JS object.
status = napi_wrap(env, result, dbHandle, NULL, NULL, NULL);
if (status != napi_ok) return NULL;
return result;
}
// Later when we receive a JavaScript object purporting to be a database handle
// we can use `napi_check_object_type_tag()` to ensure that it is indeed such a
// handle.
static napi_value
query(napi_env env, napi_callback_info info) {
napi_status status;
size_t argc = 2;
napi_value argv[2];
bool is_db_handle;
status = napi_get_cb_info(env, info, &argc, argv, NULL, NULL);
if (status != napi_ok) return NULL;
// Check that the object passed as the first parameter has the previously
// applied tag.
status = napi_check_object_type_tag(env,
argv[0],
&DatabaseHandleTypeTag,
&is_db_handle);
if (status != napi_ok) return NULL;
// Throw a `TypeError` if it doesn't.
if (!is_db_handle) {
// Throw a TypeError.
return NULL;
}
}
napi_define_class
#
napi_status napi_define_class(napi_env env,
const char* utf8name,
size_t length,
napi_callback constructor,
void* data,
size_t property_count,
const napi_property_descriptor* properties,
napi_value* result);
[in] env
: The environment that the API is invoked under.[in] utf8name
: Name of the JavaScript constructor function; When wrapping a C++ class, we recommend for clarity that this name be the same as that of the C++ class.[in] length
: The length of theutf8name
in bytes, orNAPI_AUTO_LENGTH
if it is null-terminated.[in] constructor
: Callback function that handles constructing instances of the class. When wrapping a C++ class, this method must be a static member with thenapi_callback
signature. A C++ class constructor cannot be used.napi_callback
provides more details.[in] data
: Optional data to be passed to the constructor callback as thedata
property of the callback info.[in] property_count
: Number of items in theproperties
array argument.[in] properties
: Array of property descriptors describing static and instance data properties, accessors, and methods on the class Seenapi_property_descriptor
.[out] result
: Anapi_value
representing the constructor function for the class.
Returns napi_ok
if the API succeeded.
Defines a JavaScript class, including:
- A JavaScript constructor function that has the class name. When wrapping a
corresponding C++ class, the callback passed via
constructor
can be used to instantiate a new C++ class instance, which can then be placed inside the JavaScript object instance being constructed usingnapi_wrap
. - Properties on the constructor function whose implementation can call
corresponding static data properties, accessors, and methods of the C++
class (defined by property descriptors with the
napi_static
attribute). - Properties on the constructor function's
prototype
object. When wrapping a C++ class, non-static data properties, accessors, and methods of the C++ class can be called from the static functions given in the property descriptors without thenapi_static
attribute after retrieving the C++ class instance placed inside the JavaScript object instance by usingnapi_unwrap
.
When wrapping a C++ class, the C++ constructor callback passed via constructor
should be a static method on the class that calls the actual class constructor,
then wraps the new C++ instance in a JavaScript object, and returns the wrapper
object. See napi_wrap
for details.
The JavaScript constructor function returned from napi_define_class
is
often saved and used later to construct new instances of the class from native
code, and/or to check whether provided values are instances of the class. In
that case, to prevent the function value from being garbage-collected, a
strong persistent reference to it can be created using
napi_create_reference
, ensuring that the reference count is kept >= 1.
Any non-NULL
data which is passed to this API via the data
parameter or via
the data
field of the napi_property_descriptor
array items can be associated
with the resulting JavaScript constructor (which is returned in the result
parameter) and freed whenever the class is garbage-collected by passing both
the JavaScript function and the data to napi_add_finalizer
.
napi_wrap
#
napi_status napi_wrap(napi_env env,
napi_value js_object,
void* native_object,
napi_finalize finalize_cb,
void* finalize_hint,
napi_ref* result);
[in] env
: The environment that the API is invoked under.[in] js_object
: The JavaScript object that will be the wrapper for the native object.[in] native_object
: The native instance that will be wrapped in the JavaScript object.[in] finalize_cb
: Optional native callback that can be used to free the native instance when the JavaScript object has been garbage-collected.napi_finalize
provides more details.[in] finalize_hint
: Optional contextual hint that is passed to the finalize callback.[out] result
: Optional reference to the wrapped object.
Returns napi_ok
if the API succeeded.
Wraps a native instance in a JavaScript object. The native instance can be
retrieved later using napi_unwrap()
.
When JavaScript code invokes a constructor for a class that was defined using
napi_define_class()
, the napi_callback
for the constructor is invoked.
After constructing an instance of the native class, the callback must then call
napi_wrap()
to wrap the newly constructed instance in the already-created
JavaScript object that is the this
argument to the constructor callback.
(That this
object was created from the constructor function's prototype
,
so it already has definitions of all the instance properties and methods.)
Typically when wrapping a class instance, a finalize callback should be
provided that simply deletes the native instance that is received as the data
argument to the finalize callback.
The optional returned reference is initially a weak reference, meaning it has a reference count of 0. Typically this reference count would be incremented temporarily during async operations that require the instance to remain valid.
Caution: The optional returned reference (if obtained) should be deleted via
napi_delete_reference
ONLY in response to the finalize callback
invocation. If it is deleted before then, then the finalize callback may never
be invoked. Therefore, when obtaining a reference a finalize callback is also
required in order to enable correct disposal of the reference.
Finalizer callbacks may be deferred, leaving a window where the object has
been garbage collected (and the weak reference is invalid) but the finalizer
hasn't been called yet. When using napi_get_reference_value()
on weak
references returned by napi_wrap()
, you should still handle an empty result.
Calling napi_wrap()
a second time on an object will return an error. To
associate another native instance with the object, use napi_remove_wrap()
first.
napi_unwrap
#
napi_status napi_unwrap(napi_env env,
napi_value js_object,
void** result);
[in] env
: The environment that the API is invoked under.[in] js_object
: The object associated with the native instance.[out] result
: Pointer to the wrapped native instance.
Returns napi_ok
if the API succeeded.
Retrieves a native instance that was previously wrapped in a JavaScript
object using napi_wrap()
.
When JavaScript code invokes a method or property accessor on the class, the
corresponding napi_callback
is invoked. If the callback is for an instance
method or accessor, then the this
argument to the callback is the wrapper
object; the wrapped C++ instance that is the target of the call can be obtained
then by calling napi_unwrap()
on the wrapper object.
napi_remove_wrap
#
napi_status napi_remove_wrap(napi_env env,
napi_value js_object,
void** result);
[in] env
: The environment that the API is invoked under.[in] js_object
: The object associated with the native instance.[out] result
: Pointer to the wrapped native instance.
Returns napi_ok
if the API succeeded.
Retrieves a native instance that was previously wrapped in the JavaScript
object js_object
using napi_wrap()
and removes the wrapping. If a finalize
callback was associated with the wrapping, it will no longer be called when the
JavaScript object becomes garbage-collected.
napi_type_tag_object
#
napi_status napi_type_tag_object(napi_env env,
napi_value js_object,
const napi_type_tag* type_tag);
[in] env
: The environment that the API is invoked under.[in] js_object
: The JavaScript object or external to be marked.[in] type_tag
: The tag with which the object is to be marked.
Returns napi_ok
if the API succeeded.
Associates the value of the type_tag
pointer with the JavaScript object or
external. napi_check_object_type_tag()
can then be used to compare the tag
that was attached to the object with one owned by the addon to ensure that the
object has the right type.
If the object already has an associated type tag, this API will return
napi_invalid_arg
.
napi_check_object_type_tag
#
napi_status napi_check_object_type_tag(napi_env env,
napi_value js_object,
const napi_type_tag* type_tag,
bool* result);
[in] env
: The environment that the API is invoked under.[in] js_object
: The JavaScript object or external whose type tag to examine.[in] type_tag
: The tag with which to compare any tag found on the object.[out] result
: Whether the type tag given matched the type tag on the object.false
is also returned if no type tag was found on the object.
Returns napi_ok
if the API succeeded.
Compares the pointer given as type_tag
with any that can be found on
js_object
. If no tag is found on js_object
or, if a tag is found but it does
not match type_tag
, then result
is set to false
. If a tag is found and it
matches type_tag
, then result
is set to true
.
napi_add_finalizer
#
napi_status napi_add_finalizer(napi_env env,
napi_value js_object,
void* finalize_data,
napi_finalize finalize_cb,
void* finalize_hint,
napi_ref* result);
[in] env
: The environment that the API is invoked under.[in] js_object
: The JavaScript object to which the native data will be attached.[in] finalize_data
: Optional data to be passed tofinalize_cb
.[in] finalize_cb
: Native callback that will be used to free the native data when the JavaScript object has been garbage-collected.napi_finalize
provides more details.[in] finalize_hint
: Optional contextual hint that is passed to the finalize callback.[out] result
: Optional reference to the JavaScript object.
Returns napi_ok
if the API succeeded.
Adds a napi_finalize
callback which will be called when the JavaScript object
in js_object
has been garbage-collected.
This API can be called multiple times on a single JavaScript object.
Caution: The optional returned reference (if obtained) should be deleted via
napi_delete_reference
ONLY in response to the finalize callback
invocation. If it is deleted before then, then the finalize callback may never
be invoked. Therefore, when obtaining a reference a finalize callback is also
required in order to enable correct disposal of the reference.
Simple asynchronous operations#
Addon modules often need to leverage async helpers from libuv as part of their implementation. This allows them to schedule work to be executed asynchronously so that their methods can return in advance of the work being completed. This allows them to avoid blocking overall execution of the Node.js application.
Node-API provides an ABI-stable interface for these supporting functions which covers the most common asynchronous use cases.
Node-API defines the napi_async_work
structure which is used to manage
asynchronous workers. Instances are created/deleted with
napi_create_async_work
and napi_delete_async_work
.
The execute
and complete
callbacks are functions that will be
invoked when the executor is ready to execute and when it completes its
task respectively.
The execute
function should avoid making any Node-API calls
that could result in the execution of JavaScript or interaction with
JavaScript objects. Most often, any code that needs to make Node-API
calls should be made in complete
callback instead.
Avoid using the napi_env
parameter in the execute callback as
it will likely execute JavaScript.
These functions implement the following interfaces:
typedef void (*napi_async_execute_callback)(napi_env env,
void* data);
typedef void (*napi_async_complete_callback)(napi_env env,
napi_status status,
void* data);
When these methods are invoked, the data
parameter passed will be the
addon-provided void*
data that was passed into the
napi_create_async_work
call.
Once created the async worker can be queued
for execution using the napi_queue_async_work
function:
napi_status napi_queue_async_work(napi_env env,
napi_async_work work);
napi_cancel_async_work
can be used if the work needs
to be cancelled before the work has started execution.
After calling napi_cancel_async_work
, the complete
callback
will be invoked with a status value of napi_cancelled
.
The work should not be deleted before the complete
callback invocation, even when it was cancelled.
napi_create_async_work
#
napi_status napi_create_async_work(napi_env env,
napi_value async_resource,
napi_value async_resource_name,
napi_async_execute_callback execute,
napi_async_complete_callback complete,
void* data,
napi_async_work* result);
[in] env
: The environment that the API is invoked under.[in] async_resource
: An optional object associated with the async work that will be passed to possibleasync_hooks
init
hooks.[in] async_resource_name
: Identifier for the kind of resource that is being provided for diagnostic information exposed by theasync_hooks
API.[in] execute
: The native function which should be called to execute the logic asynchronously. The given function is called from a worker pool thread and can execute in parallel with the main event loop thread.[in] complete
: The native function which will be called when the asynchronous logic is completed or is cancelled. The given function is called from the main event loop thread.napi_async_complete_callback
provides more details.[in] data
: User-provided data context. This will be passed back into the execute and complete functions.[out] result
:napi_async_work*
which is the handle to the newly created async work.
Returns napi_ok
if the API succeeded.
This API allocates a work object that is used to execute logic asynchronously.
It should be freed using napi_delete_async_work
once the work is no longer
required.
async_resource_name
should be a null-terminated, UTF-8-encoded string.
The async_resource_name
identifier is provided by the user and should be
representative of the type of async work being performed. It is also recommended
to apply namespacing to the identifier, e.g. by including the module name. See
the async_hooks
documentation for more information.
napi_delete_async_work
#
napi_status napi_delete_async_work(napi_env env,
napi_async_work work);
[in] env
: The environment that the API is invoked under.[in] work
: The handle returned by the call tonapi_create_async_work
.
Returns napi_ok
if the API succeeded.
This API frees a previously allocated work object.
This API can be called even if there is a pending JavaScript exception.
napi_queue_async_work
#
napi_status napi_queue_async_work(napi_env env,
napi_async_work work);
[in] env
: The environment that the API is invoked under.[in] work
: The handle returned by the call tonapi_create_async_work
.
Returns napi_ok
if the API succeeded.
This API requests that the previously allocated work be scheduled
for execution. Once it returns successfully, this API must not be called again
with the same napi_async_work
item or the result will be undefined.
napi_cancel_async_work
#
napi_status napi_cancel_async_work(napi_env env,
napi_async_work work);
[in] env
: The environment that the API is invoked under.[in] work
: The handle returned by the call tonapi_create_async_work
.
Returns napi_ok
if the API succeeded.
This API cancels queued work if it has not yet
been started. If it has already started executing, it cannot be
cancelled and napi_generic_failure
will be returned. If successful,
the complete
callback will be invoked with a status value of
napi_cancelled
. The work should not be deleted before the complete
callback invocation, even if it has been successfully cancelled.
This API can be called even if there is a pending JavaScript exception.
Custom asynchronous operations#
The simple asynchronous work APIs above may not be appropriate for every scenario. When using any other asynchronous mechanism, the following APIs are necessary to ensure an asynchronous operation is properly tracked by the runtime.
napi_async_init
#
napi_status napi_async_init(napi_env env,
napi_value async_resource,
napi_value async_resource_name,
napi_async_context* result)
[in] env
: The environment that the API is invoked under.[in] async_resource
: Object associated with the async work that will be passed to possibleasync_hooks
init
hooks and can be accessed byasync_hooks.executionAsyncResource()
.[in] async_resource_name
: Identifier for the kind of resource that is being provided for diagnostic information exposed by theasync_hooks
API.[out] result
: The initialized async context.
Returns napi_ok
if the API succeeded.
The async_resource
object needs to be kept alive until
napi_async_destroy
to keep async_hooks
related API acts correctly. In
order to retain ABI compatibility with previous versions, napi_async_context
s
are not maintaining the strong reference to the async_resource
objects to
avoid introducing causing memory leaks. However, if the async_resource
is
garbage collected by JavaScript engine before the napi_async_context
was
destroyed by napi_async_destroy
, calling napi_async_context
related APIs
like napi_open_callback_scope
and napi_make_callback
can cause
problems like loss of async context when using the AsyncLocalStorage
API.
In order to retain ABI compatibility with previous versions, passing NULL
for async_resource
does not result in an error. However, this is not
recommended as this will result poor results with async_hooks
init
hooks and async_hooks.executionAsyncResource()
as the resource is
now required by the underlying async_hooks
implementation in order to provide
the linkage between async callbacks.
napi_async_destroy
#
napi_status napi_async_destroy(napi_env env,
napi_async_context async_context);
[in] env
: The environment that the API is invoked under.[in] async_context
: The async context to be destroyed.
Returns napi_ok
if the API succeeded.
This API can be called even if there is a pending JavaScript exception.
napi_make_callback
#
NAPI_EXTERN napi_status napi_make_callback(napi_env env,
napi_async_context async_context,
napi_value recv,
napi_value func,
size_t argc,
const napi_value* argv,
napi_value* result);
[in] env
: The environment that the API is invoked under.[in] async_context
: Context for the async operation that is invoking the callback. This should normally be a value previously obtained fromnapi_async_init
. In order to retain ABI compatibility with previous versions, passingNULL
forasync_context
does not result in an error. However, this results in incorrect operation of async hooks. Potential issues include loss of async context when using theAsyncLocalStorage
API.[in] recv
: Thethis
value passed to the called function.[in] func
:napi_value
representing the JavaScript function to be invoked.[in] argc
: The count of elements in theargv
array.[in] argv
: Array of JavaScript values asnapi_value
representing the arguments to the function. Ifargc
is zero this parameter may be omitted by passing inNULL
.[out] result
:napi_value
representing the JavaScript object returned.
Returns napi_ok
if the API succeeded.
This method allows a JavaScript function object to be called from a native
add-on. This API is similar to napi_call_function
. However, it is used to call
from native code back into JavaScript after returning from an async
operation (when there is no other script on the stack). It is a fairly simple
wrapper around node::MakeCallback
.
Note it is not necessary to use napi_make_callback
from within a
napi_async_complete_callback
; in that situation the callback's async
context has already been set up, so a direct call to napi_call_function
is sufficient and appropriate. Use of the napi_make_callback
function
may be required when implementing custom async behavior that does not use
napi_create_async_work
.
Any process.nextTick
s or Promises scheduled on the microtask queue by
JavaScript during the callback are ran before returning back to C/C++.
napi_open_callback_scope
#
NAPI_EXTERN napi_status napi_open_callback_scope(napi_env env,
napi_value resource_object,
napi_async_context context,
napi_callback_scope* result)
[in] env
: The environment that the API is invoked under.[in] resource_object
: An object associated with the async work that will be passed to possibleasync_hooks
init
hooks. This parameter has been deprecated and is ignored at runtime. Use theasync_resource
parameter innapi_async_init
instead.[in] context
: Context for the async operation that is invoking the callback. This should be a value previously obtained fromnapi_async_init
.[out] result
: The newly created scope.
There are cases (for example, resolving promises) where it is
necessary to have the equivalent of the scope associated with a callback
in place when making certain Node-API calls. If there is no other script on
the stack the napi_open_callback_scope
and
napi_close_callback_scope
functions can be used to open/close
the required scope.
napi_close_callback_scope
#
NAPI_EXTERN napi_status napi_close_callback_scope(napi_env env,
napi_callback_scope scope)
[in] env
: The environment that the API is invoked under.[in] scope
: The scope to be closed.
This API can be called even if there is a pending JavaScript exception.
Version management#
napi_get_node_version
#
typedef struct {
uint32_t major;
uint32_t minor;
uint32_t patch;
const char* release;
} napi_node_version;
napi_status napi_get_node_version(napi_env env,
const napi_node_version** version);
[in] env
: The environment that the API is invoked under.[out] version
: A pointer to version information for Node.js itself.
Returns napi_ok
if the API succeeded.
This function fills the version
struct with the major, minor, and patch
version of Node.js that is currently running, and the release
field with the
value of process.release.name
.
The returned buffer is statically allocated and does not need to be freed.
napi_get_version
#
napi_status napi_get_version(napi_env env,
uint32_t* result);
[in] env
: The environment that the API is invoked under.[out] result
: The highest version of Node-API supported.
Returns napi_ok
if the API succeeded.
This API returns the highest Node-API version supported by the Node.js runtime. Node-API is planned to be additive such that newer releases of Node.js may support additional API functions. In order to allow an addon to use a newer function when running with versions of Node.js that support it, while providing fallback behavior when running with Node.js versions that don't support it:
- Call
napi_get_version()
to determine if the API is available. - If available, dynamically load a pointer to the function using
uv_dlsym()
. - Use the dynamically loaded pointer to invoke the function.
- If the function is not available, provide an alternate implementation that does not use the function.
Memory management#
napi_adjust_external_memory
#
NAPI_EXTERN napi_status napi_adjust_external_memory(napi_env env,
int64_t change_in_bytes,
int64_t* result);
[in] env
: The environment that the API is invoked under.[in] change_in_bytes
: The change in externally allocated memory that is kept alive by JavaScript objects.[out] result
: The adjusted value
Returns napi_ok
if the API succeeded.
This function gives V8 an indication of the amount of externally allocated memory that is kept alive by JavaScript objects (i.e. a JavaScript object that points to its own memory allocated by a native addon). Registering externally allocated memory will trigger global garbage collections more often than it would otherwise.
Promises#
Node-API provides facilities for creating Promise
objects as described in
Section 25.4 of the ECMA specification. It implements promises as a pair of
objects. When a promise is created by napi_create_promise()
, a "deferred"
object is created and returned alongside the Promise
. The deferred object is
bound to the created Promise
and is the only means to resolve or reject the
Promise
using napi_resolve_deferred()
or napi_reject_deferred()
. The
deferred object that is created by napi_create_promise()
is freed by
napi_resolve_deferred()
or napi_reject_deferred()
. The Promise
object may
be returned to JavaScript where it can be used in the usual fashion.
For example, to create a promise and pass it to an asynchronous worker:
napi_deferred deferred;
napi_value promise;
napi_status status;
// Create the promise.
status = napi_create_promise(env, &deferred, &promise);
if (status != napi_ok) return NULL;
// Pass the deferred to a function that performs an asynchronous action.
do_something_asynchronous(deferred);
// Return the promise to JS
return promise;
The above function do_something_asynchronous()
would perform its asynchronous
action and then it would resolve or reject the deferred, thereby concluding the
promise and freeing the deferred:
napi_deferred deferred;
napi_value undefined;
napi_status status;
// Create a value with which to conclude the deferred.
status = napi_get_undefined(env, &undefined);
if (status != napi_ok) return NULL;
// Resolve or reject the promise associated with the deferred depending on
// whether the asynchronous action succeeded.
if (asynchronous_action_succeeded) {
status = napi_resolve_deferred(env, deferred, undefined);
} else {
status = napi_reject_deferred(env, deferred, undefined);
}
if (status != napi_ok) return NULL;
// At this point the deferred has been freed, so we should assign NULL to it.
deferred = NULL;
napi_create_promise
#
napi_status napi_create_promise(napi_env env,
napi_deferred* deferred,
napi_value* promise);
[in] env
: The environment that the API is invoked under.[out] deferred
: A newly created deferred object which can later be passed tonapi_resolve_deferred()
ornapi_reject_deferred()
to resolve resp. reject the associated promise.[out] promise
: The JavaScript promise associated with the deferred object.
Returns napi_ok
if the API succeeded.
This API creates a deferred object and a JavaScript promise.
napi_resolve_deferred
#
napi_status napi_resolve_deferred(napi_env env,
napi_deferred deferred,
napi_value resolution);
[in] env
: The environment that the API is invoked under.[in] deferred
: The deferred object whose associated promise to resolve.[in] resolution
: The value with which to resolve the promise.
This API resolves a JavaScript promise by way of the deferred object
with which it is associated. Thus, it can only be used to resolve JavaScript
promises for which the corresponding deferred object is available. This
effectively means that the promise must have been created using
napi_create_promise()
and the deferred object returned from that call must
have been retained in order to be passed to this API.
The deferred object is freed upon successful completion.
napi_reject_deferred
#
napi_status napi_reject_deferred(napi_env env,
napi_deferred deferred,
napi_value rejection);
[in] env
: The environment that the API is invoked under.[in] deferred
: The deferred object whose associated promise to resolve.[in] rejection
: The value with which to reject the promise.
This API rejects a JavaScript promise by way of the deferred object
with which it is associated. Thus, it can only be used to reject JavaScript
promises for which the corresponding deferred object is available. This
effectively means that the promise must have been created using
napi_create_promise()
and the deferred object returned from that call must
have been retained in order to be passed to this API.
The deferred object is freed upon successful completion.
napi_is_promise
#
napi_status napi_is_promise(napi_env env,
napi_value value,
bool* is_promise);
[in] env
: The environment that the API is invoked under.[in] value
: The value to examine[out] is_promise
: Flag indicating whetherpromise
is a native promise object (that is, a promise object created by the underlying engine).
Script execution#
Node-API provides an API for executing a string containing JavaScript using the underlying JavaScript engine.
napi_run_script
#
NAPI_EXTERN napi_status napi_run_script(napi_env env,
napi_value script,
napi_value* result);
[in] env
: The environment that the API is invoked under.[in] script
: A JavaScript string containing the script to execute.[out] result
: The value resulting from having executed the script.
This function executes a string of JavaScript code and returns its result with the following caveats:
- Unlike
eval
, this function does not allow the script to access the current lexical scope, and therefore also does not allow to access the module scope, meaning that pseudo-globals such asrequire
will not be available. - The script can access the global scope. Function and
var
declarations in the script will be added to theglobal
object. Variable declarations made usinglet
andconst
will be visible globally, but will not be added to theglobal
object. - The value of
this
isglobal
within the script.
libuv event loop#
Node-API provides a function for getting the current event loop associated with
a specific napi_env
.
napi_get_uv_event_loop
#
NAPI_EXTERN napi_status napi_get_uv_event_loop(napi_env env,
struct uv_loop_s** loop);
[in] env
: The environment that the API is invoked under.[out] loop
: The current libuv loop instance.
Asynchronous thread-safe function calls#
JavaScript functions can normally only be called from a native addon's main
thread. If an addon creates additional threads, then Node-API functions that
require a napi_env
, napi_value
, or napi_ref
must not be called from those
threads.
When an addon has additional threads and JavaScript functions need to be invoked based on the processing completed by those threads, those threads must communicate with the addon's main thread so that the main thread can invoke the JavaScript function on their behalf. The thread-safe function APIs provide an easy way to do this.
These APIs provide the type napi_threadsafe_function
as well as APIs to
create, destroy, and call objects of this type.
napi_create_threadsafe_function()
creates a persistent reference to a
napi_value
that holds a JavaScript function which can be called from multiple
threads. The calls happen asynchronously. This means that values with which the
JavaScript callback is to be called will be placed in a queue, and, for each
value in the queue, a call will eventually be made to the JavaScript function.
Upon creation of a napi_threadsafe_function
a napi_finalize
callback can be
provided. This callback will be invoked on the main thread when the thread-safe
function is about to be destroyed. It receives the context and the finalize data
given during construction, and provides an opportunity for cleaning up after the
threads e.g. by calling uv_thread_join()
. Aside from the main loop thread,
no threads should be using the thread-safe function after the finalize callback
completes.
The context
given during the call to napi_create_threadsafe_function()
can
be retrieved from any thread with a call to
napi_get_threadsafe_function_context()
.
Calling a thread-safe function#
napi_call_threadsafe_function()
can be used for initiating a call into
JavaScript. napi_call_threadsafe_function()
accepts a parameter which controls
whether the API behaves blockingly. If set to napi_tsfn_nonblocking
, the API
behaves non-blockingly, returning napi_queue_full
if the queue was full,
preventing data from being successfully added to the queue. If set to
napi_tsfn_blocking
, the API blocks until space becomes available in the queue.
napi_call_threadsafe_function()
never blocks if the thread-safe function was
created with a maximum queue size of 0.
napi_call_threadsafe_function()
should not be called with napi_tsfn_blocking
from a JavaScript thread, because, if the queue is full, it may cause the
JavaScript thread to deadlock.
The actual call into JavaScript is controlled by the callback given via the
call_js_cb
parameter. call_js_cb
is invoked on the main thread once for each
value that was placed into the queue by a successful call to
napi_call_threadsafe_function()
. If such a callback is not given, a default
callback will be used, and the resulting JavaScript call will have no arguments.
The call_js_cb
callback receives the JavaScript function to call as a
napi_value
in its parameters, as well as the void*
context pointer used when
creating the napi_threadsafe_function
, and the next data pointer that was
created by one of the secondary threads. The callback can then use an API such
as napi_call_function()
to call into JavaScript.
The callback may also be invoked with env
and call_js_cb
both set to NULL
to indicate that calls into JavaScript are no longer possible, while items
remain in the queue that may need to be freed. This normally occurs when the
Node.js process exits while there is a thread-safe function still active.
It is not necessary to call into JavaScript via napi_make_callback()
because
Node-API runs call_js_cb
in a context appropriate for callbacks.
Reference counting of thread-safe functions#
Threads can be added to and removed from a napi_threadsafe_function
object
during its existence. Thus, in addition to specifying an initial number of
threads upon creation, napi_acquire_threadsafe_function
can be called to
indicate that a new thread will start making use of the thread-safe function.
Similarly, napi_release_threadsafe_function
can be called to indicate that an
existing thread will stop making use of the thread-safe function.
napi_threadsafe_function
objects are destroyed when every thread which uses
the object has called napi_release_threadsafe_function()
or has received a
return status of napi_closing
in response to a call to
napi_call_threadsafe_function
. The queue is emptied before the
napi_threadsafe_function
is destroyed. napi_release_threadsafe_function()
should be the last API call made in conjunction with a given
napi_threadsafe_function
, because after the call completes, there is no
guarantee that the napi_threadsafe_function
is still allocated. For the same
reason, do not use a thread-safe function
after receiving a return value of napi_closing
in response to a call to
napi_call_threadsafe_function
. Data associated with the
napi_threadsafe_function
can be freed in its napi_finalize
callback which
was passed to napi_create_threadsafe_function()
. The parameter
initial_thread_count
of napi_create_threadsafe_function
marks the initial
number of acquisitions of the thread-safe functions, instead of calling
napi_acquire_threadsafe_function
multiple times at creation.
Once the number of threads making use of a napi_threadsafe_function
reaches
zero, no further threads can start making use of it by calling
napi_acquire_threadsafe_function()
. In fact, all subsequent API calls
associated with it, except napi_release_threadsafe_function()
, will return an
error value of napi_closing
.
The thread-safe function can be "aborted" by giving a value of napi_tsfn_abort
to napi_release_threadsafe_function()
. This will cause all subsequent APIs
associated with the thread-safe function except
napi_release_threadsafe_function()
to return napi_closing
even before its
reference count reaches zero. In particular, napi_call_threadsafe_function()
will return napi_closing
, thus informing the threads that it is no longer
possible to make asynchronous calls to the thread-safe function. This can be
used as a criterion for terminating the thread. Upon receiving a return value
of napi_closing
from napi_call_threadsafe_function()
a thread must not use
the thread-safe function anymore because it is no longer guaranteed to
be allocated.
Deciding whether to keep the process running#
Similarly to libuv handles, thread-safe functions can be "referenced" and
"unreferenced". A "referenced" thread-safe function will cause the event loop on
the thread on which it is created to remain alive until the thread-safe function
is destroyed. In contrast, an "unreferenced" thread-safe function will not
prevent the event loop from exiting. The APIs napi_ref_threadsafe_function
and
napi_unref_threadsafe_function
exist for this purpose.
Neither does napi_unref_threadsafe_function
mark the thread-safe functions as
able to be destroyed nor does napi_ref_threadsafe_function
prevent it from
being destroyed.
napi_create_threadsafe_function
#
NAPI_EXTERN napi_status
napi_create_threadsafe_function(napi_env env,
napi_value func,
napi_value async_resource,
napi_value async_resource_name,
size_t max_queue_size,
size_t initial_thread_count,
void* thread_finalize_data,
napi_finalize thread_finalize_cb,
void* context,
napi_threadsafe_function_call_js call_js_cb,
napi_threadsafe_function* result);
[in] env
: The environment that the API is invoked under.[in] func
: An optional JavaScript function to call from another thread. It must be provided ifNULL
is passed tocall_js_cb
.[in] async_resource
: An optional object associated with the async work that will be passed to possibleasync_hooks
init
hooks.[in] async_resource_name
: A JavaScript string to provide an identifier for the kind of resource that is being provided for diagnostic information exposed by theasync_hooks
API.[in] max_queue_size
: Maximum size of the queue.0
for no limit.[in] initial_thread_count
: The initial number of acquisitions, i.e. the initial number of threads, including the main thread, which will be making use of this function.[in] thread_finalize_data
: Optional data to be passed tothread_finalize_cb
.[in] thread_finalize_cb
: Optional function to call when thenapi_threadsafe_function
is being destroyed.[in] context
: Optional data to attach to the resultingnapi_threadsafe_function
.[in] call_js_cb
: Optional callback which calls the JavaScript function in response to a call on a different thread. This callback will be called on the main thread. If not given, the JavaScript function will be called with no parameters and withundefined
as itsthis
value.napi_threadsafe_function_call_js
provides more details.[out] result
: The asynchronous thread-safe JavaScript function.
napi_get_threadsafe_function_context
#
NAPI_EXTERN napi_status
napi_get_threadsafe_function_context(napi_threadsafe_function func,
void** result);
[in] func
: The thread-safe function for which to retrieve the context.[out] result
: The location where to store the context.
This API may be called from any thread which makes use of func
.
napi_call_threadsafe_function
#
NAPI_EXTERN napi_status
napi_call_threadsafe_function(napi_threadsafe_function func,
void* data,
napi_threadsafe_function_call_mode is_blocking);
[in] func
: The asynchronous thread-safe JavaScript function to invoke.[in] data
: Data to send into JavaScript via the callbackcall_js_cb
provided during the creation of the thread-safe JavaScript function.[in] is_blocking
: Flag whose value can be eithernapi_tsfn_blocking
to indicate that the call should block if the queue is full ornapi_tsfn_nonblocking
to indicate that the call should return immediately with a status ofnapi_queue_full
whenever the queue is full.
This API should not be called with napi_tsfn_blocking
from a JavaScript
thread, because, if the queue is full, it may cause the JavaScript thread to
deadlock.
This API will return napi_closing
if napi_release_threadsafe_function()
was
called with abort
set to napi_tsfn_abort
from any thread. The value is only
added to the queue if the API returns napi_ok
.
This API may be called from any thread which makes use of func
.
napi_acquire_threadsafe_function
#
NAPI_EXTERN napi_status
napi_acquire_threadsafe_function(napi_threadsafe_function func);
[in] func
: The asynchronous thread-safe JavaScript function to start making use of.
A thread should call this API before passing func
to any other thread-safe
function APIs to indicate that it will be making use of func
. This prevents
func
from being destroyed when all other threads have stopped making use of
it.
This API may be called from any thread which will start making use of func
.
napi_release_threadsafe_function
#
NAPI_EXTERN napi_status
napi_release_threadsafe_function(napi_threadsafe_function func,
napi_threadsafe_function_release_mode mode);
[in] func
: The asynchronous thread-safe JavaScript function whose reference count to decrement.[in] mode
: Flag whose value can be eithernapi_tsfn_release
to indicate that the current thread will make no further calls to the thread-safe function, ornapi_tsfn_abort
to indicate that in addition to the current thread, no other thread should make any further calls to the thread-safe function. If set tonapi_tsfn_abort
, further calls tonapi_call_threadsafe_function()
will returnnapi_closing
, and no further values will be placed in the queue.
A thread should call this API when it stops making use of func
. Passing func
to any thread-safe APIs after having called this API has undefined results, as
func
may have been destroyed.
This API may be called from any thread which will stop making use of func
.
napi_ref_threadsafe_function
#
NAPI_EXTERN napi_status
napi_ref_threadsafe_function(napi_env env, napi_threadsafe_function func);
[in] env
: The environment that the API is invoked under.[in] func
: The thread-safe function to reference.
This API is used to indicate that the event loop running on the main thread
should not exit until func
has been destroyed. Similar to uv_ref
it is
also idempotent.
Neither does napi_unref_threadsafe_function
mark the thread-safe functions as
able to be destroyed nor does napi_ref_threadsafe_function
prevent it from
being destroyed. napi_acquire_threadsafe_function
and
napi_release_threadsafe_function
are available for that purpose.
This API may only be called from the main thread.
napi_unref_threadsafe_function
#
NAPI_EXTERN napi_status
napi_unref_threadsafe_function(napi_env env, napi_threadsafe_function func);
[in] env
: The environment that the API is invoked under.[in] func
: The thread-safe function to unreference.
This API is used to indicate that the event loop running on the main thread
may exit before func
is destroyed. Similar to uv_unref
it is also
idempotent.
This API may only be called from the main thread.
Miscellaneous utilities#
node_api_get_module_file_name
#
NAPI_EXTERN napi_status
node_api_get_module_file_name(napi_env env, const char** result);
[in] env
: The environment that the API is invoked under.[out] result
: A URL containing the absolute path of the location from which the add-on was loaded. For a file on the local file system it will start withfile://
. The string is null-terminated and owned byenv
and must thus not be modified or freed.
result
may be an empty string if the add-on loading process fails to establish
the add-on's file name during loading.
C++ embedder API#
Node.js provides a number of C++ APIs that can be used to execute JavaScript in a Node.js environment from other C++ software.
The documentation for these APIs can be found in src/node.h in the Node.js source tree. In addition to the APIs exposed by Node.js, some required concepts are provided by the V8 embedder API.
Because using Node.js as an embedded library is different from writing code that is executed by Node.js, breaking changes do not follow typical Node.js deprecation policy and may occur on each semver-major release without prior warning.
Example embedding application#
The following sections will provide an overview over how to use these APIs
to create an application from scratch that will perform the equivalent of
node -e <code>
, i.e. that will take a piece of JavaScript and run it in
a Node.js-specific environment.
The full code can be found in the Node.js source tree.
Setting up per-process state#
Node.js requires some per-process state management in order to run:
- Arguments parsing for Node.js CLI options,
- V8 per-process requirements, such as a
v8::Platform
instance.
The following example shows how these can be set up. Some class names are from
the node
and v8
C++ namespaces, respectively.
int main(int argc, char** argv) {
argv = uv_setup_args(argc, argv);
std::vector<std::string> args(argv, argv + argc);
// Parse Node.js CLI options, and print any errors that have occurred while
// trying to parse them.
std::unique_ptr<node::InitializationResult> result =
node::InitializeOncePerProcess(args, {
node::ProcessInitializationFlags::kNoInitializeV8,
node::ProcessInitializationFlags::kNoInitializeNodeV8Platform
});
for (const std::string& error : result->errors())
fprintf(stderr, "%s: %s\n", args[0].c_str(), error.c_str());
if (result->early_return() != 0) {
return result->exit_code();
}
// Create a v8::Platform instance. `MultiIsolatePlatform::Create()` is a way
// to create a v8::Platform instance that Node.js can use when creating
// Worker threads. When no `MultiIsolatePlatform` instance is present,
// Worker threads are disabled.
std::unique_ptr<MultiIsolatePlatform> platform =
MultiIsolatePlatform::Create(4);
V8::InitializePlatform(platform.get());
V8::Initialize();
// See below for the contents of this function.
int ret = RunNodeInstance(
platform.get(), result->args(), result->exec_args());
V8::Dispose();
V8::DisposePlatform();
node::TearDownOncePerProcess();
return ret;
}
Per-instance state#
Node.js has a concept of a “Node.js instance”, that is commonly being referred
to as node::Environment
. Each node::Environment
is associated with:
- Exactly one
v8::Isolate
, i.e. one JS Engine instance, - Exactly one
uv_loop_t
, i.e. one event loop, and - A number of
v8::Context
s, but exactly one mainv8::Context
. - One
node::IsolateData
instance that contains information that could be shared by multiplenode::Environment
s that use the samev8::Isolate
. Currently, no testing if performed for this scenario.
In order to set up a v8::Isolate
, an v8::ArrayBuffer::Allocator
needs
to be provided. One possible choice is the default Node.js allocator, which
can be created through node::ArrayBufferAllocator::Create()
. Using the Node.js
allocator allows minor performance optimizations when addons use the Node.js
C++ Buffer
API, and is required in order to track ArrayBuffer
memory in
process.memoryUsage()
.
Additionally, each v8::Isolate
that is used for a Node.js instance needs to
be registered and unregistered with the MultiIsolatePlatform
instance, if one
is being used, in order for the platform to know which event loop to use
for tasks scheduled by the v8::Isolate
.
The node::NewIsolate()
helper function creates a v8::Isolate
,
sets it up with some Node.js-specific hooks (e.g. the Node.js error handler),
and registers it with the platform automatically.
int RunNodeInstance(MultiIsolatePlatform* platform,
const std::vector<std::string>& args,
const std::vector<std::string>& exec_args) {
int exit_code = 0;
// Setup up a libuv event loop, v8::Isolate, and Node.js Environment.
std::vector<std::string> errors;
std::unique_ptr<CommonEnvironmentSetup> setup =
CommonEnvironmentSetup::Create(platform, &errors, args, exec_args);
if (!setup) {
for (const std::string& err : errors)
fprintf(stderr, "%s: %s\n", args[0].c_str(), err.c_str());
return 1;
}
Isolate* isolate = setup->isolate();
Environment* env = setup->env();
{
Locker locker(isolate);
Isolate::Scope isolate_scope(isolate);
HandleScope handle_scope(isolate);
// The v8::Context needs to be entered when node::CreateEnvironment() and
// node::LoadEnvironment() are being called.
Context::Scope context_scope(setup->context());
// Set up the Node.js instance for execution, and run code inside of it.
// There is also a variant that takes a callback and provides it with
// the `require` and `process` objects, so that it can manually compile
// and run scripts as needed.
// The `require` function inside this script does *not* access the file
// system, and can only load built-in Node.js modules.
// `module.createRequire()` is being used to create one that is able to
// load files from the disk, and uses the standard CommonJS file loader
// instead of the internal-only `require` function.
MaybeLocal<Value> loadenv_ret = node::LoadEnvironment(
env,
"const publicRequire ="
" require('node:module').createRequire(process.cwd() + '/');"
"globalThis.require = publicRequire;"
"require('node:vm').runInThisContext(process.argv[1]);");
if (loadenv_ret.IsEmpty()) // There has been a JS exception.
return 1;
exit_code = node::SpinEventLoop(env).FromMaybe(1);
// node::Stop() can be used to explicitly stop the event loop and keep
// further JavaScript from running. It can be called from any thread,
// and will act like worker.terminate() if called from another thread.
node::Stop(env);
}
return exit_code;
}
Child process#
Source Code: lib/child_process.js
The node:child_process
module provides the ability to spawn subprocesses in
a manner that is similar, but not identical, to popen(3)
. This capability
is primarily provided by the child_process.spawn()
function:
const { spawn } = require('node:child_process');
const ls = spawn('ls', ['-lh', '/usr']);
ls.stdout.on('data', (data) => {
console.log(`stdout: ${data}`);
});
ls.stderr.on('data', (data) => {
console.error(`stderr: ${data}`);
});
ls.on('close', (code) => {
console.log(`child process exited with code ${code}`);
});
By default, pipes for stdin
, stdout
, and stderr
are established between
the parent Node.js process and the spawned subprocess. These pipes have
limited (and platform-specific) capacity. If the subprocess writes to
stdout in excess of that limit without the output being captured, the
subprocess blocks waiting for the pipe buffer to accept more data. This is
identical to the behavior of pipes in the shell. Use the { stdio: 'ignore' }
option if the output will not be consumed.
The command lookup is performed using the options.env.PATH
environment
variable if env
is in the options
object. Otherwise, process.env.PATH
is
used. If options.env
is set without PATH
, lookup on Unix is performed
on a default search path search of /usr/bin:/bin
(see your operating system's
manual for execvpe/execvp), on Windows the current processes environment
variable PATH
is used.
On Windows, environment variables are case-insensitive. Node.js
lexicographically sorts the env
keys and uses the first one that
case-insensitively matches. Only first (in lexicographic order) entry will be
passed to the subprocess. This might lead to issues on Windows when passing
objects to the env
option that have multiple variants of the same key, such as
PATH
and Path
.
The child_process.spawn()
method spawns the child process asynchronously,
without blocking the Node.js event loop. The child_process.spawnSync()
function provides equivalent functionality in a synchronous manner that blocks
the event loop until the spawned process either exits or is terminated.
For convenience, the node:child_process
module provides a handful of
synchronous and asynchronous alternatives to child_process.spawn()
and
child_process.spawnSync()
. Each of these alternatives are implemented on
top of child_process.spawn()
or child_process.spawnSync()
.
child_process.exec()
: spawns a shell and runs a command within that shell, passing thestdout
andstderr
to a callback function when complete.child_process.execFile()
: similar tochild_process.exec()
except that it spawns the command directly without first spawning a shell by default.child_process.fork()
: spawns a new Node.js process and invokes a specified module with an IPC communication channel established that allows sending messages between parent and child.child_process.execSync()
: a synchronous version ofchild_process.exec()
that will block the Node.js event loop.child_process.execFileSync()
: a synchronous version ofchild_process.execFile()
that will block the Node.js event loop.
For certain use cases, such as automating shell scripts, the synchronous counterparts may be more convenient. In many cases, however, the synchronous methods can have significant impact on performance due to stalling the event loop while spawned processes complete.
Asynchronous process creation#
The child_process.spawn()
, child_process.fork()
, child_process.exec()
,
and child_process.execFile()
methods all follow the idiomatic asynchronous
programming pattern typical of other Node.js APIs.
Each of the methods returns a ChildProcess
instance. These objects
implement the Node.js EventEmitter
API, allowing the parent process to
register listener functions that are called when certain events occur during
the life cycle of the child process.
The child_process.exec()
and child_process.execFile()
methods
additionally allow for an optional callback
function to be specified that is
invoked when the child process terminates.
Spawning .bat
and .cmd
files on Windows#
The importance of the distinction between child_process.exec()
and
child_process.execFile()
can vary based on platform. On Unix-type
operating systems (Unix, Linux, macOS) child_process.execFile()
can be
more efficient because it does not spawn a shell by default. On Windows,
however, .bat
and .cmd
files are not executable on their own without a
terminal, and therefore cannot be launched using child_process.execFile()
.
When running on Windows, .bat
and .cmd
files can be invoked using
child_process.spawn()
with the shell
option set, with
child_process.exec()
, or by spawning cmd.exe
and passing the .bat
or
.cmd
file as an argument (which is what the shell
option and
child_process.exec()
do). In any case, if the script filename contains
spaces it needs to be quoted.
// On Windows Only...
const { spawn } = require('node:child_process');
const bat = spawn('cmd.exe', ['/c', 'my.bat']);
bat.stdout.on('data', (data) => {
console.log(data.toString());
});
bat.stderr.on('data', (data) => {
console.error(data.toString());
});
bat.on('exit', (code) => {
console.log(`Child exited with code ${code}`);
});
// OR...
const { exec, spawn } = require('node:child_process');
exec('my.bat', (err, stdout, stderr) => {
if (err) {
console.error(err);
return;
}
console.log(stdout);
});
// Script with spaces in the filename:
const bat = spawn('"my script.cmd"', ['a', 'b'], { shell: true });
// or:
exec('"my script.cmd" a b', (err, stdout, stderr) => {
// ...
});
child_process.exec(command[, options][, callback])
#
command
<string> The command to run, with space-separated arguments.options
<Object>cwd
<string> | <URL> Current working directory of the child process. Default:process.cwd()
.env
<Object> Environment key-value pairs. Default:process.env
.encoding
<string> Default:'utf8'
shell
<string> Shell to execute the command with. See Shell requirements and Default Windows shell. Default:'/bin/sh'
on Unix,process.env.ComSpec
on Windows.signal
<AbortSignal> allows aborting the child process using an AbortSignal.timeout
<number> Default:0
maxBuffer
<number> Largest amount of data in bytes allowed on stdout or stderr. If exceeded, the child process is terminated and any output is truncated. See caveat atmaxBuffer
and Unicode. Default:1024 * 1024
.killSignal
<string> | <integer> Default:'SIGTERM'
uid
<number> Sets the user identity of the process (seesetuid(2)
).gid
<number> Sets the group identity of the process (seesetgid(2)
).windowsHide
<boolean> Hide the subprocess console window that would normally be created on Windows systems. Default:false
.
callback
<Function> called with the output when process terminates.- Returns: <ChildProcess>
Spawns a shell then executes the command
within that shell, buffering any
generated output. The command
string passed to the exec function is processed
directly by the shell and special characters (vary based on
shell)
need to be dealt with accordingly:
const { exec } = require('node:child_process');
exec('"/path/to/test file/test.sh" arg1 arg2');
// Double quotes are used so that the space in the path is not interpreted as
// a delimiter of multiple arguments.
exec('echo "The \\$HOME variable is $HOME"');
// The $HOME variable is escaped in the first instance, but not in the second.
Never pass unsanitized user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.
If a callback
function is provided, it is called with the arguments
(error, stdout, stderr)
. On success, error
will be null
. On error,
error
will be an instance of Error
. The error.code
property will be
the exit code of the process. By convention, any exit code other than 0
indicates an error. error.signal
will be the signal that terminated the
process.
The stdout
and stderr
arguments passed to the callback will contain the
stdout and stderr output of the child process. By default, Node.js will decode
the output as UTF-8 and pass strings to the callback. The encoding
option
can be used to specify the character encoding used to decode the stdout and
stderr output. If encoding
is 'buffer'
, or an unrecognized character
encoding, Buffer
objects will be passed to the callback instead.
const { exec } = require('node:child_process');
exec('cat *.js missing_file | wc -l', (error, stdout, stderr) => {
if (error) {
console.error(`exec error: ${error}`);
return;
}
console.log(`stdout: ${stdout}`);
console.error(`stderr: ${stderr}`);
});
If timeout
is greater than 0
, the parent will send the signal
identified by the killSignal
property (the default is 'SIGTERM'
) if the
child runs longer than timeout
milliseconds.
Unlike the exec(3)
POSIX system call, child_process.exec()
does not replace
the existing process and uses a shell to execute the command.
If this method is invoked as its util.promisify()
ed version, it returns
a Promise
for an Object
with stdout
and stderr
properties. The returned
ChildProcess
instance is attached to the Promise
as a child
property. In
case of an error (including any error resulting in an exit code other than 0), a
rejected promise is returned, with the same error
object given in the
callback, but with two additional properties stdout
and stderr
.
const util = require('node:util');
const exec = util.promisify(require('node:child_process').exec);
async function lsExample() {
const { stdout, stderr } = await exec('ls');
console.log('stdout:', stdout);
console.error('stderr:', stderr);
}
lsExample();
If the signal
option is enabled, calling .abort()
on the corresponding
AbortController
is similar to calling .kill()
on the child process except
the error passed to the callback will be an AbortError
:
const { exec } = require('node:child_process');
const controller = new AbortController();
const { signal } = controller;
const child = exec('grep ssh', { signal }, (error) => {
console.error(error); // an AbortError
});
controller.abort();
child_process.execFile(file[, args][, options][, callback])
#
file
<string> The name or path of the executable file to run.args
<string[]> List of string arguments.options
<Object>cwd
<string> | <URL> Current working directory of the child process.env
<Object> Environment key-value pairs. Default:process.env
.encoding
<string> Default:'utf8'
timeout
<number> Default:0
maxBuffer
<number> Largest amount of data in bytes allowed on stdout or stderr. If exceeded, the child process is terminated and any output is truncated. See caveat atmaxBuffer
and Unicode. Default:1024 * 1024
.killSignal
<string> | <integer> Default:'SIGTERM'
uid
<number> Sets the user identity of the process (seesetuid(2)
).gid
<number> Sets the group identity of the process (seesetgid(2)
).windowsHide
<boolean> Hide the subprocess console window that would normally be created on Windows systems. Default:false
.windowsVerbatimArguments
<boolean> No quoting or escaping of arguments is done on Windows. Ignored on Unix. Default:false
.shell
<boolean> | <string> Iftrue
, runscommand
inside of a shell. Uses'/bin/sh'
on Unix, andprocess.env.ComSpec
on Windows. A different shell can be specified as a string. See Shell requirements and Default Windows shell. Default:false
(no shell).signal
<AbortSignal> allows aborting the child process using an AbortSignal.
callback
<Function> Called with the output when process terminates.- Returns: <ChildProcess>
The child_process.execFile()
function is similar to child_process.exec()
except that it does not spawn a shell by default. Rather, the specified
executable file
is spawned directly as a new process making it slightly more
efficient than child_process.exec()
.
The same options as child_process.exec()
are supported. Since a shell is
not spawned, behaviors such as I/O redirection and file globbing are not
supported.
const { execFile } = require('node:child_process');
const child = execFile('node', ['--version'], (error, stdout, stderr) => {
if (error) {
throw error;
}
console.log(stdout);
});
The stdout
and stderr
arguments passed to the callback will contain the
stdout and stderr output of the child process. By default, Node.js will decode
the output as UTF-8 and pass strings to the callback. The encoding
option
can be used to specify the character encoding used to decode the stdout and
stderr output. If encoding
is 'buffer'
, or an unrecognized character
encoding, Buffer
objects will be passed to the callback instead.
If this method is invoked as its util.promisify()
ed version, it returns
a Promise
for an Object
with stdout
and stderr
properties. The returned
ChildProcess
instance is attached to the Promise
as a child
property. In
case of an error (including any error resulting in an exit code other than 0), a
rejected promise is returned, with the same error
object given in the
callback, but with two additional properties stdout
and stderr
.
const util = require('node:util');
const execFile = util.promisify(require('node:child_process').execFile);
async function getVersion() {
const { stdout } = await execFile('node', ['--version']);
console.log(stdout);
}
getVersion();
If the shell
option is enabled, do not pass unsanitized user input to this
function. Any input containing shell metacharacters may be used to trigger
arbitrary command execution.
If the signal
option is enabled, calling .abort()
on the corresponding
AbortController
is similar to calling .kill()
on the child process except
the error passed to the callback will be an AbortError
:
const { execFile } = require('node:child_process');
const controller = new AbortController();
const { signal } = controller;
const child = execFile('node', ['--version'], { signal }, (error) => {
console.error(error); // an AbortError
});
controller.abort();
child_process.fork(modulePath[, args][, options])
#
modulePath
<string> | <URL> The module to run in the child.args
<string[]> List of string arguments.options
<Object>cwd
<string> | <URL> Current working directory of the child process.detached
<boolean> Prepare child to run independently of its parent process. Specific behavior depends on the platform, seeoptions.detached
).env
<Object> Environment key-value pairs. Default:process.env
.execPath
<string> Executable used to create the child process.execArgv
<string[]> List of string arguments passed to the executable. Default:process.execArgv
.gid
<number> Sets the group identity of the process (seesetgid(2)
).serialization
<string> Specify the kind of serialization used for sending messages between processes. Possible values are'json'
and'advanced'
. See Advanced serialization for more details. Default:'json'
.signal
<AbortSignal> Allows closing the child process using an AbortSignal.killSignal
<string> | <integer> The signal value to be used when the spawned process will be killed by timeout or abort signal. Default:'SIGTERM'
.silent
<boolean> Iftrue
, stdin, stdout, and stderr of the child will be piped to the parent, otherwise they will be inherited from the parent, see the'pipe'
and'inherit'
options forchild_process.spawn()
'sstdio
for more details. Default:false
.stdio
<Array> | <string> Seechild_process.spawn()
'sstdio
. When this option is provided, it overridessilent
. If the array variant is used, it must contain exactly one item with value'ipc'
or an error will be thrown. For instance[0, 1, 2, 'ipc']
.uid
<number> Sets the user identity of the process (seesetuid(2)
).windowsVerbatimArguments
<boolean> No quoting or escaping of arguments is done on Windows. Ignored on Unix. Default:false
.timeout
<number> In milliseconds the maximum amount of time the process is allowed to run. Default:undefined
.
- Returns: <ChildProcess>
The child_process.fork()
method is a special case of
child_process.spawn()
used specifically to spawn new Node.js processes.
Like child_process.spawn()
, a ChildProcess
object is returned. The
returned ChildProcess
will have an additional communication channel
built-in that allows messages to be passed back and forth between the parent and
child. See subprocess.send()
for details.
Keep in mind that spawned Node.js child processes are independent of the parent with exception of the IPC communication channel that is established between the two. Each process has its own memory, with their own V8 instances. Because of the additional resource allocations required, spawning a large number of child Node.js processes is not recommended.
By default, child_process.fork()
will spawn new Node.js instances using the
process.execPath
of the parent process. The execPath
property in the
options
object allows for an alternative execution path to be used.
Node.js processes launched with a custom execPath
will communicate with the
parent process using the file descriptor (fd) identified using the
environment variable NODE_CHANNEL_FD
on the child process.
Unlike the fork(2)
POSIX system call, child_process.fork()
does not clone the
current process.
The shell
option available in child_process.spawn()
is not supported by
child_process.fork()
and will be ignored if set.
If the signal
option is enabled, calling .abort()
on the corresponding
AbortController
is similar to calling .kill()
on the child process except
the error passed to the callback will be an AbortError
:
if (process.argv[2] === 'child') {
setTimeout(() => {
console.log(`Hello from ${process.argv[2]}!`);
}, 1_000);
} else {
const { fork } = require('node:child_process');
const controller = new AbortController();
const { signal } = controller;
const child = fork(__filename, ['child'], { signal });
child.on('error', (err) => {
// This will be called with err being an AbortError if the controller aborts
});
controller.abort(); // Stops the child process
}
child_process.spawn(command[, args][, options])
#
command
<string> The command to run.args
<string[]> List of string arguments.options
<Object>cwd
<string> | <URL> Current working directory of the child process.env
<Object> Environment key-value pairs. Default:process.env
.argv0
<string> Explicitly set the value ofargv[0]
sent to the child process. This will be set tocommand
if not specified.stdio
<Array> | <string> Child's stdio configuration (seeoptions.stdio
).detached
<boolean> Prepare child to run independently of its parent process. Specific behavior depends on the platform, seeoptions.detached
).uid
<number> Sets the user identity of the process (seesetuid(2)
).gid
<number> Sets the group identity of the process (seesetgid(2)
).serialization
<string> Specify the kind of serialization used for sending messages between processes. Possible values are'json'
and'advanced'
. See Advanced serialization for more details. Default:'json'
.shell
<boolean> | <string> Iftrue
, runscommand
inside of a shell. Uses'/bin/sh'
on Unix, andprocess.env.ComSpec
on Windows. A different shell can be specified as a string. See Shell requirements and Default Windows shell. Default:false
(no shell).windowsVerbatimArguments
<boolean> No quoting or escaping of arguments is done on Windows. Ignored on Unix. This is set totrue
automatically whenshell
is specified and is CMD. Default:false
.windowsHide
<boolean> Hide the subprocess console window that would normally be created on Windows systems. Default:false
.signal
<AbortSignal> allows aborting the child process using an AbortSignal.timeout
<number> In milliseconds the maximum amount of time the process is allowed to run. Default:undefined
.killSignal
<string> | <integer> The signal value to be used when the spawned process will be killed by timeout or abort signal. Default:'SIGTERM'
.
- Returns: <ChildProcess>
The child_process.spawn()
method spawns a new process using the given
command
, with command-line arguments in args
. If omitted, args
defaults
to an empty array.
If the shell
option is enabled, do not pass unsanitized user input to this
function. Any input containing shell metacharacters may be used to trigger
arbitrary command execution.
A third argument may be used to specify additional options, with these defaults:
const defaults = {
cwd: undefined,
env: process.env,
};
Use cwd
to specify the working directory from which the process is spawned.
If not given, the default is to inherit the current working directory. If given,
but the path does not exist, the child process emits an ENOENT
error
and exits immediately. ENOENT
is also emitted when the command
does not exist.
Use env
to specify environment variables that will be visible to the new
process, the default is process.env
.
undefined
values in env
will be ignored.
Example of running ls -lh /usr
, capturing stdout
, stderr
, and the
exit code:
const { spawn } = require('node:child_process');
const ls = spawn('ls', ['-lh', '/usr']);
ls.stdout.on('data', (data) => {
console.log(`stdout: ${data}`);
});
ls.stderr.on('data', (data) => {
console.error(`stderr: ${data}`);
});
ls.on('close', (code) => {
console.log(`child process exited with code ${code}`);
});
Example: A very elaborate way to run ps ax | grep ssh
const { spawn } = require('node:child_process');
const ps = spawn('ps', ['ax']);
const grep = spawn('grep', ['ssh']);
ps.stdout.on('data', (data) => {
grep.stdin.write(data);
});
ps.stderr.on('data', (data) => {
console.error(`ps stderr: ${data}`);
});
ps.on('close', (code) => {
if (code !== 0) {
console.log(`ps process exited with code ${code}`);
}
grep.stdin.end();
});
grep.stdout.on('data', (data) => {
console.log(data.toString());
});
grep.stderr.on('data', (data) => {
console.error(`grep stderr: ${data}`);
});
grep.on('close', (code) => {
if (code !== 0) {
console.log(`grep process exited with code ${code}`);
}
});
Example of checking for failed spawn
:
const { spawn } = require('node:child_process');
const subprocess = spawn('bad_command');
subprocess.on('error', (err) => {
console.error('Failed to start subprocess.');
});
Certain platforms (macOS, Linux) will use the value of argv[0]
for the process
title while others (Windows, SunOS) will use command
.
Node.js overwrites argv[0]
with process.execPath
on startup, so
process.argv[0]
in a Node.js child process will not match the argv0
parameter passed to spawn
from the parent. Retrieve it with the
process.argv0
property instead.
If the signal
option is enabled, calling .abort()
on the corresponding
AbortController
is similar to calling .kill()
on the child process except
the error passed to the callback will be an AbortError
:
const { spawn } = require('node:child_process');
const controller = new AbortController();
const { signal } = controller;
const grep = spawn('grep', ['ssh'], { signal });
grep.on('error', (err) => {
// This will be called with err being an AbortError if the controller aborts
});
controller.abort(); // Stops the child process
options.detached
#
On Windows, setting options.detached
to true
makes it possible for the
child process to continue running after the parent exits. The child will have
its own console window. Once enabled for a child process, it cannot be
disabled.
On non-Windows platforms, if options.detached
is set to true
, the child
process will be made the leader of a new process group and session. Child
processes may continue running after the parent exits regardless of whether
they are detached or not. See setsid(2)
for more information.
By default, the parent will wait for the detached child to exit. To prevent the
parent from waiting for a given subprocess
to exit, use the
subprocess.unref()
method. Doing so will cause the parent's event loop to not
include the child in its reference count, allowing the parent to exit
independently of the child, unless there is an established IPC channel between
the child and the parent.
When using the detached
option to start a long-running process, the process
will not stay running in the background after the parent exits unless it is
provided with a stdio
configuration that is not connected to the parent.
If the parent's stdio
is inherited, the child will remain attached to the
controlling terminal.
Example of a long-running process, by detaching and also ignoring its parent
stdio
file descriptors, in order to ignore the parent's termination:
const { spawn } = require('node:child_process');
const subprocess = spawn(process.argv[0], ['child_program.js'], {
detached: true,
stdio: 'ignore',
});
subprocess.unref();
Alternatively one can redirect the child process' output into files:
const fs = require('node:fs');
const { spawn } = require('node:child_process');
const out = fs.openSync('./out.log', 'a');
const err = fs.openSync('./out.log', 'a');
const subprocess = spawn('prg', [], {
detached: true,
stdio: [ 'ignore', out, err ],
});
subprocess.unref();
options.stdio
#
The options.stdio
option is used to configure the pipes that are established
between the parent and child process. By default, the child's stdin, stdout,
and stderr are redirected to corresponding subprocess.stdin
,
subprocess.stdout
, and subprocess.stderr
streams on the
ChildProcess
object. This is equivalent to setting the options.stdio
equal to ['pipe', 'pipe', 'pipe']
.
For convenience, options.stdio
may be one of the following strings:
'pipe'
: equivalent to['pipe', 'pipe', 'pipe']
(the default)'overlapped'
: equivalent to['overlapped', 'overlapped', 'overlapped']
'ignore'
: equivalent to['ignore', 'ignore', 'ignore']
'inherit'
: equivalent to['inherit', 'inherit', 'inherit']
or[0, 1, 2]
Otherwise, the value of options.stdio
is an array where each index corresponds
to an fd in the child. The fds 0, 1, and 2 correspond to stdin, stdout,
and stderr, respectively. Additional fds can be specified to create additional
pipes between the parent and child. The value is one of the following:
-
'pipe'
: Create a pipe between the child process and the parent process. The parent end of the pipe is exposed to the parent as a property on thechild_process
object assubprocess.stdio[fd]
. Pipes created for fds 0, 1, and 2 are also available assubprocess.stdin
,subprocess.stdout
andsubprocess.stderr
, respectively. These are not actual Unix pipes and therefore the child process can not use them by their descriptor files, e.g./dev/fd/2
or/dev/stdout
. -
'overlapped'
: Same as'pipe'
except that theFILE_FLAG_OVERLAPPED
flag is set on the handle. This is necessary for overlapped I/O on the child process's stdio handles. See the docs for more details. This is exactly the same as'pipe'
on non-Windows systems. -
'ipc'
: Create an IPC channel for passing messages/file descriptors between parent and child. AChildProcess
may have at most one IPC stdio file descriptor. Setting this option enables thesubprocess.send()
method. If the child is a Node.js process, the presence of an IPC channel will enableprocess.send()
andprocess.disconnect()
methods, as well as'disconnect'
and'message'
events within the child.Accessing the IPC channel fd in any way other than
process.send()
or using the IPC channel with a child process that is not a Node.js instance is not supported. -
'ignore'
: Instructs Node.js to ignore the fd in the child. While Node.js will always open fds 0, 1, and 2 for the processes it spawns, setting the fd to'ignore'
will cause Node.js to open/dev/null
and attach it to the child's fd. -
'inherit'
: Pass through the corresponding stdio stream to/from the parent process. In the first three positions, this is equivalent toprocess.stdin
,process.stdout
, andprocess.stderr
, respectively. In any other position, equivalent to'ignore'
. -
<Stream> object: Share a readable or writable stream that refers to a tty, file, socket, or a pipe with the child process. The stream's underlying file descriptor is duplicated in the child process to the fd that corresponds to the index in the
stdio
array. The stream must have an underlying descriptor (file streams do not until the'open'
event has occurred). -
Positive integer: The integer value is interpreted as a file descriptor that is open in the parent process. It is shared with the child process, similar to how <Stream> objects can be shared. Passing sockets is not supported on Windows.
-
null
,undefined
: Use default value. For stdio fds 0, 1, and 2 (in other words, stdin, stdout, and stderr) a pipe is created. For fd 3 and up, the default is'ignore'
.
const { spawn } = require('node:child_process');
// Child will use parent's stdios.
spawn('prg', [], { stdio: 'inherit' });
// Spawn child sharing only stderr.
spawn('prg', [], { stdio: ['pipe', 'pipe', process.stderr] });
// Open an extra fd=4, to interact with programs presenting a
// startd-style interface.
spawn('prg', [], { stdio: ['pipe', null, null, null, 'pipe'] });
It is worth noting that when an IPC channel is established between the
parent and child processes, and the child is a Node.js process, the child
is launched with the IPC channel unreferenced (using unref()
) until the
child registers an event handler for the 'disconnect'
event
or the 'message'
event. This allows the child to exit
normally without the process being held open by the open IPC channel.
On Unix-like operating systems, the child_process.spawn()
method
performs memory operations synchronously before decoupling the event loop
from the child. Applications with a large memory footprint may find frequent
child_process.spawn()
calls to be a bottleneck. For more information,
see V8 issue 7381.
See also: child_process.exec()
and child_process.fork()
.
Synchronous process creation#
The child_process.spawnSync()
, child_process.execSync()
, and
child_process.execFileSync()
methods are synchronous and will block the
Node.js event loop, pausing execution of any additional code until the spawned
process exits.
Blocking calls like these are mostly useful for simplifying general-purpose scripting tasks and for simplifying the loading/processing of application configuration at startup.
child_process.execFileSync(file[, args][, options])
#
file
<string> The name or path of the executable file to run.args
<string[]> List of string arguments.options
<Object>cwd
<string> | <URL> Current working directory of the child process.input
<string> | <Buffer> | <TypedArray> | <DataView> The value which will be passed as stdin to the spawned process. Supplying this value will overridestdio[0]
.stdio
<string> | <Array> Child's stdio configuration.stderr
by default will be output to the parent process' stderr unlessstdio
is specified. Default:'pipe'
.env
<Object> Environment key-value pairs. Default:process.env
.uid
<number> Sets the user identity of the process (seesetuid(2)
).gid
<number> Sets the group identity of the process (seesetgid(2)
).timeout
<number> In milliseconds the maximum amount of time the process is allowed to run. Default:undefined
.killSignal
<string> | <integer> The signal value to be used when the spawned process will be killed. Default:'SIGTERM'
.maxBuffer
<number> Largest amount of data in bytes allowed on stdout or stderr. If exceeded, the child process is terminated. See caveat atmaxBuffer
and Unicode. Default:1024 * 1024
.encoding
<string> The encoding used for all stdio inputs and outputs. Default:'buffer'
.windowsHide
<boolean> Hide the subprocess console window that would normally be created on Windows systems. Default:false
.shell
<boolean> | <string> Iftrue
, runscommand
inside of a shell. Uses'/bin/sh'
on Unix, andprocess.env.ComSpec
on Windows. A different shell can be specified as a string. See Shell requirements and Default Windows shell. Default:false
(no shell).
- Returns: <Buffer> | <string> The stdout from the command.
The child_process.execFileSync()
method is generally identical to
child_process.execFile()
with the exception that the method will not
return until the child process has fully closed. When a timeout has been
encountered and killSignal
is sent, the method won't return until the process
has completely exited.
If the child process intercepts and handles the SIGTERM
signal and
does not exit, the parent process will still wait until the child process has
exited.
If the process times out or has a non-zero exit code, this method will throw an
Error
that will include the full result of the underlying
child_process.spawnSync()
.
If the shell
option is enabled, do not pass unsanitized user input to this
function. Any input containing shell metacharacters may be used to trigger
arbitrary command execution.
child_process.execSync(command[, options])
#
command
<string> The command to run.options
<Object>cwd
<string> | <URL> Current working directory of the child process.input
<string> | <Buffer> | <TypedArray> | <DataView> The value which will be passed as stdin to the spawned process. Supplying this value will overridestdio[0]
.stdio
<string> | <Array> Child's stdio configuration.stderr
by default will be output to the parent process' stderr unlessstdio
is specified. Default:'pipe'
.env
<Object> Environment key-value pairs. Default:process.env
.shell
<string> Shell to execute the command with. See Shell requirements and Default Windows shell. Default:'/bin/sh'
on Unix,process.env.ComSpec
on Windows.uid
<number> Sets the user identity of the process. (Seesetuid(2)
).gid
<number> Sets the group identity of the process. (Seesetgid(2)
).timeout
<number> In milliseconds the maximum amount of time the process is allowed to run. Default:undefined
.killSignal
<string> | <integer> The signal value to be used when the spawned process will be killed. Default:'SIGTERM'
.maxBuffer
<number> Largest amount of data in bytes allowed on stdout or stderr. If exceeded, the child process is terminated and any output is truncated. See caveat atmaxBuffer
and Unicode. Default:1024 * 1024
.encoding
<string> The encoding used for all stdio inputs and outputs. Default:'buffer'
.windowsHide
<boolean> Hide the subprocess console window that would normally be created on Windows systems. Default:false
.
- Returns: <Buffer> | <string> The stdout from the command.
The child_process.execSync()
method is generally identical to
child_process.exec()
with the exception that the method will not return
until the child process has fully closed. When a timeout has been encountered
and killSignal
is sent, the method won't return until the process has
completely exited. If the child process intercepts and handles the SIGTERM
signal and doesn't exit, the parent process will wait until the child process
has exited.
If the process times out or has a non-zero exit code, this method will throw.
The Error
object will contain the entire result from
child_process.spawnSync()
.
Never pass unsanitized user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.
child_process.spawnSync(command[, args][, options])
#
command
<string> The command to run.args
<string[]> List of string arguments.options
<Object>cwd
<string> | <URL> Current working directory of the child process.input
<string> | <Buffer> | <TypedArray> | <DataView> The value which will be passed as stdin to the spawned process. Supplying this value will overridestdio[0]
.argv0
<string> Explicitly set the value ofargv[0]
sent to the child process. This will be set tocommand
if not specified.stdio
<string> | <Array> Child's stdio configuration.env
<Object> Environment key-value pairs. Default:process.env
.uid
<number> Sets the user identity of the process (seesetuid(2)
).gid
<number> Sets the group identity of the process (seesetgid(2)
).timeout
<number> In milliseconds the maximum amount of time the process is allowed to run. Default:undefined
.killSignal
<string> | <integer> The signal value to be used when the spawned process will be killed. Default:'SIGTERM'
.maxBuffer
<number> Largest amount of data in bytes allowed on stdout or stderr. If exceeded, the child process is terminated and any output is truncated. See caveat atmaxBuffer
and Unicode. Default:1024 * 1024
.encoding
<string> The encoding used for all stdio inputs and outputs. Default:'buffer'
.shell
<boolean> | <string> Iftrue
, runscommand
inside of a shell. Uses'/bin/sh'
on Unix, andprocess.env.ComSpec
on Windows. A different shell can be specified as a string. See Shell requirements and Default Windows shell. Default:false
(no shell).windowsVerbatimArguments
<boolean> No quoting or escaping of arguments is done on Windows. Ignored on Unix. This is set totrue
automatically whenshell
is specified and is CMD. Default:false
.windowsHide
<boolean> Hide the subprocess console window that would normally be created on Windows systems. Default:false
.
- Returns: <Object>
pid
<number> Pid of the child process.output
<Array> Array of results from stdio output.stdout
<Buffer> | <string> The contents ofoutput[1]
.stderr
<Buffer> | <string> The contents ofoutput[2]
.status
<number> | <null> The exit code of the subprocess, ornull
if the subprocess terminated due to a signal.signal
<string> | <null> The signal used to kill the subprocess, ornull
if the subprocess did not terminate due to a signal.error
<Error> The error object if the child process failed or timed out.
The child_process.spawnSync()
method is generally identical to
child_process.spawn()
with the exception that the function will not return
until the child process has fully closed. When a timeout has been encountered
and killSignal
is sent, the method won't return until the process has
completely exited. If the process intercepts and handles the SIGTERM
signal
and doesn't exit, the parent process will wait until the child process has
exited.
If the shell
option is enabled, do not pass unsanitized user input to this
function. Any input containing shell metacharacters may be used to trigger
arbitrary command execution.
Class: ChildProcess
#
- Extends: <EventEmitter>
Instances of the ChildProcess
represent spawned child processes.
Instances of ChildProcess
are not intended to be created directly. Rather,
use the child_process.spawn()
, child_process.exec()
,
child_process.execFile()
, or child_process.fork()
methods to create
instances of ChildProcess
.
Event: 'close'
#
code
<number> The exit code if the child exited on its own.signal
<string> The signal by which the child process was terminated.
The 'close'
event is emitted after a process has ended and the stdio
streams of a child process have been closed. This is distinct from the
'exit'
event, since multiple processes might share the same stdio
streams. The 'close'
event will always emit after 'exit'
was
already emitted, or 'error'
if the child failed to spawn.
const { spawn } = require('node:child_process');
const ls = spawn('ls', ['-lh', '/usr']);
ls.stdout.on('data', (data) => {
console.log(`stdout: ${data}`);
});
ls.on('close', (code) => {
console.log(`child process close all stdio with code ${code}`);
});
ls.on('exit', (code) => {
console.log(`child process exited with code ${code}`);
});
Event: 'disconnect'
#
The 'disconnect'
event is emitted after calling the
subprocess.disconnect()
method in parent process or
process.disconnect()
in child process. After disconnecting it is no longer
possible to send or receive messages, and the subprocess.connected
property is false
.
Event: 'error'
#
err
<Error> The error.
The 'error'
event is emitted whenever:
- The process could not be spawned.
- The process could not be killed.
- Sending a message to the child process failed.
- The child process was aborted via the
signal
option.
The 'exit'
event may or may not fire after an error has occurred. When
listening to both the 'exit'
and 'error'
events, guard
against accidentally invoking handler functions multiple times.
See also subprocess.kill()
and subprocess.send()
.
Event: 'exit'
#
code
<number> The exit code if the child exited on its own.signal
<string> The signal by which the child process was terminated.
The 'exit'
event is emitted after the child process ends. If the process
exited, code
is the final exit code of the process, otherwise null
. If the
process terminated due to receipt of a signal, signal
is the string name of
the signal, otherwise null
. One of the two will always be non-null
.
When the 'exit'
event is triggered, child process stdio streams might still be
open.
Node.js establishes signal handlers for SIGINT
and SIGTERM
and Node.js
processes will not terminate immediately due to receipt of those signals.
Rather, Node.js will perform a sequence of cleanup actions and then will
re-raise the handled signal.
See waitpid(2)
.
Event: 'message'
#
message
<Object> A parsed JSON object or primitive value.sendHandle
<Handle> Anet.Socket
ornet.Server
object, or undefined.
The 'message'
event is triggered when a child process uses
process.send()
to send messages.
The message goes through serialization and parsing. The resulting message might not be the same as what is originally sent.
If the serialization
option was set to 'advanced'
used when spawning the
child process, the message
argument can contain data that JSON is not able
to represent.
See Advanced serialization for more details.
Event: 'spawn'
#
The 'spawn'
event is emitted once the child process has spawned successfully.
If the child process does not spawn successfully, the 'spawn'
event is not
emitted and the 'error'
event is emitted instead.
If emitted, the 'spawn'
event comes before all other events and before any
data is received via stdout
or stderr
.
The 'spawn'
event will fire regardless of whether an error occurs within
the spawned process. For example, if bash some-command
spawns successfully,
the 'spawn'
event will fire, though bash
may fail to spawn some-command
.
This caveat also applies when using { shell: true }
.
subprocess.channel
#
- <Object> A pipe representing the IPC channel to the child process.
The subprocess.channel
property is a reference to the child's IPC channel. If
no IPC channel exists, this property is undefined
.
subprocess.channel.ref()
#
This method makes the IPC channel keep the event loop of the parent process
running if .unref()
has been called before.
subprocess.channel.unref()
#
This method makes the IPC channel not keep the event loop of the parent process running, and lets it finish even while the channel is open.
subprocess.connected
#
- <boolean> Set to
false
aftersubprocess.disconnect()
is called.
The subprocess.connected
property indicates whether it is still possible to
send and receive messages from a child process. When subprocess.connected
is
false
, it is no longer possible to send or receive messages.
subprocess.disconnect()
#
Closes the IPC channel between parent and child, allowing the child to exit
gracefully once there are no other connections keeping it alive. After calling
this method the subprocess.connected
and process.connected
properties in
both the parent and child (respectively) will be set to false
, and it will be
no longer possible to pass messages between the processes.
The 'disconnect'
event will be emitted when there are no messages in the
process of being received. This will most often be triggered immediately after
calling subprocess.disconnect()
.
When the child process is a Node.js instance (e.g. spawned using
child_process.fork()
), the process.disconnect()
method can be invoked
within the child process to close the IPC channel as well.
subprocess.exitCode
#
The subprocess.exitCode
property indicates the exit code of the child process.
If the child process is still running, the field will be null
.
subprocess.kill([signal])
#
The subprocess.kill()
method sends a signal to the child process. If no
argument is given, the process will be sent the 'SIGTERM'
signal. See
signal(7)
for a list of available signals. This function returns true
if
kill(2)
succeeds, and false
otherwise.
const { spawn } = require('node:child_process');
const grep = spawn('grep', ['ssh']);
grep.on('close', (code, signal) => {
console.log(
`child process terminated due to receipt of signal ${signal}`);
});
// Send SIGHUP to process.
grep.kill('SIGHUP');
The ChildProcess
object may emit an 'error'
event if the signal
cannot be delivered. Sending a signal to a child process that has already exited
is not an error but may have unforeseen consequences. Specifically, if the
process identifier (PID) has been reassigned to another process, the signal will
be delivered to that process instead which can have unexpected results.
While the function is called kill
, the signal delivered to the child process
may not actually terminate the process.
See kill(2)
for reference.
On Windows, where POSIX signals do not exist, the signal
argument will be
ignored, and the process will be killed forcefully and abruptly (similar to
'SIGKILL'
).
See Signal Events for more details.
On Linux, child processes of child processes will not be terminated
when attempting to kill their parent. This is likely to happen when running a
new process in a shell or with the use of the shell
option of ChildProcess
:
'use strict';
const { spawn } = require('node:child_process');
const subprocess = spawn(
'sh',
[
'-c',
`node -e "setInterval(() => {
console.log(process.pid, 'is alive')
}, 500);"`,
], {
stdio: ['inherit', 'inherit', 'inherit'],
},
);
setTimeout(() => {
subprocess.kill(); // Does not terminate the Node.js process in the shell.
}, 2000);
subprocess.killed
#
- <boolean> Set to
true
aftersubprocess.kill()
is used to successfully send a signal to the child process.
The subprocess.killed
property indicates whether the child process
successfully received a signal from subprocess.kill()
. The killed
property
does not indicate that the child process has been terminated.
subprocess.pid
#
Returns the process identifier (PID) of the child process. If the child process
fails to spawn due to errors, then the value is undefined
and error
is
emitted.
const { spawn } = require('node:child_process');
const grep = spawn('grep', ['ssh']);
console.log(`Spawned child pid: ${grep.pid}`);
grep.stdin.end();
subprocess.ref()
#
Calling subprocess.ref()
after making a call to subprocess.unref()
will
restore the removed reference count for the child process, forcing the parent
to wait for the child to exit before exiting itself.
const { spawn } = require('node:child_process');
const subprocess = spawn(process.argv[0], ['child_program.js'], {
detached: true,
stdio: 'ignore',
});
subprocess.unref();
subprocess.ref();
subprocess.send(message[, sendHandle[, options]][, callback])
#
message
<Object>sendHandle
<Handle>options
<Object> Theoptions
argument, if present, is an object used to parameterize the sending of certain types of handles.options
supports the following properties:keepOpen
<boolean> A value that can be used when passing instances ofnet.Socket
. Whentrue
, the socket is kept open in the sending process. Default:false
.
callback
<Function>- Returns: <boolean>
When an IPC channel has been established between the parent and child (
i.e. when using child_process.fork()
), the subprocess.send()
method can
be used to send messages to the child process. When the child process is a
Node.js instance, these messages can be received via the 'message'
event.
The message goes through serialization and parsing. The resulting message might not be the same as what is originally sent.
For example, in the parent script:
const cp = require('node:child_process');
const n = cp.fork(`${__dirname}/sub.js`);
n.on('message', (m) => {
console.log('PARENT got message:', m);
});
// Causes the child to print: CHILD got message: { hello: 'world' }
n.send({ hello: 'world' });
And then the child script, 'sub.js'
might look like this:
process.on('message', (m) => {
console.log('CHILD got message:', m);
});
// Causes the parent to print: PARENT got message: { foo: 'bar', baz: null }
process.send({ foo: 'bar', baz: NaN });
Child Node.js processes will have a process.send()
method of their own
that allows the child to send messages back to the parent.
There is a special case when sending a {cmd: 'NODE_foo'}
message. Messages
containing a NODE_
prefix in the cmd
property are reserved for use within
Node.js core and will not be emitted in the child's 'message'
event. Rather, such messages are emitted using the
'internalMessage'
event and are consumed internally by Node.js.
Applications should avoid using such messages or listening for
'internalMessage'
events as it is subject to change without notice.
The optional sendHandle
argument that may be passed to subprocess.send()
is
for passing a TCP server or socket object to the child process. The child will
receive the object as the second argument passed to the callback function
registered on the 'message'
event. Any data that is received
and buffered in the socket will not be sent to the child.
The optional callback
is a function that is invoked after the message is
sent but before the child may have received it. The function is called with a
single argument: null
on success, or an Error
object on failure.
If no callback
function is provided and the message cannot be sent, an
'error'
event will be emitted by the ChildProcess
object. This can
happen, for instance, when the child process has already exited.
subprocess.send()
will return false
if the channel has closed or when the
backlog of unsent messages exceeds a threshold that makes it unwise to send
more. Otherwise, the method returns true
. The callback
function can be
used to implement flow control.
Example: sending a server object#
The sendHandle
argument can be used, for instance, to pass the handle of
a TCP server object to the child process as illustrated in the example below:
const subprocess = require('node:child_process').fork('subprocess.js');
// Open up the server object and send the handle.
const server = require('node:net').createServer();
server.on('connection', (socket) => {
socket.end('handled by parent');
});
server.listen(1337, () => {
subprocess.send('server', server);
});
The child would then receive the server object as:
process.on('message', (m, server) => {
if (m === 'server') {
server.on('connection', (socket) => {
socket.end('handled by child');
});
}
});
Once the server is now shared between the parent and child, some connections can be handled by the parent and some by the child.
While the example above uses a server created using the node:net
module,
node:dgram
module servers use exactly the same workflow with the exceptions of
listening on a 'message'
event instead of 'connection'
and using
server.bind()
instead of server.listen()
. This is, however, only
supported on Unix platforms.
Example: sending a socket object#
Similarly, the sendHandler
argument can be used to pass the handle of a
socket to the child process. The example below spawns two children that each
handle connections with "normal" or "special" priority:
const { fork } = require('node:child_process');
const normal = fork('subprocess.js', ['normal']);
const special = fork('subprocess.js', ['special']);
// Open up the server and send sockets to child. Use pauseOnConnect to prevent
// the sockets from being read before they are sent to the child process.
const server = require('node:net').createServer({ pauseOnConnect: true });
server.on('connection', (socket) => {
// If this is special priority...
if (socket.remoteAddress === '74.125.127.100') {
special.send('socket', socket);
return;
}
// This is normal priority.
normal.send('socket', socket);
});
server.listen(1337);
The subprocess.js
would receive the socket handle as the second argument
passed to the event callback function:
process.on('message', (m, socket) => {
if (m === 'socket') {
if (socket) {
// Check that the client socket exists.
// It is possible for the socket to be closed between the time it is
// sent and the time it is received in the child process.
socket.end(`Request handled with ${process.argv[2]} priority`);
}
}
});
Do not use .maxConnections
on a socket that has been passed to a subprocess.
The parent cannot track when the socket is destroyed.
Any 'message'
handlers in the subprocess should verify that socket
exists,
as the connection may have been closed during the time it takes to send the
connection to the child.
subprocess.signalCode
#
The subprocess.signalCode
property indicates the signal received by
the child process if any, else null
.
subprocess.spawnargs
#
The subprocess.spawnargs
property represents the full list of command-line
arguments the child process was launched with.
subprocess.spawnfile
#
The subprocess.spawnfile
property indicates the executable file name of
the child process that is launched.
For child_process.fork()
, its value will be equal to
process.execPath
.
For child_process.spawn()
, its value will be the name of
the executable file.
For child_process.exec()
, its value will be the name of the shell
in which the child process is launched.
subprocess.stderr
#
A Readable Stream
that represents the child process's stderr
.
If the child was spawned with stdio[2]
set to anything other than 'pipe'
,
then this will be null
.
subprocess.stderr
is an alias for subprocess.stdio[2]
. Both properties will
refer to the same value.
The subprocess.stderr
property can be null
or undefined
if the child process could not be successfully spawned.
subprocess.stdin
#
A Writable Stream
that represents the child process's stdin
.
If a child process waits to read all of its input, the child will not continue
until this stream has been closed via end()
.
If the child was spawned with stdio[0]
set to anything other than 'pipe'
,
then this will be null
.
subprocess.stdin
is an alias for subprocess.stdio[0]
. Both properties will
refer to the same value.
The subprocess.stdin
property can be null
or undefined
if the child process could not be successfully spawned.
subprocess.stdio
#
A sparse array of pipes to the child process, corresponding with positions in
the stdio
option passed to child_process.spawn()
that have been set
to the value 'pipe'
. subprocess.stdio[0]
, subprocess.stdio[1]
, and
subprocess.stdio[2]
are also available as subprocess.stdin
,
subprocess.stdout
, and subprocess.stderr
, respectively.
In the following example, only the child's fd 1
(stdout) is configured as a
pipe, so only the parent's subprocess.stdio[1]
is a stream, all other values
in the array are null
.
const assert = require('node:assert');
const fs = require('node:fs');
const child_process = require('node:child_process');
const subprocess = child_process.spawn('ls', {
stdio: [
0, // Use parent's stdin for child.
'pipe', // Pipe child's stdout to parent.
fs.openSync('err.out', 'w'), // Direct child's stderr to a file.
],
});
assert.strictEqual(subprocess.stdio[0], null);
assert.strictEqual(subprocess.stdio[0], subprocess.stdin);
assert(subprocess.stdout);
assert.strictEqual(subprocess.stdio[1], subprocess.stdout);
assert.strictEqual(subprocess.stdio[2], null);
assert.strictEqual(subprocess.stdio[2], subprocess.stderr);
The subprocess.stdio
property can be undefined
if the child process could
not be successfully spawned.
subprocess.stdout
#
A Readable Stream
that represents the child process's stdout
.
If the child was spawned with stdio[1]
set to anything other than 'pipe'
,
then this will be null
.
subprocess.stdout
is an alias for subprocess.stdio[1]
. Both properties will
refer to the same value.
const { spawn } = require('node:child_process');
const subprocess = spawn('ls');
subprocess.stdout.on('data', (data) => {
console.log(`Received chunk ${data}`);
});
The subprocess.stdout
property can be null
or undefined
if the child process could not be successfully spawned.
subprocess.unref()
#
By default, the parent will wait for the detached child to exit. To prevent the
parent from waiting for a given subprocess
to exit, use the
subprocess.unref()
method. Doing so will cause the parent's event loop to not
include the child in its reference count, allowing the parent to exit
independently of the child, unless there is an established IPC channel between
the child and the parent.
const { spawn } = require('node:child_process');
const subprocess = spawn(process.argv[0], ['child_program.js'], {
detached: true,
stdio: 'ignore',
});
subprocess.unref();
maxBuffer
and Unicode#
The maxBuffer
option specifies the largest number of bytes allowed on stdout
or stderr
. If this value is exceeded, then the child process is terminated.
This impacts output that includes multibyte character encodings such as UTF-8 or
UTF-16. For instance, console.log('中文测试')
will send 13 UTF-8 encoded bytes
to stdout
although there are only 4 characters.
Shell requirements#
The shell should understand the -c
switch. If the shell is 'cmd.exe'
, it
should understand the /d /s /c
switches and command-line parsing should be
compatible.
Default Windows shell#
Although Microsoft specifies %COMSPEC%
must contain the path to
'cmd.exe'
in the root environment, child processes are not always subject to
the same requirement. Thus, in child_process
functions where a shell can be
spawned, 'cmd.exe'
is used as a fallback if process.env.ComSpec
is
unavailable.
Advanced serialization#
Child processes support a serialization mechanism for IPC that is based on the
serialization API of the node:v8
module, based on the
HTML structured clone algorithm. This is generally more powerful and
supports more built-in JavaScript object types, such as BigInt
, Map
and Set
, ArrayBuffer
and TypedArray
, Buffer
, Error
, RegExp
etc.
However, this format is not a full superset of JSON, and e.g. properties set on
objects of such built-in types will not be passed on through the serialization
step. Additionally, performance may not be equivalent to that of JSON, depending
on the structure of the passed data.
Therefore, this feature requires opting in by setting the
serialization
option to 'advanced'
when calling child_process.spawn()
or child_process.fork()
.
Cluster#
Source Code: lib/cluster.js
Clusters of Node.js processes can be used to run multiple instances of Node.js
that can distribute workloads among their application threads. When process
isolation is not needed, use the worker_threads
module instead, which
allows running multiple application threads within a single Node.js instance.
The cluster module allows easy creation of child processes that all share server ports.
import cluster from 'node:cluster';
import http from 'node:http';
import { availableParallelism } from 'node:os';
import process from 'node:process';
const numCPUs = availableParallelism();
if (cluster.isPrimary) {
console.log(`Primary ${process.pid} is running`);
// Fork workers.
for (let i = 0; i < numCPUs; i++) {
cluster.fork();
}
cluster.on('exit', (worker, code, signal) => {
console.log(`worker ${worker.process.pid} died`);
});
} else {
// Workers can share any TCP connection
// In this case it is an HTTP server
http.createServer((req, res) => {
res.writeHead(200);
res.end('hello world\n');
}).listen(8000);
console.log(`Worker ${process.pid} started`);
}
const cluster = require('node:cluster');
const http = require('node:http');
const numCPUs = require('node:os').availableParallelism();
const process = require('node:process');
if (cluster.isPrimary) {
console.log(`Primary ${process.pid} is running`);
// Fork workers.
for (let i = 0; i < numCPUs; i++) {
cluster.fork();
}
cluster.on('exit', (worker, code, signal) => {
console.log(`worker ${worker.process.pid} died`);
});
} else {
// Workers can share any TCP connection
// In this case it is an HTTP server
http.createServer((req, res) => {
res.writeHead(200);
res.end('hello world\n');
}).listen(8000);
console.log(`Worker ${process.pid} started`);
}
Running Node.js will now share port 8000 between the workers:
$ node server.js
Primary 3596 is running
Worker 4324 started
Worker 4520 started
Worker 6056 started
Worker 5644 started
On Windows, it is not yet possible to set up a named pipe server in a worker.
How it works#
The worker processes are spawned using the child_process.fork()
method,
so that they can communicate with the parent via IPC and pass server
handles back and forth.
The cluster module supports two methods of distributing incoming connections.
The first one (and the default one on all platforms except Windows) is the round-robin approach, where the primary process listens on a port, accepts new connections and distributes them across the workers in a round-robin fashion, with some built-in smarts to avoid overloading a worker process.
The second approach is where the primary process creates the listen socket and sends it to interested workers. The workers then accept incoming connections directly.
The second approach should, in theory, give the best performance. In practice however, distribution tends to be very unbalanced due to operating system scheduler vagaries. Loads have been observed where over 70% of all connections ended up in just two processes, out of a total of eight.
Because server.listen()
hands off most of the work to the primary
process, there are three cases where the behavior between a normal
Node.js process and a cluster worker differs:
server.listen({fd: 7})
Because the message is passed to the primary, file descriptor 7 in the parent will be listened on, and the handle passed to the worker, rather than listening to the worker's idea of what the number 7 file descriptor references.server.listen(handle)
Listening on handles explicitly will cause the worker to use the supplied handle, rather than talk to the primary process.server.listen(0)
Normally, this will cause servers to listen on a random port. However, in a cluster, each worker will receive the same "random" port each time they dolisten(0)
. In essence, the port is random the first time, but predictable thereafter. To listen on a unique port, generate a port number based on the cluster worker ID.
Node.js does not provide routing logic. It is therefore important to design an application such that it does not rely too heavily on in-memory data objects for things like sessions and login.
Because workers are all separate processes, they can be killed or re-spawned depending on a program's needs, without affecting other workers. As long as there are some workers still alive, the server will continue to accept connections. If no workers are alive, existing connections will be dropped and new connections will be refused. Node.js does not automatically manage the number of workers, however. It is the application's responsibility to manage the worker pool based on its own needs.
Although a primary use case for the node:cluster
module is networking, it can
also be used for other use cases requiring worker processes.
Class: Worker
#
- Extends: <EventEmitter>
A Worker
object contains all public information and method about a worker.
In the primary it can be obtained using cluster.workers
. In a worker
it can be obtained using cluster.worker
.
Event: 'disconnect'
#
Similar to the cluster.on('disconnect')
event, but specific to this worker.
cluster.fork().on('disconnect', () => {
// Worker has disconnected
});
Event: 'error'
#
This event is the same as the one provided by child_process.fork()
.
Within a worker, process.on('error')
may also be used.
Event: 'exit'
#
code
<number> The exit code, if it exited normally.signal
<string> The name of the signal (e.g.'SIGHUP'
) that caused the process to be killed.
Similar to the cluster.on('exit')
event, but specific to this worker.
import cluster from 'node:cluster';
if (cluster.isPrimary) {
const worker = cluster.fork();
worker.on('exit', (code, signal) => {
if (signal) {
console.log(`worker was killed by signal: ${signal}`);
} else if (code !== 0) {
console.log(`worker exited with error code: ${code}`);
} else {
console.log('worker success!');
}
});
}
const cluster = require('node:cluster');
if (cluster.isPrimary) {
const worker = cluster.fork();
worker.on('exit', (code, signal) => {
if (signal) {
console.log(`worker was killed by signal: ${signal}`);
} else if (code !== 0) {
console.log(`worker exited with error code: ${code}`);
} else {
console.log('worker success!');
}
});
}
Event: 'listening'
#
address
<Object>
Similar to the cluster.on('listening')
event, but specific to this worker.
cluster.fork().on('listening', (address) => {
// Worker is listening
});
cluster.fork().on('listening', (address) => {
// Worker is listening
});
It is not emitted in the worker.
Event: 'message'
#
message
<Object>handle
<undefined> | <Object>
Similar to the 'message'
event of cluster
, but specific to this worker.
Within a worker, process.on('message')
may also be used.
Here is an example using the message system. It keeps a count in the primary process of the number of HTTP requests received by the workers:
import cluster from 'node:cluster';
import http from 'node:http';
import { availableParallelism } from 'node:os';
import process from 'node:process';
if (cluster.isPrimary) {
// Keep track of http requests
let numReqs = 0;
setInterval(() => {
console.log(`numReqs = ${numReqs}`);
}, 1000);
// Count requests
function messageHandler(msg) {
if (msg.cmd && msg.cmd === 'notifyRequest') {
numReqs += 1;
}
}
// Start workers and listen for messages containing notifyRequest
const numCPUs = availableParallelism();
for (let i = 0; i < numCPUs; i++) {
cluster.fork();
}
for (const id in cluster.workers) {
cluster.workers[id].on('message', messageHandler);
}
} else {
// Worker processes have a http server.
http.Server((req, res) => {
res.writeHead(200);
res.end('hello world\n');
// Notify primary about the request
process.send({ cmd: 'notifyRequest' });
}).listen(8000);
}
const cluster = require('node:cluster');
const http = require('node:http');
const process = require('node:process');
if (cluster.isPrimary) {
// Keep track of http requests
let numReqs = 0;
setInterval(() => {
console.log(`numReqs = ${numReqs}`);
}, 1000);
// Count requests
function messageHandler(msg) {
if (msg.cmd && msg.cmd === 'notifyRequest') {
numReqs += 1;
}
}
// Start workers and listen for messages containing notifyRequest
const numCPUs = require('node:os').availableParallelism();
for (let i = 0; i < numCPUs; i++) {
cluster.fork();
}
for (const id in cluster.workers) {
cluster.workers[id].on('message', messageHandler);
}
} else {
// Worker processes have a http server.
http.Server((req, res) => {
res.writeHead(200);
res.end('hello world\n');
// Notify primary about the request
process.send({ cmd: 'notifyRequest' });
}).listen(8000);
}
Event: 'online'
#
Similar to the cluster.on('online')
event, but specific to this worker.
cluster.fork().on('online', () => {
// Worker is online
});
It is not emitted in the worker.
worker.disconnect()
#
- Returns: <cluster.Worker> A reference to
worker
.
In a worker, this function will close all servers, wait for the 'close'
event
on those servers, and then disconnect the IPC channel.
In the primary, an internal message is sent to the worker causing it to call
.disconnect()
on itself.
Causes .exitedAfterDisconnect
to be set.
After a server is closed, it will no longer accept new connections,
but connections may be accepted by any other listening worker. Existing
connections will be allowed to close as usual. When no more connections exist,
see server.close()
, the IPC channel to the worker will close allowing it
to die gracefully.
The above applies only to server connections, client connections are not automatically closed by workers, and disconnect does not wait for them to close before exiting.
In a worker, process.disconnect
exists, but it is not this function;
it is disconnect()
.
Because long living server connections may block workers from disconnecting, it
may be useful to send a message, so application specific actions may be taken to
close them. It also may be useful to implement a timeout, killing a worker if
the 'disconnect'
event has not been emitted after some time.
if (cluster.isPrimary) {
const worker = cluster.fork();
let timeout;
worker.on('listening', (address) => {
worker.send('shutdown');
worker.disconnect();
timeout = setTimeout(() => {
worker.kill();
}, 2000);
});
worker.on('disconnect', () => {
clearTimeout(timeout);
});
} else if (cluster.isWorker) {
const net = require('node:net');
const server = net.createServer((socket) => {
// Connections never end
});
server.listen(8000);
process.on('message', (msg) => {
if (msg === 'shutdown') {
// Initiate graceful close of any connections to server
}
});
}
worker.exitedAfterDisconnect
#
This property is true
if the worker exited due to .disconnect()
.
If the worker exited any other way, it is false
. If the
worker has not exited, it is undefined
.
The boolean worker.exitedAfterDisconnect
allows distinguishing between
voluntary and accidental exit, the primary may choose not to respawn a worker
based on this value.
cluster.on('exit', (worker, code, signal) => {
if (worker.exitedAfterDisconnect === true) {
console.log('Oh, it was just voluntary – no need to worry');
}
});
// kill worker
worker.kill();
worker.id
#
Each new worker is given its own unique id, this id is stored in the
id
.
While a worker is alive, this is the key that indexes it in
cluster.workers
.
worker.isConnected()
#
This function returns true
if the worker is connected to its primary via its
IPC channel, false
otherwise. A worker is connected to its primary after it
has been created. It is disconnected after the 'disconnect'
event is emitted.
worker.isDead()
#
This function returns true
if the worker's process has terminated (either
because of exiting or being signaled). Otherwise, it returns false
.
import cluster from 'node:cluster';
import http from 'node:http';
import { availableParallelism } from 'node:os';
import process from 'node:process';
const numCPUs = availableParallelism();
if (cluster.isPrimary) {
console.log(`Primary ${process.pid} is running`);
// Fork workers.
for (let i = 0; i < numCPUs; i++) {
cluster.fork();
}
cluster.on('fork', (worker) => {
console.log('worker is dead:', worker.isDead());
});
cluster.on('exit', (worker, code, signal) => {
console.log('worker is dead:', worker.isDead());
});
} else {
// Workers can share any TCP connection. In this case, it is an HTTP server.
http.createServer((req, res) => {
res.writeHead(200);
res.end(`Current process\n ${process.pid}`);
process.kill(process.pid);
}).listen(8000);
}
const cluster = require('node:cluster');
const http = require('node:http');
const numCPUs = require('node:os').availableParallelism();
const process = require('node:process');
if (cluster.isPrimary) {
console.log(`Primary ${process.pid} is running`);
// Fork workers.
for (let i = 0; i < numCPUs; i++) {
cluster.fork();
}
cluster.on('fork', (worker) => {
console.log('worker is dead:', worker.isDead());
});
cluster.on('exit', (worker, code, signal) => {
console.log('worker is dead:', worker.isDead());
});
} else {
// Workers can share any TCP connection. In this case, it is an HTTP server.
http.createServer((req, res) => {
res.writeHead(200);
res.end(`Current process\n ${process.pid}`);
process.kill(process.pid);
}).listen(8000);
}
worker.kill([signal])
#
signal
<string> Name of the kill signal to send to the worker process. Default:'SIGTERM'
This function will kill the worker. In the primary worker, it does this by
disconnecting the worker.process
, and once disconnected, killing with
signal
. In the worker, it does it by killing the process with signal
.
The kill()
function kills the worker process without waiting for a graceful
disconnect, it has the same behavior as worker.process.kill()
.
This method is aliased as worker.destroy()
for backwards compatibility.
In a worker, process.kill()
exists, but it is not this function;
it is kill()
.
worker.process
#
All workers are created using child_process.fork()
, the returned object
from this function is stored as .process
. In a worker, the global process
is stored.
See: Child Process module.
Workers will call process.exit(0)
if the 'disconnect'
event occurs
on process
and .exitedAfterDisconnect
is not true
. This protects against
accidental disconnection.
worker.send(message[, sendHandle[, options]][, callback])
#
message
<Object>sendHandle
<Handle>options
<Object> Theoptions
argument, if present, is an object used to parameterize the sending of certain types of handles.options
supports the following properties:keepOpen
<boolean> A value that can be used when passing instances ofnet.Socket
. Whentrue
, the socket is kept open in the sending process. Default:false
.
callback
<Function>- Returns: <boolean>
Send a message to a worker or primary, optionally with a handle.
In the primary, this sends a message to a specific worker. It is identical to
ChildProcess.send()
.
In a worker, this sends a message to the primary. It is identical to
process.send()
.
This example will echo back all messages from the primary:
if (cluster.isPrimary) {
const worker = cluster.fork();
worker.send('hi there');
} else if (cluster.isWorker) {
process.on('message', (msg) => {
process.send(msg);
});
}
Event: 'disconnect'
#
worker
<cluster.Worker>
Emitted after the worker IPC channel has disconnected. This can occur when a
worker exits gracefully, is killed, or is disconnected manually (such as with
worker.disconnect()
).
There may be a delay between the 'disconnect'
and 'exit'
events. These
events can be used to detect if the process is stuck in a cleanup or if there
are long-living connections.
cluster.on('disconnect', (worker) => {
console.log(`The worker #${worker.id} has disconnected`);
});
Event: 'exit'
#
worker
<cluster.Worker>code
<number> The exit code, if it exited normally.signal
<string> The name of the signal (e.g.'SIGHUP'
) that caused the process to be killed.
When any of the workers die the cluster module will emit the 'exit'
event.
This can be used to restart the worker by calling .fork()
again.
cluster.on('exit', (worker, code, signal) => {
console.log('worker %d died (%s). restarting...',
worker.process.pid, signal || code);
cluster.fork();
});
Event: 'fork'
#
worker
<cluster.Worker>
When a new worker is forked the cluster module will emit a 'fork'
event.
This can be used to log worker activity, and create a custom timeout.
const timeouts = [];
function errorMsg() {
console.error('Something must be wrong with the connection ...');
}
cluster.on('fork', (worker) => {
timeouts[worker.id] = setTimeout(errorMsg, 2000);
});
cluster.on('listening', (worker, address) => {
clearTimeout(timeouts[worker.id]);
});
cluster.on('exit', (worker, code, signal) => {
clearTimeout(timeouts[worker.id]);
errorMsg();
});
Event: 'listening'
#
worker
<cluster.Worker>address
<Object>
After calling listen()
from a worker, when the 'listening'
event is emitted
on the server, a 'listening'
event will also be emitted on cluster
in the
primary.
The event handler is executed with two arguments, the worker
contains the
worker object and the address
object contains the following connection
properties: address
, port
, and addressType
. This is very useful if the
worker is listening on more than one address.
cluster.on('listening', (worker, address) => {
console.log(
`A worker is now connected to ${address.address}:${address.port}`);
});
The addressType
is one of:
4
(TCPv4)6
(TCPv6)-1
(Unix domain socket)'udp4'
or'udp6'
(UDPv4 or UDPv6)
Event: 'message'
#
worker
<cluster.Worker>message
<Object>handle
<undefined> | <Object>
Emitted when the cluster primary receives a message from any worker.
Event: 'online'
#
worker
<cluster.Worker>
After forking a new worker, the worker should respond with an online message.
When the primary receives an online message it will emit this event.
The difference between 'fork'
and 'online'
is that fork is emitted when the
primary forks a worker, and 'online'
is emitted when the worker is running.
cluster.on('online', (worker) => {
console.log('Yay, the worker responded after it was forked');
});
Event: 'setup'
#
settings
<Object>
Emitted every time .setupPrimary()
is called.
The settings
object is the cluster.settings
object at the time
.setupPrimary()
was called and is advisory only, since multiple calls to
.setupPrimary()
can be made in a single tick.
If accuracy is important, use cluster.settings
.
cluster.disconnect([callback])
#
callback
<Function> Called when all workers are disconnected and handles are closed.
Calls .disconnect()
on each worker in cluster.workers
.
When they are disconnected all internal handles will be closed, allowing the primary process to die gracefully if no other event is waiting.
The method takes an optional callback argument which will be called when finished.
This can only be called from the primary process.
cluster.fork([env])
#
env
<Object> Key/value pairs to add to worker process environment.- Returns: <cluster.Worker>
Spawn a new worker process.
This can only be called from the primary process.
cluster.isMaster
#
Deprecated alias for cluster.isPrimary
.
cluster.isPrimary
#
True if the process is a primary. This is determined
by the process.env.NODE_UNIQUE_ID
. If process.env.NODE_UNIQUE_ID
is
undefined, then isPrimary
is true
.
cluster.isWorker
#
True if the process is not a primary (it is the negation of cluster.isPrimary
).
cluster.schedulingPolicy
#
The scheduling policy, either cluster.SCHED_RR
for round-robin or
cluster.SCHED_NONE
to leave it to the operating system. This is a
global setting and effectively frozen once either the first worker is spawned,
or .setupPrimary()
is called, whichever comes first.
SCHED_RR
is the default on all operating systems except Windows.
Windows will change to SCHED_RR
once libuv is able to effectively
distribute IOCP handles without incurring a large performance hit.
cluster.schedulingPolicy
can also be set through the
NODE_CLUSTER_SCHED_POLICY
environment variable. Valid
values are 'rr'
and 'none'
.
cluster.settings
#
- <Object>
execArgv
<string[]> List of string arguments passed to the Node.js executable. Default:process.execArgv
.exec
<string> File path to worker file. Default:process.argv[1]
.args
<string[]> String arguments passed to worker. Default:process.argv.slice(2)
.cwd
<string> Current working directory of the worker process. Default:undefined
(inherits from parent process).serialization
<string> Specify the kind of serialization used for sending messages between processes. Possible values are'json'
and'advanced'
. See Advanced serialization forchild_process
for more details. Default:false
.silent
<boolean> Whether or not to send output to parent's stdio. Default:false
.stdio
<Array> Configures the stdio of forked processes. Because the cluster module relies on IPC to function, this configuration must contain an'ipc'
entry. When this option is provided, it overridessilent
.uid
<number> Sets the user identity of the process. (Seesetuid(2)
.)gid
<number> Sets the group identity of the process. (Seesetgid(2)
.)inspectPort
<number> | <Function> Sets inspector port of worker. This can be a number, or a function that takes no arguments and returns a number. By default each worker gets its own port, incremented from the primary'sprocess.debugPort
.windowsHide
<boolean> Hide the forked processes console window that would normally be created on Windows systems. Default:false
.
After calling .setupPrimary()
(or .fork()
) this settings object will
contain the settings, including the default values.
This object is not intended to be changed or set manually.
cluster.setupMaster([settings])
#
Deprecated alias for .setupPrimary()
.
cluster.setupPrimary([settings])
#
settings
<Object> Seecluster.settings
.
setupPrimary
is used to change the default 'fork' behavior. Once called,
the settings will be present in cluster.settings
.
Any settings changes only affect future calls to .fork()
and have no
effect on workers that are already running.
The only attribute of a worker that cannot be set via .setupPrimary()
is
the env
passed to .fork()
.
The defaults above apply to the first call only; the defaults for later
calls are the current values at the time of cluster.setupPrimary()
is called.
import cluster from 'node:cluster';
cluster.setupPrimary({
exec: 'worker.js',
args: ['--use', 'https'],
silent: true,
});
cluster.fork(); // https worker
cluster.setupPrimary({
exec: 'worker.js',
args: ['--use', 'http'],
});
cluster.fork(); // http worker
const cluster = require('node:cluster');
cluster.setupPrimary({
exec: 'worker.js',
args: ['--use', 'https'],
silent: true,
});
cluster.fork(); // https worker
cluster.setupPrimary({
exec: 'worker.js',
args: ['--use', 'http'],
});
cluster.fork(); // http worker
This can only be called from the primary process.
cluster.worker
#
A reference to the current worker object. Not available in the primary process.
import cluster from 'node:cluster';
if (cluster.isPrimary) {
console.log('I am primary');
cluster.fork();
cluster.fork();
} else if (cluster.isWorker) {
console.log(`I am worker #${cluster.worker.id}`);
}
const cluster = require('node:cluster');
if (cluster.isPrimary) {
console.log('I am primary');
cluster.fork();
cluster.fork();
} else if (cluster.isWorker) {
console.log(`I am worker #${cluster.worker.id}`);
}
cluster.workers
#
A hash that stores the active worker objects, keyed by id
field. This makes it
easy to loop through all the workers. It is only available in the primary
process.
A worker is removed from cluster.workers
after the worker has disconnected
and exited. The order between these two events cannot be determined in
advance. However, it is guaranteed that the removal from the cluster.workers
list happens before the last 'disconnect'
or 'exit'
event is emitted.
import cluster from 'node:cluster';
for (const worker of Object.values(cluster.workers)) {
worker.send('big announcement to all workers');
}
const cluster = require('node:cluster');
for (const worker of Object.values(cluster.workers)) {
worker.send('big announcement to all workers');
}
Command-line API#
Node.js comes with a variety of CLI options. These options expose built-in debugging, multiple ways to execute scripts, and other helpful runtime options.
To view this documentation as a manual page in a terminal, run man node
.
Synopsis#
node [options] [V8 options] [<program-entry-point> | -e "script" | -] [--] [arguments]
node inspect [<program-entry-point> | -e "script" | <host>:<port>] …
node --v8-options
Execute without arguments to start the REPL.
For more info about node inspect
, see the debugger documentation.
Program entry point#
The program entry point is a specifier-like string. If the string is not an absolute path, it's resolved as a relative path from the current working directory. That path is then resolved by CommonJS module loader. If no corresponding file is found, an error is thrown.
If a file is found, its path will be passed to the ECMAScript module loader under any of the following conditions:
- The program was started with a command-line flag that forces the entry point to be loaded with ECMAScript module loader.
- The file has an
.mjs
extension. - The file does not have a
.cjs
extension, and the nearest parentpackage.json
file contains a top-level"type"
field with a value of"module"
.
Otherwise, the file is loaded using the CommonJS module loader. See Modules loaders for more details.
ECMAScript modules loader entry point caveat#
When loading ECMAScript module loader loads the program entry point, the node
command will only accept as input only files with .js
, .mjs
, or .cjs
extensions; and with .wasm
extensions when
--experimental-wasm-modules
is enabled.
Options#
All options, including V8 options, allow words to be separated by both
dashes (-
) or underscores (_
). For example, --pending-deprecation
is
equivalent to --pending_deprecation
.
If an option that takes a single value (such as --max-http-header-size
) is
passed more than once, then the last passed value is used. Options from the
command line take precedence over options passed through the NODE_OPTIONS
environment variable.
-
#
Alias for stdin. Analogous to the use of -
in other command-line utilities,
meaning that the script is read from stdin, and the rest of the options
are passed to that script.
--
#
Indicate the end of node options. Pass the rest of the arguments to the script. If no script filename or eval/print script is supplied prior to this, then the next argument is used as a script filename.
--abort-on-uncaught-exception
#
Aborting instead of exiting causes a core file to be generated for post-mortem
analysis using a debugger (such as lldb
, gdb
, and mdb
).
If this flag is passed, the behavior can still be set to not abort through
process.setUncaughtExceptionCaptureCallback()
(and through usage of the
node:domain
module that uses it).
--allow-child-process
#
When using the Permission Model, the process will not be able to spawn any
child process by default.
Attempts to do so will throw an ERR_ACCESS_DENIED
unless the
user explicitly passes the --allow-child-process
flag when starting Node.js.
Example:
const childProcess = require('node:child_process');
// Attempt to bypass the permission
childProcess.spawn('node', ['-e', 'require("fs").writeFileSync("/new-file", "example")']);
$ node --experimental-permission --allow-fs-read=* index.js
node:internal/child_process:388
const err = this._handle.spawn(options);
^
Error: Access to this API has been restricted
at ChildProcess.spawn (node:internal/child_process:388:28)
at Object.spawn (node:child_process:723:9)
at Object.<anonymous> (/home/index.js:3:14)
at Module._compile (node:internal/modules/cjs/loader:1120:14)
at Module._extensions..js (node:internal/modules/cjs/loader:1174:10)
at Module.load (node:internal/modules/cjs/loader:998:32)
at Module._load (node:internal/modules/cjs/loader:839:12)
at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:81:12)
at node:internal/main/run_main_module:17:47 {
code: 'ERR_ACCESS_DENIED',
permission: 'ChildProcess'
}
--allow-fs-read
#
This flag configures file system read permissions using the Permission Model.
The valid arguments for the --allow-fs-read
flag are:
*
- To allow theFileSystemRead
operations.- Paths delimited by comma (,) to manage
FileSystemRead
(reading) operations.
Examples can be found in the File System Permissions documentation.
Relative paths are NOT yet supported by the CLI flag.
The initializer module also needs to be allowed. Consider the following example:
$ node --experimental-permission t.js
node:internal/modules/cjs/loader:162
const result = internalModuleStat(filename);
^
Error: Access to this API has been restricted
at stat (node:internal/modules/cjs/loader:162:18)
at Module._findPath (node:internal/modules/cjs/loader:640:16)
at resolveMainPath (node:internal/modules/run_main:15:25)
at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:53:24)
at node:internal/main/run_main_module:23:47 {
code: 'ERR_ACCESS_DENIED',
permission: 'FileSystemRead',
resource: '/Users/rafaelgss/repos/os/node/t.js'
}
The process needs to have access to the index.js
module:
$ node --experimental-permission --allow-fs-read=/path/to/index.js index.js
--allow-fs-write
#
This flag configures file system write permissions using the Permission Model.
The valid arguments for the --allow-fs-write
flag are:
*
- To allow theFileSystemWrite
operations.- Paths delimited by comma (,) to manage
FileSystemWrite
(writing) operations.
Examples can be found in the File System Permissions documentation.
Relative paths are NOT supported through the CLI flag.
--allow-worker
#
When using the Permission Model, the process will not be able to create any
worker threads by default.
For security reasons, the call will throw an ERR_ACCESS_DENIED
unless the
user explicitly pass the flag --allow-worker
in the main Node.js process.
Example:
const { Worker } = require('node:worker_threads');
// Attempt to bypass the permission
new Worker(__filename);
$ node --experimental-permission --allow-fs-read=* index.js
node:internal/worker:188
this[kHandle] = new WorkerImpl(url,
^
Error: Access to this API has been restricted
at new Worker (node:internal/worker:188:21)
at Object.<anonymous> (/home/index.js.js:3:1)
at Module._compile (node:internal/modules/cjs/loader:1120:14)
at Module._extensions..js (node:internal/modules/cjs/loader:1174:10)
at Module.load (node:internal/modules/cjs/loader:998:32)
at Module._load (node:internal/modules/cjs/loader:839:12)
at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:81:12)
at node:internal/main/run_main_module:17:47 {
code: 'ERR_ACCESS_DENIED',
permission: 'WorkerThreads'
}
--build-snapshot
#
Generates a snapshot blob when the process exits and writes it to
disk, which can be loaded later with --snapshot-blob
.
When building the snapshot, if --snapshot-blob
is not specified,
the generated blob will be written, by default, to snapshot.blob
in the current working directory. Otherwise it will be written to
the path specified by --snapshot-blob
.
$ echo "globalThis.foo = 'I am from the snapshot'" > snapshot.js
# Run snapshot.js to intialize the application and snapshot the
# state of it into snapshot.blob.
$ node --snapshot-blob snapshot.blob --build-snapshot snapshot.js
$ echo "console.log(globalThis.foo)" > index.js
# Load the generated snapshot and start the application from index.js.
$ node --snapshot-blob snapshot.blob index.js
I am from the snapshot
The v8.startupSnapshot
API can be used to specify an entry point at
snapshot building time, thus avoiding the need of an additional entry
script at deserialization time:
$ echo "require('v8').startupSnapshot.setDeserializeMainFunction(() => console.log('I am from the snapshot'))" > snapshot.js
$ node --snapshot-blob snapshot.blob --build-snapshot snapshot.js
$ node --snapshot-blob snapshot.blob
I am from the snapshot
For more information, check out the v8.startupSnapshot
API documentation.
Currently the support for run-time snapshot is experimental in that:
- User-land modules are not yet supported in the snapshot, so only one single file can be snapshotted. Users can bundle their applications into a single script with their bundler of choice before building a snapshot, however.
- Only a subset of the built-in modules work in the snapshot, though the Node.js core test suite checks that a few fairly complex applications can be snapshotted. Support for more modules are being added. If any crashes or buggy behaviors occur when building a snapshot, please file a report in the Node.js issue tracker and link to it in the tracking issue for user-land snapshots.
--completion-bash
#
Print source-able bash completion script for Node.js.
$ node --completion-bash > node_bash_completion
$ source node_bash_completion
-C condition
, --conditions=condition
#
Enable experimental support for custom conditional exports resolution conditions.
Any number of custom string condition names are permitted.
The default Node.js conditions of "node"
, "default"
, "import"
, and
"require"
will always apply as defined.
For example, to run a module with "development" resolutions:
$ node -C development app.js
--cpu-prof
#
Starts the V8 CPU profiler on start up, and writes the CPU profile to disk before exit.
If --cpu-prof-dir
is not specified, the generated profile is placed
in the current working directory.
If --cpu-prof-name
is not specified, the generated profile is
named CPU.${yyyymmdd}.${hhmmss}.${pid}.${tid}.${seq}.cpuprofile
.
$ node --cpu-prof index.js
$ ls *.cpuprofile
CPU.20190409.202950.15293.0.0.cpuprofile
--cpu-prof-dir
#
Specify the directory where the CPU profiles generated by --cpu-prof
will
be placed.
The default value is controlled by the
--diagnostic-dir
command-line option.
--cpu-prof-interval
#
Specify the sampling interval in microseconds for the CPU profiles generated
by --cpu-prof
. The default is 1000 microseconds.
--cpu-prof-name
#
Specify the file name of the CPU profile generated by --cpu-prof
.
--diagnostic-dir=directory
#
Set the directory to which all diagnostic output files are written. Defaults to current working directory.
Affects the default output directory of:
--disable-proto=mode
#
Disable the Object.prototype.__proto__
property. If mode
is delete
, the
property is removed entirely. If mode
is throw
, accesses to the
property throw an exception with the code ERR_PROTO_ACCESS
.
--disallow-code-generation-from-strings
#
Make built-in language features like eval
and new Function
that generate
code from strings throw an exception instead. This does not affect the Node.js
node:vm
module.
--dns-result-order=order
#
Set the default value of verbatim
in dns.lookup()
and
dnsPromises.lookup()
. The value could be:
ipv4first
: sets defaultverbatim
false
.verbatim
: sets defaultverbatim
true
.
The default is verbatim
and dns.setDefaultResultOrder()
have higher
priority than --dns-result-order
.
--enable-fips
#
Enable FIPS-compliant crypto at startup. (Requires Node.js to be built against FIPS-compatible OpenSSL.)
--enable-network-family-autoselection
#
Enables the family autoselection algorithm unless connection options explicitly disables it.
--enable-source-maps
#
Enable Source Map v3 support for stack traces.
When using a transpiler, such as TypeScript, stack traces thrown by an
application reference the transpiled code, not the original source position.
--enable-source-maps
enables caching of Source Maps and makes a best
effort to report stack traces relative to the original source file.
Overriding Error.prepareStackTrace
prevents --enable-source-maps
from
modifying the stack trace.
Note, enabling source maps can introduce latency to your application
when Error.stack
is accessed. If you access Error.stack
frequently
in your application, take into account the performance implications
of --enable-source-maps
.
--experimental-import-meta-resolve
#
Enable experimental import.meta.resolve()
support.
--experimental-loader=module
#
Specify the module
of a custom experimental ECMAScript module loader.
module
may be any string accepted as an import
specifier.
--experimental-network-imports
#
Enable experimental support for the https:
protocol in import
specifiers.
--experimental-permission
#
Enable the Permission Model for current process. When enabled, the following permissions are restricted:
- File System - manageable through
[
--allow-fs-read
][],[allow-fs-write
][] flags - Child Process - manageable through [
--allow-child-process
][] flag - Worker Threads - manageable through [
--allow-worker
][] flag
--experimental-policy
#
Use the specified file as a security policy.
--no-experimental-fetch
#
Disable experimental support for the Fetch API.
--no-experimental-global-webcrypto
#
Disable exposition of Web Crypto API on the global scope.
--no-experimental-global-customevent
#
Disable exposition of CustomEvent Web API on the global scope.
--no-experimental-repl-await
#
Use this flag to disable top-level await in REPL.
--experimental-shadow-realm
#
Use this flag to enable ShadowRealm support.
--experimental-test-coverage
#
When used in conjunction with the node:test
module, a code coverage report is
generated as part of the test runner output. If no tests are run, a coverage
report is not generated. See the documentation on
collecting code coverage from tests for more details.
--experimental-vm-modules
#
Enable experimental ES Module support in the node:vm
module.
--experimental-wasi-unstable-preview1
#
Enable experimental WebAssembly System Interface (WASI) support.
--experimental-wasm-modules
#
Enable experimental WebAssembly module support.
--force-context-aware
#
Disable loading native addons that are not context-aware.
--force-fips
#
Force FIPS-compliant crypto on startup. (Cannot be disabled from script code.)
(Same requirements as --enable-fips
.)
--frozen-intrinsics
#
Enable experimental frozen intrinsics like Array
and Object
.
Only the root context is supported. There is no guarantee that
globalThis.Array
is indeed the default intrinsic reference. Code may break
under this flag.
To allow polyfills to be added,
--require
and --import
both run before freezing intrinsics.
--force-node-api-uncaught-exceptions-policy
#
Enforces uncaughtException
event on Node-API asynchronous callbacks.
To prevent from an existing add-on from crashing the process, this flag is not enabled by default. In the future, this flag will be enabled by default to enforce the correct behavior.
--heapsnapshot-near-heap-limit=max_count
#
Writes a V8 heap snapshot to disk when the V8 heap usage is approaching the
heap limit. count
should be a non-negative integer (in which case
Node.js will write no more than max_count
snapshots to disk).
When generating snapshots, garbage collection may be triggered and bring
the heap usage down. Therefore multiple snapshots may be written to disk
before the Node.js instance finally runs out of memory. These heap snapshots
can be compared to determine what objects are being allocated during the
time consecutive snapshots are taken. It's not guaranteed that Node.js will
write exactly max_count
snapshots to disk, but it will try
its best to generate at least one and up to max_count
snapshots before the
Node.js instance runs out of memory when max_count
is greater than 0
.
Generating V8 snapshots takes time and memory (both memory managed by the V8 heap and native memory outside the V8 heap). The bigger the heap is, the more resources it needs. Node.js will adjust the V8 heap to accommodate the additional V8 heap memory overhead, and try its best to avoid using up all the memory available to the process. When the process uses more memory than the system deems appropriate, the process may be terminated abruptly by the system, depending on the system configuration.
$ node --max-old-space-size=100 --heapsnapshot-near-heap-limit=3 index.js
Wrote snapshot to Heap.20200430.100036.49580.0.001.heapsnapshot
Wrote snapshot to Heap.20200430.100037.49580.0.002.heapsnapshot
Wrote snapshot to Heap.20200430.100038.49580.0.003.heapsnapshot
<--- Last few GCs --->
[49580:0x110000000] 4826 ms: Mark-sweep 130.6 (147.8) -> 130.5 (147.8) MB, 27.4 / 0.0 ms (average mu = 0.126, current mu = 0.034) allocation failure scavenge might not succeed
[49580:0x110000000] 4845 ms: Mark-sweep 130.6 (147.8) -> 130.6 (147.8) MB, 18.8 / 0.0 ms (average mu = 0.088, current mu = 0.031) allocation failure scavenge might not succeed
<--- JS stacktrace --->
FATAL ERROR: Ineffective mark-compacts near heap limit Allocation failed - JavaScript heap out of memory
....
--heapsnapshot-signal=signal
#
Enables a signal handler that causes the Node.js process to write a heap dump
when the specified signal is received. signal
must be a valid signal name.
Disabled by default.
$ node --heapsnapshot-signal=SIGUSR2 index.js &
$ ps aux
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
node 1 5.5 6.1 787252 247004 ? Ssl 16:43 0:02 node --heapsnapshot-signal=SIGUSR2 index.js
$ kill -USR2 1
$ ls
Heap.20190718.133405.15554.0.001.heapsnapshot
--heap-prof
#
Starts the V8 heap profiler on start up, and writes the heap profile to disk before exit.
If --heap-prof-dir
is not specified, the generated profile is placed
in the current working directory.
If --heap-prof-name
is not specified, the generated profile is
named Heap.${yyyymmdd}.${hhmmss}.${pid}.${tid}.${seq}.heapprofile
.
$ node --heap-prof index.js
$ ls *.heapprofile
Heap.20190409.202950.15293.0.001.heapprofile
--heap-prof-dir
#
Specify the directory where the heap profiles generated by --heap-prof
will
be placed.
The default value is controlled by the
--diagnostic-dir
command-line option.
--heap-prof-interval
#
Specify the average sampling interval in bytes for the heap profiles generated
by --heap-prof
. The default is 512 * 1024 bytes.
--heap-prof-name
#
Specify the file name of the heap profile generated by --heap-prof
.
--icu-data-dir=file
#
Specify ICU data load path. (Overrides NODE_ICU_DATA
.)
--import=module
#
Preload the specified module at startup.
Follows ECMAScript module resolution rules.
Use --require
to load a CommonJS module.
Modules preloaded with --require
will run before modules preloaded with --import
.
--input-type=type
#
This configures Node.js to interpret string input as CommonJS or as an ES
module. String input is input via --eval
, --print
, or STDIN
.
Valid values are "commonjs"
and "module"
. The default is "commonjs"
.
The REPL does not support this option.
--inspect-brk[=[host:]port]
#
Activate inspector on host:port
and break at start of user script.
Default host:port
is 127.0.0.1:9229
.
--inspect-port=[host:]port
#
Set the host:port
to be used when the inspector is activated.
Useful when activating the inspector by sending the SIGUSR1
signal.
Default host is 127.0.0.1
.
See the security warning below regarding the host
parameter usage.
--inspect[=[host:]port]
#
Activate inspector on host:port
. Default is 127.0.0.1:9229
.
V8 inspector integration allows tools such as Chrome DevTools and IDEs to debug and profile Node.js instances. The tools attach to Node.js instances via a tcp port and communicate using the Chrome DevTools Protocol.
Warning: binding inspector to a public IP:port combination is insecure#
Binding the inspector to a public IP (including 0.0.0.0
) with an open port is
insecure, as it allows external hosts to connect to the inspector and perform
a remote code execution attack.
If specifying a host, make sure that either:
- The host is not accessible from public networks.
- A firewall disallows unwanted connections on the port.
More specifically, --inspect=0.0.0.0
is insecure if the port (9229
by
default) is not firewall-protected.
See the debugging security implications section for more information.
--inspect-publish-uid=stderr,http
#
Specify ways of the inspector web socket url exposure.
By default inspector websocket url is available in stderr and under /json/list
endpoint on http://host:port/json/list
.
--insecure-http-parser
#
Use an insecure HTTP parser that accepts invalid HTTP headers. This may allow interoperability with non-conformant HTTP implementations. It may also allow request smuggling and other HTTP attacks that rely on invalid headers being accepted. Avoid using this option.
--jitless
#
Disable runtime allocation of executable memory. This may be required on some platforms for security reasons. It can also reduce attack surface on other platforms, but the performance impact may be severe.
This flag is inherited from V8 and is subject to change upstream. It may disappear in a non-semver-major release.
--max-http-header-size=size
#
Specify the maximum size, in bytes, of HTTP headers. Defaults to 16 KiB.
--napi-modules
#
This option is a no-op. It is kept for compatibility.
--no-addons
#
Disable the node-addons
exports condition as well as disable loading
native addons. When --no-addons
is specified, calling process.dlopen
or
requiring a native C++ addon will fail and throw an exception.
--no-deprecation
#
Silence deprecation warnings.
--no-extra-info-on-fatal-exception
#
Hide extra information on fatal exception that causes exit.
--no-force-async-hooks-checks
#
Disables runtime checks for async_hooks
. These will still be enabled
dynamically when async_hooks
is enabled.
--no-global-search-paths
#
Do not search modules from global paths like $HOME/.node_modules
and
$NODE_PATH
.
--no-warnings
#
Silence all process warnings (including deprecations).
--node-memory-debug
#
Enable extra debug checks for memory leaks in Node.js internals. This is usually only useful for developers debugging Node.js itself.
--openssl-config=file
#
Load an OpenSSL configuration file on startup. Among other uses, this can be used to enable FIPS-compliant crypto if Node.js is built against FIPS-enabled OpenSSL.
--openssl-shared-config
#
Enable OpenSSL default configuration section, openssl_conf
to be read from
the OpenSSL configuration file. The default configuration file is named
openssl.cnf
but this can be changed using the environment variable
OPENSSL_CONF
, or by using the command line option --openssl-config
.
The location of the default OpenSSL configuration file depends on how OpenSSL
is being linked to Node.js. Sharing the OpenSSL configuration may have unwanted
implications and it is recommended to use a configuration section specific to
Node.js which is nodejs_conf
and is default when this option is not used.
--openssl-legacy-provider
#
Enable OpenSSL 3.0 legacy provider. For more information please see OSSL_PROVIDER-legacy.
--pending-deprecation
#
Emit pending deprecation warnings.
Pending deprecations are generally identical to a runtime deprecation with the
notable exception that they are turned off by default and will not be emitted
unless either the --pending-deprecation
command-line flag, or the
NODE_PENDING_DEPRECATION=1
environment variable, is set. Pending deprecations
are used to provide a kind of selective "early warning" mechanism that
developers may leverage to detect deprecated API usage.
--policy-integrity=sri
#
Instructs Node.js to error prior to running any code if the policy does not have the specified integrity. It expects a Subresource Integrity string as a parameter.
--preserve-symlinks
#
Instructs the module loader to preserve symbolic links when resolving and caching modules.
By default, when Node.js loads a module from a path that is symbolically linked
to a different on-disk location, Node.js will dereference the link and use the
actual on-disk "real path" of the module as both an identifier and as a root
path to locate other dependency modules. In most cases, this default behavior
is acceptable. However, when using symbolically linked peer dependencies, as
illustrated in the example below, the default behavior causes an exception to
be thrown if moduleA
attempts to require moduleB
as a peer dependency:
{appDir}
├── app
│ ├── index.js
│ └── node_modules
│ ├── moduleA -> {appDir}/moduleA
│ └── moduleB
│ ├── index.js
│ └── package.json
└── moduleA
├── index.js
└── package.json
The --preserve-symlinks
command-line flag instructs Node.js to use the
symlink path for modules as opposed to the real path, allowing symbolically
linked peer dependencies to be found.
Note, however, that using --preserve-symlinks
can have other side effects.
Specifically, symbolically linked native modules can fail to load if those
are linked from more than one location in the dependency tree (Node.js would
see those as two separate modules and would attempt to load the module multiple
times, causing an exception to be thrown).
The --preserve-symlinks
flag does not apply to the main module, which allows
node --preserve-symlinks node_module/.bin/<foo>
to work. To apply the same
behavior for the main module, also use --preserve-symlinks-main
.
--preserve-symlinks-main
#
Instructs the module loader to preserve symbolic links when resolving and
caching the main module (require.main
).
This flag exists so that the main module can be opted-in to the same behavior
that --preserve-symlinks
gives to all other imports; they are separate flags,
however, for backward compatibility with older Node.js versions.
--preserve-symlinks-main
does not imply --preserve-symlinks
; use
--preserve-symlinks-main
in addition to
--preserve-symlinks
when it is not desirable to follow symlinks before
resolving relative paths.
See --preserve-symlinks
for more information.
--prof
#
Generate V8 profiler output.
--prof-process
#
Process V8 profiler output generated using the V8 option --prof
.
--redirect-warnings=file
#
Write process warnings to the given file instead of printing to stderr. The file will be created if it does not exist, and will be appended to if it does. If an error occurs while attempting to write the warning to the file, the warning will be written to stderr instead.
The file
name may be an absolute path. If it is not, the default directory it
will be written to is controlled by the
--diagnostic-dir
command-line option.
--report-compact
#
Write reports in a compact format, single-line JSON, more easily consumable by log processing systems than the default multi-line format designed for human consumption.
--report-dir=directory
, report-directory=directory
#
Location at which the report will be generated.
--report-filename=filename
#
Name of the file to which the report will be written.
If the filename is set to 'stdout'
or 'stderr'
, the report is written to
the stdout or stderr of the process respectively.
--report-on-fatalerror
#
Enables the report to be triggered on fatal errors (internal errors within the Node.js runtime such as out of memory) that lead to termination of the application. Useful to inspect various diagnostic data elements such as heap, stack, event loop state, resource consumption etc. to reason about the fatal error.
--report-on-signal
#
Enables report to be generated upon receiving the specified (or predefined)
signal to the running Node.js process. The signal to trigger the report is
specified through --report-signal
.
--report-signal=signal
#
Sets or resets the signal for report generation (not supported on Windows).
Default signal is SIGUSR2
.
--report-uncaught-exception
#
Enables report to be generated when the process exits due to an uncaught exception. Useful when inspecting the JavaScript stack in conjunction with native stack and other runtime environment data.
--secure-heap=n
#
Initializes an OpenSSL secure heap of n
bytes. When initialized, the
secure heap is used for selected types of allocations within OpenSSL
during key generation and other operations. This is useful, for instance,
to prevent sensitive information from leaking due to pointer overruns
or underruns.
The secure heap is a fixed size and cannot be resized at runtime so, if used, it is important to select a large enough heap to cover all application uses.
The heap size given must be a power of two. Any value less than 2 will disable the secure heap.
The secure heap is disabled by default.
The secure heap is not available on Windows.
See CRYPTO_secure_malloc_init
for more details.
--secure-heap-min=n
#
When using --secure-heap
, the --secure-heap-min
flag specifies the
minimum allocation from the secure heap. The minimum value is 2
.
The maximum value is the lesser of --secure-heap
or 2147483647
.
The value given must be a power of two.
--snapshot-blob=path
#
When used with --build-snapshot
, --snapshot-blob
specifies the path
where the generated snapshot blob is written to. If not specified, the
generated blob is written to snapshot.blob
in the current working directory.
When used without --build-snapshot
, --snapshot-blob
specifies the
path to the blob that is used to restore the application state.
When loading a snapshot, Node.js checks that:
- The version, architecture, and platform of the running Node.js binary are exactly the same as that of the binary that generates the snapshot.
- The V8 flags and CPU features are compatible with that of the binary that generates the snapshot.
If they don't match, Node.js refuses to load the snapshot and exits with status code 1.
--test
#
Starts the Node.js command line test runner. This flag cannot be combined with
--watch-path
, --check
, --eval
, --interactive
, or the inspector.
See the documentation on running tests from the command line
for more details.
--test-name-pattern
#
A regular expression that configures the test runner to only execute tests whose name matches the provided pattern. See the documentation on filtering tests by name for more details.
--test-reporter
#
A test reporter to use when running tests. See the documentation on test reporters for more details.
--test-reporter-destination
#
The destination for the corresponding test reporter. See the documentation on test reporters for more details.
--test-only
#
Configures the test runner to only execute top level tests that have the only
option set.
--throw-deprecation
#
Throw errors for deprecations.
--title=title
#
Set process.title
on startup.
--tls-cipher-list=list
#
Specify an alternative default TLS cipher list. Requires Node.js to be built with crypto support (default).
--tls-keylog=file
#
Log TLS key material to a file. The key material is in NSS SSLKEYLOGFILE
format and can be used by software (such as Wireshark) to decrypt the TLS
traffic.
--tls-max-v1.2
#
Set tls.DEFAULT_MAX_VERSION
to 'TLSv1.2'. Use to disable support for
TLSv1.3.
--tls-max-v1.3
#
Set default tls.DEFAULT_MAX_VERSION
to 'TLSv1.3'. Use to enable support
for TLSv1.3.
--tls-min-v1.0
#
Set default tls.DEFAULT_MIN_VERSION
to 'TLSv1'. Use for compatibility with
old TLS clients or servers.
--tls-min-v1.1
#
Set default tls.DEFAULT_MIN_VERSION
to 'TLSv1.1'. Use for compatibility
with old TLS clients or servers.
--tls-min-v1.2
#
Set default tls.DEFAULT_MIN_VERSION
to 'TLSv1.2'. This is the default for
12.x and later, but the option is supported for compatibility with older Node.js
versions.
--tls-min-v1.3
#
Set default tls.DEFAULT_MIN_VERSION
to 'TLSv1.3'. Use to disable support
for TLSv1.2, which is not as secure as TLSv1.3.
--trace-atomics-wait
#
Print short summaries of calls to Atomics.wait()
to stderr.
The output could look like this:
(node:15701) [Thread 0] Atomics.wait(<address> + 0, 1, inf) started
(node:15701) [Thread 0] Atomics.wait(<address> + 0, 1, inf) did not wait because the values mismatched
(node:15701) [Thread 0] Atomics.wait(<address> + 0, 0, 10) started
(node:15701) [Thread 0] Atomics.wait(<address> + 0, 0, 10) timed out
(node:15701) [Thread 0] Atomics.wait(<address> + 4, 0, inf) started
(node:15701) [Thread 1] Atomics.wait(<address> + 4, -1, inf) started
(node:15701) [Thread 0] Atomics.wait(<address> + 4, 0, inf) was woken up by another thread
(node:15701) [Thread 1] Atomics.wait(<address> + 4, -1, inf) was woken up by another thread
The fields here correspond to:
- The thread id as given by
worker_threads.threadId
- The base address of the
SharedArrayBuffer
in question, as well as the byte offset corresponding to the index passed toAtomics.wait()
- The expected value that was passed to
Atomics.wait()
- The timeout passed to
Atomics.wait
--trace-deprecation
#
Print stack traces for deprecations.
--trace-event-categories
#
A comma separated list of categories that should be traced when trace event
tracing is enabled using --trace-events-enabled
.
--trace-event-file-pattern
#
Template string specifying the filepath for the trace event data, it
supports ${rotation}
and ${pid}
.
--trace-events-enabled
#
Enables the collection of trace event tracing information.
--trace-exit
#
Prints a stack trace whenever an environment is exited proactively,
i.e. invoking process.exit()
.
--trace-sigint
#
Prints a stack trace on SIGINT.
--trace-sync-io
#
Prints a stack trace whenever synchronous I/O is detected after the first turn of the event loop.
--trace-tls
#
Prints TLS packet trace information to stderr
. This can be used to debug TLS
connection problems.
--trace-uncaught
#
Print stack traces for uncaught exceptions; usually, the stack trace associated
with the creation of an Error
is printed, whereas this makes Node.js also
print the stack trace associated with throwing the value (which does not need
to be an Error
instance).
Enabling this option may affect garbage collection behavior negatively.
--trace-warnings
#
Print stack traces for process warnings (including deprecations).
--track-heap-objects
#
Track heap object allocations for heap snapshots.
--unhandled-rejections=mode
#
Using this flag allows to change what should happen when an unhandled rejection occurs. One of the following modes can be chosen:
throw
: EmitunhandledRejection
. If this hook is not set, raise the unhandled rejection as an uncaught exception. This is the default.strict
: Raise the unhandled rejection as an uncaught exception. If the exception is handled,unhandledRejection
is emitted.warn
: Always trigger a warning, no matter if theunhandledRejection
hook is set or not but do not print the deprecation warning.warn-with-error-code
: EmitunhandledRejection
. If this hook is not set, trigger a warning, and set the process exit code to 1.none
: Silence all warnings.
If a rejection happens during the command line entry point's ES module static loading phase, it will always raise it as an uncaught exception.
--use-bundled-ca
, --use-openssl-ca
#
Use bundled Mozilla CA store as supplied by current Node.js version or use OpenSSL's default CA store. The default store is selectable at build-time.
The bundled CA store, as supplied by Node.js, is a snapshot of Mozilla CA store that is fixed at release time. It is identical on all supported platforms.
Using OpenSSL store allows for external modifications of the store. For most Linux and BSD distributions, this store is maintained by the distribution maintainers and system administrators. OpenSSL CA store location is dependent on configuration of the OpenSSL library but this can be altered at runtime using environment variables.
See SSL_CERT_DIR
and SSL_CERT_FILE
.
--use-largepages=mode
#
Re-map the Node.js static code to large memory pages at startup. If supported on the target system, this will cause the Node.js static code to be moved onto 2 MiB pages instead of 4 KiB pages.
The following values are valid for mode
:
off
: No mapping will be attempted. This is the default.on
: If supported by the OS, mapping will be attempted. Failure to map will be ignored and a message will be printed to standard error.silent
: If supported by the OS, mapping will be attempted. Failure to map will be ignored and will not be reported.
--v8-options
#
Print V8 command-line options.
--v8-pool-size=num
#
Set V8's thread pool size which will be used to allocate background jobs.
If set to 0
then Node.js will choose an appropriate size of the thread pool
based on an estimate of the amount of parallelism.
The amount of parallelism refers to the number of computations that can be carried out simultaneously in a given machine. In general, it's the same as the amount of CPUs, but it may diverge in environments such as VMs or containers.
--watch
#
Starts Node.js in watch mode.
When in watch mode, changes in the watched files cause the Node.js process to
restart.
By default, watch mode will watch the entry point
and any required or imported module.
Use --watch-path
to specify what paths to watch.
This flag cannot be combined with
--check
, --eval
, --interactive
, or the REPL.
$ node --watch index.js
--watch-path
#
Starts Node.js in watch mode and specifies what paths to watch.
When in watch mode, changes in the watched paths cause the Node.js process to
restart.
This will turn off watching of required or imported modules, even when used in
combination with --watch
.
This flag cannot be combined with
--check
, --eval
, --interactive
, --test
, or the REPL.
$ node --watch-path=./src --watch-path=./tests index.js
This option is only supported on macOS and Windows.
An ERR_FEATURE_UNAVAILABLE_ON_PLATFORM
exception will be thrown
when the option is used on a platform that does not support it.
--watch-preserve-output
#
Disable the clearing of the console when watch mode restarts the process.
$ node --watch --watch-preserve-output test.js
--zero-fill-buffers
#
Automatically zero-fills all newly allocated Buffer
and SlowBuffer
instances.
-c
, --check
#
Syntax check the script without executing.
-e
, --eval "script"
#
Evaluate the following argument as JavaScript. The modules which are
predefined in the REPL can also be used in script
.
On Windows, using cmd.exe
a single quote will not work correctly because it
only recognizes double "
for quoting. In Powershell or Git bash, both '
and "
are usable.
-h
, --help
#
Print node command-line options. The output of this option is less detailed than this document.
-i
, --interactive
#
Opens the REPL even if stdin does not appear to be a terminal.
-p
, --print "script"
#
Identical to -e
but prints the result.
-r
, --require module
#
Preload the specified module at startup.
Follows require()
's module resolution
rules. module
may be either a path to a file, or a node module name.
Only CommonJS modules are supported.
Use --import
to preload an ECMAScript module.
Modules preloaded with --require
will run before modules preloaded with --import
.
-v
, --version
#
Print node's version.
Environment variables#
FORCE_COLOR=[1, 2, 3]
#
The FORCE_COLOR
environment variable is used to
enable ANSI colorized output. The value may be:
1
,true
, or the empty string''
indicate 16-color support,2
to indicate 256-color support, or3
to indicate 16 million-color support.
When FORCE_COLOR
is used and set to a supported value, both the NO_COLOR
,
and NODE_DISABLE_COLORS
environment variables are ignored.
Any other value will result in colorized output being disabled.
NODE_DEBUG=module[,…]
#
','
-separated list of core modules that should print debug information.
NODE_DEBUG_NATIVE=module[,…]
#
','
-separated list of core C++ modules that should print debug information.
NODE_DISABLE_COLORS=1
#
When set, colors will not be used in the REPL.
NODE_EXTRA_CA_CERTS=file
#
When set, the well known "root" CAs (like VeriSign) will be extended with the
extra certificates in file
. The file should consist of one or more trusted
certificates in PEM format. A message will be emitted (once) with
process.emitWarning()
if the file is missing or
malformed, but any errors are otherwise ignored.
Neither the well known nor extra certificates are used when the ca
options property is explicitly specified for a TLS or HTTPS client or server.
This environment variable is ignored when node
runs as setuid root or
has Linux file capabilities set.
The NODE_EXTRA_CA_CERTS
environment variable is only read when the Node.js
process is first launched. Changing the value at runtime using
process.env.NODE_EXTRA_CA_CERTS
has no effect on the current process.
NODE_ICU_DATA=file
#
Data path for ICU (Intl
object) data. Will extend linked-in data when compiled
with small-icu support.
NODE_NO_WARNINGS=1
#
When set to 1
, process warnings are silenced.
NODE_OPTIONS=options...
#
A space-separated list of command-line options. options...
are interpreted
before command-line options, so command-line options will override or
compound after anything in options...
. Node.js will exit with an error if
an option that is not allowed in the environment is used, such as -p
or a
script file.
If an option value contains a space, it can be escaped using double quotes:
NODE_OPTIONS='--require "./my path/file.js"'
A singleton flag passed as a command-line option will override the same flag
passed into NODE_OPTIONS
:
# The inspector will be available on port 5555
NODE_OPTIONS='--inspect=localhost:4444' node --inspect=localhost:5555
A flag that can be passed multiple times will be treated as if its
NODE_OPTIONS
instances were passed first, and then its command-line
instances afterwards:
NODE_OPTIONS='--require "./a.js"' node --require "./b.js"
# is equivalent to:
node --require "./a.js" --require "./b.js"
Node.js options that are allowed are:
--allow-child-process
--allow-fs-read
--allow-fs-write
--allow-worker
--conditions
,-C
--diagnostic-dir
--disable-proto
--dns-result-order
--enable-fips
--enable-network-family-autoselection
--enable-source-maps
--experimental-abortcontroller
--experimental-import-meta-resolve
--experimental-json-modules
--experimental-loader
--experimental-modules
--experimental-network-imports
--experimental-permission
--experimental-policy
--experimental-shadow-realm
--experimental-specifier-resolution
--experimental-top-level-await
--experimental-vm-modules
--experimental-wasi-unstable-preview1
--experimental-wasm-modules
--force-context-aware
--force-fips
--force-node-api-uncaught-exceptions-policy
--frozen-intrinsics
--heapsnapshot-near-heap-limit
--heapsnapshot-signal
--http-parser
--icu-data-dir
--import
--input-type
--insecure-http-parser
--inspect-brk
--inspect-port
,--debug-port
--inspect-publish-uid
--inspect
--max-http-header-size
--napi-modules
--no-addons
--no-deprecation
--no-experimental-fetch
--no-experimental-global-customevent
--no-experimental-global-webcrypto
--no-experimental-repl-await
--no-extra-info-on-fatal-exception
--no-force-async-hooks-checks
--no-global-search-paths
--no-warnings
--node-memory-debug
--openssl-config
--openssl-legacy-provider
--openssl-shared-config
--pending-deprecation
--policy-integrity
--preserve-symlinks-main
--preserve-symlinks
--prof-process
--redirect-warnings
--report-compact
--report-dir
,--report-directory
--report-filename
--report-on-fatalerror
--report-on-signal
--report-signal
--report-uncaught-exception
--require
,-r
--secure-heap-min
--secure-heap
--snapshot-blob
--test-only
--test-reporter-destination
--test-reporter
--throw-deprecation
--title
--tls-cipher-list
--tls-keylog
--tls-max-v1.2
--tls-max-v1.3
--tls-min-v1.0
--tls-min-v1.1
--tls-min-v1.2
--tls-min-v1.3
--trace-atomics-wait
--trace-deprecation
--trace-event-categories
--trace-event-file-pattern
--trace-events-enabled
--trace-exit
--trace-sigint
--trace-sync-io
--trace-tls
--trace-uncaught
--trace-warnings
--track-heap-objects
--unhandled-rejections
--use-bundled-ca
--use-largepages
--use-openssl-ca
--v8-pool-size
--watch-path
--watch-preserve-output
--watch
--zero-fill-buffers
V8 options that are allowed are:
--abort-on-uncaught-exception
--disallow-code-generation-from-strings
--enable-etw-stack-walking
--huge-max-old-generation-size
--interpreted-frames-native-stack
--jitless
--max-old-space-size
--max-semi-space-size
--perf-basic-prof-only-functions
--perf-basic-prof
--perf-prof-unwinding-info
--perf-prof
--stack-trace-limit
--perf-basic-prof-only-functions
, --perf-basic-prof
,
--perf-prof-unwinding-info
, and --perf-prof
are only available on Linux.
--enable-etw-stack-walking
is only available on Windows.
NODE_PATH=path[:…]
#
':'
-separated list of directories prefixed to the module search path.
On Windows, this is a ';'
-separated list instead.
NODE_PENDING_DEPRECATION=1
#
When set to 1
, emit pending deprecation warnings.
Pending deprecations are generally identical to a runtime deprecation with the
notable exception that they are turned off by default and will not be emitted
unless either the --pending-deprecation
command-line flag, or the
NODE_PENDING_DEPRECATION=1
environment variable, is set. Pending deprecations
are used to provide a kind of selective "early warning" mechanism that
developers may leverage to detect deprecated API usage.
NODE_PENDING_PIPE_INSTANCES=instances
#
Set the number of pending pipe instance handles when the pipe server is waiting for connections. This setting applies to Windows only.
NODE_PRESERVE_SYMLINKS=1
#
When set to 1
, instructs the module loader to preserve symbolic links when
resolving and caching modules.
NODE_REDIRECT_WARNINGS=file
#
When set, process warnings will be emitted to the given file instead of
printing to stderr. The file will be created if it does not exist, and will be
appended to if it does. If an error occurs while attempting to write the
warning to the file, the warning will be written to stderr instead. This is
equivalent to using the --redirect-warnings=file
command-line flag.
NODE_REPL_HISTORY=file
#
Path to the file used to store the persistent REPL history. The default path is
~/.node_repl_history
, which is overridden by this variable. Setting the value
to an empty string (''
or ' '
) disables persistent REPL history.
NODE_REPL_EXTERNAL_MODULE=file
#
Path to a Node.js module which will be loaded in place of the built-in REPL.
Overriding this value to an empty string (''
) will use the built-in REPL.
NODE_SKIP_PLATFORM_CHECK=value
#
If value
equals '1'
, the check for a supported platform is skipped during
Node.js startup. Node.js might not execute correctly. Any issues encountered
on unsupported platforms will not be fixed.
NODE_TEST_CONTEXT=value
#
If value
equals 'child'
, test reporter options will be overridden and test
output will be sent to stdout in the TAP format.
NODE_TLS_REJECT_UNAUTHORIZED=value
#
If value
equals '0'
, certificate validation is disabled for TLS connections.
This makes TLS, and HTTPS by extension, insecure. The use of this environment
variable is strongly discouraged.
NODE_V8_COVERAGE=dir
#
When set, Node.js will begin outputting V8 JavaScript code coverage and
Source Map data to the directory provided as an argument (coverage
information is written as JSON to files with a coverage
prefix).
NODE_V8_COVERAGE
will automatically propagate to subprocesses, making it
easier to instrument applications that call the child_process.spawn()
family
of functions. NODE_V8_COVERAGE
can be set to an empty string, to prevent
propagation.
Coverage output#
Coverage is output as an array of ScriptCoverage objects on the top-level
key result
:
{
"result": [
{
"scriptId": "67",
"url": "internal/tty.js",
"functions": []
}
]
}
Source map cache#
If found, source map data is appended to the top-level key source-map-cache
on the JSON coverage object.
source-map-cache
is an object with keys representing the files source maps
were extracted from, and values which include the raw source-map URL
(in the key url
), the parsed Source Map v3 information (in the key data
),
and the line lengths of the source file (in the key lineLengths
).
{
"result": [
{
"scriptId": "68",
"url": "file:///absolute/path/to/source.js",
"functions": []
}
],
"source-map-cache": {
"file:///absolute/path/to/source.js": {
"url": "./path-to-map.json",
"data": {
"version": 3,
"sources": [
"file:///absolute/path/to/original.js"
],
"names": [
"Foo",
"console",
"info"
],
"mappings": "MAAMA,IACJC,YAAaC",
"sourceRoot": "./"
},
"lineLengths": [
13,
62,
38,
27
]
}
}
}
NO_COLOR=<any>
#
NO_COLOR
is an alias for NODE_DISABLE_COLORS
. The value of the
environment variable is arbitrary.
OPENSSL_CONF=file
#
Load an OpenSSL configuration file on startup. Among other uses, this can be
used to enable FIPS-compliant crypto if Node.js is built with
./configure --openssl-fips
.
If the --openssl-config
command-line option is used, the environment
variable is ignored.
SSL_CERT_DIR=dir
#
If --use-openssl-ca
is enabled, this overrides and sets OpenSSL's directory
containing trusted certificates.
Be aware that unless the child environment is explicitly set, this environment variable will be inherited by any child processes, and if they use OpenSSL, it may cause them to trust the same CAs as node.
SSL_CERT_FILE=file
#
If --use-openssl-ca
is enabled, this overrides and sets OpenSSL's file
containing trusted certificates.
Be aware that unless the child environment is explicitly set, this environment variable will be inherited by any child processes, and if they use OpenSSL, it may cause them to trust the same CAs as node.
TZ
#
The TZ
environment variable is used to specify the timezone configuration.
While Node.js does not support all of the various ways that TZ
is handled in
other environments, it does support basic timezone IDs (such as
'Etc/UTC'
, 'Europe/Paris'
, or 'America/New_York'
).
It may support a few other abbreviations or aliases, but these are strongly
discouraged and not guaranteed.
$ TZ=Europe/Dublin node -pe "new Date().toString()"
Wed May 12 2021 20:30:48 GMT+0100 (Irish Standard Time)
UV_THREADPOOL_SIZE=size
#
Set the number of threads used in libuv's threadpool to size
threads.
Asynchronous system APIs are used by Node.js whenever possible, but where they do not exist, libuv's threadpool is used to create asynchronous node APIs based on synchronous system APIs. Node.js APIs that use the threadpool are:
- all
fs
APIs, other than the file watcher APIs and those that are explicitly synchronous - asynchronous crypto APIs such as
crypto.pbkdf2()
,crypto.scrypt()
,crypto.randomBytes()
,crypto.randomFill()
,crypto.generateKeyPair()
dns.lookup()
- all
zlib
APIs, other than those that are explicitly synchronous
Because libuv's threadpool has a fixed size, it means that if for whatever
reason any of these APIs takes a long time, other (seemingly unrelated) APIs
that run in libuv's threadpool will experience degraded performance. In order to
mitigate this issue, one potential solution is to increase the size of libuv's
threadpool by setting the 'UV_THREADPOOL_SIZE'
environment variable to a value
greater than 4
(its current default value). For more information, see the
libuv threadpool documentation.
Useful V8 options#
V8 has its own set of CLI options. Any V8 CLI option that is provided to node
will be passed on to V8 to handle. V8's options have no stability guarantee.
The V8 team themselves don't consider them to be part of their formal API,
and reserve the right to change them at any time. Likewise, they are not
covered by the Node.js stability guarantees. Many of the V8
options are of interest only to V8 developers. Despite this, there is a small
set of V8 options that are widely applicable to Node.js, and they are
documented here:
--max-old-space-size=SIZE
(in megabytes)#
Sets the max memory size of V8's old memory section. As memory consumption approaches the limit, V8 will spend more time on garbage collection in an effort to free unused memory.
On a machine with 2 GiB of memory, consider setting this to 1536 (1.5 GiB) to leave some memory for other uses and avoid swapping.
$ node --max-old-space-size=1536 index.js
--max-semi-space-size=SIZE
(in megabytes)#
Sets the maximum semi-space size for V8's scavenge garbage collector in MiB (megabytes). Increasing the max size of a semi-space may improve throughput for Node.js at the cost of more memory consumption.
Since the young generation size of the V8 heap is three times (see
YoungGenerationSizeFromSemiSpaceSize
in V8) the size of the semi-space,
an increase of 1 MiB to semi-space applies to each of the three individual
semi-spaces and causes the heap size to increase by 3 MiB. The throughput
improvement depends on your workload (see #42511).
The default value is 16 MiB for 64-bit systems and 8 MiB for 32-bit systems. To get the best configuration for your application, you should try different max-semi-space-size values when running benchmarks for your application.
For example, benchmark on a 64-bit systems:
for MiB in 16 32 64 128; do
node --max-semi-space-size=$MiB index.js
done
Console#
Source Code: lib/console.js
The node:console
module provides a simple debugging console that is similar to
the JavaScript console mechanism provided by web browsers.
The module exports two specific components:
- A
Console
class with methods such asconsole.log()
,console.error()
, andconsole.warn()
that can be used to write to any Node.js stream. - A global
console
instance configured to write toprocess.stdout
andprocess.stderr
. The globalconsole
can be used without callingrequire('node:console')
.
Warning: The global console object's methods are neither consistently synchronous like the browser APIs they resemble, nor are they consistently asynchronous like all other Node.js streams. See the note on process I/O for more information.
Example using the global console
:
console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints error message and stack trace to stderr:
// Error: Whoops, something bad happened
// at [eval]:5:15
// at Script.runInThisContext (node:vm:132:18)
// at Object.runInThisContext (node:vm:309:38)
// at node:internal/process/execution:77:19
// at [eval]-wrapper:6:22
// at evalScript (node:internal/process/execution:76:60)
// at node:internal/main/eval_string:23:3
const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr
Example using the Console
class:
const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);
myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err
const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err
Class: Console
#
The Console
class can be used to create a simple logger with configurable
output streams and can be accessed using either require('node:console').Console
or console.Console
(or their destructured counterparts):
const { Console } = require('node:console');
const { Console } = console;
new Console(stdout[, stderr][, ignoreErrors])
#
new Console(options)
#
options
<Object>stdout
<stream.Writable>stderr
<stream.Writable>ignoreErrors
<boolean> Ignore errors when writing to the underlying streams. Default:true
.colorMode
<boolean> | <string> Set color support for thisConsole
instance. Setting totrue
enables coloring while inspecting values. Setting tofalse
disables coloring while inspecting values. Setting to'auto'
makes color support depend on the value of theisTTY
property and the value returned bygetColorDepth()
on the respective stream. This option can not be used, ifinspectOptions.colors
is set as well. Default:'auto'
.inspectOptions
<Object> Specifies options that are passed along toutil.inspect()
.groupIndentation
<number> Set group indentation. Default:2
.
Creates a new Console
with one or two writable stream instances. stdout
is a
writable stream to print log or info output. stderr
is used for warning or
error output. If stderr
is not provided, stdout
is used for stderr
.
const output = fs.createWriteStream('./stdout.log');
const errorOutput = fs.createWriteStream('./stderr.log');
// Custom simple logger
const logger = new Console({ stdout: output, stderr: errorOutput });
// use it like console
const count = 5;
logger.log('count: %d', count);
// In stdout.log: count 5
The global console
is a special Console
whose output is sent to
process.stdout
and process.stderr
. It is equivalent to calling:
new Console({ stdout: process.stdout, stderr: process.stderr });
console.assert(value[, ...message])
#
value
<any> The value tested for being truthy....message
<any> All arguments besidesvalue
are used as error message.
console.assert()
writes a message if value
is falsy or omitted. It only
writes a message and does not otherwise affect execution. The output always
starts with "Assertion failed"
. If provided, message
is formatted using
util.format()
.
If value
is truthy, nothing happens.
console.assert(true, 'does nothing');
console.assert(false, 'Whoops %s work', 'didn\'t');
// Assertion failed: Whoops didn't work
console.assert();
// Assertion failed
console.clear()
#
When stdout
is a TTY, calling console.clear()
will attempt to clear the
TTY. When stdout
is not a TTY, this method does nothing.
The specific operation of console.clear()
can vary across operating systems
and terminal types. For most Linux operating systems, console.clear()
operates similarly to the clear
shell command. On Windows, console.clear()
will clear only the output in the current terminal viewport for the Node.js
binary.
console.count([label])
#
label
<string> The display label for the counter. Default:'default'
.
Maintains an internal counter specific to label
and outputs to stdout
the
number of times console.count()
has been called with the given label
.
> console.count()
default: 1
undefined
> console.count('default')
default: 2
undefined
> console.count('abc')
abc: 1
undefined
> console.count('xyz')
xyz: 1
undefined
> console.count('abc')
abc: 2
undefined
> console.count()
default: 3
undefined
>
console.countReset([label])
#
label
<string> The display label for the counter. Default:'default'
.
Resets the internal counter specific to label
.
> console.count('abc');
abc: 1
undefined
> console.countReset('abc');
undefined
> console.count('abc');
abc: 1
undefined
>
console.debug(data[, ...args])
#
The console.debug()
function is an alias for console.log()
.
console.dir(obj[, options])
#
obj
<any>options
<Object>showHidden
<boolean> Iftrue
then the object's non-enumerable and symbol properties will be shown too. Default:false
.depth
<number> Tellsutil.inspect()
how many times to recurse while formatting the object. This is useful for inspecting large complicated objects. To make it recurse indefinitely, passnull
. Default:2
.colors
<boolean> Iftrue
, then the output will be styled with ANSI color codes. Colors are customizable; see customizingutil.inspect()
colors. Default:false
.
Uses util.inspect()
on obj
and prints the resulting string to stdout
.
This function bypasses any custom inspect()
function defined on obj
.
console.dirxml(...data)
#
...data
<any>
This method calls console.log()
passing it the arguments received.
This method does not produce any XML formatting.
console.error([data][, ...args])
#
Prints to stderr
with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to printf(3)
(the arguments are all passed to
util.format()
).
const code = 5;
console.error('error #%d', code);
// Prints: error #5, to stderr
console.error('error', code);
// Prints: error 5, to stderr
If formatting elements (e.g. %d
) are not found in the first string then
util.inspect()
is called on each argument and the resulting string
values are concatenated. See util.format()
for more information.
console.group([...label])
#
...label
<any>
Increases indentation of subsequent lines by spaces for groupIndentation
length.
If one or more label
s are provided, those are printed first without the
additional indentation.
console.groupCollapsed()
#
An alias for console.group()
.
console.groupEnd()
#
Decreases indentation of subsequent lines by spaces for groupIndentation
length.
console.info([data][, ...args])
#
The console.info()
function is an alias for console.log()
.
console.log([data][, ...args])
#
Prints to stdout
with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to printf(3)
(the arguments are all passed to
util.format()
).
const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout
See util.format()
for more information.
console.table(tabularData[, properties])
#
tabularData
<any>properties
<string[]> Alternate properties for constructing the table.
Try to construct a table with the columns of the properties of tabularData
(or use properties
) and rows of tabularData
and log it. Falls back to just
logging the argument if it can't be parsed as tabular.
// These can't be parsed as tabular data
console.table(Symbol());
// Symbol()
console.table(undefined);
// undefined
console.table([{ a: 1, b: 'Y' }, { a: 'Z', b: 2 }]);
// ┌─────────┬─────┬─────┐
// │ (index) │ a │ b │
// ├─────────┼─────┼─────┤
// │ 0 │ 1 │ 'Y' │
// │ 1 │ 'Z' │ 2 │
// └─────────┴─────┴─────┘
console.table([{ a: 1, b: 'Y' }, { a: 'Z', b: 2 }], ['a']);
// ┌─────────┬─────┐
// │ (index) │ a │
// ├─────────┼─────┤
// │ 0 │ 1 │
// │ 1 │ 'Z' │
// └─────────┴─────┘
console.time([label])
#
label
<string> Default:'default'
Starts a timer that can be used to compute the duration of an operation. Timers
are identified by a unique label
. Use the same label
when calling
console.timeEnd()
to stop the timer and output the elapsed time in
suitable time units to stdout
. For example, if the elapsed
time is 3869ms, console.timeEnd()
displays "3.869s".
console.timeEnd([label])
#
label
<string> Default:'default'
Stops a timer that was previously started by calling console.time()
and
prints the result to stdout
:
console.time('bunch-of-stuff');
// Do a bunch of stuff.
console.timeEnd('bunch-of-stuff');
// Prints: bunch-of-stuff: 225.438ms
console.timeLog([label][, ...data])
#
For a timer that was previously started by calling console.time()
, prints
the elapsed time and other data
arguments to stdout
:
console.time('process');
const value = expensiveProcess1(); // Returns 42
console.timeLog('process', value);
// Prints "process: 365.227ms 42".
doExpensiveProcess2(value);
console.timeEnd('process');
console.trace([message][, ...args])
#
Prints to stderr
the string 'Trace: '
, followed by the util.format()
formatted message and stack trace to the current position in the code.
console.trace('Show me');
// Prints: (stack trace will vary based on where trace is called)
// Trace: Show me
// at repl:2:9
// at REPLServer.defaultEval (repl.js:248:27)
// at bound (domain.js:287:14)
// at REPLServer.runBound [as eval] (domain.js:300:12)
// at REPLServer.<anonymous> (repl.js:412:12)
// at emitOne (events.js:82:20)
// at REPLServer.emit (events.js:169:7)
// at REPLServer.Interface._onLine (readline.js:210:10)
// at REPLServer.Interface._line (readline.js:549:8)
// at REPLServer.Interface._ttyWrite (readline.js:826:14)
console.warn([data][, ...args])
#
The console.warn()
function is an alias for console.error()
.
Inspector only methods#
The following methods are exposed by the V8 engine in the general API but do
not display anything unless used in conjunction with the inspector
(--inspect
flag).
console.profile([label])
#
label
<string>
This method does not display anything unless used in the inspector. The
console.profile()
method starts a JavaScript CPU profile with an optional
label until console.profileEnd()
is called. The profile is then added to
the Profile panel of the inspector.
console.profile('MyLabel');
// Some code
console.profileEnd('MyLabel');
// Adds the profile 'MyLabel' to the Profiles panel of the inspector.
console.profileEnd([label])
#
label
<string>
This method does not display anything unless used in the inspector. Stops the
current JavaScript CPU profiling session if one has been started and prints
the report to the Profiles panel of the inspector. See
console.profile()
for an example.
If this method is called without a label, the most recently started profile is stopped.
console.timeStamp([label])
#
label
<string>
This method does not display anything unless used in the inspector. The
console.timeStamp()
method adds an event with the label 'label'
to the
Timeline panel of the inspector.
Corepack#
Corepack is an experimental tool to help with managing versions of your package managers. It exposes binary proxies for each supported package manager that, when called, will identify whatever package manager is configured for the current project, transparently install it if needed, and finally run it without requiring explicit user interactions.
This feature simplifies two core workflows:
-
It eases new contributor onboarding, since they won't have to follow system-specific installation processes anymore just to have the package manager you want them to.
-
It allows you to ensure that everyone in your team will use exactly the package manager version you intend them to, without them having to manually synchronize it each time you need to make an update.
Workflows#
Enabling the feature#
Due to its experimental status, Corepack currently needs to be explicitly
enabled to have any effect. To do that, run corepack enable
, which
will set up the symlinks in your environment next to the node
binary
(and overwrite the existing symlinks if necessary).
From this point forward, any call to the supported binaries will work
without further setup. Should you experience a problem, run
corepack disable
to remove the proxies from your system (and consider
opening an issue on the Corepack repository to let us know).
Configuring a package#
The Corepack proxies will find the closest package.json
file in your
current directory hierarchy to extract its "packageManager"
property.
If the value corresponds to a supported package manager, Corepack will make sure that all calls to the relevant binaries are run against the requested version, downloading it on demand if needed, and aborting if it cannot be successfully retrieved.
Upgrading the global versions#
When running outside of an existing project (for example when running
yarn init
), Corepack will by default use predefined versions roughly
corresponding to the latest stable releases from each tool. Those versions can
be overridden by running the corepack prepare
command along with the
package manager version you wish to set:
corepack prepare [email protected] --activate
Alternately, a tag or range may be used:
corepack prepare pnpm@latest --activate
corepack prepare yarn@stable --activate
Offline workflow#
Many production environments don't have network access. Since Corepack
usually downloads the package manager releases straight from their registries,
it can conflict with such environments. To avoid that happening, call the
corepack prepare
command while you still have network access (typically at
the same time you're preparing your deploy image). This will ensure that the
required package managers are available even without network access.
The prepare
command has various flags. Consult the detailed
Corepack documentation for more information.
Supported package managers#
The following binaries are provided through Corepack:
Package manager | Binary names |
---|---|
Yarn | yarn , yarnpkg |
pnpm | pnpm , pnpx |
Common questions#
How does Corepack interact with npm?#
While Corepack could support npm like any other package manager, its shims aren't enabled by default. This has a few consequences:
-
It's always possible to run a
npm
command within a project configured to be used with another package manager, since Corepack cannot intercept it. -
While
npm
is a valid option in the"packageManager"
property, the lack of shim will cause the global npm to be used.
Running npm install -g yarn
doesn't work#
npm prevents accidentally overriding the Corepack binaries when doing a global install. To avoid this problem, consider one of the following options:
-
Don't run this command; Corepack will provide the package manager binaries anyway and will ensure that the requested versions are always available, so installing the package managers explicitly isn't needed.
-
Add the
--force
flag tonpm install
; this will tell npm that it's fine to override binaries, but you'll erase the Corepack ones in the process. (Runcorepack enable
to add them back.)
Crypto#
Source Code: lib/crypto.js
The node:crypto
module provides cryptographic functionality that includes a
set of wrappers for OpenSSL's hash, HMAC, cipher, decipher, sign, and verify
functions.
const { createHmac } = await import('node:crypto');
const secret = 'abcdefg';
const hash = createHmac('sha256', secret)
.update('I love cupcakes')
.digest('hex');
console.log(hash);
// Prints:
// c0fa1bc00531bd78ef38c628449c5102aeabd49b5dc3a2a516ea6ea959d6658e
const { createHmac } = require('node:crypto');
const secret = 'abcdefg';
const hash = createHmac('sha256', secret)
.update('I love cupcakes')
.digest('hex');
console.log(hash);
// Prints:
// c0fa1bc00531bd78ef38c628449c5102aeabd49b5dc3a2a516ea6ea959d6658e
Determining if crypto support is unavailable#
It is possible for Node.js to be built without including support for the
node:crypto
module. In such cases, attempting to import
from crypto
or
calling require('node:crypto')
will result in an error being thrown.
When using CommonJS, the error thrown can be caught using try/catch:
let crypto;
try {
crypto = require('node:crypto');
} catch (err) {
console.error('crypto support is disabled!');
}
When using the lexical ESM import
keyword, the error can only be
caught if a handler for process.on('uncaughtException')
is registered
before any attempt to load the module is made (using, for instance,
a preload module).
When using ESM, if there is a chance that the code may be run on a build
of Node.js where crypto support is not enabled, consider using the
import()
function instead of the lexical import
keyword:
let crypto;
try {
crypto = await import('node:crypto');
} catch (err) {
console.error('crypto support is disabled!');
}
Class: Certificate
#
SPKAC is a Certificate Signing Request mechanism originally implemented by
Netscape and was specified formally as part of HTML5's keygen
element.
<keygen>
is deprecated since HTML 5.2 and new projects
should not use this element anymore.
The node:crypto
module provides the Certificate
class for working with SPKAC
data. The most common usage is handling output generated by the HTML5
<keygen>
element. Node.js uses OpenSSL's SPKAC implementation internally.
Static method: Certificate.exportChallenge(spkac[, encoding])
#
spkac
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>encoding
<string> The encoding of thespkac
string.- Returns: <Buffer> The challenge component of the
spkac
data structure, which includes a public key and a challenge.
const { Certificate } = await import('node:crypto');
const spkac = getSpkacSomehow();
const challenge = Certificate.exportChallenge(spkac);
console.log(challenge.toString('utf8'));
// Prints: the challenge as a UTF8 string
const { Certificate } = require('node:crypto');
const spkac = getSpkacSomehow();
const challenge = Certificate.exportChallenge(spkac);
console.log(challenge.toString('utf8'));
// Prints: the challenge as a UTF8 string
Static method: Certificate.exportPublicKey(spkac[, encoding])
#
spkac
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>encoding
<string> The encoding of thespkac
string.- Returns: <Buffer> The public key component of the
spkac
data structure, which includes a public key and a challenge.
const { Certificate } = await import('node:crypto');
const spkac = getSpkacSomehow();
const publicKey = Certificate.exportPublicKey(spkac);
console.log(publicKey);
// Prints: the public key as <Buffer ...>
const { Certificate } = require('node:crypto');
const spkac = getSpkacSomehow();
const publicKey = Certificate.exportPublicKey(spkac);
console.log(publicKey);
// Prints: the public key as <Buffer ...>
Static method: Certificate.verifySpkac(spkac[, encoding])
#
spkac
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>encoding
<string> The encoding of thespkac
string.- Returns: <boolean>
true
if the givenspkac
data structure is valid,false
otherwise.
import { Buffer } from 'node:buffer';
const { Certificate } = await import('node:crypto');
const spkac = getSpkacSomehow();
console.log(Certificate.verifySpkac(Buffer.from(spkac)));
// Prints: true or false
const { Buffer } = require('node:buffer');
const { Certificate } = require('node:crypto');
const spkac = getSpkacSomehow();
console.log(Certificate.verifySpkac(Buffer.from(spkac)));
// Prints: true or false
Legacy API#
As a legacy interface, it is possible to create new instances of
the crypto.Certificate
class as illustrated in the examples below.
new crypto.Certificate()
#
Instances of the Certificate
class can be created using the new
keyword
or by calling crypto.Certificate()
as a function:
const { Certificate } = await import('node:crypto');
const cert1 = new Certificate();
const cert2 = Certificate();
const { Certificate } = require('node:crypto');
const cert1 = new Certificate();
const cert2 = Certificate();
certificate.exportChallenge(spkac[, encoding])
#
spkac
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>encoding
<string> The encoding of thespkac
string.- Returns: <Buffer> The challenge component of the
spkac
data structure, which includes a public key and a challenge.
const { Certificate } = await import('node:crypto');
const cert = Certificate();
const spkac = getSpkacSomehow();
const challenge = cert.exportChallenge(spkac);
console.log(challenge.toString('utf8'));
// Prints: the challenge as a UTF8 string
const { Certificate } = require('node:crypto');
const cert = Certificate();
const spkac = getSpkacSomehow();
const challenge = cert.exportChallenge(spkac);
console.log(challenge.toString('utf8'));
// Prints: the challenge as a UTF8 string
certificate.exportPublicKey(spkac[, encoding])
#
spkac
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>encoding
<string> The encoding of thespkac
string.- Returns: <Buffer> The public key component of the
spkac
data structure, which includes a public key and a challenge.
const { Certificate } = await import('node:crypto');
const cert = Certificate();
const spkac = getSpkacSomehow();
const publicKey = cert.exportPublicKey(spkac);
console.log(publicKey);
// Prints: the public key as <Buffer ...>
const { Certificate } = require('node:crypto');
const cert = Certificate();
const spkac = getSpkacSomehow();
const publicKey = cert.exportPublicKey(spkac);
console.log(publicKey);
// Prints: the public key as <Buffer ...>
certificate.verifySpkac(spkac[, encoding])
#
spkac
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>encoding
<string> The encoding of thespkac
string.- Returns: <boolean>
true
if the givenspkac
data structure is valid,false
otherwise.
import { Buffer } from 'node:buffer';
const { Certificate } = await import('node:crypto');
const cert = Certificate();
const spkac = getSpkacSomehow();
console.log(cert.verifySpkac(Buffer.from(spkac)));
// Prints: true or false
const { Buffer } = require('node:buffer');
const { Certificate } = require('node:crypto');
const cert = Certificate();
const spkac = getSpkacSomehow();
console.log(cert.verifySpkac(Buffer.from(spkac)));
// Prints: true or false
Class: Cipher
#
- Extends: <stream.Transform>
Instances of the Cipher
class are used to encrypt data. The class can be
used in one of two ways:
- As a stream that is both readable and writable, where plain unencrypted data is written to produce encrypted data on the readable side, or
- Using the
cipher.update()
andcipher.final()
methods to produce the encrypted data.
The crypto.createCipher()
or crypto.createCipheriv()
methods are
used to create Cipher
instances. Cipher
objects are not to be created
directly using the new
keyword.
Example: Using Cipher
objects as streams:
const {
scrypt,
randomFill,
createCipheriv,
} = await import('node:crypto');
const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';
// First, we'll generate the key. The key length is dependent on the algorithm.
// In this case for aes192, it is 24 bytes (192 bits).
scrypt(password, 'salt', 24, (err, key) => {
if (err) throw err;
// Then, we'll generate a random initialization vector
randomFill(new Uint8Array(16), (err, iv) => {
if (err) throw err;
// Once we have the key and iv, we can create and use the cipher...
const cipher = createCipheriv(algorithm, key, iv);
let encrypted = '';
cipher.setEncoding('hex');
cipher.on('data', (chunk) => encrypted += chunk);
cipher.on('end', () => console.log(encrypted));
cipher.write('some clear text data');
cipher.end();
});
});
const {
scrypt,
randomFill,
createCipheriv,
} = require('node:crypto');
const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';
// First, we'll generate the key. The key length is dependent on the algorithm.
// In this case for aes192, it is 24 bytes (192 bits).
scrypt(password, 'salt', 24, (err, key) => {
if (err) throw err;
// Then, we'll generate a random initialization vector
randomFill(new Uint8Array(16), (err, iv) => {
if (err) throw err;
// Once we have the key and iv, we can create and use the cipher...
const cipher = createCipheriv(algorithm, key, iv);
let encrypted = '';
cipher.setEncoding('hex');
cipher.on('data', (chunk) => encrypted += chunk);
cipher.on('end', () => console.log(encrypted));
cipher.write('some clear text data');
cipher.end();
});
});
Example: Using Cipher
and piped streams:
import {
createReadStream,
createWriteStream,
} from 'node:fs';
import {
pipeline,
} from 'node:stream';
const {
scrypt,
randomFill,
createCipheriv,
} = await import('node:crypto');
const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';
// First, we'll generate the key. The key length is dependent on the algorithm.
// In this case for aes192, it is 24 bytes (192 bits).
scrypt(password, 'salt', 24, (err, key) => {
if (err) throw err;
// Then, we'll generate a random initialization vector
randomFill(new Uint8Array(16), (err, iv) => {
if (err) throw err;
const cipher = createCipheriv(algorithm, key, iv);
const input = createReadStream('test.js');
const output = createWriteStream('test.enc');
pipeline(input, cipher, output, (err) => {
if (err) throw err;
});
});
});
const {
createReadStream,
createWriteStream,
} = require('node:fs');
const {
pipeline,
} = require('node:stream');
const {
scrypt,
randomFill,
createCipheriv,
} = require('node:crypto');
const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';
// First, we'll generate the key. The key length is dependent on the algorithm.
// In this case for aes192, it is 24 bytes (192 bits).
scrypt(password, 'salt', 24, (err, key) => {
if (err) throw err;
// Then, we'll generate a random initialization vector
randomFill(new Uint8Array(16), (err, iv) => {
if (err) throw err;
const cipher = createCipheriv(algorithm, key, iv);
const input = createReadStream('test.js');
const output = createWriteStream('test.enc');
pipeline(input, cipher, output, (err) => {
if (err) throw err;
});
});
});
Example: Using the cipher.update()
and cipher.final()
methods:
const {
scrypt,
randomFill,
createCipheriv,
} = await import('node:crypto');
const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';
// First, we'll generate the key. The key length is dependent on the algorithm.
// In this case for aes192, it is 24 bytes (192 bits).
scrypt(password, 'salt', 24, (err, key) => {
if (err) throw err;
// Then, we'll generate a random initialization vector
randomFill(new Uint8Array(16), (err, iv) => {
if (err) throw err;
const cipher = createCipheriv(algorithm, key, iv);
let encrypted = cipher.update('some clear text data', 'utf8', 'hex');
encrypted += cipher.final('hex');
console.log(encrypted);
});
});
const {
scrypt,
randomFill,
createCipheriv,
} = require('node:crypto');
const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';
// First, we'll generate the key. The key length is dependent on the algorithm.
// In this case for aes192, it is 24 bytes (192 bits).
scrypt(password, 'salt', 24, (err, key) => {
if (err) throw err;
// Then, we'll generate a random initialization vector
randomFill(new Uint8Array(16), (err, iv) => {
if (err) throw err;
const cipher = createCipheriv(algorithm, key, iv);
let encrypted = cipher.update('some clear text data', 'utf8', 'hex');
encrypted += cipher.final('hex');
console.log(encrypted);
});
});
cipher.final([outputEncoding])
#
outputEncoding
<string> The encoding of the return value.- Returns: <Buffer> | <string> Any remaining enciphered contents.
If
outputEncoding
is specified, a string is returned. If anoutputEncoding
is not provided, aBuffer
is returned.
Once the cipher.final()
method has been called, the Cipher
object can no
longer be used to encrypt data. Attempts to call cipher.final()
more than
once will result in an error being thrown.
cipher.getAuthTag()
#
- Returns: <Buffer> When using an authenticated encryption mode (
GCM
,CCM
,OCB
, andchacha20-poly1305
are currently supported), thecipher.getAuthTag()
method returns aBuffer
containing the authentication tag that has been computed from the given data.
The cipher.getAuthTag()
method should only be called after encryption has
been completed using the cipher.final()
method.
If the authTagLength
option was set during the cipher
instance's creation,
this function will return exactly authTagLength
bytes.
cipher.setAAD(buffer[, options])
#
buffer
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>options
<Object>stream.transform
options- Returns: <Cipher> for method chaining.
When using an authenticated encryption mode (GCM
, CCM
, OCB
, and
chacha20-poly1305
are
currently supported), the cipher.setAAD()
method sets the value used for the
additional authenticated data (AAD) input parameter.
The plaintextLength
option is optional for GCM
and OCB
. When using CCM
,
the plaintextLength
option must be specified and its value must match the
length of the plaintext in bytes. See CCM mode.
The cipher.setAAD()
method must be called before cipher.update()
.
cipher.setAutoPadding([autoPadding])
#
When using block encryption algorithms, the Cipher
class will automatically
add padding to the input data to the appropriate block size. To disable the
default padding call cipher.setAutoPadding(false)
.
When autoPadding
is false
, the length of the entire input data must be a
multiple of the cipher's block size or cipher.final()
will throw an error.
Disabling automatic padding is useful for non-standard padding, for instance
using 0x0
instead of PKCS padding.
The cipher.setAutoPadding()
method must be called before
cipher.final()
.
cipher.update(data[, inputEncoding][, outputEncoding])
#
data
<string> | <Buffer> | <TypedArray> | <DataView>inputEncoding
<string> The encoding of the data.outputEncoding
<string> The encoding of the return value.- Returns: <Buffer> | <string>
Updates the cipher with data
. If the inputEncoding
argument is given,
the data
argument is a string using the specified encoding. If the inputEncoding
argument is not given, data
must be a Buffer
, TypedArray
, or
DataView
. If data
is a Buffer
, TypedArray
, or DataView
, then
inputEncoding
is ignored.
The outputEncoding
specifies the output format of the enciphered
data. If the outputEncoding
is specified, a string using the specified encoding is returned. If no
outputEncoding
is provided, a Buffer
is returned.
The cipher.update()
method can be called multiple times with new data until
cipher.final()
is called. Calling cipher.update()
after
cipher.final()
will result in an error being thrown.
Class: Decipher
#
- Extends: <stream.Transform>
Instances of the Decipher
class are used to decrypt data. The class can be
used in one of two ways:
- As a stream that is both readable and writable, where plain encrypted data is written to produce unencrypted data on the readable side, or
- Using the
decipher.update()
anddecipher.final()
methods to produce the unencrypted data.
The crypto.createDecipher()
or crypto.createDecipheriv()
methods are
used to create Decipher
instances. Decipher
objects are not to be created
directly using the new
keyword.
Example: Using Decipher
objects as streams:
import { Buffer } from 'node:buffer';
const {
scryptSync,
createDecipheriv,
} = await import('node:crypto');
const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';
// Key length is dependent on the algorithm. In this case for aes192, it is
// 24 bytes (192 bits).
// Use the async `crypto.scrypt()` instead.
const key = scryptSync(password, 'salt', 24);
// The IV is usually passed along with the ciphertext.
const iv = Buffer.alloc(16, 0); // Initialization vector.
const decipher = createDecipheriv(algorithm, key, iv);
let decrypted = '';
decipher.on('readable', () => {
let chunk;
while (null !== (chunk = decipher.read())) {
decrypted += chunk.toString('utf8');
}
});
decipher.on('end', () => {
console.log(decrypted);
// Prints: some clear text data
});
// Encrypted with same algorithm, key and iv.
const encrypted =
'e5f79c5915c02171eec6b212d5520d44480993d7d622a7c4c2da32f6efda0ffa';
decipher.write(encrypted, 'hex');
decipher.end();
const {
scryptSync,
createDecipheriv,
} = require('node:crypto');
const { Buffer } = require('node:buffer');
const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';
// Key length is dependent on the algorithm. In this case for aes192, it is
// 24 bytes (192 bits).
// Use the async `crypto.scrypt()` instead.
const key = scryptSync(password, 'salt', 24);
// The IV is usually passed along with the ciphertext.
const iv = Buffer.alloc(16, 0); // Initialization vector.
const decipher = createDecipheriv(algorithm, key, iv);
let decrypted = '';
decipher.on('readable', () => {
let chunk;
while (null !== (chunk = decipher.read())) {
decrypted += chunk.toString('utf8');
}
});
decipher.on('end', () => {
console.log(decrypted);
// Prints: some clear text data
});
// Encrypted with same algorithm, key and iv.
const encrypted =
'e5f79c5915c02171eec6b212d5520d44480993d7d622a7c4c2da32f6efda0ffa';
decipher.write(encrypted, 'hex');
decipher.end();
Example: Using Decipher
and piped streams:
import {
createReadStream,
createWriteStream,
} from 'node:fs';
import { Buffer } from 'node:buffer';
const {
scryptSync,
createDecipheriv,
} = await import('node:crypto');
const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';
// Use the async `crypto.scrypt()` instead.
const key = scryptSync(password, 'salt', 24);
// The IV is usually passed along with the ciphertext.
const iv = Buffer.alloc(16, 0); // Initialization vector.
const decipher = createDecipheriv(algorithm, key, iv);
const input = createReadStream('test.enc');
const output = createWriteStream('test.js');
input.pipe(decipher).pipe(output);
const {
createReadStream,
createWriteStream,
} = require('node:fs');
const {
scryptSync,
createDecipheriv,
} = require('node:crypto');
const { Buffer } = require('node:buffer');
const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';
// Use the async `crypto.scrypt()` instead.
const key = scryptSync(password, 'salt', 24);
// The IV is usually passed along with the ciphertext.
const iv = Buffer.alloc(16, 0); // Initialization vector.
const decipher = createDecipheriv(algorithm, key, iv);
const input = createReadStream('test.enc');
const output = createWriteStream('test.js');
input.pipe(decipher).pipe(output);
Example: Using the decipher.update()
and decipher.final()
methods:
import { Buffer } from 'node:buffer';
const {
scryptSync,
createDecipheriv,
} = await import('node:crypto');
const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';
// Use the async `crypto.scrypt()` instead.
const key = scryptSync(password, 'salt', 24);
// The IV is usually passed along with the ciphertext.
const iv = Buffer.alloc(16, 0); // Initialization vector.
const decipher = createDecipheriv(algorithm, key, iv);
// Encrypted using same algorithm, key and iv.
const encrypted =
'e5f79c5915c02171eec6b212d5520d44480993d7d622a7c4c2da32f6efda0ffa';
let decrypted = decipher.update(encrypted, 'hex', 'utf8');
decrypted += decipher.final('utf8');
console.log(decrypted);
// Prints: some clear text data
const {
scryptSync,
createDecipheriv,
} = require('node:crypto');
const { Buffer } = require('node:buffer');
const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';
// Use the async `crypto.scrypt()` instead.
const key = scryptSync(password, 'salt', 24);
// The IV is usually passed along with the ciphertext.
const iv = Buffer.alloc(16, 0); // Initialization vector.
const decipher = createDecipheriv(algorithm, key, iv);
// Encrypted using same algorithm, key and iv.
const encrypted =
'e5f79c5915c02171eec6b212d5520d44480993d7d622a7c4c2da32f6efda0ffa';
let decrypted = decipher.update(encrypted, 'hex', 'utf8');
decrypted += decipher.final('utf8');
console.log(decrypted);
// Prints: some clear text data
decipher.final([outputEncoding])
#
outputEncoding
<string> The encoding of the return value.- Returns: <Buffer> | <string> Any remaining deciphered contents.
If
outputEncoding
is specified, a string is returned. If anoutputEncoding
is not provided, aBuffer
is returned.
Once the decipher.final()
method has been called, the Decipher
object can
no longer be used to decrypt data. Attempts to call decipher.final()
more
than once will result in an error being thrown.
decipher.setAAD(buffer[, options])
#
buffer
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>options
<Object>stream.transform
options- Returns: <Decipher> for method chaining.
When using an authenticated encryption mode (GCM
, CCM
, OCB
, and
chacha20-poly1305
are
currently supported), the decipher.setAAD()
method sets the value used for the
additional authenticated data (AAD) input parameter.
The options
argument is optional for GCM
. When using CCM
, the
plaintextLength
option must be specified and its value must match the length
of the ciphertext in bytes. See CCM mode.
The decipher.setAAD()
method must be called before decipher.update()
.
When passing a string as the buffer
, please consider
caveats when using strings as inputs to cryptographic APIs.
decipher.setAuthTag(buffer[, encoding])
#
buffer
<string> | <Buffer> | <ArrayBuffer> | <TypedArray> | <DataView>encoding
<string> String encoding to use whenbuffer
is a string.- Returns: <Decipher> for method chaining.
When using an authenticated encryption mode (GCM
, CCM
, OCB
, and
chacha20-poly1305
are
currently supported), the decipher.setAuthTag()
method is used to pass in the
received authentication tag. If no tag is provided, or if the cipher text
has been tampered with, decipher.final()
will throw, indicating that the
cipher text should be discarded due to failed authentication. If the tag length
is invalid according to NIST SP 800-38D or does not match the value of the
authTagLength
option, decipher.setAuthTag()
will throw an error.
The decipher.setAuthTag()
method must be called before decipher.update()
for CCM
mode or before decipher.final()
for GCM
and OCB
modes and
chacha20-poly1305
.
decipher.setAuthTag()
can only be called once.
When passing a string as the authentication tag, please consider caveats when using strings as inputs to cryptographic APIs.
decipher.setAutoPadding([autoPadding])
#
autoPadding
<boolean> Default:true
- Returns: <Decipher> for method chaining.
When data has been encrypted without standard block padding, calling
decipher.setAutoPadding(false)
will disable automatic padding to prevent
decipher.final()
from checking for and removing padding.
Turning auto padding off will only work if the input data's length is a multiple of the ciphers block size.
The decipher.setAutoPadding()
method must be called before
decipher.final()
.
decipher.update(data[, inputEncoding][, outputEncoding])
#
data
<string> | <Buffer> | <TypedArray> | <DataView>inputEncoding
<string> The encoding of thedata
string.outputEncoding
<string> The encoding of the return value.- Returns: <Buffer> | <string>
Updates the decipher with data
. If the inputEncoding
argument is given,
the data
argument is a string using the specified encoding. If the inputEncoding
argument is not given, data
must be a Buffer
. If data
is a
Buffer
then inputEncoding
is ignored.
The outputEncoding
specifies the output format of the enciphered
data. If the outputEncoding
is specified, a string using the specified encoding is returned. If no
outputEncoding
is provided, a Buffer
is returned.
The decipher.update()
method can be called multiple times with new data until
decipher.final()
is called. Calling decipher.update()
after
decipher.final()
will result in an error being thrown.
Class: DiffieHellman
#
The DiffieHellman
class is a utility for creating Diffie-Hellman key
exchanges.
Instances of the DiffieHellman
class can be created using the
crypto.createDiffieHellman()
function.
import assert from 'node:assert';
const {
createDiffieHellman,
} = await import('node:crypto');
// Generate Alice's keys...
const alice = createDiffieHellman(2048);
const aliceKey = alice.generateKeys();
// Generate Bob's keys...
const bob = createDiffieHellman(alice.getPrime(), alice.getGenerator());
const bobKey = bob.generateKeys();
// Exchange and generate the secret...
const aliceSecret = alice.computeSecret(bobKey);
const bobSecret = bob.computeSecret(aliceKey);
// OK
assert.strictEqual(aliceSecret.toString('hex'), bobSecret.toString('hex'));
const assert = require('node:assert');
const {
createDiffieHellman,
} = require('node:crypto');
// Generate Alice's keys...
const alice = createDiffieHellman(2048);
const aliceKey = alice.generateKeys();
// Generate Bob's keys...
const bob = createDiffieHellman(alice.getPrime(), alice.getGenerator());
const bobKey = bob.generateKeys();
// Exchange and generate the secret...
const aliceSecret = alice.computeSecret(bobKey);
const bobSecret = bob.computeSecret(aliceKey);
// OK
assert.strictEqual(aliceSecret.toString('hex'), bobSecret.toString('hex'));
diffieHellman.computeSecret(otherPublicKey[, inputEncoding][, outputEncoding])
#
otherPublicKey
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>inputEncoding
<string> The encoding of anotherPublicKey
string.outputEncoding
<string> The encoding of the return value.- Returns: <Buffer> | <string>
Computes the shared secret using otherPublicKey
as the other
party's public key and returns the computed shared secret. The supplied
key is interpreted using the specified inputEncoding
, and secret is
encoded using specified outputEncoding
.
If the inputEncoding
is not
provided, otherPublicKey
is expected to be a Buffer
,
TypedArray
, or DataView
.
If outputEncoding
is given a string is returned; otherwise, a
Buffer
is returned.
diffieHellman.generateKeys([encoding])
#
Generates private and public Diffie-Hellman key values, and returns
the public key in the specified encoding
. This key should be
transferred to the other party.
If encoding
is provided a string is returned; otherwise a
Buffer
is returned.
diffieHellman.getGenerator([encoding])
#
Returns the Diffie-Hellman generator in the specified encoding
.
If encoding
is provided a string is
returned; otherwise a Buffer
is returned.
diffieHellman.getPrime([encoding])
#
Returns the Diffie-Hellman prime in the specified encoding
.
If encoding
is provided a string is
returned; otherwise a Buffer
is returned.
diffieHellman.getPrivateKey([encoding])
#
Returns the Diffie-Hellman private key in the specified encoding
.
If encoding
is provided a
string is returned; otherwise a Buffer
is returned.
diffieHellman.getPublicKey([encoding])
#
Returns the Diffie-Hellman public key in the specified encoding
.
If encoding
is provided a
string is returned; otherwise a Buffer
is returned.
diffieHellman.setPrivateKey(privateKey[, encoding])
#
privateKey
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>encoding
<string> The encoding of theprivateKey
string.
Sets the Diffie-Hellman private key. If the encoding
argument is provided,
privateKey
is expected
to be a string. If no encoding
is provided, privateKey
is expected
to be a Buffer
, TypedArray
, or DataView
.
diffieHellman.setPublicKey(publicKey[, encoding])
#
publicKey
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>encoding
<string> The encoding of thepublicKey
string.
Sets the Diffie-Hellman public key. If the encoding
argument is provided,
publicKey
is expected
to be a string. If no encoding
is provided, publicKey
is expected
to be a Buffer
, TypedArray
, or DataView
.
diffieHellman.verifyError
#
A bit field containing any warnings and/or errors resulting from a check
performed during initialization of the DiffieHellman
object.
The following values are valid for this property (as defined in node:constants
module):
DH_CHECK_P_NOT_SAFE_PRIME
DH_CHECK_P_NOT_PRIME
DH_UNABLE_TO_CHECK_GENERATOR
DH_NOT_SUITABLE_GENERATOR
Class: DiffieHellmanGroup
#
The DiffieHellmanGroup
class takes a well-known modp group as its argument.
It works the same as DiffieHellman
, except that it does not allow changing
its keys after creation. In other words, it does not implement setPublicKey()
or setPrivateKey()
methods.
const { createDiffieHellmanGroup } = await import('node:crypto');
const dh = createDiffieHellmanGroup('modp16');
const { createDiffieHellmanGroup } = require('node:crypto');
const dh = createDiffieHellmanGroup('modp16');
The following groups are supported:
'modp14'
(2048 bits, RFC 3526 Section 3)'modp15'
(3072 bits, RFC 3526 Section 4)'modp16'
(4096 bits, RFC 3526 Section 5)'modp17'
(6144 bits, RFC 3526 Section 6)'modp18'
(8192 bits, RFC 3526 Section 7)
The following groups are still supported but deprecated (see Caveats):
'modp1'
(768 bits, RFC 2409 Section 6.1)'modp2'
(1024 bits, RFC 2409 Section 6.2)'modp5'
(1536 bits, RFC 3526 Section 2)
These deprecated groups might be removed in future versions of Node.js.
Class: ECDH
#
The ECDH
class is a utility for creating Elliptic Curve Diffie-Hellman (ECDH)
key exchanges.
Instances of the ECDH
class can be created using the
crypto.createECDH()
function.
import assert from 'node:assert';
const {
createECDH,
} = await import('node:crypto');
// Generate Alice's keys...
const alice = createECDH('secp521r1');
const aliceKey = alice.generateKeys();
// Generate Bob's keys...
const bob = createECDH('secp521r1');
const bobKey = bob.generateKeys();
// Exchange and generate the secret...
const aliceSecret = alice.computeSecret(bobKey);
const bobSecret = bob.computeSecret(aliceKey);
assert.strictEqual(aliceSecret.toString('hex'), bobSecret.toString('hex'));
// OK
const assert = require('node:assert');
const {
createECDH,
} = require('node:crypto');
// Generate Alice's keys...
const alice = createECDH('secp521r1');
const aliceKey = alice.generateKeys();
// Generate Bob's keys...
const bob = createECDH('secp521r1');
const bobKey = bob.generateKeys();
// Exchange and generate the secret...
const aliceSecret = alice.computeSecret(bobKey);
const bobSecret = bob.computeSecret(aliceKey);
assert.strictEqual(aliceSecret.toString('hex'), bobSecret.toString('hex'));
// OK
Static method: ECDH.convertKey(key, curve[, inputEncoding[, outputEncoding[, format]]])
#
key
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>curve
<string>inputEncoding
<string> The encoding of thekey
string.outputEncoding
<string> The encoding of the return value.format
<string> Default:'uncompressed'
- Returns: <Buffer> | <string>
Converts the EC Diffie-Hellman public key specified by key
and curve
to the
format specified by format
. The format
argument specifies point encoding
and can be 'compressed'
, 'uncompressed'
or 'hybrid'
. The supplied key is
interpreted using the specified inputEncoding
, and the returned key is encoded
using the specified outputEncoding
.
Use crypto.getCurves()
to obtain a list of available curve names.
On recent OpenSSL releases, openssl ecparam -list_curves
will also display
the name and description of each available elliptic curve.
If format
is not specified the point will be returned in 'uncompressed'
format.
If the inputEncoding
is not provided, key
is expected to be a Buffer
,
TypedArray
, or DataView
.
Example (uncompressing a key):
const {
createECDH,
ECDH,
} = await import('node:crypto');
const ecdh = createECDH('secp256k1');
ecdh.generateKeys();
const compressedKey = ecdh.getPublicKey('hex', 'compressed');
const uncompressedKey = ECDH.convertKey(compressedKey,
'secp256k1',
'hex',
'hex',
'uncompressed');
// The converted key and the uncompressed public key should be the same
console.log(uncompressedKey === ecdh.getPublicKey('hex'));
const {
createECDH,
ECDH,
} = require('node:crypto');
const ecdh = createECDH('secp256k1');
ecdh.generateKeys();
const compressedKey = ecdh.getPublicKey('hex', 'compressed');
const uncompressedKey = ECDH.convertKey(compressedKey,
'secp256k1',
'hex',
'hex',
'uncompressed');
// The converted key and the uncompressed public key should be the same
console.log(uncompressedKey === ecdh.getPublicKey('hex'));
ecdh.computeSecret(otherPublicKey[, inputEncoding][, outputEncoding])
#
otherPublicKey
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>inputEncoding
<string> The encoding of theotherPublicKey
string.outputEncoding
<string> The encoding of the return value.- Returns: <Buffer> | <string>
Computes the shared secret using otherPublicKey
as the other
party's public key and returns the computed shared secret. The supplied
key is interpreted using specified inputEncoding
, and the returned secret
is encoded using the specified outputEncoding
.
If the inputEncoding
is not
provided, otherPublicKey
is expected to be a Buffer
, TypedArray
, or
DataView
.
If outputEncoding
is given a string will be returned; otherwise a
Buffer
is returned.
ecdh.computeSecret
will throw an
ERR_CRYPTO_ECDH_INVALID_PUBLIC_KEY
error when otherPublicKey
lies outside of the elliptic curve. Since otherPublicKey
is
usually supplied from a remote user over an insecure network,
be sure to handle this exception accordingly.
ecdh.generateKeys([encoding[, format]])
#
encoding
<string> The encoding of the return value.format
<string> Default:'uncompressed'
- Returns: <Buffer> | <string>
Generates private and public EC Diffie-Hellman key values, and returns
the public key in the specified format
and encoding
. This key should be
transferred to the other party.
The format
argument specifies point encoding and can be 'compressed'
or
'uncompressed'
. If format
is not specified, the point will be returned in
'uncompressed'
format.
If encoding
is provided a string is returned; otherwise a Buffer
is returned.
ecdh.getPrivateKey([encoding])
#
encoding
<string> The encoding of the return value.- Returns: <Buffer> | <string> The EC Diffie-Hellman in the specified
encoding
.
If encoding
is specified, a string is returned; otherwise a Buffer
is
returned.
ecdh.getPublicKey([encoding][, format])
#
encoding
<string> The encoding of the return value.format
<string> Default:'uncompressed'
- Returns: <Buffer> | <string> The EC Diffie-Hellman public key in the specified
encoding
andformat
.
The format
argument specifies point encoding and can be 'compressed'
or
'uncompressed'
. If format
is not specified the point will be returned in
'uncompressed'
format.
If encoding
is specified, a string is returned; otherwise a Buffer
is
returned.
ecdh.setPrivateKey(privateKey[, encoding])
#
privateKey
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>encoding
<string> The encoding of theprivateKey
string.
Sets the EC Diffie-Hellman private key.
If encoding
is provided, privateKey
is expected
to be a string; otherwise privateKey
is expected to be a Buffer
,
TypedArray
, or DataView
.
If privateKey
is not valid for the curve specified when the ECDH
object was
created, an error is thrown. Upon setting the private key, the associated
public point (key) is also generated and set in the ECDH
object.
ecdh.setPublicKey(publicKey[, encoding])
#
publicKey
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>encoding
<string> The encoding of thepublicKey
string.
Sets the EC Diffie-Hellman public key.
If encoding
is provided publicKey
is expected to
be a string; otherwise a Buffer
, TypedArray
, or DataView
is expected.
There is not normally a reason to call this method because ECDH
only requires a private key and the other party's public key to compute the
shared secret. Typically either ecdh.generateKeys()
or
ecdh.setPrivateKey()
will be called. The ecdh.setPrivateKey()
method
attempts to generate the public point/key associated with the private key being
set.
Example (obtaining a shared secret):
const {
createECDH,
createHash,
} = await import('node:crypto');
const alice = createECDH('secp256k1');
const bob = createECDH('secp256k1');
// This is a shortcut way of specifying one of Alice's previous private
// keys. It would be unwise to use such a predictable private key in a real
// application.
alice.setPrivateKey(
createHash('sha256').update('alice', 'utf8').digest(),
);
// Bob uses a newly generated cryptographically strong
// pseudorandom key pair
bob.generateKeys();
const aliceSecret = alice.computeSecret(bob.getPublicKey(), null, 'hex');
const bobSecret = bob.computeSecret(alice.getPublicKey(), null, 'hex');
// aliceSecret and bobSecret should be the same shared secret value
console.log(aliceSecret === bobSecret);
const {
createECDH,
createHash,
} = require('node:crypto');
const alice = createECDH('secp256k1');
const bob = createECDH('secp256k1');
// This is a shortcut way of specifying one of Alice's previous private
// keys. It would be unwise to use such a predictable private key in a real
// application.
alice.setPrivateKey(
createHash('sha256').update('alice', 'utf8').digest(),
);
// Bob uses a newly generated cryptographically strong
// pseudorandom key pair
bob.generateKeys();
const aliceSecret = alice.computeSecret(bob.getPublicKey(), null, 'hex');
const bobSecret = bob.computeSecret(alice.getPublicKey(), null, 'hex');
// aliceSecret and bobSecret should be the same shared secret value
console.log(aliceSecret === bobSecret);
Class: Hash
#
- Extends: <stream.Transform>
The Hash
class is a utility for creating hash digests of data. It can be
used in one of two ways:
- As a stream that is both readable and writable, where data is written to produce a computed hash digest on the readable side, or
- Using the
hash.update()
andhash.digest()
methods to produce the computed hash.
The crypto.createHash()
method is used to create Hash
instances. Hash
objects are not to be created directly using the new
keyword.
Example: Using Hash
objects as streams:
const {
createHash,
} = await import('node:crypto');
const hash = createHash('sha256');
hash.on('readable', () => {
// Only one element is going to be produced by the
// hash stream.
const data = hash.read();
if (data) {
console.log(data.toString('hex'));
// Prints:
// 6a2da20943931e9834fc12cfe5bb47bbd9ae43489a30726962b576f4e3993e50
}
});
hash.write('some data to hash');
hash.end();
const {
createHash,
} = require('node:crypto');
const hash = createHash('sha256');
hash.on('readable', () => {
// Only one element is going to be produced by the
// hash stream.
const data = hash.read();
if (data) {
console.log(data.toString('hex'));
// Prints:
// 6a2da20943931e9834fc12cfe5bb47bbd9ae43489a30726962b576f4e3993e50
}
});
hash.write('some data to hash');
hash.end();
Example: Using Hash
and piped streams:
import { createReadStream } from 'node:fs';
import { stdout } from 'node:process';
const { createHash } = await import('node:crypto');
const hash = createHash('sha256');
const input = createReadStream('test.js');
input.pipe(hash).setEncoding('hex').pipe(stdout);
const { createReadStream } = require('node:fs');
const { createHash } = require('node:crypto');
const { stdout } = require('node:process');
const hash = createHash('sha256');
const input = createReadStream('test.js');
input.pipe(hash).setEncoding('hex').pipe(stdout);
Example: Using the hash.update()
and hash.digest()
methods:
const {
createHash,
} = await import('node:crypto');
const hash = createHash('sha256');
hash.update('some data to hash');
console.log(hash.digest('hex'));
// Prints:
// 6a2da20943931e9834fc12cfe5bb47bbd9ae43489a30726962b576f4e3993e50
const {
createHash,
} = require('node:crypto');
const hash = createHash('sha256');
hash.update('some data to hash');
console.log(hash.digest('hex'));
// Prints:
// 6a2da20943931e9834fc12cfe5bb47bbd9ae43489a30726962b576f4e3993e50
hash.copy([options])
#
options
<Object>stream.transform
options- Returns: <Hash>
Creates a new Hash
object that contains a deep copy of the internal state
of the current Hash
object.
The optional options
argument controls stream behavior. For XOF hash
functions such as 'shake256'
, the outputLength
option can be used to
specify the desired output length in bytes.
An error is thrown when an attempt is made to copy the Hash
object after
its hash.digest()
method has been called.
// Calculate a rolling hash.
const {
createHash,
} = await import('node:crypto');
const hash = createHash('sha256');
hash.update('one');
console.log(hash.copy().digest('hex'));
hash.update('two');
console.log(hash.copy().digest('hex'));
hash.update('three');
console.log(hash.copy().digest('hex'));
// Etc.
// Calculate a rolling hash.
const {
createHash,
} = require('node:crypto');
const hash = createHash('sha256');
hash.update('one');
console.log(hash.copy().digest('hex'));
hash.update('two');
console.log(hash.copy().digest('hex'));
hash.update('three');
console.log(hash.copy().digest('hex'));
// Etc.
hash.digest([encoding])
#
Calculates the digest of all of the data passed to be hashed (using the
hash.update()
method).
If encoding
is provided a string will be returned; otherwise
a Buffer
is returned.
The Hash
object can not be used again after hash.digest()
method has been
called. Multiple calls will cause an error to be thrown.
hash.update(data[, inputEncoding])
#
data
<string> | <Buffer> | <TypedArray> | <DataView>inputEncoding
<string> The encoding of thedata
string.
Updates the hash content with the given data
, the encoding of which
is given in inputEncoding
.
If encoding
is not provided, and the data
is a string, an
encoding of 'utf8'
is enforced. If data
is a Buffer
, TypedArray
, or
DataView
, then inputEncoding
is ignored.
This can be called many times with new data as it is streamed.
Class: Hmac
#
- Extends: <stream.Transform>
The Hmac
class is a utility for creating cryptographic HMAC digests. It can
be used in one of two ways:
- As a stream that is both readable and writable, where data is written to produce a computed HMAC digest on the readable side, or
- Using the
hmac.update()
andhmac.digest()
methods to produce the computed HMAC digest.
The crypto.createHmac()
method is used to create Hmac
instances. Hmac
objects are not to be created directly using the new
keyword.
Example: Using Hmac
objects as streams:
const {
createHmac,
} = await import('node:crypto');
const hmac = createHmac('sha256', 'a secret');
hmac.on('readable', () => {
// Only one element is going to be produced by the
// hash stream.
const data = hmac.read();
if (data) {
console.log(data.toString('hex'));
// Prints:
// 7fd04df92f636fd450bc841c9418e5825c17f33ad9c87c518115a45971f7f77e
}
});
hmac.write('some data to hash');
hmac.end();
const {
createHmac,
} = require('node:crypto');
const hmac = createHmac('sha256', 'a secret');
hmac.on('readable', () => {
// Only one element is going to be produced by the
// hash stream.
const data = hmac.read();
if (data) {
console.log(data.toString('hex'));
// Prints:
// 7fd04df92f636fd450bc841c9418e5825c17f33ad9c87c518115a45971f7f77e
}
});
hmac.write('some data to hash');
hmac.end();
Example: Using Hmac
and piped streams:
import { createReadStream } from 'node:fs';
import { stdout } from 'node:process';
const {
createHmac,
} = await import('node:crypto');
const hmac = createHmac('sha256', 'a secret');
const input = createReadStream('test.js');
input.pipe(hmac).pipe(stdout);
const {
createReadStream,
} = require('node:fs');
const {
createHmac,
} = require('node:crypto');
const { stdout } = require('node:process');
const hmac = createHmac('sha256', 'a secret');
const input = createReadStream('test.js');
input.pipe(hmac).pipe(stdout);
Example: Using the hmac.update()
and hmac.digest()
methods:
const {
createHmac,
} = await import('node:crypto');
const hmac = createHmac('sha256', 'a secret');
hmac.update('some data to hash');
console.log(hmac.digest('hex'));
// Prints:
// 7fd04df92f636fd450bc841c9418e5825c17f33ad9c87c518115a45971f7f77e
const {
createHmac,
} = require('node:crypto');
const hmac = createHmac('sha256', 'a secret');
hmac.update('some data to hash');
console.log(hmac.digest('hex'));
// Prints:
// 7fd04df92f636fd450bc841c9418e5825c17f33ad9c87c518115a45971f7f77e
hmac.digest([encoding])
#
Calculates the HMAC digest of all of the data passed using hmac.update()
.
If encoding
is
provided a string is returned; otherwise a Buffer
is returned;
The Hmac
object can not be used again after hmac.digest()
has been
called. Multiple calls to hmac.digest()
will result in an error being thrown.
hmac.update(data[, inputEncoding])
#
data
<string> | <Buffer> | <TypedArray> | <DataView>inputEncoding
<string> The encoding of thedata
string.
Updates the Hmac
content with the given data
, the encoding of which
is given in inputEncoding
.
If encoding
is not provided, and the data
is a string, an
encoding of 'utf8'
is enforced. If data
is a Buffer
, TypedArray
, or
DataView
, then inputEncoding
is ignored.
This can be called many times with new data as it is streamed.
Class: KeyObject
#
Node.js uses a KeyObject
class to represent a symmetric or asymmetric key,
and each kind of key exposes different functions. The
crypto.createSecretKey()
, crypto.createPublicKey()
and
crypto.createPrivateKey()
methods are used to create KeyObject
instances. KeyObject
objects are not to be created directly using the new
keyword.
Most applications should consider using the new KeyObject
API instead of
passing keys as strings or Buffer
s due to improved security features.
KeyObject
instances can be passed to other threads via postMessage()
.
The receiver obtains a cloned KeyObject
, and the KeyObject
does not need to
be listed in the transferList
argument.
Static method: KeyObject.from(key)
#
key
<CryptoKey>- Returns: <KeyObject>
Example: Converting a CryptoKey
instance to a KeyObject
:
const { KeyObject } = await import('node:crypto');
const { subtle } = globalThis.crypto;
const key = await subtle.generateKey({
name: 'HMAC',
hash: 'SHA-256',
length: 256,
}, true, ['sign', 'verify']);
const keyObject = KeyObject.from(key);
console.log(keyObject.symmetricKeySize);
// Prints: 32 (symmetric key size in bytes)
const { KeyObject } = require('node:crypto');
const { subtle } = globalThis.crypto;
(async function() {
const key = await subtle.generateKey({
name: 'HMAC',
hash: 'SHA-256',
length: 256,
}, true, ['sign', 'verify']);
const keyObject = KeyObject.from(key);
console.log(keyObject.symmetricKeySize);
// Prints: 32 (symmetric key size in bytes)
})();
keyObject.asymmetricKeyDetails
#
- <Object>
modulusLength
: <number> Key size in bits (RSA, DSA).publicExponent
: <bigint> Public exponent (RSA).hashAlgorithm
: <string> Name of the message digest (RSA-PSS).mgf1HashAlgorithm
: <string> Name of the message digest used by MGF1 (RSA-PSS).saltLength
: <number> Minimal salt length in bytes (RSA-PSS).divisorLength
: <number> Size ofq
in bits (DSA).namedCurve
: <string> Name of the curve (EC).
This property exists only on asymmetric keys. Depending on the type of the key, this object contains information about the key. None of the information obtained through this property can be used to uniquely identify a key or to compromise the security of the key.
For RSA-PSS keys, if the key material contains a RSASSA-PSS-params
sequence,
the hashAlgorithm
, mgf1HashAlgorithm
, and saltLength
properties will be
set.
Other key details might be exposed via this API using additional attributes.
keyObject.asymmetricKeyType
#
For asymmetric keys, this property represents the type of the key. Supported key types are:
'rsa'
(OID 1.2.840.113549.1.1.1)'rsa-pss'
(OID 1.2.840.113549.1.1.10)'dsa'
(OID 1.2.840.10040.4.1)'ec'
(OID 1.2.840.10045.2.1)'x25519'
(OID 1.3.101.110)'x448'
(OID 1.3.101.111)'ed25519'
(OID 1.3.101.112)'ed448'
(OID 1.3.101.113)'dh'
(OID 1.2.840.113549.1.3.1)
This property is undefined
for unrecognized KeyObject
types and symmetric
keys.
keyObject.export([options])
#
For symmetric keys, the following encoding options can be used:
format
: <string> Must be'buffer'
(default) or'jwk'
.
For public keys, the following encoding options can be used:
type
: <string> Must be one of'pkcs1'
(RSA only) or'spki'
.format
: <string> Must be'pem'
,'der'
, or'jwk'
.
For private keys, the following encoding options can be used:
type
: <string> Must be one of'pkcs1'
(RSA only),'pkcs8'
or'sec1'
(EC only).format
: <string> Must be'pem'
,'der'
, or'jwk'
.cipher
: <string> If specified, the private key will be encrypted with the givencipher
andpassphrase
using PKCS#5 v2.0 password based encryption.passphrase
: <string> | <Buffer> The passphrase to use for encryption, seecipher
.
The result type depends on the selected encoding format, when PEM the result is a string, when DER it will be a buffer containing the data encoded as DER, when JWK it will be an object.
When JWK encoding format was selected, all other encoding options are ignored.
PKCS#1, SEC1, and PKCS#8 type keys can be encrypted by using a combination of
the cipher
and format
options. The PKCS#8 type
can be used with any
format
to encrypt any key algorithm (RSA, EC, or DH) by specifying a
cipher
. PKCS#1 and SEC1 can only be encrypted by specifying a cipher
when the PEM format
is used. For maximum compatibility, use PKCS#8 for
encrypted private keys. Since PKCS#8 defines its own
encryption mechanism, PEM-level encryption is not supported when encrypting
a PKCS#8 key. See RFC 5208 for PKCS#8 encryption and RFC 1421 for
PKCS#1 and SEC1 encryption.
keyObject.equals(otherKeyObject)
#
otherKeyObject
: <KeyObject> AKeyObject
with which to comparekeyObject
.- Returns: <boolean>
Returns true
or false
depending on whether the keys have exactly the same
type, value, and parameters. This method is not
constant time.
keyObject.symmetricKeySize
#
For secret keys, this property represents the size of the key in bytes. This
property is undefined
for asymmetric keys.
keyObject.type
#
Depending on the type of this KeyObject
, this property is either
'secret'
for secret (symmetric) keys, 'public'
for public (asymmetric) keys
or 'private'
for private (asymmetric) keys.
Class: Sign
#
- Extends: <stream.Writable>
The Sign
class is a utility for generating signatures. It can be used in one
of two ways:
- As a writable stream, where data to be signed is written and the
sign.sign()
method is used to generate and return the signature, or - Using the
sign.update()
andsign.sign()
methods to produce the signature.
The crypto.createSign()
method is used to create Sign
instances. The
argument is the string name of the hash function to use. Sign
objects are not
to be created directly using the new
keyword.
Example: Using Sign
and Verify
objects as streams:
const {
generateKeyPairSync,
createSign,
createVerify,
} = await import('node:crypto');
const { privateKey, publicKey } = generateKeyPairSync('ec', {
namedCurve: 'sect239k1',
});
const sign = createSign('SHA256');
sign.write('some data to sign');
sign.end();
const signature = sign.sign(privateKey, 'hex');
const verify = createVerify('SHA256');
verify.write('some data to sign');
verify.end();
console.log(verify.verify(publicKey, signature, 'hex'));
// Prints: true
const {
generateKeyPairSync,
createSign,
createVerify,
} = require('node:crypto');
const { privateKey, publicKey } = generateKeyPairSync('ec', {
namedCurve: 'sect239k1',
});
const sign = createSign('SHA256');
sign.write('some data to sign');
sign.end();
const signature = sign.sign(privateKey, 'hex');
const verify = createVerify('SHA256');
verify.write('some data to sign');
verify.end();
console.log(verify.verify(publicKey, signature, 'hex'));
// Prints: true
Example: Using the sign.update()
and verify.update()
methods:
const {
generateKeyPairSync,
createSign,
createVerify,
} = await import('node:crypto');
const { privateKey, publicKey } = generateKeyPairSync('rsa', {
modulusLength: 2048,
});
const sign = createSign('SHA256');
sign.update('some data to sign');
sign.end();
const signature = sign.sign(privateKey);
const verify = createVerify('SHA256');
verify.update('some data to sign');
verify.end();
console.log(verify.verify(publicKey, signature));
// Prints: true
const {
generateKeyPairSync,
createSign,
createVerify,
} = require('node:crypto');
const { privateKey, publicKey } = generateKeyPairSync('rsa', {
modulusLength: 2048,
});
const sign = createSign('SHA256');
sign.update('some data to sign');
sign.end();
const signature = sign.sign(privateKey);
const verify = createVerify('SHA256');
verify.update('some data to sign');
verify.end();
console.log(verify.verify(publicKey, signature));
// Prints: true
sign.sign(privateKey[, outputEncoding])
#
privateKey
<Object> | <string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> | <KeyObject> | <CryptoKey>outputEncoding
<string> The encoding of the return value.- Returns: <Buffer> | <string>
Calculates the signature on all the data passed through using either
sign.update()
or sign.write()
.
If privateKey
is not a KeyObject
, this function behaves as if
privateKey
had been passed to crypto.createPrivateKey()
. If it is an
object, the following additional properties can be passed:
-
dsaEncoding
<string> For DSA and ECDSA, this option specifies the format of the generated signature. It can be one of the following:'der'
(default): DER-encoded ASN.1 signature structure encoding(r, s)
.'ieee-p1363'
: Signature formatr || s
as proposed in IEEE-P1363.
-
padding
<integer> Optional padding value for RSA, one of the following:crypto.constants.RSA_PKCS1_PADDING
(default)crypto.constants.RSA_PKCS1_PSS_PADDING
RSA_PKCS1_PSS_PADDING
will use MGF1 with the same hash function used to sign the message as specified in section 3.1 of RFC 4055, unless an MGF1 hash function has been specified as part of the key in compliance with section 3.3 of RFC 4055. -
saltLength
<integer> Salt length for when padding isRSA_PKCS1_PSS_PADDING
. The special valuecrypto.constants.RSA_PSS_SALTLEN_DIGEST
sets the salt length to the digest size,crypto.constants.RSA_PSS_SALTLEN_MAX_SIGN
(default) sets it to the maximum permissible value.
If outputEncoding
is provided a string is returned; otherwise a Buffer
is returned.
The Sign
object can not be again used after sign.sign()
method has been
called. Multiple calls to sign.sign()
will result in an error being thrown.
sign.update(data[, inputEncoding])
#
data
<string> | <Buffer> | <TypedArray> | <DataView>inputEncoding
<string> The encoding of thedata
string.
Updates the Sign
content with the given data
, the encoding of which
is given in inputEncoding
.
If encoding
is not provided, and the data
is a string, an
encoding of 'utf8'
is enforced. If data
is a Buffer
, TypedArray
, or
DataView
, then inputEncoding
is ignored.
This can be called many times with new data as it is streamed.
Class: Verify
#
- Extends: <stream.Writable>
The Verify
class is a utility for verifying signatures. It can be used in one
of two ways:
- As a writable stream where written data is used to validate against the supplied signature, or
- Using the
verify.update()
andverify.verify()
methods to verify the signature.
The crypto.createVerify()
method is used to create Verify
instances.
Verify
objects are not to be created directly using the new
keyword.
See Sign
for examples.
verify.update(data[, inputEncoding])
#
data
<string> | <Buffer> | <TypedArray> | <DataView>inputEncoding
<string> The encoding of thedata
string.
Updates the Verify
content with the given data
, the encoding of which
is given in inputEncoding
.
If inputEncoding
is not provided, and the data
is a string, an
encoding of 'utf8'
is enforced. If data
is a Buffer
, TypedArray
, or
DataView
, then inputEncoding
is ignored.
This can be called many times with new data as it is streamed.
verify.verify(object, signature[, signatureEncoding])
#
object
<Object> | <string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> | <KeyObject> | <CryptoKey>signature
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>signatureEncoding
<string> The encoding of thesignature
string.- Returns: <boolean>
true
orfalse
depending on the validity of the signature for the data and public key.
Verifies the provided data using the given object
and signature
.
If object
is not a KeyObject
, this function behaves as if
object
had been passed to crypto.createPublicKey()
. If it is an
object, the following additional properties can be passed:
-
dsaEncoding
<string> For DSA and ECDSA, this option specifies the format of the signature. It can be one of the following:'der'
(default): DER-encoded ASN.1 signature structure encoding(r, s)
.'ieee-p1363'
: Signature formatr || s
as proposed in IEEE-P1363.
-
padding
<integer> Optional padding value for RSA, one of the following:crypto.constants.RSA_PKCS1_PADDING
(default)crypto.constants.RSA_PKCS1_PSS_PADDING
RSA_PKCS1_PSS_PADDING
will use MGF1 with the same hash function used to verify the message as specified in section 3.1 of RFC 4055, unless an MGF1 hash function has been specified as part of the key in compliance with section 3.3 of RFC 4055. -
saltLength
<integer> Salt length for when padding isRSA_PKCS1_PSS_PADDING
. The special valuecrypto.constants.RSA_PSS_SALTLEN_DIGEST
sets the salt length to the digest size,crypto.constants.RSA_PSS_SALTLEN_AUTO
(default) causes it to be determined automatically.
The signature
argument is the previously calculated signature for the data, in
the signatureEncoding
.
If a signatureEncoding
is specified, the signature
is expected to be a
string; otherwise signature
is expected to be a Buffer
,
TypedArray
, or DataView
.
The verify
object can not be used again after verify.verify()
has been
called. Multiple calls to verify.verify()
will result in an error being
thrown.
Because public keys can be derived from private keys, a private key may be passed instead of a public key.
Class: X509Certificate
#
Encapsulates an X509 certificate and provides read-only access to its information.
const { X509Certificate } = await import('node:crypto');
const x509 = new X509Certificate('{... pem encoded cert ...}');
console.log(x509.subject);
const { X509Certificate } = require('node:crypto');
const x509 = new X509Certificate('{... pem encoded cert ...}');
console.log(x509.subject);
new X509Certificate(buffer)
#
buffer
<string> | <TypedArray> | <Buffer> | <DataView> A PEM or DER encoded X509 Certificate.
x509.ca
#
- Type: <boolean> Will be
true
if this is a Certificate Authority (CA) certificate.
x509.checkEmail(email[, options])
#
email
<string>options
<Object>subject
<string>'default'
,'always'
, or'never'
. Default:'default'
.
- Returns: <string> | <undefined> Returns
email
if the certificate matches,undefined
if it does not.
Checks whether the certificate matches the given email address.
If the 'subject'
option is undefined or set to 'default'
, the certificate
subject is only considered if the subject alternative name extension either does
not exist or does not contain any email addresses.
If the 'subject'
option is set to 'always'
and if the subject alternative
name extension either does not exist or does not contain a matching email
address, the certificate subject is considered.
If the 'subject'
option is set to 'never'
, the certificate subject is never
considered, even if the certificate contains no subject alternative names.
x509.checkHost(name[, options])
#
name
<string>options
<Object>- Returns: <string> | <undefined> Returns a subject name that matches
name
, orundefined
if no subject name matchesname
.
Checks whether the certificate matches the given host name.
If the certificate matches the given host name, the matching subject name is
returned. The returned name might be an exact match (e.g., foo.example.com
)
or it might contain wildcards (e.g., *.example.com
). Because host name
comparisons are case-insensitive, the returned subject name might also differ
from the given name
in capitalization.
If the 'subject'
option is undefined or set to 'default'
, the certificate
subject is only considered if the subject alternative name extension either does
not exist or does not contain any DNS names. This behavior is consistent with
RFC 2818 ("HTTP Over TLS").
If the 'subject'
option is set to 'always'
and if the subject alternative
name extension either does not exist or does not contain a matching DNS name,
the certificate subject is considered.
If the 'subject'
option is set to 'never'
, the certificate subject is never
considered, even if the certificate contains no subject alternative names.
x509.checkIP(ip)
#
ip
<string>- Returns: <string> | <undefined> Returns
ip
if the certificate matches,undefined
if it does not.
Checks whether the certificate matches the given IP address (IPv4 or IPv6).
Only RFC 5280 iPAddress
subject alternative names are considered, and they
must match the given ip
address exactly. Other subject alternative names as
well as the subject field of the certificate are ignored.
x509.checkIssued(otherCert)
#
otherCert
<X509Certificate>- Returns: <boolean>
Checks whether this certificate was issued by the given otherCert
.
x509.checkPrivateKey(privateKey)
#
privateKey
<KeyObject> A private key.- Returns: <boolean>
Checks whether the public key for this certificate is consistent with the given private key.
x509.fingerprint
#
- Type: <string>
The SHA-1 fingerprint of this certificate.
Because SHA-1 is cryptographically broken and because the security of SHA-1 is
significantly worse than that of algorithms that are commonly used to sign
certificates, consider using x509.fingerprint256
instead.
x509.fingerprint256
#
- Type: <string>
The SHA-256 fingerprint of this certificate.
x509.fingerprint512
#
- Type: <string>
The SHA-512 fingerprint of this certificate.
Because computing the SHA-256 fingerprint is usually faster and because it is
only half the size of the SHA-512 fingerprint, x509.fingerprint256
may be
a better choice. While SHA-512 presumably provides a higher level of security in
general, the security of SHA-256 matches that of most algorithms that are
commonly used to sign certificates.
x509.infoAccess
#
- Type: <string>
A textual representation of the certificate's authority information access extension.
This is a line feed separated list of access descriptions. Each line begins with the access method and the kind of the access location, followed by a colon and the value associated with the access location.
After the prefix denoting the access method and the kind of the access location, the remainder of each line might be enclosed in quotes to indicate that the value is a JSON string literal. For backward compatibility, Node.js only uses JSON string literals within this property when necessary to avoid ambiguity. Third-party code should be prepared to handle both possible entry formats.
x509.issuer
#
- Type: <string>
The issuer identification included in this certificate.
x509.issuerCertificate
#
- Type: <X509Certificate>
The issuer certificate or undefined
if the issuer certificate is not
available.
x509.keyUsage
#
- Type: <string[]>
An array detailing the key usages for this certificate.
x509.publicKey
#
- Type: <KeyObject>
The public key <KeyObject> for this certificate.
x509.raw
#
- Type: <Buffer>
A Buffer
containing the DER encoding of this certificate.
x509.serialNumber
#
- Type: <string>
The serial number of this certificate.
Serial numbers are assigned by certificate authorities and do not uniquely
identify certificates. Consider using x509.fingerprint256
as a unique
identifier instead.
x509.subject
#
- Type: <string>
The complete subject of this certificate.
x509.subjectAltName
#
- Type: <string>
The subject alternative name specified for this certificate.
This is a comma-separated list of subject alternative names. Each entry begins with a string identifying the kind of the subject alternative name followed by a colon and the value associated with the entry.
Earlier versions of Node.js incorrectly assumed that it is safe to split this
property at the two-character sequence ', '
(see CVE-2021-44532). However,
both malicious and legitimate certificates can contain subject alternative names
that include this sequence when represented as a string.
After the prefix denoting the type of the entry, the remainder of each entry might be enclosed in quotes to indicate that the value is a JSON string literal. For backward compatibility, Node.js only uses JSON string literals within this property when necessary to avoid ambiguity. Third-party code should be prepared to handle both possible entry formats.
x509.toJSON()
#
- Type: <string>
There is no standard JSON encoding for X509 certificates. The
toJSON()
method returns a string containing the PEM encoded
certificate.
x509.toLegacyObject()
#
- Type: <Object>
Returns information about this certificate using the legacy certificate object encoding.
x509.toString()
#
- Type: <string>
Returns the PEM-encoded certificate.
x509.validFrom
#
- Type: <string>
The date/time from which this certificate is considered valid.
x509.validTo
#
- Type: <string>
The date/time until which this certificate is considered valid.
x509.verify(publicKey)
#
publicKey
<KeyObject> A public key.- Returns: <boolean>
Verifies that this certificate was signed by the given public key. Does not perform any other validation checks on the certificate.
node:crypto
module methods and properties#
crypto.constants
#
An object containing commonly used constants for crypto and security related operations. The specific constants currently defined are described in Crypto constants.
crypto.fips
#
Property for checking and controlling whether a FIPS compliant crypto provider is currently in use. Setting to true requires a FIPS build of Node.js.
This property is deprecated. Please use crypto.setFips()
and
crypto.getFips()
instead.
crypto.checkPrime(candidate[, options], callback)
#
candidate
<ArrayBuffer> | <SharedArrayBuffer> | <TypedArray> | <Buffer> | <DataView> | <bigint> A possible prime encoded as a sequence of big endian octets of arbitrary length.options
<Object>checks
<number> The number of Miller-Rabin probabilistic primality iterations to perform. When the value is0
(zero), a number of checks is used that yields a false positive rate of at most 2-64 for random input. Care must be used when selecting a number of checks. Refer to the OpenSSL documentation for theBN_is_prime_ex
functionnchecks
options for more details. Default:0
callback
<Function>
Checks the primality of the candidate
.
crypto.checkPrimeSync(candidate[, options])
#
candidate
<ArrayBuffer> | <SharedArrayBuffer> | <TypedArray> | <Buffer> | <DataView> | <bigint> A possible prime encoded as a sequence of big endian octets of arbitrary length.options
<Object>checks
<number> The number of Miller-Rabin probabilistic primality iterations to perform. When the value is0
(zero), a number of checks is used that yields a false positive rate of at most 2-64 for random input. Care must be used when selecting a number of checks. Refer to the OpenSSL documentation for theBN_is_prime_ex
functionnchecks
options for more details. Default:0
- Returns: <boolean>
true
if the candidate is a prime with an error probability less than0.25 ** options.checks
.
Checks the primality of the candidate
.
crypto.createCipher(algorithm, password[, options])
#
crypto.createCipheriv()
instead.algorithm
<string>password
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>options
<Object>stream.transform
options- Returns: <Cipher>
Creates and returns a Cipher
object that uses the given algorithm
and
password
.
The options
argument controls stream behavior and is optional except when a
cipher in CCM or OCB mode (e.g. 'aes-128-ccm'
) is used. In that case, the
authTagLength
option is required and specifies the length of the
authentication tag in bytes, see CCM mode. In GCM mode, the authTagLength
option is not required but can be used to set the length of the authentication
tag that will be returned by getAuthTag()
and defaults to 16 bytes.
For chacha20-poly1305
, the authTagLength
option defaults to 16 bytes.
The algorithm
is dependent on OpenSSL, examples are 'aes192'
, etc. On
recent OpenSSL releases, openssl list -cipher-algorithms
will
display the available cipher algorithms.
The password
is used to derive the cipher key and initialization vector (IV).
The value must be either a 'latin1'
encoded string, a Buffer
, a
TypedArray
, or a DataView
.
This function is semantically insecure for all supported ciphers and fatally flawed for ciphers in counter mode (such as CTR, GCM, or CCM).
The implementation of crypto.createCipher()
derives keys using the OpenSSL
function EVP_BytesToKey
with the digest algorithm set to MD5, one
iteration, and no salt. The lack of salt allows dictionary attacks as the same
password always creates the same key. The low iteration count and
non-cryptographically secure hash algorithm allow passwords to be tested very
rapidly.
In line with OpenSSL's recommendation to use a more modern algorithm instead of
EVP_BytesToKey
it is recommended that developers derive a key and IV on
their own using crypto.scrypt()
and to use crypto.createCipheriv()
to create the Cipher
object. Users should not use ciphers with counter mode
(e.g. CTR, GCM, or CCM) in crypto.createCipher()
. A warning is emitted when
they are used in order to avoid the risk of IV reuse that causes
vulnerabilities. For the case when IV is reused in GCM, see Nonce-Disrespecting
Adversaries for details.
crypto.createCipheriv(algorithm, key, iv[, options])
#
algorithm
<string>key
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> | <KeyObject> | <CryptoKey>iv
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> | <null>options
<Object>stream.transform
options- Returns: <Cipher>
Creates and returns a Cipher
object, with the given algorithm
, key
and
initialization vector (iv
).
The options
argument controls stream behavior and is optional except when a
cipher in CCM or OCB mode (e.g. 'aes-128-ccm'
) is used. In that case, the
authTagLength
option is required and specifies the length of the
authentication tag in bytes, see CCM mode. In GCM mode, the authTagLength
option is not required but can be used to set the length of the authentication
tag that will be returned by getAuthTag()
and defaults to 16 bytes.
For chacha20-poly1305
, the authTagLength
option defaults to 16 bytes.
The algorithm
is dependent on OpenSSL, examples are 'aes192'
, etc. On
recent OpenSSL releases, openssl list -cipher-algorithms
will
display the available cipher algorithms.
The key
is the raw key used by the algorithm
and iv
is an
initialization vector. Both arguments must be 'utf8'
encoded strings,
Buffers, TypedArray
, or DataView
s. The key
may optionally be
a KeyObject
of type secret
. If the cipher does not need
an initialization vector, iv
may be null
.
When passing strings for key
or iv
, please consider
caveats when using strings as inputs to cryptographic APIs.
Initialization vectors should be unpredictable and unique; ideally, they will be cryptographically random. They do not have to be secret: IVs are typically just added to ciphertext messages unencrypted. It may sound contradictory that something has to be unpredictable and unique, but does not have to be secret; remember that an attacker must not be able to predict ahead of time what a given IV will be.
crypto.createDecipher(algorithm, password[, options])
#
crypto.createDecipheriv()
instead.algorithm
<string>password
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>options
<Object>stream.transform
options- Returns: <Decipher>
Creates and returns a Decipher
object that uses the given algorithm
and
password
(key).
The options
argument controls stream behavior and is optional except when a
cipher in CCM or OCB mode (e.g. 'aes-128-ccm'
) is used. In that case, the
authTagLength
option is required and specifies the length of the
authentication tag in bytes, see CCM mode.
For chacha20-poly1305
, the authTagLength
option defaults to 16 bytes.
This function is semantically insecure for all supported ciphers and fatally flawed for ciphers in counter mode (such as CTR, GCM, or CCM).
The implementation of crypto.createDecipher()
derives keys using the OpenSSL
function EVP_BytesToKey
with the digest algorithm set to MD5, one
iteration, and no salt. The lack of salt allows dictionary attacks as the same
password always creates the same key. The low iteration count and
non-cryptographically secure hash algorithm allow passwords to be tested very
rapidly.
In line with OpenSSL's recommendation to use a more modern algorithm instead of
EVP_BytesToKey
it is recommended that developers derive a key and IV on
their own using crypto.scrypt()
and to use crypto.createDecipheriv()
to create the Decipher
object.
crypto.createDecipheriv(algorithm, key, iv[, options])
#
algorithm
<string>key
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> | <KeyObject> | <CryptoKey>iv
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> | <null>options
<Object>stream.transform
options- Returns: <Decipher>
Creates and returns a Decipher
object that uses the given algorithm
, key
and initialization vector (iv
).
The options
argument controls stream behavior and is optional except when a
cipher in CCM or OCB mode (e.g. 'aes-128-ccm'
) is used. In that case, the
authTagLength
option is required and specifies the length of the
authentication tag in bytes, see CCM mode. In GCM mode, the authTagLength
option is not required but can be used to restrict accepted authentication tags
to those with the specified length.
For chacha20-poly1305
, the authTagLength
option defaults to 16 bytes.
The algorithm
is dependent on OpenSSL, examples are 'aes192'
, etc. On
recent OpenSSL releases, openssl list -cipher-algorithms
will
display the available cipher algorithms.
The key
is the raw key used by the algorithm
and iv
is an
initialization vector. Both arguments must be 'utf8'
encoded strings,
Buffers, TypedArray
, or DataView
s. The key
may optionally be
a KeyObject
of type secret
. If the cipher does not need
an initialization vector, iv
may be null
.
When passing strings for key
or iv
, please consider
caveats when using strings as inputs to cryptographic APIs.
Initialization vectors should be unpredictable and unique; ideally, they will be cryptographically random. They do not have to be secret: IVs are typically just added to ciphertext messages unencrypted. It may sound contradictory that something has to be unpredictable and unique, but does not have to be secret; remember that an attacker must not be able to predict ahead of time what a given IV will be.
crypto.createDiffieHellman(prime[, primeEncoding][, generator][, generatorEncoding])
#
prime
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>primeEncoding
<string> The encoding of theprime
string.generator
<number> | <string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> Default:2
generatorEncoding
<string> The encoding of thegenerator
string.- Returns: <DiffieHellman>
Creates a DiffieHellman
key exchange object using the supplied prime
and an
optional specific generator
.
The generator
argument can be a number, string, or Buffer
. If
generator
is not specified, the value 2
is used.
If primeEncoding
is specified, prime
is expected to be a string; otherwise
a Buffer
, TypedArray
, or DataView
is expected.
If generatorEncoding
is specified, generator
is expected to be a string;
otherwise a number, Buffer
, TypedArray
, or DataView
is expected.
crypto.createDiffieHellman(primeLength[, generator])
#
primeLength
<number>generator
<number> Default:2
- Returns: <DiffieHellman>
Creates a DiffieHellman
key exchange object and generates a prime of
primeLength
bits using an optional specific numeric generator
.
If generator
is not specified, the value 2
is used.
crypto.createDiffieHellmanGroup(name)
#
name
<string>- Returns: <DiffieHellmanGroup>
An alias for crypto.getDiffieHellman()
crypto.createECDH(curveName)
#
Creates an Elliptic Curve Diffie-Hellman (ECDH
) key exchange object using a
predefined curve specified by the curveName
string. Use
crypto.getCurves()
to obtain a list of available curve names. On recent
OpenSSL releases, openssl ecparam -list_curves
will also display the name
and description of each available elliptic curve.
crypto.createHash(algorithm[, options])
#
algorithm
<string>options
<Object>stream.transform
options- Returns: <Hash>
Creates and returns a Hash
object that can be used to generate hash digests
using the given algorithm
. Optional options
argument controls stream
behavior. For XOF hash functions such as 'shake256'
, the outputLength
option
can be used to specify the desired output length in bytes.
The algorithm
is dependent on the available algorithms supported by the
version of OpenSSL on the platform. Examples are 'sha256'
, 'sha512'
, etc.
On recent releases of OpenSSL, openssl list -digest-algorithms
will
display the available digest algorithms.
Example: generating the sha256 sum of a file
import {
createReadStream,
} from 'node:fs';
import { argv } from 'node:process';
const {
createHash,
} = await import('node:crypto');
const filename = argv[2];
const hash = createHash('sha256');
const input = createReadStream(filename);
input.on('readable', () => {
// Only one element is going to be produced by the
// hash stream.
const data = input.read();
if (data)
hash.update(data);
else {
console.log(`${hash.digest('hex')} ${filename}`);
}
});
const {
createReadStream,
} = require('node:fs');
const {
createHash,
} = require('node:crypto');
const { argv } = require('node:process');
const filename = argv[2];
const hash = createHash('sha256');
const input = createReadStream(filename);
input.on('readable', () => {
// Only one element is going to be produced by the
// hash stream.
const data = input.read();
if (data)
hash.update(data);
else {
console.log(`${hash.digest('hex')} ${filename}`);
}
});
crypto.createHmac(algorithm, key[, options])
#
algorithm
<string>key
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> | <KeyObject> | <CryptoKey>options
<Object>stream.transform
optionsencoding
<string> The string encoding to use whenkey
is a string.
- Returns: <Hmac>
Creates and returns an Hmac
object that uses the given algorithm
and key
.
Optional options
argument controls stream behavior.
The algorithm
is dependent on the available algorithms supported by the
version of OpenSSL on the platform. Examples are 'sha256'
, 'sha512'
, etc.
On recent releases of OpenSSL, openssl list -digest-algorithms
will
display the available digest algorithms.
The key
is the HMAC key used to generate the cryptographic HMAC hash. If it is
a KeyObject
, its type must be secret
.
Example: generating the sha256 HMAC of a file
import {
createReadStream,
} from 'node:fs';
import { argv } from 'node:process';
const {
createHmac,
} = await import('node:crypto');
const filename = argv[2];
const hmac = createHmac('sha256', 'a secret');
const input = createReadStream(filename);
input.on('readable', () => {
// Only one element is going to be produced by the
// hash stream.
const data = input.read();
if (data)
hmac.update(data);
else {
console.log(`${hmac.digest('hex')} ${filename}`);
}
});
const {
createReadStream,
} = require('node:fs');
const {
createHmac,
} = require('node:crypto');
const { argv } = require('node:process');
const filename = argv[2];
const hmac = createHmac('sha256', 'a secret');
const input = createReadStream(filename);
input.on('readable', () => {
// Only one element is going to be produced by the
// hash stream.
const data = input.read();
if (data)
hmac.update(data);
else {
console.log(`${hmac.digest('hex')} ${filename}`);
}
});
crypto.createPrivateKey(key)
#
key
<Object> | <string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>key
: <string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> | <Object> The key material, either in PEM, DER, or JWK format.format
: <string> Must be'pem'
,'der'
, or ''jwk'
. Default:'pem'
.type
: <string> Must be'pkcs1'
,'pkcs8'
or'sec1'
. This option is required only if theformat
is'der'
and ignored otherwise.passphrase
: <string> | <Buffer> The passphrase to use for decryption.encoding
: <string> The string encoding to use whenkey
is a string.
- Returns: <KeyObject>
Creates and returns a new key object containing a private key. If key
is a
string or Buffer
, format
is assumed to be 'pem'
; otherwise, key
must be an object with the properties described above.
If the private key is encrypted, a passphrase
must be specified. The length
of the passphrase is limited to 1024 bytes.
crypto.createPublicKey(key)
#
key
<Object> | <string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>key
: <string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> | <Object> The key material, either in PEM, DER, or JWK format.format
: <string> Must be'pem'
,'der'
, or'jwk'
. Default:'pem'
.type
: <string> Must be'pkcs1'
or'spki'
. This option is required only if theformat
is'der'
and ignored otherwise.encoding
<string> The string encoding to use whenkey
is a string.
- Returns: <KeyObject>
Creates and returns a new key object containing a public key. If key
is a
string or Buffer
, format
is assumed to be 'pem'
; if key
is a KeyObject
with type 'private'
, the public key is derived from the given private key;
otherwise, key
must be an object with the properties described above.
If the format is 'pem'
, the 'key'
may also be an X.509 certificate.
Because public keys can be derived from private keys, a private key may be
passed instead of a public key. In that case, this function behaves as if
crypto.createPrivateKey()
had been called, except that the type of the
returned KeyObject
will be 'public'
and that the private key cannot be
extracted from the returned KeyObject
. Similarly, if a KeyObject
with type
'private'
is given, a new KeyObject
with type 'public'
will be returned
and it will be impossible to extract the private key from the returned object.
crypto.createSecretKey(key[, encoding])
#
key
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>encoding
<string> The string encoding whenkey
is a string.- Returns: <KeyObject>
Creates and returns a new key object containing a secret key for symmetric
encryption or Hmac
.
crypto.createSign(algorithm[, options])
#
algorithm
<string>options
<Object>stream.Writable
options- Returns: <Sign>
Creates and returns a Sign
object that uses the given algorithm
. Use
crypto.getHashes()
to obtain the names of the available digest algorithms.
Optional options
argument controls the stream.Writable
behavior.
In some cases, a Sign
instance can be created using the name of a signature
algorithm, such as 'RSA-SHA256'
, instead of a digest algorithm. This will use
the corresponding digest algorithm. This does not work for all signature
algorithms, such as 'ecdsa-with-SHA256'
, so it is best to always use digest
algorithm names.
crypto.createVerify(algorithm[, options])
#
algorithm
<string>options
<Object>stream.Writable
options- Returns: <Verify>
Creates and returns a Verify
object that uses the given algorithm.
Use crypto.getHashes()
to obtain an array of names of the available
signing algorithms. Optional options
argument controls the
stream.Writable
behavior.
In some cases, a Verify
instance can be created using the name of a signature
algorithm, such as 'RSA-SHA256'
, instead of a digest algorithm. This will use
the corresponding digest algorithm. This does not work for all signature
algorithms, such as 'ecdsa-with-SHA256'
, so it is best to always use digest
algorithm names.
crypto.diffieHellman(options)
#
options
: <Object>privateKey
: <KeyObject>publicKey
: <KeyObject>
- Returns: <Buffer>
Computes the Diffie-Hellman secret based on a privateKey
and a publicKey
.
Both keys must have the same asymmetricKeyType
, which must be one of 'dh'
(for Diffie-Hellman), 'ec'
(for ECDH), 'x448'
, or 'x25519'
(for ECDH-ES).
crypto.generateKey(type, options, callback)
#
type
: <string> The intended use of the generated secret key. Currently accepted values are'hmac'
and'aes'
.options
: <Object>length
: <number> The bit length of the key to generate. This must be a value greater than 0.- If
type
is'hmac'
, the minimum is 8, and the maximum length is 231-1. If the value is not a multiple of 8, the generated key will be truncated toMath.floor(length / 8)
. - If
type
is'aes'
, the length must be one of128
,192
, or256
.
- If
callback
: <Function>err
: <Error>key
: <KeyObject>
Asynchronously generates a new random secret key of the given length
. The
type
will determine which validations will be performed on the length
.
const {
generateKey,
} = await import('node:crypto');
generateKey('hmac', { length: 64 }, (err, key) => {
if (err) throw err;
console.log(key.export().toString('hex')); // 46e..........620
});
const {
generateKey,
} = require('node:crypto');
generateKey('hmac', { length: 64 }, (err, key) => {
if (err) throw err;
console.log(key.export().toString('hex')); // 46e..........620
});
crypto.generateKeyPair(type, options, callback)
#
type
: <string> Must be'rsa'
,'rsa-pss'
,'dsa'
,'ec'
,'ed25519'
,'ed448'
,'x25519'
,'x448'
, or'dh'
.options
: <Object>modulusLength
: <number> Key size in bits (RSA, DSA).publicExponent
: <number> Public exponent (RSA). Default:0x10001
.hashAlgorithm
: <string> Name of the message digest (RSA-PSS).mgf1HashAlgorithm
: <string> Name of the message digest used by MGF1 (RSA-PSS).saltLength
: <number> Minimal salt length in bytes (RSA-PSS).divisorLength
: <number> Size ofq
in bits (DSA).namedCurve
: <string> Name of the curve to use (EC).prime
: <Buffer> The prime parameter (DH).primeLength
: <number> Prime length in bits (DH).generator
: <number> Custom generator (DH). Default:2
.groupName
: <string> Diffie-Hellman group name (DH). Seecrypto.getDiffieHellman()
.paramEncoding
: <string> Must be'named'
or'explicit'
(EC). Default:'named'
.publicKeyEncoding
: <Object> SeekeyObject.export()
.privateKeyEncoding
: <Object> SeekeyObject.export()
.
callback
: <Function>err
: <Error>publicKey
: <string> | <Buffer> | <KeyObject>privateKey
: <string> | <Buffer> | <KeyObject>
Generates a new asymmetric key pair of the given type
. RSA, RSA-PSS, DSA, EC,
Ed25519, Ed448, X25519, X448, and DH are currently supported.
If a publicKeyEncoding
or privateKeyEncoding
was specified, this function
behaves as if keyObject.export()
had been called on its result. Otherwise,
the respective part of the key is returned as a KeyObject
.
It is recommended to encode public keys as 'spki'
and private keys as
'pkcs8'
with encryption for long-term storage:
const {
generateKeyPair,
} = await import('node:crypto');
generateKeyPair('rsa', {
modulusLength: 4096,
publicKeyEncoding: {
type: 'spki',
format: 'pem',
},
privateKeyEncoding: {
type: 'pkcs8',
format: 'pem',
cipher: 'aes-256-cbc',
passphrase: 'top secret',
},
}, (err, publicKey, privateKey) => {
// Handle errors and use the generated key pair.
});
const {
generateKeyPair,
} = require('node:crypto');
generateKeyPair('rsa', {
modulusLength: 4096,
publicKeyEncoding: {
type: 'spki',
format: 'pem',
},
privateKeyEncoding: {
type: 'pkcs8',
format: 'pem',
cipher: 'aes-256-cbc',
passphrase: 'top secret',
},
}, (err, publicKey, privateKey) => {
// Handle errors and use the generated key pair.
});
On completion, callback
will be called with err
set to undefined
and
publicKey
/ privateKey
representing the generated key pair.
If this method is invoked as its util.promisify()
ed version, it returns
a Promise
for an Object
with publicKey
and privateKey
properties.
crypto.generateKeyPairSync(type, options)
#
type
: <string> Must be'rsa'
,'rsa-pss'
,'dsa'
,'ec'
,'ed25519'
,'ed448'
,'x25519'
,'x448'
, or'dh'
.options
: <Object>modulusLength
: <number> Key size in bits (RSA, DSA).publicExponent
: <number> Public exponent (RSA). Default:0x10001
.hashAlgorithm
: <string> Name of the message digest (RSA-PSS).mgf1HashAlgorithm
: <string> Name of the message digest used by MGF1 (RSA-PSS).saltLength
: <number> Minimal salt length in bytes (RSA-PSS).divisorLength
: <number> Size ofq
in bits (DSA).namedCurve
: <string> Name of the curve to use (EC).prime
: <Buffer> The prime parameter (DH).primeLength
: <number> Prime length in bits (DH).generator
: <number> Custom generator (DH). Default:2
.groupName
: <string> Diffie-Hellman group name (DH). Seecrypto.getDiffieHellman()
.paramEncoding
: <string> Must be'named'
or'explicit'
(EC). Default:'named'
.publicKeyEncoding
: <Object> SeekeyObject.export()
.privateKeyEncoding
: <Object> SeekeyObject.export()
.
- Returns: <Object>
publicKey
: <string> | <Buffer> | <KeyObject>privateKey
: <string> | <Buffer> | <KeyObject>
Generates a new asymmetric key pair of the given type
. RSA, RSA-PSS, DSA, EC,
Ed25519, Ed448, X25519, X448, and DH are currently supported.
If a publicKeyEncoding
or privateKeyEncoding
was specified, this function
behaves as if keyObject.export()
had been called on its result. Otherwise,
the respective part of the key is returned as a KeyObject
.
When encoding public keys, it is recommended to use 'spki'
. When encoding
private keys, it is recommended to use 'pkcs8'
with a strong passphrase,
and to keep the passphrase confidential.
const {
generateKeyPairSync,
} = await import('node:crypto');
const {
publicKey,
privateKey,
} = generateKeyPairSync('rsa', {
modulusLength: 4096,
publicKeyEncoding: {
type: 'spki',
format: 'pem',
},
privateKeyEncoding: {
type: 'pkcs8',
format: 'pem',
cipher: 'aes-256-cbc',
passphrase: 'top secret',
},
});
const {
generateKeyPairSync,
} = require('node:crypto');
const {
publicKey,
privateKey,
} = generateKeyPairSync('rsa', {
modulusLength: 4096,
publicKeyEncoding: {
type: 'spki',
format: 'pem',
},
privateKeyEncoding: {
type: 'pkcs8',
format: 'pem',
cipher: 'aes-256-cbc',
passphrase: 'top secret',
},
});
The return value { publicKey, privateKey }
represents the generated key pair.
When PEM encoding was selected, the respective key will be a string, otherwise
it will be a buffer containing the data encoded as DER.
crypto.generateKeySync(type, options)
#
type
: <string> The intended use of the generated secret key. Currently accepted values are'hmac'
and'aes'
.options
: <Object>length
: <number> The bit length of the key to generate.- If
type
is'hmac'
, the minimum is 8, and the maximum length is 231-1. If the value is not a multiple of 8, the generated key will be truncated toMath.floor(length / 8)
. - If
type
is'aes'
, the length must be one of128
,192
, or256
.
- If
- Returns: <KeyObject>
Synchronously generates a new random secret key of the given length
. The
type
will determine which validations will be performed on the length
.
const {
generateKeySync,
} = await import('node:crypto');
const key = generateKeySync('hmac', { length: 64 });
console.log(key.export().toString('hex')); // e89..........41e
const {
generateKeySync,
} = require('node:crypto');
const key = generateKeySync('hmac', { length: 64 });
console.log(key.export().toString('hex')); // e89..........41e
crypto.generatePrime(size[, options[, callback]])
#
size
<number> The size (in bits) of the prime to generate.options
<Object>add
<ArrayBuffer> | <SharedArrayBuffer> | <TypedArray> | <Buffer> | <DataView> | <bigint>rem
<ArrayBuffer> | <SharedArrayBuffer> | <TypedArray> | <Buffer> | <DataView> | <bigint>safe
<boolean> Default:false
.bigint
<boolean> Whentrue
, the generated prime is returned as abigint
.
callback
<Function>err
<Error>prime
<ArrayBuffer> | <bigint>
Generates a pseudorandom prime of size
bits.
If options.safe
is true
, the prime will be a safe prime -- that is,
(prime - 1) / 2
will also be a prime.
The options.add
and options.rem
parameters can be used to enforce additional
requirements, e.g., for Diffie-Hellman:
- If
options.add
andoptions.rem
are both set, the prime will satisfy the condition thatprime % add = rem
. - If only
options.add
is set andoptions.safe
is nottrue
, the prime will satisfy the condition thatprime % add = 1
. - If only
options.add
is set andoptions.safe
is set totrue
, the prime will instead satisfy the condition thatprime % add = 3
. This is necessary becauseprime % add = 1
foroptions.add > 2
would contradict the condition enforced byoptions.safe
. options.rem
is ignored ifoptions.add
is not given.
Both options.add
and options.rem
must be encoded as big-endian sequences
if given as an ArrayBuffer
, SharedArrayBuffer
, TypedArray
, Buffer
, or
DataView
.
By default, the prime is encoded as a big-endian sequence of octets
in an <ArrayBuffer>. If the bigint
option is true
, then a <bigint>
is provided.
crypto.generatePrimeSync(size[, options])
#
size
<number> The size (in bits) of the prime to generate.options
<Object>add
<ArrayBuffer> | <SharedArrayBuffer> | <TypedArray> | <Buffer> | <DataView> | <bigint>rem
<ArrayBuffer> | <SharedArrayBuffer> | <TypedArray> | <Buffer> | <DataView> | <bigint>safe
<boolean> Default:false
.bigint
<boolean> Whentrue
, the generated prime is returned as abigint
.
- Returns: <ArrayBuffer> | <bigint>
Generates a pseudorandom prime of size
bits.
If options.safe
is true
, the prime will be a safe prime -- that is,
(prime - 1) / 2
will also be a prime.
The options.add
and options.rem
parameters can be used to enforce additional
requirements, e.g., for Diffie-Hellman:
- If
options.add
andoptions.rem
are both set, the prime will satisfy the condition thatprime % add = rem
. - If only
options.add
is set andoptions.safe
is nottrue
, the prime will satisfy the condition thatprime % add = 1
. - If only
options.add
is set andoptions.safe
is set totrue
, the prime will instead satisfy the condition thatprime % add = 3
. This is necessary becauseprime % add = 1
foroptions.add > 2
would contradict the condition enforced byoptions.safe
. options.rem
is ignored ifoptions.add
is not given.
Both options.add
and options.rem
must be encoded as big-endian sequences
if given as an ArrayBuffer
, SharedArrayBuffer
, TypedArray
, Buffer
, or
DataView
.
By default, the prime is encoded as a big-endian sequence of octets
in an <ArrayBuffer>. If the bigint
option is true
, then a <bigint>
is provided.
crypto.getCipherInfo(nameOrNid[, options])
#
nameOrNid
: <string> | <number> The name or nid of the cipher to query.options
: <Object>- Returns: <Object>
name
<string> The name of the ciphernid
<number> The nid of the cipherblockSize
<number> The block size of the cipher in bytes. This property is omitted whenmode
is'stream'
.ivLength
<number> The expected or default initialization vector length in bytes. This property is omitted if the cipher does not use an initialization vector.keyLength
<number> The expected or default key length in bytes.mode
<string> The cipher mode. One of'cbc'
,'ccm'
,'cfb'
,'ctr'
,'ecb'
,'gcm'
,'ocb'
,'ofb'
,'stream'
,'wrap'
,'xts'
.
Returns information about a given cipher.
Some ciphers accept variable length keys and initialization vectors. By default,
the crypto.getCipherInfo()
method will return the default values for these
ciphers. To test if a given key length or iv length is acceptable for given
cipher, use the keyLength
and ivLength
options. If the given values are
unacceptable, undefined
will be returned.
crypto.getCiphers()
#
- Returns: <string[]> An array with the names of the supported cipher algorithms.
const {
getCiphers,
} = await import('node:crypto');
console.log(getCiphers()); // ['aes-128-cbc', 'aes-128-ccm', ...]
const {
getCiphers,
} = require('node:crypto');
console.log(getCiphers()); // ['aes-128-cbc', 'aes-128-ccm', ...]
crypto.getCurves()
#
- Returns: <string[]> An array with the names of the supported elliptic curves.
const {
getCurves,
} = await import('node:crypto');
console.log(getCurves()); // ['Oakley-EC2N-3', 'Oakley-EC2N-4', ...]
const {
getCurves,
} = require('node:crypto');
console.log(getCurves()); // ['Oakley-EC2N-3', 'Oakley-EC2N-4', ...]
crypto.getDiffieHellman(groupName)
#
groupName
<string>- Returns: <DiffieHellmanGroup>
Creates a predefined DiffieHellmanGroup
key exchange object. The
supported groups are listed in the documentation for DiffieHellmanGroup
.
The returned object mimics the interface of objects created by
crypto.createDiffieHellman()
, but will not allow changing
the keys (with diffieHellman.setPublicKey()
, for example). The
advantage of using this method is that the parties do not have to
generate nor exchange a group modulus beforehand, saving both processor
and communication time.
Example (obtaining a shared secret):
const {
getDiffieHellman,
} = await import('node:crypto');
const alice = getDiffieHellman('modp14');
const bob = getDiffieHellman('modp14');
alice.generateKeys();
bob.generateKeys();
const aliceSecret = alice.computeSecret(bob.getPublicKey(), null, 'hex');
const bobSecret = bob.computeSecret(alice.getPublicKey(), null, 'hex');
/* aliceSecret and bobSecret should be the same */
console.log(aliceSecret === bobSecret);
const {
getDiffieHellman,
} = require('node:crypto');
const alice = getDiffieHellman('modp14');
const bob = getDiffieHellman('modp14');
alice.generateKeys();
bob.generateKeys();
const aliceSecret = alice.computeSecret(bob.getPublicKey(), null, 'hex');
const bobSecret = bob.computeSecret(alice.getPublicKey(), null, 'hex');
/* aliceSecret and bobSecret should be the same */
console.log(aliceSecret === bobSecret);
crypto.getFips()
#
- Returns: <number>
1
if and only if a FIPS compliant crypto provider is currently in use,0
otherwise. A future semver-major release may change the return type of this API to a <boolean>.
crypto.getHashes()
#
- Returns: <string[]> An array of the names of the supported hash algorithms,
such as
'RSA-SHA256'
. Hash algorithms are also called "digest" algorithms.
const {
getHashes,
} = await import('node:crypto');
console.log(getHashes()); // ['DSA', 'DSA-SHA', 'DSA-SHA1', ...]
const {
getHashes,
} = require('node:crypto');
console.log(getHashes()); // ['DSA', 'DSA-SHA', 'DSA-SHA1', ...]
crypto.getRandomValues(typedArray)
#
typedArray
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer>- Returns: <Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> Returns
typedArray
.
A convenient alias for crypto.webcrypto.getRandomValues()
. This
implementation is not compliant with the Web Crypto spec, to write
web-compatible code use crypto.webcrypto.getRandomValues()
instead.
crypto.hkdf(digest, ikm, salt, info, keylen, callback)
#
digest
<string> The digest algorithm to use.ikm
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> | <KeyObject> The input keying material. Must be provided but can be zero-length.salt
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> The salt value. Must be provided but can be zero-length.info
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> Additional info value. Must be provided but can be zero-length, and cannot be more than 1024 bytes.keylen
<number> The length of the key to generate. Must be greater than 0. The maximum allowable value is255
times the number of bytes produced by the selected digest function (e.g.sha512
generates 64-byte hashes, making the maximum HKDF output 16320 bytes).callback
<Function>err
<Error>derivedKey
<ArrayBuffer>
HKDF is a simple key derivation function defined in RFC 5869. The given ikm
,
salt
and info
are used with the digest
to derive a key of keylen
bytes.
The supplied callback
function is called with two arguments: err
and
derivedKey
. If an errors occurs while deriving the key, err
will be set;
otherwise err
will be null
. The successfully generated derivedKey
will
be passed to the callback as an <ArrayBuffer>. An error will be thrown if any
of the input arguments specify invalid values or types.
import { Buffer } from 'node:buffer';
const {
hkdf,
} = await import('node:crypto');
hkdf('sha512', 'key', 'salt', 'info', 64, (err, derivedKey) => {
if (err) throw err;
console.log(Buffer.from(derivedKey).toString('hex')); // '24156e2...5391653'
});
const {
hkdf,
} = require('node:crypto');
const { Buffer } = require('node:buffer');
hkdf('sha512', 'key', 'salt', 'info', 64, (err, derivedKey) => {
if (err) throw err;
console.log(Buffer.from(derivedKey).toString('hex')); // '24156e2...5391653'
});
crypto.hkdfSync(digest, ikm, salt, info, keylen)
#
digest
<string> The digest algorithm to use.ikm
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> | <KeyObject> The input keying material. Must be provided but can be zero-length.salt
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> The salt value. Must be provided but can be zero-length.info
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> Additional info value. Must be provided but can be zero-length, and cannot be more than 1024 bytes.keylen
<number> The length of the key to generate. Must be greater than 0. The maximum allowable value is255
times the number of bytes produced by the selected digest function (e.g.sha512
generates 64-byte hashes, making the maximum HKDF output 16320 bytes).- Returns: <ArrayBuffer>
Provides a synchronous HKDF key derivation function as defined in RFC 5869. The
given ikm
, salt
and info
are used with the digest
to derive a key of
keylen
bytes.
The successfully generated derivedKey
will be returned as an <ArrayBuffer>.
An error will be thrown if any of the input arguments specify invalid values or types, or if the derived key cannot be generated.
import { Buffer } from 'node:buffer';
const {
hkdfSync,
} = await import('node:crypto');
const derivedKey = hkdfSync('sha512', 'key', 'salt', 'info', 64);
console.log(Buffer.from(derivedKey).toString('hex')); // '24156e2...5391653'
const {
hkdfSync,
} = require('node:crypto');
const { Buffer } = require('node:buffer');
const derivedKey = hkdfSync('sha512', 'key', 'salt', 'info', 64);
console.log(Buffer.from(derivedKey).toString('hex')); // '24156e2...5391653'
crypto.pbkdf2(password, salt, iterations, keylen, digest, callback)
#
password
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>salt
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>iterations
<number>keylen
<number>digest
<string>callback
<Function>
Provides an asynchronous Password-Based Key Derivation Function 2 (PBKDF2)
implementation. A selected HMAC digest algorithm specified by digest
is
applied to derive a key of the requested byte length (keylen
) from the
password
, salt
and iterations
.
The supplied callback
function is called with two arguments: err
and
derivedKey
. If an error occurs while deriving the key, err
will be set;
otherwise err
will be null
. By default, the successfully generated
derivedKey
will be passed to the callback as a Buffer
. An error will be
thrown if any of the input arguments specify invalid values or types.
The iterations
argument must be a number set as high as possible. The
higher the number of iterations, the more secure the derived key will be,
but will take a longer amount of time to complete.
The salt
should be as unique as possible. It is recommended that a salt is
random and at least 16 bytes long. See NIST SP 800-132 for details.
When passing strings for password
or salt
, please consider
caveats when using strings as inputs to cryptographic APIs.
const {
pbkdf2,
} = await import('node:crypto');
pbkdf2('secret', 'salt', 100000, 64, 'sha512', (err, derivedKey) => {
if (err) throw err;
console.log(derivedKey.toString('hex')); // '3745e48...08d59ae'
});
const {
pbkdf2,
} = require('node:crypto');
pbkdf2('secret', 'salt', 100000, 64, 'sha512', (err, derivedKey) => {
if (err) throw err;
console.log(derivedKey.toString('hex')); // '3745e48...08d59ae'
});
An array of supported digest functions can be retrieved using
crypto.getHashes()
.
This API uses libuv's threadpool, which can have surprising and
negative performance implications for some applications; see the
UV_THREADPOOL_SIZE
documentation for more information.
crypto.pbkdf2Sync(password, salt, iterations, keylen, digest)
#
password
<string> | <Buffer> | <TypedArray> | <DataView>salt
<string> | <Buffer> | <TypedArray> | <DataView>iterations
<number>keylen
<number>digest
<string>- Returns: <Buffer>
Provides a synchronous Password-Based Key Derivation Function 2 (PBKDF2)
implementation. A selected HMAC digest algorithm specified by digest
is
applied to derive a key of the requested byte length (keylen
) from the
password
, salt
and iterations
.
If an error occurs an Error
will be thrown, otherwise the derived key will be
returned as a Buffer
.
The iterations
argument must be a number set as high as possible. The
higher the number of iterations, the more secure the derived key will be,
but will take a longer amount of time to complete.
The salt
should be as unique as possible. It is recommended that a salt is
random and at least 16 bytes long. See NIST SP 800-132 for details.
When passing strings for password
or salt
, please consider
caveats when using strings as inputs to cryptographic APIs.
const {
pbkdf2Sync,
} = await import('node:crypto');
const key = pbkdf2Sync('secret', 'salt', 100000, 64, 'sha512');
console.log(key.toString('hex')); // '3745e48...08d59ae'
const {
pbkdf2Sync,
} = require('node:crypto');
const key = pbkdf2Sync('secret', 'salt', 100000, 64, 'sha512');
console.log(key.toString('hex')); // '3745e48...08d59ae'
An array of supported digest functions can be retrieved using
crypto.getHashes()
.
crypto.privateDecrypt(privateKey, buffer)
#
privateKey
<Object> | <string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> | <KeyObject> | <CryptoKey>oaepHash
<string> The hash function to use for OAEP padding and MGF1. Default:'sha1'
oaepLabel
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> The label to use for OAEP padding. If not specified, no label is used.padding
<crypto.constants> An optional padding value defined incrypto.constants
, which may be:crypto.constants.RSA_NO_PADDING
,crypto.constants.RSA_PKCS1_PADDING
, orcrypto.constants.RSA_PKCS1_OAEP_PADDING
.
buffer
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>- Returns: <Buffer> A new
Buffer
with the decrypted content.
Decrypts buffer
with privateKey
. buffer
was previously encrypted using
the corresponding public key, for example using crypto.publicEncrypt()
.
If privateKey
is not a KeyObject
, this function behaves as if
privateKey
had been passed to crypto.createPrivateKey()
. If it is an
object, the padding
property can be passed. Otherwise, this function uses
RSA_PKCS1_OAEP_PADDING
.
crypto.privateEncrypt(privateKey, buffer)
#
privateKey
<Object> | <string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> | <KeyObject> | <CryptoKey>key
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> | <KeyObject> | <CryptoKey> A PEM encoded private key.passphrase
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> An optional passphrase for the private key.padding
<crypto.constants> An optional padding value defined incrypto.constants
, which may be:crypto.constants.RSA_NO_PADDING
orcrypto.constants.RSA_PKCS1_PADDING
.encoding
<string> The string encoding to use whenbuffer
,key
, orpassphrase
are strings.
buffer
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>- Returns: <Buffer> A new
Buffer
with the encrypted content.
Encrypts buffer
with privateKey
. The returned data can be decrypted using
the corresponding public key, for example using crypto.publicDecrypt()
.
If privateKey
is not a KeyObject
, this function behaves as if
privateKey
had been passed to crypto.createPrivateKey()
. If it is an
object, the padding
property can be passed. Otherwise, this function uses
RSA_PKCS1_PADDING
.
crypto.publicDecrypt(key, buffer)
#
key
<Object> | <string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> | <KeyObject> | <CryptoKey>passphrase
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> An optional passphrase for the private key.padding
<crypto.constants> An optional padding value defined incrypto.constants
, which may be:crypto.constants.RSA_NO_PADDING
orcrypto.constants.RSA_PKCS1_PADDING
.encoding
<string> The string encoding to use whenbuffer
,key
, orpassphrase
are strings.
buffer
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>- Returns: <Buffer> A new
Buffer
with the decrypted content.
Decrypts buffer
with key
.buffer
was previously encrypted using
the corresponding private key, for example using crypto.privateEncrypt()
.
If key
is not a KeyObject
, this function behaves as if
key
had been passed to crypto.createPublicKey()
. If it is an
object, the padding
property can be passed. Otherwise, this function uses
RSA_PKCS1_PADDING
.
Because RSA public keys can be derived from private keys, a private key may be passed instead of a public key.
crypto.publicEncrypt(key, buffer)
#
key
<Object> | <string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> | <KeyObject> | <CryptoKey>key
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> | <KeyObject> | <CryptoKey> A PEM encoded public or private key, <KeyObject>, or <CryptoKey>.oaepHash
<string> The hash function to use for OAEP padding and MGF1. Default:'sha1'
oaepLabel
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> The label to use for OAEP padding. If not specified, no label is used.passphrase
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> An optional passphrase for the private key.padding
<crypto.constants> An optional padding value defined incrypto.constants
, which may be:crypto.constants.RSA_NO_PADDING
,crypto.constants.RSA_PKCS1_PADDING
, orcrypto.constants.RSA_PKCS1_OAEP_PADDING
.encoding
<string> The string encoding to use whenbuffer
,key
,oaepLabel
, orpassphrase
are strings.
buffer
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>- Returns: <Buffer> A new
Buffer
with the encrypted content.
Encrypts the content of buffer
with key
and returns a new
Buffer
with encrypted content. The returned data can be decrypted using
the corresponding private key, for example using crypto.privateDecrypt()
.
If key
is not a KeyObject
, this function behaves as if
key
had been passed to crypto.createPublicKey()
. If it is an
object, the padding
property can be passed. Otherwise, this function uses
RSA_PKCS1_OAEP_PADDING
.
Because RSA public keys can be derived from private keys, a private key may be passed instead of a public key.
crypto.randomBytes(size[, callback])
#
size
<number> The number of bytes to generate. Thesize
must not be larger than2**31 - 1
.callback
<Function>- Returns: <Buffer> if the
callback
function is not provided.
Generates cryptographically strong pseudorandom data. The size
argument
is a number indicating the number of bytes to generate.
If a callback
function is provided, the bytes are generated asynchronously
and the callback
function is invoked with two arguments: err
and buf
.
If an error occurs, err
will be an Error
object; otherwise it is null
. The
buf
argument is a Buffer
containing the generated bytes.
// Asynchronous
const {
randomBytes,
} = await import('node:crypto');
randomBytes(256, (err, buf) => {
if (err) throw err;
console.log(`${buf.length} bytes of random data: ${buf.toString('hex')}`);
});
// Asynchronous
const {
randomBytes,
} = require('node:crypto');
randomBytes(256, (err, buf) => {
if (err) throw err;
console.log(`${buf.length} bytes of random data: ${buf.toString('hex')}`);
});
If the callback
function is not provided, the random bytes are generated
synchronously and returned as a Buffer
. An error will be thrown if
there is a problem generating the bytes.
// Synchronous
const {
randomBytes,
} = await import('node:crypto');
const buf = randomBytes(256);
console.log(
`${buf.length} bytes of random data: ${buf.toString('hex')}`);
// Synchronous
const {
randomBytes,
} = require('node:crypto');
const buf = randomBytes(256);
console.log(
`${buf.length} bytes of random data: ${buf.toString('hex')}`);
The crypto.randomBytes()
method will not complete until there is
sufficient entropy available.
This should normally never take longer than a few milliseconds. The only time
when generating the random bytes may conceivably block for a longer period of
time is right after boot, when the whole system is still low on entropy.
This API uses libuv's threadpool, which can have surprising and
negative performance implications for some applications; see the
UV_THREADPOOL_SIZE
documentation for more information.
The asynchronous version of crypto.randomBytes()
is carried out in a single
threadpool request. To minimize threadpool task length variation, partition
large randomBytes
requests when doing so as part of fulfilling a client
request.
crypto.randomFillSync(buffer[, offset][, size])
#
buffer
<ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> Must be supplied. The size of the providedbuffer
must not be larger than2**31 - 1
.offset
<number> Default:0
size
<number> Default:buffer.length - offset
. Thesize
must not be larger than2**31 - 1
.- Returns: <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> The object passed as
buffer
argument.
Synchronous version of crypto.randomFill()
.
import { Buffer } from 'node:buffer';
const { randomFillSync } = await import('node:crypto');
const buf = Buffer.alloc(10);
console.log(randomFillSync(buf).toString('hex'));
randomFillSync(buf, 5);
console.log(buf.toString('hex'));
// The above is equivalent to the following:
randomFillSync(buf, 5, 5);
console.log(buf.toString('hex'));
const { randomFillSync } = require('node:crypto');
const { Buffer } = require('node:buffer');
const buf = Buffer.alloc(10);
console.log(randomFillSync(buf).toString('hex'));
randomFillSync(buf, 5);
console.log(buf.toString('hex'));
// The above is equivalent to the following:
randomFillSync(buf, 5, 5);
console.log(buf.toString('hex'));
Any ArrayBuffer
, TypedArray
or DataView
instance may be passed as
buffer
.
import { Buffer } from 'node:buffer';
const { randomFillSync } = await import('node:crypto');
const a = new Uint32Array(10);
console.log(Buffer.from(randomFillSync(a).buffer,
a.byteOffset, a.byteLength).toString('hex'));
const b = new DataView(new ArrayBuffer(10));
console.log(Buffer.from(randomFillSync(b).buffer,
b.byteOffset, b.byteLength).toString('hex'));
const c = new ArrayBuffer(10);
console.log(Buffer.from(randomFillSync(c)).toString('hex'));
const { randomFillSync } = require('node:crypto');
const { Buffer } = require('node:buffer');
const a = new Uint32Array(10);
console.log(Buffer.from(randomFillSync(a).buffer,
a.byteOffset, a.byteLength).toString('hex'));
const b = new DataView(new ArrayBuffer(10));
console.log(Buffer.from(randomFillSync(b).buffer,
b.byteOffset, b.byteLength).toString('hex'));
const c = new ArrayBuffer(10);
console.log(Buffer.from(randomFillSync(c)).toString('hex'));
crypto.randomFill(buffer[, offset][, size], callback)
#
buffer
<ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> Must be supplied. The size of the providedbuffer
must not be larger than2**31 - 1
.offset
<number> Default:0
size
<number> Default:buffer.length - offset
. Thesize
must not be larger than2**31 - 1
.callback
<Function>function(err, buf) {}
.
This function is similar to crypto.randomBytes()
but requires the first
argument to be a Buffer
that will be filled. It also
requires that a callback is passed in.
If the callback
function is not provided, an error will be thrown.
import { Buffer } from 'node:buffer';
const { randomFill } = await import('node:crypto');
const buf = Buffer.alloc(10);
randomFill(buf, (err, buf) => {
if (err) throw err;
console.log(buf.toString('hex'));
});
randomFill(buf, 5, (err, buf) => {
if (err) throw err;
console.log(buf.toString('hex'));
});
// The above is equivalent to the following:
randomFill(buf, 5, 5, (err, buf) => {
if (err) throw err;
console.log(buf.toString('hex'));
});
const { randomFill } = require('node:crypto');
const { Buffer } = require('node:buffer');
const buf = Buffer.alloc(10);
randomFill(buf, (err, buf) => {
if (err) throw err;
console.log(buf.toString('hex'));
});
randomFill(buf, 5, (err, buf) => {
if (err) throw err;
console.log(buf.toString('hex'));
});
// The above is equivalent to the following:
randomFill(buf, 5, 5, (err, buf) => {
if (err) throw err;
console.log(buf.toString('hex'));
});
Any ArrayBuffer
, TypedArray
, or DataView
instance may be passed as
buffer
.
While this includes instances of Float32Array
and Float64Array
, this
function should not be used to generate random floating-point numbers. The
result may contain +Infinity
, -Infinity
, and NaN
, and even if the array
contains finite numbers only, they are not drawn from a uniform random
distribution and have no meaningful lower or upper bounds.
import { Buffer } from 'node:buffer';
const { randomFill } = await import('node:crypto');
const a = new Uint32Array(10);
randomFill(a, (err, buf) => {
if (err) throw err;
console.log(Buffer.from(buf.buffer, buf.byteOffset, buf.byteLength)
.toString('hex'));
});
const b = new DataView(new ArrayBuffer(10));
randomFill(b, (err, buf) => {
if (err) throw err;
console.log(Buffer.from(buf.buffer, buf.byteOffset, buf.byteLength)
.toString('hex'));
});
const c = new ArrayBuffer(10);
randomFill(c, (err, buf) => {
if (err) throw err;
console.log(Buffer.from(buf).toString('hex'));
});
const { randomFill } = require('node:crypto');
const { Buffer } = require('node:buffer');
const a = new Uint32Array(10);
randomFill(a, (err, buf) => {
if (err) throw err;
console.log(Buffer.from(buf.buffer, buf.byteOffset, buf.byteLength)
.toString('hex'));
});
const b = new DataView(new ArrayBuffer(10));
randomFill(b, (err, buf) => {
if (err) throw err;
console.log(Buffer.from(buf.buffer, buf.byteOffset, buf.byteLength)
.toString('hex'));
});
const c = new ArrayBuffer(10);
randomFill(c, (err, buf) => {
if (err) throw err;
console.log(Buffer.from(buf).toString('hex'));
});
This API uses libuv's threadpool, which can have surprising and
negative performance implications for some applications; see the
UV_THREADPOOL_SIZE
documentation for more information.
The asynchronous version of crypto.randomFill()
is carried out in a single
threadpool request. To minimize threadpool task length variation, partition
large randomFill
requests when doing so as part of fulfilling a client
request.
crypto.randomInt([min, ]max[, callback])
#
min
<integer> Start of random range (inclusive). Default:0
.max
<integer> End of random range (exclusive).callback
<Function>function(err, n) {}
.
Return a random integer n
such that min <= n < max
. This
implementation avoids modulo bias.
The range (max - min
) must be less than 248. min
and max
must
be safe integers.
If the callback
function is not provided, the random integer is
generated synchronously.
// Asynchronous
const {
randomInt,
} = await import('node:crypto');
randomInt(3, (err, n) => {
if (err) throw err;
console.log(`Random number chosen from (0, 1, 2): ${n}`);
});
// Asynchronous
const {
randomInt,
} = require('node:crypto');
randomInt(3, (err, n) => {
if (err) throw err;
console.log(`Random number chosen from (0, 1, 2): ${n}`);
});
// Synchronous
const {
randomInt,
} = await import('node:crypto');
const n = randomInt(3);
console.log(`Random number chosen from (0, 1, 2): ${n}`);
// Synchronous
const {
randomInt,
} = require('node:crypto');
const n = randomInt(3);
console.log(`Random number chosen from (0, 1, 2): ${n}`);
// With `min` argument
const {
randomInt,
} = await import('node:crypto');
const n = randomInt(1, 7);
console.log(`The dice rolled: ${n}`);
// With `min` argument
const {
randomInt,
} = require('node:crypto');
const n = randomInt(1, 7);
console.log(`The dice rolled: ${n}`);
crypto.randomUUID([options])
#
options
<Object>disableEntropyCache
<boolean> By default, to improve performance, Node.js generates and caches enough random data to generate up to 128 random UUIDs. To generate a UUID without using the cache, setdisableEntropyCache
totrue
. Default:false
.
- Returns: <string>
Generates a random RFC 4122 version 4 UUID. The UUID is generated using a cryptographic pseudorandom number generator.
crypto.scrypt(password, salt, keylen[, options], callback)
#
password
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>salt
<string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>keylen
<number>options
<Object>cost
<number> CPU/memory cost parameter. Must be a power of two greater than one. Default:16384
.blockSize
<number> Block size parameter. Default:8
.parallelization
<number> Parallelization parameter. Default:1
.N
<number> Alias forcost
. Only one of both may be specified.r
<number> Alias forblockSize
. Only one of both may be specified.p
<number> Alias forparallelization
. Only one of both may be specified.maxmem
<number> Memory upper bound. It is an error when (approximately)128 * N * r > maxmem
. Default:32 * 1024 * 1024
.
callback
<Function>
Provides an asynchronous scrypt implementation. Scrypt is a password-based key derivation function that is designed to be expensive computationally and memory-wise in order to make brute-force attacks unrewarding.
The salt
should be as unique as possible. It is recommended that a salt is
random and at least 16 bytes long. See NIST SP 800-132 for details.
When passing strings for password
or salt
, please consider
caveats when using strings as inputs to cryptographic APIs.
The callback
function is called with two arguments: err
and derivedKey
.
err
is an exception object when key derivation fails, otherwise err
is
null
. derivedKey
is passed to the callback as a Buffer
.
An exception is thrown when any of the input arguments specify invalid values or types.
const {
scrypt,
} = await import('node:crypto');
// Using the factory defaults.
scrypt('password', 'salt', 64, (err, derivedKey) => {
if (err) throw err;
console.log(derivedKey.toString('hex')); // '3745e48...08d59ae'
});
// Using a custom N parameter. Must be a power of two.
scrypt('password', 'salt', 64, { N: 1024 }, (err, derivedKey) => {
if (err) throw err;
console.log(derivedKey.toString('hex')); // '3745e48...aa39b34'
});
const {
scrypt,
} = require('node:crypto');
// Using the factory defaults.
scrypt('password', 'salt', 64, (err, derivedKey) => {
if (err) throw err;
console.log(derivedKey.toString('hex')); // '3745e48...08d59ae'
});
// Using a custom N parameter. Must be a power of two.
scrypt('password', 'salt', 64, { N: 1024 }, (err, derivedKey) => {
if (err) throw err;
console.log(derivedKey.toString('hex')); // '3745e48...aa39b34'
});
crypto.scryptSync(password, salt, keylen[, options])
#
password
<string> | <Buffer> | <TypedArray> | <DataView>salt
<string> | <Buffer> | <TypedArray> | <DataView>keylen
<number>options
<Object>cost
<number> CPU/memory cost parameter. Must be a power of two greater than one. Default:16384
.blockSize
<number> Block size parameter. Default:8
.parallelization
<number> Parallelization parameter. Default:1
.N
<number> Alias forcost
. Only one of both may be specified.r
<number> Alias forblockSize
. Only one of both may be specified.p
<number> Alias forparallelization
. Only one of both may be specified.maxmem
<number> Memory upper bound. It is an error when (approximately)128 * N * r > maxmem
. Default:32 * 1024 * 1024
.
- Returns: <Buffer>
Provides a synchronous scrypt implementation. Scrypt is a password-based key derivation function that is designed to be expensive computationally and memory-wise in order to make brute-force attacks unrewarding.
The salt
should be as unique as possible. It is recommended that a salt is
random and at least 16 bytes long. See NIST SP 800-132 for details.
When passing strings for password
or salt
, please consider
caveats when using strings as inputs to cryptographic APIs.
An exception is thrown when key derivation fails, otherwise the derived key is
returned as a Buffer
.
An exception is thrown when any of the input arguments specify invalid values or types.
const {
scryptSync,
} = await import('node:crypto');
// Using the factory defaults.
const key1 = scryptSync('password', 'salt', 64);
console.log(key1.toString('hex')); // '3745e48...08d59ae'
// Using a custom N parameter. Must be a power of two.
const key2 = scryptSync('password', 'salt', 64, { N: 1024 });
console.log(key2.toString('hex')); // '3745e48...aa39b34'
const {
scryptSync,
} = require('node:crypto');
// Using the factory defaults.
const key1 = scryptSync('password', 'salt', 64);
console.log(key1.toString('hex')); // '3745e48...08d59ae'
// Using a custom N parameter. Must be a power of two.
const key2 = scryptSync('password', 'salt', 64, { N: 1024 });
console.log(key2.toString('hex')); // '3745e48...aa39b34'
crypto.secureHeapUsed()
#
- Returns: <Object>
total
<number> The total allocated secure heap size as specified using the--secure-heap=n
command-line flag.min
<number> The minimum allocation from the secure heap as specified using the--secure-heap-min
command-line flag.used
<number> The total number of bytes currently allocated from the secure heap.utilization
<number> The calculated ratio ofused
tototal
allocated bytes.
crypto.setEngine(engine[, flags])
#
engine
<string>flags
<crypto.constants> Default:crypto.constants.ENGINE_METHOD_ALL
Load and set the engine
for some or all OpenSSL functions (selected by flags).
engine
could be either an id or a path to the engine's shared library.
The optional flags
argument uses ENGINE_METHOD_ALL
by default. The flags
is a bit field taking one of or a mix of the following flags (defined in
crypto.constants
):
crypto.constants.ENGINE_METHOD_RSA
crypto.constants.ENGINE_METHOD_DSA
crypto.constants.ENGINE_METHOD_DH
crypto.constants.ENGINE_METHOD_RAND
crypto.constants.ENGINE_METHOD_EC
crypto.constants.ENGINE_METHOD_CIPHERS
crypto.constants.ENGINE_METHOD_DIGESTS
crypto.constants.ENGINE_METHOD_PKEY_METHS
crypto.constants.ENGINE_METHOD_PKEY_ASN1_METHS
crypto.constants.ENGINE_METHOD_ALL
crypto.constants.ENGINE_METHOD_NONE
crypto.setFips(bool)
#
bool
<boolean>true
to enable FIPS mode.
Enables the FIPS compliant crypto provider in a FIPS-enabled Node.js build. Throws an error if FIPS mode is not available.
crypto.sign(algorithm, data, key[, callback])
#
algorithm
<string> | <null> | <undefined>data
<ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>key
<Object> | <string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> | <KeyObject> | <CryptoKey>callback
<Function>- Returns: <Buffer> if the
callback
function is not provided.
Calculates and returns the signature for data
using the given private key and
algorithm. If algorithm
is null
or undefined
, then the algorithm is
dependent upon the key type (especially Ed25519 and Ed448).
If key
is not a KeyObject
, this function behaves as if key
had been
passed to crypto.createPrivateKey()
. If it is an object, the following
additional properties can be passed:
-
dsaEncoding
<string> For DSA and ECDSA, this option specifies the format of the generated signature. It can be one of the following:'der'
(default): DER-encoded ASN.1 signature structure encoding(r, s)
.'ieee-p1363'
: Signature formatr || s
as proposed in IEEE-P1363.
-
padding
<integer> Optional padding value for RSA, one of the following:crypto.constants.RSA_PKCS1_PADDING
(default)crypto.constants.RSA_PKCS1_PSS_PADDING
RSA_PKCS1_PSS_PADDING
will use MGF1 with the same hash function used to sign the message as specified in section 3.1 of RFC 4055. -
saltLength
<integer> Salt length for when padding isRSA_PKCS1_PSS_PADDING
. The special valuecrypto.constants.RSA_PSS_SALTLEN_DIGEST
sets the salt length to the digest size,crypto.constants.RSA_PSS_SALTLEN_MAX_SIGN
(default) sets it to the maximum permissible value.
If the callback
function is provided this function uses libuv's threadpool.
crypto.subtle
#
- Type: <SubtleCrypto>
A convenient alias for crypto.webcrypto.subtle
.
crypto.timingSafeEqual(a, b)
#
a
<ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>b
<ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>- Returns: <boolean>
This function compares the underlying bytes that represent the given
ArrayBuffer
, TypedArray
, or DataView
instances using a constant-time
algorithm.
This function does not leak timing information that would allow an attacker to guess one of the values. This is suitable for comparing HMAC digests or secret values like authentication cookies or capability urls.
a
and b
must both be Buffer
s, TypedArray
s, or DataView
s, and they
must have the same byte length. An error is thrown if a
and b
have
different byte lengths.
If at least one of a
and b
is a TypedArray
with more than one byte per
entry, such as Uint16Array
, the result will be computed using the platform
byte order.
When both of the inputs are Float32Array
s or
Float64Array
s, this function might return unexpected results due to IEEE 754
encoding of floating-point numbers. In particular, neither x === y
nor
Object.is(x, y)
implies that the byte representations of two floating-point
numbers x
and y
are equal.
Use of crypto.timingSafeEqual
does not guarantee that the surrounding code
is timing-safe. Care should be taken to ensure that the surrounding code does
not introduce timing vulnerabilities.
crypto.verify(algorithm, data, key, signature[, callback])
#
algorithm
<string> | <null> | <undefined>data
<ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>key
<Object> | <string> | <ArrayBuffer> | <Buffer> | <TypedArray> | <DataView> | <KeyObject> | <CryptoKey>signature
<ArrayBuffer> | <Buffer> | <TypedArray> | <DataView>callback
<Function>- Returns: <boolean>
true
orfalse
depending on the validity of the signature for the data and public key if thecallback
function is not provided.
Verifies the given signature for data
using the given key and algorithm. If
algorithm
is null
or undefined
, then the algorithm is dependent upon the
key type (especially Ed25519 and Ed448).
If key
is not a KeyObject
, this function behaves as if key
had been
passed to crypto.createPublicKey()
. If it is an object, the following
additional properties can be passed:
-
dsaEncoding
<string> For DSA and ECDSA, this option specifies the format of the signature. It can be one of the following:'der'
(default): DER-encoded ASN.1 signature structure encoding(r, s)
.'ieee-p1363'
: Signature formatr || s
as proposed in IEEE-P1363.
-
padding
<integer> Optional padding value for RSA, one of the following:crypto.constants.RSA_PKCS1_PADDING
(default)crypto.constants.RSA_PKCS1_PSS_PADDING
RSA_PKCS1_PSS_PADDING
will use MGF1 with the same hash function used to sign the message as specified in section 3.1 of RFC 4055. -
saltLength
<integer> Salt length for when padding isRSA_PKCS1_PSS_PADDING
. The special valuecrypto.constants.RSA_PSS_SALTLEN_DIGEST
sets the salt length to the digest size,crypto.constants.RSA_PSS_SALTLEN_MAX_SIGN
(default) sets it to the maximum permissible value.
The signature
argument is the previously calculated signature for the data
.
Because public keys can be derived from private keys, a private key or a public
key may be passed for key
.
If the callback
function is provided this function uses libuv's threadpool.
crypto.webcrypto
#
Type: <Crypto> An implementation of the Web Crypto API standard.
See the Web Crypto API documentation for details.
Notes#
Using strings as inputs to cryptographic APIs#
For historical reasons, many cryptographic APIs provided by Node.js accept strings as inputs where the underlying cryptographic algorithm works on byte sequences. These instances include plaintexts, ciphertexts, symmetric keys, initialization vectors, passphrases, salts, authentication tags, and additional authenticated data.
When passing strings to cryptographic APIs, consider the following factors.
-
Not all byte sequences are valid UTF-8 strings. Therefore, when a byte sequence of length
n
is derived from a string, its entropy is generally lower than the entropy of a random or pseudorandomn
byte sequence. For example, no UTF-8 string will result in the byte sequencec0 af
. Secret keys should almost exclusively be random or pseudorandom byte sequences. -
Similarly, when converting random or pseudorandom byte sequences to UTF-8 strings, subsequences that do not represent valid code points may be replaced by the Unicode replacement character (
U+FFFD
). The byte representation of the resulting Unicode string may, therefore, not be equal to the byte sequence that the string was created from.const original = [0xc0, 0xaf]; const bytesAsString = Buffer.from(original).toString('utf8'); const stringAsBytes = Buffer.from(bytesAsString, 'utf8'); console.log(stringAsBytes); // Prints '<Buffer ef bf bd ef bf bd>'.
The outputs of ciphers, hash functions, signature algorithms, and key derivation functions are pseudorandom byte sequences and should not be used as Unicode strings.
-
When strings are obtained from user input, some Unicode characters can be represented in multiple equivalent ways that result in different byte sequences. For example, when passing a user passphrase to a key derivation function, such as PBKDF2 or scrypt, the result of the key derivation function depends on whether the string uses composed or decomposed characters. Node.js does not normalize character representations. Developers should consider using
String.prototype.normalize()
on user inputs before passing them to cryptographic APIs.
Legacy streams API (prior to Node.js 0.10)#
The Crypto module was added to Node.js before there was the concept of a
unified Stream API, and before there were Buffer
objects for handling
binary data. As such, the many of the crypto
defined classes have methods not
typically found on other Node.js classes that implement the streams
API (e.g. update()
, final()
, or digest()
). Also, many methods accepted
and returned 'latin1'
encoded strings by default rather than Buffer
s. This
default was changed after Node.js v0.8 to use Buffer
objects by default
instead.
Support for weak or compromised algorithms#
The node:crypto
module still supports some algorithms which are already
compromised and are not currently recommended for use. The API also allows
the use of ciphers and hashes with a small key size that are too weak for safe
use.
Users should take full responsibility for selecting the crypto algorithm and key size according to their security requirements.
Based on the recommendations of NIST SP 800-131A:
- MD5 and SHA-1 are no longer acceptable where collision resistance is required such as digital signatures.
- The key used with RSA, DSA, and DH algorithms is recommended to have at least 2048 bits and that of the curve of ECDSA and ECDH at least 224 bits, to be safe to use for several years.
- The DH groups of
modp1
,modp2
andmodp5
have a key size smaller than 2048 bits and are not recommended.
See the reference for other recommendations and details.
Some algorithms that have known weaknesses and are of little relevance in practice are only available through the legacy provider, which is not enabled by default.
CCM mode#
CCM is one of the supported AEAD algorithms. Applications which use this mode must adhere to certain restrictions when using the cipher API:
- The authentication tag length must be specified during cipher creation by
setting the
authTagLength
option and must be one of 4, 6, 8, 10, 12, 14 or 16 bytes. - The length of the initialization vector (nonce)
N
must be between 7 and 13 bytes (7 ≤ N ≤ 13
). - The length of the plaintext is limited to
2 ** (8 * (15 - N))
bytes. - When decrypting, the authentication tag must be set via
setAuthTag()
before callingupdate()
. Otherwise, decryption will fail andfinal()
will throw an error in compliance with section 2.6 of RFC 3610. - Using stream methods such as
write(data)
,end(data)
orpipe()
in CCM mode might fail as CCM cannot handle more than one chunk of data per instance. - When passing additional authenticated data (AAD), the length of the actual
message in bytes must be passed to
setAAD()
via theplaintextLength
option. Many crypto libraries include the authentication tag in the ciphertext, which means that they produce ciphertexts of the lengthplaintextLength + authTagLength
. Node.js does not include the authentication tag, so the ciphertext length is alwaysplaintextLength
. This is not necessary if no AAD is used. - As CCM processes the whole message at once,
update()
must be called exactly once. - Even though calling
update()
is sufficient to encrypt/decrypt the message, applications must callfinal()
to compute or verify the authentication tag.
import { Buffer } from 'node:buffer';
const {
createCipheriv,
createDecipheriv,
randomBytes,
} = await import('node:crypto');
const key = 'keykeykeykeykeykeykeykey';
const nonce = randomBytes(12);
const aad = Buffer.from('0123456789', 'hex');
const cipher = createCipheriv('aes-192-ccm', key, nonce, {
authTagLength: 16,
});
const plaintext = 'Hello world';
cipher.setAAD(aad, {
plaintextLength: Buffer.byteLength(plaintext),
});
const ciphertext = cipher.update(plaintext, 'utf8');
cipher.final();
const tag = cipher.getAuthTag();
// Now transmit { ciphertext, nonce, tag }.
const decipher = createDecipheriv('aes-192-ccm', key, nonce, {
authTagLength: 16,
});
decipher.setAuthTag(tag);
decipher.setAAD(aad, {
plaintextLength: ciphertext.length,
});
const receivedPlaintext = decipher.update(ciphertext, null, 'utf8');
try {
decipher.final();
} catch (err) {
throw new Error('Authentication failed!', { cause: err });
}
console.log(receivedPlaintext);
const { Buffer } = require('node:buffer');
const {
createCipheriv,
createDecipheriv,
randomBytes,
} = require('node:crypto');
const key = 'keykeykeykeykeykeykeykey';
const nonce = randomBytes(12);
const aad = Buffer.from('0123456789', 'hex');
const cipher = createCipheriv('aes-192-ccm', key, nonce, {
authTagLength: 16,
});
const plaintext = 'Hello world';
cipher.setAAD(aad, {
plaintextLength: Buffer.byteLength(plaintext),
});
const ciphertext = cipher.update(plaintext, 'utf8');
cipher.final();
const tag = cipher.getAuthTag();
// Now transmit { ciphertext, nonce, tag }.
const decipher = createDecipheriv('aes-192-ccm', key, nonce, {
authTagLength: 16,
});
decipher.setAuthTag(tag);
decipher.setAAD(aad, {
plaintextLength: ciphertext.length,
});
const receivedPlaintext = decipher.update(ciphertext, null, 'utf8');
try {
decipher.final();
} catch (err) {
throw new Error('Authentication failed!', { cause: err });
}
console.log(receivedPlaintext);
Crypto constants#
The following constants exported by crypto.constants
apply to various uses of
the node:crypto
, node:tls
, and node:https
modules and are generally
specific to OpenSSL.
OpenSSL options#
See the list of SSL OP Flags for details.
Constant | Description |
---|---|
SSL_OP_ALL |
Applies multiple bug workarounds within OpenSSL. See https://www.openssl.org/docs/man3.0/man3/SSL_CTX_set_options.html for detail. |
SSL_OP_ALLOW_NO_DHE_KEX |
Instructs OpenSSL to allow a non-[EC]DHE-based key exchange mode for TLS v1.3 |
SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION |
Allows legacy insecure renegotiation between OpenSSL and unpatched clients or servers. See https://www.openssl.org/docs/man3.0/man3/SSL_CTX_set_options.html. |
SSL_OP_CIPHER_SERVER_PREFERENCE |
Attempts to use the server's preferences instead of the client's when selecting a cipher. Behavior depends on protocol version. See https://www.openssl.org/docs/man3.0/man3/SSL_CTX_set_options.html. |
SSL_OP_CISCO_ANYCONNECT |
Instructs OpenSSL to use Cisco's "speshul" version of DTLS_BAD_VER. |
SSL_OP_COOKIE_EXCHANGE |
Instructs OpenSSL to turn on cookie exchange. |
SSL_OP_CRYPTOPRO_TLSEXT_BUG |
Instructs OpenSSL to add server-hello extension from an early version of the cryptopro draft. |
SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS |
Instructs OpenSSL to disable a SSL 3.0/TLS 1.0 vulnerability workaround added in OpenSSL 0.9.6d. |
SSL_OP_LEGACY_SERVER_CONNECT |
Allows initial connection to servers that do not support RI. |
SSL_OP_NO_COMPRESSION |
Instructs OpenSSL to disable support for SSL/TLS compression. |
SSL_OP_NO_ENCRYPT_THEN_MAC |
Instructs OpenSSL to disable encrypt-then-MAC. |
SSL_OP_NO_QUERY_MTU |
|
SSL_OP_NO_RENEGOTIATION |
Instructs OpenSSL to disable renegotiation. |
SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION |
Instructs OpenSSL to always start a new session when performing renegotiation. |
SSL_OP_NO_SSLv2 |
Instructs OpenSSL to turn off SSL v2 |
SSL_OP_NO_SSLv3 |
Instructs OpenSSL to turn off SSL v3 |
SSL_OP_NO_TICKET |
Instructs OpenSSL to disable use of RFC4507bis tickets. |
SSL_OP_NO_TLSv1 |
Instructs OpenSSL to turn off TLS v1 |
SSL_OP_NO_TLSv1_1 |
Instructs OpenSSL to turn off TLS v1.1 |
SSL_OP_NO_TLSv1_2 |
Instructs OpenSSL to turn off TLS v1.2 |
SSL_OP_NO_TLSv1_3 |
Instructs OpenSSL to turn off TLS v1.3 |
SSL_OP_PRIORITIZE_CHACHA |
Instructs OpenSSL server to prioritize ChaCha20-Poly1305
when the client does.
This option has no effect if
SSL_OP_CIPHER_SERVER_PREFERENCE
is not enabled. |
SSL_OP_TLS_ROLLBACK_BUG |
Instructs OpenSSL to disable version rollback attack detection. |
OpenSSL engine constants#
Constant | Description |
---|---|
ENGINE_METHOD_RSA |
Limit engine usage to RSA |
ENGINE_METHOD_DSA |
Limit engine usage to DSA |
ENGINE_METHOD_DH |
Limit engine usage to DH |
ENGINE_METHOD_RAND |
Limit engine usage to RAND |
ENGINE_METHOD_EC |
Limit engine usage to EC |
ENGINE_METHOD_CIPHERS |
Limit engine usage to CIPHERS |
ENGINE_METHOD_DIGESTS |
Limit engine usage to DIGESTS |
ENGINE_METHOD_PKEY_METHS |
Limit engine usage to PKEY_METHDS |
ENGINE_METHOD_PKEY_ASN1_METHS |
Limit engine usage to PKEY_ASN1_METHS |
ENGINE_METHOD_ALL |
|
ENGINE_METHOD_NONE |
Other OpenSSL constants#
Constant | Description |
---|---|
DH_CHECK_P_NOT_SAFE_PRIME |
|
DH_CHECK_P_NOT_PRIME |
|
DH_UNABLE_TO_CHECK_GENERATOR |
|
DH_NOT_SUITABLE_GENERATOR |
|
RSA_PKCS1_PADDING |
|
RSA_SSLV23_PADDING |
|
RSA_NO_PADDING |
|
RSA_PKCS1_OAEP_PADDING |
|
RSA_X931_PADDING |
|
RSA_PKCS1_PSS_PADDING |
|
RSA_PSS_SALTLEN_DIGEST |
Sets the salt length for RSA_PKCS1_PSS_PADDING to the
digest size when signing or verifying. |
RSA_PSS_SALTLEN_MAX_SIGN |
Sets the salt length for RSA_PKCS1_PSS_PADDING to the
maximum permissible value when signing data. |
RSA_PSS_SALTLEN_AUTO |
Causes the salt length for RSA_PKCS1_PSS_PADDING to be
determined automatically when verifying a signature. |
POINT_CONVERSION_COMPRESSED |
|
POINT_CONVERSION_UNCOMPRESSED |
|
POINT_CONVERSION_HYBRID |
Node.js crypto constants#
Constant | Description |
---|---|
defaultCoreCipherList |
Specifies the built-in default cipher list used by Node.js. |
defaultCipherList |
Specifies the active default cipher list used by the current Node.js process. |
Debugger#
Node.js includes a command-line debugging utility. The Node.js debugger client is not a full-featured debugger, but simple stepping and inspection are possible.
To use it, start Node.js with the inspect
argument followed by the path to the
script to debug.
$ node inspect myscript.js
< Debugger listening on ws://127.0.0.1:9229/621111f9-ffcb-4e82-b718-48a145fa5db8
< For help, see: https://nodejs.org/en/docs/inspector
<
connecting to 127.0.0.1:9229 ... ok
< Debugger attached.
<
ok
Break on start in myscript.js:2
1 // myscript.js
> 2 global.x = 5;
3 setTimeout(() => {
4 debugger;
debug>
The debugger automatically breaks on the first executable line. To instead
run until the first breakpoint (specified by a debugger
statement), set
the NODE_INSPECT_RESUME_ON_START
environment variable to 1
.
$ cat myscript.js
// myscript.js
global.x = 5;
setTimeout(() => {
debugger;
console.log('world');
}, 1000);
console.log('hello');
$ NODE_INSPECT_RESUME_ON_START=1 node inspect myscript.js
< Debugger listening on ws://127.0.0.1:9229/f1ed133e-7876-495b-83ae-c32c6fc319c2
< For help, see: https://nodejs.org/en/docs/inspector
<
connecting to 127.0.0.1:9229 ... ok
< Debugger attached.
<
< hello
<
break in myscript.js:4
2 global.x = 5;
3 setTimeout(() => {
> 4 debugger;
5 console.log('world');
6 }, 1000);
debug> next
break in myscript.js:5
3 setTimeout(() => {
4 debugger;
> 5 console.log('world');
6 }, 1000);
7 console.log('hello');
debug> repl
Press Ctrl+C to leave debug repl
> x
5
> 2 + 2
4
debug> next
< world
<
break in myscript.js:6
4 debugger;
5 console.log('world');
> 6 }, 1000);
7 console.log('hello');
8
debug> .exit
$
The repl
command allows code to be evaluated remotely. The next
command
steps to the next line. Type help
to see what other commands are available.
Pressing enter
without typing a command will repeat the previous debugger
command.
Watchers#
It is possible to watch expression and variable values while debugging. On every breakpoint, each expression from the watchers list will be evaluated in the current context and displayed immediately before the breakpoint's source code listing.
To begin watching an expression, type watch('my_expression')
. The command
watchers
will print the active watchers. To remove a watcher, type
unwatch('my_expression')
.
Command reference#
Stepping#
cont
,c
: Continue executionnext
,n
: Step nextstep
,s
: Step inout
,o
: Step outpause
: Pause running code (like pause button in Developer Tools)
Breakpoints#
setBreakpoint()
,sb()
: Set breakpoint on current linesetBreakpoint(line)
,sb(line)
: Set breakpoint on specific linesetBreakpoint('fn()')
,sb(...)
: Set breakpoint on a first statement in function's bodysetBreakpoint('script.js', 1)
,sb(...)
: Set breakpoint on first line ofscript.js
setBreakpoint('script.js', 1, 'num < 4')
,sb(...)
: Set conditional breakpoint on first line ofscript.js
that only breaks whennum < 4
evaluates totrue
clearBreakpoint('script.js', 1)
,cb(...)
: Clear breakpoint inscript.js
on line 1
It is also possible to set a breakpoint in a file (module) that is not loaded yet:
$ node inspect main.js
< Debugger listening on ws://127.0.0.1:9229/48a5b28a-550c-471b-b5e1-d13dd7165df9
< For help, see: https://nodejs.org/en/docs/inspector
<
connecting to 127.0.0.1:9229 ... ok
< Debugger attached.
<
Break on start in main.js:1
> 1 const mod = require('./mod.js');
2 mod.hello();
3 mod.hello();
debug> setBreakpoint('mod.js', 22)
Warning: script 'mod.js' was not loaded yet.
debug> c
break in mod.js:22
20 // USE OR OTHER DEALINGS IN THE SOFTWARE.
21
>22 exports.hello = function() {
23 return 'hello from module';
24 };
debug>
It is also possible to set a conditional breakpoint that only breaks when a
given expression evaluates to true
:
$ node inspect main.js
< Debugger listening on ws://127.0.0.1:9229/ce24daa8-3816-44d4-b8ab-8273c8a66d35
< For help, see: https://nodejs.org/en/docs/inspector
<
connecting to 127.0.0.1:9229 ... ok
< Debugger attached.
Break on start in main.js:7
5 }
6
> 7 addOne(10);
8 addOne(-1);
9
debug> setBreakpoint('main.js', 4, 'num < 0')
1 'use strict';
2
3 function addOne(num) {
> 4 return num + 1;
5 }
6
7 addOne(10);
8 addOne(-1);
9
debug> cont
break in main.js:4
2
3 function addOne(num) {
> 4 return num + 1;
5 }
6
debug> exec('num')
-1
debug>
Information#
backtrace
,bt
: Print backtrace of current execution framelist(5)
: List scripts source code with 5 line context (5 lines before and after)watch(expr)
: Add expression to watch listunwatch(expr)
: Remove expression from watch listunwatch(index)
: Remove expression at specific index from watch listwatchers
: List all watchers and their values (automatically listed on each breakpoint)repl
: Open debugger's repl for evaluation in debugging script's contextexec expr
,p expr
: Execute an expression in debugging script's context and print its valueprofile
: Start CPU profiling sessionprofileEnd
: Stop current CPU profiling sessionprofiles
: List all completed CPU profiling sessionsprofiles[n].save(filepath = 'node.cpuprofile')
: Save CPU profiling session to disk as JSONtakeHeapSnapshot(filepath = 'node.heapsnapshot')
: Take a heap snapshot and save to disk as JSON
Execution control#
run
: Run script (automatically runs on debugger's start)restart
: Restart scriptkill
: Kill script
Various#
scripts
: List all loaded scriptsversion
: Display V8's version
Advanced usage#
V8 inspector integration for Node.js#
V8 Inspector integration allows attaching Chrome DevTools to Node.js instances for debugging and profiling. It uses the Chrome DevTools Protocol.
V8 Inspector can be enabled by passing the --inspect
flag when starting a
Node.js application. It is also possible to supply a custom port with that flag,
e.g. --inspect=9222
will accept DevTools connections on port 9222.
To break on the first line of the application code, pass the --inspect-brk
flag instead of --inspect
.
$ node --inspect index.js
Debugger listening on ws://127.0.0.1:9229/dc9010dd-f8b8-4ac5-a510-c1a114ec7d29
For help, see: https://nodejs.org/en/docs/inspector
(In the example above, the UUID dc9010dd-f8b8-4ac5-a510-c1a114ec7d29 at the end of the URL is generated on the fly, it varies in different debugging sessions.)
If the Chrome browser is older than 66.0.3345.0,
use inspector.html
instead of js_app.html
in the above URL.
Chrome DevTools doesn't support debugging worker threads yet. ndb can be used to debug them.
Deprecated APIs#
Node.js APIs might be deprecated for any of the following reasons:
- Use of the API is unsafe.
- An improved alternative API is available.
- Breaking changes to the API are expected in a future major release.
Node.js uses three kinds of Deprecations:
- Documentation-only
- Runtime
- End-of-Life
A Documentation-only deprecation is one that is expressed only within the
Node.js API docs. These generate no side-effects while running Node.js.
Some Documentation-only deprecations trigger a runtime warning when launched
with --pending-deprecation
flag (or its alternative,
NODE_PENDING_DEPRECATION=1
environment variable), similarly to Runtime
deprecations below. Documentation-only deprecations that support that flag
are explicitly labeled as such in the
list of Deprecated APIs.
A Runtime deprecation will, by default, generate a process warning that will
be printed to stderr
the first time the deprecated API is used. When the
--throw-deprecation
command-line flag is used, a Runtime deprecation will
cause an error to be thrown.
An End-of-Life deprecation is used when functionality is or will soon be removed from Node.js.
Revoking deprecations#
Occasionally, the deprecation of an API might be reversed. In such situations, this document will be updated with information relevant to the decision. However, the deprecation identifier will not be modified.
List of deprecated APIs#
DEP0001: http.OutgoingMessage.prototype.flush
#
Type: End-of-Life
OutgoingMessage.prototype.flush()
has been removed. Use
OutgoingMessage.prototype.flushHeaders()
instead.
DEP0002: require('_linklist')
#
Type: End-of-Life
The _linklist
module is deprecated. Please use a userland alternative.
DEP0003: _writableState.buffer
#
Type: End-of-Life
The _writableState.buffer
has been removed. Use _writableState.getBuffer()
instead.
DEP0004: CryptoStream.prototype.readyState
#
Type: End-of-Life
The CryptoStream.prototype.readyState
property was removed.
DEP0005: Buffer()
constructor#
Type: Runtime (supports --pending-deprecation
)
The Buffer()
function and new Buffer()
constructor are deprecated due to
API usability issues that can lead to accidental security issues.
As an alternative, use one of the following methods of constructing Buffer
objects:
Buffer.alloc(size[, fill[, encoding]])
: Create aBuffer
with initialized memory.Buffer.allocUnsafe(size)
: Create aBuffer
with uninitialized memory.Buffer.allocUnsafeSlow(size)
: Create aBuffer
with uninitialized memory.Buffer.from(array)
: Create aBuffer
with a copy ofarray
Buffer.from(arrayBuffer[, byteOffset[, length]])
- Create aBuffer
that wraps the givenarrayBuffer
.Buffer.from(buffer)
: Create aBuffer
that copiesbuffer
.Buffer.from(string[, encoding])
: Create aBuffer
that copiesstring
.
Without --pending-deprecation
, runtime warnings occur only for code not in
node_modules
. This means there will not be deprecation warnings for
Buffer()
usage in dependencies. With --pending-deprecation
, a runtime
warning results no matter where the Buffer()
usage occurs.
DEP0006: child_process
options.customFds
#
Type: End-of-Life
Within the child_process
module's spawn()
, fork()
, and exec()
methods, the options.customFds
option is deprecated. The options.stdio
option should be used instead.
DEP0007: Replace cluster
worker.suicide
with worker.exitedAfterDisconnect
#
Type: End-of-Life
In an earlier version of the Node.js cluster
, a boolean property with the name
suicide
was added to the Worker
object. The intent of this property was to
provide an indication of how and why the Worker
instance exited. In Node.js
6.0.0, the old property was deprecated and replaced with a new
worker.exitedAfterDisconnect
property. The old property name did not
precisely describe the actual semantics and was unnecessarily emotion-laden.
DEP0008: require('node:constants')
#
Type: Documentation-only
The node:constants
module is deprecated. When requiring access to constants
relevant to specific Node.js builtin modules, developers should instead refer
to the constants
property exposed by the relevant module. For instance,
require('node:fs').constants
and require('node:os').constants
.
DEP0009: crypto.pbkdf2
without digest#
Type: End-of-Life
Use of the crypto.pbkdf2()
API without specifying a digest was deprecated
in Node.js 6.0 because the method defaulted to using the non-recommended
'SHA1'
digest. Previously, a deprecation warning was printed. Starting in
Node.js 8.0.0, calling crypto.pbkdf2()
or crypto.pbkdf2Sync()
with
digest
set to undefined
will throw a TypeError
.
Beginning in Node.js v11.0.0, calling these functions with digest
set to
null
would print a deprecation warning to align with the behavior when digest
is undefined
.
Now, however, passing either undefined
or null
will throw a TypeError
.
DEP0010: crypto.createCredentials
#
Type: End-of-Life
The crypto.createCredentials()
API was removed. Please use
tls.createSecureContext()
instead.
DEP0011: crypto.Credentials
#
Type: End-of-Life
The crypto.Credentials
class was removed. Please use tls.SecureContext
instead.
DEP0012: Domain.dispose
#
Type: End-of-Life
Domain.dispose()
has been removed. Recover from failed I/O actions
explicitly via error event handlers set on the domain instead.
DEP0013: fs
asynchronous function without callback#
Type: End-of-Life
Calling an asynchronous function without a callback throws a TypeError
in Node.js 10.0.0 onwards. See https://github.com/nodejs/node/pull/12562.
DEP0014: fs.read
legacy String interface#
Type: End-of-Life
The fs.read()
legacy String
interface is deprecated. Use the Buffer
API as mentioned in the documentation instead.
DEP0015: fs.readSync
legacy String interface#
Type: End-of-Life
The fs.readSync()
legacy String
interface is deprecated. Use the
Buffer
API as mentioned in the documentation instead.
DEP0016: GLOBAL
/root
#
Type: End-of-Life
The GLOBAL
and root
aliases for the global
property were deprecated
in Node.js 6.0.0 and have since been removed.
DEP0017: Intl.v8BreakIterator
#
Type: End-of-Life
Intl.v8BreakIterator
was a non-standard extension and has been removed.
See Intl.Segmenter
.
DEP0018: Unhandled promise rejections#
Type: End-of-Life
Unhandled promise rejections are deprecated. By default, promise rejections
that are not handled terminate the Node.js process with a non-zero exit
code. To change the way Node.js treats unhandled rejections, use the
--unhandled-rejections
command-line option.
DEP0019: require('.')
resolved outside directory#
Type: End-of-Life
In certain cases, require('.')
could resolve outside the package directory.
This behavior has been removed.
DEP0020: Server.connections
#
Type: End-of-Life
The Server.connections
property was deprecated in Node.js v0.9.7 and has
been removed. Please use the Server.getConnections()
method instead.
DEP0021: Server.listenFD
#
Type: End-of-Life
The Server.listenFD()
method was deprecated and removed. Please use
Server.listen({fd: <number>})
instead.
DEP0022: os.tmpDir()
#
Type: End-of-Life
The os.tmpDir()
API was deprecated in Node.js 7.0.0 and has since been
removed. Please use os.tmpdir()
instead.
DEP0023: os.getNetworkInterfaces()
#
Type: End-of-Life
The os.getNetworkInterfaces()
method is deprecated. Please use the
os.networkInterfaces()
method instead.
DEP0024: REPLServer.prototype.convertToContext()
#
Type: End-of-Life
The REPLServer.prototype.convertToContext()
API has been removed.
DEP0025: require('node:sys')
#
Type: Runtime
The node:sys
module is deprecated. Please use the util
module instead.
DEP0026: util.print()
#
Type: End-of-Life
util.print()
has been removed. Please use console.log()
instead.
DEP0027: util.puts()
#
Type: End-of-Life
util.puts()
has been removed. Please use console.log()
instead.
DEP0028: util.debug()
#
Type: End-of-Life
util.debug()
has been removed. Please use console.error()
instead.
DEP0029: util.error()
#
Type: End-of-Life
util.error()
has been removed. Please use console.error()
instead.
DEP0030: SlowBuffer
#
Type: Documentation-only
The SlowBuffer
class is deprecated. Please use
Buffer.allocUnsafeSlow(size)
instead.
DEP0031: ecdh.setPublicKey()
#
Type: Documentation-only
The ecdh.setPublicKey()
method is now deprecated as its inclusion in the
API is not useful.
DEP0032: node:domain
module#
Type: Documentation-only
The domain
module is deprecated and should not be used.
DEP0033: EventEmitter.listenerCount()
#
Type: Documentation-only
The events.listenerCount(emitter, eventName)
API is
deprecated. Please use emitter.listenerCount(eventName)
instead.
DEP0034: fs.exists(path, callback)
#
Type: Documentation-only
The fs.exists(path, callback)
API is deprecated. Please use
fs.stat()
or fs.access()
instead.
DEP0035: fs.lchmod(path, mode, callback)
#
Type: Documentation-only
The fs.lchmod(path, mode, callback)
API is deprecated.
DEP0036: fs.lchmodSync(path, mode)
#
Type: Documentation-only
The fs.lchmodSync(path, mode)
API is deprecated.
DEP0037: fs.lchown(path, uid, gid, callback)
#
Type: Deprecation revoked
The fs.lchown(path, uid, gid, callback)
API was deprecated. The
deprecation was revoked because the requisite supporting APIs were added in
libuv.
DEP0038: fs.lchownSync(path, uid, gid)
#
Type: Deprecation revoked
The fs.lchownSync(path, uid, gid)
API was deprecated. The deprecation was
revoked because the requisite supporting APIs were added in libuv.
DEP0039: require.extensions
#
Type: Documentation-only
The require.extensions
property is deprecated.
DEP0040: node:punycode
module#
Type: Documentation-only (supports --pending-deprecation
)
The punycode
module is deprecated. Please use a userland alternative
instead.
DEP0041: NODE_REPL_HISTORY_FILE
environment variable#
Type: End-of-Life
The NODE_REPL_HISTORY_FILE
environment variable was removed. Please use
NODE_REPL_HISTORY
instead.
DEP0042: tls.CryptoStream
#
Type: End-of-Life
The tls.CryptoStream
class was removed. Please use
tls.TLSSocket
instead.
DEP0043: tls.SecurePair
#
Type: Documentation-only
The tls.SecurePair
class is deprecated. Please use
tls.TLSSocket
instead.
DEP0044: util.isArray()
#
Type: Documentation-only
The util.isArray()
API is deprecated. Please use Array.isArray()
instead.
DEP0045: util.isBoolean()
#
Type: Documentation-only
The util.isBoolean()
API is deprecated.
DEP0046: util.isBuffer()
#
Type: Documentation-only
The util.isBuffer()
API is deprecated. Please use
Buffer.isBuffer()
instead.
DEP0047: util.isDate()
#
Type: Documentation-only
The util.isDate()
API is deprecated.
DEP0048: util.isError()
#
Type: Documentation-only
The util.isError()
API is deprecated.
DEP0049: util.isFunction()
#
Type: Documentation-only
The util.isFunction()
API is deprecated.
DEP0050: util.isNull()
#
Type: Documentation-only
The util.isNull()
API is deprecated.
DEP0051: util.isNullOrUndefined()
#
Type: Documentation-only
The util.isNullOrUndefined()
API is deprecated.
DEP0052: util.isNumber()
#
Type: Documentation-only
The util.isNumber()
API is deprecated.
DEP0053: util.isObject()
#
Type: Documentation-only
The util.isObject()
API is deprecated.
DEP0054: util.isPrimitive()
#
Type: Documentation-only
The util.isPrimitive()
API is deprecated.
DEP0055: util.isRegExp()
#
Type: Documentation-only
The util.isRegExp()
API is deprecated.
DEP0056: util.isString()
#
Type: Documentation-only
The util.isString()
API is deprecated.
DEP0057: util.isSymbol()
#
Type: Documentation-only
The util.isSymbol()
API is deprecated.
DEP0058: util.isUndefined()
#
Type: Documentation-only
The util.isUndefined()
API is deprecated.
DEP0059: util.log()
#
Type: Documentation-only
The util.log()
API is deprecated.
DEP0060: util._extend()
#
Type: Documentation-only
The util._extend()
API is deprecated.
DEP0061: fs.SyncWriteStream
#
Type: End-of-Life
The fs.SyncWriteStream
class was never intended to be a publicly accessible
API and has been removed. No alternative API is available. Please use a userland
alternative.
DEP0062: node --debug
#
Type: End-of-Life
--debug
activates the legacy V8 debugger interface, which was removed as
of V8 5.8. It is replaced by Inspector which is activated with --inspect
instead.
DEP0063: ServerResponse.prototype.writeHeader()
#
Type: Documentation-only
The node:http
module ServerResponse.prototype.writeHeader()
API is
deprecated. Please use ServerResponse.prototype.writeHead()
instead.
The ServerResponse.prototype.writeHeader()
method was never documented as an
officially supported API.
DEP0064: tls.createSecurePair()
#
Type: Runtime
The tls.createSecurePair()
API was deprecated in documentation in Node.js
0.11.3. Users should use tls.Socket
instead.
DEP0065: repl.REPL_MODE_MAGIC
and NODE_REPL_MODE=magic
#
Type: End-of-Life
The node:repl
module's REPL_MODE_MAGIC
constant, used for replMode
option,
has been removed. Its behavior has been functionally identical to that of
REPL_MODE_SLOPPY
since Node.js 6.0.0, when V8 5.0 was imported. Please use
REPL_MODE_SLOPPY
instead.
The NODE_REPL_MODE
environment variable is used to set the underlying
replMode
of an interactive node
session. Its value, magic
, is also
removed. Please use sloppy
instead.
DEP0066: OutgoingMessage.prototype._headers, OutgoingMessage.prototype._headerNames
#
Type: Runtime
The node:http
module OutgoingMessage.prototype._headers
and
OutgoingMessage.prototype._headerNames
properties are deprecated. Use one of
the public methods (e.g. OutgoingMessage.prototype.getHeader()
,
OutgoingMessage.prototype.getHeaders()
,
OutgoingMessage.prototype.getHeaderNames()
,
OutgoingMessage.prototype.getRawHeaderNames()
,
OutgoingMessage.prototype.hasHeader()
,
OutgoingMessage.prototype.removeHeader()
,
OutgoingMessage.prototype.setHeader()
) for working with outgoing headers.
The OutgoingMessage.prototype._headers
and
OutgoingMessage.prototype._headerNames
properties were never documented as
officially supported properties.
DEP0067: OutgoingMessage.prototype._renderHeaders
#
Type: Documentation-only
The node:http
module OutgoingMessage.prototype._renderHeaders()
API is
deprecated.
The OutgoingMessage.prototype._renderHeaders
property was never documented as
an officially supported API.
DEP0068: node debug
#
Type: End-of-Life
node debug
corresponds to the legacy CLI debugger which has been replaced with
a V8-inspector based CLI debugger available through node inspect
.
DEP0069: vm.runInDebugContext(string)
#
Type: End-of-Life
DebugContext has been removed in V8 and is not available in Node.js 10+.
DebugContext was an experimental API.
DEP0070: async_hooks.currentId()
#
Type: End-of-Life
async_hooks.currentId()
was renamed to async_hooks.executionAsyncId()
for
clarity.
This change was made while async_hooks
was an experimental API.
DEP0071: async_hooks.triggerId()
#
Type: End-of-Life
async_hooks.triggerId()
was renamed to async_hooks.triggerAsyncId()
for
clarity.
This change was made while async_hooks
was an experimental API.
DEP0072: async_hooks.AsyncResource.triggerId()
#
Type: End-of-Life
async_hooks.AsyncResource.triggerId()
was renamed to
async_hooks.AsyncResource.triggerAsyncId()
for clarity.
This change was made while async_hooks
was an experimental API.
DEP0073: Several internal properties of net.Server
#
Type: End-of-Life
Accessing several internal, undocumented properties of net.Server
instances
with inappropriate names is deprecated.
As the original API was undocumented and not generally useful for non-internal code, no replacement API is provided.
DEP0074: REPLServer.bufferedCommand
#
Type: End-of-Life
The REPLServer.bufferedCommand
property was deprecated in favor of
REPLServer.clearBufferedCommand()
.
DEP0075: REPLServer.parseREPLKeyword()
#
Type: End-of-Life
REPLServer.parseREPLKeyword()
was removed from userland visibility.
DEP0076: tls.parseCertString()
#
Type: End-of-Life
tls.parseCertString()
was a trivial parsing helper that was made public by
mistake. While it was supposed to parse certificate subject and issuer strings,
it never handled multi-value Relative Distinguished Names correctly.
Earlier versions of this document suggested using querystring.parse()
as an
alternative to tls.parseCertString()
. However, querystring.parse()
also does
not handle all certificate subjects correctly and should not be used.
DEP0077: Module._debug()
#
Type: Runtime
Module._debug()
is deprecated.
The Module._debug()
function was never documented as an officially
supported API.
DEP0078: REPLServer.turnOffEditorMode()
#
Type: End-of-Life
REPLServer.turnOffEditorMode()
was removed from userland visibility.
DEP0079: Custom inspection function on objects via .inspect()
#
Type: End-of-Life
Using a property named inspect
on an object to specify a custom inspection
function for util.inspect()
is deprecated. Use util.inspect.custom
instead. For backward compatibility with Node.js prior to version 6.4.0, both
can be specified.
DEP0080: path._makeLong()
#
Type: Documentation-only
The internal path._makeLong()
was not intended for public use. However,
userland modules have found it useful. The internal API is deprecated
and replaced with an identical, public path.toNamespacedPath()
method.
DEP0081: fs.truncate()
using a file descriptor#
Type: Runtime
fs.truncate()
fs.truncateSync()
usage with a file descriptor is
deprecated. Please use fs.ftruncate()
or fs.ftruncateSync()
to work with
file descriptors.
DEP0082: REPLServer.prototype.memory()
#
Type: End-of-Life
REPLServer.prototype.memory()
is only necessary for the internal mechanics of
the REPLServer
itself. Do not use this function.
DEP0083: Disabling ECDH by setting ecdhCurve
to false
#
Type: End-of-Life.
The ecdhCurve
option to tls.createSecureContext()
and tls.TLSSocket
could
be set to false
to disable ECDH entirely on the server only. This mode was
deprecated in preparation for migrating to OpenSSL 1.1.0 and consistency with
the client and is now unsupported. Use the ciphers
parameter instead.
DEP0084: requiring bundled internal dependencies#
Type: End-of-Life
Since Node.js versions 4.4.0 and 5.2.0, several modules only intended for
internal usage were mistakenly exposed to user code through require()
. These
modules were:
v8/tools/codemap
v8/tools/consarray
v8/tools/csvparser
v8/tools/logreader
v8/tools/profile_view
v8/tools/profile
v8/tools/SourceMap
v8/tools/splaytree
v8/tools/tickprocessor-driver
v8/tools/tickprocessor
node-inspect/lib/_inspect
(from 7.6.0)node-inspect/lib/internal/inspect_client
(from 7.6.0)node-inspect/lib/internal/inspect_repl
(from 7.6.0)
The v8/*
modules do not have any exports, and if not imported in a specific
order would in fact throw errors. As such there are virtually no legitimate use
cases for importing them through require()
.
On the other hand, node-inspect
can be installed locally through a package
manager, as it is published on the npm registry under the same name. No source
code modification is necessary if that is done.
DEP0085: AsyncHooks sensitive API#
Type: End-of-Life
The AsyncHooks sensitive API was never documented and had various minor issues.
Use the AsyncResource
API instead. See
https://github.com/nodejs/node/issues/15572.
DEP0086: Remove runInAsyncIdScope
#
Type: End-of-Life
runInAsyncIdScope
doesn't emit the 'before'
or 'after'
event and can thus
cause a lot of issues. See https://github.com/nodejs/node/issues/14328.
DEP0089: require('node:assert')
#
Type: Deprecation revoked
Importing assert directly was not recommended as the exposed functions use
loose equality checks. The deprecation was revoked because use of the
node:assert
module is not discouraged, and the deprecation caused developer
confusion.
DEP0090: Invalid GCM authentication tag lengths#
Type: End-of-Life
Node.js used to support all GCM authentication tag lengths which are accepted by
OpenSSL when calling decipher.setAuthTag()
. Beginning with Node.js
v11.0.0, only authentication tag lengths of 128, 120, 112, 104, 96, 64, and 32
bits are allowed. Authentication tags of other lengths are invalid per
NIST SP 800-38D.
DEP0091: crypto.DEFAULT_ENCODING
#
Type: End-of-Life
The crypto.DEFAULT_ENCODING
property only existed for compatibility with
Node.js releases prior to versions 0.9.3 and has been removed.
DEP0092: Top-level this
bound to module.exports
#
Type: Documentation-only
Assigning properties to the top-level this
as an alternative
to module.exports
is deprecated. Developers should use exports
or module.exports
instead.
DEP0093: crypto.fips
is deprecated and replaced#
Type: Documentation-only
The crypto.fips
property is deprecated. Please use crypto.setFips()
and crypto.getFips()
instead.
DEP0094: Using assert.fail()
with more than one argument#
Type: Runtime
Using assert.fail()
with more than one argument is deprecated. Use
assert.fail()
with only one argument or use a different node:assert
module
method.
DEP0095: timers.enroll()
#
Type: Runtime
timers.enroll()
is deprecated. Please use the publicly documented
setTimeout()
or setInterval()
instead.
DEP0096: timers.unenroll()
#
Type: Runtime
timers.unenroll()
is deprecated. Please use the publicly documented
clearTimeout()
or clearInterval()
instead.
DEP0097: MakeCallback
with domain
property#
Type: Runtime
Users of MakeCallback
that add the domain
property to carry context,
should start using the async_context
variant of MakeCallback
or
CallbackScope
, or the high-level AsyncResource
class.
DEP0098: AsyncHooks embedder AsyncResource.emitBefore
and AsyncResource.emitAfter
APIs#
Type: End-of-Life
The embedded API provided by AsyncHooks exposes .emitBefore()
and
.emitAfter()
methods which are very easy to use incorrectly which can lead
to unrecoverable errors.
Use asyncResource.runInAsyncScope()
API instead which provides a much
safer, and more convenient, alternative. See
https://github.com/nodejs/node/pull/18513.
DEP0099: Async context-unaware node::MakeCallback
C++ APIs#
Type: Compile-time
Certain versions of node::MakeCallback
APIs available to native addons are
deprecated. Please use the versions of the API that accept an async_context
parameter.
DEP0100: process.assert()
#
Type: Runtime
process.assert()
is deprecated. Please use the assert
module instead.
This was never a documented feature.
DEP0101: --with-lttng
#
Type: End-of-Life
The --with-lttng
compile-time option has been removed.
DEP0102: Using noAssert
in Buffer#(read|write)
operations#
Type: End-of-Life
Using the noAssert
argument has no functionality anymore. All input is
verified regardless of the value of noAssert
. Skipping the verification
could lead to hard-to-find errors and crashes.
DEP0103: process.binding('util').is[...]
typechecks#
Type: Documentation-only (supports --pending-deprecation
)
Using process.binding()
in general should be avoided. The type checking
methods in particular can be replaced by using util.types
.
This deprecation has been superseded by the deprecation of the
process.binding()
API (DEP0111).
DEP0104: process.env
string coercion#
Type: Documentation-only (supports --pending-deprecation
)
When assigning a non-string property to process.env
, the assigned value is
implicitly converted to a string. This behavior is deprecated if the assigned
value is not a string, boolean, or number. In the future, such assignment might
result in a thrown error. Please convert the property to a string before
assigning it to process.env
.
DEP0105: decipher.finaltol
#
Type: End-of-Life
decipher.finaltol()
has never been documented and was an alias for
decipher.final()
. This API has been removed, and it is recommended to use
decipher.final()
instead.
DEP0106: crypto.createCipher
and crypto.createDecipher
#
Type: Runtime
Using crypto.createCipher()
and crypto.createDecipher()
must be
avoided as they use a weak key derivation function (MD5 with no salt) and static
initialization vectors. It is recommended to derive a key using
crypto.pbkdf2()
or crypto.scrypt()
with random salts and to use
crypto.createCipheriv()
and crypto.createDecipheriv()
to obtain the
Cipher
and Decipher
objects respectively.
DEP0107: tls.convertNPNProtocols()
#
Type: End-of-Life
This was an undocumented helper function not intended for use outside Node.js core and obsoleted by the removal of NPN (Next Protocol Negotiation) support.
DEP0108: zlib.bytesRead
#
Type: Runtime
Deprecated alias for zlib.bytesWritten
. This original name was chosen
because it also made sense to interpret the value as the number of bytes
read by the engine, but is inconsistent with other streams in Node.js that
expose values under these names.
DEP0109: http
, https
, and tls
support for invalid URLs#
Type: End-of-Life
Some previously supported (but strictly invalid) URLs were accepted through the
http.request()
, http.get()
, https.request()
,
https.get()
, and tls.checkServerIdentity()
APIs because those were
accepted by the legacy url.parse()
API. The mentioned APIs now use the WHATWG
URL parser that requires strictly valid URLs. Passing an invalid URL is
deprecated and support will be removed in the future.
DEP0110: vm.Script
cached data#
Type: Documentation-only
The produceCachedData
option is deprecated. Use
script.createCachedData()
instead.
DEP0111: process.binding()
#
Type: Documentation-only (supports --pending-deprecation
)
process.binding()
is for use by Node.js internal code only.
DEP0112: dgram
private APIs#
Type: Runtime
The node:dgram
module previously contained several APIs that were never meant
to accessed outside of Node.js core: Socket.prototype._handle
,
Socket.prototype._receiving
, Socket.prototype._bindState
,
Socket.prototype._queue
, Socket.prototype._reuseAddr
,
Socket.prototype._healthCheck()
, Socket.prototype._stopReceiving()
, and
dgram._createSocketHandle()
.
DEP0113: Cipher.setAuthTag()
, Decipher.getAuthTag()
#
Type: End-of-Life
Cipher.setAuthTag()
and Decipher.getAuthTag()
are no longer available. They
were never documented and would throw when called.
DEP0114: crypto._toBuf()
#
Type: End-of-Life
The crypto._toBuf()
function was not designed to be used by modules outside
of Node.js core and was removed.
DEP0115: crypto.prng()
, crypto.pseudoRandomBytes()
, crypto.rng()
#
Type: Documentation-only (supports --pending-deprecation
)
In recent versions of Node.js, there is no difference between
crypto.randomBytes()
and crypto.pseudoRandomBytes()
. The latter is
deprecated along with the undocumented aliases crypto.prng()
and
crypto.rng()
in favor of crypto.randomBytes()
and might be removed in a
future release.
DEP0116: Legacy URL API#
Type: Deprecation revoked
The legacy URL API is deprecated. This includes url.format()
,
url.parse()
, url.resolve()
, and the legacy urlObject
. Please
use the WHATWG URL API instead.
DEP0117: Native crypto handles#
Type: End-of-Life
Previous versions of Node.js exposed handles to internal native objects through
the _handle
property of the Cipher
, Decipher
, DiffieHellman
,
DiffieHellmanGroup
, ECDH
, Hash
, Hmac
, Sign
, and Verify
classes.
The _handle
property has been removed because improper use of the native
object can lead to crashing the application.
DEP0118: dns.lookup()
support for a falsy host name#
Type: Runtime
Previous versions of Node.js supported dns.lookup()
with a falsy host name
like dns.lookup(false)
due to backward compatibility.
This behavior is undocumented and is thought to be unused in real world apps.
It will become an error in future versions of Node.js.
DEP0119: process.binding('uv').errname()
private API#
Type: Documentation-only (supports --pending-deprecation
)
process.binding('uv').errname()
is deprecated. Please use
util.getSystemErrorName()
instead.
DEP0120: Windows Performance Counter support#
Type: End-of-Life
Windows Performance Counter support has been removed from Node.js. The
undocumented COUNTER_NET_SERVER_CONNECTION()
,
COUNTER_NET_SERVER_CONNECTION_CLOSE()
, COUNTER_HTTP_SERVER_REQUEST()
,
COUNTER_HTTP_SERVER_RESPONSE()
, COUNTER_HTTP_CLIENT_REQUEST()
, and
COUNTER_HTTP_CLIENT_RESPONSE()
functions have been deprecated.
DEP0121: net._setSimultaneousAccepts()
#
Type: Runtime
The undocumented net._setSimultaneousAccepts()
function was originally
intended for debugging and performance tuning when using the
node:child_process
and node:cluster
modules on Windows. The function is not
generally useful and is being removed. See discussion here:
https://github.com/nodejs/node/issues/18391
DEP0122: tls
Server.prototype.setOptions()
#
Type: Runtime
Please use Server.prototype.setSecureContext()
instead.
DEP0123: setting the TLS ServerName to an IP address#
Type: Runtime
Setting the TLS ServerName to an IP address is not permitted by RFC 6066. This will be ignored in a future version.
DEP0124: using REPLServer.rli
#
Type: End-of-Life
This property is a reference to the instance itself.
DEP0125: require('node:_stream_wrap')
#
Type: Runtime
The node:_stream_wrap
module is deprecated.
DEP0126: timers.active()
#
Type: Runtime
The previously undocumented timers.active()
is deprecated.
Please use the publicly documented timeout.refresh()
instead.
If re-referencing the timeout is necessary, timeout.ref()
can be used
with no performance impact since Node.js 10.
DEP0127: timers._unrefActive()
#
Type: Runtime
The previously undocumented and "private" timers._unrefActive()
is deprecated.
Please use the publicly documented timeout.refresh()
instead.
If unreferencing the timeout is necessary, timeout.unref()
can be used
with no performance impact since Node.js 10.
DEP0128: modules with an invalid main
entry and an index.js
file#
Type: Runtime
Modules that have an invalid main
entry (e.g., ./does-not-exist.js
) and
also have an index.js
file in the top level directory will resolve the
index.js
file. That is deprecated and is going to throw an error in future
Node.js versions.
DEP0129: ChildProcess._channel
#
Type: Runtime
The _channel
property of child process objects returned by spawn()
and
similar functions is not intended for public use. Use ChildProcess.channel
instead.
DEP0130: Module.createRequireFromPath()
#
Type: End-of-Life
Use module.createRequire()
instead.
DEP0131: Legacy HTTP parser#
Type: End-of-Life
The legacy HTTP parser, used by default in versions of Node.js prior to 12.0.0,
is deprecated and has been removed in v13.0.0. Prior to v13.0.0, the
--http-parser=legacy
command-line flag could be used to revert to using the
legacy parser.
DEP0132: worker.terminate()
with callback#
Type: Runtime
Passing a callback to worker.terminate()
is deprecated. Use the returned
Promise
instead, or a listener to the worker's 'exit'
event.
DEP0133: http
connection
#
Type: Documentation-only
Prefer response.socket
over response.connection
and
request.socket
over request.connection
.
DEP0134: process._tickCallback
#
Type: Documentation-only (supports --pending-deprecation
)
The process._tickCallback
property was never documented as
an officially supported API.
DEP0135: WriteStream.open()
and ReadStream.open()
are internal#
Type: Runtime
WriteStream.open()
and ReadStream.open()
are undocumented internal
APIs that do not make sense to use in userland. File streams should always be
opened through their corresponding factory methods fs.createWriteStream()
and fs.createReadStream()
) or by passing a file descriptor in options.
DEP0136: http
finished
#
Type: Documentation-only
response.finished
indicates whether response.end()
has been
called, not whether 'finish'
has been emitted and the underlying data
is flushed.
Use response.writableFinished
or response.writableEnded
accordingly instead to avoid the ambiguity.
To maintain existing behavior response.finished
should be replaced with
response.writableEnded
.
DEP0137: Closing fs.FileHandle on garbage collection#
Type: Runtime
Allowing a fs.FileHandle
object to be closed on garbage collection is
deprecated. In the future, doing so might result in a thrown error that will
terminate the process.
Please ensure that all fs.FileHandle
objects are explicitly closed using
FileHandle.prototype.close()
when the fs.FileHandle
is no longer needed:
const fsPromises = require('node:fs').promises;
async function openAndClose() {
let filehandle;
try {
filehandle = await fsPromises.open('thefile.txt', 'r');
} finally {
if (filehandle !== undefined)
await filehandle.close();
}
}
DEP0138: process.mainModule
#
Type: Documentation-only
process.mainModule
is a CommonJS-only feature while process
global
object is shared with non-CommonJS environment. Its use within ECMAScript
modules is unsupported.
It is deprecated in favor of require.main
, because it serves the same
purpose and is only available on CommonJS environment.
DEP0139: process.umask()
with no arguments#
Type: Documentation-only
Calling process.umask()
with no argument causes the process-wide umask to be
written twice. This introduces a race condition between threads, and is a
potential security vulnerability. There is no safe, cross-platform alternative
API.
DEP0140: Use request.destroy()
instead of request.abort()
#
Type: Documentation-only
Use request.destroy()
instead of request.abort()
.
DEP0141: repl.inputStream
and repl.outputStream
#
Type: Documentation-only (supports --pending-deprecation
)
The node:repl
module exported the input and output stream twice. Use .input
instead of .inputStream
and .output
instead of .outputStream
.
DEP0142: repl._builtinLibs
#
Type: Documentation-only
The node:repl
module exports a _builtinLibs
property that contains an array
of built-in modules. It was incomplete so far and instead it's better to rely
upon require('node:module').builtinModules
.
DEP0143: Transform._transformState
#
Type: Runtime
Transform._transformState
will be removed in future versions where it is
no longer required due to simplification of the implementation.
DEP0144: module.parent
#
Type: Documentation-only (supports --pending-deprecation
)
A CommonJS module can access the first module that required it using
module.parent
. This feature is deprecated because it does not work
consistently in the presence of ECMAScript modules and because it gives an
inaccurate representation of the CommonJS module graph.
Some modules use it to check if they are the entry point of the current process.
Instead, it is recommended to compare require.main
and module
:
if (require.main === module) {
// Code section that will run only if current file is the entry point.
}
When looking for the CommonJS modules that have required the current one,
require.cache
and module.children
can be used:
const moduleParents = Object.values(require.cache)
.filter((m) => m.children.includes(module));
DEP0145: socket.bufferSize
#
Type: Documentation-only
socket.bufferSize
is just an alias for writable.writableLength
.
DEP0146: new crypto.Certificate()
#
Type: Documentation-only
The crypto.Certificate()
constructor is deprecated. Use
static methods of crypto.Certificate()
instead.
DEP0147: fs.rmdir(path, { recursive: true })
#
Type: Runtime
In future versions of Node.js, recursive
option will be ignored for
fs.rmdir
, fs.rmdirSync
, and fs.promises.rmdir
.
Use fs.rm(path, { recursive: true, force: true })
,
fs.rmSync(path, { recursive: true, force: true })
or
fs.promises.rm(path, { recursive: true, force: true })
instead.
DEP0148: Folder mappings in "exports"
(trailing "/"
)#
Type: Runtime
Using a trailing "/"
to define subpath folder mappings in the
subpath exports or subpath imports fields is deprecated. Use
subpath patterns instead.
DEP0149: http.IncomingMessage#connection
#
Type: Documentation-only.
Prefer message.socket
over message.connection
.
DEP0150: Changing the value of process.config
#
Type: End-of-Life
The process.config
property provides access to Node.js compile-time settings.
However, the property is mutable and therefore subject to tampering. The ability
to change the value will be removed in a future version of Node.js.
DEP0151: Main index lookup and extension searching#
Type: Runtime
Previously, index.js
and extension searching lookups would apply to
import 'pkg'
main entry point resolution, even when resolving ES modules.
With this deprecation, all ES module main entry point resolutions require
an explicit "exports"
or "main"
entry with the exact file extension.
DEP0152: Extension PerformanceEntry properties#
Type: Runtime
The 'gc'
, 'http2'
, and 'http'
<PerformanceEntry> object types have
additional properties assigned to them that provide additional information.
These properties are now available within the standard detail
property
of the PerformanceEntry
object. The existing accessors have been
deprecated and should no longer be used.
DEP0153: dns.lookup
and dnsPromises.lookup
options type coercion#
Type: End-of-Life
Using a non-nullish non-integer value for family
option, a non-nullish
non-number value for hints
option, a non-nullish non-boolean value for all
option, or a non-nullish non-boolean value for verbatim
option in
dns.lookup()
and dnsPromises.lookup()
throws an
ERR_INVALID_ARG_TYPE
error.
DEP0154: RSA-PSS generate key pair options#
Type: Runtime
The 'hash'
and 'mgf1Hash'
options are replaced with 'hashAlgorithm'
and 'mgf1HashAlgorithm'
.
DEP0155: Trailing slashes in pattern specifier resolutions#
Type: Runtime
The remapping of specifiers ending in "/"
like import 'pkg/x/'
is deprecated
for package "exports"
and "imports"
pattern resolutions.
DEP0156: .aborted
property and 'abort'
, 'aborted'
event in http
#
Type: Documentation-only
Move to <Stream> API instead, as the http.ClientRequest
,
http.ServerResponse
, and http.IncomingMessage
are all stream-based.
Check stream.destroyed
instead of the .aborted
property, and listen for
'close'
instead of 'abort'
, 'aborted'
event.
The .aborted
property and 'abort'
event are only useful for detecting
.abort()
calls. For closing a request early, use the Stream
.destroy([error])
then check the .destroyed
property and 'close'
event
should have the same effect. The receiving end should also check the
readable.readableEnded
value on http.IncomingMessage
to get whether
it was an aborted or graceful destroy.
DEP0157: Thenable support in streams#
Type: End-of-Life
An undocumented feature of Node.js streams was to support thenables in implementation methods. This is now deprecated, use callbacks instead and avoid use of async function for streams implementation methods.
This feature caused users to encounter unexpected problems where the user implements the function in callback style but uses e.g. an async method which would cause an error since mixing promise and callback semantics is not valid.
const w = new Writable({
async final(callback) {
await someOp();
callback();
},
});
DEP0158: buffer.slice(start, end)
#
Type: Documentation-only
This method was deprecated because it is not compatible with
Uint8Array.prototype.slice()
, which is a superclass of Buffer
.
Use buffer.subarray
which does the same thing instead.
DEP0159: ERR_INVALID_CALLBACK
#
Type: End-of-Life
This error code was removed due to adding more confusion to the errors used for value type validation.
DEP0160: process.on('multipleResolves', handler)
#
Type: Runtime.
This event was deprecated because it did not work with V8 promise combinators which diminished its usefulness.
DEP0161: process._getActiveRequests()
and process._getActiveHandles()
#
Type: Documentation-only
The process._getActiveHandles()
and process._getActiveRequests()
functions are not intended for public use and can be removed in future
releases.
Use process.getActiveResourcesInfo()
to get a list of types of active
resources and not the actual references.
DEP0162: fs.write()
, fs.writeFileSync()
coercion to string#
Type: End-of-Life
Implicit coercion of objects with own toString
property, passed as second
parameter in fs.write()
, fs.writeFile()
, fs.appendFile()
,
fs.writeFileSync()
, and fs.appendFileSync()
is deprecated.
Convert them to primitive strings.
DEP0163: channel.subscribe(onMessage)
, channel.unsubscribe(onMessage)
#
Type: Documentation-only
These methods were deprecated because they can be used in a way which does not hold the channel reference alive long enough to receive the events.
Use diagnostics_channel.subscribe(name, onMessage)
or
diagnostics_channel.unsubscribe(name, onMessage)
which does the same
thing instead.
DEP0164: process.exit(code)
, process.exitCode
coercion to integer#
Type: End-of-Life
Values other than undefined
, null
, integer numbers, and integer strings
(e.g., '1'
) are deprecated as value for the code
parameter in
process.exit()
and as value to assign to process.exitCode
.
DEP0165: --trace-atomics-wait
#
Type: Documentation-only
The --trace-atomics-wait
flag is deprecated.
DEP0166: Double slashes in imports and exports targets#
Type: Runtime
Package imports and exports targets mapping into paths including a double slash (of "/" or "\") are deprecated and will fail with a resolution validation error in a future release. This same deprecation also applies to pattern matches starting or ending in a slash.
DEP0167: Weak DiffieHellmanGroup
instances (modp1
, modp2
, modp5
)#
Type: Documentation-only
The well-known MODP groups modp1
, modp2
, and modp5
are deprecated because
they are not secure against practical attacks. See RFC 8247 Section 2.4 for
details.
These groups might be removed in future versions of Node.js. Applications that rely on these groups should evaluate using stronger MODP groups instead.
DEP0168: Unhandled exception in Node-API callbacks#
Type: Runtime
The implicit suppression of uncaught exceptions in Node-API callbacks is now deprecated.
Set the flag --force-node-api-uncaught-exceptions-policy
to force Node.js
to emit an 'uncaughtException'
event if the exception is not handled in
Node-API callbacks.
DEP0169: Insecure url.parse()#
Type: Documentation-only (supports --pending-deprecation
)
url.parse()
behavior is not standardized and prone to errors that
have security implications. Use the WHATWG URL API instead. CVEs are not
issued for url.parse()
vulnerabilities.
DEP0170: Invalid port when using url.parse()
#
Type: Runtime
url.parse()
accepts URLs with ports that are not numbers. This behavior
might result in host name spoofing with unexpected input. These URLs will throw
an error in future versions of Node.js, as the WHATWG URL API does already.
DEP0171: Setters for http.IncomingMessage
headers and trailers#
Type: Documentation-only
In a future version of Node.js, message.headers
,
message.headersDistinct
, message.trailers
, and
message.trailersDistinct
will be read-only.
DEP0172: The asyncResource
property of AsyncResource
bound functions#
Type: Runtime
In a future version of Node.js, the asyncResource
property will no longer
be added when a function is bound to an AsyncResource
.
Diagnostics Channel#
Source Code: lib/diagnostics_channel.js
The node:diagnostics_channel
module provides an API to create named channels
to report arbitrary message data for diagnostics purposes.
It can be accessed using:
import diagnostics_channel from 'node:diagnostics_channel';
const diagnostics_channel = require('node:diagnostics_channel');
It is intended that a module writer wanting to report diagnostics messages will create one or many top-level channels to report messages through. Channels may also be acquired at runtime but it is not encouraged due to the additional overhead of doing so. Channels may be exported for convenience, but as long as the name is known it can be acquired anywhere.
If you intend for your module to produce diagnostics data for others to consume it is recommended that you include documentation of what named channels are used along with the shape of the message data. Channel names should generally include the module name to avoid collisions with data from other modules.
Public API#
Overview#
Following is a simple overview of the public API.
import diagnostics_channel from 'node:diagnostics_channel';
// Get a reusable channel object
const channel = diagnostics_channel.channel('my-channel');
function onMessage(message, name) {
// Received data
}
// Subscribe to the channel
diagnostics_channel.subscribe('my-channel', onMessage);
// Check if the channel has an active subscriber
if (channel.hasSubscribers) {
// Publish data to the channel
channel.publish({
some: 'data',
});
}
// Unsubscribe from the channel
diagnostics_channel.unsubscribe('my-channel', onMessage);
const diagnostics_channel = require('node:diagnostics_channel');
// Get a reusable channel object
const channel = diagnostics_channel.channel('my-channel');
function onMessage(message, name) {
// Received data
}
// Subscribe to the channel
diagnostics_channel.subscribe('my-channel', onMessage);
// Check if the channel has an active subscriber
if (channel.hasSubscribers) {
// Publish data to the channel
channel.publish({
some: 'data',
});
}
// Unsubscribe from the channel
diagnostics_channel.unsubscribe('my-channel', onMessage);
diagnostics_channel.hasSubscribers(name)
#
Check if there are active subscribers to the named channel. This is helpful if the message you want to send might be expensive to prepare.
This API is optional but helpful when trying to publish messages from very performance-sensitive code.
import diagnostics_channel from 'node:diagnostics_channel';
if (diagnostics_channel.hasSubscribers('my-channel')) {
// There are subscribers, prepare and publish message
}
const diagnostics_channel = require('node:diagnostics_channel');
if (diagnostics_channel.hasSubscribers('my-channel')) {
// There are subscribers, prepare and publish message
}
diagnostics_channel.channel(name)
#
This is the primary entry-point for anyone wanting to publish to a named channel. It produces a channel object which is optimized to reduce overhead at publish time as much as possible.
import diagnostics_channel from 'node:diagnostics_channel';
const channel = diagnostics_channel.channel('my-channel');
const diagnostics_channel = require('node:diagnostics_channel');
const channel = diagnostics_channel.channel('my-channel');
diagnostics_channel.subscribe(name, onMessage)
#
name
<string> | <symbol> The channel nameonMessage
<Function> The handler to receive channel messages
Register a message handler to subscribe to this channel. This message handler
will be run synchronously whenever a message is published to the channel. Any
errors thrown in the message handler will trigger an 'uncaughtException'
.
import diagnostics_channel from 'node:diagnostics_channel';
diagnostics_channel.subscribe('my-channel', (message, name) => {
// Received data
});
const diagnostics_channel = require('node:diagnostics_channel');
diagnostics_channel.subscribe('my-channel', (message, name) => {
// Received data
});
diagnostics_channel.unsubscribe(name, onMessage)
#
name
<string> | <symbol> The channel nameonMessage
<Function> The previous subscribed handler to remove- Returns: <boolean>
true
if the handler was found,false
otherwise.
Remove a message handler previously registered to this channel with
diagnostics_channel.subscribe(name, onMessage)
.
import diagnostics_channel from 'node:diagnostics_channel';
function onMessage(message, name) {
// Received data
}
diagnostics_channel.subscribe('my-channel', onMessage);
diagnostics_channel.unsubscribe('my-channel', onMessage);
const diagnostics_channel = require('node:diagnostics_channel');
function onMessage(message, name) {
// Received data
}
diagnostics_channel.subscribe('my-channel', onMessage);
diagnostics_channel.unsubscribe('my-channel', onMessage);
Class: Channel
#
The class Channel
represents an individual named channel within the data
pipeline. It is used to track subscribers and to publish messages when there
are subscribers present. It exists as a separate object to avoid channel
lookups at publish time, enabling very fast publish speeds and allowing
for heavy use while incurring very minimal cost. Channels are created with
diagnostics_channel.channel(name)
, constructing a channel directly
with new Channel(name)
is not supported.
channel.hasSubscribers
#
- Returns: <boolean> If there are active subscribers
Check if there are active subscribers to this channel. This is helpful if the message you want to send might be expensive to prepare.
This API is optional but helpful when trying to publish messages from very performance-sensitive code.
import diagnostics_channel from 'node:diagnostics_channel';
const channel = diagnostics_channel.channel('my-channel');
if (channel.hasSubscribers) {
// There are subscribers, prepare and publish message
}
const diagnostics_channel = require('node:diagnostics_channel');
const channel = diagnostics_channel.channel('my-channel');
if (channel.hasSubscribers) {
// There are subscribers, prepare and publish message
}
channel.publish(message)
#
message
<any> The message to send to the channel subscribers
Publish a message to any subscribers to the channel. This will trigger message handlers synchronously so they will execute within the same context.
import diagnostics_channel from 'node:diagnostics_channel';
const channel = diagnostics_channel.channel('my-channel');
channel.publish({
some: 'message',
});
const diagnostics_channel = require('node:diagnostics_channel');
const channel = diagnostics_channel.channel('my-channel');
channel.publish({
some: 'message',
});
channel.subscribe(onMessage)
#
diagnostics_channel.subscribe(name, onMessage)
onMessage
<Function> The handler to receive channel messages
Register a message handler to subscribe to this channel. This message handler
will be run synchronously whenever a message is published to the channel. Any
errors thrown in the message handler will trigger an 'uncaughtException'
.
import diagnostics_channel from 'node:diagnostics_channel';
const channel = diagnostics_channel.channel('my-channel');
channel.subscribe((message, name) => {
// Received data
});
const diagnostics_channel = require('node:diagnostics_channel');
const channel = diagnostics_channel.channel('my-channel');
channel.subscribe((message, name) => {
// Received data
});
channel.unsubscribe(onMessage)
#
diagnostics_channel.unsubscribe(name, onMessage)
onMessage
<Function> The previous subscribed handler to remove- Returns: <boolean>
true
if the handler was found,false
otherwise.
Remove a message handler previously registered to this channel with
channel.subscribe(onMessage)
.
import diagnostics_channel from 'node:diagnostics_channel';
const channel = diagnostics_channel.channel('my-channel');
function onMessage(message, name) {
// Received data
}
channel.subscribe(onMessage);
channel.unsubscribe(onMessage);
const diagnostics_channel = require('node:diagnostics_channel');
const channel = diagnostics_channel.channel('my-channel');
function onMessage(message, name) {
// Received data
}
channel.subscribe(onMessage);
channel.unsubscribe(onMessage);
Built-in Channels#
While the diagnostics_channel API is now considered stable, the built-in channels currently available are not. Each channel must be declared stable independently.
HTTP#
http.client.request.start
request
<http.ClientRequest>
Emitted when client starts a request.
http.client.response.finish
request
<http.ClientRequest>response
<http.IncomingMessage>
Emitted when client receives a response.
http.server.request.start
request
<http.IncomingMessage>response
<http.ServerResponse>socket
<net.Socket>server
<http.Server>
Emitted when server receives a request.
http.server.response.finish
request
<http.IncomingMessage>response
<http.ServerResponse>socket
<net.Socket>server
<http.Server>
Emitted when server sends a response.
NET#
net.client.socket
socket
<net.Socket>
Emitted when a new TCP or pipe client socket is created.
net.server.socket
socket
<net.Socket>
Emitted when a new TCP or pipe connection is received.
UDP#
udp.socket
socket
<dgram.Socket>
Emitted when a new UDP socket is created.
Process#
child_process
process
<ChildProcess>
Emitted when a new process is created.
Worker Thread#
worker_threads
worker
Worker
Emitted when a new thread is created.
DNS#
Source Code: lib/dns.js
The node:dns
module enables name resolution. For example, use it to look up IP
addresses of host names.
Although named for the Domain Name System (DNS), it does not always use the
DNS protocol for lookups. dns.lookup()
uses the operating system
facilities to perform name resolution. It may not need to perform any network
communication. To perform name resolution the way other applications on the same
system do, use dns.lookup()
.
const dns = require('node:dns');
dns.lookup('example.org', (err, address, family) => {
console.log('address: %j family: IPv%s', address, family);
});
// address: "93.184.216.34" family: IPv4
All other functions in the node:dns
module connect to an actual DNS server to
perform name resolution. They will always use the network to perform DNS
queries. These functions do not use the same set of configuration files used by
dns.lookup()
(e.g. /etc/hosts
). Use these functions to always perform
DNS queries, bypassing other name-resolution facilities.
const dns = require('node:dns');
dns.resolve4('archive.org', (err, addresses) => {
if (err) throw err;
console.log(`addresses: ${JSON.stringify(addresses)}`);
addresses.forEach((a) => {
dns.reverse(a, (err, hostnames) => {
if (err) {
throw err;
}
console.log(`reverse for ${a}: ${JSON.stringify(hostnames)}`);
});
});
});
See the Implementation considerations section for more information.
Class: dns.Resolver
#
An independent resolver for DNS requests.
Creating a new resolver uses the default server settings. Setting
the servers used for a resolver using
resolver.setServers()
does not affect
other resolvers:
const { Resolver } = require('node:dns');
const resolver = new Resolver();
resolver.setServers(['4.4.4.4']);
// This request will use the server at 4.4.4.4, independent of global settings.
resolver.resolve4('example.org', (err, addresses) => {
// ...
});
The following methods from the node:dns
module are available:
resolver.getServers()
resolver.resolve()
resolver.resolve4()
resolver.resolve6()
resolver.resolveAny()
resolver.resolveCaa()
resolver.resolveCname()
resolver.resolveMx()
resolver.resolveNaptr()
resolver.resolveNs()
resolver.resolvePtr()
resolver.resolveSoa()
resolver.resolveSrv()
resolver.resolveTxt()
resolver.reverse()
resolver.setServers()
Resolver([options])
#
Create a new resolver.
options
<Object>
resolver.cancel()
#
Cancel all outstanding DNS queries made by this resolver. The corresponding
callbacks will be called with an error with code ECANCELLED
.
resolver.setLocalAddress([ipv4][, ipv6])
#
ipv4
<string> A string representation of an IPv4 address. Default:'0.0.0.0'
ipv6
<string> A string representation of an IPv6 address. Default:'::0'
The resolver instance will send its requests from the specified IP address. This allows programs to specify outbound interfaces when used on multi-homed systems.
If a v4 or v6 address is not specified, it is set to the default and the operating system will choose a local address automatically.
The resolver will use the v4 local address when making requests to IPv4 DNS
servers, and the v6 local address when making requests to IPv6 DNS servers.
The rrtype
of resolution requests has no impact on the local address used.
dns.getServers()
#
- Returns: <string[]>
Returns an array of IP address strings, formatted according to RFC 5952, that are currently configured for DNS resolution. A string will include a port section if a custom port is used.
[
'4.4.4.4',
'2001:4860:4860::8888',
'4.4.4.4:1053',
'[2001:4860:4860::8888]:1053',
]
dns.lookup(hostname[, options], callback)
#
hostname
<string>options
<integer> | <Object>family
<integer> | <string> The record family. Must be4
,6
, or0
. For backward compatibility reasons,'IPv4'
and'IPv6'
are interpreted as4
and6
respectively. The value0
indicates that IPv4 and IPv6 addresses are both returned. Default:0
.hints
<number> One or more supportedgetaddrinfo
flags. Multiple flags may be passed by bitwiseOR
ing their values.all
<boolean> Whentrue
, the callback returns all resolved addresses in an array. Otherwise, returns a single address. Default:false
.verbatim
<boolean> Whentrue
, the callback receives IPv4 and IPv6 addresses in the order the DNS resolver returned them. Whenfalse
, IPv4 addresses are placed before IPv6 addresses. Default:true
(addresses are not reordered). Default value is configurable usingdns.setDefaultResultOrder()
or--dns-result-order
.
callback
<Function>
Resolves a host name (e.g. 'nodejs.org'
) into the first found A (IPv4) or
AAAA (IPv6) record. All option
properties are optional. If options
is an
integer, then it must be 4
or 6
– if options
is 0
or not provided, then
IPv4 and IPv6 addresses are both returned if found.
With the all
option set to true
, the arguments for callback
change to
(err, addresses)
, with addresses
being an array of objects with the
properties address
and family
.
On error, err
is an Error
object, where err.code
is the error code.
Keep in mind that err.code
will be set to 'ENOTFOUND'
not only when
the host name does not exist but also when the lookup fails in other ways
such as no available file descriptors.
dns.lookup()
does not necessarily have anything to do with the DNS protocol.
The implementation uses an operating system facility that can associate names
with addresses and vice versa. This implementation can have subtle but
important consequences on the behavior of any Node.js program. Please take some
time to consult the Implementation considerations section before using
dns.lookup()
.
Example usage:
const dns = require('node:dns');
const options = {
family: 6,
hints: dns.ADDRCONFIG | dns.V4MAPPED,
};
dns.lookup('example.com', options, (err, address, family) =>
console.log('address: %j family: IPv%s', address, family));
// address: "2606:2800:220:1:248:1893:25c8:1946" family: IPv6
// When options.all is true, the result will be an Array.
options.all = true;
dns.lookup('example.com', options, (err, addresses) =>
console.log('addresses: %j', addresses));
// addresses: [{"address":"2606:2800:220:1:248:1893:25c8:1946","family":6}]
If this method is invoked as its util.promisify()
ed version, and all
is not set to true
, it returns a Promise
for an Object
with address
and
family
properties.
Supported getaddrinfo flags#
The following flags can be passed as hints to dns.lookup()
.
dns.ADDRCONFIG
: Limits returned address types to the types of non-loopback addresses configured on the system. For example, IPv4 addresses are only returned if the current system has at least one IPv4 address configured.dns.V4MAPPED
: If the IPv6 family was specified, but no IPv6 addresses were found, then return IPv4 mapped IPv6 addresses. It is not supported on some operating systems (e.g. FreeBSD 10.1).dns.ALL
: Ifdns.V4MAPPED
is specified, return resolved IPv6 addresses as well as IPv4 mapped IPv6 addresses.
dns.lookupService(address, port, callback)
#
address
<string>port
<number>callback
<Function>
Resolves the given address
and port
into a host name and service using
the operating system's underlying getnameinfo
implementation.
If address
is not a valid IP address, a TypeError
will be thrown.
The port
will be coerced to a number. If it is not a legal port, a TypeError
will be thrown.
On an error, err
is an Error
object, where err.code
is the error code.
const dns = require('node:dns');
dns.lookupService('127.0.0.1', 22, (err, hostname, service) => {
console.log(hostname, service);
// Prints: localhost ssh
});
If this method is invoked as its util.promisify()
ed version, it returns a
Promise
for an Object
with hostname
and service
properties.
dns.resolve(hostname[, rrtype], callback)
#
hostname
<string> Host name to resolve.rrtype
<string> Resource record type. Default:'A'
.callback
<Function>err
<Error>records
<string[]> | <Object[]> | <Object>
Uses the DNS protocol to resolve a host name (e.g. 'nodejs.org'
) into an array
of the resource records. The callback
function has arguments
(err, records)
. When successful, records
will be an array of resource
records. The type and structure of individual results varies based on rrtype
:
rrtype | records contains | Result type | Shorthand method |
---|---|---|---|
'A' | IPv4 addresses (default) | <string> | dns.resolve4() |
'AAAA' | IPv6 addresses | <string> | dns.resolve6() |
'ANY' | any records | <Object> | dns.resolveAny() |
'CAA' | CA authorization records | <Object> | dns.resolveCaa() |
'CNAME' | canonical name records | <string> | dns.resolveCname() |
'MX' | mail exchange records | <Object> | dns.resolveMx() |
'NAPTR' | name authority pointer records | <Object> | dns.resolveNaptr() |
'NS' | name server records | <string> | dns.resolveNs() |
'PTR' | pointer records | <string> | dns.resolvePtr() |
'SOA' | start of authority records | <Object> | dns.resolveSoa() |
'SRV' | service records | <Object> | dns.resolveSrv() |
'TXT' | text records | <string[]> | dns.resolveTxt() |
On error, err
is an Error
object, where err.code
is one of the
DNS error codes.
dns.resolve4(hostname[, options], callback)
#
hostname
<string> Host name to resolve.options
<Object>ttl
<boolean> Retrieves the Time-To-Live value (TTL) of each record. Whentrue
, the callback receives an array of{ address: '1.2.3.4', ttl: 60 }
objects rather than an array of strings, with the TTL expressed in seconds.
callback
<Function>err
<Error>addresses
<string[]> | <Object[]>
Uses the DNS protocol to resolve a IPv4 addresses (A
records) for the
hostname
. The addresses
argument passed to the callback
function
will contain an array of IPv4 addresses (e.g.
['74.125.79.104', '74.125.79.105', '74.125.79.106']
).
dns.resolve6(hostname[, options], callback)
#
hostname
<string> Host name to resolve.options
<Object>ttl
<boolean> Retrieve the Time-To-Live value (TTL) of each record. Whentrue
, the callback receives an array of{ address: '0:1:2:3:4:5:6:7', ttl: 60 }
objects rather than an array of strings, with the TTL expressed in seconds.
callback
<Function>err
<Error>addresses
<string[]> | <Object[]>
Uses the DNS protocol to resolve IPv6 addresses (AAAA
records) for the
hostname
. The addresses
argument passed to the callback
function
will contain an array of IPv6 addresses.
dns.resolveAny(hostname, callback)
#
hostname
<string>callback
<Function>err
<Error>ret
<Object[]>
Uses the DNS protocol to resolve all records (also known as ANY
or *
query).
The ret
argument passed to the callback
function will be an array containing
various types of records. Each object has a property type
that indicates the
type of the current record. And depending on the type
, additional properties
will be present on the object:
Type | Properties |
---|---|
'A' | address /ttl |
'AAAA' | address /ttl |
'CNAME' | value |
'MX' | Refer to dns.resolveMx() |
'NAPTR' | Refer to dns.resolveNaptr() |
'NS' | value |
'PTR' | value |
'SOA' | Refer to dns.resolveSoa() |
'SRV' | Refer to dns.resolveSrv() |
'TXT' | This type of record contains an array property called entries which refers to dns.resolveTxt() , e.g. { entries: ['...'], type: 'TXT' } |
Here is an example of the ret
object passed to the callback:
[ { type: 'A', address: '127.0.0.1', ttl: 299 },
{ type: 'CNAME', value: 'example.com' },
{ type: 'MX', exchange: 'alt4.aspmx.l.example.com', priority: 50 },
{ type: 'NS', value: 'ns1.example.com' },
{ type: 'TXT', entries: [ 'v=spf1 include:_spf.example.com ~all' ] },
{ type: 'SOA',
nsname: 'ns1.example.com',
hostmaster: 'admin.example.com',
serial: 156696742,
refresh: 900,
retry: 900,
expire: 1800,
minttl: 60 } ]
DNS server operators may choose not to respond to ANY
queries. It may be better to call individual methods like dns.resolve4()
,
dns.resolveMx()
, and so on. For more details, see RFC 8482.
dns.resolveCname(hostname, callback)
#
hostname
<string>callback
<Function>err
<Error>addresses
<string[]>
Uses the DNS protocol to resolve CNAME
records for the hostname
. The
addresses
argument passed to the callback
function
will contain an array of canonical name records available for the hostname
(e.g. ['bar.example.com']
).
dns.resolveCaa(hostname, callback)
#
hostname
<string>callback
<Function>err
<Error>records
<Object[]>
Uses the DNS protocol to resolve CAA
records for the hostname
. The
addresses
argument passed to the callback
function
will contain an array of certification authority authorization records
available for the hostname
(e.g. [{critical: 0, iodef: 'mailto:[email protected]'}, {critical: 128, issue: 'pki.example.com'}]
).
dns.resolveMx(hostname, callback)
#
hostname
<string>callback
<Function>err
<Error>addresses
<Object[]>
Uses the DNS protocol to resolve mail exchange records (MX
records) for the
hostname
. The addresses
argument passed to the callback
function will
contain an array of objects containing both a priority
and exchange
property (e.g. [{priority: 10, exchange: 'mx.example.com'}, ...]
).
dns.resolveNaptr(hostname, callback)
#
hostname
<string>callback
<Function>err
<Error>addresses
<Object[]>
Uses the DNS protocol to resolve regular expression-based records (NAPTR
records) for the hostname
. The addresses
argument passed to the callback
function will contain an array of objects with the following properties:
flags
service
regexp
replacement
order
preference
{
flags: 's',
service: 'SIP+D2U',
regexp: '',
replacement: '_sip._udp.example.com',
order: 30,
preference: 100
}
dns.resolveNs(hostname, callback)
#
hostname
<string>callback
<Function>err
<Error>addresses
<string[]>
Uses the DNS protocol to resolve name server records (NS
records) for the
hostname
. The addresses
argument passed to the callback
function will
contain an array of name server records available for hostname
(e.g. ['ns1.example.com', 'ns2.example.com']
).
dns.resolvePtr(hostname, callback)
#
hostname
<string>callback
<Function>err
<Error>addresses
<string[]>
Uses the DNS protocol to resolve pointer records (PTR
records) for the
hostname
. The addresses
argument passed to the callback
function will
be an array of strings containing the reply records.
dns.resolveSoa(hostname, callback)
#
hostname
<string>callback
<Function>
Uses the DNS protocol to resolve a start of authority record (SOA
record) for
the hostname
. The address
argument passed to the callback
function will
be an object with the following properties:
nsname
hostmaster
serial
refresh
retry
expire
minttl
{
nsname: 'ns.example.com',
hostmaster: 'root.example.com',
serial: 2013101809,
refresh: 10000,
retry: 2400,
expire: 604800,
minttl: 3600
}
dns.resolveSrv(hostname, callback)
#
hostname
<string>callback
<Function>err
<Error>addresses
<Object[]>
Uses the DNS protocol to resolve service records (SRV
records) for the
hostname
. The addresses
argument passed to the callback
function will
be an array of objects with the following properties:
priority
weight
port
name
{
priority: 10,
weight: 5,
port: 21223,
name: 'service.example.com'
}
dns.resolveTxt(hostname, callback)
#
hostname
<string>callback
<Function>err
<Error>records
<string[][]>
Uses the DNS protocol to resolve text queries (TXT
records) for the
hostname
. The records
argument passed to the callback
function is a
two-dimensional array of the text records available for hostname
(e.g.
[ ['v=spf1 ip4:0.0.0.0 ', '~all' ] ]
). Each sub-array contains TXT chunks of
one record. Depending on the use case, these could be either joined together or
treated separately.
dns.reverse(ip, callback)
#
ip
<string>callback
<Function>err
<Error>hostnames
<string[]>
Performs a reverse DNS query that resolves an IPv4 or IPv6 address to an array of host names.
On error, err
is an Error
object, where err.code
is
one of the DNS error codes.
dns.setDefaultResultOrder(order)
#
order
<string> must be'ipv4first'
or'verbatim'
.
Set the default value of verbatim
in dns.lookup()
and
dnsPromises.lookup()
. The value could be:
ipv4first
: sets defaultverbatim
false
.verbatim
: sets defaultverbatim
true
.
The default is verbatim
and dns.setDefaultResultOrder()
have higher
priority than --dns-result-order
. When using worker threads,
dns.setDefaultResultOrder()
from the main thread won't affect the default
dns orders in workers.
dns.setServers(servers)
#
servers
<string[]> array of RFC 5952 formatted addresses
Sets the IP address and port of servers to be used when performing DNS
resolution. The servers
argument is an array of RFC 5952 formatted
addresses. If the port is the IANA default DNS port (53) it can be omitted.
dns.setServers([
'4.4.4.4',
'[2001:4860:4860::8888]',
'4.4.4.4:1053',
'[2001:4860:4860::8888]:1053',
]);
An error will be thrown if an invalid address is provided.
The dns.setServers()
method must not be called while a DNS query is in
progress.
The dns.setServers()
method affects only dns.resolve()
,
dns.resolve*()
and dns.reverse()
(and specifically not
dns.lookup()
).
This method works much like
resolve.conf.
That is, if attempting to resolve with the first server provided results in a
NOTFOUND
error, the resolve()
method will not attempt to resolve with
subsequent servers provided. Fallback DNS servers will only be used if the
earlier ones time out or result in some other error.
DNS promises API#
The dns.promises
API provides an alternative set of asynchronous DNS methods
that return Promise
objects rather than using callbacks. The API is accessible
via require('node:dns').promises
or require('node:dns/promises')
.
Class: dnsPromises.Resolver
#
An independent resolver for DNS requests.
Creating a new resolver uses the default server settings. Setting
the servers used for a resolver using
resolver.setServers()
does not affect
other resolvers:
const { Resolver } = require('node:dns').promises;
const resolver = new Resolver();
resolver.setServers(['4.4.4.4']);
// This request will use the server at 4.4.4.4, independent of global settings.
resolver.resolve4('example.org').then((addresses) => {
// ...
});
// Alternatively, the same code can be written using async-await style.
(async function() {
const addresses = await resolver.resolve4('example.org');
})();
The following methods from the dnsPromises
API are available:
resolver.getServers()
resolver.resolve()
resolver.resolve4()
resolver.resolve6()
resolver.resolveAny()
resolver.resolveCaa()
resolver.resolveCname()
resolver.resolveMx()
resolver.resolveNaptr()
resolver.resolveNs()
resolver.resolvePtr()
resolver.resolveSoa()
resolver.resolveSrv()
resolver.resolveTxt()
resolver.reverse()
resolver.setServers()
resolver.cancel()
#
Cancel all outstanding DNS queries made by this resolver. The corresponding
promises will be rejected with an error with the code ECANCELLED
.
dnsPromises.getServers()
#
- Returns: <string[]>
Returns an array of IP address strings, formatted according to RFC 5952, that are currently configured for DNS resolution. A string will include a port section if a custom port is used.
[
'4.4.4.4',
'2001:4860:4860::8888',
'4.4.4.4:1053',
'[2001:4860:4860::8888]:1053',
]
dnsPromises.lookup(hostname[, options])
#
hostname
<string>options
<integer> | <Object>family
<integer> The record family. Must be4
,6
, or0
. The value0
indicates that IPv4 and IPv6 addresses are both returned. Default:0
.hints
<number> One or more supportedgetaddrinfo
flags. Multiple flags may be passed by bitwiseOR
ing their values.all
<boolean> Whentrue
, thePromise
is resolved with all addresses in an array. Otherwise, returns a single address. Default:false
.verbatim
<boolean> Whentrue
, thePromise
is resolved with IPv4 and IPv6 addresses in the order the DNS resolver returned them. Whenfalse
, IPv4 addresses are placed before IPv6 addresses. Default: currentlyfalse
(addresses are reordered) but this is expected to change in the not too distant future. Default value is configurable usingdns.setDefaultResultOrder()
or--dns-result-order
. New code should use{ verbatim: true }
.
Resolves a host name (e.g. 'nodejs.org'
) into the first found A (IPv4) or
AAAA (IPv6) record. All option
properties are optional. If options
is an
integer, then it must be 4
or 6
– if options
is not provided, then IPv4
and IPv6 addresses are both returned if found.
With the all
option set to true
, the Promise
is resolved with addresses
being an array of objects with the properties address
and family
.
On error, the Promise
is rejected with an Error
object, where err.code
is the error code.
Keep in mind that err.code
will be set to 'ENOTFOUND'
not only when
the host name does not exist but also when the lookup fails in other ways
such as no available file descriptors.
dnsPromises.lookup()
does not necessarily have anything to do with the DNS
protocol. The implementation uses an operating system facility that can
associate names with addresses and vice versa. This implementation can have
subtle but important consequences on the behavior of any Node.js program. Please
take some time to consult the Implementation considerations section before
using dnsPromises.lookup()
.
Example usage:
const dns = require('node:dns');
const dnsPromises = dns.promises;
const options = {
family: 6,
hints: dns.ADDRCONFIG | dns.V4MAPPED,
};
dnsPromises.lookup('example.com', options).then((result) => {
console.log('address: %j family: IPv%s', result.address, result.family);
// address: "2606:2800:220:1:248:1893:25c8:1946" family: IPv6
});
// When options.all is true, the result will be an Array.
options.all = true;
dnsPromises.lookup('example.com', options).then((result) => {
console.log('addresses: %j', result);
// addresses: [{"address":"2606:2800:220:1:248:1893:25c8:1946","family":6}]
});
dnsPromises.lookupService(address, port)
#
Resolves the given address
and port
into a host name and service using
the operating system's underlying getnameinfo
implementation.
If address
is not a valid IP address, a TypeError
will be thrown.
The port
will be coerced to a number. If it is not a legal port, a TypeError
will be thrown.
On error, the Promise
is rejected with an Error
object, where err.code
is the error code.
const dnsPromises = require('node:dns').promises;
dnsPromises.lookupService('127.0.0.1', 22).then((result) => {
console.log(result.hostname, result.service);
// Prints: localhost ssh
});
dnsPromises.resolve(hostname[, rrtype])
#
Uses the DNS protocol to resolve a host name (e.g. 'nodejs.org'
) into an array
of the resource records. When successful, the Promise
is resolved with an
array of resource records. The type and structure of individual results vary
based on rrtype
:
rrtype | records contains | Result type | Shorthand method |
---|---|---|---|
'A' | IPv4 addresses (default) | <string> | dnsPromises.resolve4() |
'AAAA' | IPv6 addresses | <string> | dnsPromises.resolve6() |
'ANY' | any records | <Object> | dnsPromises.resolveAny() |
'CAA' | CA authorization records | <Object> | dnsPromises.resolveCaa() |
'CNAME' | canonical name records | <string> | dnsPromises.resolveCname() |
'MX' | mail exchange records | <Object> | dnsPromises.resolveMx() |
'NAPTR' | name authority pointer records | <Object> | dnsPromises.resolveNaptr() |
'NS' | name server records | <string> | dnsPromises.resolveNs() |
'PTR' | pointer records | <string> | dnsPromises.resolvePtr() |
'SOA' | start of authority records | <Object> | dnsPromises.resolveSoa() |
'SRV' | service records | <Object> | dnsPromises.resolveSrv() |
'TXT' | text records | <string[]> | dnsPromises.resolveTxt() |
On error, the Promise
is rejected with an Error
object, where err.code
is one of the DNS error codes.
dnsPromises.resolve4(hostname[, options])
#
hostname
<string> Host name to resolve.options
<Object>ttl
<boolean> Retrieve the Time-To-Live value (TTL) of each record. Whentrue
, thePromise
is resolved with an array of{ address: '1.2.3.4', ttl: 60 }
objects rather than an array of strings, with the TTL expressed in seconds.
Uses the DNS protocol to resolve IPv4 addresses (A
records) for the
hostname
. On success, the Promise
is resolved with an array of IPv4
addresses (e.g. ['74.125.79.104', '74.125.79.105', '74.125.79.106']
).
dnsPromises.resolve6(hostname[, options])
#
hostname
<string> Host name to resolve.options
<Object>ttl
<boolean> Retrieve the Time-To-Live value (TTL) of each record. Whentrue
, thePromise
is resolved with an array of{ address: '0:1:2:3:4:5:6:7', ttl: 60 }
objects rather than an array of strings, with the TTL expressed in seconds.
Uses the DNS protocol to resolve IPv6 addresses (AAAA
records) for the
hostname
. On success, the Promise
is resolved with an array of IPv6
addresses.
dnsPromises.resolveAny(hostname)
#
hostname
<string>
Uses the DNS protocol to resolve all records (also known as ANY
or *
query).
On success, the Promise
is resolved with an array containing various types of
records. Each object has a property type
that indicates the type of the
current record. And depending on the type
, additional properties will be
present on the object:
Type | Properties |
---|---|
'A' | address /ttl |
'AAAA' | address /ttl |
'CNAME' | value |
'MX' | Refer to dnsPromises.resolveMx() |
'NAPTR' | Refer to dnsPromises.resolveNaptr() |
'NS' | value |
'PTR' | value |
'SOA' | Refer to dnsPromises.resolveSoa() |
'SRV' | Refer to dnsPromises.resolveSrv() |
'TXT' | This type of record contains an array property called entries which refers to dnsPromises.resolveTxt() , e.g. { entries: ['...'], type: 'TXT' } |
Here is an example of the result object:
[ { type: 'A', address: '127.0.0.1', ttl: 299 },
{ type: 'CNAME', value: 'example.com' },
{ type: 'MX', exchange: 'alt4.aspmx.l.example.com', priority: 50 },
{ type: 'NS', value: 'ns1.example.com' },
{ type: 'TXT', entries: [ 'v=spf1 include:_spf.example.com ~all' ] },
{ type: 'SOA',
nsname: 'ns1.example.com',
hostmaster: 'admin.example.com',
serial: 156696742,
refresh: 900,
retry: 900,
expire: 1800,
minttl: 60 } ]
dnsPromises.resolveCaa(hostname)
#
hostname
<string>
Uses the DNS protocol to resolve CAA
records for the hostname
. On success,
the Promise
is resolved with an array of objects containing available
certification authority authorization records available for the hostname
(e.g. [{critical: 0, iodef: 'mailto:[email protected]'},{critical: 128, issue: 'pki.example.com'}]
).
dnsPromises.resolveCname(hostname)
#
hostname
<string>
Uses the DNS protocol to resolve CNAME
records for the hostname
. On success,
the Promise
is resolved with an array of canonical name records available for
the hostname
(e.g. ['bar.example.com']
).
dnsPromises.resolveMx(hostname)
#
hostname
<string>
Uses the DNS protocol to resolve mail exchange records (MX
records) for the
hostname
. On success, the Promise
is resolved with an array of objects
containing both a priority
and exchange
property (e.g.
[{priority: 10, exchange: 'mx.example.com'}, ...]
).
dnsPromises.resolveNaptr(hostname)
#
hostname
<string>
Uses the DNS protocol to resolve regular expression-based records (NAPTR
records) for the hostname
. On success, the Promise
is resolved with an array
of objects with the following properties:
flags
service
regexp
replacement
order
preference
{
flags: 's',
service: 'SIP+D2U',
regexp: '',
replacement: '_sip._udp.example.com',
order: 30,
preference: 100
}
dnsPromises.resolveNs(hostname)
#
hostname
<string>
Uses the DNS protocol to resolve name server records (NS
records) for the
hostname
. On success, the Promise
is resolved with an array of name server
records available for hostname
(e.g.
['ns1.example.com', 'ns2.example.com']
).
dnsPromises.resolvePtr(hostname)
#
hostname
<string>
Uses the DNS protocol to resolve pointer records (PTR
records) for the
hostname
. On success, the Promise
is resolved with an array of strings
containing the reply records.
dnsPromises.resolveSoa(hostname)
#
hostname
<string>
Uses the DNS protocol to resolve a start of authority record (SOA
record) for
the hostname
. On success, the Promise
is resolved with an object with the
following properties:
nsname
hostmaster
serial
refresh
retry
expire
minttl
{
nsname: 'ns.example.com',
hostmaster: 'root.example.com',
serial: 2013101809,
refresh: 10000,
retry: 2400,
expire: 604800,
minttl: 3600
}
dnsPromises.resolveSrv(hostname)
#
hostname
<string>
Uses the DNS protocol to resolve service records (SRV
records) for the
hostname
. On success, the Promise
is resolved with an array of objects with
the following properties:
priority
weight
port
name
{
priority: 10,
weight: 5,
port: 21223,
name: 'service.example.com'
}
dnsPromises.resolveTxt(hostname)
#
hostname
<string>
Uses the DNS protocol to resolve text queries (TXT
records) for the
hostname
. On success, the Promise
is resolved with a two-dimensional array
of the text records available for hostname
(e.g.
[ ['v=spf1 ip4:0.0.0.0 ', '~all' ] ]
). Each sub-array contains TXT chunks of
one record. Depending on the use case, these could be either joined together or
treated separately.
dnsPromises.reverse(ip)
#
ip
<string>
Performs a reverse DNS query that resolves an IPv4 or IPv6 address to an array of host names.
On error, the Promise
is rejected with an Error
object, where err.code
is one of the DNS error codes.
dnsPromises.setDefaultResultOrder(order)
#
order
<string> must be'ipv4first'
or'verbatim'
.
Set the default value of verbatim
in dns.lookup()
and
dnsPromises.lookup()
. The value could be:
ipv4first
: sets defaultverbatim
false
.verbatim
: sets defaultverbatim
true
.
The default is verbatim
and dnsPromises.setDefaultResultOrder()
have
higher priority than --dns-result-order
. When using worker threads,
dnsPromises.setDefaultResultOrder()
from the main thread won't affect the
default dns orders in workers.
dnsPromises.setServers(servers)
#
servers
<string[]> array of RFC 5952 formatted addresses
Sets the IP address and port of servers to be used when performing DNS
resolution. The servers
argument is an array of RFC 5952 formatted
addresses. If the port is the IANA default DNS port (53) it can be omitted.
dnsPromises.setServers([
'4.4.4.4',
'[2001:4860:4860::8888]',
'4.4.4.4:1053',
'[2001:4860:4860::8888]:1053',
]);
An error will be thrown if an invalid address is provided.
The dnsPromises.setServers()
method must not be called while a DNS query is in
progress.
This method works much like
resolve.conf.
That is, if attempting to resolve with the first server provided results in a
NOTFOUND
error, the resolve()
method will not attempt to resolve with
subsequent servers provided. Fallback DNS servers will only be used if the
earlier ones time out or result in some other error.
Error codes#
Each DNS query can return one of the following error codes:
dns.NODATA
: DNS server returned an answer with no data.dns.FORMERR
: DNS server claims query was misformatted.dns.SERVFAIL
: DNS server returned general failure.dns.NOTFOUND
: Domain name not found.dns.NOTIMP
: DNS server does not implement the requested operation.dns.REFUSED
: DNS server refused query.dns.BADQUERY
: Misformatted DNS query.dns.BADNAME
: Misformatted host name.dns.BADFAMILY
: Unsupported address family.dns.BADRESP
: Misformatted DNS reply.dns.CONNREFUSED
: Could not contact DNS servers.dns.TIMEOUT
: Timeout while contacting DNS servers.dns.EOF
: End of file.dns.FILE
: Error reading file.dns.NOMEM
: Out of memory.dns.DESTRUCTION
: Channel is being destroyed.dns.BADSTR
: Misformatted string.dns.BADFLAGS
: Illegal flags specified.dns.NONAME
: Given host name is not numeric.dns.BADHINTS
: Illegal hints flags specified.dns.NOTINITIALIZED
: c-ares library initialization not yet performed.dns.LOADIPHLPAPI
: Error loadingiphlpapi.dll
.dns.ADDRGETNETWORKPARAMS
: Could not findGetNetworkParams
function.dns.CANCELLED
: DNS query cancelled.
The dnsPromises
API also exports the above error codes, e.g., dnsPromises.NODATA
.
Implementation considerations#
Although dns.lookup()
and the various dns.resolve*()/dns.reverse()
functions have the same goal of associating a network name with a network
address (or vice versa), their behavior is quite different. These differences
can have subtle but significant consequences on the behavior of Node.js
programs.
dns.lookup()
#
Under the hood, dns.lookup()
uses the same operating system facilities
as most other programs. For instance, dns.lookup()
will almost always
resolve a given name the same way as the ping
command. On most POSIX-like
operating systems, the behavior of the dns.lookup()
function can be
modified by changing settings in nsswitch.conf(5)
and/or resolv.conf(5)
,
but changing these files will change the behavior of all other
programs running on the same operating system.
Though the call to dns.lookup()
will be asynchronous from JavaScript's
perspective, it is implemented as a synchronous call to getaddrinfo(3)
that runs
on libuv's threadpool. This can have surprising negative performance
implications for some applications, see the UV_THREADPOOL_SIZE
documentation for more information.
Various networking APIs will call dns.lookup()
internally to resolve
host names. If that is an issue, consider resolving the host name to an address
using dns.resolve()
and using the address instead of a host name. Also, some
networking APIs (such as socket.connect()
and dgram.createSocket()
)
allow the default resolver, dns.lookup()
, to be replaced.
dns.resolve()
, dns.resolve*()
, and dns.reverse()
#
These functions are implemented quite differently than dns.lookup()
. They
do not use getaddrinfo(3)
and they always perform a DNS query on the
network. This network communication is always done asynchronously and does not
use libuv's threadpool.
As a result, these functions cannot have the same negative impact on other
processing that happens on libuv's threadpool that dns.lookup()
can have.
They do not use the same set of configuration files that dns.lookup()
uses. For instance, they do not use the configuration from /etc/hosts
.
Domain#
Source Code: lib/domain.js
This module is pending deprecation. Once a replacement API has been finalized, this module will be fully deprecated. Most developers should not have cause to use this module. Users who absolutely must have the functionality that domains provide may rely on it for the time being but should expect to have to migrate to a different solution in the future.
Domains provide a way to handle multiple different IO operations as a
single group. If any of the event emitters or callbacks registered to a
domain emit an 'error'
event, or throw an error, then the domain object
will be notified, rather than losing the context of the error in the
process.on('uncaughtException')
handler, or causing the program to
exit immediately with an error code.
Warning: Don't ignore errors!#
Domain error handlers are not a substitute for closing down a process when an error occurs.
By the very nature of how throw
works in JavaScript, there is almost
never any way to safely "pick up where it left off", without leaking
references, or creating some other sort of undefined brittle state.
The safest way to respond to a thrown error is to shut down the process. Of course, in a normal web server, there may be many open connections, and it is not reasonable to abruptly shut those down because an error was triggered by someone else.
The better approach is to send an error response to the request that triggered the error, while letting the others finish in their normal time, and stop listening for new requests in that worker.
In this way, domain
usage goes hand-in-hand with the cluster module,
since the primary process can fork a new worker when a worker
encounters an error. For Node.js programs that scale to multiple
machines, the terminating proxy or service registry can take note of
the failure, and react accordingly.
For example, this is not a good idea:
// XXX WARNING! BAD IDEA!
const d = require('node:domain').create();
d.on('error', (er) => {
// The error won't crash the process, but what it does is worse!
// Though we've prevented abrupt process restarting, we are leaking
// a lot of resources if this ever happens.
// This is no better than process.on('uncaughtException')!
console.log(`error, but oh well ${er.message}`);
});
d.run(() => {
require('node:http').createServer((req, res) => {
handleRequest(req, res);
}).listen(PORT);
});
By using the context of a domain, and the resilience of separating our program into multiple worker processes, we can react more appropriately, and handle errors with much greater safety.
// Much better!
const cluster = require('node:cluster');
const PORT = +process.env.PORT || 1337;
if (cluster.isPrimary) {
// A more realistic scenario would have more than 2 workers,
// and perhaps not put the primary and worker in the same file.
//
// It is also possible to get a bit fancier about logging, and
// implement whatever custom logic is needed to prevent DoS
// attacks and other bad behavior.
//
// See the options in the cluster documentation.
//
// The important thing is that the primary does very little,
// increasing our resilience to unexpected errors.
cluster.fork();
cluster.fork();
cluster.on('disconnect', (worker) => {
console.error('disconnect!');
cluster.fork();
});
} else {
// the worker
//
// This is where we put our bugs!
const domain = require('node:domain');
// See the cluster documentation for more details about using
// worker processes to serve requests. How it works, caveats, etc.
const server = require('node:http').createServer((req, res) => {
const d = domain.create();
d.on('error', (er) => {
console.error(`error ${er.stack}`);
// We're in dangerous territory!
// By definition, something unexpected occurred,
// which we probably didn't want.
// Anything can happen now! Be very careful!
try {
// Make sure we close down within 30 seconds
const killtimer = setTimeout(() => {
process.exit(1);
}, 30000);
// But don't keep the process open just for that!
killtimer.unref();
// Stop taking new requests.
server.close();
// Let the primary know we're dead. This will trigger a
// 'disconnect' in the cluster primary, and then it will fork
// a new worker.
cluster.worker.disconnect();
// Try to send an error to the request that triggered the problem
res.statusCode = 500;
res.setHeader('content-type', 'text/plain');
res.end('Oops, there was a problem!\n');
} catch (er2) {
// Oh well, not much we can do at this point.
console.error(`Error sending 500! ${er2.stack}`);
}
});
// Because req and res were created before this domain existed,
// we need to explicitly add them.
// See the explanation of implicit vs explicit binding below.
d.add(req);
d.add(res);
// Now run the handler function in the domain.
d.run(() => {
handleRequest(req, res);
});
});
server.listen(PORT);
}
// This part is not important. Just an example routing thing.
// Put fancy application logic here.
function handleRequest(req, res) {
switch (req.url) {
case '/error':
// We do some async stuff, and then...
setTimeout(() => {
// Whoops!
flerb.bark();
}, timeout);
break;
default:
res.end('ok');
}
}
Additions to Error
objects#
Any time an Error
object is routed through a domain, a few extra fields
are added to it.
error.domain
The domain that first handled the error.error.domainEmitter
The event emitter that emitted an'error'
event with the error object.error.domainBound
The callback function which was bound to the domain, and passed an error as its first argument.error.domainThrown
A boolean indicating whether the error was thrown, emitted, or passed to a bound callback function.
Implicit binding#
If domains are in use, then all new EventEmitter
objects (including
Stream objects, requests, responses, etc.) will be implicitly bound to
the active domain at the time of their creation.
Additionally, callbacks passed to lowlevel event loop requests (such as
to fs.open()
, or other callback-taking methods) will automatically be
bound to the active domain. If they throw, then the domain will catch
the error.
In order to prevent excessive memory usage, Domain
objects themselves
are not implicitly added as children of the active domain. If they
were, then it would be too easy to prevent request and response objects
from being properly garbage collected.
To nest Domain
objects as children of a parent Domain
they must be
explicitly added.
Implicit binding routes thrown errors and 'error'
events to the
Domain
's 'error'
event, but does not register the EventEmitter
on the
Domain
.
Implicit binding only takes care of thrown errors and 'error'
events.
Explicit binding#
Sometimes, the domain in use is not the one that ought to be used for a specific event emitter. Or, the event emitter could have been created in the context of one domain, but ought to instead be bound to some other domain.
For example, there could be one domain in use for an HTTP server, but perhaps we would like to have a separate domain to use for each request.
That is possible via explicit binding.
// Create a top-level domain for the server
const domain = require('node:domain');
const http = require('node:http');
const serverDomain = domain.create();
serverDomain.run(() => {
// Server is created in the scope of serverDomain
http.createServer((req, res) => {
// Req and res are also created in the scope of serverDomain
// however, we'd prefer to have a separate domain for each request.
// create it first thing, and add req and res to it.
const reqd = domain.create();
reqd.add(req);
reqd.add(res);
reqd.on('error', (er) => {
console.error('Error', er, req.url);
try {
res.writeHead(500);
res.end('Error occurred, sorry.');
} catch (er2) {
console.error('Error sending 500', er2, req.url);
}
});
}).listen(1337);
});
domain.create()
#
- Returns: <Domain>
Class: Domain
#
- Extends: <EventEmitter>
The Domain
class encapsulates the functionality of routing errors and
uncaught exceptions to the active Domain
object.
To handle the errors that it catches, listen to its 'error'
event.
domain.members
#
An array of timers and event emitters that have been explicitly added to the domain.
domain.add(emitter)
#
emitter
<EventEmitter> | <Timer> emitter or timer to be added to the domain
Explicitly adds an emitter to the domain. If any event handlers called by
the emitter throw an error, or if the emitter emits an 'error'
event, it
will be routed to the domain's 'error'
event, just like with implicit
binding.
This also works with timers that are returned from setInterval()
and
setTimeout()
. If their callback function throws, it will be caught by
the domain 'error'
handler.
If the Timer or EventEmitter
was already bound to a domain, it is removed
from that one, and bound to this one instead.
domain.bind(callback)
#
callback
<Function> The callback function- Returns: <Function> The bound function
The returned function will be a wrapper around the supplied callback
function. When the returned function is called, any errors that are
thrown will be routed to the domain's 'error'
event.
const d = domain.create();
function readSomeFile(filename, cb) {
fs.readFile(filename, 'utf8', d.bind((er, data) => {
// If this throws, it will also be passed to the domain.
return cb(er, data ? JSON.parse(data) : null);
}));
}
d.on('error', (er) => {
// An error occurred somewhere. If we throw it now, it will crash the program
// with the normal line number and stack message.
});
domain.enter()
#
The enter()
method is plumbing used by the run()
, bind()
, and
intercept()
methods to set the active domain. It sets domain.active
and
process.domain
to the domain, and implicitly pushes the domain onto the domain
stack managed by the domain module (see domain.exit()
for details on the
domain stack). The call to enter()
delimits the beginning of a chain of
asynchronous calls and I/O operations bound to a domain.
Calling enter()
changes only the active domain, and does not alter the domain
itself. enter()
and exit()
can be called an arbitrary number of times on a
single domain.
domain.exit()
#
The exit()
method exits the current domain, popping it off the domain stack.
Any time execution is going to switch to the context of a different chain of
asynchronous calls, it's important to ensure that the current domain is exited.
The call to exit()
delimits either the end of or an interruption to the chain
of asynchronous calls and I/O operations bound to a domain.
If there are multiple, nested domains bound to the current execution context,
exit()
will exit any domains nested within this domain.
Calling exit()
changes only the active domain, and does not alter the domain
itself. enter()
and exit()
can be called an arbitrary number of times on a
single domain.
domain.intercept(callback)
#
callback
<Function> The callback function- Returns: <Function> The intercepted function
This method is almost identical to domain.bind(callback)
. However, in
addition to catching thrown errors, it will also intercept Error
objects sent as the first argument to the function.
In this way, the common if (err) return callback(err);
pattern can be replaced
with a single error handler in a single place.
const d = domain.create();
function readSomeFile(filename, cb) {
fs.readFile(filename, 'utf8', d.intercept((data) => {
// Note, the first argument is never passed to the
// callback since it is assumed to be the 'Error' argument
// and thus intercepted by the domain.
// If this throws, it will also be passed to the domain
// so the error-handling logic can be moved to the 'error'
// event on the domain instead of being repeated throughout
// the program.
return cb(null, JSON.parse(data));
}));
}
d.on('error', (er) => {
// An error occurred somewhere. If we throw it now, it will crash the program
// with the normal line number and stack message.
});
domain.remove(emitter)
#
emitter
<EventEmitter> | <Timer> emitter or timer to be removed from the domain
The opposite of domain.add(emitter)
. Removes domain handling from the
specified emitter.
domain.run(fn[, ...args])
#
fn
<Function>...args
<any>
Run the supplied function in the context of the domain, implicitly binding all event emitters, timers, and lowlevel requests that are created in that context. Optionally, arguments can be passed to the function.
This is the most basic way to use a domain.
const domain = require('node:domain');
const fs = require('node:fs');
const d = domain.create();
d.on('error', (er) => {
console.error('Caught error!', er);
});
d.run(() => {
process.nextTick(() => {
setTimeout(() => { // Simulating some various async stuff
fs.open('non-existent file', 'r', (er, fd) => {
if (er) throw er;
// proceed...
});
}, 100);
});
});
In this example, the d.on('error')
handler will be triggered, rather
than crashing the program.
Domains and promises#
As of Node.js 8.0.0, the handlers of promises are run inside the domain in
which the call to .then()
or .catch()
itself was made:
const d1 = domain.create();
const d2 = domain.create();
let p;
d1.run(() => {
p = Promise.resolve(42);
});
d2.run(() => {
p.then((v) => {
// running in d2
});
});
A callback may be bound to a specific domain using domain.bind(callback)
:
const d1 = domain.create();
const d2 = domain.create();
let p;
d1.run(() => {
p = Promise.resolve(42);
});
d2.run(() => {
p.then(p.domain.bind((v) => {
// running in d1
}));
});
Domains will not interfere with the error handling mechanisms for
promises. In other words, no 'error'
event will be emitted for unhandled
Promise
rejections.
Errors#
Applications running in Node.js will generally experience four categories of errors:
- Standard JavaScript errors such as <EvalError>, <SyntaxError>, <RangeError>, <ReferenceError>, <TypeError>, and <URIError>.
- System errors triggered by underlying operating system constraints such as attempting to open a file that does not exist or attempting to send data over a closed socket.
- User-specified errors triggered by application code.
AssertionError
s are a special class of error that can be triggered when Node.js detects an exceptional logic violation that should never occur. These are raised typically by thenode:assert
module.
All JavaScript and system errors raised by Node.js inherit from, or are instances of, the standard JavaScript <Error> class and are guaranteed to provide at least the properties available on that class.
Error propagation and interception#
Node.js supports several mechanisms for propagating and handling errors that
occur while an application is running. How these errors are reported and
handled depends entirely on the type of Error
and the style of the API that is
called.
All JavaScript errors are handled as exceptions that immediately generate
and throw an error using the standard JavaScript throw
mechanism. These
are handled using the try…catch
construct provided by the
JavaScript language.
// Throws with a ReferenceError because z is not defined.
try {
const m = 1;
const n = m + z;
} catch (err) {
// Handle the error here.
}
Any use of the JavaScript throw
mechanism will raise an exception that
must be handled using try…catch
or the Node.js process will exit
immediately.
With few exceptions, Synchronous APIs (any blocking method that does not
accept a callback
function, such as fs.readFileSync
), will use throw
to report errors.
Errors that occur within Asynchronous APIs may be reported in multiple ways:
-
Most asynchronous methods that accept a
callback
function will accept anError
object passed as the first argument to that function. If that first argument is notnull
and is an instance ofError
, then an error occurred that should be handled.const fs = require('node:fs'); fs.readFile('a file that does not exist', (err, data) => { if (err) { console.error('There was an error reading the file!', err); return; } // Otherwise handle the data });
-
When an asynchronous method is called on an object that is an
EventEmitter
, errors can be routed to that object's'error'
event.const net = require('node:net'); const connection = net.connect('localhost'); // Adding an 'error' event handler to a stream: connection.on('error', (err) => { // If the connection is reset by the server, or if it can't // connect at all, or on any sort of error encountered by // the connection, the error will be sent here. console.error(err); }); connection.pipe(process.stdout);
-
A handful of typically asynchronous methods in the Node.js API may still use the
throw
mechanism to raise exceptions that must be handled usingtry…catch
. There is no comprehensive list of such methods; please refer to the documentation of each method to determine the appropriate error handling mechanism required.
The use of the 'error'
event mechanism is most common for stream-based
and event emitter-based APIs, which themselves represent a series of
asynchronous operations over time (as opposed to a single operation that may
pass or fail).
For all EventEmitter
objects, if an 'error'
event handler is not
provided, the error will be thrown, causing the Node.js process to report an
uncaught exception and crash unless either: The domain
module is
used appropriately or a handler has been registered for the
'uncaughtException'
event.
const EventEmitter = require('node:events');
const ee = new EventEmitter();
setImmediate(() => {
// This will crash the process because no 'error' event
// handler has been added.
ee.emit('error', new Error('This will crash'));
});
Errors generated in this way cannot be intercepted using try…catch
as
they are thrown after the calling code has already exited.
Developers must refer to the documentation for each method to determine exactly how errors raised by those methods are propagated.
Error-first callbacks#
Most asynchronous methods exposed by the Node.js core API follow an idiomatic
pattern referred to as an error-first callback. With this pattern, a callback
function is passed to the method as an argument. When the operation either
completes or an error is raised, the callback function is called with the
Error
object (if any) passed as the first argument. If no error was raised,
the first argument will be passed as null
.
const fs = require('node:fs');
function errorFirstCallback(err, data) {
if (err) {
console.error('There was an error', err);
return;
}
console.log(data);
}
fs.readFile('/some/file/that/does-not-exist', errorFirstCallback);
fs.readFile('/some/file/that/does-exist', errorFirstCallback);
The JavaScript try…catch
mechanism cannot be used to intercept errors
generated by asynchronous APIs. A common mistake for beginners is to try to
use throw
inside an error-first callback:
// THIS WILL NOT WORK:
const fs = require('node:fs');
try {
fs.readFile('/some/file/that/does-not-exist', (err, data) => {
// Mistaken assumption: throwing here...
if (err) {
throw err;
}
});
} catch (err) {
// This will not catch the throw!
console.error(err);
}
This will not work because the callback function passed to fs.readFile()
is
called asynchronously. By the time the callback has been called, the
surrounding code, including the try…catch
block, will have already exited.
Throwing an error inside the callback can crash the Node.js process in most
cases. If domains are enabled, or a handler has been registered with
process.on('uncaughtException')
, such errors can be intercepted.
Class: Error
#
A generic JavaScript <Error> object that does not denote any specific
circumstance of why the error occurred. Error
objects capture a "stack trace"
detailing the point in the code at which the Error
was instantiated, and may
provide a text description of the error.
All errors generated by Node.js, including all system and JavaScript errors,
will either be instances of, or inherit from, the Error
class.
new Error(message[, options])
#
Creates a new Error
object and sets the error.message
property to the
provided text message. If an object is passed as message
, the text message
is generated by calling String(message)
. If the cause
option is provided,
it is assigned to the error.cause
property. The error.stack
property will
represent the point in the code at which new Error()
was called. Stack traces
are dependent on V8's stack trace API. Stack traces extend only to either
(a) the beginning of synchronous code execution, or (b) the number of frames
given by the property Error.stackTraceLimit
, whichever is smaller.
Error.captureStackTrace(targetObject[, constructorOpt])
#
targetObject
<Object>constructorOpt
<Function>
Creates a .stack
property on targetObject
, which when accessed returns
a string representing the location in the code at which
Error.captureStackTrace()
was called.
const myObject = {};
Error.captureStackTrace(myObject);
myObject.stack; // Similar to `new Error().stack`
The first line of the trace will be prefixed with
${myObject.name}: ${myObject.message}
.
The optional constructorOpt
argument accepts a function. If given, all frames
above constructorOpt
, including constructorOpt
, will be omitted from the
generated stack trace.
The constructorOpt
argument is useful for hiding implementation
details of error generation from the user. For instance:
function a() {
b();
}
function b() {
c();
}
function c() {
// Create an error without stack trace to avoid calculating the stack trace twice.
const { stackTraceLimit } = Error;
Error.stackTraceLimit = 0;
const error = new Error();
Error.stackTraceLimit = stackTraceLimit;
// Capture the stack trace above function b
Error.captureStackTrace(error, b); // Neither function c, nor b is included in the stack trace
throw error;
}
a();
Error.stackTraceLimit
#
The Error.stackTraceLimit
property specifies the number of stack frames
collected by a stack trace (whether generated by new Error().stack
or
Error.captureStackTrace(obj)
).
The default value is 10
but may be set to any valid JavaScript number. Changes
will affect any stack trace captured after the value has been changed.
If set to a non-number value, or set to a negative number, stack traces will not capture any frames.
error.cause
#
If present, the error.cause
property is the underlying cause of the Error
.
It is used when catching an error and throwing a new one with a different
message or code in order to still have access to the original error.
The error.cause
property is typically set by calling
new Error(message, { cause })
. It is not set by the constructor if the
cause
option is not provided.
This property allows errors to be chained. When serializing Error
objects,
util.inspect()
recursively serializes error.cause
if it is set.
const cause = new Error('The remote HTTP server responded with a 500 status');
const symptom = new Error('The message failed to send', { cause });
console.log(symptom);
// Prints:
// Error: The message failed to send
// at REPL2:1:17
// at Script.runInThisContext (node:vm:130:12)
// ... 7 lines matching cause stack trace ...
// at [_line] [as _line] (node:internal/readline/interface:886:18) {
// [cause]: Error: The remote HTTP server responded with a 500 status
// at REPL1:1:15
// at Script.runInThisContext (node:vm:130:12)
// at REPLServer.defaultEval (node:repl:574:29)
// at bound (node:domain:426:15)
// at REPLServer.runBound [as eval] (node:domain:437:12)
// at REPLServer.onLine (node:repl:902:10)
// at REPLServer.emit (node:events:549:35)
// at REPLServer.emit (node:domain:482:12)
// at [_onLine] [as _onLine] (node:internal/readline/interface:425:12)
// at [_line] [as _line] (node:internal/readline/interface:886:18)
error.code
#
The error.code
property is a string label that identifies the kind of error.
error.code
is the most stable way to identify an error. It will only change
between major versions of Node.js. In contrast, error.message
strings may
change between any versions of Node.js. See Node.js error codes for details
about specific codes.
error.message
#
The error.message
property is the string description of the error as set by
calling new Error(message)
. The message
passed to the constructor will also
appear in the first line of the stack trace of the Error
, however changing
this property after the Error
object is created may not change the first
line of the stack trace (for example, when error.stack
is read before this
property is changed).
const err = new Error('The message');
console.error(err.message);
// Prints: The message
error.stack
#
The error.stack
property is a string describing the point in the code at which
the Error
was instantiated.
Error: Things keep happening!
at /home/gbusey/file.js:525:2
at Frobnicator.refrobulate (/home/gbusey/business-logic.js:424:21)
at Actor.<anonymous> (/home/gbusey/actors.js:400:8)
at increaseSynergy (/home/gbusey/actors.js:701:6)
The first line is formatted as <error class name>: <error message>
, and
is followed by a series of stack frames (each line beginning with "at ").
Each frame describes a call site within the code that lead to the error being
generated. V8 attempts to display a name for each function (by variable name,
function name, or object method name), but occasionally it will not be able to
find a suitable name. If V8 cannot determine a name for the function, only
location information will be displayed for that frame. Otherwise, the
determined function name will be displayed with location information appended
in parentheses.
Frames are only generated for JavaScript functions. If, for example, execution
synchronously passes through a C++ addon function called cheetahify
which
itself calls a JavaScript function, the frame representing the cheetahify
call
will not be present in the stack traces:
const cheetahify = require('./native-binding.node');
function makeFaster() {
// `cheetahify()` *synchronously* calls speedy.
cheetahify(function speedy() {
throw new Error('oh no!');
});
}
makeFaster();
// will throw:
// /home/gbusey/file.js:6
// throw new Error('oh no!');
// ^
// Error: oh no!
// at speedy (/home/gbusey/file.js:6:11)
// at makeFaster (/home/gbusey/file.js:5:3)
// at Object.<anonymous> (/home/gbusey/file.js:10:1)
// at Module._compile (module.js:456:26)
// at Object.Module._extensions..js (module.js:474:10)
// at Module.load (module.js:356:32)
// at Function.Module._load (module.js:312:12)
// at Function.Module.runMain (module.js:497:10)
// at startup (node.js:119:16)
// at node.js:906:3
The location information will be one of:
native
, if the frame represents a call internal to V8 (as in[].forEach
).plain-filename.js:line:column
, if the frame represents a call internal to Node.js./absolute/path/to/file.js:line:column
, if the frame represents a call in a user program (using CommonJS module system), or its dependencies.<transport-protocol>:///url/to/module/file.mjs:line:column
, if the frame represents a call in a user program (using ES module system), or its dependencies.
The string representing the stack trace is lazily generated when the
error.stack
property is accessed.
The number of frames captured by the stack trace is bounded by the smaller of
Error.stackTraceLimit
or the number of available frames on the current event
loop tick.
Class: AssertionError
#
- Extends: <errors.Error>
Indicates the failure of an assertion. For details, see
Class: assert.AssertionError
.
Class: RangeError
#
- Extends: <errors.Error>
Indicates that a provided argument was not within the set or range of acceptable values for a function; whether that is a numeric range, or outside the set of options for a given function parameter.
require('node:net').connect(-1);
// Throws "RangeError: "port" option should be >= 0 and < 65536: -1"
Node.js will generate and throw RangeError
instances immediately as a form
of argument validation.
Class: ReferenceError
#
- Extends: <errors.Error>
Indicates that an attempt is being made to access a variable that is not defined. Such errors commonly indicate typos in code, or an otherwise broken program.
While client code may generate and propagate these errors, in practice, only V8 will do so.
doesNotExist;
// Throws ReferenceError, doesNotExist is not a variable in this program.
Unless an application is dynamically generating and running code,
ReferenceError
instances indicate a bug in the code or its dependencies.
Class: SyntaxError
#
- Extends: <errors.Error>
Indicates that a program is not valid JavaScript. These errors may only be
generated and propagated as a result of code evaluation. Code evaluation may
happen as a result of eval
, Function
, require
, or vm. These errors
are almost always indicative of a broken program.
try {
require('node:vm').runInThisContext('binary ! isNotOk');
} catch (err) {
// 'err' will be a SyntaxError.
}
SyntaxError
instances are unrecoverable in the context that created them –
they may only be caught by other contexts.
Class: SystemError
#
- Extends: <errors.Error>
Node.js generates system errors when exceptions occur within its runtime environment. These usually occur when an application violates an operating system constraint. For example, a system error will occur if an application attempts to read a file that does not exist.
address
<string> If present, the address to which a network connection failedcode
<string> The string error codedest
<string> If present, the file path destination when reporting a file system errorerrno
<number> The system-provided error numberinfo
<Object> If present, extra details about the error conditionmessage
<string> A system-provided human-readable description of the errorpath
<string> If present, the file path when reporting a file system errorport
<number> If present, the network connection port that is not availablesyscall
<string> The name of the system call that triggered the error
error.address
#
If present, error.address
is a string describing the address to which a
network connection failed.
error.code
#
The error.code
property is a string representing the error code.
error.dest
#
If present, error.dest
is the file path destination when reporting a file
system error.
error.errno
#
The error.errno
property is a negative number which corresponds
to the error code defined in libuv Error handling
.
On Windows the error number provided by the system will be normalized by libuv.
To get the string representation of the error code, use
util.getSystemErrorName(error.errno)
.
error.info
#
If present, error.info
is an object with details about the error condition.
error.message
#
error.message
is a system-provided human-readable description of the error.
error.path
#
If present, error.path
is a string containing a relevant invalid pathname.
error.port
#
If present, error.port
is the network connection port that is not available.
error.syscall
#
The error.syscall
property is a string describing the syscall that failed.
Common system errors#
This is a list of system errors commonly-encountered when writing a Node.js
program. For a comprehensive list, see the errno
(3) man page.
-
EACCES
(Permission denied): An attempt was made to access a file in a way forbidden by its file access permissions. -
EADDRINUSE
(Address already in use): An attempt to bind a server (net
,http
, orhttps
) to a local address failed due to another server on the local system already occupying that address. -
ECONNREFUSED
(Connection refused): No connection could be made because the target machine actively refused it. This usually results from trying to connect to a service that is inactive on the foreign host. -
ECONNRESET
(Connection reset by peer): A connection was forcibly closed by a peer. This normally results from a loss of the connection on the remote socket due to a timeout or reboot. Commonly encountered via thehttp
andnet
modules. -
EEXIST
(File exists): An existing file was the target of an operation that required that the target not exist. -
EISDIR
(Is a directory): An operation expected a file, but the given pathname was a directory. -
EMFILE
(Too many open files in system): Maximum number of file descriptors allowable on the system has been reached, and requests for another descriptor cannot be fulfilled until at least one has been closed. This is encountered when opening many files at once in parallel, especially on systems (in particular, macOS) where there is a low file descriptor limit for processes. To remedy a low limit, runulimit -n 2048
in the same shell that will run the Node.js process. -
ENOENT
(No such file or directory): Commonly raised byfs
operations to indicate that a component of the specified pathname does not exist. No entity (file or directory) could be found by the given path. -
ENOTDIR
(Not a directory): A component of the given pathname existed, but was not a directory as expected. Commonly raised byfs.readdir
. -
ENOTEMPTY
(Directory not empty): A directory with entries was the target of an operation that requires an empty directory, usuallyfs.unlink
. -
ENOTFOUND
(DNS lookup failed): Indicates a DNS failure of eitherEAI_NODATA
orEAI_NONAME
. This is not a standard POSIX error. -
EPERM
(Operation not permitted): An attempt was made to perform an operation that requires elevated privileges. -
EPIPE
(Broken pipe): A write on a pipe, socket, or FIFO for which there is no process to read the data. Commonly encountered at thenet
andhttp
layers, indicative that the remote side of the stream being written to has been closed. -
ETIMEDOUT
(Operation timed out): A connect or send request failed because the connected party did not properly respond after a period of time. Usually encountered byhttp
ornet
. Often a sign that asocket.end()
was not properly called.
Class: TypeError
#
- Extends <errors.Error>
Indicates that a provided argument is not an allowable type. For example,
passing a function to a parameter which expects a string would be a TypeError
.
require('node:url').parse(() => { });
// Throws TypeError, since it expected a string.
Node.js will generate and throw TypeError
instances immediately as a form
of argument validation.
Exceptions vs. errors#
A JavaScript exception is a value that is thrown as a result of an invalid
operation or as the target of a throw
statement. While it is not required
that these values are instances of Error
or classes which inherit from
Error
, all exceptions thrown by Node.js or the JavaScript runtime will be
instances of Error
.
Some exceptions are unrecoverable at the JavaScript layer. Such exceptions
will always cause the Node.js process to crash. Examples include assert()
checks or abort()
calls in the C++ layer.
OpenSSL errors#
Errors originating in crypto
or tls
are of class Error
, and in addition to
the standard .code
and .message
properties, may have some additional
OpenSSL-specific properties.
error.opensslErrorStack
#
An array of errors that can give context to where in the OpenSSL library an error originates from.
error.function
#
The OpenSSL function the error originates in.
error.library
#
The OpenSSL library the error originates in.
error.reason
#
A human-readable string describing the reason for the error.
Node.js error codes#
ABORT_ERR
#
Used when an operation has been aborted (typically using an AbortController
).
APIs not using AbortSignal
s typically do not raise an error with this code.
This code does not use the regular ERR_*
convention Node.js errors use in
order to be compatible with the web platform's AbortError
.
ERR_ACCESS_DENIED
#
A special type of error that is triggered whenever Node.js tries to get access to a resource restricted by the Permission Model.
ERR_AMBIGUOUS_ARGUMENT
#
A function argument is being used in a way that suggests that the function
signature may be misunderstood. This is thrown by the node:assert
module when
the message
parameter in assert.throws(block, message)
matches the error
message thrown by block
because that usage suggests that the user believes
message
is the expected message rather than the message the AssertionError
will display if block
does not throw.
ERR_ARG_NOT_ITERABLE
#
An iterable argument (i.e. a value that works with for...of
loops) was
required, but not provided to a Node.js API.
ERR_ASSERTION
#
A special type of error that can be triggered whenever Node.js detects an
exceptional logic violation that should never occur. These are raised typically
by the node:assert
module.
ERR_ASYNC_CALLBACK
#
An attempt was made to register something that is not a function as an
AsyncHooks
callback.
ERR_ASYNC_TYPE
#
The type of an asynchronous resource was invalid. Users are also able to define their own types if using the public embedder API.
ERR_BROTLI_COMPRESSION_FAILED
#
Data passed to a Brotli stream was not successfully compressed.
ERR_BROTLI_INVALID_PARAM
#
An invalid parameter key was passed during construction of a Brotli stream.
ERR_BUFFER_CONTEXT_NOT_AVAILABLE
#
An attempt was made to create a Node.js Buffer
instance from addon or embedder
code, while in a JS engine Context that is not associated with a Node.js
instance. The data passed to the Buffer
method will have been released
by the time the method returns.
When encountering this error, a possible alternative to creating a Buffer
instance is to create a normal Uint8Array
, which only differs in the
prototype of the resulting object. Uint8Array
s are generally accepted in all
Node.js core APIs where Buffer
s are; they are available in all Contexts.
ERR_BUFFER_OUT_OF_BOUNDS
#
An operation outside the bounds of a Buffer
was attempted.
ERR_BUFFER_TOO_LARGE
#
An attempt has been made to create a Buffer
larger than the maximum allowed
size.
ERR_CANNOT_WATCH_SIGINT
#
Node.js was unable to watch for the SIGINT
signal.
ERR_CHILD_CLOSED_BEFORE_REPLY
#
A child process was closed before the parent received a reply.
ERR_CHILD_PROCESS_IPC_REQUIRED
#
Used when a child process is being forked without specifying an IPC channel.
ERR_CHILD_PROCESS_STDIO_MAXBUFFER
#
Used when the main process is trying to read data from the child process's
STDERR/STDOUT, and the data's length is longer than the maxBuffer
option.
ERR_CLOSED_MESSAGE_PORT
#
There was an attempt to use a MessagePort
instance in a closed
state, usually after .close()
has been called.
ERR_CONSOLE_WRITABLE_STREAM
#
Console
was instantiated without stdout
stream, or Console
has a
non-writable stdout
or stderr
stream.
ERR_CONSTRUCT_CALL_INVALID
#
A class constructor was called that is not callable.
ERR_CONSTRUCT_CALL_REQUIRED
#
A constructor for a class was called without new
.
ERR_CONTEXT_NOT_INITIALIZED
#
The vm context passed into the API is not yet initialized. This could happen when an error occurs (and is caught) during the creation of the context, for example, when the allocation fails or the maximum call stack size is reached when the context is created.
ERR_CRYPTO_CUSTOM_ENGINE_NOT_SUPPORTED
#
A client certificate engine was requested that is not supported by the version of OpenSSL being used.
ERR_CRYPTO_ECDH_INVALID_FORMAT
#
An invalid value for the format
argument was passed to the crypto.ECDH()
class getPublicKey()
method.
ERR_CRYPTO_ECDH_INVALID_PUBLIC_KEY
#
An invalid value for the key
argument has been passed to the
crypto.ECDH()
class computeSecret()
method. It means that the public
key lies outside of the elliptic curve.
ERR_CRYPTO_ENGINE_UNKNOWN
#
An invalid crypto engine identifier was passed to
require('node:crypto').setEngine()
.
ERR_CRYPTO_FIPS_FORCED
#
The --force-fips
command-line argument was used but there was an attempt
to enable or disable FIPS mode in the node:crypto
module.
ERR_CRYPTO_FIPS_UNAVAILABLE
#
An attempt was made to enable or disable FIPS mode, but FIPS mode was not available.
ERR_CRYPTO_HASH_FINALIZED
#
hash.digest()
was called multiple times. The hash.digest()
method must
be called no more than one time per instance of a Hash
object.
ERR_CRYPTO_HASH_UPDATE_FAILED
#
hash.update()
failed for any reason. This should rarely, if ever, happen.
ERR_CRYPTO_INCOMPATIBLE_KEY
#
The given crypto keys are incompatible with the attempted operation.
ERR_CRYPTO_INCOMPATIBLE_KEY_OPTIONS
#
The selected public or private key encoding is incompatible with other options.
ERR_CRYPTO_INITIALIZATION_FAILED
#
Initialization of the crypto subsystem failed.
ERR_CRYPTO_INVALID_AUTH_TAG
#
An invalid authentication tag was provided.
ERR_CRYPTO_INVALID_COUNTER
#
An invalid counter was provided for a counter-mode cipher.
ERR_CRYPTO_INVALID_CURVE
#
An invalid elliptic-curve was provided.
ERR_CRYPTO_INVALID_DIGEST
#
An invalid crypto digest algorithm was specified.
ERR_CRYPTO_INVALID_IV
#
An invalid initialization vector was provided.
ERR_CRYPTO_INVALID_JWK
#
An invalid JSON Web Key was provided.
ERR_CRYPTO_INVALID_KEY_OBJECT_TYPE
#
The given crypto key object's type is invalid for the attempted operation.
ERR_CRYPTO_INVALID_KEYLEN
#
An invalid key length was provided.
ERR_CRYPTO_INVALID_KEYPAIR
#
An invalid key pair was provided.
ERR_CRYPTO_INVALID_KEYTYPE
#
An invalid key type was provided.
ERR_CRYPTO_INVALID_MESSAGELEN
#
An invalid message length was provided.
ERR_CRYPTO_INVALID_SCRYPT_PARAMS
#
Invalid scrypt algorithm parameters were provided.
ERR_CRYPTO_INVALID_STATE
#
A crypto method was used on an object that was in an invalid state. For
instance, calling cipher.getAuthTag()
before calling cipher.final()
.
ERR_CRYPTO_INVALID_TAG_LENGTH
#
An invalid authentication tag length was provided.
ERR_CRYPTO_JOB_INIT_FAILED
#
Initialization of an asynchronous crypto operation failed.
ERR_CRYPTO_JWK_UNSUPPORTED_CURVE
#
Key's Elliptic Curve is not registered for use in the JSON Web Key Elliptic Curve Registry.
ERR_CRYPTO_JWK_UNSUPPORTED_KEY_TYPE
#
Key's Asymmetric Key Type is not registered for use in the JSON Web Key Types Registry.
ERR_CRYPTO_OPERATION_FAILED
#
A crypto operation failed for an otherwise unspecified reason.
ERR_CRYPTO_PBKDF2_ERROR
#
The PBKDF2 algorithm failed for unspecified reasons. OpenSSL does not provide more details and therefore neither does Node.js.
ERR_CRYPTO_SCRYPT_INVALID_PARAMETER
#
One or more crypto.scrypt()
or crypto.scryptSync()
parameters are
outside their legal range.
ERR_CRYPTO_SCRYPT_NOT_SUPPORTED
#
Node.js was compiled without scrypt
support. Not possible with the official
release binaries but can happen with custom builds, including distro builds.
ERR_CRYPTO_SIGN_KEY_REQUIRED
#
A signing key
was not provided to the sign.sign()
method.
ERR_CRYPTO_TIMING_SAFE_EQUAL_LENGTH
#
crypto.timingSafeEqual()
was called with Buffer
, TypedArray
, or
DataView
arguments of different lengths.
ERR_CRYPTO_UNKNOWN_CIPHER
#
An unknown cipher was specified.
ERR_CRYPTO_UNKNOWN_DH_GROUP
#
An unknown Diffie-Hellman group name was given. See
crypto.getDiffieHellman()
for a list of valid group names.
ERR_CRYPTO_UNSUPPORTED_OPERATION
#
An attempt to invoke an unsupported crypto operation was made.
ERR_DEBUGGER_ERROR
#
An error occurred with the debugger.
ERR_DEBUGGER_STARTUP_ERROR
#
The debugger timed out waiting for the required host/port to be free.
ERR_DLOPEN_DISABLED
#
Loading native addons has been disabled using --no-addons
.
ERR_DLOPEN_FAILED
#
A call to process.dlopen()
failed.
ERR_DIR_CLOSED
#
The fs.Dir
was previously closed.
ERR_DIR_CONCURRENT_OPERATION
#
A synchronous read or close call was attempted on an fs.Dir
which has
ongoing asynchronous operations.
ERR_DNS_SET_SERVERS_FAILED
#
c-ares
failed to set the DNS server.
ERR_DOMAIN_CALLBACK_NOT_AVAILABLE
#
The node:domain
module was not usable since it could not establish the
required error handling hooks, because
process.setUncaughtExceptionCaptureCallback()
had been called at an
earlier point in time.
ERR_DOMAIN_CANNOT_SET_UNCAUGHT_EXCEPTION_CAPTURE
#
process.setUncaughtExceptionCaptureCallback()
could not be called
because the node:domain
module has been loaded at an earlier point in time.
The stack trace is extended to include the point in time at which the
node:domain
module had been loaded.
ERR_DUPLICATE_STARTUP_SNAPSHOT_MAIN_FUNCTION
#
v8.startupSnapshot.setDeserializeMainFunction()
could not be called
because it had already been called before.
ERR_ENCODING_INVALID_ENCODED_DATA
#
Data provided to TextDecoder()
API was invalid according to the encoding
provided.
ERR_ENCODING_NOT_SUPPORTED
#
Encoding provided to TextDecoder()
API was not one of the
WHATWG Supported Encodings.
ERR_EVAL_ESM_CANNOT_PRINT
#
--print
cannot be used with ESM input.
ERR_EVENT_RECURSION
#
Thrown when an attempt is made to recursively dispatch an event on EventTarget
.
ERR_EXECUTION_ENVIRONMENT_NOT_AVAILABLE
#
The JS execution context is not associated with a Node.js environment. This may occur when Node.js is used as an embedded library and some hooks for the JS engine are not set up properly.
ERR_FALSY_VALUE_REJECTION
#
A Promise
that was callbackified via util.callbackify()
was rejected with a
falsy value.
ERR_FEATURE_UNAVAILABLE_ON_PLATFORM
#
Used when a feature that is not available to the current platform which is running Node.js is used.
ERR_FS_CP_DIR_TO_NON_DIR
#
An attempt was made to copy a directory to a non-directory (file, symlink,
etc.) using fs.cp()
.
ERR_FS_CP_EEXIST
#
An attempt was made to copy over a file that already existed with
fs.cp()
, with the force
and errorOnExist
set to true
.
ERR_FS_CP_EINVAL
#
When using fs.cp()
, src
or dest
pointed to an invalid path.
ERR_HTTP_CONTENT_LENGTH_MISMATCH
#
Response body size doesn't match with the specified content-length header value.
ERR_FS_CP_FIFO_PIPE
#
An attempt was made to copy a named pipe with fs.cp()
.
ERR_FS_CP_NON_DIR_TO_DIR
#
An attempt was made to copy a non-directory (file, symlink, etc.) to a directory
using fs.cp()
.
ERR_FS_CP_SOCKET
#
An attempt was made to copy to a socket with fs.cp()
.
ERR_FS_CP_SYMLINK_TO_SUBDIRECTORY
#
When using fs.cp()
, a symlink in dest
pointed to a subdirectory
of src
.
ERR_FS_CP_UNKNOWN
#
An attempt was made to copy to an unknown file type with fs.cp()
.
ERR_FS_EISDIR
#
Path is a directory.
ERR_FS_FILE_TOO_LARGE
#
An attempt has been made to read a file whose size is larger than the maximum
allowed size for a Buffer
.
ERR_FS_INVALID_SYMLINK_TYPE
#
An invalid symlink type was passed to the fs.symlink()
or
fs.symlinkSync()
methods.
ERR_HTTP_HEADERS_SENT
#
An attempt was made to add more headers after the headers had already been sent.
ERR_HTTP_INVALID_HEADER_VALUE
#
An invalid HTTP header value was specified.
ERR_HTTP_INVALID_STATUS_CODE
#
Status code was outside the regular status code range (100-999).
ERR_HTTP_REQUEST_TIMEOUT
#
The client has not sent the entire request within the allowed time.
ERR_HTTP_SOCKET_ENCODING
#
Changing the socket encoding is not allowed per RFC 7230 Section 3.
ERR_HTTP_TRAILER_INVALID
#
The Trailer
header was set even though the transfer encoding does not support
that.
ERR_HTTP2_ALTSVC_INVALID_ORIGIN
#
HTTP/2 ALTSVC frames require a valid origin.
ERR_HTTP2_ALTSVC_LENGTH
#
HTTP/2 ALTSVC frames are limited to a maximum of 16,382 payload bytes.
ERR_HTTP2_CONNECT_AUTHORITY
#
For HTTP/2 requests using the CONNECT
method, the :authority
pseudo-header
is required.
ERR_HTTP2_CONNECT_PATH
#
For HTTP/2 requests using the CONNECT
method, the :path
pseudo-header is
forbidden.
ERR_HTTP2_CONNECT_SCHEME
#
For HTTP/2 requests using the CONNECT
method, the :scheme
pseudo-header is
forbidden.
ERR_HTTP2_ERROR
#
A non-specific HTTP/2 error has occurred.
ERR_HTTP2_GOAWAY_SESSION
#
New HTTP/2 Streams may not be opened after the Http2Session
has received a
GOAWAY
frame from the connected peer.
ERR_HTTP2_HEADER_SINGLE_VALUE
#
Multiple values were provided for an HTTP/2 header field that was required to have only a single value.
ERR_HTTP2_HEADERS_AFTER_RESPOND
#
An additional headers was specified after an HTTP/2 response was initiated.
ERR_HTTP2_HEADERS_SENT
#
An attempt was made to send multiple response headers.
ERR_HTTP2_INFO_STATUS_NOT_ALLOWED
#
Informational HTTP status codes (1xx
) may not be set as the response status
code on HTTP/2 responses.
ERR_HTTP2_INVALID_CONNECTION_HEADERS
#
HTTP/1 connection specific headers are forbidden to be used in HTTP/2 requests and responses.
ERR_HTTP2_INVALID_HEADER_VALUE
#
An invalid HTTP/2 header value was specified.
ERR_HTTP2_INVALID_INFO_STATUS
#
An invalid HTTP informational status code has been specified. Informational
status codes must be an integer between 100
and 199
(inclusive).
ERR_HTTP2_INVALID_ORIGIN
#
HTTP/2 ORIGIN
frames require a valid origin.
ERR_HTTP2_INVALID_PACKED_SETTINGS_LENGTH
#
Input Buffer
and Uint8Array
instances passed to the
http2.getUnpackedSettings()
API must have a length that is a multiple of
six.
ERR_HTTP2_INVALID_PSEUDOHEADER
#
Only valid HTTP/2 pseudoheaders (:status
, :path
, :authority
, :scheme
,
and :method
) may be used.
ERR_HTTP2_INVALID_SESSION
#
An action was performed on an Http2Session
object that had already been
destroyed.
ERR_HTTP2_INVALID_SETTING_VALUE
#
An invalid value has been specified for an HTTP/2 setting.
ERR_HTTP2_INVALID_STREAM
#
An operation was performed on a stream that had already been destroyed.
ERR_HTTP2_MAX_PENDING_SETTINGS_ACK
#
Whenever an HTTP/2 SETTINGS
frame is sent to a connected peer, the peer is
required to send an acknowledgment that it has received and applied the new
SETTINGS
. By default, a maximum number of unacknowledged SETTINGS
frames may
be sent at any given time. This error code is used when that limit has been
reached.
ERR_HTTP2_NESTED_PUSH
#
An attempt was made to initiate a new push stream from within a push stream. Nested push streams are not permitted.
ERR_HTTP2_NO_MEM
#
Out of memory when using the http2session.setLocalWindowSize(windowSize)
API.
ERR_HTTP2_NO_SOCKET_MANIPULATION
#
An attempt was made to directly manipulate (read, write, pause, resume, etc.) a
socket attached to an Http2Session
.
ERR_HTTP2_ORIGIN_LENGTH
#
HTTP/2 ORIGIN
frames are limited to a length of 16382 bytes.
ERR_HTTP2_OUT_OF_STREAMS
#
The number of streams created on a single HTTP/2 session reached the maximum limit.
ERR_HTTP2_PAYLOAD_FORBIDDEN
#
A message payload was specified for an HTTP response code for which a payload is forbidden.
ERR_HTTP2_PING_CANCEL
#
An HTTP/2 ping was canceled.
ERR_HTTP2_PING_LENGTH
#
HTTP/2 ping payloads must be exactly 8 bytes in length.
ERR_HTTP2_PSEUDOHEADER_NOT_ALLOWED
#
An HTTP/2 pseudo-header has been used inappropriately. Pseudo-headers are header
key names that begin with the :
prefix.
ERR_HTTP2_PUSH_DISABLED
#
An attempt was made to create a push stream, which had been disabled by the client.
ERR_HTTP2_SEND_FILE
#
An attempt was made to use the Http2Stream.prototype.responseWithFile()
API to
send a directory.
ERR_HTTP2_SEND_FILE_NOSEEK
#
An attempt was made to use the Http2Stream.prototype.responseWithFile()
API to
send something other than a regular file, but offset
or length
options were
provided.
ERR_HTTP2_SESSION_ERROR
#
The Http2Session
closed with a non-zero error code.
ERR_HTTP2_SETTINGS_CANCEL
#
The Http2Session
settings canceled.
ERR_HTTP2_SOCKET_BOUND
#
An attempt was made to connect a Http2Session
object to a net.Socket
or
tls.TLSSocket
that had already been bound to another Http2Session
object.
ERR_HTTP2_SOCKET_UNBOUND
#
An attempt was made to use the socket
property of an Http2Session
that
has already been closed.
ERR_HTTP2_STATUS_101
#
Use of the 101
Informational status code is forbidden in HTTP/2.
ERR_HTTP2_STATUS_INVALID
#
An invalid HTTP status code has been specified. Status codes must be an integer
between 100
and 599
(inclusive).
ERR_HTTP2_STREAM_CANCEL
#
An Http2Stream
was destroyed before any data was transmitted to the connected
peer.
ERR_HTTP2_STREAM_ERROR
#
A non-zero error code was been specified in an RST_STREAM
frame.
ERR_HTTP2_STREAM_SELF_DEPENDENCY
#
When setting the priority for an HTTP/2 stream, the stream may be marked as a dependency for a parent stream. This error code is used when an attempt is made to mark a stream and dependent of itself.
ERR_HTTP2_TOO_MANY_INVALID_FRAMES
#
The limit of acceptable invalid HTTP/2 protocol frames sent by the peer,
as specified through the maxSessionInvalidFrames
option, has been exceeded.
ERR_HTTP2_TRAILERS_ALREADY_SENT
#
Trailing headers have already been sent on the Http2Stream
.
ERR_HTTP2_TRAILERS_NOT_READY
#
The http2stream.sendTrailers()
method cannot be called until after the
'wantTrailers'
event is emitted on an Http2Stream
object. The
'wantTrailers'
event will only be emitted if the waitForTrailers
option
is set for the Http2Stream
.
ERR_HTTP2_UNSUPPORTED_PROTOCOL
#
http2.connect()
was passed a URL that uses any protocol other than http:
or
https:
.
ERR_ILLEGAL_CONSTRUCTOR
#
An attempt was made to construct an object using a non-public constructor.
ERR_IMPORT_ASSERTION_TYPE_FAILED
#
An import assertion has failed, preventing the specified module to be imported.
ERR_IMPORT_ASSERTION_TYPE_MISSING
#
An import assertion is missing, preventing the specified module to be imported.
ERR_IMPORT_ASSERTION_TYPE_UNSUPPORTED
#
An import assertion is not supported by this version of Node.js.
ERR_INCOMPATIBLE_OPTION_PAIR
#
An option pair is incompatible with each other and cannot be used at the same time.
ERR_INPUT_TYPE_NOT_ALLOWED
#
The --input-type
flag was used to attempt to execute a file. This flag can
only be used with input via --eval
, --print
, or STDIN
.
ERR_INSPECTOR_ALREADY_ACTIVATED
#
While using the node:inspector
module, an attempt was made to activate the
inspector when it already started to listen on a port. Use inspector.close()
before activating it on a different address.
ERR_INSPECTOR_ALREADY_CONNECTED
#
While using the node:inspector
module, an attempt was made to connect when the
inspector was already connected.
ERR_INSPECTOR_CLOSED
#
While using the node:inspector
module, an attempt was made to use the
inspector after the session had already closed.
ERR_INSPECTOR_COMMAND
#
An error occurred while issuing a command via the node:inspector
module.
ERR_INSPECTOR_NOT_ACTIVE
#
The inspector
is not active when inspector.waitForDebugger()
is called.
ERR_INSPECTOR_NOT_AVAILABLE
#
The node:inspector
module is not available for use.
ERR_INSPECTOR_NOT_CONNECTED
#
While using the node:inspector
module, an attempt was made to use the
inspector before it was connected.
ERR_INSPECTOR_NOT_WORKER
#
An API was called on the main thread that can only be used from the worker thread.
ERR_INTERNAL_ASSERTION
#
There was a bug in Node.js or incorrect usage of Node.js internals. To fix the error, open an issue at https://github.com/nodejs/node/issues.
ERR_INVALID_ADDRESS_FAMILY
#
The provided address family is not understood by the Node.js API.
ERR_INVALID_ARG_TYPE
#
An argument of the wrong type was passed to a Node.js API.
ERR_INVALID_ARG_VALUE
#
An invalid or unsupported value was passed for a given argument.
ERR_INVALID_ASYNC_ID
#
An invalid asyncId
or triggerAsyncId
was passed using AsyncHooks
. An id
less than -1 should never happen.
ERR_INVALID_BUFFER_SIZE
#
A swap was performed on a Buffer
but its size was not compatible with the
operation.
ERR_INVALID_CHAR
#
Invalid characters were detected in headers.
ERR_INVALID_CURSOR_POS
#
A cursor on a given stream cannot be moved to a specified row without a specified column.
ERR_INVALID_FD
#
A file descriptor ('fd') was not valid (e.g. it was a negative value).
ERR_INVALID_FD_TYPE
#
A file descriptor ('fd') type was not valid.
ERR_INVALID_FILE_URL_HOST
#
A Node.js API that consumes file:
URLs (such as certain functions in the
fs
module) encountered a file URL with an incompatible host. This
situation can only occur on Unix-like systems where only localhost
or an empty
host is supported.
ERR_INVALID_FILE_URL_PATH
#
A Node.js API that consumes file:
URLs (such as certain functions in the
fs
module) encountered a file URL with an incompatible path. The exact
semantics for determining whether a path can be used is platform-dependent.
ERR_INVALID_HANDLE_TYPE
#
An attempt was made to send an unsupported "handle" over an IPC communication
channel to a child process. See subprocess.send()
and process.send()
for more information.
ERR_INVALID_HTTP_TOKEN
#
An invalid HTTP token was supplied.
ERR_INVALID_IP_ADDRESS
#
An IP address is not valid.
ERR_INVALID_MIME_SYNTAX
#
The syntax of a MIME is not valid.
ERR_INVALID_MODULE
#
An attempt was made to load a module that does not exist or was otherwise not valid.
ERR_INVALID_MODULE_SPECIFIER
#
The imported module string is an invalid URL, package name, or package subpath specifier.
ERR_INVALID_OBJECT_DEFINE_PROPERTY
#
An error occurred while setting an invalid attribute on the property of an object.
ERR_INVALID_PACKAGE_CONFIG
#
An invalid package.json
file failed parsing.
ERR_INVALID_PACKAGE_TARGET
#
The package.json
"exports"
field contains an invalid target mapping
value for the attempted module resolution.
ERR_INVALID_PERFORMANCE_MARK
#
While using the Performance Timing API (perf_hooks
), a performance mark is
invalid.
ERR_INVALID_PROTOCOL
#
An invalid options.protocol
was passed to http.request()
.
ERR_INVALID_REPL_EVAL_CONFIG
#
Both breakEvalOnSigint
and eval
options were set in the REPL
config,
which is not supported.
ERR_INVALID_REPL_INPUT
#
The input may not be used in the REPL
. The conditions under which this
error is used are described in the REPL
documentation.
ERR_INVALID_RETURN_PROPERTY
#
Thrown in case a function option does not provide a valid value for one of its returned object properties on execution.
ERR_INVALID_RETURN_PROPERTY_VALUE
#
Thrown in case a function option does not provide an expected value type for one of its returned object properties on execution.
ERR_INVALID_RETURN_VALUE
#
Thrown in case a function option does not return an expected value type on execution, such as when a function is expected to return a promise.
ERR_INVALID_STATE
#
Indicates that an operation cannot be completed due to an invalid state. For instance, an object may have already been destroyed, or may be performing another operation.
ERR_INVALID_SYNC_FORK_INPUT
#
A Buffer
, TypedArray
, DataView
, or string
was provided as stdio input to
an asynchronous fork. See the documentation for the child_process
module
for more information.
ERR_INVALID_THIS
#
A Node.js API function was called with an incompatible this
value.
const urlSearchParams = new URLSearchParams('foo=bar&baz=new');
const buf = Buffer.alloc(1);
urlSearchParams.has.call(buf, 'foo');
// Throws a TypeError with code 'ERR_INVALID_THIS'
ERR_INVALID_TRANSFER_OBJECT
#
An invalid transfer object was passed to postMessage()
.
ERR_INVALID_TUPLE
#
An element in the iterable
provided to the WHATWG
URLSearchParams
constructor did not
represent a [name, value]
tuple – that is, if an element is not iterable, or
does not consist of exactly two elements.
ERR_INVALID_URI
#
An invalid URI was passed.
ERR_INVALID_URL
#
An invalid URL was passed to the WHATWG URL
constructor or the legacy url.parse()
to be parsed.
The thrown error object typically has an additional property 'input'
that
contains the URL that failed to parse.
ERR_INVALID_URL_SCHEME
#
An attempt was made to use a URL of an incompatible scheme (protocol) for a
specific purpose. It is only used in the WHATWG URL API support in the
fs
module (which only accepts URLs with 'file'
scheme), but may be used
in other Node.js APIs as well in the future.
ERR_IPC_CHANNEL_CLOSED
#
An attempt was made to use an IPC communication channel that was already closed.
ERR_IPC_DISCONNECTED
#
An attempt was made to disconnect an IPC communication channel that was already
disconnected. See the documentation for the child_process
module
for more information.
ERR_IPC_ONE_PIPE
#
An attempt was made to create a child Node.js process using more than one IPC
communication channel. See the documentation for the child_process
module
for more information.
ERR_IPC_SYNC_FORK
#
An attempt was made to open an IPC communication channel with a synchronously
forked Node.js process. See the documentation for the child_process
module
for more information.
ERR_LOADER_CHAIN_INCOMPLETE
#
An ESM loader hook returned without calling next()
and without explicitly
signaling a short circuit.
ERR_MANIFEST_ASSERT_INTEGRITY
#
An attempt was made to load a resource, but the resource did not match the integrity defined by the policy manifest. See the documentation for policy manifests for more information.
ERR_MANIFEST_DEPENDENCY_MISSING
#
An attempt was made to load a resource, but the resource was not listed as a dependency from the location that attempted to load it. See the documentation for policy manifests for more information.
ERR_MANIFEST_INTEGRITY_MISMATCH
#
An attempt was made to load a policy manifest, but the manifest had multiple entries for a resource which did not match each other. Update the manifest entries to match in order to resolve this error. See the documentation for policy manifests for more information.
ERR_MANIFEST_INVALID_RESOURCE_FIELD
#
A policy manifest resource had an invalid value for one of its fields. Update the manifest entry to match in order to resolve this error. See the documentation for policy manifests for more information.
ERR_MANIFEST_INVALID_SPECIFIER
#
A policy manifest resource had an invalid value for one of its dependency mappings. Update the manifest entry to match to resolve this error. See the documentation for policy manifests for more information.
ERR_MANIFEST_PARSE_POLICY
#
An attempt was made to load a policy manifest, but the manifest was unable to be parsed. See the documentation for policy manifests for more information.
ERR_MANIFEST_TDZ
#
An attempt was made to read from a policy manifest, but the manifest initialization has not yet taken place. This is likely a bug in Node.js.
ERR_MANIFEST_UNKNOWN_ONERROR
#
A policy manifest was loaded, but had an unknown value for its "onerror" behavior. See the documentation for policy manifests for more information.
ERR_MEMORY_ALLOCATION_FAILED
#
An attempt was made to allocate memory (usually in the C++ layer) but it failed.
ERR_MESSAGE_TARGET_CONTEXT_UNAVAILABLE
#
A message posted to a MessagePort
could not be deserialized in the target
vm Context
. Not all Node.js objects can be successfully instantiated in
any context at this time, and attempting to transfer them using postMessage()
can fail on the receiving side in that case.
ERR_METHOD_NOT_IMPLEMENTED
#
A method is required but not implemented.
ERR_MISSING_ARGS
#
A required argument of a Node.js API was not passed. This is only used for
strict compliance with the API specification (which in some cases may accept
func(undefined)
but not func()
). In most native Node.js APIs,
func(undefined)
and func()
are treated identically, and the
ERR_INVALID_ARG_TYPE
error code may be used instead.
ERR_MISSING_OPTION
#
For APIs that accept options objects, some options might be mandatory. This code is thrown if a required option is missing.
ERR_MISSING_PASSPHRASE
#
An attempt was made to read an encrypted key without specifying a passphrase.
ERR_MISSING_PLATFORM_FOR_WORKER
#
The V8 platform used by this instance of Node.js does not support creating Workers. This is caused by lack of embedder support for Workers. In particular, this error will not occur with standard builds of Node.js.
ERR_MISSING_TRANSFERABLE_IN_TRANSFER_LIST
#
An object that needs to be explicitly listed in the transferList
argument
is in the object passed to a postMessage()
call, but is not provided
in the transferList
for that call. Usually, this is a MessagePort
.
In Node.js versions prior to v15.0.0, the error code being used here was
ERR_MISSING_MESSAGE_PORT_IN_TRANSFER_LIST
. However, the set of
transferable object types has been expanded to cover more types than
MessagePort
.
ERR_MODULE_NOT_FOUND
#
A module file could not be resolved by the ECMAScript modules loader while
attempting an import
operation or when loading the program entry point.
ERR_MULTIPLE_CALLBACK
#
A callback was called more than once.
A callback is almost always meant to only be called once as the query can either be fulfilled or rejected but not both at the same time. The latter would be possible by calling a callback more than once.
ERR_NAPI_CONS_FUNCTION
#
While using Node-API
, a constructor passed was not a function.
ERR_NAPI_INVALID_DATAVIEW_ARGS
#
While calling napi_create_dataview()
, a given offset
was outside the bounds
of the dataview or offset + length
was larger than a length of given buffer
.
ERR_NAPI_INVALID_TYPEDARRAY_ALIGNMENT
#
While calling napi_create_typedarray()
, the provided offset
was not a
multiple of the element size.
ERR_NAPI_INVALID_TYPEDARRAY_LENGTH
#
While calling napi_create_typedarray()
, (length * size_of_element) + byte_offset
was larger than the length of given buffer
.
ERR_NAPI_TSFN_CALL_JS
#
An error occurred while invoking the JavaScript portion of the thread-safe function.
ERR_NAPI_TSFN_GET_UNDEFINED
#
An error occurred while attempting to retrieve the JavaScript undefined
value.
ERR_NAPI_TSFN_START_IDLE_LOOP
#
On the main thread, values are removed from the queue associated with the thread-safe function in an idle loop. This error indicates that an error has occurred when attempting to start the loop.
ERR_NAPI_TSFN_STOP_IDLE_LOOP
#
Once no more items are left in the queue, the idle loop must be suspended. This error indicates that the idle loop has failed to stop.
ERR_NOT_BUILDING_SNAPSHOT
#
An attempt was made to use operations that can only be used when building V8 startup snapshot even though Node.js isn't building one.
ERR_NO_CRYPTO
#
An attempt was made to use crypto features while Node.js was not compiled with OpenSSL crypto support.
ERR_NO_ICU
#
An attempt was made to use features that require ICU, but Node.js was not compiled with ICU support.
ERR_NON_CONTEXT_AWARE_DISABLED
#
A non-context-aware native addon was loaded in a process that disallows them.
ERR_OUT_OF_RANGE
#
A given value is out of the accepted range.
ERR_PACKAGE_IMPORT_NOT_DEFINED
#
The package.json
"imports"
field does not define the given internal
package specifier mapping.
ERR_PACKAGE_PATH_NOT_EXPORTED
#
The package.json
"exports"
field does not export the requested subpath.
Because exports are encapsulated, private internal modules that are not exported
cannot be imported through the package resolution, unless using an absolute URL.
ERR_PARSE_ARGS_INVALID_OPTION_VALUE
#
When strict
set to true
, thrown by util.parseArgs()
if a <boolean>
value is provided for an option of type <string>, or if a <string>
value is provided for an option of type <boolean>.
ERR_PARSE_ARGS_UNEXPECTED_POSITIONAL
#
Thrown by util.parseArgs()
, when a positional argument is provided and
allowPositionals
is set to false
.
ERR_PARSE_ARGS_UNKNOWN_OPTION
#
When strict
set to true
, thrown by util.parseArgs()
if an argument
is not configured in options
.
ERR_PERFORMANCE_INVALID_TIMESTAMP
#
An invalid timestamp value was provided for a performance mark or measure.
ERR_PERFORMANCE_MEASURE_INVALID_OPTIONS
#
Invalid options were provided for a performance measure.
ERR_PROTO_ACCESS
#
Accessing Object.prototype.__proto__
has been forbidden using
--disable-proto=throw
. Object.getPrototypeOf
and
Object.setPrototypeOf
should be used to get and set the prototype of an
object.
ERR_REQUIRE_ESM
#
An attempt was made to require()
an ES Module.
ERR_SCRIPT_EXECUTION_INTERRUPTED
#
Script execution was interrupted by SIGINT
(For
example, Ctrl+C was pressed.)
ERR_SCRIPT_EXECUTION_TIMEOUT
#
Script execution timed out, possibly due to bugs in the script being executed.
ERR_SERVER_ALREADY_LISTEN
#
The server.listen()
method was called while a net.Server
was already
listening. This applies to all instances of net.Server
, including HTTP, HTTPS,
and HTTP/2 Server
instances.
ERR_SERVER_NOT_RUNNING
#
The server.close()
method was called when a net.Server
was not
running. This applies to all instances of net.Server
, including HTTP, HTTPS,
and HTTP/2 Server
instances.
ERR_SOCKET_ALREADY_BOUND
#
An attempt was made to bind a socket that has already been bound.
ERR_SOCKET_BAD_BUFFER_SIZE
#
An invalid (negative) size was passed for either the recvBufferSize
or
sendBufferSize
options in dgram.createSocket()
.
ERR_SOCKET_BAD_PORT
#
An API function expecting a port >= 0 and < 65536 received an invalid value.
ERR_SOCKET_BAD_TYPE
#
An API function expecting a socket type (udp4
or udp6
) received an invalid
value.
ERR_SOCKET_BUFFER_SIZE
#
While using dgram.createSocket()
, the size of the receive or send Buffer
could not be determined.
ERR_SOCKET_CLOSED
#
An attempt was made to operate on an already closed socket.
ERR_SOCKET_CLOSED_BEFORE_CONNECTION
#
When calling net.Socket.write()
on a connecting socket and the socket was
closed before the connection was established.
ERR_SOCKET_DGRAM_IS_CONNECTED
#
A dgram.connect()
call was made on an already connected socket.
ERR_SOCKET_DGRAM_NOT_CONNECTED
#
A dgram.disconnect()
or dgram.remoteAddress()
call was made on a
disconnected socket.
ERR_SOCKET_DGRAM_NOT_RUNNING
#
A call was made and the UDP subsystem was not running.
ERR_SRI_PARSE
#
A string was provided for a Subresource Integrity check, but was unable to be parsed. Check the format of integrity attributes by looking at the Subresource Integrity specification.
ERR_STREAM_ALREADY_FINISHED
#
A stream method was called that cannot complete because the stream was finished.
ERR_STREAM_CANNOT_PIPE
#
An attempt was made to call stream.pipe()
on a Writable
stream.
ERR_STREAM_DESTROYED
#
A stream method was called that cannot complete because the stream was
destroyed using stream.destroy()
.
ERR_STREAM_NULL_VALUES
#
An attempt was made to call stream.write()
with a null
chunk.
ERR_STREAM_PREMATURE_CLOSE
#
An error returned by stream.finished()
and stream.pipeline()
, when a stream
or a pipeline ends non gracefully with no explicit error.
ERR_STREAM_PUSH_AFTER_EOF
#
An attempt was made to call stream.push()
after a null
(EOF) had been
pushed to the stream.
ERR_STREAM_UNSHIFT_AFTER_END_EVENT
#
An attempt was made to call stream.unshift()
after the 'end'
event was
emitted.
ERR_STREAM_WRAP
#
Prevents an abort if a string decoder was set on the Socket or if the decoder
is in objectMode
.
const Socket = require('node:net').Socket;
const instance = new Socket();
instance.setEncoding('utf8');
ERR_STREAM_WRITE_AFTER_END
#
An attempt was made to call stream.write()
after stream.end()
has been
called.
ERR_STRING_TOO_LONG
#
An attempt has been made to create a string longer than the maximum allowed length.
ERR_SYNTHETIC
#
An artificial error object used to capture the call stack for diagnostic reports.
ERR_SYSTEM_ERROR
#
An unspecified or non-specific system error has occurred within the Node.js
process. The error object will have an err.info
object property with
additional details.
ERR_TAP_LEXER_ERROR
#
An error representing a failing lexer state.
ERR_TAP_PARSER_ERROR
#
An error representing a failing parser state. Additional information about
the token causing the error is available via the cause
property.
ERR_TAP_VALIDATION_ERROR
#
This error represents a failed TAP validation.
ERR_TEST_FAILURE
#
This error represents a failed test. Additional information about the failure
is available via the cause
property. The failureType
property specifies
what the test was doing when the failure occurred.
ERR_TLS_CERT_ALTNAME_FORMAT
#
This error is thrown by checkServerIdentity
if a user-supplied
subjectaltname
property violates encoding rules. Certificate objects produced
by Node.js itself always comply with encoding rules and will never cause
this error.
ERR_TLS_CERT_ALTNAME_INVALID
#
While using TLS, the host name/IP of the peer did not match any of the
subjectAltNames
in its certificate.
ERR_TLS_DH_PARAM_SIZE
#
While using TLS, the parameter offered for the Diffie-Hellman (DH
)
key-agreement protocol is too small. By default, the key length must be greater
than or equal to 1024 bits to avoid vulnerabilities, even though it is strongly
recommended to use 2048 bits or larger for stronger security.
ERR_TLS_HANDSHAKE_TIMEOUT
#
A TLS/SSL handshake timed out. In this case, the server must also abort the connection.
ERR_TLS_INVALID_CONTEXT
#
The context must be a SecureContext
.
ERR_TLS_INVALID_PROTOCOL_METHOD
#
The specified secureProtocol
method is invalid. It is either unknown, or
disabled because it is insecure.
ERR_TLS_INVALID_PROTOCOL_VERSION
#
Valid TLS protocol versions are 'TLSv1'
, 'TLSv1.1'
, or 'TLSv1.2'
.
ERR_TLS_INVALID_STATE
#
The TLS socket must be connected and securely established. Ensure the 'secure' event is emitted before continuing.
ERR_TLS_PROTOCOL_VERSION_CONFLICT
#
Attempting to set a TLS protocol minVersion
or maxVersion
conflicts with an
attempt to set the secureProtocol
explicitly. Use one mechanism or the other.
ERR_TLS_PSK_SET_IDENTIY_HINT_FAILED
#
Failed to set PSK identity hint. Hint may be too long.
ERR_TLS_RENEGOTIATION_DISABLED
#
An attempt was made to renegotiate TLS on a socket instance with renegotiation disabled.
ERR_TLS_REQUIRED_SERVER_NAME
#
While using TLS, the server.addContext()
method was called without providing
a host name in the first parameter.
ERR_TLS_SESSION_ATTACK
#
An excessive amount of TLS renegotiations is detected, which is a potential vector for denial-of-service attacks.
ERR_TLS_SNI_FROM_SERVER
#
An attempt was made to issue Server Name Indication from a TLS server-side socket, which is only valid from a client.
ERR_TRACE_EVENTS_CATEGORY_REQUIRED
#
The trace_events.createTracing()
method requires at least one trace event
category.
ERR_TRACE_EVENTS_UNAVAILABLE
#
The node:trace_events
module could not be loaded because Node.js was compiled
with the --without-v8-platform
flag.
ERR_TRANSFORM_ALREADY_TRANSFORMING
#
A Transform
stream finished while it was still transforming.
ERR_TRANSFORM_WITH_LENGTH_0
#
A Transform
stream finished with data still in the write buffer.
ERR_TTY_INIT_FAILED
#
The initialization of a TTY failed due to a system error.
ERR_UNAVAILABLE_DURING_EXIT
#
Function was called within a process.on('exit')
handler that shouldn't be
called within process.on('exit')
handler.
ERR_UNCAUGHT_EXCEPTION_CAPTURE_ALREADY_SET
#
process.setUncaughtExceptionCaptureCallback()
was called twice,
without first resetting the callback to null
.
This error is designed to prevent accidentally overwriting a callback registered from another module.
ERR_UNESCAPED_CHARACTERS
#
A string that contained unescaped characters was received.
ERR_UNHANDLED_ERROR
#
An unhandled error occurred (for instance, when an 'error'
event is emitted
by an EventEmitter
but an 'error'
handler is not registered).
ERR_UNKNOWN_BUILTIN_MODULE
#
Used to identify a specific kind of internal Node.js error that should not typically be triggered by user code. Instances of this error point to an internal bug within the Node.js binary itself.
ERR_UNKNOWN_CREDENTIAL
#
A Unix group or user identifier that does not exist was passed.
ERR_UNKNOWN_ENCODING
#
An invalid or unknown encoding option was passed to an API.
ERR_UNKNOWN_FILE_EXTENSION
#
An attempt was made to load a module with an unknown or unsupported file extension.
ERR_UNKNOWN_MODULE_FORMAT
#
An attempt was made to load a module with an unknown or unsupported format.
ERR_UNKNOWN_SIGNAL
#
An invalid or unknown process signal was passed to an API expecting a valid
signal (such as subprocess.kill()
).
ERR_UNSUPPORTED_DIR_IMPORT
#
import
a directory URL is unsupported. Instead,
self-reference a package using its name and define a custom subpath in
the "exports"
field of the package.json
file.
import './'; // unsupported
import './index.js'; // supported
import 'package-name'; // supported
ERR_UNSUPPORTED_ESM_URL_SCHEME
#
import
with URL schemes other than file
and data
is unsupported.
ERR_USE_AFTER_CLOSE
#
An attempt was made to use something that was already closed.
ERR_VALID_PERFORMANCE_ENTRY_TYPE
#
While using the Performance Timing API (perf_hooks
), no valid performance
entry types are found.
ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING
#
A dynamic import callback was not specified.
ERR_VM_MODULE_ALREADY_LINKED
#
The module attempted to be linked is not eligible for linking, because of one of the following reasons:
- It has already been linked (
linkingStatus
is'linked'
) - It is being linked (
linkingStatus
is'linking'
) - Linking has failed for this module (
linkingStatus
is'errored'
)
ERR_VM_MODULE_CACHED_DATA_REJECTED
#
The cachedData
option passed to a module constructor is invalid.
ERR_VM_MODULE_CANNOT_CREATE_CACHED_DATA
#
Cached data cannot be created for modules which have already been evaluated.
ERR_VM_MODULE_DIFFERENT_CONTEXT
#
The module being returned from the linker function is from a different context than the parent module. Linked modules must share the same context.
ERR_VM_MODULE_LINK_FAILURE
#
The module was unable to be linked due to a failure.
ERR_VM_MODULE_NOT_MODULE
#
The fulfilled value of a linking promise is not a vm.Module
object.
ERR_VM_MODULE_STATUS
#
The current module's status does not allow for this operation. The specific meaning of the error depends on the specific function.
ERR_WASI_ALREADY_STARTED
#
The WASI instance has already started.
ERR_WASI_NOT_STARTED
#
The WASI instance has not been started.
ERR_WEBASSEMBLY_RESPONSE
#
The Response
that has been passed to WebAssembly.compileStreaming
or to
WebAssembly.instantiateStreaming
is not a valid WebAssembly response.
ERR_WORKER_INIT_FAILED
#
The Worker
initialization failed.
ERR_WORKER_INVALID_EXEC_ARGV
#
The execArgv
option passed to the Worker
constructor contains
invalid flags.
ERR_WORKER_NOT_RUNNING
#
An operation failed because the Worker
instance is not currently running.
ERR_WORKER_OUT_OF_MEMORY
#
The Worker
instance terminated because it reached its memory limit.
ERR_WORKER_PATH
#
The path for the main script of a worker is neither an absolute path
nor a relative path starting with ./
or ../
.
ERR_WORKER_UNSERIALIZABLE_ERROR
#
All attempts at serializing an uncaught exception from a worker thread failed.
ERR_WORKER_UNSUPPORTED_OPERATION
#
The requested functionality is not supported in worker threads.
ERR_ZLIB_INITIALIZATION_FAILED
#
Creation of a zlib
object failed due to incorrect configuration.
HPE_HEADER_OVERFLOW
#
Too much HTTP header data was received. In order to protect against malicious or
malconfigured clients, if more than 8 KiB of HTTP header data is received then
HTTP parsing will abort without a request or response object being created, and
an Error
with this code will be emitted.
HPE_UNEXPECTED_CONTENT_LENGTH
#
Server is sending both a Content-Length
header and Transfer-Encoding: chunked
.
Transfer-Encoding: chunked
allows the server to maintain an HTTP persistent
connection for dynamically generated content.
In this case, the Content-Length
HTTP header cannot be used.
Use Content-Length
or Transfer-Encoding: chunked
.
MODULE_NOT_FOUND
#
A module file could not be resolved by the CommonJS modules loader while
attempting a require()
operation or when loading the program entry point.
Legacy Node.js error codes#
ERR_CANNOT_TRANSFER_OBJECT
#
The value passed to postMessage()
contained an object that is not supported
for transferring.
ERR_CRYPTO_HASH_DIGEST_NO_UTF16
#
The UTF-16 encoding was used with hash.digest()
. While the
hash.digest()
method does allow an encoding
argument to be passed in,
causing the method to return a string rather than a Buffer
, the UTF-16
encoding (e.g. ucs
or utf16le
) is not supported.
ERR_HTTP2_FRAME_ERROR
#
Used when a failure occurs sending an individual frame on the HTTP/2 session.
ERR_HTTP2_HEADERS_OBJECT
#
Used when an HTTP/2 Headers Object is expected.
ERR_HTTP2_HEADER_REQUIRED
#
Used when a required header is missing in an HTTP/2 message.
ERR_HTTP2_INFO_HEADERS_AFTER_RESPOND
#
HTTP/2 informational headers must only be sent prior to calling the
Http2Stream.prototype.respond()
method.
ERR_HTTP2_STREAM_CLOSED
#
Used when an action has been performed on an HTTP/2 Stream that has already been closed.
ERR_HTTP_INVALID_CHAR
#
Used when an invalid character is found in an HTTP response status message (reason phrase).
ERR_INDEX_OUT_OF_RANGE
#
A given index was out of the accepted range (e.g. negative offsets).
ERR_INVALID_OPT_VALUE
#
An invalid or unexpected value was passed in an options object.
ERR_INVALID_OPT_VALUE_ENCODING
#
An invalid or unknown file encoding was passed.
ERR_MISSING_MESSAGE_PORT_IN_TRANSFER_LIST
#
This error code was replaced by ERR_MISSING_TRANSFERABLE_IN_TRANSFER_LIST
in Node.js v15.0.0, because it is no longer accurate as other types of
transferable objects also exist now.
ERR_NAPI_CONS_PROTOTYPE_OBJECT
#
Used by the Node-API
when Constructor.prototype
is not an object.
ERR_NETWORK_IMPORT_BAD_RESPONSE
#
Response was received but was invalid when importing a module over the network.
ERR_NETWORK_IMPORT_DISALLOWED
#
A network module attempted to load another module that it is not allowed to load. Likely this restriction is for security reasons.
ERR_NO_LONGER_SUPPORTED
#
A Node.js API was called in an unsupported manner, such as
Buffer.write(string, encoding, offset[, length])
.
ERR_OPERATION_FAILED
#
An operation failed. This is typically used to signal the general failure of an asynchronous operation.
ERR_OUTOFMEMORY
#
Used generically to identify that an operation caused an out of memory condition.
ERR_PARSE_HISTORY_DATA
#
The node:repl
module was unable to parse data from the REPL history file.
ERR_SOCKET_CANNOT_SEND
#
Data could not be sent on a socket.
ERR_STDERR_CLOSE
#
An attempt was made to close the process.stderr
stream. By design, Node.js
does not allow stdout
or stderr
streams to be closed by user code.
ERR_STDOUT_CLOSE
#
An attempt was made to close the process.stdout
stream. By design, Node.js
does not allow stdout
or stderr
streams to be closed by user code.
ERR_STREAM_READ_NOT_IMPLEMENTED
#
Used when an attempt is made to use a readable stream that has not implemented
readable._read()
.
ERR_TLS_RENEGOTIATION_FAILED
#
Used when a TLS renegotiation request has failed in a non-specific way.
ERR_TRANSFERRING_EXTERNALIZED_SHAREDARRAYBUFFER
#
A SharedArrayBuffer
whose memory is not managed by the JavaScript engine
or by Node.js was encountered during serialization. Such a SharedArrayBuffer
cannot be serialized.
This can only happen when native addons create SharedArrayBuffer
s in
"externalized" mode, or put existing SharedArrayBuffer
into externalized mode.
ERR_UNKNOWN_STDIN_TYPE
#
An attempt was made to launch a Node.js process with an unknown stdin
file
type. This error is usually an indication of a bug within Node.js itself,
although it is possible for user code to trigger it.
ERR_UNKNOWN_STREAM_TYPE
#
An attempt was made to launch a Node.js process with an unknown stdout
or
stderr
file type. This error is usually an indication of a bug within Node.js
itself, although it is possible for user code to trigger it.
ERR_V8BREAKITERATOR
#
The V8 BreakIterator
API was used but the full ICU data set is not installed.
ERR_VALUE_OUT_OF_RANGE
#
Used when a given value is out of the accepted range.
ERR_VM_MODULE_NOT_LINKED
#
The module must be successfully linked before instantiation.
ERR_VM_MODULE_LINKING_ERRORED
#
The linker function returned a module for which linking has failed.
ERR_WORKER_UNSUPPORTED_EXTENSION
#
The pathname used for the main script of a worker has an unknown file extension.
ERR_ZLIB_BINDING_CLOSED
#
Used when an attempt is made to use a zlib
object after it has already been
closed.
ERR_CPU_USAGE
#
The native call from process.cpuUsage
could not be processed.
Events#
Source Code: lib/events.js
Much of the Node.js core API is built around an idiomatic asynchronous
event-driven architecture in which certain kinds of objects (called "emitters")
emit named events that cause Function
objects ("listeners") to be called.
For instance: a net.Server
object emits an event each time a peer
connects to it; a fs.ReadStream
emits an event when the file is opened;
a stream emits an event whenever data is available to be read.
All objects that emit events are instances of the EventEmitter
class. These
objects expose an eventEmitter.on()
function that allows one or more
functions to be attached to named events emitted by the object. Typically,
event names are camel-cased strings but any valid JavaScript property key
can be used.
When the EventEmitter
object emits an event, all of the functions attached
to that specific event are called synchronously. Any values returned by the
called listeners are ignored and discarded.
The following example shows a simple EventEmitter
instance with a single
listener. The eventEmitter.on()
method is used to register listeners, while
the eventEmitter.emit()
method is used to trigger the event.
import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('event', () => {
console.log('an event occurred!');
});
myEmitter.emit('event');
const EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('event', () => {
console.log('an event occurred!');
});
myEmitter.emit('event');
Passing arguments and this
to listeners#
The eventEmitter.emit()
method allows an arbitrary set of arguments to be
passed to the listener functions. Keep in mind that when
an ordinary listener function is called, the standard this
keyword
is intentionally set to reference the EventEmitter
instance to which the
listener is attached.
import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('event', function(a, b) {
console.log(a, b, this, this === myEmitter);
// Prints:
// a b MyEmitter {
// _events: [Object: null prototype] { event: [Function (anonymous)] },
// _eventsCount: 1,
// _maxListeners: undefined,
// [Symbol(kCapture)]: false
// } true
});
myEmitter.emit('event', 'a', 'b');
const EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('event', function(a, b) {
console.log(a, b, this, this === myEmitter);
// Prints:
// a b MyEmitter {
// _events: [Object: null prototype] { event: [Function (anonymous)] },
// _eventsCount: 1,
// _maxListeners: undefined,
// [Symbol(kCapture)]: false
// } true
});
myEmitter.emit('event', 'a', 'b');
It is possible to use ES6 Arrow Functions as listeners, however, when doing so,
the this
keyword will no longer reference the EventEmitter
instance:
import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('event', (a, b) => {
console.log(a, b, this);
// Prints: a b {}
});
myEmitter.emit('event', 'a', 'b');
const EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('event', (a, b) => {
console.log(a, b, this);
// Prints: a b {}
});
myEmitter.emit('event', 'a', 'b');
Asynchronous vs. synchronous#
The EventEmitter
calls all listeners synchronously in the order in which
they were registered. This ensures the proper sequencing of
events and helps avoid race conditions and logic errors. When appropriate,
listener functions can switch to an asynchronous mode of operation using
the setImmediate()
or process.nextTick()
methods:
import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('event', (a, b) => {
setImmediate(() => {
console.log('this happens asynchronously');
});
});
myEmitter.emit('event', 'a', 'b');
const EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('event', (a, b) => {
setImmediate(() => {
console.log('this happens asynchronously');
});
});
myEmitter.emit('event', 'a', 'b');
Handling events only once#
When a listener is registered using the eventEmitter.on()
method, that
listener is invoked every time the named event is emitted.
import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
let m = 0;
myEmitter.on('event', () => {
console.log(++m);
});
myEmitter.emit('event');
// Prints: 1
myEmitter.emit('event');
// Prints: 2
const EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
let m = 0;
myEmitter.on('event', () => {
console.log(++m);
});
myEmitter.emit('event');
// Prints: 1
myEmitter.emit('event');
// Prints: 2
Using the eventEmitter.once()
method, it is possible to register a listener
that is called at most once for a particular event. Once the event is emitted,
the listener is unregistered and then called.
import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
let m = 0;
myEmitter.once('event', () => {
console.log(++m);
});
myEmitter.emit('event');
// Prints: 1
myEmitter.emit('event');
// Ignored
const EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
let m = 0;
myEmitter.once('event', () => {
console.log(++m);
});
myEmitter.emit('event');
// Prints: 1
myEmitter.emit('event');
// Ignored
Error events#
When an error occurs within an EventEmitter
instance, the typical action is
for an 'error'
event to be emitted. These are treated as special cases
within Node.js.
If an EventEmitter
does not have at least one listener registered for the
'error'
event, and an 'error'
event is emitted, the error is thrown, a
stack trace is printed, and the Node.js process exits.
import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.emit('error', new Error('whoops!'));
// Throws and crashes Node.js
const EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.emit('error', new Error('whoops!'));
// Throws and crashes Node.js
To guard against crashing the Node.js process the domain
module can be
used. (Note, however, that the node:domain
module is deprecated.)
As a best practice, listeners should always be added for the 'error'
events.
import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('error', (err) => {
console.error('whoops! there was an error');
});
myEmitter.emit('error', new Error('whoops!'));
// Prints: whoops! there was an error
const EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('error', (err) => {
console.error('whoops! there was an error');
});
myEmitter.emit('error', new Error('whoops!'));
// Prints: whoops! there was an error
It is possible to monitor 'error'
events without consuming the emitted error
by installing a listener using the symbol events.errorMonitor
.
import { EventEmitter, errorMonitor } from 'node:events';
const myEmitter = new EventEmitter();
myEmitter.on(errorMonitor, (err) => {
MyMonitoringTool.log(err);
});
myEmitter.emit('error', new Error('whoops!'));
// Still throws and crashes Node.js
const { EventEmitter, errorMonitor } = require('node:events');
const myEmitter = new EventEmitter();
myEmitter.on(errorMonitor, (err) => {
MyMonitoringTool.log(err);
});
myEmitter.emit('error', new Error('whoops!'));
// Still throws and crashes Node.js
Capture rejections of promises#
Using async
functions with event handlers is problematic, because it
can lead to an unhandled rejection in case of a thrown exception:
import { EventEmitter } from 'node:events';
const ee = new EventEmitter();
ee.on('something', async (value) => {
throw new Error('kaboom');
});
const EventEmitter = require('node:events');
const ee = new EventEmitter();
ee.on('something', async (value) => {
throw new Error('kaboom');
});
The captureRejections
option in the EventEmitter
constructor or the global
setting change this behavior, installing a .then(undefined, handler)
handler on the Promise
. This handler routes the exception
asynchronously to the Symbol.for('nodejs.rejection')
method
if there is one, or to 'error'
event handler if there is none.
import { EventEmitter } from 'node:events';
const ee1 = new EventEmitter({ captureRejections: true });
ee1.on('something', async (value) => {
throw new Error('kaboom');
});
ee1.on('error', console.log);
const ee2 = new EventEmitter({ captureRejections: true });
ee2.on('something', async (value) => {
throw new Error('kaboom');
});
ee2[Symbol.for('nodejs.rejection')] = console.log;
const EventEmitter = require('node:events');
const ee1 = new EventEmitter({ captureRejections: true });
ee1.on('something', async (value) => {
throw new Error('kaboom');
});
ee1.on('error', console.log);
const ee2 = new EventEmitter({ captureRejections: true });
ee2.on('something', async (value) => {
throw new Error('kaboom');
});
ee2[Symbol.for('nodejs.rejection')] = console.log;
Setting events.captureRejections = true
will change the default for all
new instances of EventEmitter
.
import { EventEmitter } from 'node:events';
EventEmitter.captureRejections = true;
const ee1 = new EventEmitter();
ee1.on('something', async (value) => {
throw new Error('kaboom');
});
ee1.on('error', console.log);
const events = require('node:events');
events.captureRejections = true;
const ee1 = new events.EventEmitter();
ee1.on('something', async (value) => {
throw new Error('kaboom');
});
ee1.on('error', console.log);
The 'error'
events that are generated by the captureRejections
behavior
do not have a catch handler to avoid infinite error loops: the
recommendation is to not use async
functions as 'error'
event handlers.
Class: EventEmitter
#
The EventEmitter
class is defined and exposed by the node:events
module:
import { EventEmitter } from 'node:events';
const EventEmitter = require('node:events');
All EventEmitter
s emit the event 'newListener'
when new listeners are
added and 'removeListener'
when existing listeners are removed.
It supports the following option:
captureRejections
<boolean> It enables automatic capturing of promise rejection. Default:false
.
Event: 'newListener'
#
eventName
<string> | <symbol> The name of the event being listened forlistener
<Function> The event handler function
The EventEmitter
instance will emit its own 'newListener'
event before
a listener is added to its internal array of listeners.
Listeners registered for the 'newListener'
event are passed the event
name and a reference to the listener being added.
The fact that the event is triggered before adding the listener has a subtle
but important side effect: any additional listeners registered to the same
name
within the 'newListener'
callback are inserted before the
listener that is in the process of being added.
import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
// Only do this once so we don't loop forever
myEmitter.once('newListener', (event, listener) => {
if (event === 'event') {
// Insert a new listener in front
myEmitter.on('event', () => {
console.log('B');
});
}
});
myEmitter.on('event', () => {
console.log('A');
});
myEmitter.emit('event');
// Prints:
// B
// A
const EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
// Only do this once so we don't loop forever
myEmitter.once('newListener', (event, listener) => {
if (event === 'event') {
// Insert a new listener in front
myEmitter.on('event', () => {
console.log('B');
});
}
});
myEmitter.on('event', () => {
console.log('A');
});
myEmitter.emit('event');
// Prints:
// B
// A
Event: 'removeListener'
#
eventName
<string> | <symbol> The event namelistener
<Function> The event handler function
The 'removeListener'
event is emitted after the listener
is removed.
emitter.addListener(eventName, listener)
#
eventName
<string> | <symbol>listener
<Function>
Alias for emitter.on(eventName, listener)
.
emitter.emit(eventName[, ...args])
#
Synchronously calls each of the listeners registered for the event named
eventName
, in the order they were registered, passing the supplied arguments
to each.
Returns true
if the event had listeners, false
otherwise.
import { EventEmitter } from 'node:events';
const myEmitter = new EventEmitter();
// First listener
myEmitter.on('event', function firstListener() {
console.log('Helloooo! first listener');
});
// Second listener
myEmitter.on('event', function secondListener(arg1, arg2) {
console.log(`event with parameters ${arg1}, ${arg2} in second listener`);
});
// Third listener
myEmitter.on('event', function thirdListener(...args) {
const parameters = args.join(', ');
console.log(`event with parameters ${parameters} in third listener`);
});
console.log(myEmitter.listeners('event'));
myEmitter.emit('event', 1, 2, 3, 4, 5);
// Prints:
// [
// [Function: firstListener],
// [Function: secondListener],
// [Function: thirdListener]
// ]
// Helloooo! first listener
// event with parameters 1, 2 in second listener
// event with parameters 1, 2, 3, 4, 5 in third listener
const EventEmitter = require('node:events');
const myEmitter = new EventEmitter();
// First listener
myEmitter.on('event', function firstListener() {
console.log('Helloooo! first listener');
});
// Second listener
myEmitter.on('event', function secondListener(arg1, arg2) {
console.log(`event with parameters ${arg1}, ${arg2} in second listener`);
});
// Third listener
myEmitter.on('event', function thirdListener(...args) {
const parameters = args.join(', ');
console.log(`event with parameters ${parameters} in third listener`);
});
console.log(myEmitter.listeners('event'));
myEmitter.emit('event', 1, 2, 3, 4, 5);
// Prints:
// [
// [Function: firstListener],
// [Function: secondListener],
// [Function: thirdListener]
// ]
// Helloooo! first listener
// event with parameters 1, 2 in second listener
// event with parameters 1, 2, 3, 4, 5 in third listener
emitter.eventNames()
#
- Returns: <Array>
Returns an array listing the events for which the emitter has registered
listeners. The values in the array are strings or Symbol
s.
import { EventEmitter } from 'node:events';
const myEE = new EventEmitter();
myEE.on('foo', () => {});
myEE.on('bar', () => {});
const sym = Symbol('symbol');
myEE.on(sym, () => {});
console.log(myEE.eventNames());
// Prints: [ 'foo', 'bar', Symbol(symbol) ]
const EventEmitter = require('node:events');
const myEE = new EventEmitter();
myEE.on('foo', () => {});
myEE.on('bar', () => {});
const sym = Symbol('symbol');
myEE.on(sym, () => {});
console.log(myEE.eventNames());
// Prints: [ 'foo', 'bar', Symbol(symbol) ]
emitter.getMaxListeners()
#
- Returns: <integer>
Returns the current max listener value for the EventEmitter
which is either
set by emitter.setMaxListeners(n)
or defaults to
events.defaultMaxListeners
.
emitter.listenerCount(eventName[, listener])
#
eventName
<string> | <symbol> The name of the event being listened forlistener
<Function> The event handler function- Returns: <integer>
Returns the number of listeners listening for the event named eventName
.
If listener
is provided, it will return how many times the listener is found
in the list of the listeners of the event.
emitter.listeners(eventName)
#
eventName
<string> | <symbol>- Returns: <Function[]>
Returns a copy of the array of listeners for the event named eventName
.
server.on('connection', (stream) => {
console.log('someone connected!');
});
console.log(util.inspect(server.listeners('connection')));
// Prints: [ [Function] ]
emitter.off(eventName, listener)
#
eventName
<string> | <symbol>listener
<Function>- Returns: <EventEmitter>
Alias for emitter.removeListener()
.
emitter.on(eventName, listener)
#
eventName
<string> | <symbol> The name of the event.listener
<Function> The callback function- Returns: <EventEmitter>
Adds the listener
function to the end of the listeners array for the
event named eventName
. No checks are made to see if the listener
has
already been added. Multiple calls passing the same combination of eventName
and listener
will result in the listener
being added, and called, multiple
times.
server.on('connection', (stream) => {
console.log('someone connected!');
});
Returns a reference to the EventEmitter
, so that calls can be chained.
By default, event listeners are invoked in the order they are added. The
emitter.prependListener()
method can be used as an alternative to add the
event listener to the beginning of the listeners array.
import { EventEmitter } from 'node:events';
const myEE = new EventEmitter();
myEE.on('foo', () => console.log('a'));
myEE.prependListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
// b
// a
const EventEmitter = require('node:events');
const myEE = new EventEmitter();
myEE.on('foo', () => console.log('a'));
myEE.prependListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
// b
// a
emitter.once(eventName, listener)
#
eventName
<string> | <symbol> The name of the event.listener
<Function> The callback function- Returns: <EventEmitter>
Adds a one-time listener
function for the event named eventName
. The
next time eventName
is triggered, this listener is removed and then invoked.
server.once('connection', (stream) => {
console.log('Ah, we have our first user!');
});
Returns a reference to the EventEmitter
, so that calls can be chained.
By default, event listeners are invoked in the order they are added. The
emitter.prependOnceListener()
method can be used as an alternative to add the
event listener to the beginning of the listeners array.
import { EventEmitter } from 'node:events';
const myEE = new EventEmitter();
myEE.once('foo', () => console.log('a'));
myEE.prependOnceListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
// b
// a
const EventEmitter = require('node:events');
const myEE = new EventEmitter();
myEE.once('foo', () => console.log('a'));
myEE.prependOnceListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
// b
// a
emitter.prependListener(eventName, listener)
#
eventName
<string> | <symbol> The name of the event.listener
<Function> The callback function- Returns: <EventEmitter>
Adds the listener
function to the beginning of the listeners array for the
event named eventName
. No checks are made to see if the listener
has
already been added. Multiple calls passing the same combination of eventName
and listener
will result in the listener
being added, and called, multiple
times.
server.prependListener('connection', (stream) => {
console.log('someone connected!');
});
Returns a reference to the EventEmitter
, so that calls can be chained.
emitter.prependOnceListener(eventName, listener)
#
eventName
<string> | <symbol> The name of the event.listener
<Function> The callback function- Returns: <EventEmitter>
Adds a one-time listener
function for the event named eventName
to the
beginning of the listeners array. The next time eventName
is triggered, this
listener is removed, and then invoked.
server.prependOnceListener('connection', (stream) => {
console.log('Ah, we have our first user!');
});
Returns a reference to the EventEmitter
, so that calls can be chained.
emitter.removeAllListeners([eventName])
#
eventName
<string> | <symbol>- Returns: <EventEmitter>
Removes all listeners, or those of the specified eventName
.
It is bad practice to remove listeners added elsewhere in the code,
particularly when the EventEmitter
instance was created by some other
component or module (e.g. sockets or file streams).
Returns a reference to the EventEmitter
, so that calls can be chained.
emitter.removeListener(eventName, listener)
#
eventName
<string> | <symbol>listener
<Function>- Returns: <EventEmitter>
Removes the specified listener
from the listener array for the event named
eventName
.
const callback = (stream) => {
console.log('someone connected!');
};
server.on('connection', callback);
// ...
server.removeListener('connection', callback);
removeListener()
will remove, at most, one instance of a listener from the
listener array. If any single listener has been added multiple times to the
listener array for the specified eventName
, then removeListener()
must be
called multiple times to remove each instance.
Once an event is emitted, all listeners attached to it at the
time of emitting are called in order. This implies that any
removeListener()
or removeAllListeners()
calls after emitting and
before the last listener finishes execution will not remove them from
emit()
in progress. Subsequent events behave as expected.
import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
const callbackA = () => {
console.log('A');
myEmitter.removeListener('event', callbackB);
};
const callbackB = () => {
console.log('B');
};
myEmitter.on('event', callbackA);
myEmitter.on('event', callbackB);
// callbackA removes listener callbackB but it will still be called.
// Internal listener array at time of emit [callbackA, callbackB]
myEmitter.emit('event');
// Prints:
// A
// B
// callbackB is now removed.
// Internal listener array [callbackA]
myEmitter.emit('event');
// Prints:
// A
const EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
const callbackA = () => {
console.log('A');
myEmitter.removeListener('event', callbackB);
};
const callbackB = () => {
console.log('B');
};
myEmitter.on('event', callbackA);
myEmitter.on('event', callbackB);
// callbackA removes listener callbackB but it will still be called.
// Internal listener array at time of emit [callbackA, callbackB]
myEmitter.emit('event');
// Prints:
// A
// B
// callbackB is now removed.
// Internal listener array [callbackA]
myEmitter.emit('event');
// Prints:
// A
Because listeners are managed using an internal array, calling this will
change the position indices of any listener registered after the listener
being removed. This will not impact the order in which listeners are called,
but it means that any copies of the listener array as returned by
the emitter.listeners()
method will need to be recreated.
When a single function has been added as a handler multiple times for a single
event (as in the example below), removeListener()
will remove the most
recently added instance. In the example the once('ping')
listener is removed:
import { EventEmitter } from 'node:events';
const ee = new EventEmitter();
function pong() {
console.log('pong');
}
ee.on('ping', pong);
ee.once('ping', pong);
ee.removeListener('ping', pong);
ee.emit('ping');
ee.emit('ping');
const EventEmitter = require('node:events');
const ee = new EventEmitter();
function pong() {
console.log('pong');
}
ee.on('ping', pong);
ee.once('ping', pong);
ee.removeListener('ping', pong);
ee.emit('ping');
ee.emit('ping');
Returns a reference to the EventEmitter
, so that calls can be chained.
emitter.setMaxListeners(n)
#
n
<integer>- Returns: <EventEmitter>
By default EventEmitter
s will print a warning if more than 10
listeners are
added for a particular event. This is a useful default that helps finding
memory leaks. The emitter.setMaxListeners()
method allows the limit to be
modified for this specific EventEmitter
instance. The value can be set to
Infinity
(or 0
) to indicate an unlimited number of listeners.
Returns a reference to the EventEmitter
, so that calls can be chained.
emitter.rawListeners(eventName)
#
eventName
<string> | <symbol>- Returns: <Function[]>
Returns a copy of the array of listeners for the event named eventName
,
including any wrappers (such as those created by .once()
).
import { EventEmitter } from 'node:events';
const emitter = new EventEmitter();
emitter.once('log', () => console.log('log once'));
// Returns a new Array with a function `onceWrapper` which has a property
// `listener` which contains the original listener bound above
const listeners = emitter.rawListeners('log');
const logFnWrapper = listeners[0];
// Logs "log once" to the console and does not unbind the `once` event
logFnWrapper.listener();
// Logs "log once" to the console and removes the listener
logFnWrapper();
emitter.on('log', () => console.log('log persistently'));
// Will return a new Array with a single function bound by `.on()` above
const newListeners = emitter.rawListeners('log');
// Logs "log persistently" twice
newListeners[0]();
emitter.emit('log');
const EventEmitter = require('node:events');
const emitter = new EventEmitter();
emitter.once('log', () => console.log('log once'));
// Returns a new Array with a function `onceWrapper` which has a property
// `listener` which contains the original listener bound above
const listeners = emitter.rawListeners('log');
const logFnWrapper = listeners[0];
// Logs "log once" to the console and does not unbind the `once` event
logFnWrapper.listener();
// Logs "log once" to the console and removes the listener
logFnWrapper();
emitter.on('log', () => console.log('log persistently'));
// Will return a new Array with a single function bound by `.on()` above
const newListeners = emitter.rawListeners('log');
// Logs "log persistently" twice
newListeners[0]();
emitter.emit('log');
emitter[Symbol.for('nodejs.rejection')](err, eventName[, ...args])
#
The Symbol.for('nodejs.rejection')
method is called in case a
promise rejection happens when emitting an event and
captureRejections
is enabled on the emitter.
It is possible to use events.captureRejectionSymbol
in
place of Symbol.for('nodejs.rejection')
.
import { EventEmitter, captureRejectionSymbol } from 'node:events';
class MyClass extends EventEmitter {
constructor() {
super({ captureRejections: true });
}
[captureRejectionSymbol](err, event, ...args) {
console.log('rejection happened for', event, 'with', err, ...args);
this.destroy(err);
}
destroy(err) {
// Tear the resource down here.
}
}
const { EventEmitter, captureRejectionSymbol } = require('node:events');
class MyClass extends EventEmitter {
constructor() {
super({ captureRejections: true });
}
[captureRejectionSymbol](err, event, ...args) {
console.log('rejection happened for', event, 'with', err, ...args);
this.destroy(err);
}
destroy(err) {
// Tear the resource down here.
}
}
events.defaultMaxListeners
#
By default, a maximum of 10
listeners can be registered for any single
event. This limit can be changed for individual EventEmitter
instances
using the emitter.setMaxListeners(n)
method. To change the default
for all EventEmitter
instances, the events.defaultMaxListeners
property can be used. If this value is not a positive number, a RangeError
is thrown.
Take caution when setting the events.defaultMaxListeners
because the
change affects all EventEmitter
instances, including those created before
the change is made. However, calling emitter.setMaxListeners(n)
still has
precedence over events.defaultMaxListeners
.
This is not a hard limit. The EventEmitter
instance will allow
more listeners to be added but will output a trace warning to stderr indicating
that a "possible EventEmitter memory leak" has been detected. For any single
EventEmitter
, the emitter.getMaxListeners()
and emitter.setMaxListeners()
methods can be used to temporarily avoid this warning:
import { EventEmitter } from 'node:events';
const emitter = new EventEmitter();
emitter.setMaxListeners(emitter.getMaxListeners() + 1);
emitter.once('event', () => {
// do stuff
emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0));
});
const EventEmitter = require('node:events');
const emitter = new EventEmitter();
emitter.setMaxListeners(emitter.getMaxListeners() + 1);
emitter.once('event', () => {
// do stuff
emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0));
});
The --trace-warnings
command-line flag can be used to display the
stack trace for such warnings.
The emitted warning can be inspected with process.on('warning')
and will
have the additional emitter
, type
, and count
properties, referring to
the event emitter instance, the event's name and the number of attached
listeners, respectively.
Its name
property is set to 'MaxListenersExceededWarning'
.
events.errorMonitor
#
This symbol shall be used to install a listener for only monitoring 'error'
events. Listeners installed using this symbol are called before the regular
'error'
listeners are called.
Installing a listener using this symbol does not change the behavior once an
'error'
event is emitted. Therefore, the process will still crash if no
regular 'error'
listener is installed.
events.getEventListeners(emitterOrTarget, eventName)
#
emitterOrTarget
<EventEmitter> | <EventTarget>eventName
<string> | <symbol>- Returns: <Function[]>
Returns a copy of the array of listeners for the event named eventName
.
For EventEmitter
s this behaves exactly the same as calling .listeners
on
the emitter.
For EventTarget
s this is the only way to get the event listeners for the
event target. This is useful for debugging and diagnostic purposes.
import { getEventListeners, EventEmitter } from 'node:events';
{
const ee = new EventEmitter();
const listener = () => console.log('Events are fun');
ee.on('foo', listener);
console.log(getEventListeners(ee, 'foo')); // [ [Function: listener] ]
}
{
const et = new EventTarget();
const listener = () => console.log('Events are fun');
et.addEventListener('foo', listener);
console.log(getEventListeners(et, 'foo')); // [ [Function: listener] ]
}
const { getEventListeners, EventEmitter } = require('node:events');
{
const ee = new EventEmitter();
const listener = () => console.log('Events are fun');
ee.on('foo', listener);
console.log(getEventListeners(ee, 'foo')); // [ [Function: listener] ]
}
{
const et = new EventTarget();
const listener = () => console.log('Events are fun');
et.addEventListener('foo', listener);
console.log(getEventListeners(et, 'foo')); // [ [Function: listener] ]
}
events.getMaxListeners(emitterOrTarget)
#
emitterOrTarget
<EventEmitter> | <EventTarget>- Returns: <number>
Returns the currently set max amount of listeners.
For EventEmitter
s this behaves exactly the same as calling .getMaxListeners
on
the emitter.
For EventTarget
s this is the only way to get the max event listeners for the
event target. If the number of event handlers on a single EventTarget exceeds
the max set, the EventTarget will print a warning.
import { getMaxListeners, setMaxListeners, EventEmitter } from 'node:events';
{
const ee = new EventEmitter();
console.log(getMaxListeners(ee)); // 10
setMaxListeners(11, ee);
console.log(getMaxListeners(ee)); // 11
}
{
const et = new EventTarget();
console.log(getMaxListeners(et)); // 10
setMaxListeners(11, et);
console.log(getMaxListeners(et)); // 11
}
const { getMaxListeners, setMaxListeners, EventEmitter } = require('node:events');
{
const ee = new EventEmitter();
console.log(getMaxListeners(ee)); // 10
setMaxListeners(11, ee);
console.log(getMaxListeners(ee)); // 11
}
{
const et = new EventTarget();
console.log(getMaxListeners(et)); // 10
setMaxListeners(11, et);
console.log(getMaxListeners(et)); // 11
}
events.once(emitter, name[, options])
#
emitter
<EventEmitter>name
<string>options
<Object>signal
<AbortSignal> Can be used to cancel waiting for the event.
- Returns: <Promise>
Creates a Promise
that is fulfilled when the EventEmitter
emits the given
event or that is rejected if the EventEmitter
emits 'error'
while waiting.
The Promise
will resolve with an array of all the arguments emitted to the
given event.
This method is intentionally generic and works with the web platform
EventTarget interface, which has no special
'error'
event semantics and does not listen to the 'error'
event.
import { once, EventEmitter } from 'node:events';
import process from 'node:process';
const ee = new EventEmitter();
process.nextTick(() => {
ee.emit('myevent', 42);
});
const [value] = await once(ee, 'myevent');
console.log(value);
const err = new Error('kaboom');
process.nextTick(() => {
ee.emit('error', err);
});
try {
await once(ee, 'myevent');
} catch (err) {
console.error('error happened', err);
}
const { once, EventEmitter } = require('node:events');
async function run() {
const ee = new EventEmitter();
process.nextTick(() => {
ee.emit('myevent', 42);
});
const [value] = await once(ee, 'myevent');
console.log(value);
const err = new Error('kaboom');
process.nextTick(() => {
ee.emit('error', err);
});
try {
await once(ee, 'myevent');
} catch (err) {
console.error('error happened', err);
}
}
run();
The special handling of the 'error'
event is only used when events.once()
is used to wait for another event. If events.once()
is used to wait for the
'error'
event itself, then it is treated as any other kind of event without
special handling:
import { EventEmitter, once } from 'node:events';
const ee = new EventEmitter();
once(ee, 'error')
.then(([err]) => console.log('ok', err.message))
.catch((err) => console.error('error', err.message));
ee.emit('error', new Error('boom'));
// Prints: ok boom
const { EventEmitter, once } = require('node:events');
const ee = new EventEmitter();
once(ee, 'error')
.then(([err]) => console.log('ok', err.message))
.catch((err) => console.error('error', err.message));
ee.emit('error', new Error('boom'));
// Prints: ok boom
An <AbortSignal> can be used to cancel waiting for the event:
import { EventEmitter, once } from 'node:events';
const ee = new EventEmitter();
const ac = new AbortController();
async function foo(emitter, event, signal) {
try {
await once(emitter, event, { signal });
console.log('event emitted!');
} catch (error) {
if (error.name === 'AbortError') {
console.error('Waiting for the event was canceled!');
} else {
console.error('There was an error', error.message);
}
}
}
foo(ee, 'foo', ac.signal);
ac.abort(); // Abort waiting for the event
ee.emit('foo'); // Prints: Waiting for the event was canceled!
const { EventEmitter, once } = require('node:events');
const ee = new EventEmitter();
const ac = new AbortController();
async function foo(emitter, event, signal) {
try {
await once(emitter, event, { signal });
console.log('event emitted!');
} catch (error) {
if (error.name === 'AbortError') {
console.error('Waiting for the event was canceled!');
} else {
console.error('There was an error', error.message);
}
}
}
foo(ee, 'foo', ac.signal);
ac.abort(); // Abort waiting for the event
ee.emit('foo'); // Prints: Waiting for the event was canceled!
Awaiting multiple events emitted on process.nextTick()
#
There is an edge case worth noting when using the events.once()
function
to await multiple events emitted on in the same batch of process.nextTick()
operations, or whenever multiple events are emitted synchronously. Specifically,
because the process.nextTick()
queue is drained before the Promise
microtask
queue, and because EventEmitter
emits all events synchronously, it is possible
for events.once()
to miss an event.
import { EventEmitter, once } from 'node:events';
import process from 'node:process';
const myEE = new EventEmitter();
async function foo() {
await once(myEE, 'bar');
console.log('bar');
// This Promise will never resolve because the 'foo' event will
// have already been emitted before the Promise is created.
await once(myEE, 'foo');
console.log('foo');
}
process.nextTick(() => {
myEE.emit('bar');
myEE.emit('foo');
});
foo().then(() => console.log('done'));
const { EventEmitter, once } = require('node:events');
const myEE = new EventEmitter();
async function foo() {
await once(myEE, 'bar');
console.log('bar');
// This Promise will never resolve because the 'foo' event will
// have already been emitted before the Promise is created.
await once(myEE, 'foo');
console.log('foo');
}
process.nextTick(() => {
myEE.emit('bar');
myEE.emit('foo');
});
foo().then(() => console.log('done'));
To catch both events, create each of the Promises before awaiting either
of them, then it becomes possible to use Promise.all()
, Promise.race()
,
or Promise.allSettled()
:
import { EventEmitter, once } from 'node:events';
import process from 'node:process';
const myEE = new EventEmitter();
async function foo() {
await Promise.all([once(myEE, 'bar'), once(myEE, 'foo')]);
console.log('foo', 'bar');
}
process.nextTick(() => {
myEE.emit('bar');
myEE.emit('foo');
});
foo().then(() => console.log('done'));
const { EventEmitter, once } = require('node:events');
const myEE = new EventEmitter();
async function foo() {
await Promise.all([once(myEE, 'bar'), once(myEE, 'foo')]);
console.log('foo', 'bar');
}
process.nextTick(() => {
myEE.emit('bar');
myEE.emit('foo');
});
foo().then(() => console.log('done'));
events.captureRejections
#
Value: <boolean>
Change the default captureRejections
option on all new EventEmitter
objects.
events.captureRejectionSymbol
#
Value: Symbol.for('nodejs.rejection')
See how to write a custom rejection handler.
events.listenerCount(emitter, eventName)
#
emitter.listenerCount()
instead.emitter
<EventEmitter> The emitter to queryeventName
<string> | <symbol> The event name
A class method that returns the number of listeners for the given eventName
registered on the given emitter
.
import { EventEmitter, listenerCount } from 'node:events';
const myEmitter = new EventEmitter();
myEmitter.on('event', () => {});
myEmitter.on('event', () => {});
console.log(listenerCount(myEmitter, 'event'));
// Prints: 2
const { EventEmitter, listenerCount } = require('node:events');
const myEmitter = new EventEmitter();
myEmitter.on('event', () => {});
myEmitter.on('event', () => {});
console.log(listenerCount(myEmitter, 'event'));
// Prints: 2
events.on(emitter, eventName[, options])
#
emitter
<EventEmitter>eventName
<string> | <symbol> The name of the event being listened foroptions
<Object>signal
<AbortSignal> Can be used to cancel awaiting events.
- Returns: <AsyncIterator> that iterates
eventName
events emitted by theemitter
import { on, EventEmitter } from 'node:events';
import process from 'node:process';
const ee = new EventEmitter();
// Emit later on
process.nextTick(() => {
ee.emit('foo', 'bar');
ee.emit('foo', 42);
});
for await (const event of on(ee, 'foo')) {
// The execution of this inner block is synchronous and it
// processes one event at a time (even with await). Do not use
// if concurrent execution is required.
console.log(event); // prints ['bar'] [42]
}
// Unreachable here
const { on, EventEmitter } = require('node:events');
(async () => {
const ee = new EventEmitter();
// Emit later on
process.nextTick(() => {
ee.emit('foo', 'bar');
ee.emit('foo', 42);
});
for await (const event of on(ee, 'foo')) {
// The execution of this inner block is synchronous and it
// processes one event at a time (even with await). Do not use
// if concurrent execution is required.
console.log(event); // prints ['bar'] [42]
}
// Unreachable here
})();
Returns an AsyncIterator
that iterates eventName
events. It will throw
if the EventEmitter
emits 'error'
. It removes all listeners when
exiting the loop. The value
returned by each iteration is an array
composed of the emitted event arguments.
An <AbortSignal> can be used to cancel waiting on events:
import { on, EventEmitter } from 'node:events';
import process from 'node:process';
const ac = new AbortController();
(async () => {
const ee = new EventEmitter();
// Emit later on
process.nextTick(() => {
ee.emit('foo', 'bar');
ee.emit('foo', 42);
});
for await (const event of on(ee, 'foo', { signal: ac.signal })) {
// The execution of this inner block is synchronous and it
// processes one event at a time (even with await). Do not use
// if concurrent execution is required.
console.log(event); // prints ['bar'] [42]
}
// Unreachable here
})();
process.nextTick(() => ac.abort());
const { on, EventEmitter } = require('node:events');
const ac = new AbortController();
(async () => {
const ee = new EventEmitter();
// Emit later on
process.nextTick(() => {
ee.emit('foo', 'bar');
ee.emit('foo', 42);
});
for await (const event of on(ee, 'foo', { signal: ac.signal })) {
// The execution of this inner block is synchronous and it
// processes one event at a time (even with await). Do not use
// if concurrent execution is required.
console.log(event); // prints ['bar'] [42]
}
// Unreachable here
})();
process.nextTick(() => ac.abort());
events.setMaxListeners(n[, ...eventTargets])
#
n
<number> A non-negative number. The maximum number of listeners perEventTarget
event....eventsTargets
<EventTarget[]> | <EventEmitter[]> Zero or more <EventTarget> or <EventEmitter> instances. If none are specified,n
is set as the default max for all newly created <EventTarget> and <EventEmitter> objects.
import { setMaxListeners, EventEmitter } from 'node:events';
const target = new EventTarget();
const emitter = new EventEmitter();
setMaxListeners(5, target, emitter);
const {
setMaxListeners,
EventEmitter,
} = require('node:events');
const target = new EventTarget();
const emitter = new EventEmitter();
setMaxListeners(5, target, emitter);
Class: events.EventEmitterAsyncResource extends EventEmitter
#
Integrates EventEmitter
with <AsyncResource> for EventEmitter
s that
require manual async tracking. Specifically, all events emitted by instances
of events.EventEmitterAsyncResource
will run within its async context.
import { EventEmitterAsyncResource, EventEmitter } from 'node:events';
import { notStrictEqual, strictEqual } from 'node:assert';
import { executionAsyncId, triggerAsyncId } from 'node:async_hooks';
// Async tracking tooling will identify this as 'Q'.
const ee1 = new EventEmitterAsyncResource({ name: 'Q' });
// 'foo' listeners will run in the EventEmitters async context.
ee1.on('foo', () => {
strictEqual(executionAsyncId(), ee1.asyncId);
strictEqual(triggerAsyncId(), ee1.triggerAsyncId);
});
const ee2 = new EventEmitter();
// 'foo' listeners on ordinary EventEmitters that do not track async
// context, however, run in the same async context as the emit().
ee2.on('foo', () => {
notStrictEqual(executionAsyncId(), ee2.asyncId);
notStrictEqual(triggerAsyncId(), ee2.triggerAsyncId);
});
Promise.resolve().then(() => {
ee1.emit('foo');
ee2.emit('foo');
});
const { EventEmitterAsyncResource, EventEmitter } = require('node:events');
const { notStrictEqual, strictEqual } = require('node:assert');
const { executionAsyncId, triggerAsyncId } = require('node:async_hooks');
// Async tracking tooling will identify this as 'Q'.
const ee1 = new EventEmitterAsyncResource({ name: 'Q' });
// 'foo' listeners will run in the EventEmitters async context.
ee1.on('foo', () => {
strictEqual(executionAsyncId(), ee1.asyncId);
strictEqual(triggerAsyncId(), ee1.triggerAsyncId);
});
const ee2 = new EventEmitter();
// 'foo' listeners on ordinary EventEmitters that do not track async
// context, however, run in the same async context as the emit().
ee2.on('foo', () => {
notStrictEqual(executionAsyncId(), ee2.asyncId);
notStrictEqual(triggerAsyncId(), ee2.triggerAsyncId);
});
Promise.resolve().then(() => {
ee1.emit('foo');
ee2.emit('foo');
});
The EventEmitterAsyncResource
class has the same methods and takes the
same options as EventEmitter
and AsyncResource
themselves.
new events.EventEmitterAsyncResource([options])
#
options
<Object>captureRejections
<boolean> It enables automatic capturing of promise rejection. Default:false
.name
<string> The type of async event. Default::new.target.name
.triggerAsyncId
<number> The ID of the execution context that created this async event. Default:executionAsyncId()
.requireManualDestroy
<boolean> If set totrue
, disablesemitDestroy
when the object is garbage collected. This usually does not need to be set (even ifemitDestroy
is called manually), unless the resource'sasyncId
is retrieved and the sensitive API'semitDestroy
is called with it. When set tofalse
, theemitDestroy
call on garbage collection will only take place if there is at least one activedestroy
hook. Default:false
.
eventemitterasyncresource.asyncId
#
- Type: <number> The unique
asyncId
assigned to the resource.
eventemitterasyncresource.asyncResource
#
- Type: The underlying <AsyncResource>.
The returned AsyncResource
object has an additional eventEmitter
property
that provides a reference to this EventEmitterAsyncResource
.
eventemitterasyncresource.emitDestroy()
#
Call all destroy
hooks. This should only ever be called once. An error will
be thrown if it is called more than once. This must be manually called. If
the resource is left to be collected by the GC then the destroy
hooks will
never be called.
eventemitterasyncresource.triggerAsyncId
#
- Type: <number> The same
triggerAsyncId
that is passed to theAsyncResource
constructor.
EventTarget
and Event
API#
The EventTarget
and Event
objects are a Node.js-specific implementation
of the EventTarget
Web API that are exposed by some Node.js core APIs.
const target = new EventTarget();
target.addEventListener('foo', (event) => {
console.log('foo event happened!');
});
Node.js EventTarget
vs. DOM EventTarget
#
There are two key differences between the Node.js EventTarget
and the
EventTarget
Web API:
- Whereas DOM
EventTarget
instances may be hierarchical, there is no concept of hierarchy and event propagation in Node.js. That is, an event dispatched to anEventTarget
does not propagate through a hierarchy of nested target objects that may each have their own set of handlers for the event. - In the Node.js
EventTarget
, if an event listener is an async function or returns aPromise
, and the returnedPromise
rejects, the rejection is automatically captured and handled the same way as a listener that throws synchronously (seeEventTarget
error handling for details).
NodeEventTarget
vs. EventEmitter
#
The NodeEventTarget
object implements a modified subset of the
EventEmitter
API that allows it to closely emulate an EventEmitter
in
certain situations. A NodeEventTarget
is not an instance of EventEmitter
and cannot be used in place of an EventEmitter
in most cases.
- Unlike
EventEmitter
, any givenlistener
can be registered at most once per eventtype
. Attempts to register alistener
multiple times are ignored. - The
NodeEventTarget
does not emulate the fullEventEmitter
API. Specifically theprependListener()
,prependOnceListener()
,rawListeners()
, anderrorMonitor
APIs are not emulated. The'newListener'
and'removeListener'
events will also not be emitted. - The
NodeEventTarget
does not implement any special default behavior for events with type'error'
. - The
NodeEventTarget
supportsEventListener
objects as well as functions as handlers for all event types.
Event listener#
Event listeners registered for an event type
may either be JavaScript
functions or objects with a handleEvent
property whose value is a function.
In either case, the handler function is invoked with the event
argument
passed to the eventTarget.dispatchEvent()
function.
Async functions may be used as event listeners. If an async handler function
rejects, the rejection is captured and handled as described in
EventTarget
error handling.
An error thrown by one handler function does not prevent the other handlers from being invoked.
The return value of a handler function is ignored.
Handlers are always invoked in the order they were added.
Handler functions may mutate the event
object.
function handler1(event) {
console.log(event.type); // Prints 'foo'
event.a = 1;
}
async function handler2(event) {
console.log(event.type); // Prints 'foo'
console.log(event.a); // Prints 1
}
const handler3 = {
handleEvent(event) {
console.log(event.type); // Prints 'foo'
},
};
const handler4 = {
async handleEvent(event) {
console.log(event.type); // Prints 'foo'
},
};
const target = new EventTarget();
target.addEventListener('foo', handler1);
target.addEventListener('foo', handler2);
target.addEventListener('foo', handler3);
target.addEventListener('foo', handler4, { once: true });
EventTarget
error handling#
When a registered event listener throws (or returns a Promise that rejects),
by default the error is treated as an uncaught exception on
process.nextTick()
. This means uncaught exceptions in EventTarget
s will
terminate the Node.js process by default.
Throwing within an event listener will not stop the other registered handlers from being invoked.
The EventTarget
does not implement any special default handling for 'error'
type events like EventEmitter
.
Currently errors are first forwarded to the process.on('error')
event
before reaching process.on('uncaughtException')
. This behavior is
deprecated and will change in a future release to align EventTarget
with
other Node.js APIs. Any code relying on the process.on('error')
event should
be aligned with the new behavior.
Class: Event
#
The Event
object is an adaptation of the Event
Web API. Instances
are created internally by Node.js.
event.bubbles
#
- Type: <boolean> Always returns
false
.
This is not used in Node.js and is provided purely for completeness.
event.cancelBubble
#
event.stopPropagation()
instead.- Type: <boolean>
Alias for event.stopPropagation()
if set to true
. This is not used
in Node.js and is provided purely for completeness.
event.cancelable
#
- Type: <boolean> True if the event was created with the
cancelable
option.
event.composed
#
- Type: <boolean> Always returns
false
.
This is not used in Node.js and is provided purely for completeness.
event.composedPath()
#
Returns an array containing the current EventTarget
as the only entry or
empty if the event is not being dispatched. This is not used in
Node.js and is provided purely for completeness.
event.currentTarget
#
- Type: <EventTarget> The
EventTarget
dispatching the event.
Alias for event.target
.
event.defaultPrevented
#
- Type: <boolean>
Is true
if cancelable
is true
and event.preventDefault()
has been
called.
event.eventPhase
#
- Type: <number> Returns
0
while an event is not being dispatched,2
while it is being dispatched.
This is not used in Node.js and is provided purely for completeness.
event.initEvent(type[, bubbles[, cancelable]])
#
Redundant with event constructors and incapable of setting composed
.
This is not used in Node.js and is provided purely for completeness.
event.isTrusted
#
- Type: <boolean>
The <AbortSignal> "abort"
event is emitted with isTrusted
set to true
. The
value is false
in all other cases.
event.preventDefault()
#
Sets the defaultPrevented
property to true
if cancelable
is true
.
event.returnValue
#
event.defaultPrevented
instead.- Type: <boolean> True if the event has not been canceled.
The value of event.returnValue
is always the opposite of event.defaultPrevented
.
This is not used in Node.js and is provided purely for completeness.
event.srcElement
#
event.target
instead.- Type: <EventTarget> The
EventTarget
dispatching the event.
Alias for event.target
.
event.stopImmediatePropagation()
#
Stops the invocation of event listeners after the current one completes.
event.stopPropagation()
#
This is not used in Node.js and is provided purely for completeness.
event.target
#
- Type: <EventTarget> The
EventTarget
dispatching the event.
event.timeStamp
#
- Type: <number>
The millisecond timestamp when the Event
object was created.
event.type
#
- Type: <string>
The event type identifier.
Class: EventTarget
#
eventTarget.addEventListener(type, listener[, options])
#
type
<string>listener
<Function> | <EventListener>options
<Object>once
<boolean> Whentrue
, the listener is automatically removed when it is first invoked. Default:false
.passive
<boolean> Whentrue
, serves as a hint that the listener will not call theEvent
object'spreventDefault()
method. Default:false
.capture
<boolean> Not directly used by Node.js. Added for API completeness. Default:false
.signal
<AbortSignal> The listener will be removed when the given AbortSignal object'sabort()
method is called.
Adds a new handler for the type
event. Any given listener
is added
only once per type
and per capture
option value.
If the once
option is true
, the listener
is removed after the
next time a type
event is dispatched.
The capture
option is not used by Node.js in any functional way other than
tracking registered event listeners per the EventTarget
specification.
Specifically, the capture
option is used as part of the key when registering
a listener
. Any individual listener
may be added once with
capture = false
, and once with capture = true
.
function handler(event) {}
const target = new EventTarget();
target.addEventListener('foo', handler, { capture: true }); // first
target.addEventListener('foo', handler, { capture: false }); // second
// Removes the second instance of handler
target.removeEventListener('foo', handler);
// Removes the first instance of handler
target.removeEventListener('foo', handler, { capture: true });
eventTarget.dispatchEvent(event)
#
event
<Event>- Returns: <boolean>
true
if either event'scancelable
attribute value is false or itspreventDefault()
method was not invoked, otherwisefalse
.
Dispatches the event
to the list of handlers for event.type
.
The registered event listeners is synchronously invoked in the order they were registered.
eventTarget.removeEventListener(type, listener[, options])
#
type
<string>listener
<Function> | <EventListener>options
<Object>capture
<boolean>
Removes the listener
from the list of handlers for event type
.
Class: CustomEvent
#
- Extends: <Event>
The CustomEvent
object is an adaptation of the CustomEvent
Web API.
Instances are created internally by Node.js.
event.detail
#
- Type: <any> Returns custom data passed when initializing.
Read-only.
Class: NodeEventTarget
#
- Extends: <EventTarget>
The NodeEventTarget
is a Node.js-specific extension to EventTarget
that emulates a subset of the EventEmitter
API.
nodeEventTarget.addListener(type, listener)
#
-
type
<string> -
listener
<Function> | <EventListener> -
Returns: <EventTarget> this
Node.js-specific extension to the EventTarget
class that emulates the
equivalent EventEmitter
API. The only difference between addListener()
and
addEventListener()
is that addListener()
will return a reference to the
EventTarget
.
nodeEventTarget.emit(type, arg)
#
type
<string>arg
<any>- Returns: <boolean>
true
if event listeners registered for thetype
exist, otherwisefalse
.
Node.js-specific extension to the EventTarget
class that dispatches the
arg
to the list of handlers for type
.
nodeEventTarget.eventNames()
#
- Returns: <string[]>
Node.js-specific extension to the EventTarget
class that returns an array
of event type
names for which event listeners are registered.
nodeEventTarget.listenerCount(type)
#
Node.js-specific extension to the EventTarget
class that returns the number
of event listeners registered for the type
.
nodeEventTarget.setMaxListeners(n)
#
n
<number>
Node.js-specific extension to the EventTarget
class that sets the number
of max event listeners as n
.
nodeEventTarget.getMaxListeners()
#
- Returns: <number>
Node.js-specific extension to the EventTarget
class that returns the number
of max event listeners.
nodeEventTarget.off(type, listener[, options])
#
-
type
<string> -
listener
<Function> | <EventListener> -
options
<Object>capture
<boolean>
-
Returns: <EventTarget> this
Node.js-specific alias for eventTarget.removeEventListener()
.
nodeEventTarget.on(type, listener)
#
-
type
<string> -
listener
<Function> | <EventListener> -
Returns: <EventTarget> this
Node.js-specific alias for eventTarget.addEventListener()
.
nodeEventTarget.once(type, listener)
#
-
type
<string> -
listener
<Function> | <EventListener> -
Returns: <EventTarget> this
Node.js-specific extension to the EventTarget
class that adds a once
listener for the given event type
. This is equivalent to calling on
with the once
option set to true
.
nodeEventTarget.removeAllListeners([type])
#
-
type
<string> -
Returns: <EventTarget> this
Node.js-specific extension to the EventTarget
class. If type
is specified,
removes all registered listeners for type
, otherwise removes all registered
listeners.
nodeEventTarget.removeListener(type, listener[, options])
#
-
type
<string> -
listener
<Function> | <EventListener> -
options
<Object>capture
<boolean>
-
Returns: <EventTarget> this
Node.js-specific extension to the EventTarget
class that removes the
listener
for the given type
. The only difference between removeListener()
and removeEventListener()
is that removeListener()
will return a reference
to the EventTarget
.
File system#
Source Code: lib/fs.js
The node:fs
module enables interacting with the file system in a
way modeled on standard POSIX functions.
To use the promise-based APIs:
import * as fs from 'node:fs/promises';
const fs = require('node:fs/promises');
To use the callback and sync APIs:
import * as fs from 'node:fs';
const fs = require('node:fs');
All file system operations have synchronous, callback, and promise-based forms, and are accessible using both CommonJS syntax and ES6 Modules (ESM).
Promise example#
Promise-based operations return a promise that is fulfilled when the asynchronous operation is complete.
import { unlink } from 'node:fs/promises';
try {
await unlink('/tmp/hello');
console.log('successfully deleted /tmp/hello');
} catch (error) {
console.error('there was an error:', error.message);
}
const { unlink } = require('node:fs/promises');
(async function(path) {
try {
await unlink(path);
console.log(`successfully deleted ${path}`);
} catch (error) {
console.error('there was an error:', error.message);
}
})('/tmp/hello');
Callback example#
The callback form takes a completion callback function as its last
argument and invokes the operation asynchronously. The arguments passed to
the completion callback depend on the method, but the first argument is always
reserved for an exception. If the operation is completed successfully, then
the first argument is null
or undefined
.
import { unlink } from 'node:fs';
unlink('/tmp/hello', (err) => {
if (err) throw err;
console.log('successfully deleted /tmp/hello');
});
const { unlink } = require('node:fs');
unlink('/tmp/hello', (err) => {
if (err) throw err;
console.log('successfully deleted /tmp/hello');
});
The callback-based versions of the node:fs
module APIs are preferable over
the use of the promise APIs when maximal performance (both in terms of
execution time and memory allocation) is required.
Synchronous example#
The synchronous APIs block the Node.js event loop and further JavaScript
execution until the operation is complete. Exceptions are thrown immediately
and can be handled using try…catch
, or can be allowed to bubble up.
import { unlinkSync } from 'node:fs';
try {
unlinkSync('/tmp/hello');
console.log('successfully deleted /tmp/hello');
} catch (err) {
// handle the error
}
const { unlinkSync } = require('node:fs');
try {
unlinkSync('/tmp/hello');
console.log('successfully deleted /tmp/hello');
} catch (err) {
// handle the error
}
Promises API#
The fs/promises
API provides asynchronous file system methods that return
promises.
The promise APIs use the underlying Node.js threadpool to perform file system operations off the event loop thread. These operations are not synchronized or threadsafe. Care must be taken when performing multiple concurrent modifications on the same file or data corruption may occur.
Class: FileHandle
#
A <FileHandle> object is an object wrapper for a numeric file descriptor.
Instances of the <FileHandle> object are created by the fsPromises.open()
method.
All <FileHandle> objects are <EventEmitter>s.
If a <FileHandle> is not closed using the filehandle.close()
method, it will
try to automatically close the file descriptor and emit a process warning,
helping to prevent memory leaks. Please do not rely on this behavior because
it can be unreliable and the file may not be closed. Instead, always explicitly
close <FileHandle>s. Node.js may change this behavior in the future.
Event: 'close'
#
The 'close'
event is emitted when the <FileHandle> has been closed and can no
longer be used.
filehandle.appendFile(data[, options])
#
data
<string> | <Buffer> | <TypedArray> | <DataView> | <AsyncIterable> | <Iterable> | <Stream>options
<Object> | <string>- Returns: <Promise> Fulfills with
undefined
upon success.
Alias of filehandle.writeFile()
.
When operating on file handles, the mode cannot be changed from what it was set
to with fsPromises.open()
. Therefore, this is equivalent to
filehandle.writeFile()
.
filehandle.chmod(mode)
#
Modifies the permissions on the file. See chmod(2)
.
filehandle.chown(uid, gid)
#
uid
<integer> The file's new owner's user id.gid
<integer> The file's new group's group id.- Returns: <Promise> Fulfills with
undefined
upon success.
Changes the ownership of the file. A wrapper for chown(2)
.
filehandle.close()
#
- Returns: <Promise> Fulfills with
undefined
upon success.
Closes the file handle after waiting for any pending operation on the handle to complete.
import { open } from 'node:fs/promises';
let filehandle;
try {
filehandle = await open('thefile.txt', 'r');
} finally {
await filehandle?.close();
}
filehandle.createReadStream([options])
#
options
<Object>- Returns: <fs.ReadStream>
Unlike the 16 KiB default highWaterMark
for a <stream.Readable>, the stream
returned by this method has a default highWaterMark
of 64 KiB.
options
can include start
and end
values to read a range of bytes from
the file instead of the entire file. Both start
and end
are inclusive and
start counting at 0, allowed values are in the
[0, Number.MAX_SAFE_INTEGER
] range. If start
is
omitted or undefined
, filehandle.createReadStream()
reads sequentially from
the current file position. The encoding
can be any one of those accepted by
<Buffer>.
If the FileHandle
points to a character device that only supports blocking
reads (such as keyboard or sound card), read operations do not finish until data
is available. This can prevent the process from exiting and the stream from
closing naturally.
By default, the stream will emit a 'close'
event after it has been
destroyed. Set the emitClose
option to false
to change this behavior.
import { open } from 'node:fs/promises';
const fd = await open('/dev/input/event0');
// Create a stream from some character device.
const stream = fd.createReadStream();
setTimeout(() => {
stream.close(); // This may not close the stream.
// Artificially marking end-of-stream, as if the underlying resource had
// indicated end-of-file by itself, allows the stream to close.
// This does not cancel pending read operations, and if there is such an
// operation, the process may still not be able to exit successfully
// until it finishes.
stream.push(null);
stream.read(0);
}, 100);
If autoClose
is false, then the file descriptor won't be closed, even if
there's an error. It is the application's responsibility to close it and make
sure there's no file descriptor leak. If autoClose
is set to true (default
behavior), on 'error'
or 'end'
the file descriptor will be closed
automatically.
An example to read the last 10 bytes of a file which is 100 bytes long:
import { open } from 'node:fs/promises';
const fd = await open('sample.txt');
fd.createReadStream({ start: 90, end: 99 });
filehandle.createWriteStream([options])
#
options
<Object>- Returns: <fs.WriteStream>
options
may also include a start
option to allow writing data at some
position past the beginning of the file, allowed values are in the
[0, Number.MAX_SAFE_INTEGER
] range. Modifying a file rather than
replacing it may require the flags
open
option to be set to r+
rather than
the default r
. The encoding
can be any one of those accepted by <Buffer>.
If autoClose
is set to true (default behavior) on 'error'
or 'finish'
the file descriptor will be closed automatically. If autoClose
is false,
then the file descriptor won't be closed, even if there's an error.
It is the application's responsibility to close it and make sure there's no
file descriptor leak.
By default, the stream will emit a 'close'
event after it has been
destroyed. Set the emitClose
option to false
to change this behavior.
filehandle.datasync()
#
- Returns: <Promise> Fulfills with
undefined
upon success.
Forces all currently queued I/O operations associated with the file to the
operating system's synchronized I/O completion state. Refer to the POSIX
fdatasync(2)
documentation for details.
Unlike filehandle.sync
this method does not flush modified metadata.
filehandle.fd
#
- <number> The numeric file descriptor managed by the <FileHandle> object.
filehandle.read(buffer, offset, length, position)
#
buffer
<Buffer> | <TypedArray> | <DataView> A buffer that will be filled with the file data read.offset
<integer> The location in the buffer at which to start filling.length
<integer> The number of bytes to read.position
<integer> | <null> The location where to begin reading data from the file. Ifnull
, data will be read from the current file position, and the position will be updated. Ifposition
is an integer, the current file position will remain unchanged.- Returns: <Promise> Fulfills upon success with an object with two properties:
bytesRead
<integer> The number of bytes readbuffer
<Buffer> | <TypedArray> | <DataView> A reference to the passed inbuffer
argument.
Reads data from the file and stores that in the given buffer.
If the file is not modified concurrently, the end-of-file is reached when the number of bytes read is zero.
filehandle.read([options])
#
options
<Object>buffer
<Buffer> | <TypedArray> | <DataView> A buffer that will be filled with the file data read. Default:Buffer.alloc(16384)
offset
<integer> The location in the buffer at which to start filling. Default:0
length
<integer> The number of bytes to read. Default:buffer.byteLength - offset
position
<integer> | <null> The location where to begin reading data from the file. Ifnull
, data will be read from the current file position, and the position will be updated. Ifposition
is an integer, the current file position will remain unchanged. Default::null
- Returns: <Promise> Fulfills upon success with an object with two properties:
bytesRead
<integer> The number of bytes readbuffer
<Buffer> | <TypedArray> | <DataView> A reference to the passed inbuffer
argument.
Reads data from the file and stores that in the given buffer.
If the file is not modified concurrently, the end-of-file is reached when the number of bytes read is zero.
filehandle.read(buffer[, options])
#
buffer
<Buffer> | <TypedArray> | <DataView> A buffer that will be filled with the file data read.options
<Object>offset
<integer> The location in the buffer at which to start filling. Default:0
length
<integer> The number of bytes to read. Default:buffer.byteLength - offset
position
<integer> The location where to begin reading data from the file. Ifnull
, data will be read from the current file position, and the position will be updated. Ifposition
is an integer, the current file position will remain unchanged. Default::null
- Returns: <Promise> Fulfills upon success with an object with two properties:
bytesRead
<integer> The number of bytes readbuffer
<Buffer> | <TypedArray> | <DataView> A reference to the passed inbuffer
argument.
Reads data from the file and stores that in the given buffer.
If the file is not modified concurrently, the end-of-file is reached when the number of bytes read is zero.
filehandle.readableWebStream()
#
- Returns: <ReadableStream>
Returns a ReadableStream
that may be used to read the files data.
An error will be thrown if this method is called more than once or is called
after the FileHandle
is closed or closing.
import {
open,
} from 'node:fs/promises';
const file = await open('./some/file/to/read');
for await (const chunk of file.readableWebStream())
console.log(chunk);
await file.close();
const {
open,
} = require('node:fs/promises');
(async () => {
const file = await open('./some/file/to/read');
for await (const chunk of file.readableWebStream())
console.log(chunk);
await file.close();
})();
While the ReadableStream
will read the file to completion, it will not
close the FileHandle
automatically. User code must still call the
fileHandle.close()
method.
filehandle.readFile(options)
#
options
<Object> | <string>encoding
<string> | <null> Default:null
signal
<AbortSignal> allows aborting an in-progress readFile
- Returns: <Promise> Fulfills upon a successful read with the contents of the
file. If no encoding is specified (using
options.encoding
), the data is returned as a <Buffer> object. Otherwise, the data will be a string.
Asynchronously reads the entire contents of a file.
If options
is a string, then it specifies the encoding
.
The <FileHandle> has to support reading.
If one or more filehandle.read()
calls are made on a file handle and then a
filehandle.readFile()
call is made, the data will be read from the current
position till the end of the file. It doesn't always read from the beginning
of the file.
filehandle.readLines([options])
#
options
<Object>- Returns: <readline.InterfaceConstructor>
Convenience method to create a readline
interface and stream over the file.
See filehandle.createReadStream()
for the options.
import { open } from 'node:fs/promises';
const file = await open('./some/file/to/read');
for await (const line of file.readLines()) {
console.log(line);
}
const { open } = require('node:fs/promises');
(async () => {
const file = await open('./some/file/to/read');
for await (const line of file.readLines()) {
console.log(line);
}
})();
filehandle.readv(buffers[, position])
#
buffers
<Buffer[]> | <TypedArray[]> | <DataView[]>position
<integer> | <null> The offset from the beginning of the file where the data should be read from. Ifposition
is not anumber
, the data will be read from the current position. Default:null
- Returns: <Promise> Fulfills upon success an object containing two properties:
bytesRead
<integer> the number of bytes readbuffers
<Buffer[]> | <TypedArray[]> | <DataView[]> property containing a reference to thebuffers
input.
Read from a file and write to an array of <ArrayBufferView>s
filehandle.stat([options])
#
options
<Object>bigint
<boolean> Whether the numeric values in the returned <fs.Stats> object should bebigint
. Default:false
.
- Returns: <Promise> Fulfills with an <fs.Stats> for the file.
filehandle.sync()
#
- Returns: <Promise> Fulfills with
undefined
upon success.
Request that all data for the open file descriptor is flushed to the storage
device. The specific implementation is operating system and device specific.
Refer to the POSIX fsync(2)
documentation for more detail.
filehandle.truncate(len)
#
Truncates the file.
If the file was larger than len
bytes, only the first len
bytes will be
retained in the file.
The following example retains only the first four bytes of the file:
import { open } from 'node:fs/promises';
let filehandle = null;
try {
filehandle = await open('temp.txt', 'r+');
await filehandle.truncate(4);
} finally {
await filehandle?.close();
}
If the file previously was shorter than len
bytes, it is extended, and the
extended part is filled with null bytes ('\0'
):
If len
is negative then 0
will be used.
filehandle.utimes(atime, mtime)
#
Change the file system timestamps of the object referenced by the <FileHandle> then resolves the promise with no arguments upon success.
filehandle.write(buffer, offset[, length[, position]])
#
buffer
<Buffer> | <TypedArray> | <DataView>offset
<integer> The start position from withinbuffer
where the data to write begins.length
<integer> The number of bytes frombuffer
to write. Default:buffer.byteLength - offset
position
<integer> | <null> The offset from the beginning of the file where the data frombuffer
should be written. Ifposition
is not anumber
, the data will be written at the current position. See the POSIXpwrite(2)
documentation for more detail. Default:null
- Returns: <Promise>
Write buffer
to the file.
The promise is resolved with an object containing two properties:
bytesWritten
<integer> the number of bytes writtenbuffer
<Buffer> | <TypedArray> | <DataView> a reference to thebuffer
written.
It is unsafe to use filehandle.write()
multiple times on the same file
without waiting for the promise to be resolved (or rejected). For this
scenario, use filehandle.createWriteStream()
.
On Linux, positional writes do not work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.
filehandle.write(buffer[, options])
#
buffer
<Buffer> | <TypedArray> | <DataView>options
<Object>- Returns: <Promise>
Write buffer
to the file.
Similar to the above filehandle.write
function, this version takes an
optional options
object. If no options
object is specified, it will
default with the above values.
filehandle.write(string[, position[, encoding]])
#
string
<string>position
<integer> | <null> The offset from the beginning of the file where the data fromstring
should be written. Ifposition
is not anumber
the data will be written at the current position. See the POSIXpwrite(2)
documentation for more detail. Default:null
encoding
<string> The expected string encoding. Default:'utf8'
- Returns: <Promise>
Write string
to the file. If string
is not a string, the promise is
rejected with an error.
The promise is resolved with an object containing two properties:
bytesWritten
<integer> the number of bytes writtenbuffer
<string> a reference to thestring
written.
It is unsafe to use filehandle.write()
multiple times on the same file
without waiting for the promise to be resolved (or rejected). For this
scenario, use filehandle.createWriteStream()
.
On Linux, positional writes do not work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.
filehandle.writeFile(data, options)
#
data
<string> | <Buffer> | <TypedArray> | <DataView> | <AsyncIterable> | <Iterable> | <Stream>options
<Object> | <string>- Returns: <Promise>
Asynchronously writes data to a file, replacing the file if it already exists.
data
can be a string, a buffer, an <AsyncIterable>, or an <Iterable> object.
The promise is resolved with no arguments upon success.
If options
is a string, then it specifies the encoding
.
The <FileHandle> has to support writing.
It is unsafe to use filehandle.writeFile()
multiple times on the same file
without waiting for the promise to be resolved (or rejected).
If one or more filehandle.write()
calls are made on a file handle and then a
filehandle.writeFile()
call is made, the data will be written from the
current position till the end of the file. It doesn't always write from the
beginning of the file.
filehandle.writev(buffers[, position])
#
buffers
<Buffer[]> | <TypedArray[]> | <DataView[]>position
<integer> | <null> The offset from the beginning of the file where the data frombuffers
should be written. Ifposition
is not anumber
, the data will be written at the current position. Default:null
- Returns: <Promise>
Write an array of <ArrayBufferView>s to the file.
The promise is resolved with an object containing a two properties:
bytesWritten
<integer> the number of bytes writtenbuffers
<Buffer[]> | <TypedArray[]> | <DataView[]> a reference to thebuffers
input.
It is unsafe to call writev()
multiple times on the same file without waiting
for the promise to be resolved (or rejected).
On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.
fsPromises.access(path[, mode])
#
path
<string> | <Buffer> | <URL>mode
<integer> Default:fs.constants.F_OK
- Returns: <Promise> Fulfills with
undefined
upon success.
Tests a user's permissions for the file or directory specified by path
.
The mode
argument is an optional integer that specifies the accessibility
checks to be performed. mode
should be either the value fs.constants.F_OK
or a mask consisting of the bitwise OR of any of fs.constants.R_OK
,
fs.constants.W_OK
, and fs.constants.X_OK
(e.g.
fs.constants.W_OK | fs.constants.R_OK
). Check File access constants for
possible values of mode
.
If the accessibility check is successful, the promise is resolved with no
value. If any of the accessibility checks fail, the promise is rejected
with an <Error> object. The following example checks if the file
/etc/passwd
can be read and written by the current process.
import { access, constants } from 'node:fs/promises';
try {
await access('/etc/passwd', constants.R_OK | constants.W_OK);
console.log('can access');
} catch {
console.error('cannot access');
}
Using fsPromises.access()
to check for the accessibility of a file before
calling fsPromises.open()
is not recommended. Doing so introduces a race
condition, since other processes may change the file's state between the two
calls. Instead, user code should open/read/write the file directly and handle
the error raised if the file is not accessible.
fsPromises.appendFile(path, data[, options])
#
path
<string> | <Buffer> | <URL> | <FileHandle> filename or <FileHandle>data
<string> | <Buffer>options
<Object> | <string>encoding
<string> | <null> Default:'utf8'
mode
<integer> Default:0o666
flag
<string> See support of file systemflags
. Default:'a'
.
- Returns: <Promise> Fulfills with
undefined
upon success.
Asynchronously append data to a file, creating the file if it does not yet
exist. data
can be a string or a <Buffer>.
If options
is a string, then it specifies the encoding
.
The mode
option only affects the newly created file. See fs.open()
for more details.
The path
may be specified as a <FileHandle> that has been opened
for appending (using fsPromises.open()
).
fsPromises.chmod(path, mode)
#
path
<string> | <Buffer> | <URL>mode
<string> | <integer>- Returns: <Promise> Fulfills with
undefined
upon success.
Changes the permissions of a file.
fsPromises.chown(path, uid, gid)
#
path
<string> | <Buffer> | <URL>uid
<integer>gid
<integer>- Returns: <Promise> Fulfills with
undefined
upon success.
Changes the ownership of a file.
fsPromises.copyFile(src, dest[, mode])
#
src
<string> | <Buffer> | <URL> source filename to copydest
<string> | <Buffer> | <URL> destination filename of the copy operationmode
<integer> Optional modifiers that specify the behavior of the copy operation. It is possible to create a mask consisting of the bitwise OR of two or more values (e.g.fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE
) Default:0
.fs.constants.COPYFILE_EXCL
: The copy operation will fail ifdest
already exists.fs.constants.COPYFILE_FICLONE
: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then a fallback copy mechanism is used.fs.constants.COPYFILE_FICLONE_FORCE
: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then the operation will fail.
- Returns: <Promise> Fulfills with
undefined
upon success.
Asynchronously copies src
to dest
. By default, dest
is overwritten if it
already exists.
No guarantees are made about the atomicity of the copy operation. If an error occurs after the destination file has been opened for writing, an attempt will be made to remove the destination.
import { copyFile, constants } from 'node:fs/promises';
try {
await copyFile('source.txt', 'destination.txt');
console.log('source.txt was copied to destination.txt');
} catch {
console.error('The file could not be copied');
}
// By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
try {
await copyFile('source.txt', 'destination.txt', constants.COPYFILE_EXCL);
console.log('source.txt was copied to destination.txt');
} catch {
console.error('The file could not be copied');
}
fsPromises.cp(src, dest[, options])
#
src
<string> | <URL> source path to copy.dest
<string> | <URL> destination path to copy to.options
<Object>dereference
<boolean> dereference symlinks. Default:false
.errorOnExist
<boolean> whenforce
isfalse
, and the destination exists, throw an error. Default:false
.filter
<Function> Function to filter copied files/directories. Returntrue
to copy the item,false
to ignore it. Can also return aPromise
that resolves totrue
orfalse
Default:undefined
.force
<boolean> overwrite existing file or directory. The copy operation will ignore errors if you set this to false and the destination exists. Use theerrorOnExist
option to change this behavior. Default:true
.preserveTimestamps
<boolean> Whentrue
timestamps fromsrc
will be preserved. Default:false
.recursive
<boolean> copy directories recursively Default:false
verbatimSymlinks
<boolean> Whentrue
, path resolution for symlinks will be skipped. Default:false
- Returns: <Promise> Fulfills with
undefined
upon success.
Asynchronously copies the entire directory structure from src
to dest
,
including subdirectories and files.
When copying a directory to another directory, globs are not supported and
behavior is similar to cp dir1/ dir2/
.
fsPromises.lchmod(path, mode)
#
path
<string> | <Buffer> | <URL>mode
<integer>- Returns: <Promise> Fulfills with
undefined
upon success.
Changes the permissions on a symbolic link.
This method is only implemented on macOS.
fsPromises.lchown(path, uid, gid)
#
path
<string> | <Buffer> | <URL>uid
<integer>gid
<integer>- Returns: <Promise> Fulfills with
undefined
upon success.
Changes the ownership on a symbolic link.
fsPromises.lutimes(path, atime, mtime)
#
path
<string> | <Buffer> | <URL>atime
<number> | <string> | <Date>mtime
<number> | <string> | <Date>- Returns: <Promise> Fulfills with
undefined
upon success.
Changes the access and modification times of a file in the same way as
fsPromises.utimes()
, with the difference that if the path refers to a
symbolic link, then the link is not dereferenced: instead, the timestamps of
the symbolic link itself are changed.
fsPromises.link(existingPath, newPath)
#
existingPath
<string> | <Buffer> | <URL>newPath
<string> | <Buffer> | <URL>- Returns: <Promise> Fulfills with
undefined
upon success.
Creates a new link from the existingPath
to the newPath
. See the POSIX
link(2)
documentation for more detail.
fsPromises.lstat(path[, options])
#
path
<string> | <Buffer> | <URL>options
<Object>bigint
<boolean> Whether the numeric values in the returned <fs.Stats> object should bebigint
. Default:false
.
- Returns: <Promise> Fulfills with the <fs.Stats> object for the given
symbolic link
path
.
Equivalent to fsPromises.stat()
unless path
refers to a symbolic link,
in which case the link itself is stat-ed, not the file that it refers to.
Refer to the POSIX lstat(2)
document for more detail.
fsPromises.mkdir(path[, options])
#
path
<string> | <Buffer> | <URL>options
<Object> | <integer>- Returns: <Promise> Upon success, fulfills with
undefined
ifrecursive
isfalse
, or the first directory path created ifrecursive
istrue
.
Asynchronously creates a directory.
The optional options
argument can be an integer specifying mode
(permission
and sticky bits), or an object with a mode
property and a recursive
property indicating whether parent directories should be created. Calling
fsPromises.mkdir()
when path
is a directory that exists results in a
rejection only when recursive
is false.
import { mkdir } from 'node:fs/promises';
try {
const projectFolder = new URL('./test/project/', import.meta.url);
const createDir = await mkdir(projectFolder, { recursive: true });
console.log(`created ${createDir}`);
} catch (err) {
console.error(err.message);
}
const { mkdir } = require('node:fs/promises');
const { join } = require('node:path');
async function makeDirectory() {
const projectFolder = join(__dirname, 'test', 'project');
const dirCreation = await mkdir(projectFolder, { recursive: true });
console.log(dirCreation);
return dirCreation;
}
makeDirectory().catch(console.error);
fsPromises.mkdtemp(prefix[, options])
#
prefix
<string>options
<string> | <Object>encoding
<string> Default:'utf8'
- Returns: <Promise> Fulfills with a string containing the file system path of the newly created temporary directory.
Creates a unique temporary directory. A unique directory name is generated by
appending six random characters to the end of the provided prefix
. Due to
platform inconsistencies, avoid trailing X
characters in prefix
. Some
platforms, notably the BSDs, can return more than six random characters, and
replace trailing X
characters in prefix
with random characters.
The optional options
argument can be a string specifying an encoding, or an
object with an encoding
property specifying the character encoding to use.
import { mkdtemp } from 'node:fs/promises';
import { join } from 'node:path';
import { tmpdir } from 'node:os';
try {
await mkdtemp(join(tmpdir(), 'foo-'));
} catch (err) {
console.error(err);
}
The fsPromises.mkdtemp()
method will append the six randomly selected
characters directly to the prefix
string. For instance, given a directory
/tmp
, if the intention is to create a temporary directory within /tmp
, the
prefix
must end with a trailing platform-specific path separator
(require('node:path').sep
).
fsPromises.open(path, flags[, mode])
#
path
<string> | <Buffer> | <URL>flags
<string> | <number> See support of file systemflags
. Default:'r'
.mode
<string> | <integer> Sets the file mode (permission and sticky bits) if the file is created. Default:0o666
(readable and writable)- Returns: <Promise> Fulfills with a <FileHandle> object.
Opens a <FileHandle>.
Refer to the POSIX open(2)
documentation for more detail.
Some characters (< > : " / \ | ? *
) are reserved under Windows as documented
by Naming Files, Paths, and Namespaces. Under NTFS, if the filename contains
a colon, Node.js will open a file system stream, as described by
this MSDN page.
fsPromises.opendir(path[, options])
#
Asynchronously open a directory for iterative scanning. See the POSIX
opendir(3)
documentation for more detail.
Creates an <fs.Dir>, which contains all further functions for reading from and cleaning up the directory.
The encoding
option sets the encoding for the path
while opening the
directory and subsequent read operations.
Example using async iteration:
import { opendir } from 'node:fs/promises';
try {
const dir = await opendir('./');
for await (const dirent of dir)
console.log(dirent.name);
} catch (err) {
console.error(err);
}
When using the async iterator, the <fs.Dir> object will be automatically closed after the iterator exits.
fsPromises.readdir(path[, options])
#
path
<string> | <Buffer> | <URL>options
<string> | <Object>- Returns: <Promise> Fulfills with an array of the names of the files in
the directory excluding
'.'
and'..'
.
Reads the contents of a directory.
The optional options
argument can be a string specifying an encoding, or an
object with an encoding
property specifying the character encoding to use for
the filenames. If the encoding
is set to 'buffer'
, the filenames returned
will be passed as <Buffer> objects.
If options.withFileTypes
is set to true
, the resolved array will contain
<fs.Dirent> objects.
import { readdir } from 'node:fs/promises';
try {
const files = await readdir(path);
for (const file of files)
console.log(file);
} catch (err) {
console.error(err);
}
fsPromises.readFile(path[, options])
#
path
<string> | <Buffer> | <URL> | <FileHandle> filename orFileHandle
options
<Object> | <string>encoding
<string> | <null> Default:null
flag
<string> See support of file systemflags
. Default:'r'
.signal
<AbortSignal> allows aborting an in-progress readFile
- Returns: <Promise> Fulfills with the contents of the file.
Asynchronously reads the entire contents of a file.
If no encoding is specified (using options.encoding
), the data is returned
as a <Buffer> object. Otherwise, the data will be a string.
If options
is a string, then it specifies the encoding.
When the path
is a directory, the behavior of fsPromises.readFile()
is
platform-specific. On macOS, Linux, and Windows, the promise will be rejected
with an error. On FreeBSD, a representation of the directory's contents will be
returned.
An example of reading a package.json
file located in the same directory of the
running code:
import { readFile } from 'node:fs/promises';
try {
const filePath = new URL('./package.json', import.meta.url);
const contents = await readFile(filePath, { encoding: 'utf8' });
console.log(contents);
} catch (err) {
console.error(err.message);
}
const { readFile } = require('node:fs/promises');
const { resolve } = require('node:path');
async function logFile() {
try {
const filePath = resolve('./package.json');
const contents = await readFile(filePath, { encoding: 'utf8' });
console.log(contents);
} catch (err) {
console.error(err.message);
}
}
logFile();
It is possible to abort an ongoing readFile
using an <AbortSignal>. If a
request is aborted the promise returned is rejected with an AbortError
:
import { readFile } from 'node:fs/promises';
try {
const controller = new AbortController();
const { signal } = controller;
const promise = readFile(fileName, { signal });
// Abort the request before the promise settles.
controller.abort();
await promise;
} catch (err) {
// When a request is aborted - err is an AbortError
console.error(err);
}
Aborting an ongoing request does not abort individual operating
system requests but rather the internal buffering fs.readFile
performs.
Any specified <FileHandle> has to support reading.
fsPromises.readlink(path[, options])
#
path
<string> | <Buffer> | <URL>options
<string> | <Object>encoding
<string> Default:'utf8'
- Returns: <Promise> Fulfills with the
linkString
upon success.
Reads the contents of the symbolic link referred to by path
. See the POSIX
readlink(2)
documentation for more detail. The promise is resolved with the
linkString
upon success.
The optional options
argument can be a string specifying an encoding, or an
object with an encoding
property specifying the character encoding to use for
the link path returned. If the encoding
is set to 'buffer'
, the link path
returned will be passed as a <Buffer> object.
fsPromises.realpath(path[, options])
#
path
<string> | <Buffer> | <URL>options
<string> | <Object>encoding
<string> Default:'utf8'
- Returns: <Promise> Fulfills with the resolved path upon success.
Determines the actual location of path
using the same semantics as the
fs.realpath.native()
function.
Only paths that can be converted to UTF8 strings are supported.
The optional options
argument can be a string specifying an encoding, or an
object with an encoding
property specifying the character encoding to use for
the path. If the encoding
is set to 'buffer'
, the path returned will be
passed as a <Buffer> object.
On Linux, when Node.js is linked against musl libc, the procfs file system must
be mounted on /proc
in order for this function to work. Glibc does not have
this restriction.
fsPromises.rename(oldPath, newPath)
#
oldPath
<string> | <Buffer> | <URL>newPath
<string> | <Buffer> | <URL>- Returns: <Promise> Fulfills with
undefined
upon success.
Renames oldPath
to newPath
.
fsPromises.rmdir(path[, options])
#
path
<string> | <Buffer> | <URL>options
<Object>maxRetries
<integer> If anEBUSY
,EMFILE
,ENFILE
,ENOTEMPTY
, orEPERM
error is encountered, Node.js retries the operation with a linear backoff wait ofretryDelay
milliseconds longer on each try. This option represents the number of retries. This option is ignored if therecursive
option is nottrue
. Default:0
.recursive
<boolean> Iftrue
, perform a recursive directory removal. In recursive mode, operations are retried on failure. Default:false
. Deprecated.retryDelay
<integer> The amount of time in milliseconds to wait between retries. This option is ignored if therecursive
option is nottrue
. Default:100
.
- Returns: <Promise> Fulfills with
undefined
upon success.
Removes the directory identified by path
.
Using fsPromises.rmdir()
on a file (not a directory) results in the
promise being rejected with an ENOENT
error on Windows and an ENOTDIR
error on POSIX.
To get a behavior similar to the rm -rf
Unix command, use
fsPromises.rm()
with options { recursive: true, force: true }
.
fsPromises.rm(path[, options])
#
path
<string> | <Buffer> | <URL>options
<Object>force
<boolean> Whentrue
, exceptions will be ignored ifpath
does not exist. Default:false
.maxRetries
<integer> If anEBUSY
,EMFILE
,ENFILE
,ENOTEMPTY
, orEPERM
error is encountered, Node.js will retry the operation with a linear backoff wait ofretryDelay
milliseconds longer on each try. This option represents the number of retries. This option is ignored if therecursive
option is nottrue
. Default:0
.recursive
<boolean> Iftrue
, perform a recursive directory removal. In recursive mode operations are retried on failure. Default:false
.retryDelay
<integer> The amount of time in milliseconds to wait between retries. This option is ignored if therecursive
option is nottrue
. Default:100
.
- Returns: <Promise> Fulfills with
undefined
upon success.
Removes files and directories (modeled on the standard POSIX rm
utility).
fsPromises.stat(path[, options])
#
path
<string> | <Buffer> | <URL>options
<Object>bigint
<boolean> Whether the numeric values in the returned <fs.Stats> object should bebigint
. Default:false
.
- Returns: <Promise> Fulfills with the <fs.Stats> object for the
given
path
.
fsPromises.statfs(path[, options])
#
path
<string> | <Buffer> | <URL>options
<Object>bigint
<boolean> Whether the numeric values in the returned <fs.StatFs> object should bebigint
. Default:false
.
- Returns: <Promise> Fulfills with the <fs.StatFs> object for the
given
path
.
fsPromises.symlink(target, path[, type])
#
target
<string> | <Buffer> | <URL>path
<string> | <Buffer> | <URL>type
<string> | <null> Default:null
- Returns: <Promise> Fulfills with
undefined
upon success.
Creates a symbolic link.
The type
argument is only used on Windows platforms and can be one of 'dir'
,
'file'
, or 'junction'
. If the type
argument is not a string, Node.js will
autodetect target
type and use 'file'
or 'dir'
. If the target
does not
exist, 'file'
will be used. Windows junction points require the destination
path to be absolute. When using 'junction'
, the target
argument will
automatically be normalized to absolute path.
fsPromises.truncate(path[, len])
#
path
<string> | <Buffer> | <URL>len
<integer> Default:0
- Returns: <Promise> Fulfills with
undefined
upon success.
Truncates (shortens or extends the length) of the content at path
to len
bytes.
fsPromises.unlink(path)
#
If path
refers to a symbolic link, then the link is removed without affecting
the file or directory to which that link refers. If the path
refers to a file
path that is not a symbolic link, the file is deleted. See the POSIX unlink(2)
documentation for more detail.
fsPromises.utimes(path, atime, mtime)
#
path
<string> | <Buffer> | <URL>atime
<number> | <string> | <Date>mtime
<number> | <string> | <Date>- Returns: <Promise> Fulfills with
undefined
upon success.
Change the file system timestamps of the object referenced by path
.
The atime
and mtime
arguments follow these rules:
- Values can be either numbers representing Unix epoch time,
Date
s, or a numeric string like'123456789.0'
. - If the value can not be converted to a number, or is
NaN
,Infinity
, or-Infinity
, anError
will be thrown.
fsPromises.watch(filename[, options])
#
filename
<string> | <Buffer> | <URL>options
<string> | <Object>persistent
<boolean> Indicates whether the process should continue to run as long as files are being watched. Default:true
.recursive
<boolean> Indicates whether all subdirectories should be watched, or only the current directory. This applies when a directory is specified, and only on supported platforms (See caveats). Default:false
.encoding
<string> Specifies the character encoding to be used for the filename passed to the listener. Default:'utf8'
.signal
<AbortSignal> An <AbortSignal> used to signal when the watcher should stop.
- Returns: <AsyncIterator> of objects with the properties:
Returns an async iterator that watches for changes on filename
, where filename
is either a file or a directory.
const { watch } = require('node:fs/promises');
const ac = new AbortController();
const { signal } = ac;
setTimeout(() => ac.abort(), 10000);
(async () => {
try {
const watcher = watch(__filename, { signal });
for await (const event of watcher)
console.log(event);
} catch (err) {
if (err.name === 'AbortError')
return;
throw err;
}
})();
On most platforms, 'rename'
is emitted whenever a filename appears or
disappears in the directory.
All the caveats for fs.watch()
also apply to fsPromises.watch()
.
fsPromises.writeFile(file, data[, options])
#
file
<string> | <Buffer> | <URL> | <FileHandle> filename orFileHandle
data
<string> | <Buffer> | <TypedArray> | <DataView> | <AsyncIterable> | <Iterable> | <Stream>options
<Object> | <string>encoding
<string> | <null> Default:'utf8'
mode
<integer> Default:0o666
flag
<string> See support of file systemflags
. Default:'w'
.signal
<AbortSignal> allows aborting an in-progress writeFile
- Returns: <Promise> Fulfills with
undefined
upon success.
Asynchronously writes data to a file, replacing the file if it already exists.
data
can be a string, a buffer, an <AsyncIterable>, or an <Iterable> object.
The encoding
option is ignored if data
is a buffer.
If options
is a string, then it specifies the encoding.
The mode
option only affects the newly created file. See fs.open()
for more details.
Any specified <FileHandle> has to support writing.
It is unsafe to use fsPromises.writeFile()
multiple times on the same file
without waiting for the promise to be settled.
Similarly to fsPromises.readFile
- fsPromises.writeFile
is a convenience
method that performs multiple write
calls internally to write the buffer
passed to it. For performance sensitive code consider using
fs.createWriteStream()
or filehandle.createWriteStream()
.
It is possible to use an <AbortSignal> to cancel an fsPromises.writeFile()
.
Cancelation is "best effort", and some amount of data is likely still
to be written.
import { writeFile } from 'node:fs/promises';
import { Buffer } from 'node:buffer';
try {
const controller = new AbortController();
const { signal } = controller;
const data = new Uint8Array(Buffer.from('Hello Node.js'));
const promise = writeFile('message.txt', data, { signal });
// Abort the request before the promise settles.
controller.abort();
await promise;
} catch (err) {
// When a request is aborted - err is an AbortError
console.error(err);
}
Aborting an ongoing request does not abort individual operating
system requests but rather the internal buffering fs.writeFile
performs.
fsPromises.constants
#
Returns an object containing commonly used constants for file system
operations. The object is the same as fs.constants
. See FS constants
for more details.
Callback API#
The callback APIs perform all operations asynchronously, without blocking the event loop, then invoke a callback function upon completion or error.
The callback APIs use the underlying Node.js threadpool to perform file system operations off the event loop thread. These operations are not synchronized or threadsafe. Care must be taken when performing multiple concurrent modifications on the same file or data corruption may occur.
fs.access(path[, mode], callback)
#
path
<string> | <Buffer> | <URL>mode
<integer> Default:fs.constants.F_OK
callback
<Function>err
<Error>
Tests a user's permissions for the file or directory specified by path
.
The mode
argument is an optional integer that specifies the accessibility
checks to be performed. mode
should be either the value fs.constants.F_OK
or a mask consisting of the bitwise OR of any of fs.constants.R_OK
,
fs.constants.W_OK
, and fs.constants.X_OK
(e.g.
fs.constants.W_OK | fs.constants.R_OK
). Check File access constants for
possible values of mode
.
The final argument, callback
, is a callback function that is invoked with
a possible error argument. If any of the accessibility checks fail, the error
argument will be an Error
object. The following examples check if
package.json
exists, and if it is readable or writable.
import { access, constants } from 'node:fs';
const file = 'package.json';
// Check if the file exists in the current directory.
access(file, constants.F_OK, (err) => {
console.log(`${file} ${err ? 'does not exist' : 'exists'}`);
});
// Check if the file is readable.
access(file, constants.R_OK, (err) => {
console.log(`${file} ${err ? 'is not readable' : 'is readable'}`);
});
// Check if the file is writable.
access(file, constants.W_OK, (err) => {
console.log(`${file} ${err ? 'is not writable' : 'is writable'}`);
});
// Check if the file is readable and writable.
access(file, constants.R_OK | constants.W_OK, (err) => {
console.log(`${file} ${err ? 'is not' : 'is'} readable and writable`);
});
Do not use fs.access()
to check for the accessibility of a file before calling
fs.open()
, fs.readFile()
, or fs.writeFile()
. Doing
so introduces a race condition, since other processes may change the file's
state between the two calls. Instead, user code should open/read/write the
file directly and handle the error raised if the file is not accessible.
write (NOT RECOMMENDED)
import { access, open, close } from 'node:fs';
access('myfile', (err) => {
if (!err) {
console.error('myfile already exists');
return;
}
open('myfile', 'wx', (err, fd) => {
if (err) throw err;
try {
writeMyData(fd);
} finally {
close(fd, (err) => {
if (err) throw err;
});
}
});
});
write (RECOMMENDED)
import { open, close } from 'node:fs';
open('myfile', 'wx', (err, fd) => {
if (err) {
if (err.code === 'EEXIST') {
console.error('myfile already exists');
return;
}
throw err;
}
try {
writeMyData(fd);
} finally {
close(fd, (err) => {
if (err) throw err;
});
}
});
read (NOT RECOMMENDED)
import { access, open, close } from 'node:fs';
access('myfile', (err) => {
if (err) {
if (err.code === 'ENOENT') {
console.error('myfile does not exist');
return;
}
throw err;
}
open('myfile', 'r', (err, fd) => {
if (err) throw err;
try {
readMyData(fd);
} finally {
close(fd, (err) => {
if (err) throw err;
});
}
});
});
read (RECOMMENDED)
import { open, close } from 'node:fs';
open('myfile', 'r', (err, fd) => {
if (err) {
if (err.code === 'ENOENT') {
console.error('myfile does not exist');
return;
}
throw err;
}
try {
readMyData(fd);
} finally {
close(fd, (err) => {
if (err) throw err;
});
}
});
The "not recommended" examples above check for accessibility and then use the file; the "recommended" examples are better because they use the file directly and handle the error, if any.
In general, check for the accessibility of a file only if the file will not be used directly, for example when its accessibility is a signal from another process.
On Windows, access-control policies (ACLs) on a directory may limit access to
a file or directory. The fs.access()
function, however, does not check the
ACL and therefore may report that a path is accessible even if the ACL restricts
the user from reading or writing to it.
fs.appendFile(path, data[, options], callback)
#
path
<string> | <Buffer> | <URL> | <number> filename or file descriptordata
<string> | <Buffer>options
<Object> | <string>encoding
<string> | <null> Default:'utf8'
mode
<integer> Default:0o666
flag
<string> See support of file systemflags
. Default:'a'
.
callback
<Function>err
<Error>
Asynchronously append data to a file, creating the file if it does not yet
exist. data
can be a string or a <Buffer>.
The mode
option only affects the newly created file. See fs.open()
for more details.
import { appendFile } from 'node:fs';
appendFile('message.txt', 'data to append', (err) => {
if (err) throw err;
console.log('The "data to append" was appended to file!');
});
If options
is a string, then it specifies the encoding:
import { appendFile } from 'node:fs';
appendFile('message.txt', 'data to append', 'utf8', callback);
The path
may be specified as a numeric file descriptor that has been opened
for appending (using fs.open()
or fs.openSync()
). The file descriptor will
not be closed automatically.
import { open, close, appendFile } from 'node:fs';
function closeFd(fd) {
close(fd, (err) => {
if (err) throw err;
});
}
open('message.txt', 'a', (err, fd) => {
if (err) throw err;
try {
appendFile(fd, 'data to append', 'utf8', (err) => {
closeFd(fd);
if (err) throw err;
});
} catch (err) {
closeFd(fd);
throw err;
}
});
fs.chmod(path, mode, callback)
#
Asynchronously changes the permissions of a file. No arguments other than a possible exception are given to the completion callback.
See the POSIX chmod(2)
documentation for more detail.
import { chmod } from 'node:fs';
chmod('my_file.txt', 0o775, (err) => {
if (err) throw err;
console.log('The permissions for file "my_file.txt" have been changed!');
});
File modes#
The mode
argument used in both the fs.chmod()
and fs.chmodSync()
methods is a numeric bitmask created using a logical OR of the following
constants:
Constant | Octal | Description |
---|---|---|
fs.constants.S_IRUSR | 0o400 | read by owner |
fs.constants.S_IWUSR | 0o200 | write by owner |
fs.constants.S_IXUSR | 0o100 | execute/search by owner |
fs.constants.S_IRGRP | 0o40 | read by group |
fs.constants.S_IWGRP | 0o20 | write by group |
fs.constants.S_IXGRP | 0o10 | execute/search by group |
fs.constants.S_IROTH | 0o4 | read by others |
fs.constants.S_IWOTH | 0o2 | write by others |
fs.constants.S_IXOTH | 0o1 | execute/search by others |
An easier method of constructing the mode
is to use a sequence of three
octal digits (e.g. 765
). The left-most digit (7
in the example), specifies
the permissions for the file owner. The middle digit (6
in the example),
specifies permissions for the group. The right-most digit (5
in the example),
specifies the permissions for others.
Number | Description |
---|---|
7 | read, write, and execute |
6 | read and write |
5 | read and execute |
4 | read only |
3 | write and execute |
2 | write only |
1 | execute only |
0 | no permission |
For example, the octal value 0o765
means:
- The owner may read, write, and execute the file.
- The group may read and write the file.
- Others may read and execute the file.
When using raw numbers where file modes are expected, any value larger than
0o777
may result in platform-specific behaviors that are not supported to work
consistently. Therefore constants like S_ISVTX
, S_ISGID
, or S_ISUID
are
not exposed in fs.constants
.
Caveats: on Windows only the write permission can be changed, and the distinction among the permissions of group, owner, or others is not implemented.
fs.chown(path, uid, gid, callback)
#
Asynchronously changes owner and group of a file. No arguments other than a possible exception are given to the completion callback.
See the POSIX chown(2)
documentation for more detail.
fs.close(fd[, callback])
#
fd
<integer>callback
<Function>err
<Error>
Closes the file descriptor. No arguments other than a possible exception are given to the completion callback.
Calling fs.close()
on any file descriptor (fd
) that is currently in use
through any other fs
operation may lead to undefined behavior.
See the POSIX close(2)
documentation for more detail.
fs.copyFile(src, dest[, mode], callback)
#
src
<string> | <Buffer> | <URL> source filename to copydest
<string> | <Buffer> | <URL> destination filename of the copy operationmode
<integer> modifiers for copy operation. Default:0
.callback
<Function>
Asynchronously copies src
to dest
. By default, dest
is overwritten if it
already exists. No arguments other than a possible exception are given to the
callback function. Node.js makes no guarantees about the atomicity of the copy
operation. If an error occurs after the destination file has been opened for
writing, Node.js will attempt to remove the destination.
mode
is an optional integer that specifies the behavior
of the copy operation. It is possible to create a mask consisting of the bitwise
OR of two or more values (e.g.
fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE
).
fs.constants.COPYFILE_EXCL
: The copy operation will fail ifdest
already exists.fs.constants.COPYFILE_FICLONE
: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then a fallback copy mechanism is used.fs.constants.COPYFILE_FICLONE_FORCE
: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then the operation will fail.
import { copyFile, constants } from 'node:fs';
function callback(err) {
if (err) throw err;
console.log('source.txt was copied to destination.txt');
}
// destination.txt will be created or overwritten by default.
copyFile('source.txt', 'destination.txt', callback);
// By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
copyFile('source.txt', 'destination.txt', constants.COPYFILE_EXCL, callback);
fs.cp(src, dest[, options], callback)
#
src
<string> | <URL> source path to copy.dest
<string> | <URL> destination path to copy to.options
<Object>dereference
<boolean> dereference symlinks. Default:false
.errorOnExist
<boolean> whenforce
isfalse
, and the destination exists, throw an error. Default:false
.filter
<Function> Function to filter copied files/directories. Returntrue
to copy the item,false
to ignore it. Can also return aPromise
that resolves totrue
orfalse
Default:undefined
.force
<boolean> overwrite existing file or directory. The copy operation will ignore errors if you set this to false and the destination exists. Use theerrorOnExist
option to change this behavior. Default:true
.preserveTimestamps
<boolean> Whentrue
timestamps fromsrc
will be preserved. Default:false
.recursive
<boolean> copy directories recursively Default:false
verbatimSymlinks
<boolean> Whentrue
, path resolution for symlinks will be skipped. Default:false
callback
<Function>
Asynchronously copies the entire directory structure from src
to dest
,
including subdirectories and files.
When copying a directory to another directory, globs are not supported and
behavior is similar to cp dir1/ dir2/
.
fs.createReadStream(path[, options])
#
path
<string> | <Buffer> | <URL>options
<string> | <Object>flags
<string> See support of file systemflags
. Default:'r'
.encoding
<string> Default:null
fd
<integer> | <FileHandle> Default:null
mode
<integer> Default:0o666
autoClose
<boolean> Default:true
emitClose
<boolean> Default:true
start
<integer>end
<integer> Default:Infinity
highWaterMark
<integer> Default:64 * 1024
fs
<Object> | <null> Default:null
signal
<AbortSignal> | <null> Default:null
- Returns: <fs.ReadStream>
Unlike the 16 KiB default highWaterMark
for a <stream.Readable>, the stream
returned by this method has a default highWaterMark
of 64 KiB.
options
can include start
and end
values to read a range of bytes from
the file instead of the entire file. Both start
and end
are inclusive and
start counting at 0, allowed values are in the
[0, Number.MAX_SAFE_INTEGER
] range. If fd
is specified and start
is
omitted or undefined
, fs.createReadStream()
reads sequentially from the
current file position. The encoding
can be any one of those accepted by
<Buffer>.
If fd
is specified, ReadStream
will ignore the path
argument and will use
the specified file descriptor. This means that no 'open'
event will be
emitted. fd
should be blocking; non-blocking fd
s should be passed to
<net.Socket>.
If fd
points to a character device that only supports blocking reads
(such as keyboard or sound card), read operations do not finish until data is
available. This can prevent the process from exiting and the stream from
closing naturally.
By default, the stream will emit a 'close'
event after it has been
destroyed. Set the emitClose
option to false
to change this behavior.
By providing the fs
option, it is possible to override the corresponding fs
implementations for open
, read
, and close
. When providing the fs
option,
an override for read
is required. If no fd
is provided, an override for
open
is also required. If autoClose
is true
, an override for close
is
also required.
import { createReadStream } from 'node:fs';
// Create a stream from some character device.
const stream = createReadStream('/dev/input/event0');
setTimeout(() => {
stream.close(); // This may not close the stream.
// Artificially marking end-of-stream, as if the underlying resource had
// indicated end-of-file by itself, allows the stream to close.
// This does not cancel pending read operations, and if there is such an
// operation, the process may still not be able to exit successfully
// until it finishes.
stream.push(null);
stream.read(0);
}, 100);
If autoClose
is false, then the file descriptor won't be closed, even if
there's an error. It is the application's responsibility to close it and make
sure there's no file descriptor leak. If autoClose
is set to true (default
behavior), on 'error'
or 'end'
the file descriptor will be closed
automatically.
mode
sets the file mode (permission and sticky bits), but only if the
file was created.
An example to read the last 10 bytes of a file which is 100 bytes long:
import { createReadStream } from 'node:fs';
createReadStream('sample.txt', { start: 90, end: 99 });
If options
is a string, then it specifies the encoding.
fs.createWriteStream(path[, options])
#
path
<string> | <Buffer> | <URL>options
<string> | <Object>flags
<string> See support of file systemflags
. Default:'w'
.encoding
<string> Default:'utf8'
fd
<integer> | <FileHandle> Default:null
mode
<integer> Default:0o666
autoClose
<boolean> Default:true
emitClose
<boolean> Default:true
start
<integer>fs
<Object> | <null> Default:null
signal
<AbortSignal> | <null> Default:null
- Returns: <fs.WriteStream>
options
may also include a start
option to allow writing data at some
position past the beginning of the file, allowed values are in the
[0, Number.MAX_SAFE_INTEGER
] range. Modifying a file rather than
replacing it may require the flags
option to be set to r+
rather than the
default w
. The encoding
can be any one of those accepted by <Buffer>.
If autoClose
is set to true (default behavior) on 'error'
or 'finish'
the file descriptor will be closed automatically. If autoClose
is false,
then the file descriptor won't be closed, even if there's an error.
It is the application's responsibility to close it and make sure there's no
file descriptor leak.
By default, the stream will emit a 'close'
event after it has been
destroyed. Set the emitClose
option to false
to change this behavior.
By providing the fs
option it is possible to override the corresponding fs
implementations for open
, write
, writev
, and close
. Overriding write()
without writev()
can reduce performance as some optimizations (_writev()
)
will be disabled. When providing the fs
option, overrides for at least one of
write
and writev
are required. If no fd
option is supplied, an override
for open
is also required. If autoClose
is true
, an override for close
is also required.
Like <fs.ReadStream>, if fd
is specified, <fs.WriteStream> will ignore the
path
argument and will use the specified file descriptor. This means that no
'open'
event will be emitted. fd
should be blocking; non-blocking fd
s
should be passed to <net.Socket>.
If options
is a string, then it specifies the encoding.
fs.exists(path, callback)
#
path
<string> | <Buffer> | <URL>callback
<Function>exists
<boolean>
Test whether or not the given path exists by checking with the file system.
Then call the callback
argument with either true or false:
import { exists } from 'node:fs';
exists('/etc/passwd', (e) => {
console.log(e ? 'it exists' : 'no passwd!');
});
The parameters for this callback are not consistent with other Node.js
callbacks. Normally, the first parameter to a Node.js callback is an err
parameter, optionally followed by other parameters. The fs.exists()
callback
has only one boolean parameter. This is one reason fs.access()
is recommended
instead of fs.exists()
.
Using fs.exists()
to check for the existence of a file before calling
fs.open()
, fs.readFile()
, or fs.writeFile()
is not recommended. Doing
so introduces a race condition, since other processes may change the file's
state between the two calls. Instead, user code should open/read/write the
file directly and handle the error raised if the file does not exist.
write (NOT RECOMMENDED)
import { exists, open, close } from 'node:fs';
exists('myfile', (e) => {
if (e) {
console.error('myfile already exists');
} else {
open('myfile', 'wx', (err, fd) => {
if (err) throw err;
try {
writeMyData(fd);
} finally {
close(fd, (err) => {
if (err) throw err;
});
}
});
}
});
write (RECOMMENDED)
import { open, close } from 'node:fs';
open('myfile', 'wx', (err, fd) => {
if (err) {
if (err.code === 'EEXIST') {
console.error('myfile already exists');
return;
}
throw err;
}
try {
writeMyData(fd);
} finally {
close(fd, (err) => {
if (err) throw err;
});
}
});
read (NOT RECOMMENDED)
import { open, close, exists } from 'node:fs';
exists('myfile', (e) => {
if (e) {
open('myfile', 'r', (err, fd) => {
if (err) throw err;
try {
readMyData(fd);
} finally {
close(fd, (err) => {
if (err) throw err;
});
}
});
} else {
console.error('myfile does not exist');
}
});
read (RECOMMENDED)
import { open, close } from 'node:fs';
open('myfile', 'r', (err, fd) => {
if (err) {
if (err.code === 'ENOENT') {
console.error('myfile does not exist');
return;
}
throw err;
}
try {
readMyData(fd);
} finally {
close(fd, (err) => {
if (err) throw err;
});
}
});
The "not recommended" examples above check for existence and then use the file; the "recommended" examples are better because they use the file directly and handle the error, if any.
In general, check for the existence of a file only if the file won't be used directly, for example when its existence is a signal from another process.
fs.fchmod(fd, mode, callback)
#
fd
<integer>mode
<string> | <integer>callback
<Function>err
<Error>
Sets the permissions on the file. No arguments other than a possible exception are given to the completion callback.
See the POSIX fchmod(2)
documentation for more detail.
fs.fchown(fd, uid, gid, callback)
#
fd
<integer>uid
<integer>gid
<integer>callback
<Function>err
<Error>
Sets the owner of the file. No arguments other than a possible exception are given to the completion callback.
See the POSIX fchown(2)
documentation for more detail.
fs.fdatasync(fd, callback)
#
fd
<integer>callback
<Function>err
<Error>
Forces all currently queued I/O operations associated with the file to the
operating system's synchronized I/O completion state. Refer to the POSIX
fdatasync(2)
documentation for details. No arguments other than a possible
exception are given to the completion callback.
fs.fstat(fd[, options], callback)
#
fd
<integer>options
<Object>bigint
<boolean> Whether the numeric values in the returned <fs.Stats> object should bebigint
. Default:false
.
callback
<Function>err
<Error>stats
<fs.Stats>
Invokes the callback with the <fs.Stats> for the file descriptor.
See the POSIX fstat(2)
documentation for more detail.
fs.fsync(fd, callback)
#
fd
<integer>callback
<Function>err
<Error>
Request that all data for the open file descriptor is flushed to the storage
device. The specific implementation is operating system and device specific.
Refer to the POSIX fsync(2)
documentation for more detail. No arguments other
than a possible exception are given to the completion callback.
fs.ftruncate(fd[, len], callback)
#
fd
<integer>len
<integer> Default:0
callback
<Function>err
<Error>
Truncates the file descriptor. No arguments other than a possible exception are given to the completion callback.
See the POSIX ftruncate(2)
documentation for more detail.
If the file referred to by the file descriptor was larger than len
bytes, only
the first len
bytes will be retained in the file.
For example, the following program retains only the first four bytes of the file:
import { open, close, ftruncate } from 'node:fs';
function closeFd(fd) {
close(fd, (err) => {
if (err) throw err;
});
}
open('temp.txt', 'r+', (err, fd) => {
if (err) throw err;
try {
ftruncate(fd, 4, (err) => {
closeFd(fd);
if (err) throw err;
});
} catch (err) {
closeFd(fd);
if (err) throw err;
}
});
If the file previously was shorter than len
bytes, it is extended, and the
extended part is filled with null bytes ('\0'
):
If len
is negative then 0
will be used.
fs.futimes(fd, atime, mtime, callback)
#
fd
<integer>atime
<number> | <string> | <Date>mtime
<number> | <string> | <Date>callback
<Function>err
<Error>
Change the file system timestamps of the object referenced by the supplied file
descriptor. See fs.utimes()
.
fs.lchmod(path, mode, callback)
#
path
<string> | <Buffer> | <URL>mode
<integer>callback
<Function>err
<Error> | <AggregateError>
Changes the permissions on a symbolic link. No arguments other than a possible exception are given to the completion callback.
This method is only implemented on macOS.
See the POSIX lchmod(2)
documentation for more detail.
fs.lchown(path, uid, gid, callback)
#
Set the owner of the symbolic link. No arguments other than a possible exception are given to the completion callback.
See the POSIX lchown(2)
documentation for more detail.
fs.lutimes(path, atime, mtime, callback)
#
path
<string> | <Buffer> | <URL>atime
<number> | <string> | <Date>mtime
<number> | <string> | <Date>callback
<Function>err
<Error>
Changes the access and modification times of a file in the same way as
fs.utimes()
, with the difference that if the path refers to a symbolic
link, then the link is not dereferenced: instead, the timestamps of the
symbolic link itself are changed.
No arguments other than a possible exception are given to the completion callback.
fs.link(existingPath, newPath, callback)
#
existingPath
<string> | <Buffer> | <URL>newPath
<string> | <Buffer> | <URL>callback
<Function>err
<Error>
Creates a new link from the existingPath
to the newPath
. See the POSIX
link(2)
documentation for more detail. No arguments other than a possible
exception are given to the completion callback.
fs.lstat(path[, options], callback)
#
path
<string> | <Buffer> | <URL>options
<Object>bigint
<boolean> Whether the numeric values in the returned <fs.Stats> object should bebigint
. Default:false
.
callback
<Function>err
<Error>stats
<fs.Stats>
Retrieves the <fs.Stats> for the symbolic link referred to by the path.
The callback gets two arguments (err, stats)
where stats
is a <fs.Stats>
object. lstat()
is identical to stat()
, except that if path
is a symbolic
link, then the link itself is stat-ed, not the file that it refers to.
See the POSIX lstat(2)
documentation for more details.
fs.mkdir(path[, options], callback)
#
path
<string> | <Buffer> | <URL>options
<Object> | <integer>callback
<Function>err
<Error>path
<string> | <undefined> Present only if a directory is created withrecursive
set totrue
.
Asynchronously creates a directory.
The callback is given a possible exception and, if recursive
is true
, the
first directory path created, (err[, path])
.
path
can still be undefined
when recursive
is true
, if no directory was
created.
The optional options
argument can be an integer specifying mode
(permission
and sticky bits), or an object with a mode
property and a recursive
property indicating whether parent directories should be created. Calling
fs.mkdir()
when path
is a directory that exists results in an error only
when recursive
is false.
import { mkdir } from 'node:fs';
// Creates /tmp/a/apple, regardless of whether `/tmp` and /tmp/a exist.
mkdir('/tmp/a/apple', { recursive: true }, (err) => {
if (err) throw err;
});
On Windows, using fs.mkdir()
on the root directory even with recursion will
result in an error:
import { mkdir } from 'node:fs';
mkdir('/', { recursive: true }, (err) => {
// => [Error: EPERM: operation not permitted, mkdir 'C:\']
});
See the POSIX mkdir(2)
documentation for more details.
fs.mkdtemp(prefix[, options], callback)
#
prefix
<string>options
<string> | <Object>encoding
<string> Default:'utf8'
callback
<Function>
Creates a unique temporary directory.
Generates six random characters to be appended behind a required
prefix
to create a unique temporary directory. Due to platform
inconsistencies, avoid trailing X
characters in prefix
. Some platforms,
notably the BSDs, can return more than six random characters, and replace
trailing X
characters in prefix
with random characters.
The created directory path is passed as a string to the callback's second parameter.
The optional options
argument can be a string specifying an encoding, or an
object with an encoding
property specifying the character encoding to use.
import { mkdtemp } from 'node:fs';
import { join } from 'node:path';
import { tmpdir } from 'node:os';
mkdtemp(join(tmpdir(), 'foo-'), (err, directory) => {
if (err) throw err;
console.log(directory);
// Prints: /tmp/foo-itXde2 or C:\Users\...\AppData\Local\Temp\foo-itXde2
});
The fs.mkdtemp()
method will append the six randomly selected characters
directly to the prefix
string. For instance, given a directory /tmp
, if the
intention is to create a temporary directory within /tmp
, the prefix
must end with a trailing platform-specific path separator
(require('node:path').sep
).
import { tmpdir } from 'node:os';
import { mkdtemp } from 'node:fs';
// The parent directory for the new temporary directory
const tmpDir = tmpdir();
// This method is *INCORRECT*:
mkdtemp(tmpDir, (err, directory) => {
if (err) throw err;
console.log(directory);
// Will print something similar to `/tmpabc123`.
// A new temporary directory is created at the file system root
// rather than *within* the /tmp directory.
});
// This method is *CORRECT*:
import { sep } from 'node:path';
mkdtemp(`${tmpDir}${sep}`, (err, directory) => {
if (err) throw err;
console.log(directory);
// Will print something similar to `/tmp/abc123`.
// A new temporary directory is created within
// the /tmp directory.
});
fs.open(path[, flags[, mode]], callback)
#
path
<string> | <Buffer> | <URL>flags
<string> | <number> See support of file systemflags
. Default:'r'
.mode
<string> | <integer> Default:0o666
(readable and writable)callback
<Function>
Asynchronous file open. See the POSIX open(2)
documentation for more details.
mode
sets the file mode (permission and sticky bits), but only if the file was
created. On Windows, only the write permission can be manipulated; see
fs.chmod()
.
The callback gets two arguments (err, fd)
.
Some characters (< > : " / \ | ? *
) are reserved under Windows as documented
by Naming Files, Paths, and Namespaces. Under NTFS, if the filename contains
a colon, Node.js will open a file system stream, as described by
this MSDN page.
Functions based on fs.open()
exhibit this behavior as well:
fs.writeFile()
, fs.readFile()
, etc.
fs.openAsBlob(path[, options])
#
path
<string> | <Buffer> | <URL>options
<Object>type
<string> An optional mime type for the blob.
- Return: <Promise> containing <Blob>
Returns a <Blob> whose data is backed by the given file.
The file must not be modified after the <Blob> is created. Any modifications
will cause reading the <Blob> data to fail with a DOMException
.
error. Synchronous stat operations on the file when the Blob
is created, and
before each read in order to detect whether the file data has been modified
on disk.
import { openAsBlob } from 'node:fs';
const blob = await openAsBlob('the.file.txt');
const ab = await blob.arrayBuffer();
blob.stream();
const { openAsBlob } = require('node:fs');
(async () => {
const blob = await openAsBlob('the.file.txt');
const ab = await blob.arrayBuffer();
blob.stream();
})();
fs.opendir(path[, options], callback)
#
path
<string> | <Buffer> | <URL>options
<Object>callback
<Function>
Asynchronously open a directory. See the POSIX opendir(3)
documentation for
more details.
Creates an <fs.Dir>, which contains all further functions for reading from and cleaning up the directory.
The encoding
option sets the encoding for the path
while opening the
directory and subsequent read operations.
fs.read(fd, buffer, offset, length, position, callback)
#
fd
<integer>buffer
<Buffer> | <TypedArray> | <DataView> The buffer that the data will be written to.offset
<integer> The position inbuffer
to write the data to.length
<integer> The number of bytes to read.position
<integer> | <bigint> | <null> Specifies where to begin reading from in the file. Ifposition
isnull
or-1
, data will be read from the current file position, and the file position will be updated. Ifposition
is an integer, the file position will be unchanged.callback
<Function>
Read data from the file specified by fd
.
The callback is given the three arguments, (err, bytesRead, buffer)
.
If the file is not modified concurrently, the end-of-file is reached when the number of bytes read is zero.
If this method is invoked as its util.promisify()
ed version, it returns
a promise for an Object
with bytesRead
and buffer
properties.
fs.read(fd[, options], callback)
#
fd
<integer>options
<Object>buffer
<Buffer> | <TypedArray> | <DataView> Default:Buffer.alloc(16384)
offset
<integer> Default:0
length
<integer> Default:buffer.byteLength - offset
position
<integer> | <bigint> | <null> Default:null
callback
<Function>
Similar to the fs.read()
function, this version takes an optional
options
object. If no options
object is specified, it will default with the
above values.
fs.read(fd, buffer[, options], callback)
#
fd
<integer>buffer
<Buffer> | <TypedArray> | <DataView> The buffer that the data will be written to.options
<Object>callback
<Function>
Similar to the fs.read()
function, this version takes an optional
options
object. If no options
object is specified, it will default with the
above values.
fs.readdir(path[, options], callback)
#
path
<string> | <Buffer> | <URL>options
<string> | <Object>callback
<Function>err
<Error>files
<string[]> | <Buffer[]> | <fs.Dirent[]>
Reads the contents of a directory. The callback gets two arguments (err, files)
where files
is an array of the names of the files in the directory excluding
'.'
and '..'
.
See the POSIX readdir(3)
documentation for more details.
The optional options
argument can be a string specifying an encoding, or an
object with an encoding
property specifying the character encoding to use for
the filenames passed to the callback. If the encoding
is set to 'buffer'
,
the filenames returned will be passed as <Buffer> objects.
If options.withFileTypes
is set to true
, the files
array will contain
<fs.Dirent> objects.
fs.readFile(path[, options], callback)
#
path
<string> | <Buffer> | <URL> | <integer> filename or file descriptoroptions
<Object> | <string>encoding
<string> | <null> Default:null
flag
<string> See support of file systemflags
. Default:'r'
.signal
<AbortSignal> allows aborting an in-progress readFile
callback
<Function>err
<Error> | <AggregateError>data
<string> | <Buffer>
Asynchronously reads the entire contents of a file.
import { readFile } from 'node:fs';
readFile('/etc/passwd', (err, data) => {
if (err) throw err;
console.log(data);
});
The callback is passed two arguments (err, data)
, where data
is the
contents of the file.
If no encoding is specified, then the raw buffer is returned.
If options
is a string, then it specifies the encoding:
import { readFile } from 'node:fs';
readFile('/etc/passwd', 'utf8', callback);
When the path is a directory, the behavior of fs.readFile()
and
fs.readFileSync()
is platform-specific. On macOS, Linux, and Windows, an
error will be returned. On FreeBSD, a representation of the directory's contents
will be returned.
import { readFile } from 'node:fs';
// macOS, Linux, and Windows
readFile('<directory>', (err, data) => {
// => [Error: EISDIR: illegal operation on a directory, read <directory>]
});
// FreeBSD
readFile('<directory>', (err, data) => {
// => null, <data>
});
It is possible to abort an ongoing request using an AbortSignal
. If a
request is aborted the callback is called with an AbortError
:
import { readFile } from 'node:fs';
const controller = new AbortController();
const signal = controller.signal;
readFile(fileInfo[0].name, { signal }, (err, buf) => {
// ...
});
// When you want to abort the request
controller.abort();
The fs.readFile()
function buffers the entire file. To minimize memory costs,
when possible prefer streaming via fs.createReadStream()
.
Aborting an ongoing request does not abort individual operating
system requests but rather the internal buffering fs.readFile
performs.
File descriptors#
- Any specified file descriptor has to support reading.
- If a file descriptor is specified as the
path
, it will not be closed automatically. - The reading will begin at the current position. For example, if the file
already had
'Hello World
' and six bytes are read with the file descriptor, the call tofs.readFile()
with the same file descriptor, would give'World'
, rather than'Hello World'
.
Performance Considerations#
The fs.readFile()
method asynchronously reads the contents of a file into
memory one chunk at a time, allowing the event loop to turn between each chunk.
This allows the read operation to have less impact on other activity that may
be using the underlying libuv thread pool but means that it will take longer
to read a complete file into memory.
The additional read overhead can vary broadly on different systems and depends on the type of file being read. If the file type is not a regular file (a pipe for instance) and Node.js is unable to determine an actual file size, each read operation will load on 64 KiB of data. For regular files, each read will process 512 KiB of data.
For applications that require as-fast-as-possible reading of file contents, it
is better to use fs.read()
directly and for application code to manage
reading the full contents of the file itself.
The Node.js GitHub issue #25741 provides more information and a detailed
analysis on the performance of fs.readFile()
for multiple file sizes in
different Node.js versions.
fs.readlink(path[, options], callback)
#
path
<string> | <Buffer> | <URL>options
<string> | <Object>encoding
<string> Default:'utf8'
callback
<Function>
Reads the contents of the symbolic link referred to by path
. The callback gets
two arguments (err, linkString)
.
See the POSIX readlink(2)
documentation for more details.
The optional options
argument can be a string specifying an encoding, or an
object with an encoding
property specifying the character encoding to use for
the link path passed to the callback. If the encoding
is set to 'buffer'
,
the link path returned will be passed as a <Buffer> object.
fs.readv(fd, buffers[, position], callback)
#
fd
<integer>buffers
<ArrayBufferView[]>position
<integer> | <null> Default:null
callback
<Function>err
<Error>bytesRead
<integer>buffers
<ArrayBufferView[]>
Read from a file specified by fd
and write to an array of ArrayBufferView
s
using readv()
.
position
is the offset from the beginning of the file from where data
should be read. If typeof position !== 'number'
, the data will be read
from the current position.
The callback will be given three arguments: err
, bytesRead
, and
buffers
. bytesRead
is how many bytes were read from the file.
If this method is invoked as its util.promisify()
ed version, it returns
a promise for an Object
with bytesRead
and buffers
properties.
fs.realpath(path[, options], callback)
#
path
<string> | <Buffer> | <URL>options
<string> | <Object>encoding
<string> Default:'utf8'
callback
<Function>
Asynchronously computes the canonical pathname by resolving .
, ..
, and
symbolic links.
A canonical pathname is not necessarily unique. Hard links and bind mounts can expose a file system entity through many pathnames.
This function behaves like realpath(3)
, with some exceptions:
-
No case conversion is performed on case-insensitive file systems.
-
The maximum number of symbolic links is platform-independent and generally (much) higher than what the native
realpath(3)
implementation supports.
The callback
gets two arguments (err, resolvedPath)
. May use process.cwd
to resolve relative paths.
Only paths that can be converted to UTF8 strings are supported.
The optional options
argument can be a string specifying an encoding, or an
object with an encoding
property specifying the character encoding to use for
the path passed to the callback. If the encoding
is set to 'buffer'
,
the path returned will be passed as a <Buffer> object.
If path
resolves to a socket or a pipe, the function will return a system
dependent name for that object.
fs.realpath.native(path[, options], callback)
#
path
<string> | <Buffer> | <URL>options
<string> | <Object>encoding
<string> Default:'utf8'
callback
<Function>
Asynchronous realpath(3)
.
The callback
gets two arguments (err, resolvedPath)
.
Only paths that can be converted to UTF8 strings are supported.
The optional options
argument can be a string specifying an encoding, or an
object with an encoding
property specifying the character encoding to use for
the path passed to the callback. If the encoding
is set to 'buffer'
,
the path returned will be passed as a <Buffer> object.
On Linux, when Node.js is linked against musl libc, the procfs file system must
be mounted on /proc
in order for this function to work. Glibc does not have
this restriction.
fs.rename(oldPath, newPath, callback)
#
oldPath
<string> | <Buffer> | <URL>newPath
<string> | <Buffer> | <URL>callback
<Function>err
<Error>
Asynchronously rename file at oldPath
to the pathname provided
as newPath
. In the case that newPath
already exists, it will
be overwritten. If there is a directory at newPath
, an error will
be raised instead. No arguments other than a possible exception are
given to the completion callback.
See also: rename(2)
.
import { rename } from 'node:fs';
rename('oldFile.txt', 'newFile.txt', (err) => {
if (err) throw err;
console.log('Rename complete!');
});
fs.rmdir(path[, options], callback)
#
path
<string> | <Buffer> | <URL>options
<Object>maxRetries
<integer> If anEBUSY
,EMFILE
,ENFILE
,ENOTEMPTY
, orEPERM
error is encountered, Node.js retries the operation with a linear backoff wait ofretryDelay
milliseconds longer on each try. This option represents the number of retries. This option is ignored if therecursive
option is nottrue
. Default:0
.recursive
<boolean> Iftrue
, perform a recursive directory removal. In recursive mode, operations are retried on failure. Default:false
. Deprecated.retryDelay
<integer> The amount of time in milliseconds to wait between retries. This option is ignored if therecursive
option is nottrue
. Default:100
.
callback
<Function>err
<Error>
Asynchronous rmdir(2)
. No arguments other than a possible exception are given
to the completion callback.
Using fs.rmdir()
on a file (not a directory) results in an ENOENT
error on
Windows and an ENOTDIR
error on POSIX.
To get a behavior similar to the rm -rf
Unix command, use fs.rm()
with options { recursive: true, force: true }
.
fs.rm(path[, options], callback)
#
path
<string> | <Buffer> | <URL>options
<Object>force
<boolean> Whentrue
, exceptions will be ignored ifpath
does not exist. Default:false
.maxRetries
<integer> If anEBUSY
,EMFILE
,ENFILE
,ENOTEMPTY
, orEPERM
error is encountered, Node.js will retry the operation with a linear backoff wait ofretryDelay
milliseconds longer on each try. This option represents the number of retries. This option is ignored if therecursive
option is nottrue
. Default:0
.recursive
<boolean> Iftrue
, perform a recursive removal. In recursive mode operations are retried on failure. Default:false
.retryDelay
<integer> The amount of time in milliseconds to wait between retries. This option is ignored if therecursive
option is nottrue
. Default:100
.
callback
<Function>err
<Error>
Asynchronously removes files and directories (modeled on the standard POSIX rm
utility). No arguments other than a possible exception are given to the
completion callback.
fs.stat(path[, options], callback)
#
path
<string> | <Buffer> | <URL>options
<Object>bigint
<boolean> Whether the numeric values in the returned <fs.Stats> object should bebigint
. Default:false
.
callback
<Function>err
<Error>stats
<fs.Stats>
Asynchronous stat(2)
. The callback gets two arguments (err, stats)
where
stats
is an <fs.Stats> object.
In case of an error, the err.code
will be one of Common System Errors.
Using fs.stat()
to check for the existence of a file before calling
fs.open()
, fs.readFile()
, or fs.writeFile()
is not recommended.
Instead, user code should open/read/write the file directly and handle the
error raised if the file is not available.
To check if a file exists without manipulating it afterwards, fs.access()
is recommended.
For example, given the following directory structure:
- txtDir
-- file.txt
- app.js
The next program will check for the stats of the given paths:
import { stat } from 'node:fs';
const pathsToCheck = ['./txtDir', './txtDir/file.txt'];
for (let i = 0; i < pathsToCheck.length; i++) {
stat(pathsToCheck[i], (err, stats) => {
console.log(stats.isDirectory());
console.log(stats);
});
}
The resulting output will resemble:
true
Stats {
dev: 16777220,
mode: 16877,
nlink: 3,
uid: 501,
gid: 20,
rdev: 0,
blksize: 4096,
ino: 14214262,
size: 96,
blocks: 0,
atimeMs: 1561174653071.963,
mtimeMs: 1561174614583.3518,
ctimeMs: 1561174626623.5366,
birthtimeMs: 1561174126937.2893,
atime: 2019-06-22T03:37:33.072Z,
mtime: 2019-06-22T03:36:54.583Z,
ctime: 2019-06-22T03:37:06.624Z,
birthtime: 2019-06-22T03:28:46.937Z
}
false
Stats {
dev: 16777220,
mode: 33188,
nlink: 1,
uid: 501,
gid: 20,
rdev: 0,
blksize: 4096,
ino: 14214074,
size: 8,
blocks: 8,
atimeMs: 1561174616618.8555,
mtimeMs: 1561174614584,
ctimeMs: 1561174614583.8145,
birthtimeMs: 1561174007710.7478,
atime: 2019-06-22T03:36:56.619Z,
mtime: 2019-06-22T03:36:54.584Z,
ctime: 2019-06-22T03:36:54.584Z,
birthtime: 2019-06-22T03:26:47.711Z
}
fs.statfs(path[, options], callback)
#
path
<string> | <Buffer> | <URL>options
<Object>bigint
<boolean> Whether the numeric values in the returned <fs.StatFs> object should bebigint
. Default:false
.
callback
<Function>err
<Error>stats
<fs.StatFs>
Asynchronous statfs(2)
. Returns information about the mounted file system which
contains path
. The callback gets two arguments (err, stats)
where stats
is an <fs.StatFs> object.
In case of an error, the err.code
will be one of Common System Errors.
fs.symlink(target, path[, type], callback)
#
target
<string> | <Buffer> | <URL>path
<string> | <Buffer> | <URL>type
<string> | <null> Default:null
callback
<Function>err
<Error>
Creates the link called path
pointing to target
. No arguments other than a
possible exception are given to the completion callback.
See the POSIX symlink(2)
documentation for more details.
The type
argument is only available on Windows and ignored on other platforms.
It can be set to 'dir'
, 'file'
, or 'junction'
. If the type
argument is
not a string, Node.js will autodetect target
type and use 'file'
or 'dir'
.
If the target
does not exist, 'file'
will be used. Windows junction points
require the destination path to be absolute. When using 'junction'
, the
target
argument will automatically be normalized to absolute path.
Relative targets are relative to the link's parent directory.
import { symlink } from 'node:fs';
symlink('./mew', './mewtwo', callback);
The above example creates a symbolic link mewtwo
which points to mew
in the
same directory:
$ tree .
.
├── mew
└── mewtwo -> ./mew
fs.truncate(path[, len], callback)
#
path
<string> | <Buffer> | <URL>len
<integer> Default:0
callback
<Function>err
<Error> | <AggregateError>
Truncates the file. No arguments other than a possible exception are
given to the completion callback. A file descriptor can also be passed as the
first argument. In this case, fs.ftruncate()
is called.
import { truncate } from 'node:fs';
// Assuming that 'path/file.txt' is a regular file.
truncate('path/file.txt', (err) => {
if (err) throw err;
console.log('path/file.txt was truncated');
});
const { truncate } = require('node:fs');
// Assuming that 'path/file.txt' is a regular file.
truncate('path/file.txt', (err) => {
if (err) throw err;
console.log('path/file.txt was truncated');
});
Passing a file descriptor is deprecated and may result in an error being thrown in the future.
See the POSIX truncate(2)
documentation for more details.
fs.unlink(path, callback)
#
path
<string> | <Buffer> | <URL>callback
<Function>err
<Error>
Asynchronously removes a file or symbolic link. No arguments other than a possible exception are given to the completion callback.
import { unlink } from 'node:fs';
// Assuming that 'path/file.txt' is a regular file.
unlink('path/file.txt', (err) => {
if (err) throw err;
console.log('path/file.txt was deleted');
});
fs.unlink()
will not work on a directory, empty or otherwise. To remove a
directory, use fs.rmdir()
.
See the POSIX unlink(2)
documentation for more details.
fs.unwatchFile(filename[, listener])
#
filename
<string> | <Buffer> | <URL>listener
<Function> Optional, a listener previously attached usingfs.watchFile()
Stop watching for changes on filename
. If listener
is specified, only that
particular listener is removed. Otherwise, all listeners are removed,
effectively stopping watching of filename
.
Calling fs.unwatchFile()
with a filename that is not being watched is a
no-op, not an error.
Using fs.watch()
is more efficient than fs.watchFile()
and
fs.unwatchFile()
. fs.watch()
should be used instead of fs.watchFile()
and fs.unwatchFile()
when possible.
fs.utimes(path, atime, mtime, callback)
#
path
<string> | <Buffer> | <URL>atime
<number> | <string> | <Date>mtime
<number> | <string> | <Date>callback
<Function>err
<Error>
Change the file system timestamps of the object referenced by path
.
The atime
and mtime
arguments follow these rules:
- Values can be either numbers representing Unix epoch time in seconds,
Date
s, or a numeric string like'123456789.0'
. - If the value can not be converted to a number, or is
NaN
,Infinity
, or-Infinity
, anError
will be thrown.
fs.watch(filename[, options][, listener])
#
filename
<string> | <Buffer> | <URL>options
<string> | <Object>persistent
<boolean> Indicates whether the process should continue to run as long as files are being watched. Default:true
.recursive
<boolean> Indicates whether all subdirectories should be watched, or only the current directory. This applies when a directory is specified, and only on supported platforms (See caveats). Default:false
.encoding
<string> Specifies the character encoding to be used for the filename passed to the listener. Default:'utf8'
.signal
<AbortSignal> allows closing the watcher with an AbortSignal.
listener
<Function> | <undefined> Default:undefined
- Returns: <fs.FSWatcher>
Watch for changes on filename
, where filename
is either a file or a
directory.
The second argument is optional. If options
is provided as a string, it
specifies the encoding
. Otherwise options
should be passed as an object.
The listener callback gets two arguments (eventType, filename)
. eventType
is either 'rename'
or 'change'
, and filename
is the name of the file
which triggered the event.
On most platforms, 'rename'
is emitted whenever a filename appears or
disappears in the directory.
The listener callback is attached to the 'change'
event fired by
<fs.FSWatcher>, but it is not the same thing as the 'change'
value of
eventType
.
If a signal
is passed, aborting the corresponding AbortController will close
the returned <fs.FSWatcher>.
Caveats#
The fs.watch
API is not 100% consistent across platforms, and is
unavailable in some situations.
On Windows, no events will be emitted if the watched directory is moved or
renamed. An EPERM
error is reported when the watched directory is deleted.
Availability#
This feature depends on the underlying operating system providing a way to be notified of file system changes.
- On Linux systems, this uses
inotify(7)
. - On BSD systems, this uses
kqueue(2)
. - On macOS, this uses
kqueue(2)
for files andFSEvents
for directories. - On SunOS systems (including Solaris and SmartOS), this uses
event ports
. - On Windows systems, this feature depends on
ReadDirectoryChangesW
. - On AIX systems, this feature depends on
AHAFS
, which must be enabled. - On IBM i systems, this feature is not supported.
If the underlying functionality is not available for some reason, then
fs.watch()
will not be able to function and may throw an exception.
For example, watching files or directories can be unreliable, and in some
cases impossible, on network file systems (NFS, SMB, etc) or host file systems
when using virtualization software such as Vagrant or Docker.
It is still possible to use fs.watchFile()
, which uses stat polling, but
this method is slower and less reliable.
Inodes#
On Linux and macOS systems, fs.watch()
resolves the path to an inode and
watches the inode. If the watched path is deleted and recreated, it is assigned
a new inode. The watch will emit an event for the delete but will continue
watching the original inode. Events for the new inode will not be emitted.
This is expected behavior.
AIX files retain the same inode for the lifetime of a file. Saving and closing a watched file on AIX will result in two notifications (one for adding new content, and one for truncation).
Filename argument#
Providing filename
argument in the callback is only supported on Linux,
macOS, Windows, and AIX. Even on supported platforms, filename
is not always
guaranteed to be provided. Therefore, don't assume that filename
argument is
always provided in the callback, and have some fallback logic if it is null
.
import { watch } from 'node:fs';
watch('somedir', (eventType, filename) => {
console.log(`event type is: ${eventType}`);
if (filename) {
console.log(`filename provided: ${filename}`);
} else {
console.log('filename not provided');
}
});
fs.watchFile(filename[, options], listener)
#
filename
<string> | <Buffer> | <URL>options
<Object>listener
<Function>current
<fs.Stats>previous
<fs.Stats>
- Returns: <fs.StatWatcher>
Watch for changes on filename
. The callback listener
will be called each
time the file is accessed.
The options
argument may be omitted. If provided, it should be an object. The
options
object may contain a boolean named persistent
that indicates
whether the process should continue to run as long as files are being watched.
The options
object may specify an interval
property indicating how often the
target should be polled in milliseconds.
The listener
gets two arguments the current stat object and the previous
stat object:
import { watchFile } from 'node:fs';
watchFile('message.text', (curr, prev) => {
console.log(`the current mtime is: ${curr.mtime}`);
console.log(`the previous mtime was: ${prev.mtime}`);
});
These stat objects are instances of fs.Stat
. If the bigint
option is true
,
the numeric values in these objects are specified as BigInt
s.
To be notified when the file was modified, not just accessed, it is necessary
to compare curr.mtimeMs
and prev.mtimeMs
.
When an fs.watchFile
operation results in an ENOENT
error, it
will invoke the listener once, with all the fields zeroed (or, for dates, the
Unix Epoch). If the file is created later on, the listener will be called
again, with the latest stat objects. This is a change in functionality since
v0.10.
Using fs.watch()
is more efficient than fs.watchFile
and
fs.unwatchFile
. fs.watch
should be used instead of fs.watchFile
and
fs.unwatchFile
when possible.
When a file being watched by fs.watchFile()
disappears and reappears,
then the contents of previous
in the second callback event (the file's
reappearance) will be the same as the contents of previous
in the first
callback event (its disappearance).
This happens when:
- the file is deleted, followed by a restore
- the file is renamed and then renamed a second time back to its original name
fs.write(fd, buffer, offset[, length[, position]], callback)
#
fd
<integer>buffer
<Buffer> | <TypedArray> | <DataView>offset
<integer> Default:0
length
<integer> Default:buffer.byteLength - offset
position
<integer> | <null> Default:null
callback
<Function>err
<Error>bytesWritten
<integer>buffer
<Buffer> | <TypedArray> | <DataView>
Write buffer
to the file specified by fd
.
offset
determines the part of the buffer to be written, and length
is
an integer specifying the number of bytes to write.
position
refers to the offset from the beginning of the file where this data
should be written. If typeof position !== 'number'
, the data will be written
at the current position. See pwrite(2)
.
The callback will be given three arguments (err, bytesWritten, buffer)
where
bytesWritten
specifies how many bytes were written from buffer
.
If this method is invoked as its util.promisify()
ed version, it returns
a promise for an Object
with bytesWritten
and buffer
properties.
It is unsafe to use fs.write()
multiple times on the same file without waiting
for the callback. For this scenario, fs.createWriteStream()
is
recommended.
On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.
fs.write(fd, buffer[, options], callback)
#
fd
<integer>buffer
<Buffer> | <TypedArray> | <DataView>options
<Object>callback
<Function>err
<Error>bytesWritten
<integer>buffer
<Buffer> | <TypedArray> | <DataView>
Write buffer
to the file specified by fd
.
Similar to the above fs.write
function, this version takes an
optional options
object. If no options
object is specified, it will
default with the above values.
fs.write(fd, string[, position[, encoding]], callback)
#
fd
<integer>string
<string>position
<integer> | <null> Default:null
encoding
<string> Default:'utf8'
callback
<Function>
Write string
to the file specified by fd
. If string
is not a string,
an exception is thrown.
position
refers to the offset from the beginning of the file where this data
should be written. If typeof position !== 'number'
the data will be written at
the current position. See pwrite(2)
.
encoding
is the expected string encoding.
The callback will receive the arguments (err, written, string)
where written
specifies how many bytes the passed string required to be written. Bytes
written is not necessarily the same as string characters written. See
Buffer.byteLength
.
It is unsafe to use fs.write()
multiple times on the same file without waiting
for the callback. For this scenario, fs.createWriteStream()
is
recommended.
On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.
On Windows, if the file descriptor is connected to the console (e.g. fd == 1
or stdout
) a string containing non-ASCII characters will not be rendered
properly by default, regardless of the encoding used.
It is possible to configure the console to render UTF-8 properly by changing the
active codepage with the chcp 65001
command. See the chcp docs for more
details.
fs.writeFile(file, data[, options], callback)
#
file
<string> | <Buffer> | <URL> | <integer> filename or file descriptordata
<string> | <Buffer> | <TypedArray> | <DataView>options
<Object> | <string>encoding
<string> | <null> Default:'utf8'
mode
<integer> Default:0o666
flag
<string> See support of file systemflags
. Default:'w'
.signal
<AbortSignal> allows aborting an in-progress writeFile
callback
<Function>err
<Error> | <AggregateError>
When file
is a filename, asynchronously writes data to the file, replacing the
file if it already exists. data
can be a string or a buffer.
When file
is a file descriptor, the behavior is similar to calling
fs.write()
directly (which is recommended). See the notes below on using
a file descriptor.
The encoding
option is ignored if data
is a buffer.
The mode
option only affects the newly created file. See fs.open()
for more details.
import { writeFile } from 'node:fs';
import { Buffer } from 'node:buffer';
const data = new Uint8Array(Buffer.from('Hello Node.js'));
writeFile('message.txt', data, (err) => {
if (err) throw err;
console.log('The file has been saved!');
});
If options
is a string, then it specifies the encoding:
import { writeFile } from 'node:fs';
writeFile('message.txt', 'Hello Node.js', 'utf8', callback);
It is unsafe to use fs.writeFile()
multiple times on the same file without
waiting for the callback. For this scenario, fs.createWriteStream()
is
recommended.
Similarly to fs.readFile
- fs.writeFile
is a convenience method that
performs multiple write
calls internally to write the buffer passed to it.
For performance sensitive code consider using fs.createWriteStream()
.
It is possible to use an <AbortSignal> to cancel an fs.writeFile()
.
Cancelation is "best effort", and some amount of data is likely still
to be written.
import { writeFile } from 'node:fs';
import { Buffer } from 'node:buffer';
const controller = new AbortController();
const { signal } = controller;
const data = new Uint8Array(Buffer.from('Hello Node.js'));
writeFile('message.txt', data, { signal }, (err) => {
// When a request is aborted - the callback is called with an AbortError
});
// When the request should be aborted
controller.abort();
Aborting an ongoing request does not abort individual operating
system requests but rather the internal buffering fs.writeFile
performs.
Using fs.writeFile()
with file descriptors#
When file
is a file descriptor, the behavior is almost identical to directly
calling fs.write()
like:
import { write } from 'node:fs';
import { Buffer } from 'node:buffer';
write(fd, Buffer.from(data, options.encoding), callback);
The difference from directly calling fs.write()
is that under some unusual
conditions, fs.write()
might write only part of the buffer and need to be
retried to write the remaining data, whereas fs.writeFile()
retries until
the data is entirely written (or an error occurs).
The implications of this are a common source of confusion. In the file descriptor case, the file is not replaced! The data is not necessarily written to the beginning of the file, and the file's original data may remain before and/or after the newly written data.
For example, if fs.writeFile()
is called twice in a row, first to write the
string 'Hello'
, then to write the string ', World'
, the file would contain
'Hello, World'
, and might contain some of the file's original data (depending
on the size of the original file, and the position of the file descriptor). If
a file name had been used instead of a descriptor, the file would be guaranteed
to contain only ', World'
.
fs.writev(fd, buffers[, position], callback)
#
fd
<integer>buffers
<ArrayBufferView[]>position
<integer> | <null> Default:null
callback
<Function>err
<Error>bytesWritten
<integer>buffers
<ArrayBufferView[]>
Write an array of ArrayBufferView
s to the file specified by fd
using
writev()
.
position
is the offset from the beginning of the file where this data
should be written. If typeof position !== 'number'
, the data will be written
at the current position.
The callback will be given three arguments: err
, bytesWritten
, and
buffers
. bytesWritten
is how many bytes were written from buffers
.
If this method is util.promisify()
ed, it returns a promise for an
Object
with bytesWritten
and buffers
properties.
It is unsafe to use fs.writev()
multiple times on the same file without
waiting for the callback. For this scenario, use fs.createWriteStream()
.
On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.
Synchronous API#
The synchronous APIs perform all operations synchronously, blocking the event loop until the operation completes or fails.
fs.accessSync(path[, mode])
#
Synchronously tests a user's permissions for the file or directory specified
by path
. The mode
argument is an optional integer that specifies the
accessibility checks to be performed. mode
should be either the value
fs.constants.F_OK
or a mask consisting of the bitwise OR of any of
fs.constants.R_OK
, fs.constants.W_OK
, and fs.constants.X_OK
(e.g.
fs.constants.W_OK | fs.constants.R_OK
). Check File access constants for
possible values of mode
.
If any of the accessibility checks fail, an Error
will be thrown. Otherwise,
the method will return undefined
.
import { accessSync, constants } from 'node:fs';
try {
accessSync('etc/passwd', constants.R_OK | constants.W_OK);
console.log('can read/write');
} catch (err) {
console.error('no access!');
}
fs.appendFileSync(path, data[, options])
#
path
<string> | <Buffer> | <URL> | <number> filename or file descriptordata
<string> | <Buffer>options
<Object> | <string>encoding
<string> | <null> Default:'utf8'
mode
<integer> Default:0o666
flag
<string> See support of file systemflags
. Default:'a'
.
Synchronously append data to a file, creating the file if it does not yet
exist. data
can be a string or a <Buffer>.
The mode
option only affects the newly created file. See fs.open()
for more details.
import { appendFileSync } from 'node:fs';
try {
appendFileSync('message.txt', 'data to append');
console.log('The "data to append" was appended to file!');
} catch (err) {
/* Handle the error */
}
If options
is a string, then it specifies the encoding:
import { appendFileSync } from 'node:fs';
appendFileSync('message.txt', 'data to append', 'utf8');
The path
may be specified as a numeric file descriptor that has been opened
for appending (using fs.open()
or fs.openSync()
). The file descriptor will
not be closed automatically.
import { openSync, closeSync, appendFileSync } from 'node:fs';
let fd;
try {
fd = openSync('message.txt', 'a');
appendFileSync(fd, 'data to append', 'utf8');
} catch (err) {
/* Handle the error */
} finally {
if (fd !== undefined)
closeSync(fd);
}
fs.chmodSync(path, mode)
#
For detailed information, see the documentation of the asynchronous version of
this API: fs.chmod()
.
See the POSIX chmod(2)
documentation for more detail.
fs.chownSync(path, uid, gid)
#
Synchronously changes owner and group of a file. Returns undefined
.
This is the synchronous version of fs.chown()
.
See the POSIX chown(2)
documentation for more detail.
fs.closeSync(fd)
#
fd
<integer>
Closes the file descriptor. Returns undefined
.
Calling fs.closeSync()
on any file descriptor (fd
) that is currently in use
through any other fs
operation may lead to undefined behavior.
See the POSIX close(2)
documentation for more detail.
fs.copyFileSync(src, dest[, mode])
#
src
<string> | <Buffer> | <URL> source filename to copydest
<string> | <Buffer> | <URL> destination filename of the copy operationmode
<integer> modifiers for copy operation. Default:0
.
Synchronously copies src
to dest
. By default, dest
is overwritten if it
already exists. Returns undefined
. Node.js makes no guarantees about the
atomicity of the copy operation. If an error occurs after the destination file
has been opened for writing, Node.js will attempt to remove the destination.
mode
is an optional integer that specifies the behavior
of the copy operation. It is possible to create a mask consisting of the bitwise
OR of two or more values (e.g.
fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE
).
fs.constants.COPYFILE_EXCL
: The copy operation will fail ifdest
already exists.fs.constants.COPYFILE_FICLONE
: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then a fallback copy mechanism is used.fs.constants.COPYFILE_FICLONE_FORCE
: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then the operation will fail.
import { copyFileSync, constants } from 'node:fs';
// destination.txt will be created or overwritten by default.
copyFileSync('source.txt', 'destination.txt');
console.log('source.txt was copied to destination.txt');
// By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
copyFileSync('source.txt', 'destination.txt', constants.COPYFILE_EXCL);
fs.cpSync(src, dest[, options])
#
src
<string> | <URL> source path to copy.dest
<string> | <URL> destination path to copy to.options
<Object>dereference
<boolean> dereference symlinks. Default:false
.errorOnExist
<boolean> whenforce
isfalse
, and the destination exists, throw an error. Default:false
.filter
<Function> Function to filter copied files/directories. Returntrue
to copy the item,false
to ignore it. Default:undefined
force
<boolean> overwrite existing file or directory. The copy operation will ignore errors if you set this to false and the destination exists. Use theerrorOnExist
option to change this behavior. Default:true
.preserveTimestamps
<boolean> Whentrue
timestamps fromsrc
will be preserved. Default:false
.recursive
<boolean> copy directories recursively Default:false
verbatimSymlinks
<boolean> Whentrue
, path resolution for symlinks will be skipped. Default:false
Synchronously copies the entire directory structure from src
to dest
,
including subdirectories and files.
When copying a directory to another directory, globs are not supported and
behavior is similar to cp dir1/ dir2/
.
fs.existsSync(path)
#
Returns true
if the path exists, false
otherwise.
For detailed information, see the documentation of the asynchronous version of
this API: fs.exists()
.
fs.exists()
is deprecated, but fs.existsSync()
is not. The callback
parameter to fs.exists()
accepts parameters that are inconsistent with other
Node.js callbacks. fs.existsSync()
does not use a callback.
import { existsSync } from 'node:fs';
if (existsSync('/etc/passwd'))
console.log('The path exists.');
fs.fchmodSync(fd, mode)
#
Sets the permissions on the file. Returns undefined
.
See the POSIX fchmod(2)
documentation for more detail.
fs.fchownSync(fd, uid, gid)
#
fd
<integer>uid
<integer> The file's new owner's user id.gid
<integer> The file's new group's group id.
Sets the owner of the file. Returns undefined
.
See the POSIX fchown(2)
documentation for more detail.
fs.fdatasyncSync(fd)
#
fd
<integer>
Forces all currently queued I/O operations associated with the file to the
operating system's synchronized I/O completion state. Refer to the POSIX
fdatasync(2)
documentation for details. Returns undefined
.
fs.fstatSync(fd[, options])
#
fd
<integer>options
<Object>bigint
<boolean> Whether the numeric values in the returned <fs.Stats> object should bebigint
. Default:false
.
- Returns: <fs.Stats>
Retrieves the <fs.Stats> for the file descriptor.
See the POSIX fstat(2)
documentation for more detail.
fs.fsyncSync(fd)
#
fd
<integer>
Request that all data for the open file descriptor is flushed to the storage
device. The specific implementation is operating system and device specific.
Refer to the POSIX fsync(2)
documentation for more detail. Returns undefined
.
fs.ftruncateSync(fd[, len])
#
Truncates the file descriptor. Returns undefined
.
For detailed information, see the documentation of the asynchronous version of
this API: fs.ftruncate()
.
fs.futimesSync(fd, atime, mtime)
#
Synchronous version of fs.futimes()
. Returns undefined
.
fs.lchmodSync(path, mode)
#
Changes the permissions on a symbolic link. Returns undefined
.
This method is only implemented on macOS.
See the POSIX lchmod(2)
documentation for more detail.
fs.lchownSync(path, uid, gid)
#
path
<string> | <Buffer> | <URL>uid
<integer> The file's new owner's user id.gid
<integer> The file's new group's group id.
Set the owner for the path. Returns undefined
.
See the POSIX lchown(2)
documentation for more details.
fs.lutimesSync(path, atime, mtime)
#
path
<string> | <Buffer> | <URL>atime
<number> | <string> | <Date>mtime
<number> | <string> | <Date>
Change the file system timestamps of the symbolic link referenced by path
.
Returns undefined
, or throws an exception when parameters are incorrect or
the operation fails. This is the synchronous version of fs.lutimes()
.
fs.linkSync(existingPath, newPath)
#
Creates a new link from the existingPath
to the newPath
. See the POSIX
link(2)
documentation for more detail. Returns undefined
.
fs.lstatSync(path[, options])
#
path
<string> | <Buffer> | <URL>options
<Object>bigint
<boolean> Whether the numeric values in the returned <fs.Stats> object should bebigint
. Default:false
.throwIfNoEntry
<boolean> Whether an exception will be thrown if no file system entry exists, rather than returningundefined
. Default:true
.
- Returns: <fs.Stats>
Retrieves the <fs.Stats> for the symbolic link referred to by path
.
See the POSIX lstat(2)
documentation for more details.
fs.mkdirSync(path[, options])
#
Synchronously creates a directory. Returns undefined
, or if recursive
is
true
, the first directory path created.
This is the synchronous version of fs.mkdir()
.
See the POSIX mkdir(2)
documentation for more details.
fs.mkdtempSync(prefix[, options])
#
Returns the created directory path.
For detailed information, see the documentation of the asynchronous version of
this API: fs.mkdtemp()
.
The optional options
argument can be a string specifying an encoding, or an
object with an encoding
property specifying the character encoding to use.
fs.opendirSync(path[, options])
#
Synchronously open a directory. See opendir(3)
.
Creates an <fs.Dir>, which contains all further functions for reading from and cleaning up the directory.
The encoding
option sets the encoding for the path
while opening the
directory and subsequent read operations.
fs.openSync(path[, flags[, mode]])
#
path
<string> | <Buffer> | <URL>flags
<string> | <number> Default:'r'
. See support of file systemflags
.mode
<string> | <integer> Default:0o666
- Returns: <number>
Returns an integer representing the file descriptor.
For detailed information, see the documentation of the asynchronous version of
this API: fs.open()
.
fs.readdirSync(path[, options])
#
path
<string> | <Buffer> | <URL>options
<string> | <Object>- Returns: <string[]> | <Buffer[]> | <fs.Dirent[]>
Reads the contents of the directory.
See the POSIX readdir(3)
documentation for more details.
The optional options
argument can be a string specifying an encoding, or an
object with an encoding
property specifying the character encoding to use for
the filenames returned. If the encoding
is set to 'buffer'
,
the filenames returned will be passed as <Buffer> objects.
If options.withFileTypes
is set to true
, the result will contain
<fs.Dirent> objects.
fs.readFileSync(path[, options])
#
path
<string> | <Buffer> | <URL> | <integer> filename or file descriptoroptions
<Object> | <string>encoding
<string> | <null> Default:null
flag
<string> See support of file systemflags
. Default:'r'
.
- Returns: <string> | <Buffer>
Returns the contents of the path
.
For detailed information, see the documentation of the asynchronous version of
this API: fs.readFile()
.
If the encoding
option is specified then this function returns a
string. Otherwise it returns a buffer.
Similar to fs.readFile()
, when the path is a directory, the behavior of
fs.readFileSync()
is platform-specific.
import { readFileSync } from 'node:fs';
// macOS, Linux, and Windows
readFileSync('<directory>');
// => [Error: EISDIR: illegal operation on a directory, read <directory>]
// FreeBSD
readFileSync('<directory>'); // => <data>
fs.readlinkSync(path[, options])
#
path
<string> | <Buffer> | <URL>options
<string> | <Object>encoding
<string> Default:'utf8'
- Returns: <string> | <Buffer>
Returns the symbolic link's string value.
See the POSIX readlink(2)
documentation for more details.
The optional options
argument can be a string specifying an encoding, or an
object with an encoding
property specifying the character encoding to use for
the link path returned. If the encoding
is set to 'buffer'
,
the link path returned will be passed as a <Buffer> object.
fs.readSync(fd, buffer, offset, length[, position])
#
fd
<integer>buffer
<Buffer> | <TypedArray> | <DataView>offset
<integer>length
<integer>position
<integer> | <bigint> | <null> Default:null
- Returns: <number>
Returns the number of bytesRead
.
For detailed information, see the documentation of the asynchronous version of
this API: fs.read()
.
fs.readSync(fd, buffer[, options])
#
fd
<integer>buffer
<Buffer> | <TypedArray> | <DataView>options
<Object>- Returns: <number>
Returns the number of bytesRead
.
Similar to the above fs.readSync
function, this version takes an optional options
object.
If no options
object is specified, it will default with the above values.
For detailed information, see the documentation of the asynchronous version of
this API: fs.read()
.
fs.readvSync(fd, buffers[, position])
#
fd
<integer>buffers
<ArrayBufferView[]>position
<integer> | <null> Default:null
- Returns: <number> The number of bytes read.
For detailed information, see the documentation of the asynchronous version of
this API: fs.readv()
.
fs.realpathSync(path[, options])
#
path
<string> | <Buffer> | <URL>options
<string> | <Object>encoding
<string> Default:'utf8'
- Returns: <string> | <Buffer>
Returns the resolved pathname.
For detailed information, see the documentation of the asynchronous version of
this API: fs.realpath()
.
fs.realpathSync.native(path[, options])
#
path
<string> | <Buffer> | <URL>options
<string> | <Object>encoding
<string> Default:'utf8'
- Returns: <string> | <Buffer>
Synchronous realpath(3)
.
Only paths that can be converted to UTF8 strings are supported.
The optional options
argument can be a string specifying an encoding, or an
object with an encoding
property specifying the character encoding to use for
the path returned. If the encoding
is set to 'buffer'
,
the path returned will be passed as a <Buffer> object.
On Linux, when Node.js is linked against musl libc, the procfs file system must
be mounted on /proc
in order for this function to work. Glibc does not have
this restriction.
fs.renameSync(oldPath, newPath)
#
Renames the file from oldPath
to newPath
. Returns undefined
.
See the POSIX rename(2)
documentation for more details.
fs.rmdirSync(path[, options])
#
path
<string> | <Buffer> | <URL>options
<Object>maxRetries
<integer> If anEBUSY
,EMFILE
,ENFILE
,ENOTEMPTY
, orEPERM
error is encountered, Node.js retries the operation with a linear backoff wait ofretryDelay
milliseconds longer on each try. This option represents the number of retries. This option is ignored if therecursive
option is nottrue
. Default:0
.recursive
<boolean> Iftrue
, perform a recursive directory removal. In recursive mode, operations are retried on failure. Default:false
. Deprecated.retryDelay
<integer> The amount of time in milliseconds to wait between retries. This option is ignored if therecursive
option is nottrue
. Default:100
.
Synchronous rmdir(2)
. Returns undefined
.
Using fs.rmdirSync()
on a file (not a directory) results in an ENOENT
error
on Windows and an ENOTDIR
error on POSIX.
To get a behavior similar to the rm -rf
Unix command, use fs.rmSync()
with options { recursive: true, force: true }
.
fs.rmSync(path[, options])
#
path
<string> | <Buffer> | <URL>options
<Object>force
<boolean> Whentrue
, exceptions will be ignored ifpath
does not exist. Default:false
.maxRetries
<integer> If anEBUSY
,EMFILE
,ENFILE
,ENOTEMPTY
, orEPERM
error is encountered, Node.js will retry the operation with a linear backoff wait ofretryDelay
milliseconds longer on each try. This option represents the number of retries. This option is ignored if therecursive
option is nottrue
. Default:0
.recursive
<boolean> Iftrue
, perform a recursive directory removal. In recursive mode operations are retried on failure. Default:false
.retryDelay
<integer> The amount of time in milliseconds to wait between retries. This option is ignored if therecursive
option is nottrue
. Default:100
.
Synchronously removes files and directories (modeled on the standard POSIX rm
utility). Returns undefined
.
fs.statSync(path[, options])
#
path
<string> | <Buffer> | <URL>options
<Object>bigint
<boolean> Whether the numeric values in the returned <fs.Stats> object should bebigint
. Default:false
.throwIfNoEntry
<boolean> Whether an exception will be thrown if no file system entry exists, rather than returningundefined
. Default:true
.
- Returns: <fs.Stats>
Retrieves the <fs.Stats> for the path.
fs.statfsSync(path[, options])
#
path
<string> | <Buffer> | <URL>options
<Object>bigint
<boolean> Whether the numeric values in the returned <fs.StatFs> object should bebigint
. Default:false
.
- Returns: <fs.StatFs>
Synchronous statfs(2)
. Returns information about the mounted file system which
contains path
.
In case of an error, the err.code
will be one of Common System Errors.
fs.symlinkSync(target, path[, type])
#
target
<string> | <Buffer> | <URL>path
<string> | <Buffer> | <URL>type
<string> | <null> Default:null
Returns undefined
.
For detailed information, see the documentation of the asynchronous version of
this API: fs.symlink()
.
fs.truncateSync(path[, len])
#
Truncates the file. Returns undefined
. A file descriptor can also be
passed as the first argument. In this case, fs.ftruncateSync()
is called.
Passing a file descriptor is deprecated and may result in an error being thrown in the future.
fs.unlinkSync(path)
#
Synchronous unlink(2)
. Returns undefined
.
fs.utimesSync(path, atime, mtime)
#
path
<string> | <Buffer> | <URL>atime
<number> | <string> | <Date>mtime
<number> | <string> | <Date>
Returns undefined
.
For detailed information, see the documentation of the asynchronous version of
this API: fs.utimes()
.
fs.writeFileSync(file, data[, options])
#
file
<string> | <Buffer> | <URL> | <integer> filename or file descriptordata
<string> | <Buffer> | <TypedArray> | <DataView>options
<Object> | <string>encoding
<string> | <null> Default:'utf8'
mode
<integer> Default:0o666
flag
<string> See support of file systemflags
. Default:'w'
.
Returns undefined
.
The mode
option only affects the newly created file. See fs.open()
for more details.
For detailed information, see the documentation of the asynchronous version of
this API: fs.writeFile()
.
fs.writeSync(fd, buffer, offset[, length[, position]])
#
fd
<integer>buffer
<Buffer> | <TypedArray> | <DataView>offset
<integer> Default:0
length
<integer> Default:buffer.byteLength - offset
position
<integer> | <null> Default:null
- Returns: <number> The number of bytes written.
For detailed information, see the documentation of the asynchronous version of
this API: fs.write(fd, buffer...)
.
fs.writeSync(fd, buffer[, options])
#
fd
<integer>buffer
<Buffer> | <TypedArray> | <DataView>options
<Object>- Returns: <number> The number of bytes written.
For detailed information, see the documentation of the asynchronous version of
this API: fs.write(fd, buffer...)
.
fs.writeSync(fd, string[, position[, encoding]])
#
fd
<integer>string
<string>position
<integer> | <null> Default:null
encoding
<string> Default:'utf8'
- Returns: <number> The number of bytes written.
For detailed information, see the documentation of the asynchronous version of
this API: fs.write(fd, string...)
.
fs.writevSync(fd, buffers[, position])
#
fd
<integer>buffers
<ArrayBufferView[]>position
<integer> | <null> Default:null
- Returns: <number> The number of bytes written.
For detailed information, see the documentation of the asynchronous version of
this API: fs.writev()
.
Common Objects#
The common objects are shared by all of the file system API variants (promise, callback, and synchronous).
Class: fs.Dir
#
A class representing a directory stream.
Created by fs.opendir()
, fs.opendirSync()
, or
fsPromises.opendir()
.
import { opendir } from 'node:fs/promises';
try {
const dir = await opendir('./');
for await (const dirent of dir)
console.log(dirent.name);
} catch (err) {
console.error(err);
}
When using the async iterator, the <fs.Dir> object will be automatically closed after the iterator exits.
dir.close()
#
- Returns: <Promise>
Asynchronously close the directory's underlying resource handle. Subsequent reads will result in errors.
A promise is returned that will be resolved after the resource has been closed.
dir.close(callback)
#
callback
<Function>err
<Error>
Asynchronously close the directory's underlying resource handle. Subsequent reads will result in errors.
The callback
will be called after the resource handle has been closed.
dir.closeSync()
#
Synchronously close the directory's underlying resource handle. Subsequent reads will result in errors.
dir.path
#
The read-only path of this directory as was provided to fs.opendir()
,
fs.opendirSync()
, or fsPromises.opendir()
.
dir.read()
#
- Returns: <Promise> containing <fs.Dirent> | <null>
Asynchronously read the next directory entry via readdir(3)
as an
<fs.Dirent>.
A promise is returned that will be resolved with an <fs.Dirent>, or null
if there are no more directory entries to read.
Directory entries returned by this function are in no particular order as provided by the operating system's underlying directory mechanisms. Entries added or removed while iterating over the directory might not be included in the iteration results.
dir.read(callback)
#
callback
<Function>err
<Error>dirent
<fs.Dirent> | <null>
Asynchronously read the next directory entry via readdir(3)
as an
<fs.Dirent>.
After the read is completed, the callback
will be called with an
<fs.Dirent>, or null
if there are no more directory entries to read.
Directory entries returned by this function are in no particular order as provided by the operating system's underlying directory mechanisms. Entries added or removed while iterating over the directory might not be included in the iteration results.
dir.readSync()
#
- Returns: <fs.Dirent> | <null>
Synchronously read the next directory entry as an <fs.Dirent>. See the
POSIX readdir(3)
documentation for more detail.
If there are no more directory entries to read, null
will be returned.
Directory entries returned by this function are in no particular order as provided by the operating system's underlying directory mechanisms. Entries added or removed while iterating over the directory might not be included in the iteration results.
dir[Symbol.asyncIterator]()
#
- Returns: <AsyncIterator> of <fs.Dirent>
Asynchronously iterates over the directory until all entries have
been read. Refer to the POSIX readdir(3)
documentation for more detail.
Entries returned by the async iterator are always an <fs.Dirent>.
The null
case from dir.read()
is handled internally.
See <fs.Dir> for an example.
Directory entries returned by this iterator are in no particular order as provided by the operating system's underlying directory mechanisms. Entries added or removed while iterating over the directory might not be included in the iteration results.
Class: fs.Dirent
#
A representation of a directory entry, which can be a file or a subdirectory within the directory, as returned by reading from an <fs.Dir>. The directory entry is a combination of the file name and file type pairs.
Additionally, when fs.readdir()
or fs.readdirSync()
is called with
the withFileTypes
option set to true
, the resulting array is filled with
<fs.Dirent> objects, rather than strings or <Buffer>s.
dirent.isBlockDevice()
#
- Returns: <boolean>
Returns true
if the <fs.Dirent> object describes a block device.
dirent.isCharacterDevice()
#
- Returns: <boolean>
Returns true
if the <fs.Dirent> object describes a character device.
dirent.isDirectory()
#
- Returns: <boolean>
Returns true
if the <fs.Dirent> object describes a file system
directory.
dirent.isFIFO()
#
- Returns: <boolean>
Returns true
if the <fs.Dirent> object describes a first-in-first-out
(FIFO) pipe.
dirent.isFile()
#
- Returns: <boolean>
Returns true
if the <fs.Dirent> object describes a regular file.
dirent.isSocket()
#
- Returns: <boolean>
Returns true
if the <fs.Dirent> object describes a socket.
dirent.isSymbolicLink()
#
- Returns: <boolean>
Returns true
if the <fs.Dirent> object describes a symbolic link.
dirent.name
#
The file name that this <fs.Dirent> object refers to. The type of this
value is determined by the options.encoding
passed to fs.readdir()
or
fs.readdirSync()
.
Class: fs.FSWatcher
#
- Extends <EventEmitter>
A successful call to fs.watch()
method will return a new <fs.FSWatcher>
object.
All <fs.FSWatcher> objects emit a 'change'
event whenever a specific watched
file is modified.
Event: 'change'
#
eventType
<string> The type of change event that has occurredfilename
<string> | <Buffer> The filename that changed (if relevant/available)
Emitted when something changes in a watched directory or file.
See more details in fs.watch()
.
The filename
argument may not be provided depending on operating system
support. If filename
is provided, it will be provided as a <Buffer> if
fs.watch()
is called with its encoding
option set to 'buffer'
, otherwise
filename
will be a UTF-8 string.
import { watch } from 'node:fs';
// Example when handled through fs.watch() listener
watch('./tmp', { encoding: 'buffer' }, (eventType, filename) => {
if (filename) {
console.log(filename);
// Prints: <Buffer ...>
}
});
Event: 'close'
#
Emitted when the watcher stops watching for changes. The closed <fs.FSWatcher> object is no longer usable in the event handler.
Event: 'error'
#
error
<Error>
Emitted when an error occurs while watching the file. The errored <fs.FSWatcher> object is no longer usable in the event handler.
watcher.close()
#
Stop watching for changes on the given <fs.FSWatcher>. Once stopped, the <fs.FSWatcher> object is no longer usable.
watcher.ref()
#
- Returns: <fs.FSWatcher>
When called, requests that the Node.js event loop not exit so long as the
<fs.FSWatcher> is active. Calling watcher.ref()
multiple times will have
no effect.
By default, all <fs.FSWatcher> objects are "ref'ed", making it normally
unnecessary to call watcher.ref()
unless watcher.unref()
had been
called previously.
watcher.unref()
#
- Returns: <fs.FSWatcher>
When called, the active <fs.FSWatcher> object will not require the Node.js
event loop to remain active. If there is no other activity keeping the
event loop running, the process may exit before the <fs.FSWatcher> object's
callback is invoked. Calling watcher.unref()
multiple times will have
no effect.
Class: fs.StatWatcher
#
- Extends <EventEmitter>
A successful call to fs.watchFile()
method will return a new <fs.StatWatcher>
object.
watcher.ref()
#
- Returns: <fs.StatWatcher>
When called, requests that the Node.js event loop not exit so long as the
<fs.StatWatcher> is active. Calling watcher.ref()
multiple times will have
no effect.
By default, all <fs.StatWatcher> objects are "ref'ed", making it normally
unnecessary to call watcher.ref()
unless watcher.unref()
had been
called previously.
watcher.unref()
#
- Returns: <fs.StatWatcher>
When called, the active <fs.StatWatcher> object will not require the Node.js
event loop to remain active. If there is no other activity keeping the
event loop running, the process may exit before the <fs.StatWatcher> object's
callback is invoked. Calling watcher.unref()
multiple times will have
no effect.
Class: fs.ReadStream
#
- Extends: <stream.Readable>
Instances of <fs.ReadStream> are created and returned using the
fs.createReadStream()
function.
Event: 'close'
#
Emitted when the <fs.ReadStream>'s underlying file descriptor has been closed.
Event: 'open'
#
fd
<integer> Integer file descriptor used by the <fs.ReadStream>.
Emitted when the <fs.ReadStream>'s file descriptor has been opened.
Event: 'ready'
#
Emitted when the <fs.ReadStream> is ready to be used.
Fires immediately after 'open'
.
readStream.bytesRead
#
The number of bytes that have been read so far.
readStream.path
#
The path to the file the stream is reading from as specified in the first
argument to fs.createReadStream()
. If path
is passed as a string, then
readStream.path
will be a string. If path
is passed as a <Buffer>, then
readStream.path
will be a <Buffer>. If fd
is specified, then
readStream.path
will be undefined
.
readStream.pending
#
This property is true
if the underlying file has not been opened yet,
i.e. before the 'ready'
event is emitted.
Class: fs.Stats
#
A <fs.Stats> object provides information about a file.
Objects returned from fs.stat()
, fs.lstat()
, fs.fstat()
, and
their synchronous counterparts are of this type.
If bigint
in the options
passed to those methods is true, the numeric values
will be bigint
instead of number
, and the object will contain additional
nanosecond-precision properties suffixed with Ns
.
Stats {
dev: 2114,
ino: 48064969,
mode: 33188,
nlink: 1,
uid: 85,
gid: 100,
rdev: 0,
size: 527,
blksize: 4096,
blocks: 8,
atimeMs: 1318289051000.1,
mtimeMs: 1318289051000.1,
ctimeMs: 1318289051000.1,
birthtimeMs: 1318289051000.1,
atime: Mon, 10 Oct 2011 23:24:11 GMT,
mtime: Mon, 10 Oct 2011 23:24:11 GMT,
ctime: Mon, 10 Oct 2011 23:24:11 GMT,
birthtime: Mon, 10 Oct 2011 23:24:11 GMT }
bigint
version:
BigIntStats {
dev: 2114n,
ino: 48064969n,
mode: 33188n,
nlink: 1n,
uid: 85n,
gid: 100n,
rdev: 0n,
size: 527n,
blksize: 4096n,
blocks: 8n,
atimeMs: 1318289051000n,
mtimeMs: 1318289051000n,
ctimeMs: 1318289051000n,
birthtimeMs: 1318289051000n,
atimeNs: 1318289051000000000n,
mtimeNs: 1318289051000000000n,
ctimeNs: 1318289051000000000n,
birthtimeNs: 1318289051000000000n,
atime: Mon, 10 Oct 2011 23:24:11 GMT,
mtime: Mon, 10 Oct 2011 23:24:11 GMT,
ctime: Mon, 10 Oct 2011 23:24:11 GMT,
birthtime: Mon, 10 Oct 2011 23:24:11 GMT }
stats.isBlockDevice()
#
- Returns: <boolean>
Returns true
if the <fs.Stats> object describes a block device.
stats.isCharacterDevice()
#
- Returns: <boolean>
Returns true
if the <fs.Stats> object describes a character device.
stats.isDirectory()
#
- Returns: <boolean>
Returns true
if the <fs.Stats> object describes a file system directory.
If the <fs.Stats> object was obtained from fs.lstat()
, this method will
always return false
. This is because fs.lstat()
returns information
about a symbolic link itself and not the path it resolves to.
stats.isFIFO()
#
- Returns: <boolean>
Returns true
if the <fs.Stats> object describes a first-in-first-out (FIFO)
pipe.
stats.isFile()
#
- Returns: <boolean>
Returns true
if the <fs.Stats> object describes a regular file.
stats.isSocket()
#
- Returns: <boolean>
Returns true
if the <fs.Stats> object describes a socket.
stats.isSymbolicLink()
#
- Returns: <boolean>
Returns true
if the <fs.Stats> object describes a symbolic link.
This method is only valid when using fs.lstat()
.
stats.dev
#
The numeric identifier of the device containing the file.
stats.ino
#
The file system specific "Inode" number for the file.
stats.mode
#
A bit-field describing the file type and mode.
stats.nlink
#
The number of hard-links that exist for the file.
stats.uid
#
The numeric user identifier of the user that owns the file (POSIX).
stats.gid
#
The numeric group identifier of the group that owns the file (POSIX).
stats.rdev
#
A numeric device identifier if the file represents a device.
stats.size
#
The size of the file in bytes.
If the underlying file system does not support getting the size of the file,
this will be 0
.
stats.blksize
#
The file system block size for i/o operations.
stats.blocks
#
The number of blocks allocated for this file.
stats.atimeMs
#
The timestamp indicating the last time this file was accessed expressed in milliseconds since the POSIX Epoch.
stats.mtimeMs
#
The timestamp indicating the last time this file was modified expressed in milliseconds since the POSIX Epoch.
stats.ctimeMs
#
The timestamp indicating the last time the file status was changed expressed in milliseconds since the POSIX Epoch.
stats.birthtimeMs
#
The timestamp indicating the creation time of this file expressed in milliseconds since the POSIX Epoch.
stats.atimeNs
#
Only present when bigint: true
is passed into the method that generates
the object.
The timestamp indicating the last time this file was accessed expressed in
nanoseconds since the POSIX Epoch.
stats.mtimeNs
#
Only present when bigint: true
is passed into the method that generates
the object.
The timestamp indicating the last time this file was modified expressed in
nanoseconds since the POSIX Epoch.
stats.ctimeNs
#
Only present when bigint: true
is passed into the method that generates
the object.
The timestamp indicating the last time the file status was changed expressed
in nanoseconds since the POSIX Epoch.
stats.birthtimeNs
#
Only present when bigint: true
is passed into the method that generates
the object.
The timestamp indicating the creation time of this file expressed in
nanoseconds since the POSIX Epoch.
stats.atime
#
The timestamp indicating the last time this file was accessed.
stats.mtime
#
The timestamp indicating the last time this file was modified.
stats.ctime
#
The timestamp indicating the last time the file status was changed.
stats.birthtime
#
The timestamp indicating the creation time of this file.
Stat time values#
The atimeMs
, mtimeMs
, ctimeMs
, birthtimeMs
properties are
numeric values that hold the corresponding times in milliseconds. Their
precision is platform specific. When bigint: true
is passed into the
method that generates the object, the properties will be bigints,
otherwise they will be numbers.
The atimeNs
, mtimeNs
, ctimeNs
, birthtimeNs
properties are
bigints that hold the corresponding times in nanoseconds. They are
only present when bigint: true
is passed into the method that generates
the object. Their precision is platform specific.
atime
, mtime
, ctime
, and birthtime
are
Date
object alternate representations of the various times. The
Date
and number values are not connected. Assigning a new number value, or
mutating the Date
value, will not be reflected in the corresponding alternate
representation.
The times in the stat object have the following semantics:
atime
"Access Time": Time when file data last accessed. Changed by themknod(2)
,utimes(2)
, andread(2)
system calls.mtime
"Modified Time": Time when file data last modified. Changed by themknod(2)
,utimes(2)
, andwrite(2)
system calls.ctime
"Change Time": Time when file status was last changed (inode data modification). Changed by thechmod(2)
,chown(2)
,link(2)
,mknod(2)
,rename(2)
,unlink(2)
,utimes(2)
,read(2)
, andwrite(2)
system calls.birthtime
"Birth Time": Time of file creation. Set once when the file is created. On file systems where birthtime is not available, this field may instead hold either thectime
or1970-01-01T00:00Z
(ie, Unix epoch timestamp0
). This value may be greater thanatime
ormtime
in this case. On Darwin and other FreeBSD variants, also set if theatime
is explicitly set to an earlier value than the currentbirthtime
using theutimes(2)
system call.
Prior to Node.js 0.12, the ctime
held the birthtime
on Windows systems. As
of 0.12, ctime
is not "creation time", and on Unix systems, it never was.
Class: fs.StatFs
#
Provides information about a mounted file system.
Objects returned from fs.statfs()
and its synchronous counterpart are of
this type. If bigint
in the options
passed to those methods is true
, the
numeric values will be bigint
instead of number
.
StatFs {
type: 1397114950,
bsize: 4096,
blocks: 121938943,
bfree: 61058895,
bavail: 61058895,
files: 999,
ffree: 1000000
}
bigint
version:
StatFs {
type: 1397114950n,
bsize: 4096n,
blocks: 121938943n,
bfree: 61058895n,
bavail: 61058895n,
files: 999n,
ffree: 1000000n
}
statfs.bavail
#
Free blocks available to unprivileged users.
statfs.bfree
#
Free blocks in file system.
statfs.blocks
#
Total data blocks in file system.
statfs.bsize
#
Optimal transfer block size.
statfs.ffree
#
Free file nodes in file system.
statfs.files
#
Total file nodes in file system.
statfs.type
#
Type of file system.
Class: fs.WriteStream
#
- Extends <stream.Writable>
Instances of <fs.WriteStream> are created and returned using the
fs.createWriteStream()
function.
Event: 'close'
#
Emitted when the <fs.WriteStream>'s underlying file descriptor has been closed.
Event: 'open'
#
fd
<integer> Integer file descriptor used by the <fs.WriteStream>.
Emitted when the <fs.WriteStream>'s file is opened.
Event: 'ready'
#
Emitted when the <fs.WriteStream> is ready to be used.
Fires immediately after 'open'
.
writeStream.bytesWritten
#
The number of bytes written so far. Does not include data that is still queued for writing.
writeStream.close([callback])
#
callback
<Function>err
<Error>
Closes writeStream
. Optionally accepts a
callback that will be executed once the writeStream
is closed.
writeStream.path
#
The path to the file the stream is writing to as specified in the first
argument to fs.createWriteStream()
. If path
is passed as a string, then
writeStream.path
will be a string. If path
is passed as a <Buffer>, then
writeStream.path
will be a <Buffer>.
writeStream.pending
#
This property is true
if the underlying file has not been opened yet,
i.e. before the 'ready'
event is emitted.
fs.constants
#
Returns an object containing commonly used constants for file system operations.
FS constants#
The following constants are exported by fs.constants
and fsPromises.constants
.
Not every constant will be available on every operating system; this is especially important for Windows, where many of the POSIX specific definitions are not available. For portable applications it is recommended to check for their presence before use.
To use more than one constant, use the bitwise OR |
operator.
Example:
import { open, constants } from 'node:fs';
const {
O_RDWR,
O_CREAT,
O_EXCL,
} = constants;
open('/path/to/my/file', O_RDWR | O_CREAT | O_EXCL, (err, fd) => {
// ...
});
File access constants#
The following constants are meant for use as the mode
parameter passed to
fsPromises.access()
, fs.access()
, and fs.accessSync()
.
Constant | Description |
---|---|
F_OK |
Flag indicating that the file is visible to the calling process.
This is useful for determining if a file exists, but says nothing
about rwx permissions. Default if no mode is specified. |
R_OK |
Flag indicating that the file can be read by the calling process. |
W_OK |
Flag indicating that the file can be written by the calling process. |
X_OK |
Flag indicating that the file can be executed by the calling
process. This has no effect on Windows
(will behave like fs.constants.F_OK ). |
The definitions are also available on Windows.
File copy constants#
The following constants are meant for use with fs.copyFile()
.
Constant | Description |
---|---|
COPYFILE_EXCL |
If present, the copy operation will fail with an error if the destination path already exists. |
COPYFILE_FICLONE |
If present, the copy operation will attempt to create a copy-on-write reflink. If the underlying platform does not support copy-on-write, then a fallback copy mechanism is used. |
COPYFILE_FICLONE_FORCE |
If present, the copy operation will attempt to create a copy-on-write reflink. If the underlying platform does not support copy-on-write, then the operation will fail with an error. |
The definitions are also available on Windows.
File open constants#
The following constants are meant for use with fs.open()
.
Constant | Description |
---|---|
O_RDONLY |
Flag indicating to open a file for read-only access. |
O_WRONLY |
Flag indicating to open a file for write-only access. |
O_RDWR |
Flag indicating to open a file for read-write access. |
O_CREAT |
Flag indicating to create the file if it does not already exist. |
O_EXCL |
Flag indicating that opening a file should fail if the
O_CREAT flag is set and the file already exists. |
O_NOCTTY |
Flag indicating that if path identifies a terminal device, opening the path shall not cause that terminal to become the controlling terminal for the process (if the process does not already have one). |
O_TRUNC |
Flag indicating that if the file exists and is a regular file, and the file is opened successfully for write access, its length shall be truncated to zero. |
O_APPEND |
Flag indicating that data will be appended to the end of the file. |
O_DIRECTORY |
Flag indicating that the open should fail if the path is not a directory. |
O_NOATIME |
Flag indicating reading accesses to the file system will no longer
result in an update to the atime information associated with
the file. This flag is available on Linux operating systems only. |
O_NOFOLLOW |
Flag indicating that the open should fail if the path is a symbolic link. |
O_SYNC |
Flag indicating that the file is opened for synchronized I/O with write operations waiting for file integrity. |
O_DSYNC |
Flag indicating that the file is opened for synchronized I/O with write operations waiting for data integrity. |
O_SYMLINK |
Flag indicating to open the symbolic link itself rather than the resource it is pointing to. |
O_DIRECT |
When set, an attempt will be made to minimize caching effects of file I/O. |
O_NONBLOCK |
Flag indicating to open the file in nonblocking mode when possible. |
UV_FS_O_FILEMAP |
When set, a memory file mapping is used to access the file. This flag is available on Windows operating systems only. On other operating systems, this flag is ignored. |
On Windows, only O_APPEND
, O_CREAT
, O_EXCL
, O_RDONLY
, O_RDWR
,
O_TRUNC
, O_WRONLY
, and UV_FS_O_FILEMAP
are available.
File type constants#
The following constants are meant for use with the <fs.Stats> object's
mode
property for determining a file's type.
Constant | Description |
---|---|
S_IFMT |
Bit mask used to extract the file type code. |
S_IFREG |
File type constant for a regular file. |
S_IFDIR |
File type constant for a directory. |
S_IFCHR |
File type constant for a character-oriented device file. |
S_IFBLK |
File type constant for a block-oriented device file. |
S_IFIFO |
File type constant for a FIFO/pipe. |
S_IFLNK |
File type constant for a symbolic link. |
S_IFSOCK |
File type constant for a socket. |
On Windows, only S_IFCHR
, S_IFDIR
, S_IFLNK
, S_IFMT
, and S_IFREG
,
are available.
File mode constants#
The following constants are meant for use with the <fs.Stats> object's
mode
property for determining the access permissions for a file.
Constant | Description |
---|---|
S_IRWXU |
File mode indicating readable, writable, and executable by owner. |
S_IRUSR |
File mode indicating readable by owner. |
S_IWUSR |
File mode indicating writable by owner. |
S_IXUSR |
File mode indicating executable by owner. |
S_IRWXG |
File mode indicating readable, writable, and executable by group. |
S_IRGRP |
File mode indicating readable by group. |
S_IWGRP |
File mode indicating writable by group. |
S_IXGRP |
File mode indicating executable by group. |
S_IRWXO |
File mode indicating readable, writable, and executable by others. |
S_IROTH |
File mode indicating readable by others. |
S_IWOTH |
File mode indicating writable by others. |
S_IXOTH |
File mode indicating executable by others. |
On Windows, only S_IRUSR
and S_IWUSR
are available.
Notes#
Ordering of callback and promise-based operations#
Because they are executed asynchronously by the underlying thread pool, there is no guaranteed ordering when using either the callback or promise-based methods.
For example, the following is prone to error because the fs.stat()
operation might complete before the fs.rename()
operation:
const fs = require('node:fs');
fs.rename('/tmp/hello', '/tmp/world', (err) => {
if (err) throw err;
console.log('renamed complete');
});
fs.stat('/tmp/world', (err, stats) => {
if (err) throw err;
console.log(`stats: ${JSON.stringify(stats)}`);
});
It is important to correctly order the operations by awaiting the results of one before invoking the other:
import { rename, stat } from 'node:fs/promises';
const oldPath = '/tmp/hello';
const newPath = '/tmp/world';
try {
await rename(oldPath, newPath);
const stats = await stat(newPath);
console.log(`stats: ${JSON.stringify(stats)}`);
} catch (error) {
console.error('there was an error:', error.message);
}
const { rename, stat } = require('node:fs/promises');
(async function(oldPath, newPath) {
try {
await rename(oldPath, newPath);
const stats = await stat(newPath);
console.log(`stats: ${JSON.stringify(stats)}`);
} catch (error) {
console.error('there was an error:', error.message);
}
})('/tmp/hello', '/tmp/world');
Or, when using the callback APIs, move the fs.stat()
call into the callback
of the fs.rename()
operation:
import { rename, stat } from 'node:fs';
rename('/tmp/hello', '/tmp/world', (err) => {
if (err) throw err;
stat('/tmp/world', (err, stats) => {
if (err) throw err;
console.log(`stats: ${JSON.stringify(stats)}`);
});
});
const { rename, stat } = require('node:fs/promises');
rename('/tmp/hello', '/tmp/world', (err) => {
if (err) throw err;
stat('/tmp/world', (err, stats) => {
if (err) throw err;
console.log(`stats: ${JSON.stringify(stats)}`);
});
});
File paths#
Most fs
operations accept file paths that may be specified in the form of
a string, a <Buffer>, or a <URL> object using the file:
protocol.
String paths#
String paths are interpreted as UTF-8 character sequences identifying
the absolute or relative filename. Relative paths will be resolved relative
to the current working directory as determined by calling process.cwd()
.
Example using an absolute path on POSIX:
import { open } from 'node:fs/promises';
let fd;
try {
fd = await open('/open/some/file.txt', 'r');
// Do something with the file
} finally {
await fd?.close();
}
Example using a relative path on POSIX (relative to process.cwd()
):
import { open } from 'node:fs/promises';
let fd;
try {
fd = await open('file.txt', 'r');
// Do something with the file
} finally {
await fd?.close();
}
File URL paths#
For most node:fs
module functions, the path
or filename
argument may be
passed as a <URL> object using the file:
protocol.
import { readFileSync } from 'node:fs';
readFileSync(new URL('file:///tmp/hello'));
file:
URLs are always absolute paths.
Platform-specific considerations#
On Windows, file:
<URL>s with a host name convert to UNC paths, while file:
<URL>s with drive letters convert to local absolute paths. file:
<URL>s
with no host name and no drive letter will result in an error:
import { readFileSync } from 'node:fs';
// On Windows :
// - WHATWG file URLs with hostname convert to UNC path
// file://hostname/p/a/t/h/file => \\hostname\p\a\t\h\file
readFileSync(new URL('file://hostname/p/a/t/h/file'));
// - WHATWG file URLs with drive letters convert to absolute path
// file:///C:/tmp/hello => C:\tmp\hello
readFileSync(new URL('file:///C:/tmp/hello'));
// - WHATWG file URLs without hostname must have a drive letters
readFileSync(new URL('file:///notdriveletter/p/a/t/h/file'));
readFileSync(new URL('file:///c/p/a/t/h/file'));
// TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must be absolute
file:
<URL>s with drive letters must use :
as a separator just after
the drive letter. Using another separator will result in an error.
On all other platforms, file:
<URL>s with a host name are unsupported and
will result in an error:
import { readFileSync } from 'node:fs';
// On other platforms:
// - WHATWG file URLs with hostname are unsupported
// file://hostname/p/a/t/h/file => throw!
readFileSync(new URL('file://hostname/p/a/t/h/file'));
// TypeError [ERR_INVALID_FILE_URL_PATH]: must be absolute
// - WHATWG file URLs convert to absolute path
// file:///tmp/hello => /tmp/hello
readFileSync(new URL('file:///tmp/hello'));
A file:
<URL> having encoded slash characters will result in an error on all
platforms:
import { readFileSync } from 'node:fs';
// On Windows
readFileSync(new URL('file:///C:/p/a/t/h/%2F'));
readFileSync(new URL('file:///C:/p/a/t/h/%2f'));
/* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded
\ or / characters */
// On POSIX
readFileSync(new URL('file:///p/a/t/h/%2F'));
readFileSync(new URL('file:///p/a/t/h/%2f'));
/* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded
/ characters */
On Windows, file:
<URL>s having encoded backslash will result in an error:
import { readFileSync } from 'node:fs';
// On Windows
readFileSync(new URL('file:///C:/path/%5C'));
readFileSync(new URL('file:///C:/path/%5c'));
/* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded
\ or / characters */
Buffer paths#
Paths specified using a <Buffer> are useful primarily on certain POSIX operating systems that treat file paths as opaque byte sequences. On such systems, it is possible for a single file path to contain sub-sequences that use multiple character encodings. As with string paths, <Buffer> paths may be relative or absolute:
Example using an absolute path on POSIX:
import { open } from 'node:fs/promises';
import { Buffer } from 'node:buffer';
let fd;
try {
fd = await open(Buffer.from('/open/some/file.txt'), 'r');
// Do something with the file
} finally {
await fd?.close();
}
Per-drive working directories on Windows#
On Windows, Node.js follows the concept of per-drive working directory. This
behavior can be observed when using a drive path without a backslash. For
example fs.readdirSync('C:\\')
can potentially return a different result than
fs.readdirSync('C:')
. For more information, see
this MSDN page.
File descriptors#
On POSIX systems, for every process, the kernel maintains a table of currently open files and resources. Each open file is assigned a simple numeric identifier called a file descriptor. At the system-level, all file system operations use these file descriptors to identify and track each specific file. Windows systems use a different but conceptually similar mechanism for tracking resources. To simplify things for users, Node.js abstracts away the differences between operating systems and assigns all open files a numeric file descriptor.
The callback-based fs.open()
, and synchronous fs.openSync()
methods open a
file and allocate a new file descriptor. Once allocated, the file descriptor may
be used to read data from, write data to, or request information about the file.
Operating systems limit the number of file descriptors that may be open at any given time so it is critical to close the descriptor when operations are completed. Failure to do so will result in a memory leak that will eventually cause an application to crash.
import { open, close, fstat } from 'node:fs';
function closeFd(fd) {
close(fd, (err) => {
if (err) throw err;
});
}
open('/open/some/file.txt', 'r', (err, fd) => {
if (err) throw err;
try {
fstat(fd, (err, stat) => {
if (err) {
closeFd(fd);
throw err;
}
// use stat
closeFd(fd);
});
} catch (err) {
closeFd(fd);
throw err;
}
});
The promise-based APIs use a <FileHandle> object in place of the numeric file descriptor. These objects are better managed by the system to ensure that resources are not leaked. However, it is still required that they are closed when operations are completed:
import { open } from 'node:fs/promises';
let file;
try {
file = await open('/open/some/file.txt', 'r');
const stat = await file.stat();
// use stat
} finally {
await file.close();
}
Threadpool usage#
All callback and promise-based file system APIs (with the exception of
fs.FSWatcher()
) use libuv's threadpool. This can have surprising and negative
performance implications for some applications. See the
UV_THREADPOOL_SIZE
documentation for more information.
File system flags#
The following flags are available wherever the flag
option takes a
string.
-
'a'
: Open file for appending. The file is created if it does not exist. -
'ax'
: Like'a'
but fails if the path exists. -
'a+'
: Open file for reading and appending. The file is created if it does not exist. -
'ax+'
: Like'a+'
but fails if the path exists. -
'as'
: Open file for appending in synchronous mode. The file is created if it does not exist. -
'as+'
: Open file for reading and appending in synchronous mode. The file is created if it does not exist. -
'r'
: Open file for reading. An exception occurs if the file does not exist. -
'r+'
: Open file for reading and writing. An exception occurs if the file does not exist. -
'rs+'
: Open file for reading and writing in synchronous mode. Instructs the operating system to bypass the local file system cache.This is primarily useful for opening files on NFS mounts as it allows skipping the potentially stale local cache. It has a very real impact on I/O performance so using this flag is not recommended unless it is needed.
This doesn't turn
fs.open()
orfsPromises.open()
into a synchronous blocking call. If synchronous operation is desired, something likefs.openSync()
should be used. -
'w'
: Open file for writing. The file is created (if it does not exist) or truncated (if it exists). -
'wx'
: Like'w'
but fails if the path exists. -
'w+'
: Open file for reading and writing. The file is created (if it does not exist) or truncated (if it exists). -
'wx+'
: Like'w+'
but fails if the path exists.
flag
can also be a number as documented by open(2)
; commonly used constants
are available from fs.constants
. On Windows, flags are translated to
their equivalent ones where applicable, e.g. O_WRONLY
to FILE_GENERIC_WRITE
,
or O_EXCL|O_CREAT
to CREATE_NEW
, as accepted by CreateFileW
.
The exclusive flag 'x'
(O_EXCL
flag in open(2)
) causes the operation to
return an error if the path already exists. On POSIX, if the path is a symbolic
link, using O_EXCL
returns an error even if the link is to a path that does
not exist. The exclusive flag might not work with network file systems.
On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.
Modifying a file rather than replacing it may require the flag
option to be
set to 'r+'
rather than the default 'w'
.
The behavior of some flags are platform-specific. As such, opening a directory
on macOS and Linux with the 'a+'
flag, as in the example below, will return an
error. In contrast, on Windows and FreeBSD, a file descriptor or a FileHandle
will be returned.
// macOS and Linux
fs.open('<directory>', 'a+', (err, fd) => {
// => [Error: EISDIR: illegal operation on a directory, open <directory>]
});
// Windows and FreeBSD
fs.open('<directory>', 'a+', (err, fd) => {
// => null, <fd>
});
On Windows, opening an existing hidden file using the 'w'
flag (either
through fs.open()
, fs.writeFile()
, or fsPromises.open()
) will fail with
EPERM
. Existing hidden files can be opened for writing with the 'r+'
flag.
A call to fs.ftruncate()
or filehandle.truncate()
can be used to reset
the file contents.
Global objects#
These objects are available in all modules. The following variables may appear to be global but are not. They exist only in the scope of modules, see the module system documentation:
The objects listed here are specific to Node.js. There are built-in objects that are part of the JavaScript language itself, which are also globally accessible.
Class: AbortController
#
A utility class used to signal cancelation in selected Promise
-based APIs.
The API is based on the Web API AbortController
.
const ac = new AbortController();
ac.signal.addEventListener('abort', () => console.log('Aborted!'),
{ once: true });
ac.abort();
console.log(ac.signal.aborted); // Prints true
abortController.abort([reason])
#
reason
<any> An optional reason, retrievable on theAbortSignal
'sreason
property.
Triggers the abort signal, causing the abortController.signal
to emit
the 'abort'
event.
abortController.signal
#
- Type: <AbortSignal>
Class: AbortSignal
#
- Extends: <EventTarget>
The AbortSignal
is used to notify observers when the
abortController.abort()
method is called.
Static method: AbortSignal.abort([reason])
#
reason
: <any>- Returns: <AbortSignal>
Returns a new already aborted AbortSignal
.
Static method: AbortSignal.timeout(delay)
#
delay
<number> The number of milliseconds to wait before triggering the AbortSignal.
Returns a new AbortSignal
which will be aborted in delay
milliseconds.
Event: 'abort'
#
The 'abort'
event is emitted when the abortController.abort()
method
is called. The callback is invoked with a single object argument with a
single type
property set to 'abort'
:
const ac = new AbortController();
// Use either the onabort property...
ac.signal.onabort = () => console.log('aborted!');
// Or the EventTarget API...
ac.signal.addEventListener('abort', (event) => {
console.log(event.type); // Prints 'abort'
}, { once: true });
ac.abort();
The AbortController
with which the AbortSignal
is associated will only
ever trigger the 'abort'
event once. We recommended that code check
that the abortSignal.aborted
attribute is false
before adding an 'abort'
event listener.
Any event listeners attached to the AbortSignal
should use the
{ once: true }
option (or, if using the EventEmitter
APIs to attach a
listener, use the once()
method) to ensure that the event listener is
removed as soon as the 'abort'
event is handled. Failure to do so may
result in memory leaks.
abortSignal.aborted
#
- Type: <boolean> True after the
AbortController
has been aborted.
abortSignal.onabort
#
- Type: <Function>
An optional callback function that may be set by user code to be notified
when the abortController.abort()
function has been called.
abortSignal.reason
#
- Type: <any>
An optional reason specified when the AbortSignal
was triggered.
const ac = new AbortController();
ac.abort(new Error('boom!'));
console.log(ac.signal.reason); // Error: boom!
abortSignal.throwIfAborted()
#
If abortSignal.aborted
is true
, throws abortSignal.reason
.
Class: Blob
#
See <Blob>.
Class: Buffer
#
Used to handle binary data. See the buffer section.
Class: ByteLengthQueuingStrategy
#
A browser-compatible implementation of ByteLengthQueuingStrategy
.
__dirname
#
This variable may appear to be global but is not. See __dirname
.
__filename
#
This variable may appear to be global but is not. See __filename
.
atob(data)
#
Buffer.from(data, 'base64')
instead.Global alias for buffer.atob()
.
BroadcastChannel
#
See <BroadcastChannel>.
btoa(data)
#
buf.toString('base64')
instead.Global alias for buffer.btoa()
.
clearImmediate(immediateObject)
#
clearImmediate
is described in the timers section.
clearInterval(intervalObject)
#
clearInterval
is described in the timers section.
clearTimeout(timeoutObject)
#
clearTimeout
is described in the timers section.
Class: CompressionStream
#
A browser-compatible implementation of CompressionStream
.
console
#
Used to print to stdout and stderr. See the console
section.
Class: CountQueuingStrategy
#
A browser-compatible implementation of CountQueuingStrategy
.
Crypto
#
--no-experimental-global-webcrypto
CLI flag.A browser-compatible implementation of <Crypto>. This global is available
only if the Node.js binary was compiled with including support for the
node:crypto
module.
crypto
#
--no-experimental-global-webcrypto
CLI flag.A browser-compatible implementation of the Web Crypto API.
CryptoKey
#
--no-experimental-global-webcrypto
CLI flag.A browser-compatible implementation of <CryptoKey>. This global is available
only if the Node.js binary was compiled with including support for the
node:crypto
module.
CustomEvent
#
--no-experimental-global-customevent
CLI flag.A browser-compatible implementation of the CustomEvent
Web API.
Class: DecompressionStream
#
A browser-compatible implementation of DecompressionStream
.
Event
#
A browser-compatible implementation of the Event
class. See
EventTarget
and Event
API for more details.
EventTarget
#
A browser-compatible implementation of the EventTarget
class. See
EventTarget
and Event
API for more details.
exports
#
This variable may appear to be global but is not. See exports
.
fetch
#
--no-experimental-fetch
CLI flag.A browser-compatible implementation of the fetch()
function.
Class: File
#
See <File>.
Class FormData
#
--no-experimental-fetch
CLI flag.A browser-compatible implementation of <FormData>.
global
#
- <Object> The global namespace object.
In browsers, the top-level scope is the global scope. This means that
within the browser var something
will define a new global variable. In
Node.js this is different. The top-level scope is not the global scope;
var something
inside a Node.js module will be local to that module.
Class Headers
#
--no-experimental-fetch
CLI flag.A browser-compatible implementation of <Headers>.
MessageChannel
#
The MessageChannel
class. See MessageChannel
for more details.
MessageEvent
#
The MessageEvent
class. See MessageEvent
for more details.
MessagePort
#
The MessagePort
class. See MessagePort
for more details.
module
#
This variable may appear to be global but is not. See module
.
PerformanceEntry
#
The PerformanceEntry
class. See PerformanceEntry
for more details.
PerformanceMark
#
The PerformanceMark
class. See PerformanceMark
for more details.
PerformanceMeasure
#
The PerformanceMeasure
class. See PerformanceMeasure
for more details.
PerformanceObserver
#
The PerformanceObserver
class. See PerformanceObserver
for more details.
PerformanceObserverEntryList
#
The PerformanceObserverEntryList
class. See
PerformanceObserverEntryList
for more details.
PerformanceResourceTiming
#
The PerformanceResourceTiming
class. See PerformanceResourceTiming
for
more details.
performance
#
The perf_hooks.performance
object.
process
#
The process object. See the process
object section.
queueMicrotask(callback)
#
callback
<Function> Function to be queued.
The queueMicrotask()
method queues a microtask to invoke callback
. If
callback
throws an exception, the process
object 'uncaughtException'
event will be emitted.
The microtask queue is managed by V8 and may be used in a similar manner to
the process.nextTick()
queue, which is managed by Node.js. The
process.nextTick()
queue is always processed before the microtask queue
within each turn of the Node.js event loop.
// Here, `queueMicrotask()` is used to ensure the 'load' event is always
// emitted asynchronously, and therefore consistently. Using
// `process.nextTick()` here would result in the 'load' event always emitting
// before any other promise jobs.
DataHandler.prototype.load = async function load(key) {
const hit = this._cache.get(key);
if (hit !== undefined) {
queueMicrotask(() => {
this.emit('load', hit);
});
return;
}
const data = await fetchData(key);
this._cache.set(key, data);
this.emit('load', data);
};
Class: ReadableByteStreamController
#
A browser-compatible implementation of ReadableByteStreamController
.
Class: ReadableStream
#
A browser-compatible implementation of ReadableStream
.
Class: ReadableStreamBYOBReader
#
A browser-compatible implementation of ReadableStreamBYOBReader
.
Class: ReadableStreamBYOBRequest
#
A browser-compatible implementation of ReadableStreamBYOBRequest
.
Class: ReadableStreamDefaultController
#
A browser-compatible implementation of ReadableStreamDefaultController
.
Class: ReadableStreamDefaultReader
#
A browser-compatible implementation of ReadableStreamDefaultReader
.
require()
#
This variable may appear to be global but is not. See require()
.
Response
#
--no-experimental-fetch
CLI flag.A browser-compatible implementation of <Response>.
Request
#
--no-experimental-fetch
CLI flag.A browser-compatible implementation of <Request>.
setImmediate(callback[, ...args])
#
setImmediate
is described in the timers section.
setInterval(callback, delay[, ...args])
#
setInterval
is described in the timers section.
setTimeout(callback, delay[, ...args])
#
setTimeout
is described in the timers section.
structuredClone(value[, options])
#
The WHATWG structuredClone
method.
SubtleCrypto
#
--no-experimental-global-webcrypto
CLI flag.A browser-compatible implementation of <SubtleCrypto>. This global is available
only if the Node.js binary was compiled with including support for the
node:crypto
module.
DOMException
#
The WHATWG DOMException
class. See DOMException
for more details.
TextDecoder
#
The WHATWG TextDecoder
class. See the TextDecoder
section.
Class: TextDecoderStream
#
A browser-compatible implementation of TextDecoderStream
.
TextEncoder
#
The WHATWG TextEncoder
class. See the TextEncoder
section.
Class: TextEncoderStream
#
A browser-compatible implementation of TextEncoderStream
.
Class: TransformStream
#
A browser-compatible implementation of TransformStream
.
Class: TransformStreamDefaultController
#
A browser-compatible implementation of TransformStreamDefaultController
.
URL
#
The WHATWG URL
class. See the URL
section.
URLSearchParams
#
The WHATWG URLSearchParams
class. See the URLSearchParams
section.
WebAssembly
#
The object that acts as the namespace for all W3C WebAssembly related functionality. See the Mozilla Developer Network for usage and compatibility.
Class: WritableStream
#
A browser-compatible implementation of WritableStream
.
Class: WritableStreamDefaultController
#
A browser-compatible implementation of WritableStreamDefaultController
.
Class: WritableStreamDefaultWriter
#
A browser-compatible implementation of WritableStreamDefaultWriter
.
HTTP#
Source Code: lib/http.js
To use the HTTP server and client one must require('node:http')
.
The HTTP interfaces in Node.js are designed to support many features of the protocol which have been traditionally difficult to use. In particular, large, possibly chunk-encoded, messages. The interface is careful to never buffer entire requests or responses, so the user is able to stream data.
HTTP message headers are represented by an object like this:
{ 'content-length': '123',
'content-type': 'text/plain',
'connection': 'keep-alive',
'host': 'example.com',
'accept': '*/*' }
Keys are lowercased. Values are not modified.
In order to support the full spectrum of possible HTTP applications, the Node.js HTTP API is very low-level. It deals with stream handling and message parsing only. It parses a message into headers and body but it does not parse the actual headers or the body.
See message.headers
for details on how duplicate headers are handled.
The raw headers as they were received are retained in the rawHeaders
property, which is an array of [key, value, key2, value2, ...]
. For
example, the previous message header object might have a rawHeaders
list like the following:
[ 'ConTent-Length', '123456',
'content-LENGTH', '123',
'content-type', 'text/plain',
'CONNECTION', 'keep-alive',
'Host', 'example.com',
'accepT', '*/*' ]
Class: http.Agent
#
An Agent
is responsible for managing connection persistence
and reuse for HTTP clients. It maintains a queue of pending requests
for a given host and port, reusing a single socket connection for each
until the queue is empty, at which time the socket is either destroyed
or put into a pool where it is kept to be used again for requests to the
same host and port. Whether it is destroyed or pooled depends on the
keepAlive
option.
Pooled connections have TCP Keep-Alive enabled for them, but servers may
still close idle connections, in which case they will be removed from the
pool and a new connection will be made when a new HTTP request is made for
that host and port. Servers may also refuse to allow multiple requests
over the same connection, in which case the connection will have to be
remade for every request and cannot be pooled. The Agent
will still make
the requests to that server, but each one will occur over a new connection.
When a connection is closed by the client or the server, it is removed
from the pool. Any unused sockets in the pool will be unrefed so as not
to keep the Node.js process running when there are no outstanding requests.
(see socket.unref()
).
It is good practice, to destroy()
an Agent
instance when it is no
longer in use, because unused sockets consume OS resources.
Sockets are removed from an agent when the socket emits either
a 'close'
event or an 'agentRemove'
event. When intending to keep one
HTTP request open for a long time without keeping it in the agent, something
like the following may be done:
http.get(options, (res) => {
// Do stuff
}).on('socket', (socket) => {
socket.emit('agentRemove');
});
An agent may also be used for an individual request. By providing
{agent: false}
as an option to the http.get()
or http.request()
functions, a one-time use Agent
with default options will be used
for the client connection.
agent:false
:
http.get({
hostname: 'localhost',
port: 80,
path: '/',
agent: false, // Create a new agent just for this one request
}, (res) => {
// Do stuff with response
});
new Agent([options])
#
options
<Object> Set of configurable options to set on the agent. Can have the following fields:keepAlive
<boolean> Keep sockets around even when there are no outstanding requests, so they can be used for future requests without having to reestablish a TCP connection. Not to be confused with thekeep-alive
value of theConnection
header. TheConnection: keep-alive
header is always sent when using an agent except when theConnection
header is explicitly specified or when thekeepAlive
andmaxSockets
options are respectively set tofalse
andInfinity
, in which caseConnection: close
will be used. Default:false
.keepAliveMsecs
<number> When using thekeepAlive
option, specifies the initial delay for TCP Keep-Alive packets. Ignored when thekeepAlive
option isfalse
orundefined
. Default:1000
.maxSockets
<number> Maximum number of sockets to allow per host. If the same host opens multiple concurrent connections, each request will use new socket until themaxSockets
value is reached. If the host attempts to open more connections thanmaxSockets
, the additional requests will enter into a pending request queue, and will enter active connection state when an existing connection terminates. This makes sure there are at mostmaxSockets
active connections at any point in time, from a given host. Default:Infinity
.maxTotalSockets
<number> Maximum number of sockets allowed for all hosts in total. Each request will use a new socket until the maximum is reached. Default:Infinity
.maxFreeSockets
<number> Maximum number of sockets per host to leave open in a free state. Only relevant ifkeepAlive
is set totrue
. Default:256
.scheduling
<string> Scheduling strategy to apply when picking the next free socket to use. It can be'fifo'
or'lifo'
. The main difference between the two scheduling strategies is that'lifo'
selects the most recently used socket, while'fifo'
selects the least recently used socket. In case of a low rate of request per second, the'lifo'
scheduling will lower the risk of picking a socket that might have been closed by the server due to inactivity. In case of a high rate of request per second, the'fifo'
scheduling will maximize the number of open sockets, while the'lifo'
scheduling will keep it as low as possible. Default:'lifo'
.timeout
<number> Socket timeout in milliseconds. This will set the timeout when the socket is created.
options
in socket.connect()
are also supported.
The default http.globalAgent
that is used by http.request()
has all
of these values set to their respective defaults.
To configure any of them, a custom http.Agent
instance must be created.
const http = require('node:http');
const keepAliveAgent = new http.Agent({ keepAlive: true });
options.agent = keepAliveAgent;
http.request(options, onResponseCallback);
agent.createConnection(options[, callback])
#
options
<Object> Options containing connection details. Checknet.createConnection()
for the format of the optionscallback
<Function> Callback function that receives the created socket- Returns: <stream.Duplex>
Produces a socket/stream to be used for HTTP requests.
By default, this function is the same as net.createConnection()
. However,
custom agents may override this method in case greater flexibility is desired.
A socket/stream can be supplied in one of two ways: by returning the
socket/stream from this function, or by passing the socket/stream to callback
.
This method is guaranteed to return an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specifies a socket type other than <net.Socket>.
callback
has a signature of (err, stream)
.
agent.keepSocketAlive(socket)
#
socket
<stream.Duplex>
Called when socket
is detached from a request and could be persisted by the
Agent
. Default behavior is to:
socket.setKeepAlive(true, this.keepAliveMsecs);
socket.unref();
return true;
This method can be overridden by a particular Agent
subclass. If this
method returns a falsy value, the socket will be destroyed instead of persisting
it for use with the next request.
The socket
argument can be an instance of <net.Socket>, a subclass of
<stream.Duplex>.
agent.reuseSocket(socket, request)
#
socket
<stream.Duplex>request
<http.ClientRequest>
Called when socket
is attached to request
after being persisted because of
the keep-alive options. Default behavior is to:
socket.ref();
This method can be overridden by a particular Agent
subclass.
The socket
argument can be an instance of <net.Socket>, a subclass of
<stream.Duplex>.
agent.destroy()
#
Destroy any sockets that are currently in use by the agent.
It is usually not necessary to do this. However, if using an
agent with keepAlive
enabled, then it is best to explicitly shut down
the agent when it is no longer needed. Otherwise,
sockets might stay open for quite a long time before the server
terminates them.
agent.freeSockets
#
An object which contains arrays of sockets currently awaiting use by
the agent when keepAlive
is enabled. Do not modify.
Sockets in the freeSockets
list will be automatically destroyed and
removed from the array on 'timeout'
.
agent.getName([options])
#
Get a unique name for a set of request options, to determine whether a
connection can be reused. For an HTTP agent, this returns
host:port:localAddress
or host:port:localAddress:family
. For an HTTPS agent,
the name includes the CA, cert, ciphers, and other HTTPS/TLS-specific options
that determine socket reusability.
agent.maxFreeSockets
#
By default set to 256. For agents with keepAlive
enabled, this
sets the maximum number of sockets that will be left open in the free
state.
agent.maxSockets
#
By default set to Infinity
. Determines how many concurrent sockets the agent
can have open per origin. Origin is the returned value of agent.getName()
.
agent.maxTotalSockets
#
By default set to Infinity
. Determines how many concurrent sockets the agent
can have open. Unlike maxSockets
, this parameter applies across all origins.
agent.requests
#
An object which contains queues of requests that have not yet been assigned to sockets. Do not modify.
agent.sockets
#
An object which contains arrays of sockets currently in use by the agent. Do not modify.
Class: http.ClientRequest
#
- Extends: <http.OutgoingMessage>
This object is created internally and returned from http.request()
. It
represents an in-progress request whose header has already been queued. The
header is still mutable using the setHeader(name, value)
,
getHeader(name)
, removeHeader(name)
API. The actual header will
be sent along with the first data chunk or when calling request.end()
.
To get the response, add a listener for 'response'
to the request object.
'response'
will be emitted from the request object when the response
headers have been received. The 'response'
event is executed with one
argument which is an instance of http.IncomingMessage
.
During the 'response'
event, one can add listeners to the
response object; particularly to listen for the 'data'
event.
If no 'response'
handler is added, then the response will be
entirely discarded. However, if a 'response'
event handler is added,
then the data from the response object must be consumed, either by
calling response.read()
whenever there is a 'readable'
event, or
by adding a 'data'
handler, or by calling the .resume()
method.
Until the data is consumed, the 'end'
event will not fire. Also, until
the data is read it will consume memory that can eventually lead to a
'process out of memory' error.
For backward compatibility, res
will only emit 'error'
if there is an
'error'
listener registered.
Set Content-Length
header to limit the response body size.
If response.strictContentLength
is set to true
, mismatching the
Content-Length
header value will result in an Error
being thrown,
identified by code:
'ERR_HTTP_CONTENT_LENGTH_MISMATCH'
.
Content-Length
value should be in bytes, not characters. Use
Buffer.byteLength()
to determine the length of the body in bytes.
Event: 'abort'
#
'close'
event instead.Emitted when the request has been aborted by the client. This event is only
emitted on the first call to abort()
.
Event: 'close'
#
Indicates that the request is completed, or its underlying connection was terminated prematurely (before the response completion).
Event: 'connect'
#
response
<http.IncomingMessage>socket
<stream.Duplex>head
<Buffer>
Emitted each time a server responds to a request with a CONNECT
method. If
this event is not being listened for, clients receiving a CONNECT
method will
have their connections closed.
This event is guaranteed to be passed an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specifies a socket type other than <net.Socket>.
A client and server pair demonstrating how to listen for the 'connect'
event:
const http = require('node:http');
const net = require('node:net');
const { URL } = require('node:url');
// Create an HTTP tunneling proxy
const proxy = http.createServer((req, res) => {
res.writeHead(200, { 'Content-Type': 'text/plain' });
res.end('okay');
});
proxy.on('connect', (req, clientSocket, head) => {
// Connect to an origin server
const { port, hostname } = new URL(`http://${req.url}`);
const serverSocket = net.connect(port || 80, hostname, () => {
clientSocket.write('HTTP/1.1 200 Connection Established\r\n' +
'Proxy-agent: Node.js-Proxy\r\n' +
'\r\n');
serverSocket.write(head);
serverSocket.pipe(clientSocket);
clientSocket.pipe(serverSocket);
});
});
// Now that proxy is running
proxy.listen(1337, '127.0.0.1', () => {
// Make a request to a tunneling proxy
const options = {
port: 1337,
host: '127.0.0.1',
method: 'CONNECT',
path: 'www.google.com:80',
};
const req = http.request(options);
req.end();
req.on('connect', (res, socket, head) => {
console.log('got connected!');
// Make a request over an HTTP tunnel
socket.write('GET / HTTP/1.1\r\n' +
'Host: www.google.com:80\r\n' +
'Connection: close\r\n' +
'\r\n');
socket.on('data', (chunk) => {
console.log(chunk.toString());
});
socket.on('end', () => {
proxy.close();
});
});
});
Event: 'continue'
#
Emitted when the server sends a '100 Continue' HTTP response, usually because the request contained 'Expect: 100-continue'. This is an instruction that the client should send the request body.
Event: 'finish'
#
Emitted when the request has been sent. More specifically, this event is emitted when the last segment of the response headers and body have been handed off to the operating system for transmission over the network. It does not imply that the server has received anything yet.
Event: 'information'
#
info
<Object>
Emitted when the server sends a 1xx intermediate response (excluding 101 Upgrade). The listeners of this event will receive an object containing the HTTP version, status code, status message, key-value headers object, and array with the raw header names followed by their respective values.
const http = require('node:http');
const options = {
host: '127.0.0.1',
port: 8080,
path: '/length_request',
};
// Make a request
const req = http.request(options);
req.end();
req.on('information', (info) => {
console.log(`Got information prior to main response: ${info.statusCode}`);
});
101 Upgrade statuses do not fire this event due to their break from the
traditional HTTP request/response chain, such as web sockets, in-place TLS
upgrades, or HTTP 2.0. To be notified of 101 Upgrade notices, listen for the
'upgrade'
event instead.
Event: 'response'
#
response
<http.IncomingMessage>
Emitted when a response is received to this request. This event is emitted only once.
Event: 'socket'
#
socket
<stream.Duplex>
This event is guaranteed to be passed an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specifies a socket type other than <net.Socket>.
Event: 'timeout'
#
Emitted when the underlying socket times out from inactivity. This only notifies that the socket has been idle. The request must be destroyed manually.
See also: request.setTimeout()
.
Event: 'upgrade'
#
response
<http.IncomingMessage>socket
<stream.Duplex>head
<Buffer>
Emitted each time a server responds to a request with an upgrade. If this event is not being listened for and the response status code is 101 Switching Protocols, clients receiving an upgrade header will have their connections closed.
This event is guaranteed to be passed an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specifies a socket type other than <net.Socket>.
A client server pair demonstrating how to listen for the 'upgrade'
event.
const http = require('node:http');
// Create an HTTP server
const server = http.createServer((req, res) => {
res.writeHead(200, { 'Content-Type': 'text/plain' });
res.end('okay');
});
server.on('upgrade', (req, socket, head) => {
socket.write('HTTP/1.1 101 Web Socket Protocol Handshake\r\n' +
'Upgrade: WebSocket\r\n' +
'Connection: Upgrade\r\n' +
'\r\n');
socket.pipe(socket); // echo back
});
// Now that server is running
server.listen(1337, '127.0.0.1', () => {
// make a request
const options = {
port: 1337,
host: '127.0.0.1',
headers: {
'Connection': 'Upgrade',
'Upgrade': 'websocket',
},
};
const req = http.request(options);
req.end();
req.on('upgrade', (res, socket, upgradeHead) => {
console.log('got upgraded!');
socket.end();
process.exit(0);
});
});
request.abort()
#
request.destroy()
instead.Marks the request as aborting. Calling this will cause remaining data in the response to be dropped and the socket to be destroyed.
request.aborted
#
request.destroyed
instead.The request.aborted
property will be true
if the request has
been aborted.
request.connection
#
request.socket
.See request.socket
.
request.cork()
#
See writable.cork()
.
request.end([data[, encoding]][, callback])
#
data
<string> | <Buffer> | <Uint8Array>encoding
<string>callback
<Function>- Returns: <this>
Finishes sending the request. If any parts of the body are
unsent, it will flush them to the stream. If the request is
chunked, this will send the terminating '0\r\n\r\n'
.
If data
is specified, it is equivalent to calling
request.write(data, encoding)
followed by request.end(callback)
.
If callback
is specified, it will be called when the request stream
is finished.
request.destroy([error])
#
Destroy the request. Optionally emit an 'error'
event,
and emit a 'close'
event. Calling this will cause remaining data
in the response to be dropped and the socket to be destroyed.
See writable.destroy()
for further details.
request.destroyed
#
Is true
after request.destroy()
has been called.
See writable.destroyed
for further details.
request.finished
#
request.writableEnded
.The request.finished
property will be true
if request.end()
has been called. request.end()
will automatically be called if the
request was initiated via http.get()
.
request.flushHeaders()
#
Flushes the request headers.
For efficiency reasons, Node.js normally buffers the request headers until
request.end()
is called or the first chunk of request data is written. It
then tries to pack the request headers and data into a single TCP packet.
That's usually desired (it saves a TCP round-trip), but not when the first
data is not sent until possibly much later. request.flushHeaders()
bypasses
the optimization and kickstarts the request.
request.getHeader(name)
#
Reads out a header on the request. The name is case-insensitive.
The type of the return value depends on the arguments provided to
request.setHeader()
.
request.setHeader('content-type', 'text/html');
request.setHeader('Content-Length', Buffer.byteLength(body));
request.setHeader('Cookie', ['type=ninja', 'language=javascript']);
const contentType = request.getHeader('Content-Type');
// 'contentType' is 'text/html'
const contentLength = request.getHeader('Content-Length');
// 'contentLength' is of type number
const cookie = request.getHeader('Cookie');
// 'cookie' is of type string[]
request.getHeaderNames()
#
- Returns: <string[]>
Returns an array containing the unique names of the current outgoing headers. All header names are lowercase.
request.setHeader('Foo', 'bar');
request.setHeader('Cookie', ['foo=bar', 'bar=baz']);
const headerNames = request.getHeaderNames();
// headerNames === ['foo', 'cookie']
request.getHeaders()
#
- Returns: <Object>
Returns a shallow copy of the current outgoing headers. Since a shallow copy is used, array values may be mutated without additional calls to various header-related http module methods. The keys of the returned object are the header names and the values are the respective header values. All header names are lowercase.
The object returned by the request.getHeaders()
method does not
prototypically inherit from the JavaScript Object
. This means that typical
Object
methods such as obj.toString()
, obj.hasOwnProperty()
, and others
are not defined and will not work.
request.setHeader('Foo', 'bar');
request.setHeader('Cookie', ['foo=bar', 'bar=baz']);
const headers = request.getHeaders();
// headers === { foo: 'bar', 'cookie': ['foo=bar', 'bar=baz'] }
request.getRawHeaderNames()
#
- Returns: <string[]>
Returns an array containing the unique names of the current outgoing raw headers. Header names are returned with their exact casing being set.
request.setHeader('Foo', 'bar');
request.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']);
const headerNames = request.getRawHeaderNames();
// headerNames === ['Foo', 'Set-Cookie']
request.hasHeader(name)
#
Returns true
if the header identified by name
is currently set in the
outgoing headers. The header name matching is case-insensitive.
const hasContentType = request.hasHeader('content-type');
request.maxHeadersCount
#
- <number> Default:
2000
Limits maximum response headers count. If set to 0, no limit will be applied.
request.path
#
- <string> The request path.
request.method
#
- <string> The request method.
request.host
#
- <string> The request host.
request.protocol
#
- <string> The request protocol.
request.removeHeader(name)
#
name
<string>
Removes a header that's already defined into headers object.
request.removeHeader('Content-Type');
request.reusedSocket
#
- <boolean> Whether the request is send through a reused socket.
When sending request through a keep-alive enabled agent, the underlying socket might be reused. But if server closes connection at unfortunate time, client may run into a 'ECONNRESET' error.
const http = require('node:http');
// Server has a 5 seconds keep-alive timeout by default
http
.createServer((req, res) => {
res.write('hello\n');
res.end();
})
.listen(3000);
setInterval(() => {
// Adapting a keep-alive agent
http.get('http://localhost:3000', { agent }, (res) => {
res.on('data', (data) => {
// Do nothing
});
});
}, 5000); // Sending request on 5s interval so it's easy to hit idle timeout
By marking a request whether it reused socket or not, we can do automatic error retry base on it.
const http = require('node:http');
const agent = new http.Agent({ keepAlive: true });
function retriableRequest() {
const req = http
.get('http://localhost:3000', { agent }, (res) => {
// ...
})
.on('error', (err) => {
// Check if retry is needed
if (req.reusedSocket && err.code === 'ECONNRESET') {
retriableRequest();
}
});
}
retriableRequest();
request.setHeader(name, value)
#
Sets a single header value for headers object. If this header already exists in
the to-be-sent headers, its value will be replaced. Use an array of strings
here to send multiple headers with the same name. Non-string values will be
stored without modification. Therefore, request.getHeader()
may return
non-string values. However, the non-string values will be converted to strings
for network transmission.
request.setHeader('Content-Type', 'application/json');
or
request.setHeader('Cookie', ['type=ninja', 'language=javascript']);
When the value is a string an exception will be thrown if it contains
characters outside the latin1
encoding.
If you need to pass UTF-8 characters in the value please encode the value using the RFC 8187 standard.
const filename = 'Rock 🎵.txt';
request.setHeader('Content-Disposition', `attachment; filename*=utf-8''${encodeURIComponent(filename)}`);
request.setNoDelay([noDelay])
#
noDelay
<boolean>
Once a socket is assigned to this request and is connected
socket.setNoDelay()
will be called.
request.setSocketKeepAlive([enable][, initialDelay])
#
Once a socket is assigned to this request and is connected
socket.setKeepAlive()
will be called.
request.setTimeout(timeout[, callback])
#
timeout
<number> Milliseconds before a request times out.callback
<Function> Optional function to be called when a timeout occurs. Same as binding to the'timeout'
event.- Returns: <http.ClientRequest>
Once a socket is assigned to this request and is connected
socket.setTimeout()
will be called.
request.socket
#
Reference to the underlying socket. Usually users will not want to access
this property. In particular, the socket will not emit 'readable'
events
because of how the protocol parser attaches to the socket.
const http = require('node:http');
const options = {
host: 'www.google.com',
};
const req = http.get(options);
req.end();
req.once('response', (res) => {
const ip = req.socket.localAddress;
const port = req.socket.localPort;
console.log(`Your IP address is ${ip} and your source port is ${port}.`);
// Consume response object
});
This property is guaranteed to be an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specified a socket type other than <net.Socket>.
request.uncork()
#
See writable.uncork()
.
request.writableEnded
#
Is true
after request.end()
has been called. This property
does not indicate whether the data has been flushed, for this use
request.writableFinished
instead.
request.writableFinished
#
Is true
if all data has been flushed to the underlying system, immediately
before the 'finish'
event is emitted.
request.write(chunk[, encoding][, callback])
#
chunk
<string> | <Buffer> | <Uint8Array>encoding
<string>callback
<Function>- Returns: <boolean>
Sends a chunk of the body. This method can be called multiple times. If no
Content-Length
is set, data will automatically be encoded in HTTP Chunked
transfer encoding, so that server knows when the data ends. The
Transfer-Encoding: chunked
header is added. Calling request.end()
is necessary to finish sending the request.
The encoding
argument is optional and only applies when chunk
is a string.
Defaults to 'utf8'
.
The callback
argument is optional and will be called when this chunk of data
is flushed, but only if the chunk is non-empty.
Returns true
if the entire data was flushed successfully to the kernel
buffer. Returns false
if all or part of the data was queued in user memory.
'drain'
will be emitted when the buffer is free again.
When write
function is called with empty string or buffer, it does
nothing and waits for more input.
Class: http.Server
#
- Extends: <net.Server>
Event: 'checkContinue'
#
request
<http.IncomingMessage>response
<http.ServerResponse>
Emitted each time a request with an HTTP Expect: 100-continue
is received.
If this event is not listened for, the server will automatically respond
with a 100 Continue
as appropriate.
Handling this event involves calling response.writeContinue()
if the
client should continue to send the request body, or generating an appropriate
HTTP response (e.g. 400 Bad Request) if the client should not continue to send
the request body.
When this event is emitted and handled, the 'request'
event will
not be emitted.
Event: 'checkExpectation'
#
request
<http.IncomingMessage>response
<http.ServerResponse>
Emitted each time a request with an HTTP Expect
header is received, where the
value is not 100-continue
. If this event is not listened for, the server will
automatically respond with a 417 Expectation Failed
as appropriate.
When this event is emitted and handled, the 'request'
event will
not be emitted.
Event: 'clientError'
#
exception
<Error>socket
<stream.Duplex>
If a client connection emits an 'error'
event, it will be forwarded here.
Listener of this event is responsible for closing/destroying the underlying
socket. For example, one may wish to more gracefully close the socket with a
custom HTTP response instead of abruptly severing the connection. The socket
must be closed or destroyed before the listener ends.
This event is guaranteed to be passed an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specifies a socket type other than <net.Socket>.
Default behavior is to try close the socket with a HTTP '400 Bad Request',
or a HTTP '431 Request Header Fields Too Large' in the case of a
HPE_HEADER_OVERFLOW
error. If the socket is not writable or headers
of the current attached http.ServerResponse
has been sent, it is
immediately destroyed.
socket
is the net.Socket
object that the error originated from.
const http = require('node:http');
const server = http.createServer((req, res) => {
res.end();
});
server.on('clientError', (err, socket) => {
socket.end('HTTP/1.1 400 Bad Request\r\n\r\n');
});
server.listen(8000);
When the 'clientError'
event occurs, there is no request
or response
object, so any HTTP response sent, including response headers and payload,
must be written directly to the socket
object. Care must be taken to
ensure the response is a properly formatted HTTP response message.
err
is an instance of Error
with two extra columns:
bytesParsed
: the bytes count of request packet that Node.js may have parsed correctly;rawPacket
: the raw packet of current request.
In some cases, the client has already received the response and/or the socket
has already been destroyed, like in case of ECONNRESET
errors. Before
trying to send data to the socket, it is better to check that it is still
writable.
server.on('clientError', (err, socket) => {
if (err.code === 'ECONNRESET' || !socket.writable) {
return;
}
socket.end('HTTP/1.1 400 Bad Request\r\n\r\n');
});
Event: 'close'
#
Emitted when the server closes.
Event: 'connect'
#
request
<http.IncomingMessage> Arguments for the HTTP request, as it is in the'request'
eventsocket
<stream.Duplex> Network socket between the server and clienthead
<Buffer> The first packet of the tunneling stream (may be empty)
Emitted each time a client requests an HTTP CONNECT
method. If this event is
not listened for, then clients requesting a CONNECT
method will have their
connections closed.
This event is guaranteed to be passed an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specifies a socket type other than <net.Socket>.
After this event is emitted, the request's socket will not have a 'data'
event listener, meaning it will need to be bound in order to handle data
sent to the server on that socket.
Event: 'connection'
#
socket
<stream.Duplex>
This event is emitted when a new TCP stream is established. socket
is
typically an object of type net.Socket
. Usually users will not want to
access this event. In particular, the socket will not emit 'readable'
events
because of how the protocol parser attaches to the socket. The socket
can
also be accessed at request.socket
.
This event can also be explicitly emitted by users to inject connections
into the HTTP server. In that case, any Duplex
stream can be passed.
If socket.setTimeout()
is called here, the timeout will be replaced with
server.keepAliveTimeout
when the socket has served a request (if
server.keepAliveTimeout
is non-zero).
This event is guaranteed to be passed an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specifies a socket type other than <net.Socket>.
Event: 'dropRequest'
#
request
<http.IncomingMessage> Arguments for the HTTP request, as it is in the'request'
eventsocket
<stream.Duplex> Network socket between the server and client
When the number of requests on a socket reaches the threshold of
server.maxRequestsPerSocket
, the server will drop new requests
and emit 'dropRequest'
event instead, then send 503
to client.
Event: 'request'
#
request
<http.IncomingMessage>response
<http.ServerResponse>
Emitted each time there is a request. There may be multiple requests per connection (in the case of HTTP Keep-Alive connections).
Event: 'upgrade'
#
request
<http.IncomingMessage> Arguments for the HTTP request, as it is in the'request'
eventsocket
<stream.Duplex> Network socket between the server and clienthead
<Buffer> The first packet of the upgraded stream (may be empty)
Emitted each time a client requests an HTTP upgrade. Listening to this event is optional and clients cannot insist on a protocol change.
After this event is emitted, the request's socket will not have a 'data'
event listener, meaning it will need to be bound in order to handle data
sent to the server on that socket.
This event is guaranteed to be passed an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specifies a socket type other than <net.Socket>.
server.close([callback])
#
callback
<Function>
Stops the server from accepting new connections and closes all connections
connected to this server which are not sending a request or waiting for
a response.
See net.Server.close()
.
server.closeAllConnections()
#
Closes all connections connected to this server.
server.closeIdleConnections()
#
Closes all connections connected to this server which are not sending a request or waiting for a response.
server.headersTimeout
#
- <number> Default: The minimum between
server.requestTimeout
or60000
.
Limit the amount of time the parser will wait to receive the complete HTTP headers.
If the timeout expires, the server responds with status 408 without forwarding the request to the request listener and then closes the connection.
It must be set to a non-zero value (e.g. 120 seconds) to protect against potential Denial-of-Service attacks in case the server is deployed without a reverse proxy in front.
server.listen()
#
Starts the HTTP server listening for connections.
This method is identical to server.listen()
from net.Server
.
server.listening
#
- <boolean> Indicates whether or not the server is listening for connections.
server.maxHeadersCount
#
- <number> Default:
2000
Limits maximum incoming headers count. If set to 0, no limit will be applied.
server.requestTimeout
#
- <number> Default:
300000
Sets the timeout value in milliseconds for receiving the entire request from the client.
If the timeout expires, the server responds with status 408 without forwarding the request to the request listener and then closes the connection.
It must be set to a non-zero value (e.g. 120 seconds) to protect against potential Denial-of-Service attacks in case the server is deployed without a reverse proxy in front.
server.setTimeout([msecs][, callback])
#
msecs
<number> Default: 0 (no timeout)callback
<Function>- Returns: <http.Server>
Sets the timeout value for sockets, and emits a 'timeout'
event on
the Server object, passing the socket as an argument, if a timeout
occurs.
If there is a 'timeout'
event listener on the Server object, then it
will be called with the timed-out socket as an argument.
By default, the Server does not timeout sockets. However, if a callback
is assigned to the Server's 'timeout'
event, timeouts must be handled
explicitly.
server.maxRequestsPerSocket
#
- <number> Requests per socket. Default: 0 (no limit)
The maximum number of requests socket can handle before closing keep alive connection.
A value of 0
will disable the limit.
When the limit is reached it will set the Connection
header value to close
,
but will not actually close the connection, subsequent requests sent
after the limit is reached will get 503 Service Unavailable
as a response.
server.timeout
#
- <number> Timeout in milliseconds. Default: 0 (no timeout)
The number of milliseconds of inactivity before a socket is presumed to have timed out.
A value of 0
will disable the timeout behavior on incoming connections.
The socket timeout logic is set up on connection, so changing this value only affects new connections to the server, not any existing connections.
server.keepAliveTimeout
#
- <number> Timeout in milliseconds. Default:
5000
(5 seconds).
The number of milliseconds of inactivity a server needs to wait for additional
incoming data, after it has finished writing the last response, before a socket
will be destroyed. If the server receives new data before the keep-alive
timeout has fired, it will reset the regular inactivity timeout, i.e.,
server.timeout
.
A value of 0
will disable the keep-alive timeout behavior on incoming
connections.
A value of 0
makes the http server behave similarly to Node.js versions prior
to 8.0.0, which did not have a keep-alive timeout.
The socket timeout logic is set up on connection, so changing this value only affects new connections to the server, not any existing connections.
Class: http.ServerResponse
#
- Extends: <http.OutgoingMessage>
This object is created internally by an HTTP server, not by the user. It is
passed as the second parameter to the 'request'
event.
Event: 'close'
#
Indicates that the response is completed, or its underlying connection was terminated prematurely (before the response completion).
Event: 'finish'
#
Emitted when the response has been sent. More specifically, this event is emitted when the last segment of the response headers and body have been handed off to the operating system for transmission over the network. It does not imply that the client has received anything yet.
response.addTrailers(headers)
#
headers
<Object>
This method adds HTTP trailing headers (a header but at the end of the message) to the response.
Trailers will only be emitted if chunked encoding is used for the response; if it is not (e.g. if the request was HTTP/1.0), they will be silently discarded.
HTTP requires the Trailer
header to be sent in order to
emit trailers, with a list of the header fields in its value. E.g.,
response.writeHead(200, { 'Content-Type': 'text/plain',
'Trailer': 'Content-MD5' });
response.write(fileData);
response.addTrailers({ 'Content-MD5': '7895bf4b8828b55ceaf47747b4bca667' });
response.end();
Attempting to set a header field name or value that contains invalid characters
will result in a TypeError
being thrown.
response.connection
#
response.socket
.See response.socket
.
response.cork()
#
See writable.cork()
.
response.end([data[, encoding]][, callback])
#
data
<string> | <Buffer> | <Uint8Array>encoding
<string>callback
<Function>- Returns: <this>
This method signals to the server that all of the response headers and body
have been sent; that server should consider this message complete.
The method, response.end()
, MUST be called on each response.
If data
is specified, it is similar in effect to calling
response.write(data, encoding)
followed by response.end(callback)
.
If callback
is specified, it will be called when the response stream
is finished.
response.finished
#
response.writableEnded
.The response.finished
property will be true
if response.end()
has been called.
response.flushHeaders()
#
Flushes the response headers. See also: request.flushHeaders()
.
response.getHeader(name)
#
Reads out a header that's already been queued but not sent to the client.
The name is case-insensitive. The type of the return value depends
on the arguments provided to response.setHeader()
.
response.setHeader('Content-Type', 'text/html');
response.setHeader('Content-Length', Buffer.byteLength(body));
response.setHeader('Set-Cookie', ['type=ninja', 'language=javascript']);
const contentType = response.getHeader('content-type');
// contentType is 'text/html'
const contentLength = response.getHeader('Content-Length');
// contentLength is of type number
const setCookie = response.getHeader('set-cookie');
// setCookie is of type string[]
response.getHeaderNames()
#
- Returns: <string[]>
Returns an array containing the unique names of the current outgoing headers. All header names are lowercase.
response.setHeader('Foo', 'bar');
response.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']);
const headerNames = response.getHeaderNames();
// headerNames === ['foo', 'set-cookie']
response.getHeaders()
#
- Returns: <Object>
Returns a shallow copy of the current outgoing headers. Since a shallow copy is used, array values may be mutated without additional calls to various header-related http module methods. The keys of the returned object are the header names and the values are the respective header values. All header names are lowercase.
The object returned by the response.getHeaders()
method does not
prototypically inherit from the JavaScript Object
. This means that typical
Object
methods such as obj.toString()
, obj.hasOwnProperty()
, and others
are not defined and will not work.
response.setHeader('Foo', 'bar');
response.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']);
const headers = response.getHeaders();
// headers === { foo: 'bar', 'set-cookie': ['foo=bar', 'bar=baz'] }
response.hasHeader(name)
#
Returns true
if the header identified by name
is currently set in the
outgoing headers. The header name matching is case-insensitive.
const hasContentType = response.hasHeader('content-type');
response.headersSent
#
Boolean (read-only). True if headers were sent, false otherwise.
response.removeHeader(name)
#
name
<string>
Removes a header that's queued for implicit sending.
response.removeHeader('Content-Encoding');
response.req
#
A reference to the original HTTP request
object.
response.sendDate
#
When true, the Date header will be automatically generated and sent in the response if it is not already present in the headers. Defaults to true.
This should only be disabled for testing; HTTP requires the Date header in responses.
response.setHeader(name, value)
#
name
<string>value
<any>- Returns: <http.ServerResponse>
Returns the response object.
Sets a single header value for implicit headers. If this header already exists
in the to-be-sent headers, its value will be replaced. Use an array of strings
here to send multiple headers with the same name. Non-string values will be
stored without modification. Therefore, response.getHeader()
may return
non-string values. However, the non-string values will be converted to strings
for network transmission. The same response object is returned to the caller,
to enable call chaining.
response.setHeader('Content-Type', 'text/html');
or
response.setHeader('Set-Cookie', ['type=ninja', 'language=javascript']);
Attempting to set a header field name or value that contains invalid characters
will result in a TypeError
being thrown.
When headers have been set with response.setHeader()
, they will be merged
with any headers passed to response.writeHead()
, with the headers passed
to response.writeHead()
given precedence.
// Returns content-type = text/plain
const server = http.createServer((req, res) => {
res.setHeader('Content-Type', 'text/html');
res.setHeader('X-Foo', 'bar');
res.writeHead(200, { 'Content-Type': 'text/plain' });
res.end('ok');
});
If response.writeHead()
method is called and this method has not been
called, it will directly write the supplied header values onto the network
channel without caching internally, and the response.getHeader()
on the
header will not yield the expected result. If progressive population of headers
is desired with potential future retrieval and modification, use
response.setHeader()
instead of response.writeHead()
.
response.setTimeout(msecs[, callback])
#
msecs
<number>callback
<Function>- Returns: <http.ServerResponse>
Sets the Socket's timeout value to msecs
. If a callback is
provided, then it is added as a listener on the 'timeout'
event on
the response object.
If no 'timeout'
listener is added to the request, the response, or
the server, then sockets are destroyed when they time out. If a handler is
assigned to the request, the response, or the server's 'timeout'
events,
timed out sockets must be handled explicitly.
response.socket
#
Reference to the underlying socket. Usually users will not want to access
this property. In particular, the socket will not emit 'readable'
events
because of how the protocol parser attaches to the socket. After
response.end()
, the property is nulled.
const http = require('node:http');
const server = http.createServer((req, res) => {
const ip = res.socket.remoteAddress;
const port = res.socket.remotePort;
res.end(`Your IP address is ${ip} and your source port is ${port}.`);
}).listen(3000);
This property is guaranteed to be an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specified a socket type other than <net.Socket>.
response.statusCode
#
- <number> Default:
200
When using implicit headers (not calling response.writeHead()
explicitly),
this property controls the status code that will be sent to the client when
the headers get flushed.
response.statusCode = 404;
After response header was sent to the client, this property indicates the status code which was sent out.
response.statusMessage
#
When using implicit headers (not calling response.writeHead()
explicitly),
this property controls the status message that will be sent to the client when
the headers get flushed. If this is left as undefined
then the standard
message for the status code will be used.
response.statusMessage = 'Not found';
After response header was sent to the client, this property indicates the status message which was sent out.
response.strictContentLength
#
- <boolean> Default:
false
If set to true
, Node.js will check whether the Content-Length
header value and the size of the body, in bytes, are equal.
Mismatching the Content-Length
header value will result
in an Error
being thrown, identified by code:
'ERR_HTTP_CONTENT_LENGTH_MISMATCH'
.
response.uncork()
#
See writable.uncork()
.
response.writableEnded
#
Is true
after response.end()
has been called. This property
does not indicate whether the data has been flushed, for this use
response.writableFinished
instead.
response.writableFinished
#
Is true
if all data has been flushed to the underlying system, immediately
before the 'finish'
event is emitted.
response.write(chunk[, encoding][, callback])
#
chunk
<string> | <Buffer> | <Uint8Array>encoding
<string> Default:'utf8'
callback
<Function>- Returns: <boolean>
If this method is called and response.writeHead()
has not been called,
it will switch to implicit header mode and flush the implicit headers.
This sends a chunk of the response body. This method may be called multiple times to provide successive parts of the body.
In the node:http
module, the response body is omitted when the
request is a HEAD request. Similarly, the 204
and 304
responses
must not include a message body.
chunk
can be a string or a buffer. If chunk
is a string,
the second parameter specifies how to encode it into a byte stream.
callback
will be called when this chunk of data is flushed.
This is the raw HTTP body and has nothing to do with higher-level multi-part body encodings that may be used.
The first time response.write()
is called, it will send the buffered
header information and the first chunk of the body to the client. The second
time response.write()
is called, Node.js assumes data will be streamed,
and sends the new data separately. That is, the response is buffered up to the
first chunk of the body.
Returns true
if the entire data was flushed successfully to the kernel
buffer. Returns false
if all or part of the data was queued in user memory.
'drain'
will be emitted when the buffer is free again.
response.writeContinue()
#
Sends an HTTP/1.1 100 Continue message to the client, indicating that
the request body should be sent. See the 'checkContinue'
event on
Server
.
response.writeEarlyHints(hints[, callback])
#
hints
<Object>callback
<Function>
Sends an HTTP/1.1 103 Early Hints message to the client with a Link header,
indicating that the user agent can preload/preconnect the linked resources.
The hints
is an object containing the values of headers to be sent with
early hints message. The optional callback
argument will be called when
the response message has been written.
Example
const earlyHintsLink = '</styles.css>; rel=preload; as=style';
response.writeEarlyHints({
'link': earlyHintsLink,
});
const earlyHintsLinks = [
'</styles.css>; rel=preload; as=style',
'</scripts.js>; rel=preload; as=script',
];
response.writeEarlyHints({
'link': earlyHintsLinks,
'x-trace-id': 'id for diagnostics',
});
const earlyHintsCallback = () => console.log('early hints message sent');
response.writeEarlyHints({
'link': earlyHintsLinks,
}, earlyHintsCallback);
response.writeHead(statusCode[, statusMessage][, headers])
#
statusCode
<number>statusMessage
<string>headers
<Object> | <Array>- Returns: <http.ServerResponse>
Sends a response header to the request. The status code is a 3-digit HTTP
status code, like 404
. The last argument, headers
, are the response headers.
Optionally one can give a human-readable statusMessage
as the second
argument.
headers
may be an Array
where the keys and values are in the same list.
It is not a list of tuples. So, the even-numbered offsets are key values,
and the odd-numbered offsets are the associated values. The array is in the same
format as request.rawHeaders
.
Returns a reference to the ServerResponse
, so that calls can be chained.
const body = 'hello world';
response
.writeHead(200, {
'Content-Length': Buffer.byteLength(body),
'Content-Type': 'text/plain',
})
.end(body);
This method must only be called once on a message and it must
be called before response.end()
is called.
If response.write()
or response.end()
are called before calling
this, the implicit/mutable headers will be calculated and call this function.
When headers have been set with response.setHeader()
, they will be merged
with any headers passed to response.writeHead()
, with the headers passed
to response.writeHead()
given precedence.
If this method is called and response.setHeader()
has not been called,
it will directly write the supplied header values onto the network channel
without caching internally, and the response.getHeader()
on the header
will not yield the expected result. If progressive population of headers is
desired with potential future retrieval and modification, use
response.setHeader()
instead.
// Returns content-type = text/plain
const server = http.createServer((req, res) => {
res.setHeader('Content-Type', 'text/html');
res.setHeader('X-Foo', 'bar');
res.writeHead(200, { 'Content-Type': 'text/plain' });
res.end('ok');
});
Content-Length
is read in bytes, not characters. Use
Buffer.byteLength()
to determine the length of the body in bytes. Node.js
will check whether Content-Length
and the length of the body which has
been transmitted are equal or not.
Attempting to set a header field name or value that contains invalid characters
will result in a [Error
][] being thrown.
response.writeProcessing()
#
Sends a HTTP/1.1 102 Processing message to the client, indicating that the request body should be sent.
Class: http.IncomingMessage
#
- Extends: <stream.Readable>
An IncomingMessage
object is created by http.Server
or
http.ClientRequest
and passed as the first argument to the 'request'
and 'response'
event respectively. It may be used to access response
status, headers, and data.
Different from its socket
value which is a subclass of <stream.Duplex>, the
IncomingMessage
itself extends <stream.Readable> and is created separately to
parse and emit the incoming HTTP headers and payload, as the underlying socket
may be reused multiple times in case of keep-alive.
Event: 'aborted'
#
'close'
event instead.Emitted when the request has been aborted.
Event: 'close'
#
Emitted when the request has been completed.
message.aborted
#
The message.aborted
property will be true
if the request has
been aborted.
message.complete
#
The message.complete
property will be true
if a complete HTTP message has
been received and successfully parsed.
This property is particularly useful as a means of determining if a client or server fully transmitted a message before a connection was terminated:
const req = http.request({
host: '127.0.0.1',
port: 8080,
method: 'POST',
}, (res) => {
res.resume();
res.on('end', () => {
if (!res.complete)
console.error(
'The connection was terminated while the message was still being sent');
});
});
message.connection
#
message.socket
.Alias for message.socket
.
message.destroy([error])
#
Calls destroy()
on the socket that received the IncomingMessage
. If error
is provided, an 'error'
event is emitted on the socket and error
is passed
as an argument to any listeners on the event.
message.headers
#
The request/response headers object.
Key-value pairs of header names and values. Header names are lower-cased.
// Prints something like:
//
// { 'user-agent': 'curl/7.22.0',
// host: '127.0.0.1:8000',
// accept: '*/*' }
console.log(request.headers);
Duplicates in raw headers are handled in the following ways, depending on the header name:
- Duplicates of
age
,authorization
,content-length
,content-type
,etag
,expires
,from
,host
,if-modified-since
,if-unmodified-since
,last-modified
,location
,max-forwards
,proxy-authorization
,referer
,retry-after
,server
, oruser-agent
are discarded. To allow duplicate values of the headers listed above to be joined, use the optionjoinDuplicateHeaders
inhttp.request()
andhttp.createServer()
. See RFC 9110 Section 5.3 for more information. set-cookie
is always an array. Duplicates are added to the array.- For duplicate
cookie
headers, the values are joined together with;
. - For all other headers, the values are joined together with
,
.
message.headersDistinct
#
Similar to message.headers
, but there is no join logic and the values are
always arrays of strings, even for headers received just once.
// Prints something like:
//
// { 'user-agent': ['curl/7.22.0'],
// host: ['127.0.0.1:8000'],
// accept: ['*/*'] }
console.log(request.headersDistinct);
message.httpVersion
#
In case of server request, the HTTP version sent by the client. In the case of
client response, the HTTP version of the connected-to server.
Probably either '1.1'
or '1.0'
.
Also message.httpVersionMajor
is the first integer and
message.httpVersionMinor
is the second.
message.method
#
Only valid for request obtained from http.Server
.
The request method as a string. Read only. Examples: 'GET'
, 'DELETE'
.
message.rawHeaders
#
The raw request/response headers list exactly as they were received.
The keys and values are in the same list. It is not a list of tuples. So, the even-numbered offsets are key values, and the odd-numbered offsets are the associated values.
Header names are not lowercased, and duplicates are not merged.
// Prints something like:
//
// [ 'user-agent',
// 'this is invalid because there can be only one',
// 'User-Agent',
// 'curl/7.22.0',
// 'Host',
// '127.0.0.1:8000',
// 'ACCEPT',
// '*/*' ]
console.log(request.rawHeaders);
message.rawTrailers
#
The raw request/response trailer keys and values exactly as they were
received. Only populated at the 'end'
event.
message.setTimeout(msecs[, callback])
#
msecs
<number>callback
<Function>- Returns: <http.IncomingMessage>
Calls message.socket.setTimeout(msecs, callback)
.
message.socket
#
The net.Socket
object associated with the connection.
With HTTPS support, use request.socket.getPeerCertificate()
to obtain the
client's authentication details.
This property is guaranteed to be an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specified a socket type other than <net.Socket> or internally nulled.
message.statusCode
#
Only valid for response obtained from http.ClientRequest
.
The 3-digit HTTP response status code. E.G. 404
.
message.statusMessage
#
Only valid for response obtained from http.ClientRequest
.
The HTTP response status message (reason phrase). E.G. OK
or Internal Server Error
.
message.trailers
#
The request/response trailers object. Only populated at the 'end'
event.
message.trailersDistinct
#
Similar to message.trailers
, but there is no join logic and the values are
always arrays of strings, even for headers received just once.
Only populated at the 'end'
event.
message.url
#
Only valid for request obtained from http.Server
.
Request URL string. This contains only the URL that is present in the actual HTTP request. Take the following request:
GET /status?name=ryan HTTP/1.1
Accept: text/plain
To parse the URL into its parts:
new URL(request.url, `http://${request.headers.host}`);
When request.url
is '/status?name=ryan'
and request.headers.host
is
'localhost:3000'
:
$ node
> new URL(request.url, `http://${request.headers.host}`)
URL {
href: 'http://localhost:3000/status?name=ryan',
origin: 'http://localhost:3000',
protocol: 'http:',
username: '',
password: '',
host: 'localhost:3000',
hostname: 'localhost',
port: '3000',
pathname: '/status',
search: '?name=ryan',
searchParams: URLSearchParams { 'name' => 'ryan' },
hash: ''
}
Class: http.OutgoingMessage
#
- Extends: <Stream>
This class serves as the parent class of http.ClientRequest
and http.ServerResponse
. It is an abstract outgoing message from
the perspective of the participants of an HTTP transaction.
Event: 'drain'
#
Emitted when the buffer of the message is free again.
Event: 'finish'
#
Emitted when the transmission is finished successfully.
Event: 'prefinish'
#
Emitted after outgoingMessage.end()
is called.
When the event is emitted, all data has been processed but not necessarily
completely flushed.
outgoingMessage.addTrailers(headers)
#
headers
<Object>
Adds HTTP trailers (headers but at the end of the message) to the message.
Trailers will only be emitted if the message is chunked encoded. If not, the trailers will be silently discarded.
HTTP requires the Trailer
header to be sent to emit trailers,
with a list of header field names in its value, e.g.
message.writeHead(200, { 'Content-Type': 'text/plain',
'Trailer': 'Content-MD5' });
message.write(fileData);
message.addTrailers({ 'Content-MD5': '7895bf4b8828b55ceaf47747b4bca667' });
message.end();
Attempting to set a header field name or value that contains invalid characters
will result in a TypeError
being thrown.
outgoingMessage.appendHeader(name, value)
#
name
<string> Header namevalue
<string> | <string[]> Header value- Returns: <this>
Append a single header value for the header object.
If the value is an array, this is equivalent of calling this method multiple times.
If there were no previous value for the header, this is equivalent of calling
outgoingMessage.setHeader(name, value)
.
Depending of the value of options.uniqueHeaders
when the client request or the
server were created, this will end up in the header being sent multiple times or
a single time with values joined using ;
.
outgoingMessage.connection
#
outgoingMessage.socket
instead.Alias of outgoingMessage.socket
.
outgoingMessage.cork()
#
See writable.cork()
.
outgoingMessage.destroy([error])
#
Destroys the message. Once a socket is associated with the message and is connected, that socket will be destroyed as well.
outgoingMessage.end(chunk[, encoding][, callback])
#
chunk
<string> | <Buffer> | <Uint8Array>encoding
<string> Optional, Default:utf8
callback
<Function> Optional- Returns: <this>
Finishes the outgoing message. If any parts of the body are unsent, it will
flush them to the underlying system. If the message is chunked, it will
send the terminating chunk 0\r\n\r\n
, and send the trailers (if any).
If chunk
is specified, it is equivalent to calling
outgoingMessage.write(chunk, encoding)
, followed by
outgoingMessage.end(callback)
.
If callback
is provided, it will be called when the message is finished
(equivalent to a listener of the 'finish'
event).
outgoingMessage.flushHeaders()
#
Flushes the message headers.
For efficiency reason, Node.js normally buffers the message headers
until outgoingMessage.end()
is called or the first chunk of message data
is written. It then tries to pack the headers and data into a single TCP
packet.
It is usually desired (it saves a TCP round-trip), but not when the first
data is not sent until possibly much later. outgoingMessage.flushHeaders()
bypasses the optimization and kickstarts the message.
outgoingMessage.getHeader(name)
#
name
<string> Name of header- Returns <string> | <undefined>
Gets the value of the HTTP header with the given name. If that header is not
set, the returned value will be undefined
.
outgoingMessage.getHeaderNames()
#
- Returns <string[]>
Returns an array containing the unique names of the current outgoing headers. All names are lowercase.
outgoingMessage.getHeaders()
#
- Returns: <Object>
Returns a shallow copy of the current outgoing headers. Since a shallow copy is used, array values may be mutated without additional calls to various header-related HTTP module methods. The keys of the returned object are the header names and the values are the respective header values. All header names are lowercase.
The object returned by the outgoingMessage.getHeaders()
method does
not prototypically inherit from the JavaScript Object
. This means that
typical Object
methods such as obj.toString()
, obj.hasOwnProperty()
,
and others are not defined and will not work.
outgoingMessage.setHeader('Foo', 'bar');
outgoingMessage.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']);
const headers = outgoingMessage.getHeaders();
// headers === { foo: 'bar', 'set-cookie': ['foo=bar', 'bar=baz'] }
outgoingMessage.hasHeader(name)
#
Returns true
if the header identified by name
is currently set in the
outgoing headers. The header name is case-insensitive.
const hasContentType = outgoingMessage.hasHeader('content-type');
outgoingMessage.headersSent
#
Read-only. true
if the headers were sent, otherwise false
.
outgoingMessage.pipe()
#
Overrides the stream.pipe()
method inherited from the legacy Stream
class
which is the parent class of http.OutgoingMessage
.
Calling this method will throw an Error
because outgoingMessage
is a
write-only stream.
outgoingMessage.removeHeader(name)
#
name
<string> Header name
Removes a header that is queued for implicit sending.
outgoingMessage.removeHeader('Content-Encoding');
outgoingMessage.setHeader(name, value)
#
Sets a single header value. If the header already exists in the to-be-sent headers, its value will be replaced. Use an array of strings to send multiple headers with the same name.
outgoingMessage.setHeaders(headers)
#
headers
<Headers> | <Map>- Returns: <http.ServerResponse>
Returns the response object.
Sets multiple header values for implicit headers.
headers
must be an instance of Headers
or Map
,
if a header already exists in the to-be-sent headers,
its value will be replaced.
const headers = new Headers({ foo: 'bar' });
response.setHeaders(headers);
or
const headers = new Map([['foo', 'bar']]);
res.setHeaders(headers);
When headers have been set with outgoingMessage.setHeaders()
,
they will be merged with any headers passed to response.writeHead()
,
with the headers passed to response.writeHead()
given precedence.
// Returns content-type = text/plain
const server = http.createServer((req, res) => {
const headers = new Headers({ 'Content-Type': 'text/html' });
res.setHeaders(headers);
res.writeHead(200, { 'Content-Type': 'text/plain' });
res.end('ok');
});
outgoingMessage.setTimeout(msesc[, callback])
#
msesc
<number>callback
<Function> Optional function to be called when a timeout occurs. Same as binding to thetimeout
event.- Returns: <this>
Once a socket is associated with the message and is connected,
socket.setTimeout()
will be called with msecs
as the first parameter.
outgoingMessage.socket
#
Reference to the underlying socket. Usually, users will not want to access this property.
After calling outgoingMessage.end()
, this property will be nulled.
outgoingMessage.uncork()
#
outgoingMessage.writableCorked
#
The number of times outgoingMessage.cork()
has been called.
outgoingMessage.writableEnded
#
Is true
if outgoingMessage.end()
has been called. This property does
not indicate whether the data has been flushed. For that purpose, use
message.writableFinished
instead.
outgoingMessage.writableFinished
#
Is true
if all data has been flushed to the underlying system.
outgoingMessage.writableHighWaterMark
#
The highWaterMark
of the underlying socket if assigned. Otherwise, the default
buffer level when writable.write()
starts returning false (16384
).
outgoingMessage.writableLength
#
The number of buffered bytes.
outgoingMessage.writableObjectMode
#
Always false
.
outgoingMessage.write(chunk[, encoding][, callback])
#
chunk
<string> | <Buffer> | <Uint8Array>encoding
<string> Default:utf8
callback
<Function>- Returns <boolean>
Sends a chunk of the body. This method can be called multiple times.
The encoding
argument is only relevant when chunk
is a string. Defaults to
'utf8'
.
The callback
argument is optional and will be called when this chunk of data
is flushed.
Returns true
if the entire data was flushed successfully to the kernel
buffer. Returns false
if all or part of the data was queued in the user
memory. The 'drain'
event will be emitted when the buffer is free again.
http.METHODS
#
A list of the HTTP methods that are supported by the parser.
http.STATUS_CODES
#
A collection of all the standard HTTP response status codes, and the
short description of each. For example, http.STATUS_CODES[404] === 'Not Found'
.
http.createServer([options][, requestListener])
#
-
options
<Object>connectionsCheckingInterval
: Sets the interval value in milliseconds to check for request and headers timeout in incomplete requests. Default:30000
.headersTimeout
: Sets the timeout value in milliseconds for receiving the complete HTTP headers from the client. Seeserver.headersTimeout
for more information. Default:60000
.insecureHTTPParser
<boolean> Use an insecure HTTP parser that accepts invalid HTTP headers whentrue
. Using the insecure parser should be avoided. See--insecure-http-parser
for more information. Default:false
.IncomingMessage
<http.IncomingMessage> Specifies theIncomingMessage
class to be used. Useful for extending the originalIncomingMessage
. Default:IncomingMessage
.keepAlive
<boolean> If set totrue
, it enables keep-alive functionality on the socket immediately after a new incoming connection is received, similarly on what is done in [socket.setKeepAlive([enable][, initialDelay])
][socket.setKeepAlive(enable, initialDelay)
]. Default:false
.keepAliveInitialDelay
<number> If set to a positive number, it sets the initial delay before the first keepalive probe is sent on an idle socket. Default:0
.keepAliveTimeout
: The number of milliseconds of inactivity a server needs to wait for additional incoming data, after it has finished writing the last response, before a socket will be destroyed. Seeserver.keepAliveTimeout
for more information. Default:5000
.maxHeaderSize
<number> Optionally overrides the value of--max-http-header-size
for requests received by this server, i.e. the maximum length of request headers in bytes. Default: 16384 (16 KiB).noDelay
<boolean> If set totrue
, it disables the use of Nagle's algorithm immediately after a new incoming connection is received. Default:true
.requestTimeout
: Sets the timeout value in milliseconds for receiving the entire request from the client. Seeserver.requestTimeout
for more information. Default:300000
.requireHostHeader
<boolean> It forces the server to respond with a 400 (Bad Request) status code to any HTTP/1.1 request message that lacks a Host header (as mandated by the specification). Default:true
.joinDuplicateHeaders
<boolean> It joins the field line values of multiple headers in a request with,
instead of discarding the duplicates. Seemessage.headers
for more information. Default:false
.ServerResponse
<http.ServerResponse> Specifies theServerResponse
class to be used. Useful for extending the originalServerResponse
. Default:ServerResponse
.uniqueHeaders
<Array> A list of response headers that should be sent only once. If the header's value is an array, the items will be joined using;
.
-
requestListener
<Function> -
Returns: <http.Server>
Returns a new instance of http.Server
.
The requestListener
is a function which is automatically
added to the 'request'
event.
const http = require('node:http');
// Create a local server to receive data from
const server = http.createServer((req, res) => {
res.writeHead(200, { 'Content-Type': 'application/json' });
res.end(JSON.stringify({
data: 'Hello World!',
}));
});
server.listen(8000);
const http = require('node:http');
// Create a local server to receive data from
const server = http.createServer();
// Listen to the request event
server.on('request', (request, res) => {
res.writeHead(200, { 'Content-Type': 'application/json' });
res.end(JSON.stringify({
data: 'Hello World!',
}));
});
server.listen(8000);
http.get(options[, callback])
#
http.get(url[, options][, callback])
#
url
<string> | <URL>options
<Object> Accepts the sameoptions
ashttp.request()
, with themethod
always set toGET
. Properties that are inherited from the prototype are ignored.callback
<Function>- Returns: <http.ClientRequest>
Since most requests are GET requests without bodies, Node.js provides this
convenience method. The only difference between this method and
http.request()
is that it sets the method to GET and calls req.end()
automatically. The callback must take care to consume the response
data for reasons stated in http.ClientRequest
section.
The callback
is invoked with a single argument that is an instance of
http.IncomingMessage
.
JSON fetching example:
http.get('http://localhost:8000/', (res) => {
const { statusCode } = res;
const contentType = res.headers['content-type'];
let error;
// Any 2xx status code signals a successful response but
// here we're only checking for 200.
if (statusCode !== 200) {
error = new Error('Request Failed.\n' +
`Status Code: ${statusCode}`);
} else if (!/^application\/json/.test(contentType)) {
error = new Error('Invalid content-type.\n' +
`Expected application/json but received ${contentType}`);
}
if (error) {
console.error(error.message);
// Consume response data to free up memory
res.resume();
return;
}
res.setEncoding('utf8');
let rawData = '';
res.on('data', (chunk) => { rawData += chunk; });
res.on('end', () => {
try {
const parsedData = JSON.parse(rawData);
console.log(parsedData);
} catch (e) {
console.error(e.message);
}
});
}).on('error', (e) => {
console.error(`Got error: ${e.message}`);
});
// Create a local server to receive data from
const server = http.createServer((req, res) => {
res.writeHead(200, { 'Content-Type': 'application/json' });
res.end(JSON.stringify({
data: 'Hello World!',
}));
});
server.listen(8000);
http.globalAgent
#
Global instance of Agent
which is used as the default for all HTTP client
requests.
http.maxHeaderSize
#
Read-only property specifying the maximum allowed size of HTTP headers in bytes.
Defaults to 16 KiB. Configurable using the --max-http-header-size
CLI
option.
This can be overridden for servers and client requests by passing the
maxHeaderSize
option.
http.request(options[, callback])
#
http.request(url[, options][, callback])
#
url
<string> | <URL>options
<Object>agent
<http.Agent> | <boolean> ControlsAgent
behavior. Possible values:undefined
(default): usehttp.globalAgent
for this host and port.Agent
object: explicitly use the passed inAgent
.false
: causes a newAgent
with default values to be used.
auth
<string> Basic authentication ('user:password'
) to compute an Authorization header.createConnection
<Function> A function that produces a socket/stream to use for the request when theagent
option is not used. This can be used to avoid creating a customAgent
class just to override the defaultcreateConnection
function. Seeagent.createConnection()
for more details. AnyDuplex
stream is a valid return value.defaultPort
<number> Default port for the protocol. Default:agent.defaultPort
if anAgent
is used, elseundefined
.family
<number> IP address family to use when resolvinghost
orhostname
. Valid values are4
or6
. When unspecified, both IP v4 and v6 will be used.headers
<Object> An object containing request headers.hints
<number> Optionaldns.lookup()
hints.host
<string> A domain name or IP address of the server to issue the request to. Default:'localhost'
.hostname
<string> Alias forhost
. To supporturl.parse()
,hostname
will be used if bothhost
andhostname
are specified.insecureHTTPParser
<boolean> Use an insecure HTTP parser that accepts invalid HTTP headers whentrue
. Using the insecure parser should be avoided. See--insecure-http-parser
for more information. Default:false
localAddress
<string> Local interface to bind for network connections.localPort
<number> Local port to connect from.lookup
<Function> Custom lookup function. Default:dns.lookup()
.maxHeaderSize
<number> Optionally overrides the value of--max-http-header-size
(the maximum length of response headers in bytes) for responses received from the server. Default: 16384 (16 KiB).method
<string> A string specifying the HTTP request method. Default:'GET'
.path
<string> Request path. Should include query string if any. E.G.'/index.html?page=12'
. An exception is thrown when the request path contains illegal characters. Currently, only spaces are rejected but that may change in the future. Default:'/'
.port
<number> Port of remote server. Default:defaultPort
if set, else80
.protocol
<string> Protocol to use. Default:'http:'
.setHost
<boolean>: Specifies whether or not to automatically add theHost
header. Defaults totrue
.signal
<AbortSignal>: An AbortSignal that may be used to abort an ongoing request.socketPath
<string> Unix domain socket. Cannot be used if one ofhost
orport
is specified, as those specify a TCP Socket.timeout
<number>: A number specifying the socket timeout in milliseconds. This will set the timeout before the socket is connected.uniqueHeaders
<Array> A list of request headers that should be sent only once. If the header's value is an array, the items will be joined using;
.joinDuplicateHeaders
<boolean> It joins the field line values of multiple headers in a request with,
instead of discarding the duplicates. Seemessage.headers
for more information. Default:false
.
callback
<Function>- Returns: <http.ClientRequest>
options
in socket.connect()
are also supported.
Node.js maintains several connections per server to make HTTP requests. This function allows one to transparently issue requests.
url
can be a string or a URL
object. If url
is a
string, it is automatically parsed with new URL()
. If it is a URL
object, it will be automatically converted to an ordinary options
object.
If both url
and options
are specified, the objects are merged, with the
options
properties taking precedence.
The optional callback
parameter will be added as a one-time listener for
the 'response'
event.
http.request()
returns an instance of the http.ClientRequest
class. The ClientRequest
instance is a writable stream. If one needs to
upload a file with a POST request, then write to the ClientRequest
object.
const http = require('node:http');
const postData = JSON.stringify({
'msg': 'Hello World!',
});
const options = {
hostname: 'www.google.com',
port: 80,
path: '/upload',
method: 'POST',
headers: {
'Content-Type': 'application/json',
'Content-Length': Buffer.byteLength(postData),
},
};
const req = http.request(options, (res) => {
console.log(`STATUS: ${res.statusCode}`);
console.log(`HEADERS: ${JSON.stringify(res.headers)}`);
res.setEncoding('utf8');
res.on('data', (chunk) => {
console.log(`BODY: ${chunk}`);
});
res.on('end', () => {
console.log('No more data in response.');
});
});
req.on('error', (e) => {
console.error(`problem with request: ${e.message}`);
});
// Write data to request body
req.write(postData);
req.end();
In the example req.end()
was called. With http.request()
one
must always call req.end()
to signify the end of the request -
even if there is no data being written to the request body.
If any error is encountered during the request (be that with DNS resolution,
TCP level errors, or actual HTTP parse errors) an 'error'
event is emitted
on the returned request object. As with all 'error'
events, if no listeners
are registered the error will be thrown.
There are a few special headers that should be noted.
-
Sending a 'Connection: keep-alive' will notify Node.js that the connection to the server should be persisted until the next request.
-
Sending a 'Content-Length' header will disable the default chunked encoding.
-
Sending an 'Expect' header will immediately send the request headers. Usually, when sending 'Expect: 100-continue', both a timeout and a listener for the
'continue'
event should be set. See RFC 2616 Section 8.2.3 for more information. -
Sending an Authorization header will override using the
auth
option to compute basic authentication.
Example using a URL
as options
:
const options = new URL('http://abc:[email protected]');
const req = http.request(options, (res) => {
// ...
});
In a successful request, the following events will be emitted in the following order:
'socket'
'response'
'data'
any number of times, on theres
object ('data'
will not be emitted at all if the response body is empty, for instance, in most redirects)'end'
on theres
object
'close'
In the case of a connection error, the following events will be emitted:
'socket'
'error'
'close'
In the case of a premature connection close before the response is received, the following events will be emitted in the following order:
'socket'
'error'
with an error with message'Error: socket hang up'
and code'ECONNRESET'
'close'
In the case of a premature connection close after the response is received, the following events will be emitted in the following order:
'socket'
'response'
'data'
any number of times, on theres
object
- (connection closed here)
'aborted'
on theres
object'error'
on theres
object with an error with message'Error: aborted'
and code'ECONNRESET'
'close'
'close'
on theres
object
If req.destroy()
is called before a socket is assigned, the following
events will be emitted in the following order:
- (
req.destroy()
called here) 'error'
with an error with message'Error: socket hang up'
and code'ECONNRESET'
, or the error with whichreq.destroy()
was called'close'
If req.destroy()
is called before the connection succeeds, the following
events will be emitted in the following order:
'socket'
- (
req.destroy()
called here) 'error'
with an error with message'Error: socket hang up'
and code'ECONNRESET'
, or the error with whichreq.destroy()
was called'close'
If req.destroy()
is called after the response is received, the following
events will be emitted in the following order:
'socket'
'response'
'data'
any number of times, on theres
object
- (
req.destroy()
called here) 'aborted'
on theres
object'error'
on theres
object with an error with message'Error: aborted'
and code'ECONNRESET'
, or the error with whichreq.destroy()
was called'close'
'close'
on theres
object
If req.abort()
is called before a socket is assigned, the following
events will be emitted in the following order:
- (
req.abort()
called here) 'abort'
'close'
If req.abort()
is called before the connection succeeds, the following
events will be emitted in the following order:
'socket'
- (
req.abort()
called here) 'abort'
'error'
with an error with message'Error: socket hang up'
and code'ECONNRESET'
'close'
If req.abort()
is called after the response is received, the following
events will be emitted in the following order:
'socket'
'response'
'data'
any number of times, on theres
object
- (
req.abort()
called here) 'abort'
'aborted'
on theres
object'error'
on theres
object with an error with message'Error: aborted'
and code'ECONNRESET'
.'close'
'close'
on theres
object
Setting the timeout
option or using the setTimeout()
function will
not abort the request or do anything besides add a 'timeout'
event.
Passing an AbortSignal
and then calling abort()
on the corresponding
AbortController
will behave the same way as calling .destroy()
on the
request. Specifically, the 'error'
event will be emitted with an error with
the message 'AbortError: The operation was aborted'
, the code 'ABORT_ERR'
and the cause
, if one was provided.
http.validateHeaderName(name[, label])
#
Performs the low-level validations on the provided name
that are done when
res.setHeader(name, value)
is called.
Passing illegal value as name
will result in a TypeError
being thrown,
identified by code: 'ERR_INVALID_HTTP_TOKEN'
.
It is not necessary to use this method before passing headers to an HTTP request or response. The HTTP module will automatically validate such headers. Examples:
Example:
const { validateHeaderName } = require('node:http');
try {
validateHeaderName('');
} catch (err) {
console.error(err instanceof TypeError); // --> true
console.error(err.code); // --> 'ERR_INVALID_HTTP_TOKEN'
console.error(err.message); // --> 'Header name must be a valid HTTP token [""]'
}
http.validateHeaderValue(name, value)
#
Performs the low-level validations on the provided value
that are done when
res.setHeader(name, value)
is called.
Passing illegal value as value
will result in a TypeError
being thrown.
- Undefined value error is identified by
code: 'ERR_HTTP_INVALID_HEADER_VALUE'
. - Invalid value character error is identified by
code: 'ERR_INVALID_CHAR'
.
It is not necessary to use this method before passing headers to an HTTP request or response. The HTTP module will automatically validate such headers.
Examples:
const { validateHeaderValue } = require('node:http');
try {
validateHeaderValue('x-my-header', undefined);
} catch (err) {
console.error(err instanceof TypeError); // --> true
console.error(err.code === 'ERR_HTTP_INVALID_HEADER_VALUE'); // --> true
console.error(err.message); // --> 'Invalid value "undefined" for header "x-my-header"'
}
try {
validateHeaderValue('x-my-header', 'oʊmɪɡə');
} catch (err) {
console.error(err instanceof TypeError); // --> true
console.error(err.code === 'ERR_INVALID_CHAR'); // --> true
console.error(err.message); // --> 'Invalid character in header content ["x-my-header"]'
}
http.setMaxIdleHTTPParsers(max)
#
max
<number> Default:1000
.
Set the maximum number of idle HTTP parsers.
HTTP/2#
Source Code: lib/http2.js
The node:http2
module provides an implementation of the HTTP/2 protocol.
It can be accessed using:
const http2 = require('node:http2');
Determining if crypto support is unavailable#
It is possible for Node.js to be built without including support for the
node:crypto
module. In such cases, attempting to import
from node:http2
or
calling require('node:http2')
will result in an error being thrown.
When using CommonJS, the error thrown can be caught using try/catch:
let http2;
try {
http2 = require('node:http2');
} catch (err) {
console.error('http2 support is disabled!');
}
When using the lexical ESM import
keyword, the error can only be
caught if a handler for process.on('uncaughtException')
is registered
before any attempt to load the module is made (using, for instance,
a preload module).
When using ESM, if there is a chance that the code may be run on a build
of Node.js where crypto support is not enabled, consider using the
import()
function instead of the lexical import
keyword:
let http2;
try {
http2 = await import('node:http2');
} catch (err) {
console.error('http2 support is disabled!');
}
Core API#
The Core API provides a low-level interface designed specifically around support for HTTP/2 protocol features. It is specifically not designed for compatibility with the existing HTTP/1 module API. However, the Compatibility API is.
The http2
Core API is much more symmetric between client and server than the
http
API. For instance, most events, like 'error'
, 'connect'
and
'stream'
, can be emitted either by client-side code or server-side code.
Server-side example#
The following illustrates a simple HTTP/2 server using the Core API.
Since there are no browsers known that support
unencrypted HTTP/2, the use of
http2.createSecureServer()
is necessary when communicating
with browser clients.
const http2 = require('node:http2');
const fs = require('node:fs');
const server = http2.createSecureServer({
key: fs.readFileSync('localhost-privkey.pem'),
cert: fs.readFileSync('localhost-cert.pem'),
});
server.on('error', (err) => console.error(err));
server.on('stream', (stream, headers) => {
// stream is a Duplex
stream.respond({
'content-type': 'text/html; charset=utf-8',
':status': 200,
});
stream.end('<h1>Hello World</h1>');
});
server.listen(8443);
To generate the certificate and key for this example, run:
openssl req -x509 -newkey rsa:2048 -nodes -sha256 -subj '/CN=localhost' \
-keyout localhost-privkey.pem -out localhost-cert.pem
Client-side example#
The following illustrates an HTTP/2 client:
const http2 = require('node:http2');
const fs = require('node:fs');
const client = http2.connect('https://localhost:8443', {
ca: fs.readFileSync('localhost-cert.pem'),
});
client.on('error', (err) => console.error(err));
const req = client.request({ ':path': '/' });
req.on('response', (headers, flags) => {
for (const name in headers) {
console.log(`${name}: ${headers[name]}`);
}
});
req.setEncoding('utf8');
let data = '';
req.on('data', (chunk) => { data += chunk; });
req.on('end', () => {
console.log(`\n${data}`);
client.close();
});
req.end();
Class: Http2Session
#
- Extends: <EventEmitter>
Instances of the http2.Http2Session
class represent an active communications
session between an HTTP/2 client and server. Instances of this class are not
intended to be constructed directly by user code.
Each Http2Session
instance will exhibit slightly different behaviors
depending on whether it is operating as a server or a client. The
http2session.type
property can be used to determine the mode in which an
Http2Session
is operating. On the server side, user code should rarely
have occasion to work with the Http2Session
object directly, with most
actions typically taken through interactions with either the Http2Server
or
Http2Stream
objects.
User code will not create Http2Session
instances directly. Server-side
Http2Session
instances are created by the Http2Server
instance when a
new HTTP/2 connection is received. Client-side Http2Session
instances are
created using the http2.connect()
method.
Http2Session
and sockets#
Every Http2Session
instance is associated with exactly one net.Socket
or
tls.TLSSocket
when it is created. When either the Socket
or the
Http2Session
are destroyed, both will be destroyed.
Because of the specific serialization and processing requirements imposed
by the HTTP/2 protocol, it is not recommended for user code to read data from
or write data to a Socket
instance bound to a Http2Session
. Doing so can
put the HTTP/2 session into an indeterminate state causing the session and
the socket to become unusable.
Once a Socket
has been bound to an Http2Session
, user code should rely
solely on the API of the Http2Session
.
Event: 'close'
#
The 'close'
event is emitted once the Http2Session
has been destroyed. Its
listener does not expect any arguments.
Event: 'connect'
#
session
<Http2Session>socket
<net.Socket>
The 'connect'
event is emitted once the Http2Session
has been successfully
connected to the remote peer and communication may begin.
User code will typically not listen for this event directly.
Event: 'error'
#
error
<Error>
The 'error'
event is emitted when an error occurs during the processing of
an Http2Session
.
Event: 'frameError'
#
type
<integer> The frame type.code
<integer> The error code.id
<integer> The stream id (or0
if the frame isn't associated with a stream).
The 'frameError'
event is emitted when an error occurs while attempting to
send a frame on the session. If the frame that could not be sent is associated
with a specific Http2Stream
, an attempt to emit a 'frameError'
event on the
Http2Stream
is made.
If the 'frameError'
event is associated with a stream, the stream will be
closed and destroyed immediately following the 'frameError'
event. If the
event is not associated with a stream, the Http2Session
will be shut down
immediately following the 'frameError'
event.
Event: 'goaway'
#
errorCode
<number> The HTTP/2 error code specified in theGOAWAY
frame.lastStreamID
<number> The ID of the last stream the remote peer successfully processed (or0
if no ID is specified).opaqueData
<Buffer> If additional opaque data was included in theGOAWAY
frame, aBuffer
instance will be passed containing that data.
The 'goaway'
event is emitted when a GOAWAY
frame is received.
The Http2Session
instance will be shut down automatically when the 'goaway'
event is emitted.
Event: 'localSettings'
#
settings
<HTTP/2 Settings Object> A copy of theSETTINGS
frame received.
The 'localSettings'
event is emitted when an acknowledgment SETTINGS
frame
has been received.
When using http2session.settings()
to submit new settings, the modified
settings do not take effect until the 'localSettings'
event is emitted.
session.settings({ enablePush: false });
session.on('localSettings', (settings) => {
/* Use the new settings */
});
Event: 'ping'
#
payload
<Buffer> ThePING
frame 8-byte payload
The 'ping'
event is emitted whenever a PING
frame is received from the
connected peer.
Event: 'remoteSettings'
#
settings
<HTTP/2 Settings Object> A copy of theSETTINGS
frame received.
The 'remoteSettings'
event is emitted when a new SETTINGS
frame is received
from the connected peer.
session.on('remoteSettings', (settings) => {
/* Use the new settings */
});
Event: 'stream'
#
stream
<Http2Stream> A reference to the streamheaders
<HTTP/2 Headers Object> An object describing the headersflags
<number> The associated numeric flagsrawHeaders
<Array> An array containing the raw header names followed by their respective values.
The 'stream'
event is emitted when a new Http2Stream
is created.
const http2 = require('node:http2');
session.on('stream', (stream, headers, flags) => {
const method = headers[':method'];
const path = headers[':path'];
// ...
stream.respond({
':status': 200,
'content-type': 'text/plain; charset=utf-8',
});
stream.write('hello ');
stream.end('world');
});
On the server side, user code will typically not listen for this event directly,
and would instead register a handler for the 'stream'
event emitted by the
net.Server
or tls.Server
instances returned by http2.createServer()
and
http2.createSecureServer()
, respectively, as in the example below:
const http2 = require('node:http2');
// Create an unencrypted HTTP/2 server
const server = http2.createServer();
server.on('stream', (stream, headers) => {
stream.respond({
'content-type': 'text/html; charset=utf-8',
':status': 200,
});
stream.on('error', (error) => console.error(error));
stream.end('<h1>Hello World</h1>');
});
server.listen(8000);
Even though HTTP/2 streams and network sockets are not in a 1:1 correspondence, a network error will destroy each individual stream and must be handled on the stream level, as shown above.
Event: 'timeout'
#
After the http2session.setTimeout()
method is used to set the timeout period
for this Http2Session
, the 'timeout'
event is emitted if there is no
activity on the Http2Session
after the configured number of milliseconds.
Its listener does not expect any arguments.
session.setTimeout(2000);
session.on('timeout', () => { /* .. */ });
http2session.alpnProtocol
#
Value will be undefined
if the Http2Session
is not yet connected to a
socket, h2c
if the Http2Session
is not connected to a TLSSocket
, or
will return the value of the connected TLSSocket
's own alpnProtocol
property.
http2session.close([callback])
#
callback
<Function>
Gracefully closes the Http2Session
, allowing any existing streams to
complete on their own and preventing new Http2Stream
instances from being
created. Once closed, http2session.destroy()
might be called if there
are no open Http2Stream
instances.
If specified, the callback
function is registered as a handler for the
'close'
event.
http2session.closed
#
Will be true
if this Http2Session
instance has been closed, otherwise
false
.
http2session.connecting
#
Will be true
if this Http2Session
instance is still connecting, will be set
to false
before emitting connect
event and/or calling the http2.connect
callback.
http2session.destroy([error][, code])
#
error
<Error> AnError
object if theHttp2Session
is being destroyed due to an error.code
<number> The HTTP/2 error code to send in the finalGOAWAY
frame. If unspecified, anderror
is not undefined, the default isINTERNAL_ERROR
, otherwise defaults toNO_ERROR
.
Immediately terminates the Http2Session
and the associated net.Socket
or
tls.TLSSocket
.
Once destroyed, the Http2Session
will emit the 'close'
event. If error
is not undefined, an 'error'
event will be emitted immediately before the
'close'
event.
If there are any remaining open Http2Streams
associated with the
Http2Session
, those will also be destroyed.
http2session.destroyed
#
Will be true
if this Http2Session
instance has been destroyed and must no
longer be used, otherwise false
.
http2session.encrypted
#
Value is undefined
if the Http2Session
session socket has not yet been
connected, true
if the Http2Session
is connected with a TLSSocket
,
and false
if the Http2Session
is connected to any other kind of socket
or stream.
http2session.goaway([code[, lastStreamID[, opaqueData]]])
#
code
<number> An HTTP/2 error codelastStreamID
<number> The numeric ID of the last processedHttp2Stream
opaqueData
<Buffer> | <TypedArray> | <DataView> ATypedArray
orDataView
instance containing additional data to be carried within theGOAWAY
frame.
Transmits a GOAWAY
frame to the connected peer without shutting down the
Http2Session
.
http2session.localSettings
#
A prototype-less object describing the current local settings of this
Http2Session
. The local settings are local to this Http2Session
instance.
http2session.originSet
#
If the Http2Session
is connected to a TLSSocket
, the originSet
property
will return an Array
of origins for which the Http2Session
may be
considered authoritative.
The originSet
property is only available when using a secure TLS connection.
http2session.pendingSettingsAck
#
Indicates whether the Http2Session
is currently waiting for acknowledgment of
a sent SETTINGS
frame. Will be true
after calling the
http2session.settings()
method. Will be false
once all sent SETTINGS
frames have been acknowledged.
http2session.ping([payload, ]callback)
#
payload
<Buffer> | <TypedArray> | <DataView> Optional ping payload.callback
<Function>- Returns: <boolean>
Sends a PING
frame to the connected HTTP/2 peer. A callback
function must
be provided. The method will return true
if the PING
was sent, false
otherwise.
The maximum number of outstanding (unacknowledged) pings is determined by the
maxOutstandingPings
configuration option. The default maximum is 10.
If provided, the payload
must be a Buffer
, TypedArray
, or DataView
containing 8 bytes of data that will be transmitted with the PING
and
returned with the ping acknowledgment.
The callback will be invoked with three arguments: an error argument that will
be null
if the PING
was successfully acknowledged, a duration
argument
that reports the number of milliseconds elapsed since the ping was sent and the
acknowledgment was received, and a Buffer
containing the 8-byte PING
payload.
session.ping(Buffer.from('abcdefgh'), (err, duration, payload) => {
if (!err) {
console.log(`Ping acknowledged in ${duration} milliseconds`);
console.log(`With payload '${payload.toString()}'`);
}
});
If the payload
argument is not specified, the default payload will be the
64-bit timestamp (little endian) marking the start of the PING
duration.
http2session.ref()
#
Calls ref()
on this Http2Session
instance's underlying net.Socket
.
http2session.remoteSettings
#
A prototype-less object describing the current remote settings of this
Http2Session
. The remote settings are set by the connected HTTP/2 peer.
http2session.setLocalWindowSize(windowSize)
#
windowSize
<number>
Sets the local endpoint's window size.
The windowSize
is the total window size to set, not
the delta.
const http2 = require('node:http2');
const server = http2.createServer();
const expectedWindowSize = 2 ** 20;
server.on('connect', (session) => {
// Set local window size to be 2 ** 20
session.setLocalWindowSize(expectedWindowSize);
});
http2session.setTimeout(msecs, callback)
#
msecs
<number>callback
<Function>
Used to set a callback function that is called when there is no activity on
the Http2Session
after msecs
milliseconds. The given callback
is
registered as a listener on the 'timeout'
event.
http2session.socket
#
Returns a Proxy
object that acts as a net.Socket
(or tls.TLSSocket
) but
limits available methods to ones safe to use with HTTP/2.
destroy
, emit
, end
, pause
, read
, resume
, and write
will throw
an error with code ERR_HTTP2_NO_SOCKET_MANIPULATION
. See
Http2Session
and Sockets for more information.
setTimeout
method will be called on this Http2Session
.
All other interactions will be routed directly to the socket.
http2session.state
#
Provides miscellaneous information about the current state of the
Http2Session
.
- <Object>
effectiveLocalWindowSize
<number> The current local (receive) flow control window size for theHttp2Session
.effectiveRecvDataLength
<number> The current number of bytes that have been received since the last flow controlWINDOW_UPDATE
.nextStreamID
<number> The numeric identifier to be used the next time a newHttp2Stream
is created by thisHttp2Session
.localWindowSize
<number> The number of bytes that the remote peer can send without receiving aWINDOW_UPDATE
.lastProcStreamID
<number> The numeric id of theHttp2Stream
for which aHEADERS
orDATA
frame was most recently received.remoteWindowSize
<number> The number of bytes that thisHttp2Session
may send without receiving aWINDOW_UPDATE
.outboundQueueSize
<number> The number of frames currently within the outbound queue for thisHttp2Session
.deflateDynamicTableSize
<number> The current size in bytes of the outbound header compression state table.inflateDynamicTableSize
<number> The current size in bytes of the inbound header compression state table.
An object describing the current status of this Http2Session
.
http2session.settings([settings][, callback])
#
settings
<HTTP/2 Settings Object>callback
<Function> Callback that is called once the session is connected or right away if the session is already connected.err
<Error> | <null>settings
<HTTP/2 Settings Object> The updatedsettings
object.duration
<integer>
Updates the current local settings for this Http2Session
and sends a new
SETTINGS
frame to the connected HTTP/2 peer.
Once called, the http2session.pendingSettingsAck
property will be true
while the session is waiting for the remote peer to acknowledge the new
settings.
The new settings will not become effective until the SETTINGS
acknowledgment
is received and the 'localSettings'
event is emitted. It is possible to send
multiple SETTINGS
frames while acknowledgment is still pending.
http2session.type
#
The http2session.type
will be equal to
http2.constants.NGHTTP2_SESSION_SERVER
if this Http2Session
instance is a
server, and http2.constants.NGHTTP2_SESSION_CLIENT
if the instance is a
client.
http2session.unref()
#
Calls unref()
on this Http2Session
instance's underlying net.Socket
.
Class: ServerHttp2Session
#
- Extends: <Http2Session>
serverhttp2session.altsvc(alt, originOrStream)
#
alt
<string> A description of the alternative service configuration as defined by RFC 7838.originOrStream
<number> | <string> | <URL> | <Object> Either a URL string specifying the origin (or anObject
with anorigin
property) or the numeric identifier of an activeHttp2Stream
as given by thehttp2stream.id
property.
Submits an ALTSVC
frame (as defined by RFC 7838) to the connected client.
const http2 = require('node:http2');
const server = http2.createServer();
server.on('session', (session) => {
// Set altsvc for origin https://example.org:80
session.altsvc('h2=":8000"', 'https://example.org:80');
});
server.on('stream', (stream) => {
// Set altsvc for a specific stream
stream.session.altsvc('h2=":8000"', stream.id);
});
Sending an ALTSVC
frame with a specific stream ID indicates that the alternate
service is associated with the origin of the given Http2Stream
.
The alt
and origin string must contain only ASCII bytes and are
strictly interpreted as a sequence of ASCII bytes. The special value 'clear'
may be passed to clear any previously set alternative service for a given
domain.
When a string is passed for the originOrStream
argument, it will be parsed as
a URL and the origin will be derived. For instance, the origin for the
HTTP URL 'https://example.org/foo/bar'
is the ASCII string
'https://example.org'
. An error will be thrown if either the given string
cannot be parsed as a URL or if a valid origin cannot be derived.
A URL
object, or any object with an origin
property, may be passed as
originOrStream
, in which case the value of the origin
property will be
used. The value of the origin
property must be a properly serialized
ASCII origin.
Specifying alternative services#
The format of the alt
parameter is strictly defined by RFC 7838 as an
ASCII string containing a comma-delimited list of "alternative" protocols
associated with a specific host and port.
For example, the value 'h2="example.org:81"'
indicates that the HTTP/2
protocol is available on the host 'example.org'
on TCP/IP port 81. The
host and port must be contained within the quote ("
) characters.
Multiple alternatives may be specified, for instance: 'h2="example.org:81", h2=":82"'
.
The protocol identifier ('h2'
in the examples) may be any valid
ALPN Protocol ID.
The syntax of these values is not validated by the Node.js implementation and are passed through as provided by the user or received from the peer.
serverhttp2session.origin(...origins)
#
Submits an ORIGIN
frame (as defined by RFC 8336) to the connected client
to advertise the set of origins for which the server is capable of providing
authoritative responses.
const http2 = require('node:http2');
const options = getSecureOptionsSomehow();
const server = http2.createSecureServer(options);
server.on('stream', (stream) => {
stream.respond();
stream.end('ok');
});
server.on('session', (session) => {
session.origin('https://example.com', 'https://example.org');
});
When a string is passed as an origin
, it will be parsed as a URL and the
origin will be derived. For instance, the origin for the HTTP URL
'https://example.org/foo/bar'
is the ASCII string
'https://example.org'
. An error will be thrown if either the given string
cannot be parsed as a URL or if a valid origin cannot be derived.
A URL
object, or any object with an origin
property, may be passed as
an origin
, in which case the value of the origin
property will be
used. The value of the origin
property must be a properly serialized
ASCII origin.
Alternatively, the origins
option may be used when creating a new HTTP/2
server using the http2.createSecureServer()
method:
const http2 = require('node:http2');
const options = getSecureOptionsSomehow();
options.origins = ['https://example.com', 'https://example.org'];
const server = http2.createSecureServer(options);
server.on('stream', (stream) => {
stream.respond();
stream.end('ok');
});
Class: ClientHttp2Session
#
- Extends: <Http2Session>
Event: 'altsvc'
#
The 'altsvc'
event is emitted whenever an ALTSVC
frame is received by
the client. The event is emitted with the ALTSVC
value, origin, and stream
ID. If no origin
is provided in the ALTSVC
frame, origin
will
be an empty string.
const http2 = require('node:http2');
const client = http2.connect('https://example.org');
client.on('altsvc', (alt, origin, streamId) => {
console.log(alt);
console.log(origin);
console.log(streamId);
});
Event: 'origin'
#
origins
<string[]>
The 'origin'
event is emitted whenever an ORIGIN
frame is received by
the client. The event is emitted with an array of origin
strings. The
http2session.originSet
will be updated to include the received
origins.
const http2 = require('node:http2');
const client = http2.connect('https://example.org');
client.on('origin', (origins) => {
for (let n = 0; n < origins.length; n++)
console.log(origins[n]);
});
The 'origin'
event is only emitted when using a secure TLS connection.
clienthttp2session.request(headers[, options])
#
-
headers
<HTTP/2 Headers Object> -
options
<Object>endStream
<boolean>true
if theHttp2Stream
writable side should be closed initially, such as when sending aGET
request that should not expect a payload body.exclusive
<boolean> Whentrue
andparent
identifies a parent Stream, the created stream is made the sole direct dependency of the parent, with all other existing dependents made a dependent of the newly created stream. Default:false
.parent
<number> Specifies the numeric identifier of a stream the newly created stream is dependent on.weight
<number> Specifies the relative dependency of a stream in relation to other streams with the sameparent
. The value is a number between1
and256
(inclusive).waitForTrailers
<boolean> Whentrue
, theHttp2Stream
will emit the'wantTrailers'
event after the finalDATA
frame has been sent.signal
<AbortSignal> An AbortSignal that may be used to abort an ongoing request.
-
Returns: <ClientHttp2Stream>
For HTTP/2 Client Http2Session
instances only, the http2session.request()
creates and returns an Http2Stream
instance that can be used to send an
HTTP/2 request to the connected server.
When a ClientHttp2Session
is first created, the socket may not yet be
connected. if clienthttp2session.request()
is called during this time, the
actual request will be deferred until the socket is ready to go.
If the session
is closed before the actual request be executed, an
ERR_HTTP2_GOAWAY_SESSION
is thrown.
This method is only available if http2session.type
is equal to
http2.constants.NGHTTP2_SESSION_CLIENT
.
const http2 = require('node:http2');
const clientSession = http2.connect('https://localhost:1234');
const {
HTTP2_HEADER_PATH,
HTTP2_HEADER_STATUS,
} = http2.constants;
const req = clientSession.request({ [HTTP2_HEADER_PATH]: '/' });
req.on('response', (headers) => {
console.log(headers[HTTP2_HEADER_STATUS]);
req.on('data', (chunk) => { /* .. */ });
req.on('end', () => { /* .. */ });
});
When the options.waitForTrailers
option is set, the 'wantTrailers'
event
is emitted immediately after queuing the last chunk of payload data to be sent.
The http2stream.sendTrailers()
method can then be called to send trailing
headers to the peer.
When options.waitForTrailers
is set, the Http2Stream
will not automatically
close when the final DATA
frame is transmitted. User code must call either
http2stream.sendTrailers()
or http2stream.close()
to close the
Http2Stream
.
When options.signal
is set with an AbortSignal
and then abort
on the
corresponding AbortController
is called, the request will emit an 'error'
event with an AbortError
error.
The :method
and :path
pseudo-headers are not specified within headers
,
they respectively default to:
:method
='GET'
:path
=/
Class: Http2Stream
#
- Extends: <stream.Duplex>
Each instance of the Http2Stream
class represents a bidirectional HTTP/2
communications stream over an Http2Session
instance. Any single Http2Session
may have up to 231-1 Http2Stream
instances over its lifetime.
User code will not construct Http2Stream
instances directly. Rather, these
are created, managed, and provided to user code through the Http2Session
instance. On the server, Http2Stream
instances are created either in response
to an incoming HTTP request (and handed off to user code via the 'stream'
event), or in response to a call to the http2stream.pushStream()
method.
On the client, Http2Stream
instances are created and returned when either the
http2session.request()
method is called, or in response to an incoming
'push'
event.
The Http2Stream
class is a base for the ServerHttp2Stream
and
ClientHttp2Stream
classes, each of which is used specifically by either
the Server or Client side, respectively.
All Http2Stream
instances are Duplex
streams. The Writable
side of the
Duplex
is used to send data to the connected peer, while the Readable
side
is used to receive data sent by the connected peer.
The default text character encoding for an Http2Stream
is UTF-8. When using an
Http2Stream
to send text, use the 'content-type'
header to set the character
encoding.
stream.respond({
'content-type': 'text/html; charset=utf-8',
':status': 200,
});
Http2Stream
Lifecycle#
Creation#
On the server side, instances of ServerHttp2Stream
are created either
when:
- A new HTTP/2
HEADERS
frame with a previously unused stream ID is received; - The
http2stream.pushStream()
method is called.
On the client side, instances of ClientHttp2Stream
are created when the
http2session.request()
method is called.
On the client, the Http2Stream
instance returned by http2session.request()
may not be immediately ready for use if the parent Http2Session
has not yet
been fully established. In such cases, operations called on the Http2Stream
will be buffered until the 'ready'
event is emitted. User code should rarely,
if ever, need to handle the 'ready'
event directly. The ready status of an
Http2Stream
can be determined by checking the value of http2stream.id
. If
the value is undefined
, the stream is not yet ready for use.
Destruction#
All Http2Stream
instances are destroyed either when:
- An
RST_STREAM
frame for the stream is received by the connected peer, and (for client streams only) pending data has been read. - The
http2stream.close()
method is called, and (for client streams only) pending data has been read. - The
http2stream.destroy()
orhttp2session.destroy()
methods are called.
When an Http2Stream
instance is destroyed, an attempt will be made to send an
RST_STREAM
frame to the connected peer.
When the Http2Stream
instance is destroyed, the 'close'
event will
be emitted. Because Http2Stream
is an instance of stream.Duplex
, the
'end'
event will also be emitted if the stream data is currently flowing.
The 'error'
event may also be emitted if http2stream.destroy()
was called
with an Error
passed as the first argument.
After the Http2Stream
has been destroyed, the http2stream.destroyed
property will be true
and the http2stream.rstCode
property will specify the
RST_STREAM
error code. The Http2Stream
instance is no longer usable once
destroyed.
Event: 'aborted'
#
The 'aborted'
event is emitted whenever a Http2Stream
instance is
abnormally aborted in mid-communication.
Its listener does not expect any arguments.
The 'aborted'
event will only be emitted if the Http2Stream
writable side
has not been ended.
Event: 'close'
#
The 'close'
event is emitted when the Http2Stream
is destroyed. Once
this event is emitted, the Http2Stream
instance is no longer usable.
The HTTP/2 error code used when closing the stream can be retrieved using
the http2stream.rstCode
property. If the code is any value other than
NGHTTP2_NO_ERROR
(0
), an 'error'
event will have also been emitted.
Event: 'error'
#
error
<Error>
The 'error'
event is emitted when an error occurs during the processing of
an Http2Stream
.
Event: 'frameError'
#
type
<integer> The frame type.code
<integer> The error code.id
<integer> The stream id (or0
if the frame isn't associated with a stream).
The 'frameError'
event is emitted when an error occurs while attempting to
send a frame. When invoked, the handler function will receive an integer
argument identifying the frame type, and an integer argument identifying the
error code. The Http2Stream
instance will be destroyed immediately after the
'frameError'
event is emitted.
Event: 'ready'
#
The 'ready'
event is emitted when the Http2Stream
has been opened, has
been assigned an id
, and can be used. The listener does not expect any
arguments.
Event: 'timeout'
#
The 'timeout'
event is emitted after no activity is received for this
Http2Stream
within the number of milliseconds set using
http2stream.setTimeout()
.
Its listener does not expect any arguments.
Event: 'trailers'
#
headers
<HTTP/2 Headers Object> An object describing the headersflags
<number> The associated numeric flags
The 'trailers'
event is emitted when a block of headers associated with
trailing header fields is received. The listener callback is passed the
HTTP/2 Headers Object and flags associated with the headers.
This event might not be emitted if http2stream.end()
is called
before trailers are received and the incoming data is not being read or
listened for.
stream.on('trailers', (headers, flags) => {
console.log(headers);
});
Event: 'wantTrailers'
#
The 'wantTrailers'
event is emitted when the Http2Stream
has queued the
final DATA
frame to be sent on a frame and the Http2Stream
is ready to send
trailing headers. When initiating a request or response, the waitForTrailers
option must be set for this event to be emitted.
http2stream.aborted
#
Set to true
if the Http2Stream
instance was aborted abnormally. When set,
the 'aborted'
event will have been emitted.
http2stream.bufferSize
#
This property shows the number of characters currently buffered to be written.
See net.Socket.bufferSize
for details.
http2stream.close(code[, callback])
#
code
<number> Unsigned 32-bit integer identifying the error code. Default:http2.constants.NGHTTP2_NO_ERROR
(0x00
).callback
<Function> An optional function registered to listen for the'close'
event.
Closes the Http2Stream
instance by sending an RST_STREAM
frame to the
connected HTTP/2 peer.
http2stream.closed
#
Set to true
if the Http2Stream
instance has been closed.
http2stream.destroyed
#
Set to true
if the Http2Stream
instance has been destroyed and is no longer
usable.
http2stream.endAfterHeaders
#
Set to true
if the END_STREAM
flag was set in the request or response
HEADERS frame received, indicating that no additional data should be received
and the readable side of the Http2Stream
will be closed.
http2stream.id
#
The numeric stream identifier of this Http2Stream
instance. Set to undefined
if the stream identifier has not yet been assigned.
http2stream.pending
#
Set to true
if the Http2Stream
instance has not yet been assigned a
numeric stream identifier.
http2stream.priority(options)
#
options
<Object>exclusive
<boolean> Whentrue
andparent
identifies a parent Stream, this stream is made the sole direct dependency of the parent, with all other existing dependents made a dependent of this stream. Default:false
.parent
<number> Specifies the numeric identifier of a stream this stream is dependent on.weight
<number> Specifies the relative dependency of a stream in relation to other streams with the sameparent
. The value is a number between1
and256
(inclusive).silent
<boolean> Whentrue
, changes the priority locally without sending aPRIORITY
frame to the connected peer.
Updates the priority for this Http2Stream
instance.
http2stream.rstCode
#
Set to the RST_STREAM
error code reported when the Http2Stream
is
destroyed after either receiving an RST_STREAM
frame from the connected peer,
calling http2stream.close()
, or http2stream.destroy()
. Will be
undefined
if the Http2Stream
has not been closed.
http2stream.sentHeaders
#
An object containing the outbound headers sent for this Http2Stream
.
http2stream.sentInfoHeaders
#
An array of objects containing the outbound informational (additional) headers
sent for this Http2Stream
.
http2stream.sentTrailers
#
An object containing the outbound trailers sent for this HttpStream
.
http2stream.session
#
A reference to the Http2Session
instance that owns this Http2Stream
. The
value will be undefined
after the Http2Stream
instance is destroyed.
http2stream.setTimeout(msecs, callback)
#
msecs
<number>callback
<Function>
const http2 = require('node:http2');
const client = http2.connect('http://example.org:8000');
const { NGHTTP2_CANCEL } = http2.constants;
const req = client.request({ ':path': '/' });
// Cancel the stream if there's no activity after 5 seconds
req.setTimeout(5000, () => req.close(NGHTTP2_CANCEL));
http2stream.state
#
Provides miscellaneous information about the current state of the
Http2Stream
.
- <Object>
localWindowSize
<number> The number of bytes the connected peer may send for thisHttp2Stream
without receiving aWINDOW_UPDATE
.state
<number> A flag indicating the low-level current state of theHttp2Stream
as determined bynghttp2
.localClose
<number>1
if thisHttp2Stream
has been closed locally.remoteClose
<number>1
if thisHttp2Stream
has been closed remotely.sumDependencyWeight
<number> The sum weight of allHttp2Stream
instances that depend on thisHttp2Stream
as specified usingPRIORITY
frames.weight
<number> The priority weight of thisHttp2Stream
.
A current state of this Http2Stream
.
http2stream.sendTrailers(headers)
#
headers
<HTTP/2 Headers Object>
Sends a trailing HEADERS
frame to the connected HTTP/2 peer. This method
will cause the Http2Stream
to be immediately closed and must only be
called after the 'wantTrailers'
event has been emitted. When sending a
request or sending a response, the options.waitForTrailers
option must be set
in order to keep the Http2Stream
open after the final DATA
frame so that
trailers can be sent.
const http2 = require('node:http2');
const server = http2.createServer();
server.on('stream', (stream) => {
stream.respond(undefined, { waitForTrailers: true });
stream.on('wantTrailers', () => {
stream.sendTrailers({ xyz: 'abc' });
});
stream.end('Hello World');
});
The HTTP/1 specification forbids trailers from containing HTTP/2 pseudo-header
fields (e.g. ':method'
, ':path'
, etc).
Class: ClientHttp2Stream
#
- Extends <Http2Stream>
The ClientHttp2Stream
class is an extension of Http2Stream
that is
used exclusively on HTTP/2 Clients. Http2Stream
instances on the client
provide events such as 'response'
and 'push'
that are only relevant on
the client.
Event: 'continue'
#
Emitted when the server sends a 100 Continue
status, usually because
the request contained Expect: 100-continue
. This is an instruction that
the client should send the request body.
Event: 'headers'
#
headers
<HTTP/2 Headers Object>flags
<number>
The 'headers'
event is emitted when an additional block of headers is received
for a stream, such as when a block of 1xx
informational headers is received.
The listener callback is passed the HTTP/2 Headers Object and flags
associated with the headers.
stream.on('headers', (headers, flags) => {
console.log(headers);
});
Event: 'push'
#
headers
<HTTP/2 Headers Object>flags
<number>
The 'push'
event is emitted when response headers for a Server Push stream
are received. The listener callback is passed the HTTP/2 Headers Object and
flags associated with the headers.
stream.on('push', (headers, flags) => {
console.log(headers);
});
Event: 'response'
#
headers
<HTTP/2 Headers Object>flags
<number>
The 'response'
event is emitted when a response HEADERS
frame has been
received for this stream from the connected HTTP/2 server. The listener is
invoked with two arguments: an Object
containing the received
HTTP/2 Headers Object, and flags associated with the headers.
const http2 = require('node:http2');
const client = http2.connect('https://localhost');
const req = client.request({ ':path': '/' });
req.on('response', (headers, flags) => {
console.log(headers[':status']);
});
Class: ServerHttp2Stream
#
- Extends: <Http2Stream>
The ServerHttp2Stream
class is an extension of Http2Stream
that is
used exclusively on HTTP/2 Servers. Http2Stream
instances on the server
provide additional methods such as http2stream.pushStream()
and
http2stream.respond()
that are only relevant on the server.
http2stream.additionalHeaders(headers)
#
headers
<HTTP/2 Headers Object>
Sends an additional informational HEADERS
frame to the connected HTTP/2 peer.
http2stream.headersSent
#
True if headers were sent, false otherwise (read-only).
http2stream.pushAllowed
#
Read-only property mapped to the SETTINGS_ENABLE_PUSH
flag of the remote
client's most recent SETTINGS
frame. Will be true
if the remote peer
accepts push streams, false
otherwise. Settings are the same for every
Http2Stream
in the same Http2Session
.
http2stream.pushStream(headers[, options], callback)
#
headers
<HTTP/2 Headers Object>options
<Object>exclusive
<boolean> Whentrue
andparent
identifies a parent Stream, the created stream is made the sole direct dependency of the parent, with all other existing dependents made a dependent of the newly created stream. Default:false
.parent
<number> Specifies the numeric identifier of a stream the newly created stream is dependent on.
callback
<Function> Callback that is called once the push stream has been initiated.err
<Error>pushStream
<ServerHttp2Stream> The returnedpushStream
object.headers
<HTTP/2 Headers Object> Headers object thepushStream
was initiated with.
Initiates a push stream. The callback is invoked with the new Http2Stream
instance created for the push stream passed as the second argument, or an
Error
passed as the first argument.
const http2 = require('node:http2');
const server = http2.createServer();
server.on('stream', (stream) => {
stream.respond({ ':status': 200 });
stream.pushStream({ ':path': '/' }, (err, pushStream, headers) => {
if (err) throw err;
pushStream.respond({ ':status': 200 });
pushStream.end('some pushed data');
});
stream.end('some data');
});
Setting the weight of a push stream is not allowed in the HEADERS
frame. Pass
a weight
value to http2stream.priority
with the silent
option set to
true
to enable server-side bandwidth balancing between concurrent streams.
Calling http2stream.pushStream()
from within a pushed stream is not permitted
and will throw an error.
http2stream.respond([headers[, options]])
#
headers
<HTTP/2 Headers Object>options
<Object>
const http2 = require('node:http2');
const server = http2.createServer();
server.on('stream', (stream) => {
stream.respond({ ':status': 200 });
stream.end('some data');
});
Initiates a response. When the options.waitForTrailers
option is set, the
'wantTrailers'
event will be emitted immediately after queuing the last chunk
of payload data to be sent. The http2stream.sendTrailers()
method can then be
used to sent trailing header fields to the peer.
When options.waitForTrailers
is set, the Http2Stream
will not automatically
close when the final DATA
frame is transmitted. User code must call either
http2stream.sendTrailers()
or http2stream.close()
to close the
Http2Stream
.
const http2 = require('node:http2');
const server = http2.createServer();
server.on('stream', (stream) => {
stream.respond({ ':status': 200 }, { waitForTrailers: true });
stream.on('wantTrailers', () => {
stream.sendTrailers({ ABC: 'some value to send' });
});
stream.end('some data');
});
http2stream.respondWithFD(fd[, headers[, options]])
#
fd
<number> | <FileHandle> A readable file descriptor.headers
<HTTP/2 Headers Object>options
<Object>statCheck
<Function>waitForTrailers
<boolean> Whentrue
, theHttp2Stream
will emit the'wantTrailers'
event after the finalDATA
frame has been sent.offset
<number> The offset position at which to begin reading.length
<number> The amount of data from the fd to send.
Initiates a response whose data is read from the given file descriptor. No
validation is performed on the given file descriptor. If an error occurs while
attempting to read data using the file descriptor, the Http2Stream
will be
closed using an RST_STREAM
frame using the standard INTERNAL_ERROR
code.
When used, the Http2Stream
object's Duplex
interface will be closed
automatically.
const http2 = require('node:http2');
const fs = require('node:fs');
const server = http2.createServer();
server.on('stream', (stream) => {
const fd = fs.openSync('/some/file', 'r');
const stat = fs.fstatSync(fd);
const headers = {
'content-length': stat.size,
'last-modified': stat.mtime.toUTCString(),
'content-type': 'text/plain; charset=utf-8',
};
stream.respondWithFD(fd, headers);
stream.on('close', () => fs.closeSync(fd));
});
The optional options.statCheck
function may be specified to give user code
an opportunity to set additional content headers based on the fs.Stat
details
of the given fd. If the statCheck
function is provided, the
http2stream.respondWithFD()
method will perform an fs.fstat()
call to
collect details on the provided file descriptor.
The offset
and length
options may be used to limit the response to a
specific range subset. This can be used, for instance, to support HTTP Range
requests.
The file descriptor or FileHandle
is not closed when the stream is closed,
so it will need to be closed manually once it is no longer needed.
Using the same file descriptor concurrently for multiple streams
is not supported and may result in data loss. Re-using a file descriptor
after a stream has finished is supported.
When the options.waitForTrailers
option is set, the 'wantTrailers'
event
will be emitted immediately after queuing the last chunk of payload data to be
sent. The http2stream.sendTrailers()
method can then be used to sent trailing
header fields to the peer.
When options.waitForTrailers
is set, the Http2Stream
will not automatically
close when the final DATA
frame is transmitted. User code must call either
http2stream.sendTrailers()
or http2stream.close()
to close the
Http2Stream
.
const http2 = require('node:http2');
const fs = require('node:fs');
const server = http2.createServer();
server.on('stream', (stream) => {
const fd = fs.openSync('/some/file', 'r');
const stat = fs.fstatSync(fd);
const headers = {
'content-length': stat.size,
'last-modified': stat.mtime.toUTCString(),
'content-type': 'text/plain; charset=utf-8',
};
stream.respondWithFD(fd, headers, { waitForTrailers: true });
stream.on('wantTrailers', () => {
stream.sendTrailers({ ABC: 'some value to send' });
});
stream.on('close', () => fs.closeSync(fd));
});
http2stream.respondWithFile(path[, headers[, options]])
#
path
<string> | <Buffer> | <URL>headers
<HTTP/2 Headers Object>options
<Object>statCheck
<Function>onError
<Function> Callback function invoked in the case of an error before send.waitForTrailers
<boolean> Whentrue
, theHttp2Stream
will emit the'wantTrailers'
event after the finalDATA
frame has been sent.offset
<number> The offset position at which to begin reading.length
<number> The amount of data from the fd to send.
Sends a regular file as the response. The path
must specify a regular file
or an 'error'
event will be emitted on the Http2Stream
object.
When used, the Http2Stream
object's Duplex
interface will be closed
automatically.
The optional options.statCheck
function may be specified to give user code
an opportunity to set additional content headers based on the fs.Stat
details
of the given file:
If an error occurs while attempting to read the file data, the Http2Stream
will be closed using an RST_STREAM
frame using the standard INTERNAL_ERROR
code. If the onError
callback is defined, then it will be called. Otherwise
the stream will be destroyed.
Example using a file path:
const http2 = require('node:http2');
const server = http2.createServer();
server.on('stream', (stream) => {
function statCheck(stat, headers) {
headers['last-modified'] = stat.mtime.toUTCString();
}
function onError(err) {
// stream.respond() can throw if the stream has been destroyed by
// the other side.
try {
if (err.code === 'ENOENT') {
stream.respond({ ':status': 404 });
} else {
stream.respond({ ':status': 500 });
}
} catch (err) {
// Perform actual error handling.
console.error(err);
}
stream.end();
}
stream.respondWithFile('/some/file',
{ 'content-type': 'text/plain; charset=utf-8' },
{ statCheck, onError });
});
The options.statCheck
function may also be used to cancel the send operation
by returning false
. For instance, a conditional request may check the stat
results to determine if the file has been modified to return an appropriate
304
response:
const http2 = require('node:http2');
const server = http2.createServer();
server.on('stream', (stream) => {
function statCheck(stat, headers) {
// Check the stat here...
stream.respond({ ':status': 304 });
return false; // Cancel the send operation
}
stream.respondWithFile('/some/file',
{ 'content-type': 'text/plain; charset=utf-8' },
{ statCheck });
});
The content-length
header field will be automatically set.
The offset
and length
options may be used to limit the response to a
specific range subset. This can be used, for instance, to support HTTP Range
requests.
The options.onError
function may also be used to handle all the errors
that could happen before the delivery of the file is initiated. The
default behavior is to destroy the stream.
When the options.waitForTrailers
option is set, the 'wantTrailers'
event
will be emitted immediately after queuing the last chunk of payload data to be
sent. The http2stream.sendTrailers()
method can then be used to sent trailing
header fields to the peer.
When options.waitForTrailers
is set, the Http2Stream
will not automatically
close when the final DATA
frame is transmitted. User code must call either
http2stream.sendTrailers()
or http2stream.close()
to close the
Http2Stream
.
const http2 = require('node:http2');
const server = http2.createServer();
server.on('stream', (stream) => {
stream.respondWithFile('/some/file',
{ 'content-type': 'text/plain; charset=utf-8' },
{ waitForTrailers: true });
stream.on('wantTrailers', () => {
stream.sendTrailers({ ABC: 'some value to send' });
});
});
Class: Http2Server
#
- Extends: <net.Server>
Instances of Http2Server
are created using the http2.createServer()
function. The Http2Server
class is not exported directly by the
node:http2
module.
Event: 'checkContinue'
#
request
<http2.Http2ServerRequest>response
<http2.Http2ServerResponse>
If a 'request'
listener is registered or http2.createServer()
is
supplied a callback function, the 'checkContinue'
event is emitted each time
a request with an HTTP Expect: 100-continue
is received. If this event is
not listened for, the server will automatically respond with a status
100 Continue
as appropriate.
Handling this event involves calling response.writeContinue()
if the
client should continue to send the request body, or generating an appropriate
HTTP response (e.g. 400 Bad Request) if the client should not continue to send
the request body.
When this event is emitted and handled, the 'request'
event will
not be emitted.
Event: 'connection'
#
socket
<stream.Duplex>
This event is emitted when a new TCP stream is established. socket
is
typically an object of type net.Socket
. Usually users will not want to
access this event.
This event can also be explicitly emitted by users to inject connections
into the HTTP server. In that case, any Duplex
stream can be passed.
Event: 'request'
#
request
<http2.Http2ServerRequest>response
<http2.Http2ServerResponse>
Emitted each time there is a request. There may be multiple requests per session. See the Compatibility API.
Event: 'session'
#
session
<ServerHttp2Session>
The 'session'
event is emitted when a new Http2Session
is created by the
Http2Server
.
Event: 'sessionError'
#
error
<Error>session
<ServerHttp2Session>
The 'sessionError'
event is emitted when an 'error'
event is emitted by
an Http2Session
object associated with the Http2Server
.
Event: 'stream'
#
stream
<Http2Stream> A reference to the streamheaders
<HTTP/2 Headers Object> An object describing the headersflags
<number> The associated numeric flagsrawHeaders
<Array> An array containing the raw header names followed by their respective values.
The 'stream'
event is emitted when a 'stream'
event has been emitted by
an Http2Session
associated with the server.
See also Http2Session
's 'stream'
event.
const http2 = require('node:http2');
const {
HTTP2_HEADER_METHOD,
HTTP2_HEADER_PATH,
HTTP2_HEADER_STATUS,
HTTP2_HEADER_CONTENT_TYPE,
} = http2.constants;
const server = http2.createServer();
server.on('stream', (stream, headers, flags) => {
const method = headers[HTTP2_HEADER_METHOD];
const path = headers[HTTP2_HEADER_PATH];
// ...
stream.respond({
[HTTP2_HEADER_STATUS]: 200,
[HTTP2_HEADER_CONTENT_TYPE]: 'text/plain; charset=utf-8',
});
stream.write('hello ');
stream.end('world');
});
Event: 'timeout'
#
The 'timeout'
event is emitted when there is no activity on the Server for
a given number of milliseconds set using http2server.setTimeout()
.
Default: 0 (no timeout)
server.close([callback])
#
callback
<Function>
Stops the server from establishing new sessions. This does not prevent new
request streams from being created due to the persistent nature of HTTP/2
sessions. To gracefully shut down the server, call http2session.close()
on
all active sessions.
If callback
is provided, it is not invoked until all active sessions have been
closed, although the server has already stopped allowing new sessions. See
net.Server.close()
for more details.
server.setTimeout([msecs][, callback])
#
msecs
<number> Default: 0 (no timeout)callback
<Function>- Returns: <Http2Server>
Used to set the timeout value for http2 server requests,
and sets a callback function that is called when there is no activity
on the Http2Server
after msecs
milliseconds.
The given callback is registered as a listener on the 'timeout'
event.
In case if callback
is not a function, a new ERR_INVALID_ARG_TYPE
error will be thrown.
server.timeout
#
- <number> Timeout in milliseconds. Default: 0 (no timeout)
The number of milliseconds of inactivity before a socket is presumed to have timed out.
A value of 0
will disable the timeout behavior on incoming connections.
The socket timeout logic is set up on connection, so changing this value only affects new connections to the server, not any existing connections.
server.updateSettings([settings])
#
settings
<HTTP/2 Settings Object>
Used to update the server with the provided settings.
Throws ERR_HTTP2_INVALID_SETTING_VALUE
for invalid settings
values.
Throws ERR_INVALID_ARG_TYPE
for invalid settings
argument.
Class: Http2SecureServer
#
- Extends: <tls.Server>
Instances of Http2SecureServer
are created using the
http2.createSecureServer()
function. The Http2SecureServer
class is not
exported directly by the node:http2
module.
Event: 'checkContinue'
#
request
<http2.Http2ServerRequest>response
<http2.Http2ServerResponse>
If a 'request'
listener is registered or http2.createSecureServer()
is supplied a callback function, the 'checkContinue'
event is emitted each
time a request with an HTTP Expect: 100-continue
is received. If this event
is not listened for, the server will automatically respond with a status
100 Continue
as appropriate.
Handling this event involves calling response.writeContinue()
if the
client should continue to send the request body, or generating an appropriate
HTTP response (e.g. 400 Bad Request) if the client should not continue to send
the request body.
When this event is emitted and handled, the 'request'
event will
not be emitted.
Event: 'connection'
#
socket
<stream.Duplex>
This event is emitted when a new TCP stream is established, before the TLS
handshake begins. socket
is typically an object of type net.Socket
.
Usually users will not want to access this event.
This event can also be explicitly emitted by users to inject connections
into the HTTP server. In that case, any Duplex
stream can be passed.
Event: 'request'
#
request
<http2.Http2ServerRequest>response
<http2.Http2ServerResponse>
Emitted each time there is a request. There may be multiple requests per session. See the Compatibility API.
Event: 'session'
#
session
<ServerHttp2Session>
The 'session'
event is emitted when a new Http2Session
is created by the
Http2SecureServer
.
Event: 'sessionError'
#
error
<Error>session
<ServerHttp2Session>
The 'sessionError'
event is emitted when an 'error'
event is emitted by
an Http2Session
object associated with the Http2SecureServer
.
Event: 'stream'
#
stream
<Http2Stream> A reference to the streamheaders
<HTTP/2 Headers Object> An object describing the headersflags
<number> The associated numeric flagsrawHeaders
<Array> An array containing the raw header names followed by their respective values.
The 'stream'
event is emitted when a 'stream'
event has been emitted by
an Http2Session
associated with the server.
See also Http2Session
's 'stream'
event.
const http2 = require('node:http2');
const {
HTTP2_HEADER_METHOD,
HTTP2_HEADER_PATH,
HTTP2_HEADER_STATUS,
HTTP2_HEADER_CONTENT_TYPE,
} = http2.constants;
const options = getOptionsSomehow();
const server = http2.createSecureServer(options);
server.on('stream', (stream, headers, flags) => {
const method = headers[HTTP2_HEADER_METHOD];
const path = headers[HTTP2_HEADER_PATH];
// ...
stream.respond({
[HTTP2_HEADER_STATUS]: 200,
[HTTP2_HEADER_CONTENT_TYPE]: 'text/plain; charset=utf-8',
});
stream.write('hello ');
stream.end('world');
});
Event: 'timeout'
#
The 'timeout'
event is emitted when there is no activity on the Server for
a given number of milliseconds set using http2secureServer.setTimeout()
.
Default: 2 minutes.
Event: 'unknownProtocol'
#
socket
<stream.Duplex>
The 'unknownProtocol'
event is emitted when a connecting client fails to
negotiate an allowed protocol (i.e. HTTP/2 or HTTP/1.1). The event handler
receives the socket for handling. If no listener is registered for this event,
the connection is terminated. A timeout may be specified using the
'unknownProtocolTimeout'
option passed to http2.createSecureServer()
.
In earlier versions of Node.js, this event would be emitted if allowHTTP1
is
false
and, during the TLS handshake, the client either does not send an ALPN
extension or sends an ALPN extension that does not include HTTP/2 (h2
). Newer
versions of Node.js only emit this event if allowHTTP1
is false
and the
client does not send an ALPN extension. If the client sends an ALPN extension
that does not include HTTP/2 (or HTTP/1.1 if allowHTTP1
is true
), the TLS
handshake will fail and no secure connection will be established.
See the Compatibility API.
server.close([callback])
#
callback
<Function>
Stops the server from establishing new sessions. This does not prevent new
request streams from being created due to the persistent nature of HTTP/2
sessions. To gracefully shut down the server, call http2session.close()
on
all active sessions.
If callback
is provided, it is not invoked until all active sessions have been
closed, although the server has already stopped allowing new sessions. See
tls.Server.close()
for more details.
server.setTimeout([msecs][, callback])
#
msecs
<number> Default:120000
(2 minutes)callback
<Function>- Returns: <Http2SecureServer>
Used to set the timeout value for http2 secure server requests,
and sets a callback function that is called when there is no activity
on the Http2SecureServer
after msecs
milliseconds.
The given callback is registered as a listener on the 'timeout'
event.
In case if callback
is not a function, a new ERR_INVALID_ARG_TYPE
error will be thrown.
server.timeout
#
- <number> Timeout in milliseconds. Default: 0 (no timeout)
The number of milliseconds of inactivity before a socket is presumed to have timed out.
A value of 0
will disable the timeout behavior on incoming connections.
The socket timeout logic is set up on connection, so changing this value only affects new connections to the server, not any existing connections.
server.updateSettings([settings])
#
settings
<HTTP/2 Settings Object>
Used to update the server with the provided settings.
Throws ERR_HTTP2_INVALID_SETTING_VALUE
for invalid settings
values.
Throws ERR_INVALID_ARG_TYPE
for invalid settings
argument.
http2.createServer([options][, onRequestHandler])
#
options
<Object>maxDeflateDynamicTableSize
<number> Sets the maximum dynamic table size for deflating header fields. Default:4Kib
.maxSettings
<number> Sets the maximum number of settings entries perSETTINGS
frame. The minimum value allowed is1
. Default:32
.maxSessionMemory
<number> Sets the maximum memory that theHttp2Session
is permitted to use. The value is expressed in terms of number of megabytes, e.g.1
equal 1 megabyte. The minimum value allowed is1
. This is a credit based limit, existingHttp2Stream
s may cause this limit to be exceeded, but newHttp2Stream
instances will be rejected while this limit is exceeded. The current number ofHttp2Stream
sessions, the current memory use of the header compression tables, current data queued to be sent, and unacknowledgedPING
andSETTINGS
frames are all counted towards the current limit. Default:10
.maxHeaderListPairs
<number> Sets the maximum number of header entries. This is similar toserver.maxHeadersCount
orrequest.maxHeadersCount
in thenode:http
module. The minimum value is4
. Default:128
.maxOutstandingPings
<number> Sets the maximum number of outstanding, unacknowledged pings. Default:10
.maxSendHeaderBlockLength
<number> Sets the maximum allowed size for a serialized, compressed block of headers. Attempts to send headers that exceed this limit will result in a'frameError'
event being emitted and the stream being closed and destroyed. While this sets the maximum allowed size to the entire block of headers,nghttp2
(the internal http2 library) has a limit of65536
for each decompressed key/value pair.paddingStrategy
<number> The strategy used for determining the amount of padding to use forHEADERS
andDATA
frames. Default:http2.constants.PADDING_STRATEGY_NONE
. Value may be one of:http2.constants.PADDING_STRATEGY_NONE
: No padding is applied.http2.constants.PADDING_STRATEGY_MAX
: The maximum amount of padding, determined by the internal implementation, is applied.http2.constants.PADDING_STRATEGY_ALIGNED
: Attempts to apply enough padding to ensure that the total frame length, including the 9-byte header, is a multiple of 8. For each frame, there is a maximum allowed number of padding bytes that is determined by current flow control state and settings. If this maximum is less than the calculated amount needed to ensure alignment, the maximum is used and the total frame length is not necessarily aligned at 8 bytes.
peerMaxConcurrentStreams
<number> Sets the maximum number of concurrent streams for the remote peer as if aSETTINGS
frame had been received. Will be overridden if the remote peer sets its own value formaxConcurrentStreams
. Default:100
.maxSessionInvalidFrames
<integer> Sets the maximum number of invalid frames that will be tolerated before the session is closed. Default:1000
.maxSessionRejectedStreams
<integer> Sets the maximum number of rejected upon creation streams that will be tolerated before the session is closed. Each rejection is associated with anNGHTTP2_ENHANCE_YOUR_CALM
error that should tell the peer to not open any more streams, continuing to open streams is therefore regarded as a sign of a misbehaving peer. Default:100
.settings
<HTTP/2 Settings Object> The initial settings to send to the remote peer upon connection.Http1IncomingMessage
<http.IncomingMessage> Specifies theIncomingMessage
class to used for HTTP/1 fallback. Useful for extending the originalhttp.IncomingMessage
. Default:http.IncomingMessage
.Http1ServerResponse
<http.ServerResponse> Specifies theServerResponse
class to used for HTTP/1 fallback. Useful for extending the originalhttp.ServerResponse
. Default:http.ServerResponse
.Http2ServerRequest
<http2.Http2ServerRequest> Specifies theHttp2ServerRequest
class to use. Useful for extending the originalHttp2ServerRequest
. Default:Http2ServerRequest
.Http2ServerResponse
<http2.Http2ServerResponse> Specifies theHttp2ServerResponse
class to use. Useful for extending the originalHttp2ServerResponse
. Default:Http2ServerResponse
.unknownProtocolTimeout
<number> Specifies a timeout in milliseconds that a server should wait when an'unknownProtocol'
is emitted. If the socket has not been destroyed by that time the server will destroy it. Default:10000
.- ...: Any
net.createServer()
option can be provided.
onRequestHandler
<Function> See Compatibility API- Returns: <Http2Server>
Returns a net.Server
instance that creates and manages Http2Session
instances.
Since there are no browsers known that support
unencrypted HTTP/2, the use of
http2.createSecureServer()
is necessary when communicating
with browser clients.
const http2 = require('node:http2');
// Create an unencrypted HTTP/2 server.
// Since there are no browsers known that support
// unencrypted HTTP/2, the use of `http2.createSecureServer()`
// is necessary when communicating with browser clients.
const server = http2.createServer();
server.on('stream', (stream, headers) => {
stream.respond({
'content-type': 'text/html; charset=utf-8',
':status': 200,
});
stream.end('<h1>Hello World</h1>');
});
server.listen(8000);
http2.createSecureServer(options[, onRequestHandler])
#
options
<Object>allowHTTP1
<boolean> Incoming client connections that do not support HTTP/2 will be downgraded to HTTP/1.x when set totrue
. See the'unknownProtocol'
event. See ALPN negotiation. Default:false
.maxDeflateDynamicTableSize
<number> Sets the maximum dynamic table size for deflating header fields. Default:4Kib
.maxSettings
<number> Sets the maximum number of settings entries perSETTINGS
frame. The minimum value allowed is1
. Default:32
.maxSessionMemory
<number> Sets the maximum memory that theHttp2Session
is permitted to use. The value is expressed in terms of number of megabytes, e.g.1
equal 1 megabyte. The minimum value allowed is1
. This is a credit based limit, existingHttp2Stream
s may cause this limit to be exceeded, but newHttp2Stream
instances will be rejected while this limit is exceeded. The current number ofHttp2Stream
sessions, the current memory use of the header compression tables, current data queued to be sent, and unacknowledgedPING
andSETTINGS
frames are all counted towards the current limit. Default:10
.maxHeaderListPairs
<number> Sets the maximum number of header entries. This is similar toserver.maxHeadersCount
orrequest.maxHeadersCount
in thenode:http
module. The minimum value is4
. Default:128
.maxOutstandingPings
<number> Sets the maximum number of outstanding, unacknowledged pings. Default:10
.maxSendHeaderBlockLength
<number> Sets the maximum allowed size for a serialized, compressed block of headers. Attempts to send headers that exceed this limit will result in a'frameError'
event being emitted and the stream being closed and destroyed.paddingStrategy
<number> Strategy used for determining the amount of padding to use forHEADERS
andDATA
frames. Default:http2.constants.PADDING_STRATEGY_NONE
. Value may be one of:http2.constants.PADDING_STRATEGY_NONE
: No padding is applied.http2.constants.PADDING_STRATEGY_MAX
: The maximum amount of padding, determined by the internal implementation, is applied.http2.constants.PADDING_STRATEGY_ALIGNED
: Attempts to apply enough padding to ensure that the total frame length, including the 9-byte header, is a multiple of 8. For each frame, there is a maximum allowed number of padding bytes that is determined by current flow control state and settings. If this maximum is less than the calculated amount needed to ensure alignment, the maximum is used and the total frame length is not necessarily aligned at 8 bytes.
peerMaxConcurrentStreams
<number> Sets the maximum number of concurrent streams for the remote peer as if aSETTINGS
frame had been received. Will be overridden if the remote peer sets its own value formaxConcurrentStreams
. Default:100
.maxSessionInvalidFrames
<integer> Sets the maximum number of invalid frames that will be tolerated before the session is closed. Default:1000
.maxSessionRejectedStreams
<integer> Sets the maximum number of rejected upon creation streams that will be tolerated before the session is closed. Each rejection is associated with anNGHTTP2_ENHANCE_YOUR_CALM
error that should tell the peer to not open any more streams, continuing to open streams is therefore regarded as a sign of a misbehaving peer. Default:100
.settings
<HTTP/2 Settings Object> The initial settings to send to the remote peer upon connection.- ...: Any
tls.createServer()
options can be provided. For servers, the identity options (pfx
orkey
/cert
) are usually required. origins
<string[]> An array of origin strings to send within anORIGIN
frame immediately following creation of a new serverHttp2Session
.unknownProtocolTimeout
<number> Specifies a timeout in milliseconds that a server should wait when an'unknownProtocol'
event is emitted. If the socket has not been destroyed by that time the server will destroy it. Default:10000
.
onRequestHandler
<Function> See Compatibility API- Returns: <Http2SecureServer>
Returns a tls.Server
instance that creates and manages Http2Session
instances.
const http2 = require('node:http2');
const fs = require('node:fs');
const options = {
key: fs.readFileSync('server-key.pem'),
cert: fs.readFileSync('server-cert.pem'),
};
// Create a secure HTTP/2 server
const server = http2.createSecureServer(options);
server.on('stream', (stream, headers) => {
stream.respond({
'content-type': 'text/html; charset=utf-8',
':status': 200,
});
stream.end('<h1>Hello World</h1>');
});
server.listen(8443);
http2.connect(authority[, options][, listener])
#
authority
<string> | <URL> The remote HTTP/2 server to connect to. This must be in the form of a minimal, valid URL with thehttp://
orhttps://
prefix, host name, and IP port (if a non-default port is used). Userinfo (user ID and password), path, querystring, and fragment details in the URL will be ignored.options
<Object>maxDeflateDynamicTableSize
<number> Sets the maximum dynamic table size for deflating header fields. Default:4Kib
.maxSettings
<number> Sets the maximum number of settings entries perSETTINGS
frame. The minimum value allowed is1
. Default:32
.maxSessionMemory
<number> Sets the maximum memory that theHttp2Session
is permitted to use. The value is expressed in terms of number of megabytes, e.g.1
equal 1 megabyte. The minimum value allowed is1
. This is a credit based limit, existingHttp2Stream
s may cause this limit to be exceeded, but newHttp2Stream
instances will be rejected while this limit is exceeded. The current number ofHttp2Stream
sessions, the current memory use of the header compression tables, current data queued to be sent, and unacknowledgedPING
andSETTINGS
frames are all counted towards the current limit. Default:10
.maxHeaderListPairs
<number> Sets the maximum number of header entries. This is similar toserver.maxHeadersCount
orrequest.maxHeadersCount
in thenode:http
module. The minimum value is1
. Default:128
.maxOutstandingPings
<number> Sets the maximum number of outstanding, unacknowledged pings. Default:10
.maxReservedRemoteStreams
<number> Sets the maximum number of reserved push streams the client will accept at any given time. Once the current number of currently reserved push streams exceeds reaches this limit, new push streams sent by the server will be automatically rejected. The minimum allowed value is 0. The maximum allowed value is 232-1. A negative value sets this option to the maximum allowed value. Default:200
.maxSendHeaderBlockLength
<number> Sets the maximum allowed size for a serialized, compressed block of headers. Attempts to send headers that exceed this limit will result in a'frameError'
event being emitted and the stream being closed and destroyed.paddingStrategy
<number> Strategy used for determining the amount of padding to use forHEADERS
andDATA
frames. Default:http2.constants.PADDING_STRATEGY_NONE
. Value may be one of:http2.constants.PADDING_STRATEGY_NONE
: No padding is applied.http2.constants.PADDING_STRATEGY_MAX
: The maximum amount of padding, determined by the internal implementation, is applied.http2.constants.PADDING_STRATEGY_ALIGNED
: Attempts to apply enough padding to ensure that the total frame length, including the 9-byte header, is a multiple of 8. For each frame, there is a maximum allowed number of padding bytes that is determined by current flow control state and settings. If this maximum is less than the calculated amount needed to ensure alignment, the maximum is used and the total frame length is not necessarily aligned at 8 bytes.
peerMaxConcurrentStreams
<number> Sets the maximum number of concurrent streams for the remote peer as if aSETTINGS
frame had been received. Will be overridden if the remote peer sets its own value formaxConcurrentStreams
. Default:100
.protocol
<string> The protocol to connect with, if not set in theauthority
. Value may be either'http:'
or'https:'
. Default:'https:'
settings
<HTTP/2 Settings Object> The initial settings to send to the remote peer upon connection.createConnection
<Function> An optional callback that receives theURL
instance passed toconnect
and theoptions
object, and returns anyDuplex
stream that is to be used as the connection for this session.- ...: Any
net.connect()
ortls.connect()
options can be provided. unknownProtocolTimeout
<number> Specifies a timeout in milliseconds that a server should wait when an'unknownProtocol'
event is emitted. If the socket has not been destroyed by that time the server will destroy it. Default:10000
.
listener
<Function> Will be registered as a one-time listener of the'connect'
event.- Returns: <ClientHttp2Session>
Returns a ClientHttp2Session
instance.
const http2 = require('node:http2');
const client = http2.connect('https://localhost:1234');
/* Use the client */
client.close();
http2.constants
#
Error codes for RST_STREAM
and GOAWAY
#
Value | Name | Constant |
---|---|---|
0x00 | No Error | http2.constants.NGHTTP2_NO_ERROR |
0x01 | Protocol Error | http2.constants.NGHTTP2_PROTOCOL_ERROR |
0x02 | Internal Error | http2.constants.NGHTTP2_INTERNAL_ERROR |
0x03 | Flow Control Error | http2.constants.NGHTTP2_FLOW_CONTROL_ERROR |
0x04 | Settings Timeout | http2.constants.NGHTTP2_SETTINGS_TIMEOUT |
0x05 | Stream Closed | http2.constants.NGHTTP2_STREAM_CLOSED |
0x06 | Frame Size Error | http2.constants.NGHTTP2_FRAME_SIZE_ERROR |
0x07 | Refused Stream | http2.constants.NGHTTP2_REFUSED_STREAM |
0x08 | Cancel | http2.constants.NGHTTP2_CANCEL |
0x09 | Compression Error | http2.constants.NGHTTP2_COMPRESSION_ERROR |
0x0a | Connect Error | http2.constants.NGHTTP2_CONNECT_ERROR |
0x0b | Enhance Your Calm | http2.constants.NGHTTP2_ENHANCE_YOUR_CALM |
0x0c | Inadequate Security | http2.constants.NGHTTP2_INADEQUATE_SECURITY |
0x0d | HTTP/1.1 Required | http2.constants.NGHTTP2_HTTP_1_1_REQUIRED |
The 'timeout'
event is emitted when there is no activity on the Server for
a given number of milliseconds set using http2server.setTimeout()
.
http2.getDefaultSettings()
#
- Returns: <HTTP/2 Settings Object>
Returns an object containing the default settings for an Http2Session
instance. This method returns a new object instance every time it is called
so instances returned may be safely modified for use.
http2.getPackedSettings([settings])
#
settings
<HTTP/2 Settings Object>- Returns: <Buffer>
Returns a Buffer
instance containing serialized representation of the given
HTTP/2 settings as specified in the HTTP/2 specification. This is intended
for use with the HTTP2-Settings
header field.
const http2 = require('node:http2');
const packed = http2.getPackedSettings({ enablePush: false });
console.log(packed.toString('base64'));
// Prints: AAIAAAAA
http2.getUnpackedSettings(buf)
#
buf
<Buffer> | <TypedArray> The packed settings.- Returns: <HTTP/2 Settings Object>
Returns a HTTP/2 Settings Object containing the deserialized settings from
the given Buffer
as generated by http2.getPackedSettings()
.
http2.sensitiveHeaders
#
This symbol can be set as a property on the HTTP/2 headers object with an array value in order to provide a list of headers considered sensitive. See Sensitive headers for more details.
Headers object#
Headers are represented as own-properties on JavaScript objects. The property
keys will be serialized to lower-case. Property values should be strings (if
they are not they will be coerced to strings) or an Array
of strings (in order
to send more than one value per header field).
const headers = {
':status': '200',
'content-type': 'text-plain',
'ABC': ['has', 'more', 'than', 'one', 'value'],
};
stream.respond(headers);
Header objects passed to callback functions will have a null
prototype. This
means that normal JavaScript object methods such as
Object.prototype.toString()
and Object.prototype.hasOwnProperty()
will
not work.
For incoming headers:
- The
:status
header is converted tonumber
. - Duplicates of
:status
,:method
,:authority
,:scheme
,:path
,:protocol
,age
,authorization
,access-control-allow-credentials
,access-control-max-age
,access-control-request-method
,content-encoding
,content-language
,content-length
,content-location
,content-md5
,content-range
,content-type
,date
,dnt
,etag
,expires
,from
,host
,if-match
,if-modified-since
,if-none-match
,if-range
,if-unmodified-since
,last-modified
,location
,max-forwards
,proxy-authorization
,range
,referer
,retry-after
,tk
,upgrade-insecure-requests
,user-agent
orx-content-type-options
are discarded. set-cookie
is always an array. Duplicates are added to the array.- For duplicate
cookie
headers, the values are joined together with '; '. - For all other headers, the values are joined together with ', '.
const http2 = require('node:http2');
const server = http2.createServer();
server.on('stream', (stream, headers) => {
console.log(headers[':path']);
console.log(headers.ABC);
});
Sensitive headers#
HTTP2 headers can be marked as sensitive, which means that the HTTP/2
header compression algorithm will never index them. This can make sense for
header values with low entropy and that may be considered valuable to an
attacker, for example Cookie
or Authorization
. To achieve this, add
the header name to the [http2.sensitiveHeaders]
property as an array:
const headers = {
':status': '200',
'content-type': 'text-plain',
'cookie': 'some-cookie',
'other-sensitive-header': 'very secret data',
[http2.sensitiveHeaders]: ['cookie', 'other-sensitive-header'],
};
stream.respond(headers);
For some headers, such as Authorization
and short Cookie
headers,
this flag is set automatically.
This property is also set for received headers. It will contain the names of all headers marked as sensitive, including ones marked that way automatically.
Settings object#
The http2.getDefaultSettings()
, http2.getPackedSettings()
,
http2.createServer()
, http2.createSecureServer()
,
http2session.settings()
, http2session.localSettings
, and
http2session.remoteSettings
APIs either return or receive as input an
object that defines configuration settings for an Http2Session
object.
These objects are ordinary JavaScript objects containing the following
properties.
headerTableSize
<number> Specifies the maximum number of bytes used for header compression. The minimum allowed value is 0. The maximum allowed value is 232-1. Default:4096
.enablePush
<boolean> Specifiestrue
if HTTP/2 Push Streams are to be permitted on theHttp2Session
instances. Default:true
.initialWindowSize
<number> Specifies the sender's initial window size in bytes for stream-level flow control. The minimum allowed value is 0. The maximum allowed value is 232-1. Default:65535
.maxFrameSize
<number> Specifies the size in bytes of the largest frame payload. The minimum allowed value is 16,384. The maximum allowed value is 224-1. Default:16384
.maxConcurrentStreams
<number> Specifies the maximum number of concurrent streams permitted on anHttp2Session
. There is no default value which implies, at least theoretically, 232-1 streams may be open concurrently at any given time in anHttp2Session
. The minimum value is 0. The maximum allowed value is 232-1. Default:4294967295
.maxHeaderListSize
<number> Specifies the maximum size (uncompressed octets) of header list that will be accepted. The minimum allowed value is 0. The maximum allowed value is 232-1. Default:65535
.maxHeaderSize
<number> Alias formaxHeaderListSize
.enableConnectProtocol
<boolean> Specifiestrue
if the "Extended Connect Protocol" defined by RFC 8441 is to be enabled. This setting is only meaningful if sent by the server. Once theenableConnectProtocol
setting has been enabled for a givenHttp2Session
, it cannot be disabled. Default:false
.
All additional properties on the settings object are ignored.
Error handling#
There are several types of error conditions that may arise when using the
node:http2
module:
Validation errors occur when an incorrect argument, option, or setting value is
passed in. These will always be reported by a synchronous throw
.
State errors occur when an action is attempted at an incorrect time (for
instance, attempting to send data on a stream after it has closed). These will
be reported using either a synchronous throw
or via an 'error'
event on
the Http2Stream
, Http2Session
or HTTP/2 Server objects, depending on where
and when the error occurs.
Internal errors occur when an HTTP/2 session fails unexpectedly. These will be
reported via an 'error'
event on the Http2Session
or HTTP/2 Server objects.
Protocol errors occur when various HTTP/2 protocol constraints are violated.
These will be reported using either a synchronous throw
or via an 'error'
event on the Http2Stream
, Http2Session
or HTTP/2 Server objects, depending
on where and when the error occurs.
Invalid character handling in header names and values#
The HTTP/2 implementation applies stricter handling of invalid characters in HTTP header names and values than the HTTP/1 implementation.
Header field names are case-insensitive and are transmitted over the wire
strictly as lower-case strings. The API provided by Node.js allows header
names to be set as mixed-case strings (e.g. Content-Type
) but will convert
those to lower-case (e.g. content-type
) upon transmission.
Header field-names must only contain one or more of the following ASCII
characters: a
-z
, A
-Z
, 0
-9
, !
, #
, $
, %
, &
, '
, *
, +
,
-
, .
, ^
, _
, `
(backtick), |
, and ~
.
Using invalid characters within an HTTP header field name will cause the stream to be closed with a protocol error being reported.
Header field values are handled with more leniency but should not contain new-line or carriage return characters and should be limited to US-ASCII characters, per the requirements of the HTTP specification.
Push streams on the client#
To receive pushed streams on the client, set a listener for the 'stream'
event on the ClientHttp2Session
:
const http2 = require('node:http2');
const client = http2.connect('http://localhost');
client.on('stream', (pushedStream, requestHeaders) => {
pushedStream.on('push', (responseHeaders) => {
// Process response headers
});
pushedStream.on('data', (chunk) => { /* handle pushed data */ });
});
const req = client.request({ ':path': '/' });
Supporting the CONNECT
method#
The CONNECT
method is used to allow an HTTP/2 server to be used as a proxy
for TCP/IP connections.
A simple TCP Server:
const net = require('node:net');
const server = net.createServer((socket) => {
let name = '';
socket.setEncoding('utf8');
socket.on('data', (chunk) => name += chunk);
socket.on('end', () => socket.end(`hello ${name}`));
});
server.listen(8000);
An HTTP/2 CONNECT proxy:
const http2 = require('node:http2');
const { NGHTTP2_REFUSED_STREAM } = http2.constants;
const net = require('node:net');
const proxy = http2.createServer();
proxy.on('stream', (stream, headers) => {
if (headers[':method'] !== 'CONNECT') {
// Only accept CONNECT requests
stream.close(NGHTTP2_REFUSED_STREAM);
return;
}
const auth = new URL(`tcp://${headers[':authority']}`);
// It's a very good idea to verify that hostname and port are
// things this proxy should be connecting to.
const socket = net.connect(auth.port, auth.hostname, () => {
stream.respond();
socket.pipe(stream);
stream.pipe(socket);
});
socket.on('error', (error) => {
stream.close(http2.constants.NGHTTP2_CONNECT_ERROR);
});
});
proxy.listen(8001);
An HTTP/2 CONNECT client:
const http2 = require('node:http2');
const client = http2.connect('http://localhost:8001');
// Must not specify the ':path' and ':scheme' headers
// for CONNECT requests or an error will be thrown.
const req = client.request({
':method': 'CONNECT',
':authority': 'localhost:8000',
});
req.on('response', (headers) => {
console.log(headers[http2.constants.HTTP2_HEADER_STATUS]);
});
let data = '';
req.setEncoding('utf8');
req.on('data', (chunk) => data += chunk);
req.on('end', () => {
console.log(`The server says: ${data}`);
client.close();
});
req.end('Jane');
The extended CONNECT
protocol#
RFC 8441 defines an "Extended CONNECT Protocol" extension to HTTP/2 that
may be used to bootstrap the use of an Http2Stream
using the CONNECT
method as a tunnel for other communication protocols (such as WebSockets).
The use of the Extended CONNECT Protocol is enabled by HTTP/2 servers by using
the enableConnectProtocol
setting:
const http2 = require('node:http2');
const settings = { enableConnectProtocol: true };
const server = http2.createServer({ settings });
Once the client receives the SETTINGS
frame from the server indicating that
the extended CONNECT may be used, it may send CONNECT
requests that use the
':protocol'
HTTP/2 pseudo-header:
const http2 = require('node:http2');
const client = http2.connect('http://localhost:8080');
client.on('remoteSettings', (settings) => {
if (settings.enableConnectProtocol) {
const req = client.request({ ':method': 'CONNECT', ':protocol': 'foo' });
// ...
}
});
Compatibility API#
The Compatibility API has the goal of providing a similar developer experience of HTTP/1 when using HTTP/2, making it possible to develop applications that support both HTTP/1 and HTTP/2. This API targets only the public API of the HTTP/1. However many modules use internal methods or state, and those are not supported as it is a completely different implementation.
The following example creates an HTTP/2 server using the compatibility API:
const http2 = require('node:http2');
const server = http2.createServer((req, res) => {
res.setHeader('Content-Type', 'text/html');
res.setHeader('X-Foo', 'bar');
res.writeHead(200, { 'Content-Type': 'text/plain; charset=utf-8' });
res.end('ok');
});
In order to create a mixed HTTPS and HTTP/2 server, refer to the ALPN negotiation section. Upgrading from non-tls HTTP/1 servers is not supported.
The HTTP/2 compatibility API is composed of Http2ServerRequest
and
Http2ServerResponse
. They aim at API compatibility with HTTP/1, but
they do not hide the differences between the protocols. As an example,
the status message for HTTP codes is ignored.
ALPN negotiation#
ALPN negotiation allows supporting both HTTPS and HTTP/2 over
the same socket. The req
and res
objects can be either HTTP/1 or
HTTP/2, and an application must restrict itself to the public API of
HTTP/1, and detect if it is possible to use the more advanced
features of HTTP/2.
The following example creates a server that supports both protocols:
const { createSecureServer } = require('node:http2');
const { readFileSync } = require('node:fs');
const cert = readFileSync('./cert.pem');
const key = readFileSync('./key.pem');
const server = createSecureServer(
{ cert, key, allowHTTP1: true },
onRequest,
).listen(4443);
function onRequest(req, res) {
// Detects if it is a HTTPS request or HTTP/2
const { socket: { alpnProtocol } } = req.httpVersion === '2.0' ?
req.stream.session : req;
res.writeHead(200, { 'content-type': 'application/json' });
res.end(JSON.stringify({
alpnProtocol,
httpVersion: req.httpVersion,
}));
}
The 'request'
event works identically on both HTTPS and
HTTP/2.
Class: http2.Http2ServerRequest
#
- Extends: <stream.Readable>
A Http2ServerRequest
object is created by http2.Server
or
http2.SecureServer
and passed as the first argument to the
'request'
event. It may be used to access a request status, headers, and
data.
Event: 'aborted'
#
The 'aborted'
event is emitted whenever a Http2ServerRequest
instance is
abnormally aborted in mid-communication.
The 'aborted'
event will only be emitted if the Http2ServerRequest
writable
side has not been ended.
Event: 'close'
#
Indicates that the underlying Http2Stream
was closed.
Just like 'end'
, this event occurs only once per response.
request.aborted
#
The request.aborted
property will be true
if the request has
been aborted.
request.authority
#
The request authority pseudo header field. Because HTTP/2 allows requests
to set either :authority
or host
, this value is derived from
req.headers[':authority']
if present. Otherwise, it is derived from
req.headers['host']
.
request.complete
#
The request.complete
property will be true
if the request has
been completed, aborted, or destroyed.
request.connection
#
request.socket
.See request.socket
.
request.destroy([error])
#
error
<Error>
Calls destroy()
on the Http2Stream
that received
the Http2ServerRequest
. If error
is provided, an 'error'
event
is emitted and error
is passed as an argument to any listeners on the event.
It does nothing if the stream was already destroyed.
request.headers
#
The request/response headers object.
Key-value pairs of header names and values. Header names are lower-cased.
// Prints something like:
//
// { 'user-agent': 'curl/7.22.0',
// host: '127.0.0.1:8000',
// accept: '*/*' }
console.log(request.headers);
In HTTP/2, the request path, host name, protocol, and method are represented as
special headers prefixed with the :
character (e.g. ':path'
). These special
headers will be included in the request.headers
object. Care must be taken not
to inadvertently modify these special headers or errors may occur. For instance,
removing all headers from the request will cause errors to occur:
removeAllHeaders(request.headers);
assert(request.url); // Fails because the :path header has been removed
request.httpVersion
#
In case of server request, the HTTP version sent by the client. In the case of
client response, the HTTP version of the connected-to server. Returns
'2.0'
.
Also message.httpVersionMajor
is the first integer and
message.httpVersionMinor
is the second.
request.method
#
The request method as a string. Read-only. Examples: 'GET'
, 'DELETE'
.
request.rawHeaders
#
The raw request/response headers list exactly as they were received.
The keys and values are in the same list. It is not a list of tuples. So, the even-numbered offsets are key values, and the odd-numbered offsets are the associated values.
Header names are not lowercased, and duplicates are not merged.
// Prints something like:
//
// [ 'user-agent',
// 'this is invalid because there can be only one',
// 'User-Agent',
// 'curl/7.22.0',
// 'Host',
// '127.0.0.1:8000',
// 'ACCEPT',
// '*/*' ]
console.log(request.rawHeaders);
request.rawTrailers
#
The raw request/response trailer keys and values exactly as they were
received. Only populated at the 'end'
event.
request.scheme
#
The request scheme pseudo header field indicating the scheme portion of the target URL.
request.setTimeout(msecs, callback)
#
msecs
<number>callback
<Function>- Returns: <http2.Http2ServerRequest>
Sets the Http2Stream
's timeout value to msecs
. If a callback is
provided, then it is added as a listener on the 'timeout'
event on
the response object.
If no 'timeout'
listener is added to the request, the response, or
the server, then Http2Stream
s are destroyed when they time out. If a
handler is assigned to the request, the response, or the server's 'timeout'
events, timed out sockets must be handled explicitly.
request.socket
#
Returns a Proxy
object that acts as a net.Socket
(or tls.TLSSocket
) but
applies getters, setters, and methods based on HTTP/2 logic.
destroyed
, readable
, and writable
properties will be retrieved from and
set on request.stream
.
destroy
, emit
, end
, on
and once
methods will be called on
request.stream
.
setTimeout
method will be called on request.stream.session
.
pause
, read
, resume
, and write
will throw an error with code
ERR_HTTP2_NO_SOCKET_MANIPULATION
. See Http2Session
and Sockets for
more information.
All other interactions will be routed directly to the socket. With TLS support,
use request.socket.getPeerCertificate()
to obtain the client's
authentication details.
request.stream
#
The Http2Stream
object backing the request.
request.trailers
#
The request/response trailers object. Only populated at the 'end'
event.
request.url
#
Request URL string. This contains only the URL that is present in the actual HTTP request. If the request is:
GET /status?name=ryan HTTP/1.1
Accept: text/plain
Then request.url
will be:
'/status?name=ryan'
To parse the url into its parts, new URL()
can be used:
$ node
> new URL('/status?name=ryan', 'http://example.com')
URL {
href: 'http://example.com/status?name=ryan',
origin: 'http://example.com',
protocol: 'http:',
username: '',
password: '',
host: 'example.com',
hostname: 'example.com',
port: '',
pathname: '/status',
search: '?name=ryan',
searchParams: URLSearchParams { 'name' => 'ryan' },
hash: ''
}
Class: http2.Http2ServerResponse
#
- Extends: <Stream>
This object is created internally by an HTTP server, not by the user. It is
passed as the second parameter to the 'request'
event.
Event: 'close'
#
Indicates that the underlying Http2Stream
was terminated before
response.end()
was called or able to flush.
Event: 'finish'
#
Emitted when the response has been sent. More specifically, this event is emitted when the last segment of the response headers and body have been handed off to the HTTP/2 multiplexing for transmission over the network. It does not imply that the client has received anything yet.
After this event, no more events will be emitted on the response object.
response.addTrailers(headers)
#
headers
<Object>
This method adds HTTP trailing headers (a header but at the end of the message) to the response.
Attempting to set a header field name or value that contains invalid characters
will result in a TypeError
being thrown.
response.connection
#
response.socket
.See response.socket
.
response.createPushResponse(headers, callback)
#
headers
<HTTP/2 Headers Object> An object describing the headerscallback
<Function> Called oncehttp2stream.pushStream()
is finished, or either when the attempt to create the pushedHttp2Stream
has failed or has been rejected, or the state ofHttp2ServerRequest
is closed prior to calling thehttp2stream.pushStream()
methoderr
<Error>res
<http2.Http2ServerResponse> The newly-createdHttp2ServerResponse
object
Call http2stream.pushStream()
with the given headers, and wrap the
given Http2Stream
on a newly created Http2ServerResponse
as the callback
parameter if successful. When Http2ServerRequest
is closed, the callback is
called with an error ERR_HTTP2_INVALID_STREAM
.
response.end([data[, encoding]][, callback])
#
data
<string> | <Buffer> | <Uint8Array>encoding
<string>callback
<Function>- Returns: <this>
This method signals to the server that all of the response headers and body
have been sent; that server should consider this message complete.
The method, response.end()
, MUST be called on each response.
If data
is specified, it is equivalent to calling
response.write(data, encoding)
followed by response.end(callback)
.
If callback
is specified, it will be called when the response stream
is finished.
response.finished
#
response.writableEnded
.Boolean value that indicates whether the response has completed. Starts
as false
. After response.end()
executes, the value will be true
.
response.getHeader(name)
#
Reads out a header that has already been queued but not sent to the client. The name is case-insensitive.
const contentType = response.getHeader('content-type');
response.getHeaderNames()
#
- Returns: <string[]>
Returns an array containing the unique names of the current outgoing headers. All header names are lowercase.
response.setHeader('Foo', 'bar');
response.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']);
const headerNames = response.getHeaderNames();
// headerNames === ['foo', 'set-cookie']
response.getHeaders()
#
- Returns: <Object>
Returns a shallow copy of the current outgoing headers. Since a shallow copy is used, array values may be mutated without additional calls to various header-related http module methods. The keys of the returned object are the header names and the values are the respective header values. All header names are lowercase.
The object returned by the response.getHeaders()
method does not
prototypically inherit from the JavaScript Object
. This means that typical
Object
methods such as obj.toString()
, obj.hasOwnProperty()
, and others
are not defined and will not work.
response.setHeader('Foo', 'bar');
response.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']);
const headers = response.getHeaders();
// headers === { foo: 'bar', 'set-cookie': ['foo=bar', 'bar=baz'] }
response.hasHeader(name)
#
Returns true
if the header identified by name
is currently set in the
outgoing headers. The header name matching is case-insensitive.
const hasContentType = response.hasHeader('content-type');
response.headersSent
#
True if headers were sent, false otherwise (read-only).
response.removeHeader(name)
#
name
<string>
Removes a header that has been queued for implicit sending.
response.removeHeader('Content-Encoding');
response.req
#
A reference to the original HTTP2 request
object.
response.sendDate
#
When true, the Date header will be automatically generated and sent in the response if it is not already present in the headers. Defaults to true.
This should only be disabled for testing; HTTP requires the Date header in responses.
response.setHeader(name, value)
#
name
<string>value
<string> | <string[]>
Sets a single header value for implicit headers. If this header already exists in the to-be-sent headers, its value will be replaced. Use an array of strings here to send multiple headers with the same name.
response.setHeader('Content-Type', 'text/html; charset=utf-8');
or
response.setHeader('Set-Cookie', ['type=ninja', 'language=javascript']);
Attempting to set a header field name or value that contains invalid characters
will result in a TypeError
being thrown.
When headers have been set with response.setHeader()
, they will be merged
with any headers passed to response.writeHead()
, with the headers passed
to response.writeHead()
given precedence.
// Returns content-type = text/plain
const server = http2.createServer((req, res) => {
res.setHeader('Content-Type', 'text/html; charset=utf-8');
res.setHeader('X-Foo', 'bar');
res.writeHead(200, { 'Content-Type': 'text/plain; charset=utf-8' });
res.end('ok');
});
response.setTimeout(msecs[, callback])
#
msecs
<number>callback
<Function>- Returns: <http2.Http2ServerResponse>
Sets the Http2Stream
's timeout value to msecs
. If a callback is
provided, then it is added as a listener on the 'timeout'
event on
the response object.
If no 'timeout'
listener is added to the request, the response, or
the server, then Http2Stream
s are destroyed when they time out. If a
handler is assigned to the request, the response, or the server's 'timeout'
events, timed out sockets must be handled explicitly.
response.socket
#
Returns a Proxy
object that acts as a net.Socket
(or tls.TLSSocket
) but
applies getters, setters, and methods based on HTTP/2 logic.
destroyed
, readable
, and writable
properties will be retrieved from and
set on response.stream
.
destroy
, emit
, end
, on
and once
methods will be called on
response.stream
.
setTimeout
method will be called on response.stream.session
.
pause
, read
, resume
, and write
will throw an error with code
ERR_HTTP2_NO_SOCKET_MANIPULATION
. See Http2Session
and Sockets for
more information.
All other interactions will be routed directly to the socket.
const http2 = require('node:http2');
const server = http2.createServer((req, res) => {
const ip = req.socket.remoteAddress;
const port = req.socket.remotePort;
res.end(`Your IP address is ${ip} and your source port is ${port}.`);
}).listen(3000);
response.statusCode
#
When using implicit headers (not calling response.writeHead()
explicitly),
this property controls the status code that will be sent to the client when
the headers get flushed.
response.statusCode = 404;
After response header was sent to the client, this property indicates the status code which was sent out.
response.statusMessage
#
Status message is not supported by HTTP/2 (RFC 7540 8.1.2.4). It returns an empty string.
response.stream
#
The Http2Stream
object backing the response.
response.writableEnded
#
Is true
after response.end()
has been called. This property
does not indicate whether the data has been flushed, for this use
writable.writableFinished
instead.
response.write(chunk[, encoding][, callback])
#
chunk
<string> | <Buffer> | <Uint8Array>encoding
<string>callback
<Function>- Returns: <boolean>
If this method is called and response.writeHead()
has not been called,
it will switch to implicit header mode and flush the implicit headers.
This sends a chunk of the response body. This method may be called multiple times to provide successive parts of the body.
In the node:http
module, the response body is omitted when the
request is a HEAD request. Similarly, the 204
and 304
responses
must not include a message body.
chunk
can be a string or a buffer. If chunk
is a string,
the second parameter specifies how to encode it into a byte stream.
By default the encoding
is 'utf8'
. callback
will be called when this chunk
of data is flushed.
This is the raw HTTP body and has nothing to do with higher-level multi-part body encodings that may be used.
The first time response.write()
is called, it will send the buffered
header information and the first chunk of the body to the client. The second
time response.write()
is called, Node.js assumes data will be streamed,
and sends the new data separately. That is, the response is buffered up to the
first chunk of the body.
Returns true
if the entire data was flushed successfully to the kernel
buffer. Returns false
if all or part of the data was queued in user memory.
'drain'
will be emitted when the buffer is free again.
response.writeContinue()
#
Sends a status 100 Continue
to the client, indicating that the request body
should be sent. See the 'checkContinue'
event on Http2Server
and
Http2SecureServer
.
response.writeEarlyHints(hints)
#
hints
<Object>
Sends a status 103 Early Hints
to the client with a Link header,
indicating that the user agent can preload/preconnect the linked resources.
The hints
is an object containing the values of headers to be sent with
early hints message.
Example
const earlyHintsLink = '</styles.css>; rel=preload; as=style';
response.writeEarlyHints({
'link': earlyHintsLink,
});
const earlyHintsLinks = [
'</styles.css>; rel=preload; as=style',
'</scripts.js>; rel=preload; as=script',
];
response.writeEarlyHints({
'link': earlyHintsLinks,
});
response.writeHead(statusCode[, statusMessage][, headers])
#
statusCode
<number>statusMessage
<string>headers
<Object> | <Array>- Returns: <http2.Http2ServerResponse>
Sends a response header to the request. The status code is a 3-digit HTTP
status code, like 404
. The last argument, headers
, are the response headers.
Returns a reference to the Http2ServerResponse
, so that calls can be chained.
For compatibility with HTTP/1, a human-readable statusMessage
may be
passed as the second argument. However, because the statusMessage
has no
meaning within HTTP/2, the argument will have no effect and a process warning
will be emitted.
const body = 'hello world';
response.writeHead(200, {
'Content-Length': Buffer.byteLength(body),
'Content-Type': 'text/plain; charset=utf-8',
});
Content-Length
is given in bytes not characters. The
Buffer.byteLength()
API may be used to determine the number of bytes in a
given encoding. On outbound messages, Node.js does not check if Content-Length
and the length of the body being transmitted are equal or not. However, when
receiving messages, Node.js will automatically reject messages when the
Content-Length
does not match the actual payload size.
This method may be called at most one time on a message before
response.end()
is called.
If response.write()
or response.end()
are called before calling
this, the implicit/mutable headers will be calculated and call this function.
When headers have been set with response.setHeader()
, they will be merged
with any headers passed to response.writeHead()
, with the headers passed
to response.writeHead()
given precedence.
// Returns content-type = text/plain
const server = http2.createServer((req, res) => {
res.setHeader('Content-Type', 'text/html; charset=utf-8');
res.setHeader('X-Foo', 'bar');
res.writeHead(200, { 'Content-Type': 'text/plain; charset=utf-8' });
res.end('ok');
});
Attempting to set a header field name or value that contains invalid characters
will result in a TypeError
being thrown.
Collecting HTTP/2 performance metrics#
The Performance Observer API can be used to collect basic performance
metrics for each Http2Session
and Http2Stream
instance.
const { PerformanceObserver } = require('node:perf_hooks');
const obs = new PerformanceObserver((items) => {
const entry = items.getEntries()[0];
console.log(entry.entryType); // prints 'http2'
if (entry.name === 'Http2Session') {
// Entry contains statistics about the Http2Session
} else if (entry.name === 'Http2Stream') {
// Entry contains statistics about the Http2Stream
}
});
obs.observe({ entryTypes: ['http2'] });
The entryType
property of the PerformanceEntry
will be equal to 'http2'
.
The name
property of the PerformanceEntry
will be equal to either
'Http2Stream'
or 'Http2Session'
.
If name
is equal to Http2Stream
, the PerformanceEntry
will contain the
following additional properties:
bytesRead
<number> The number ofDATA
frame bytes received for thisHttp2Stream
.bytesWritten
<number> The number ofDATA
frame bytes sent for thisHttp2Stream
.id
<number> The identifier of the associatedHttp2Stream
timeToFirstByte
<number> The number of milliseconds elapsed between thePerformanceEntry
startTime
and the reception of the firstDATA
frame.timeToFirstByteSent
<number> The number of milliseconds elapsed between thePerformanceEntry
startTime
and sending of the firstDATA
frame.timeToFirstHeader
<number> The number of milliseconds elapsed between thePerformanceEntry
startTime
and the reception of the first header.
If name
is equal to Http2Session
, the PerformanceEntry
will contain the
following additional properties:
bytesRead
<number> The number of bytes received for thisHttp2Session
.bytesWritten
<number> The number of bytes sent for thisHttp2Session
.framesReceived
<number> The number of HTTP/2 frames received by theHttp2Session
.framesSent
<number> The number of HTTP/2 frames sent by theHttp2Session
.maxConcurrentStreams
<number> The maximum number of streams concurrently open during the lifetime of theHttp2Session
.pingRTT
<number> The number of milliseconds elapsed since the transmission of aPING
frame and the reception of its acknowledgment. Only present if aPING
frame has been sent on theHttp2Session
.streamAverageDuration
<number> The average duration (in milliseconds) for allHttp2Stream
instances.streamCount
<number> The number ofHttp2Stream
instances processed by theHttp2Session
.type
<string> Either'server'
or'client'
to identify the type ofHttp2Session
.
Note on :authority
and host
#
HTTP/2 requires requests to have either the :authority
pseudo-header
or the host
header. Prefer :authority
when constructing an HTTP/2
request directly, and host
when converting from HTTP/1 (in proxies,
for instance).
The compatibility API falls back to host
if :authority
is not
present. See request.authority
for more information. However,
if you don't use the compatibility API (or use req.headers
directly),
you need to implement any fall-back behavior yourself.
HTTPS#
Source Code: lib/https.js
HTTPS is the HTTP protocol over TLS/SSL. In Node.js this is implemented as a separate module.
Determining if crypto support is unavailable#
It is possible for Node.js to be built without including support for the
node:crypto
module. In such cases, attempting to import
from https
or
calling require('node:https')
will result in an error being thrown.
When using CommonJS, the error thrown can be caught using try/catch:
let https;
try {
https = require('node:https');
} catch (err) {
console.error('https support is disabled!');
}
When using the lexical ESM import
keyword, the error can only be
caught if a handler for process.on('uncaughtException')
is registered
before any attempt to load the module is made (using, for instance,
a preload module).
When using ESM, if there is a chance that the code may be run on a build
of Node.js where crypto support is not enabled, consider using the
import()
function instead of the lexical import
keyword:
let https;
try {
https = await import('node:https');
} catch (err) {
console.error('https support is disabled!');
}
Class: https.Agent
#
An Agent
object for HTTPS similar to http.Agent
. See
https.request()
for more information.
new Agent([options])
#
options
<Object> Set of configurable options to set on the agent. Can have the same fields as forhttp.Agent(options)
, and-
maxCachedSessions
<number> maximum number of TLS cached sessions. Use0
to disable TLS session caching. Default:100
. -
servername
<string> the value of Server Name Indication extension to be sent to the server. Use empty string''
to disable sending the extension. Default: host name of the target server, unless the target server is specified using an IP address, in which case the default is''
(no extension).See
Session Resumption
for information about TLS session reuse.
-
Event: 'keylog'
#
line
<Buffer> Line of ASCII text, in NSSSSLKEYLOGFILE
format.tlsSocket
<tls.TLSSocket> Thetls.TLSSocket
instance on which it was generated.
The keylog
event is emitted when key material is generated or received by a
connection managed by this agent (typically before handshake has completed, but
not necessarily). This keying material can be stored for debugging, as it
allows captured TLS traffic to be decrypted. It may be emitted multiple times
for each socket.
A typical use case is to append received lines to a common text file, which is later used by software (such as Wireshark) to decrypt the traffic:
// ...
https.globalAgent.on('keylog', (line, tlsSocket) => {
fs.appendFileSync('/tmp/ssl-keys.log', line, { mode: 0o600 });
});
Class: https.Server
#
- Extends: <tls.Server>
See http.Server
for more information.
server.close([callback])
#
callback
<Function>- Returns: <https.Server>
See server.close()
in the node:http
module.
server.closeAllConnections()
#
See server.closeAllConnections()
in the node:http
module.
server.closeIdleConnections()
#
See server.closeIdleConnections()
in the node:http
module.
server.headersTimeout
#
- <number> Default:
60000
See server.headersTimeout
in the node:http
module.
server.listen()
#
Starts the HTTPS server listening for encrypted connections.
This method is identical to server.listen()
from net.Server
.
server.maxHeadersCount
#
- <number> Default:
2000
See server.maxHeadersCount
in the node:http
module.
server.requestTimeout
#
- <number> Default:
300000
See server.requestTimeout
in the node:http
module.
server.setTimeout([msecs][, callback])
#
msecs
<number> Default:120000
(2 minutes)callback
<Function>- Returns: <https.Server>
See server.setTimeout()
in the node:http
module.
server.timeout
#
- <number> Default: 0 (no timeout)
See server.timeout
in the node:http
module.
server.keepAliveTimeout
#
- <number> Default:
5000
(5 seconds)
See server.keepAliveTimeout
in the node:http
module.
https.createServer([options][, requestListener])
#
options
<Object> Acceptsoptions
fromtls.createServer()
,tls.createSecureContext()
andhttp.createServer()
.requestListener
<Function> A listener to be added to the'request'
event.- Returns: <https.Server>
// curl -k https://localhost:8000/
const https = require('node:https');
const fs = require('node:fs');
const options = {
key: fs.readFileSync('test/fixtures/keys/agent2-key.pem'),
cert: fs.readFileSync('test/fixtures/keys/agent2-cert.pem'),
};
https.createServer(options, (req, res) => {
res.writeHead(200);
res.end('hello world\n');
}).listen(8000);
Or
const https = require('node:https');
const fs = require('node:fs');
const options = {
pfx: fs.readFileSync('test/fixtures/test_cert.pfx'),
passphrase: 'sample',
};
https.createServer(options, (req, res) => {
res.writeHead(200);
res.end('hello world\n');
}).listen(8000);
https.get(options[, callback])
#
https.get(url[, options][, callback])
#
url
<string> | <URL>options
<Object> | <string> | <URL> Accepts the sameoptions
ashttps.request()
, with themethod
always set toGET
.callback
<Function>
Like http.get()
but for HTTPS.
options
can be an object, a string, or a URL
object. If options
is a
string, it is automatically parsed with new URL()
. If it is a URL
object, it will be automatically converted to an ordinary options
object.
const https = require('node:https');
https.get('https://encrypted.google.com/', (res) => {
console.log('statusCode:', res.statusCode);
console.log('headers:', res.headers);
res.on('data', (d) => {
process.stdout.write(d);
});
}).on('error', (e) => {
console.error(e);
});
https.globalAgent
#
Global instance of https.Agent
for all HTTPS client requests.
https.request(options[, callback])
#
https.request(url[, options][, callback])
#
url
<string> | <URL>options
<Object> | <string> | <URL> Accepts alloptions
fromhttp.request()
, with some differences in default values:protocol
Default:'https:'
port
Default:443
agent
Default:https.globalAgent
callback
<Function>- Returns: <http.ClientRequest>
Makes a request to a secure web server.
The following additional options
from tls.connect()
are also accepted:
ca
, cert
, ciphers
, clientCertEngine
, crl
, dhparam
, ecdhCurve
,
honorCipherOrder
, key
, passphrase
, pfx
, rejectUnauthorized
,
secureOptions
, secureProtocol
, servername
, sessionIdContext
,
highWaterMark
.
options
can be an object, a string, or a URL
object. If options
is a
string, it is automatically parsed with new URL()
. If it is a URL
object, it will be automatically converted to an ordinary options
object.
https.request()
returns an instance of the http.ClientRequest
class. The ClientRequest
instance is a writable stream. If one needs to
upload a file with a POST request, then write to the ClientRequest
object.
const https = require('node:https');
const options = {
hostname: 'encrypted.google.com',
port: 443,
path: '/',
method: 'GET',
};
const req = https.request(options, (res) => {
console.log('statusCode:', res.statusCode);
console.log('headers:', res.headers);
res.on('data', (d) => {
process.stdout.write(d);
});
});
req.on('error', (e) => {
console.error(e);
});
req.end();
Example using options from tls.connect()
:
const options = {
hostname: 'encrypted.google.com',
port: 443,
path: '/',
method: 'GET',
key: fs.readFileSync('test/fixtures/keys/agent2-key.pem'),
cert: fs.readFileSync('test/fixtures/keys/agent2-cert.pem'),
};
options.agent = new https.Agent(options);
const req = https.request(options, (res) => {
// ...
});
Alternatively, opt out of connection pooling by not using an Agent
.
const options = {
hostname: 'encrypted.google.com',
port: 443,
path: '/',
method: 'GET',
key: fs.readFileSync('test/fixtures/keys/agent2-key.pem'),
cert: fs.readFileSync('test/fixtures/keys/agent2-cert.pem'),
agent: false,
};
const req = https.request(options, (res) => {
// ...
});
Example using a URL
as options
:
const options = new URL('https://abc:[email protected]');
const req = https.request(options, (res) => {
// ...
});
Example pinning on certificate fingerprint, or the public key (similar to
pin-sha256
):
const tls = require('node:tls');
const https = require('node:https');
const crypto = require('node:crypto');
function sha256(s) {
return crypto.createHash('sha256').update(s).digest('base64');
}
const options = {
hostname: 'github.com',
port: 443,
path: '/',
method: 'GET',
checkServerIdentity: function(host, cert) {
// Make sure the certificate is issued to the host we are connected to
const err = tls.checkServerIdentity(host, cert);
if (err) {
return err;
}
// Pin the public key, similar to HPKP pin-sha256 pinning
const pubkey256 = 'pL1+qb9HTMRZJmuC/bB/ZI9d302BYrrqiVuRyW+DGrU=';
if (sha256(cert.pubkey) !== pubkey256) {
const msg = 'Certificate verification error: ' +
`The public key of '${cert.subject.CN}' ` +
'does not match our pinned fingerprint';
return new Error(msg);
}
// Pin the exact certificate, rather than the pub key
const cert256 = '25:FE:39:32:D9:63:8C:8A:FC:A1:9A:29:87:' +
'D8:3E:4C:1D:98:DB:71:E4:1A:48:03:98:EA:22:6A:BD:8B:93:16';
if (cert.fingerprint256 !== cert256) {
const msg = 'Certificate verification error: ' +
`The certificate of '${cert.subject.CN}' ` +
'does not match our pinned fingerprint';
return new Error(msg);
}
// This loop is informational only.
// Print the certificate and public key fingerprints of all certs in the
// chain. Its common to pin the public key of the issuer on the public
// internet, while pinning the public key of the service in sensitive
// environments.
do {
console.log('Subject Common Name:', cert.subject.CN);
console.log(' Certificate SHA256 fingerprint:', cert.fingerprint256);
hash = crypto.createHash('sha256');
console.log(' Public key ping-sha256:', sha256(cert.pubkey));
lastprint256 = cert.fingerprint256;
cert = cert.issuerCertificate;
} while (cert.fingerprint256 !== lastprint256);
},
};
options.agent = new https.Agent(options);
const req = https.request(options, (res) => {
console.log('All OK. Server matched our pinned cert or public key');
console.log('statusCode:', res.statusCode);
// Print the HPKP values
console.log('headers:', res.headers['public-key-pins']);
res.on('data', (d) => {});
});
req.on('error', (e) => {
console.error(e.message);
});
req.end();
Outputs for example:
Subject Common Name: github.com
Certificate SHA256 fingerprint: 25:FE:39:32:D9:63:8C:8A:FC:A1:9A:29:87:D8:3E:4C:1D:98:DB:71:E4:1A:48:03:98:EA:22:6A:BD:8B:93:16
Public key ping-sha256: pL1+qb9HTMRZJmuC/bB/ZI9d302BYrrqiVuRyW+DGrU=
Subject Common Name: DigiCert SHA2 Extended Validation Server CA
Certificate SHA256 fingerprint: 40:3E:06:2A:26:53:05:91:13:28:5B:AF:80:A0:D4:AE:42:2C:84:8C:9F:78:FA:D0:1F:C9:4B:C5:B8:7F:EF:1A
Public key ping-sha256: RRM1dGqnDFsCJXBTHky16vi1obOlCgFFn/yOhI/y+ho=
Subject Common Name: DigiCert High Assurance EV Root CA
Certificate SHA256 fingerprint: 74:31:E5:F4:C3:C1:CE:46:90:77:4F:0B:61:E0:54:40:88:3B:A9:A0:1E:D0:0B:A6:AB:D7:80:6E:D3:B1:18:CF
Public key ping-sha256: WoiWRyIOVNa9ihaBciRSC7XHjliYS9VwUGOIud4PB18=
All OK. Server matched our pinned cert or public key
statusCode: 200
headers: max-age=0; pin-sha256="WoiWRyIOVNa9ihaBciRSC7XHjliYS9VwUGOIud4PB18="; pin-sha256="RRM1dGqnDFsCJXBTHky16vi1obOlCgFFn/yOhI/y+ho="; pin-sha256="k2v657xBsOVe1PQRwOsHsw3bsGT2VzIqz5K+59sNQws="; pin-sha256="K87oWBWM9UZfyddvDfoxL+8lpNyoUB2ptGtn0fv6G2Q="; pin-sha256="IQBnNBEiFuhj+8x6X8XLgh01V9Ic5/V3IRQLNFFc7v4="; pin-sha256="iie1VXtL7HzAMF+/PVPR9xzT80kQxdZeJ+zduCB3uj0="; pin-sha256="LvRiGEjRqfzurezaWuj8Wie2gyHMrW5Q06LspMnox7A="; includeSubDomains
Inspector#
Source Code: lib/inspector.js
The node:inspector
module provides an API for interacting with the V8
inspector.
It can be accessed using:
import * as inspector from 'node:inspector/promises';
const inspector = require('node:inspector/promises');
or
import * as inspector from 'node:inspector';
const inspector = require('node:inspector');
Promises API#
Class: inspector.Session
#
- Extends: <EventEmitter>
The inspector.Session
is used for dispatching messages to the V8 inspector
back-end and receiving message responses and notifications.
new inspector.Session()
#
Create a new instance of the inspector.Session
class. The inspector session
needs to be connected through session.connect()
before the messages
can be dispatched to the inspector backend.
Event: 'inspectorNotification'
#
- <Object> The notification message object
Emitted when any notification from the V8 Inspector is received.
session.on('inspectorNotification', (message) => console.log(message.method));
// Debugger.paused
// Debugger.resumed
It is also possible to subscribe only to notifications with specific method:
Event: <inspector-protocol-method>
;#
- <Object> The notification message object
Emitted when an inspector notification is received that has its method field set
to the <inspector-protocol-method>
value.
The following snippet installs a listener on the 'Debugger.paused'
event, and prints the reason for program suspension whenever program
execution is suspended (through breakpoints, for example):
session.on('Debugger.paused', ({ params }) => {
console.log(params.hitBreakpoints);
});
// [ '/the/file/that/has/the/breakpoint.js:11:0' ]
session.connect()
#
Connects a session to the inspector back-end.
session.connectToMainThread()
#
Connects a session to the main thread inspector back-end. An exception will be thrown if this API was not called on a Worker thread.
session.disconnect()
#
Immediately close the session. All pending message callbacks will be called
with an error. session.connect()
will need to be called to be able to send
messages again. Reconnected session will lose all inspector state, such as
enabled agents or configured breakpoints.
session.post(method[, params])
#
Posts a message to the inspector back-end.
import { Session } from 'node:inspector/promises';
try {
const session = new Session();
session.connect();
const result = await session.post('Runtime.evaluate', { expression: '2 + 2' });
console.log(result);
} catch (error) {
console.error(error);
}
// Output: { result: { type: 'number', value: 4, description: '4' } }
The latest version of the V8 inspector protocol is published on the Chrome DevTools Protocol Viewer.
Node.js inspector supports all the Chrome DevTools Protocol domains declared by V8. Chrome DevTools Protocol domain provides an interface for interacting with one of the runtime agents used to inspect the application state and listen to the run-time events.
Example usage#
Apart from the debugger, various V8 Profilers are available through the DevTools protocol.
CPU profiler#
Here's an example showing how to use the CPU Profiler:
import { Session } from 'node:inspector/promises';
import fs from 'node:fs';
const session = new Session();
session.connect();
await session.post('Profiler.enable');
await session.post('Profiler.start');
// Invoke business logic under measurement here...
// some time later...
const { profile } = await session.post('Profiler.stop');
// Write profile to disk, upload, etc.
fs.writeFileSync('./profile.cpuprofile', JSON.stringify(profile));
Heap profiler#
Here's an example showing how to use the Heap Profiler:
import { Session } from 'node:inspector/promises';
import fs from 'node:fs';
const session = new Session();
const fd = fs.openSync('profile.heapsnapshot', 'w');
session.connect();
session.on('HeapProfiler.addHeapSnapshotChunk', (m) => {
fs.writeSync(fd, m.params.chunk);
});
const result = await session.post('HeapProfiler.takeHeapSnapshot', null);
console.log('HeapProfiler.takeHeapSnapshot done:', result);
session.disconnect();
fs.closeSync(fd);
Callback API#
Class: inspector.Session
#
- Extends: <EventEmitter>
The inspector.Session
is used for dispatching messages to the V8 inspector
back-end and receiving message responses and notifications.
new inspector.Session()
#
Create a new instance of the inspector.Session
class. The inspector session
needs to be connected through session.connect()
before the messages
can be dispatched to the inspector backend.
Event: 'inspectorNotification'
#
- <Object> The notification message object
Emitted when any notification from the V8 Inspector is received.
session.on('inspectorNotification', (message) => console.log(message.method));
// Debugger.paused
// Debugger.resumed
It is also possible to subscribe only to notifications with specific method:
Event: <inspector-protocol-method>
;#
- <Object> The notification message object
Emitted when an inspector notification is received that has its method field set
to the <inspector-protocol-method>
value.
The following snippet installs a listener on the 'Debugger.paused'
event, and prints the reason for program suspension whenever program
execution is suspended (through breakpoints, for example):
session.on('Debugger.paused', ({ params }) => {
console.log(params.hitBreakpoints);
});
// [ '/the/file/that/has/the/breakpoint.js:11:0' ]
session.connect()
#
Connects a session to the inspector back-end.
session.connectToMainThread()
#
Connects a session to the main thread inspector back-end. An exception will be thrown if this API was not called on a Worker thread.
session.disconnect()
#
Immediately close the session. All pending message callbacks will be called
with an error. session.connect()
will need to be called to be able to send
messages again. Reconnected session will lose all inspector state, such as
enabled agents or configured breakpoints.
session.post(method[, params][, callback])
#
method
<string>params
<Object>callback
<Function>
Posts a message to the inspector back-end. callback
will be notified when
a response is received. callback
is a function that accepts two optional
arguments: error and message-specific result.
session.post('Runtime.evaluate', { expression: '2 + 2' },
(error, { result }) => console.log(result));
// Output: { type: 'number', value: 4, description: '4' }
The latest version of the V8 inspector protocol is published on the Chrome DevTools Protocol Viewer.
Node.js inspector supports all the Chrome DevTools Protocol domains declared by V8. Chrome DevTools Protocol domain provides an interface for interacting with one of the runtime agents used to inspect the application state and listen to the run-time events.
You can not set reportProgress
to true
when sending a
HeapProfiler.takeHeapSnapshot
or HeapProfiler.stopTrackingHeapObjects
command to V8.
Example usage#
Apart from the debugger, various V8 Profilers are available through the DevTools protocol.
CPU profiler#
Here's an example showing how to use the CPU Profiler:
const inspector = require('node:inspector');
const fs = require('node:fs');
const session = new inspector.Session();
session.connect();
session.post('Profiler.enable', () => {
session.post('Profiler.start', () => {
// Invoke business logic under measurement here...
// some time later...
session.post('Profiler.stop', (err, { profile }) => {
// Write profile to disk, upload, etc.
if (!err) {
fs.writeFileSync('./profile.cpuprofile', JSON.stringify(profile));
}
});
});
});
Heap profiler#
Here's an example showing how to use the Heap Profiler:
const inspector = require('node:inspector');
const fs = require('node:fs');
const session = new inspector.Session();
const fd = fs.openSync('profile.heapsnapshot', 'w');
session.connect();
session.on('HeapProfiler.addHeapSnapshotChunk', (m) => {
fs.writeSync(fd, m.params.chunk);
});
session.post('HeapProfiler.takeHeapSnapshot', null, (err, r) => {
console.log('HeapProfiler.takeHeapSnapshot done:', err, r);
session.disconnect();
fs.closeSync(fd);
});
Common Objects#
inspector.close()
#
Deactivate the inspector. Blocks until there are no active connections.
inspector.console
#
- <Object> An object to send messages to the remote inspector console.
require('node:inspector').console.log('a message');
The inspector console does not have API parity with Node.js console.
inspector.open([port[, host[, wait]]])
#
port
<number> Port to listen on for inspector connections. Optional. Default: what was specified on the CLI.host
<string> Host to listen on for inspector connections. Optional. Default: what was specified on the CLI.wait
<boolean> Block until a client has connected. Optional. Default:false
.
Activate inspector on host and port. Equivalent to
node --inspect=[[host:]port]
, but can be done programmatically after node has
started.
If wait is true
, will block until a client has connected to the inspect port
and flow control has been passed to the debugger client.
See the security warning regarding the host
parameter usage.
inspector.url()
#
- Returns: <string> | <undefined>
Return the URL of the active inspector, or undefined
if there is none.
$ node --inspect -p 'inspector.url()'
Debugger listening on ws://127.0.0.1:9229/166e272e-7a30-4d09-97ce-f1c012b43c34
For help, see: https://nodejs.org/en/docs/inspector
ws://127.0.0.1:9229/166e272e-7a30-4d09-97ce-f1c012b43c34
$ node --inspect=localhost:3000 -p 'inspector.url()'
Debugger listening on ws://localhost:3000/51cf8d0e-3c36-4c59-8efd-54519839e56a
For help, see: https://nodejs.org/en/docs/inspector
ws://localhost:3000/51cf8d0e-3c36-4c59-8efd-54519839e56a
$ node -p 'inspector.url()'
undefined
inspector.waitForDebugger()
#
Blocks until a client (existing or connected later) has sent
Runtime.runIfWaitingForDebugger
command.
An exception will be thrown if there is no active inspector.
Internationalization support#
Node.js has many features that make it easier to write internationalized programs. Some of them are:
- Locale-sensitive or Unicode-aware functions in the ECMAScript Language Specification:
- All functionality described in the ECMAScript Internationalization API
Specification (aka ECMA-402):
Intl
object- Locale-sensitive methods like
String.prototype.localeCompare()
andDate.prototype.toLocaleString()
- The WHATWG URL parser's internationalized domain names (IDNs) support
require('node:buffer').transcode()
- More accurate REPL line editing
require('node:util').TextDecoder
RegExp
Unicode Property Escapes
Node.js and the underlying V8 engine use International Components for Unicode (ICU) to implement these features in native C/C++ code. The full ICU data set is provided by Node.js by default. However, due to the size of the ICU data file, several options are provided for customizing the ICU data set either when building or running Node.js.
Options for building Node.js#
To control how ICU is used in Node.js, four configure
options are available
during compilation. Additional details on how to compile Node.js are documented
in BUILDING.md.
--with-intl=none
/--without-intl
--with-intl=system-icu
--with-intl=small-icu
--with-intl=full-icu
(default)
An overview of available Node.js and JavaScript features for each configure
option:
Feature | none | system-icu | small-icu | full-icu |
---|---|---|---|---|
String.prototype.normalize() | none (function is no-op) | full | full | full |
String.prototype.to*Case() | full | full | full | full |
Intl | none (object does not exist) | partial/full (depends on OS) | partial (English-only) | full |
String.prototype.localeCompare() | partial (not locale-aware) | full | full | full |
String.prototype.toLocale*Case() | partial (not locale-aware) | full | full | full |
Number.prototype.toLocaleString() | partial (not locale-aware) | partial/full (depends on OS) | partial (English-only) | full |
Date.prototype.toLocale*String() | partial (not locale-aware) | partial/full (depends on OS) | partial (English-only) | full |
Legacy URL Parser | partial (no IDN support) | full | full | full |
WHATWG URL Parser | partial (no IDN support) | full | full | full |
require('node:buffer').transcode() | none (function does not exist) | full | full | full |
REPL | partial (inaccurate line editing) | full | full | full |
require('node:util').TextDecoder | partial (basic encodings support) | partial/full (depends on OS) | partial (Unicode-only) | full |
RegExp Unicode Property Escapes | none (invalid RegExp error) | full | full | full |
The "(not locale-aware)" designation denotes that the function carries out its
operation just like the non-Locale
version of the function, if one
exists. For example, under none
mode, Date.prototype.toLocaleString()
's
operation is identical to that of Date.prototype.toString()
.
Disable all internationalization features (none
)#
If this option is chosen, ICU is disabled and most internationalization
features mentioned above will be unavailable in the resulting node
binary.
Build with a pre-installed ICU (system-icu
)#
Node.js can link against an ICU build already installed on the system. In fact, most Linux distributions already come with ICU installed, and this option would make it possible to reuse the same set of data used by other components in the OS.
Functionalities that only require the ICU library itself, such as
String.prototype.normalize()
and the WHATWG URL parser, are fully
supported under system-icu
. Features that require ICU locale data in
addition, such as Intl.DateTimeFormat
may be fully or partially
supported, depending on the completeness of the ICU data installed on the
system.
Embed a limited set of ICU data (small-icu
)#
This option makes the resulting binary link against the ICU library statically,
and includes a subset of ICU data (typically only the English locale) within
the node
executable.
Functionalities that only require the ICU library itself, such as
String.prototype.normalize()
and the WHATWG URL parser, are fully
supported under small-icu
. Features that require ICU locale data in addition,
such as Intl.DateTimeFormat
, generally only work with the English locale:
const january = new Date(9e8);
const english = new Intl.DateTimeFormat('en', { month: 'long' });
const spanish = new Intl.DateTimeFormat('es', { month: 'long' });
console.log(english.format(january));
// Prints "January"
console.log(spanish.format(january));
// Prints either "M01" or "January" on small-icu, depending on the user’s default locale
// Should print "enero"
This mode provides a balance between features and binary size.
Providing ICU data at runtime#
If the small-icu
option is used, one can still provide additional locale data
at runtime so that the JS methods would work for all ICU locales. Assuming the
data file is stored at /some/directory
, it can be made available to ICU
through either:
-
The
NODE_ICU_DATA
environment variable:env NODE_ICU_DATA=/some/directory node
-
The
--icu-data-dir
CLI parameter:node --icu-data-dir=/some/directory
(If both are specified, the --icu-data-dir
CLI parameter takes precedence.)
ICU is able to automatically find and load a variety of data formats, but the
data must be appropriate for the ICU version, and the file correctly named.
The most common name for the data file is icudt6X[bl].dat
, where 6X
denotes
the intended ICU version, and b
or l
indicates the system's endianness.
Check "ICU Data" article in the ICU User Guide for other supported formats
and more details on ICU data in general.
The full-icu npm module can greatly simplify ICU data installation by
detecting the ICU version of the running node
executable and downloading the
appropriate data file. After installing the module through npm i full-icu
,
the data file will be available at ./node_modules/full-icu
. This path can be
then passed either to NODE_ICU_DATA
or --icu-data-dir
as shown above to
enable full Intl
support.
Embed the entire ICU (full-icu
)#
This option makes the resulting binary link against ICU statically and include
a full set of ICU data. A binary created this way has no further external
dependencies and supports all locales, but might be rather large. This is
the default behavior if no --with-intl
flag is passed. The official binaries
are also built in this mode.
Detecting internationalization support#
To verify that ICU is enabled at all (system-icu
, small-icu
, or
full-icu
), simply checking the existence of Intl
should suffice:
const hasICU = typeof Intl === 'object';
Alternatively, checking for process.versions.icu
, a property defined only
when ICU is enabled, works too:
const hasICU = typeof process.versions.icu === 'string';
To check for support for a non-English locale (i.e. full-icu
or
system-icu
), Intl.DateTimeFormat
can be a good distinguishing factor:
const hasFullICU = (() => {
try {
const january = new Date(9e8);
const spanish = new Intl.DateTimeFormat('es', { month: 'long' });
return spanish.format(january) === 'enero';
} catch (err) {
return false;
}
})();
For more verbose tests for Intl
support, the following resources may be found
to be helpful:
Modules: CommonJS modules#
CommonJS modules are the original way to package JavaScript code for Node.js. Node.js also supports the ECMAScript modules standard used by browsers and other JavaScript runtimes.
In Node.js, each file is treated as a separate module. For
example, consider a file named foo.js
:
const circle = require('./circle.js');
console.log(`The area of a circle of radius 4 is ${circle.area(4)}`);
On the first line, foo.js
loads the module circle.js
that is in the same
directory as foo.js
.
Here are the contents of circle.js
:
const { PI } = Math;
exports.area = (r) => PI * r ** 2;
exports.circumference = (r) => 2 * PI * r;
The module circle.js
has exported the functions area()
and
circumference()
. Functions and objects are added to the root of a module
by specifying additional properties on the special exports
object.
Variables local to the module will be private, because the module is wrapped
in a function by Node.js (see module wrapper).
In this example, the variable PI
is private to circle.js
.
The module.exports
property can be assigned a new value (such as a function
or object).
Below, bar.js
makes use of the square
module, which exports a Square class:
const Square = require('./square.js');
const mySquare = new Square(2);
console.log(`The area of mySquare is ${mySquare.area()}`);
The square
module is defined in square.js
:
// Assigning to exports will not modify module, must use module.exports
module.exports = class Square {
constructor(width) {
this.width = width;
}
area() {
return this.width ** 2;
}
};
The CommonJS module system is implemented in the module
core module.
Enabling#
Node.js has two module systems: CommonJS modules and ECMAScript modules.
By default, Node.js will treat the following as CommonJS modules:
-
Files with a
.cjs
extension; -
Files with a
.js
extension when the nearest parentpackage.json
file contains a top-level field"type"
with a value of"commonjs"
. -
Files with a
.js
extension when the nearest parentpackage.json
file doesn't contain a top-level field"type"
. Package authors should include the"type"
field, even in packages where all sources are CommonJS. Being explicit about thetype
of the package will make things easier for build tools and loaders to determine how the files in the package should be interpreted. -
Files with an extension that is not
.mjs
,.cjs
,.json
,.node
, or.js
(when the nearest parentpackage.json
file contains a top-level field"type"
with a value of"module"
, those files will be recognized as CommonJS modules only if they are being included viarequire()
, not when used as the command-line entry point of the program).
See Determining module system for more details.
Calling require()
always use the CommonJS module loader. Calling import()
always use the ECMAScript module loader.
Accessing the main module#
When a file is run directly from Node.js, require.main
is set to its
module
. That means that it is possible to determine whether a file has been
run directly by testing require.main === module
.
For a file foo.js
, this will be true
if run via node foo.js
, but
false
if run by require('./foo')
.
When the entry point is not a CommonJS module, require.main
is undefined
,
and the main module is out of reach.
Package manager tips#
The semantics of the Node.js require()
function were designed to be general
enough to support reasonable directory structures. Package manager programs
such as dpkg
, rpm
, and npm
will hopefully find it possible to build
native packages from Node.js modules without modification.
Below we give a suggested directory structure that could work:
Let's say that we wanted to have the folder at
/usr/lib/node/<some-package>/<some-version>
hold the contents of a
specific version of a package.
Packages can depend on one another. In order to install package foo
, it
may be necessary to install a specific version of package bar
. The bar
package may itself have dependencies, and in some cases, these may even collide
or form cyclic dependencies.
Because Node.js looks up the realpath
of any modules it loads (that is, it
resolves symlinks) and then looks for their dependencies in node_modules
folders,
this situation can be resolved with the following architecture:
/usr/lib/node/foo/1.2.3/
: Contents of thefoo
package, version 1.2.3./usr/lib/node/bar/4.3.2/
: Contents of thebar
package thatfoo
depends on./usr/lib/node/foo/1.2.3/node_modules/bar
: Symbolic link to/usr/lib/node/bar/4.3.2/
./usr/lib/node/bar/4.3.2/node_modules/*
: Symbolic links to the packages thatbar
depends on.
Thus, even if a cycle is encountered, or if there are dependency conflicts, every module will be able to get a version of its dependency that it can use.
When the code in the foo
package does require('bar')
, it will get the
version that is symlinked into /usr/lib/node/foo/1.2.3/node_modules/bar
.
Then, when the code in the bar
package calls require('quux')
, it'll get
the version that is symlinked into
/usr/lib/node/bar/4.3.2/node_modules/quux
.
Furthermore, to make the module lookup process even more optimal, rather
than putting packages directly in /usr/lib/node
, we could put them in
/usr/lib/node_modules/<name>/<version>
. Then Node.js will not bother
looking for missing dependencies in /usr/node_modules
or /node_modules
.
In order to make modules available to the Node.js REPL, it might be useful to
also add the /usr/lib/node_modules
folder to the $NODE_PATH
environment
variable. Since the module lookups using node_modules
folders are all
relative, and based on the real path of the files making the calls to
require()
, the packages themselves can be anywhere.
The .mjs
extension#
Due to the synchronous nature of require()
, it is not possible to use it to
load ECMAScript module files. Attempting to do so will throw a
ERR_REQUIRE_ESM
error. Use import()
instead.
The .mjs
extension is reserved for ECMAScript Modules which cannot be
loaded via require()
. See Determining module system section for more info
regarding which files are parsed as ECMAScript modules.
All together#
To get the exact filename that will be loaded when require()
is called, use
the require.resolve()
function.
Putting together all of the above, here is the high-level algorithm
in pseudocode of what require()
does:
require(X) from module at path Y 1. If X is a core module, a. return the core module b. STOP 2. If X begins with '/' a. set Y to be the file system root 3. If X begins with './' or '/' or '../' a. LOAD_AS_FILE(Y + X) b. LOAD_AS_DIRECTORY(Y + X) c. THROW "not found" 4. If X begins with '#' a. LOAD_PACKAGE_IMPORTS(X, dirname(Y)) 5. LOAD_PACKAGE_SELF(X, dirname(Y)) 6. LOAD_NODE_MODULES(X, dirname(Y)) 7. THROW "not found" LOAD_AS_FILE(X) 1. If X is a file, load X as its file extension format. STOP 2. If X.js is a file, load X.js as JavaScript text. STOP 3. If X.json is a file, parse X.json to a JavaScript Object. STOP 4. If X.node is a file, load X.node as binary addon. STOP LOAD_INDEX(X) 1. If X/index.js is a file, load X/index.js as JavaScript text. STOP 2. If X/index.json is a file, parse X/index.json to a JavaScript object. STOP 3. If X/index.node is a file, load X/index.node as binary addon. STOP LOAD_AS_DIRECTORY(X) 1. If X/package.json is a file, a. Parse X/package.json, and look for "main" field. b. If "main" is a falsy value, GOTO 2. c. let M = X + (json main field) d. LOAD_AS_FILE(M) e. LOAD_INDEX(M) f. LOAD_INDEX(X) DEPRECATED g. THROW "not found" 2. LOAD_INDEX(X) LOAD_NODE_MODULES(X, START) 1. let DIRS = NODE_MODULES_PATHS(START) 2. for each DIR in DIRS: a. LOAD_PACKAGE_EXPORTS(X, DIR) b. LOAD_AS_FILE(DIR/X) c. LOAD_AS_DIRECTORY(DIR/X) NODE_MODULES_PATHS(START) 1. let PARTS = path split(START) 2. let I = count of PARTS - 1 3. let DIRS = [] 4. while I >= 0, a. if PARTS[I] = "node_modules" CONTINUE b. DIR = path join(PARTS[0 .. I] + "node_modules") c. DIRS = DIR + DIRS d. let I = I - 1 5. return DIRS + GLOBAL_FOLDERS LOAD_PACKAGE_IMPORTS(X, DIR) 1. Find the closest package scope SCOPE to DIR. 2. If no scope was found, return. 3. If the SCOPE/package.json "imports" is null or undefined, return. 4. let MATCH = PACKAGE_IMPORTS_RESOLVE(X, pathToFileURL(SCOPE), ["node", "require"]) defined in the ESM resolver. 5. RESOLVE_ESM_MATCH(MATCH). LOAD_PACKAGE_EXPORTS(X, DIR) 1. Try to interpret X as a combination of NAME and SUBPATH where the name may have a @scope/ prefix and the subpath begins with a slash (`/`). 2. If X does not match this pattern or DIR/NAME/package.json is not a file, return. 3. Parse DIR/NAME/package.json, and look for "exports" field. 4. If "exports" is null or undefined, return. 5. let MATCH = PACKAGE_EXPORTS_RESOLVE(pathToFileURL(DIR/NAME), "." + SUBPATH, `package.json` "exports", ["node", "require"]) defined in the ESM resolver. 6. RESOLVE_ESM_MATCH(MATCH) LOAD_PACKAGE_SELF(X, DIR) 1. Find the closest package scope SCOPE to DIR. 2. If no scope was found, return. 3. If the SCOPE/package.json "exports" is null or undefined, return. 4. If the SCOPE/package.json "name" is not the first segment of X, return. 5. let MATCH = PACKAGE_EXPORTS_RESOLVE(pathToFileURL(SCOPE), "." + X.slice("name".length), `package.json` "exports", ["node", "require"]) defined in the ESM resolver. 6. RESOLVE_ESM_MATCH(MATCH) RESOLVE_ESM_MATCH(MATCH) 1. let RESOLVED_PATH = fileURLToPath(MATCH) 2. If the file at RESOLVED_PATH exists, load RESOLVED_PATH as its extension format. STOP 3. THROW "not found"
Caching#
Modules are cached after the first time they are loaded. This means (among other
things) that every call to require('foo')
will get exactly the same object
returned, if it would resolve to the same file.
Provided require.cache
is not modified, multiple calls to require('foo')
will not cause the module code to be executed multiple times. This is an
important feature. With it, "partially done" objects can be returned, thus
allowing transitive dependencies to be loaded even when they would cause cycles.
To have a module execute code multiple times, export a function, and call that function.
Module caching caveats#
Modules are cached based on their resolved filename. Since modules may resolve
to a different filename based on the location of the calling module (loading
from node_modules
folders), it is not a guarantee that require('foo')
will
always return the exact same object, if it would resolve to different files.
Additionally, on case-insensitive file systems or operating systems, different
resolved filenames can point to the same file, but the cache will still treat
them as different modules and will reload the file multiple times. For example,
require('./foo')
and require('./FOO')
return two different objects,
irrespective of whether or not ./foo
and ./FOO
are the same file.
Core modules#
Node.js has several modules compiled into the binary. These modules are described in greater detail elsewhere in this documentation.
The core modules are defined within the Node.js source and are located in the
lib/
folder.
Core modules can be identified using the node:
prefix, in which case
it bypasses the require
cache. For instance, require('node:http')
will
always return the built in HTTP module, even if there is require.cache
entry
by that name.
Some core modules are always preferentially loaded if their identifier is
passed to require()
. For instance, require('http')
will always
return the built-in HTTP module, even if there is a file by that name. The list
of core modules that can be loaded without using the node:
prefix is exposed
as module.builtinModules
.
Cycles#
When there are circular require()
calls, a module might not have finished
executing when it is returned.
Consider this situation:
a.js
:
console.log('a starting');
exports.done = false;
const b = require('./b.js');
console.log('in a, b.done = %j', b.done);
exports.done = true;
console.log('a done');
b.js
:
console.log('b starting');
exports.done = false;
const a = require('./a.js');
console.log('in b, a.done = %j', a.done);
exports.done = true;
console.log('b done');
main.js
:
console.log('main starting');
const a = require('./a.js');
const b = require('./b.js');
console.log('in main, a.done = %j, b.done = %j', a.done, b.done);
When main.js
loads a.js
, then a.js
in turn loads b.js
. At that
point, b.js
tries to load a.js
. In order to prevent an infinite
loop, an unfinished copy of the a.js
exports object is returned to the
b.js
module. b.js
then finishes loading, and its exports
object is
provided to the a.js
module.
By the time main.js
has loaded both modules, they're both finished.
The output of this program would thus be:
$ node main.js
main starting
a starting
b starting
in b, a.done = false
b done
in a, b.done = true
a done
in main, a.done = true, b.done = true
Careful planning is required to allow cyclic module dependencies to work correctly within an application.
File modules#
If the exact filename is not found, then Node.js will attempt to load the
required filename with the added extensions: .js
, .json
, and finally
.node
. When loading a file that has a different extension (e.g. .cjs
), its
full name must be passed to require()
, including its file extension (e.g.
require('./file.cjs')
).
.json
files are parsed as JSON text files, .node
files are interpreted as
compiled addon modules loaded with process.dlopen()
. Files using any other
extension (or no extension at all) are parsed as JavaScript text files. Refer to
the Determining module system section to understand what parse goal will be
used.
A required module prefixed with '/'
is an absolute path to the file. For
example, require('/home/marco/foo.js')
will load the file at
/home/marco/foo.js
.
A required module prefixed with './'
is relative to the file calling
require()
. That is, circle.js
must be in the same directory as foo.js
for
require('./circle')
to find it.
Without a leading '/'
, './'
, or '../'
to indicate a file, the module must
either be a core module or is loaded from a node_modules
folder.
If the given path does not exist, require()
will throw a
MODULE_NOT_FOUND
error.
Folders as modules#
There are three ways in which a folder may be passed to require()
as
an argument.
The first is to create a package.json
file in the root of the folder,
which specifies a main
module. An example package.json
file might
look like this:
{ "name" : "some-library",
"main" : "./lib/some-library.js" }
If this was in a folder at ./some-library
, then
require('./some-library')
would attempt to load
./some-library/lib/some-library.js
.
If there is no package.json
file present in the directory, or if the
"main"
entry is missing or cannot be resolved, then Node.js
will attempt to load an index.js
or index.node
file out of that
directory. For example, if there was no package.json
file in the previous
example, then require('./some-library')
would attempt to load:
./some-library/index.js
./some-library/index.node
If these attempts fail, then Node.js will report the entire module as missing with the default error:
Error: Cannot find module 'some-library'
In all three above cases, an import('./some-library')
call would result in a
ERR_UNSUPPORTED_DIR_IMPORT
error. Using package subpath exports or
subpath imports can provide the same containment organization benefits as
folders as modules, and work for both require
and import
.
Loading from node_modules
folders#
If the module identifier passed to require()
is not a
core module, and does not begin with '/'
, '../'
, or
'./'
, then Node.js starts at the directory of the current module, and
adds /node_modules
, and attempts to load the module from that location.
Node.js will not append node_modules
to a path already ending in
node_modules
.
If it is not found there, then it moves to the parent directory, and so on, until the root of the file system is reached.
For example, if the file at '/home/ry/projects/foo.js'
called
require('bar.js')
, then Node.js would look in the following locations, in
this order:
/home/ry/projects/node_modules/bar.js
/home/ry/node_modules/bar.js
/home/node_modules/bar.js
/node_modules/bar.js
This allows programs to localize their dependencies, so that they do not clash.
It is possible to require specific files or sub modules distributed with a
module by including a path suffix after the module name. For instance
require('example-module/path/to/file')
would resolve path/to/file
relative to where example-module
is located. The suffixed path follows the
same module resolution semantics.
Loading from the global folders#
If the NODE_PATH
environment variable is set to a colon-delimited list
of absolute paths, then Node.js will search those paths for modules if they
are not found elsewhere.
On Windows, NODE_PATH
is delimited by semicolons (;
) instead of colons.
NODE_PATH
was originally created to support loading modules from
varying paths before the current module resolution algorithm was defined.
NODE_PATH
is still supported, but is less necessary now that the Node.js
ecosystem has settled on a convention for locating dependent modules.
Sometimes deployments that rely on NODE_PATH
show surprising behavior
when people are unaware that NODE_PATH
must be set. Sometimes a
module's dependencies change, causing a different version (or even a
different module) to be loaded as the NODE_PATH
is searched.
Additionally, Node.js will search in the following list of GLOBAL_FOLDERS:
- 1:
$HOME/.node_modules
- 2:
$HOME/.node_libraries
- 3:
$PREFIX/lib/node
Where $HOME
is the user's home directory, and $PREFIX
is the Node.js
configured node_prefix
.
These are mostly for historic reasons.
It is strongly encouraged to place dependencies in the local node_modules
folder. These will be loaded faster, and more reliably.
The module wrapper#
Before a module's code is executed, Node.js will wrap it with a function wrapper that looks like the following:
(function(exports, require, module, __filename, __dirname) {
// Module code actually lives in here
});
By doing this, Node.js achieves a few things:
- It keeps top-level variables (defined with
var
,const
, orlet
) scoped to the module rather than the global object. - It helps to provide some global-looking variables that are actually specific
to the module, such as:
- The
module
andexports
objects that the implementor can use to export values from the module. - The convenience variables
__filename
and__dirname
, containing the module's absolute filename and directory path.
- The
The module scope#
__dirname
#
The directory name of the current module. This is the same as the
path.dirname()
of the __filename
.
Example: running node example.js
from /Users/mjr
console.log(__dirname);
// Prints: /Users/mjr
console.log(path.dirname(__filename));
// Prints: /Users/mjr
__filename
#
The file name of the current module. This is the current module file's absolute path with symlinks resolved.
For a main program this is not necessarily the same as the file name used in the command line.
See __dirname
for the directory name of the current module.
Examples:
Running node example.js
from /Users/mjr
console.log(__filename);
// Prints: /Users/mjr/example.js
console.log(__dirname);
// Prints: /Users/mjr
Given two modules: a
and b
, where b
is a dependency of
a
and there is a directory structure of:
/Users/mjr/app/a.js
/Users/mjr/app/node_modules/b/b.js
References to __filename
within b.js
will return
/Users/mjr/app/node_modules/b/b.js
while references to __filename
within
a.js
will return /Users/mjr/app/a.js
.
exports
#
A reference to the module.exports
that is shorter to type.
See the section about the exports shortcut for details on when to use
exports
and when to use module.exports
.
module
#
A reference to the current module, see the section about the
module
object. In particular, module.exports
is used for defining what
a module exports and makes available through require()
.
require(id)
#
Used to import modules, JSON
, and local files. Modules can be imported
from node_modules
. Local modules and JSON files can be imported using
a relative path (e.g. ./
, ./foo
, ./bar/baz
, ../foo
) that will be
resolved against the directory named by __dirname
(if defined) or
the current working directory. The relative paths of POSIX style are resolved
in an OS independent fashion, meaning that the examples above will work on
Windows in the same way they would on Unix systems.
// Importing a local module with a path relative to the `__dirname` or current
// working directory. (On Windows, this would resolve to .\path\myLocalModule.)
const myLocalModule = require('./path/myLocalModule');
// Importing a JSON file:
const jsonData = require('./path/filename.json');
// Importing a module from node_modules or Node.js built-in module:
const crypto = require('node:crypto');
require.cache
#
Modules are cached in this object when they are required. By deleting a key
value from this object, the next require
will reload the module.
This does not apply to native addons, for which reloading will result in an
error.
Adding or replacing entries is also possible. This cache is checked before
built-in modules and if a name matching a built-in module is added to the cache,
only node:
-prefixed require calls are going to receive the built-in module.
Use with care!
const assert = require('node:assert');
const realFs = require('node:fs');
const fakeFs = {};
require.cache.fs = { exports: fakeFs };
assert.strictEqual(require('fs'), fakeFs);
assert.strictEqual(require('node:fs'), realFs);
require.extensions
#
Instruct require
on how to handle certain file extensions.
Process files with the extension .sjs
as .js
:
require.extensions['.sjs'] = require.extensions['.js'];
Deprecated. In the past, this list has been used to load non-JavaScript modules into Node.js by compiling them on-demand. However, in practice, there are much better ways to do this, such as loading modules via some other Node.js program, or compiling them to JavaScript ahead of time.
Avoid using require.extensions
. Use could cause subtle bugs and resolving the
extensions gets slower with each registered extension.
require.main
#
The Module
object representing the entry script loaded when the Node.js
process launched, or undefined
if the entry point of the program is not a
CommonJS module.
See "Accessing the main module".
In entry.js
script:
console.log(require.main);
node entry.js
Module {
id: '.',
path: '/absolute/path/to',
exports: {},
filename: '/absolute/path/to/entry.js',
loaded: false,
children: [],
paths:
[ '/absolute/path/to/node_modules',
'/absolute/path/node_modules',
'/absolute/node_modules',
'/node_modules' ] }
require.resolve(request[, options])
#
request
<string> The module path to resolve.options
<Object>paths
<string[]> Paths to resolve module location from. If present, these paths are used instead of the default resolution paths, with the exception of GLOBAL_FOLDERS like$HOME/.node_modules
, which are always included. Each of these paths is used as a starting point for the module resolution algorithm, meaning that thenode_modules
hierarchy is checked from this location.
- Returns: <string>
Use the internal require()
machinery to look up the location of a module,
but rather than loading the module, just return the resolved filename.
If the module can not be found, a MODULE_NOT_FOUND
error is thrown.
require.resolve.paths(request)
#
request
<string> The module path whose lookup paths are being retrieved.- Returns: <string[]> | <null>
Returns an array containing the paths searched during resolution of request
or
null
if the request
string references a core module, for example http
or
fs
.
The module
object#
In each module, the module
free variable is a reference to the object
representing the current module. For convenience, module.exports
is
also accessible via the exports
module-global. module
is not actually
a global but rather local to each module.
module.children
#
The module objects required for the first time by this one.
module.exports
#
The module.exports
object is created by the Module
system. Sometimes this is
not acceptable; many want their module to be an instance of some class. To do
this, assign the desired export object to module.exports
. Assigning
the desired object to exports
will simply rebind the local exports
variable,
which is probably not what is desired.
For example, suppose we were making a module called a.js
:
const EventEmitter = require('node:events');
module.exports = new EventEmitter();
// Do some work, and after some time emit
// the 'ready' event from the module itself.
setTimeout(() => {
module.exports.emit('ready');
}, 1000);
Then in another file we could do:
const a = require('./a');
a.on('ready', () => {
console.log('module "a" is ready');
});
Assignment to module.exports
must be done immediately. It cannot be
done in any callbacks. This does not work:
x.js
:
setTimeout(() => {
module.exports = { a: 'hello' };
}, 0);
y.js
:
const x = require('./x');
console.log(x.a);
exports
shortcut#
The exports
variable is available within a module's file-level scope, and is
assigned the value of module.exports
before the module is evaluated.
It allows a shortcut, so that module.exports.f = ...
can be written more
succinctly as exports.f = ...
. However, be aware that like any variable, if a
new value is assigned to exports
, it is no longer bound to module.exports
:
module.exports.hello = true; // Exported from require of module
exports = { hello: false }; // Not exported, only available in the module
When the module.exports
property is being completely replaced by a new
object, it is common to also reassign exports
:
module.exports = exports = function Constructor() {
// ... etc.
};
To illustrate the behavior, imagine this hypothetical implementation of
require()
, which is quite similar to what is actually done by require()
:
function require(/* ... */) {
const module = { exports: {} };
((module, exports) => {
// Module code here. In this example, define a function.
function someFunc() {}
exports = someFunc;
// At this point, exports is no longer a shortcut to module.exports, and
// this module will still export an empty default object.
module.exports = someFunc;
// At this point, the module will now export someFunc, instead of the
// default object.
})(module, module.exports);
return module.exports;
}
module.filename
#
The fully resolved filename of the module.
module.id
#
The identifier for the module. Typically this is the fully resolved filename.
module.isPreloading
#
- Type: <boolean>
true
if the module is running during the Node.js preload phase.
module.loaded
#
Whether or not the module is done loading, or is in the process of loading.
module.parent
#
The module that first required this one, or null
if the current module is the
entry point of the current process, or undefined
if the module was loaded by
something that is not a CommonJS module (E.G.: REPL or import
).
module.path
#
The directory name of the module. This is usually the same as the
path.dirname()
of the module.id
.
module.paths
#
The search paths for the module.
module.require(id)
#
The module.require()
method provides a way to load a module as if
require()
was called from the original module.
In order to do this, it is necessary to get a reference to the module
object.
Since require()
returns the module.exports
, and the module
is typically
only available within a specific module's code, it must be explicitly exported
in order to be used.
The Module
object#
This section was moved to
Modules: module
core module.
Source map v3 support#
This section was moved to
Modules: module
core module.
Modules: ECMAScript modules#
Introduction#
ECMAScript modules are the official standard format to package JavaScript
code for reuse. Modules are defined using a variety of import
and
export
statements.
The following example of an ES module exports a function:
// addTwo.mjs
function addTwo(num) {
return num + 2;
}
export { addTwo };
The following example of an ES module imports the function from addTwo.mjs
:
// app.mjs
import { addTwo } from './addTwo.mjs';
// Prints: 6
console.log(addTwo(4));
Node.js fully supports ECMAScript modules as they are currently specified and provides interoperability between them and its original module format, CommonJS.
Enabling#
Node.js has two module systems: CommonJS modules and ECMAScript modules.
Authors can tell Node.js to use the ECMAScript modules loader
via the .mjs
file extension, the package.json
"type"
field, or the
--input-type
flag. Outside of those cases, Node.js will use the CommonJS
module loader. See Determining module system for more details.
Packages#
This section was moved to Modules: Packages.
import
Specifiers#
Terminology#
The specifier of an import
statement is the string after the from
keyword,
e.g. 'node:path'
in import { sep } from 'node:path'
. Specifiers are also
used in export from
statements, and as the argument to an import()
expression.
There are three types of specifiers:
-
Relative specifiers like
'./startup.js'
or'../config.mjs'
. They refer to a path relative to the location of the importing file. The file extension is always necessary for these. -
Bare specifiers like
'some-package'
or'some-package/shuffle'
. They can refer to the main entry point of a package by the package name, or a specific feature module within a package prefixed by the package name as per the examples respectively. Including the file extension is only necessary for packages without an"exports"
field. -
Absolute specifiers like
'file:///opt/nodejs/config.js'
. They refer directly and explicitly to a full path.
Bare specifier resolutions are handled by the Node.js module resolution algorithm. All other specifier resolutions are always only resolved with the standard relative URL resolution semantics.
Like in CommonJS, module files within packages can be accessed by appending a
path to the package name unless the package's package.json
contains an
"exports"
field, in which case files within packages can only be accessed
via the paths defined in "exports"
.
For details on these package resolution rules that apply to bare specifiers in the Node.js module resolution, see the packages documentation.
Mandatory file extensions#
A file extension must be provided when using the import
keyword to resolve
relative or absolute specifiers. Directory indexes (e.g. './startup/index.js'
)
must also be fully specified.
This behavior matches how import
behaves in browser environments, assuming a
typically configured server.
URLs#
ES modules are resolved and cached as URLs. This means that special characters
must be percent-encoded, such as #
with %23
and ?
with %3F
.
file:
, node:
, and data:
URL schemes are supported. A specifier like
'https://example.com/app.js'
is not supported natively in Node.js unless using
a custom HTTPS loader.
file:
URLs#
Modules are loaded multiple times if the import
specifier used to resolve
them has a different query or fragment.
import './foo.mjs?query=1'; // loads ./foo.mjs with query of "?query=1"
import './foo.mjs?query=2'; // loads ./foo.mjs with query of "?query=2"
The volume root may be referenced via /
, //
, or file:///
. Given the
differences between URL and path resolution (such as percent encoding
details), it is recommended to use url.pathToFileURL when importing a path.
data:
imports#
data:
URLs are supported for importing with the following MIME types:
text/javascript
for ES modulesapplication/json
for JSONapplication/wasm
for Wasm
import 'data:text/javascript,console.log("hello!");';
import _ from 'data:application/json,"world!"' assert { type: 'json' };
data:
URLs only resolve bare specifiers for builtin modules
and absolute specifiers. Resolving
relative specifiers does not work because data:
is not a
special scheme. For example, attempting to load ./foo
from data:text/javascript,import "./foo";
fails to resolve because there
is no concept of relative resolution for data:
URLs.
node:
imports#
node:
URLs are supported as an alternative means to load Node.js builtin
modules. This URL scheme allows for builtin modules to be referenced by valid
absolute URL strings.
import fs from 'node:fs/promises';
Import assertions#
The Import Assertions proposal adds an inline syntax for module import statements to pass on more information alongside the module specifier.
import fooData from './foo.json' assert { type: 'json' };
const { default: barData } =
await import('./bar.json', { assert: { type: 'json' } });
Node.js supports the following type
values, for which the assertion is
mandatory:
Assertion type | Needed for |
---|---|
'json' | JSON modules |
Builtin modules#
Core modules provide named exports of their public API. A
default export is also provided which is the value of the CommonJS exports.
The default export can be used for, among other things, modifying the named
exports. Named exports of builtin modules are updated only by calling
module.syncBuiltinESMExports()
.
import EventEmitter from 'node:events';
const e = new EventEmitter();
import { readFile } from 'node:fs';
readFile('./foo.txt', (err, source) => {
if (err) {
console.error(err);
} else {
console.log(source);
}
});
import fs, { readFileSync } from 'node:fs';
import { syncBuiltinESMExports } from 'node:module';
import { Buffer } from 'node:buffer';
fs.readFileSync = () => Buffer.from('Hello, ESM');
syncBuiltinESMExports();
fs.readFileSync === readFileSync;
import()
expressions#
Dynamic import()
is supported in both CommonJS and ES modules. In CommonJS
modules it can be used to load ES modules.
import.meta
#
The import.meta
meta property is an Object
that contains the following
properties.
import.meta.url
#
- <string> The absolute
file:
URL of the module.
This is defined exactly the same as it is in browsers providing the URL of the current module file.
This enables useful patterns such as relative file loading:
import { readFileSync } from 'node:fs';
const buffer = readFileSync(new URL('./data.proto', import.meta.url));
import.meta.resolve(specifier[, parent])
#
This feature is only available with the --experimental-import-meta-resolve
command flag enabled.
specifier
<string> The module specifier to resolve relative toparent
.parent
<string> | <URL> The absolute parent module URL to resolve from. If none is specified, the value ofimport.meta.url
is used as the default.- Returns: <Promise>
Provides a module-relative resolution function scoped to each module, returning the URL string.
const dependencyAsset = await import.meta.resolve('component-lib/asset.css');
import.meta.resolve
also accepts a second argument which is the parent module
from which to resolve from:
await import.meta.resolve('./dep', import.meta.url);
This function is asynchronous because the ES module resolver in Node.js is allowed to be asynchronous.
Interoperability with CommonJS#
import
statements#
An import
statement can reference an ES module or a CommonJS module.
import
statements are permitted only in ES modules, but dynamic import()
expressions are supported in CommonJS for loading ES modules.
When importing CommonJS modules, the
module.exports
object is provided as the default export. Named exports may be
available, provided by static analysis as a convenience for better ecosystem
compatibility.
require
#
The CommonJS module require
always treats the files it references as CommonJS.
Using require
to load an ES module is not supported because ES modules have
asynchronous execution. Instead, use import()
to load an ES module
from a CommonJS module.
CommonJS Namespaces#
CommonJS modules consist of a module.exports
object which can be of any type.
When importing a CommonJS module, it can be reliably imported using the ES module default import or its corresponding sugar syntax:
import { default as cjs } from 'cjs';
// The following import statement is "syntax sugar" (equivalent but sweeter)
// for `{ default as cjsSugar }` in the above import statement:
import cjsSugar from 'cjs';
console.log(cjs);
console.log(cjs === cjsSugar);
// Prints:
// <module.exports>
// true
The ECMAScript Module Namespace representation of a CommonJS module is always
a namespace with a default
export key pointing to the CommonJS
module.exports
value.
This Module Namespace Exotic Object can be directly observed either when using
import * as m from 'cjs'
or a dynamic import:
import * as m from 'cjs';
console.log(m);
console.log(m === await import('cjs'));
// Prints:
// [Module] { default: <module.exports> }
// true
For better compatibility with existing usage in the JS ecosystem, Node.js in addition attempts to determine the CommonJS named exports of every imported CommonJS module to provide them as separate ES module exports using a static analysis process.
For example, consider a CommonJS module written:
// cjs.cjs
exports.name = 'exported';
The preceding module supports named imports in ES modules:
import { name } from './cjs.cjs';
console.log(name);
// Prints: 'exported'
import cjs from './cjs.cjs';
console.log(cjs);
// Prints: { name: 'exported' }
import * as m from './cjs.cjs';
console.log(m);
// Prints: [Module] { default: { name: 'exported' }, name: 'exported' }
As can be seen from the last example of the Module Namespace Exotic Object being
logged, the name
export is copied off of the module.exports
object and set
directly on the ES module namespace when the module is imported.
Live binding updates or new exports added to module.exports
are not detected
for these named exports.
The detection of named exports is based on common syntax patterns but does not always correctly detect named exports. In these cases, using the default import form described above can be a better option.
Named exports detection covers many common export patterns, reexport patterns and build tool and transpiler outputs. See cjs-module-lexer for the exact semantics implemented.
Differences between ES modules and CommonJS#
No require
, exports
, or module.exports
#
In most cases, the ES module import
can be used to load CommonJS modules.
If needed, a require
function can be constructed within an ES module using
module.createRequire()
.
No __filename
or __dirname
#
These CommonJS variables are not available in ES modules.
__filename
and __dirname
use cases can be replicated via
import.meta.url
.
No Addon Loading#
Addons are not currently supported with ES module imports.
They can instead be loaded with module.createRequire()
or
process.dlopen
.
No require.resolve
#
Relative resolution can be handled via new URL('./local', import.meta.url)
.
For a complete require.resolve
replacement, there is a flagged experimental
import.meta.resolve
API.
Alternatively module.createRequire()
can be used.
No NODE_PATH
#
NODE_PATH
is not part of resolving import
specifiers. Please use symlinks
if this behavior is desired.
No require.extensions
#
require.extensions
is not used by import
. The expectation is that loader
hooks can provide this workflow in the future.
No require.cache
#
require.cache
is not used by import
as the ES module loader has its own
separate cache.
JSON modules#
JSON files can be referenced by import
:
import packageConfig from './package.json' assert { type: 'json' };
The assert { type: 'json' }
syntax is mandatory; see Import Assertions.
The imported JSON only exposes a default
export. There is no support for named
exports. A cache entry is created in the CommonJS cache to avoid duplication.
The same object is returned in CommonJS if the JSON module has already been
imported from the same path.
Wasm modules#
Importing WebAssembly modules is supported under the
--experimental-wasm-modules
flag, allowing any .wasm
files to be
imported as normal modules while also supporting their module imports.
This integration is in line with the ES Module Integration Proposal for WebAssembly.
For example, an index.mjs
containing:
import * as M from './module.wasm';
console.log(M);
executed under:
node --experimental-wasm-modules index.mjs
would provide the exports interface for the instantiation of module.wasm
.
Top-level await
#
The await
keyword may be used in the top level body of an ECMAScript module.
Assuming an a.mjs
with
export const five = await Promise.resolve(5);
And a b.mjs
with
import { five } from './a.mjs';
console.log(five); // Logs `5`
node b.mjs # works
If a top level await
expression never resolves, the node
process will exit
with a 13
status code.
import { spawn } from 'node:child_process';
import { execPath } from 'node:process';
spawn(execPath, [
'--input-type=module',
'--eval',
// Never-resolving Promise:
'await new Promise(() => {})',
]).once('exit', (code) => {
console.log(code); // Logs `13`
});
HTTPS and HTTP imports#
Importing network based modules using https:
and http:
is supported under
the --experimental-network-imports
flag. This allows web browser-like imports
to work in Node.js with a few differences due to application stability and
security concerns that are different when running in a privileged environment
instead of a browser sandbox.
Imports are limited to HTTP/1#
Automatic protocol negotiation for HTTP/2 and HTTP/3 is not yet supported.
HTTP is limited to loopback addresses#
http:
is vulnerable to man-in-the-middle attacks and is not allowed to be
used for addresses outside of the IPv4 address 127.0.0.0/8
(127.0.0.1
to
127.255.255.255
) and the IPv6 address ::1
. Support for http:
is intended
to be used for local development.
Authentication is never sent to the destination server.#
Authorization
, Cookie
, and Proxy-Authorization
headers are not sent to the
server. Avoid including user info in parts of imported URLs. A security model
for safely using these on the server is being worked on.
CORS is never checked on the destination server#
CORS is designed to allow a server to limit the consumers of an API to a specific set of hosts. This is not supported as it does not make sense for a server-based implementation.
Cannot load non-network dependencies#
These modules cannot access other modules that are not over http:
or https:
.
To still access local modules while avoiding the security concern, pass in
references to the local dependencies:
// file.mjs
import worker_threads from 'node:worker_threads';
import { configure, resize } from 'https://example.com/imagelib.mjs';
configure({ worker_threads });
// https://example.com/imagelib.mjs
let worker_threads;
export function configure(opts) {
worker_threads = opts.worker_threads;
}
export function resize(img, size) {
// Perform resizing in worker_thread to avoid main thread blocking
}
Network-based loading is not enabled by default#
For now, the --experimental-network-imports
flag is required to enable loading
resources over http:
or https:
. In the future, a different mechanism will be
used to enforce this. Opt-in is required to prevent transitive dependencies
inadvertently using potentially mutable state that could affect reliability
of Node.js applications.
Loaders#
This API is currently being redesigned and will still change.
To customize the default module resolution, loader hooks can optionally be
provided via a --experimental-loader ./loader-name.mjs
argument to Node.js.
When hooks are used they apply to each subsequent loader, the entry point, and
all import
calls. They won't apply to require
calls; those still follow
CommonJS rules.
Loaders follow the pattern of --require
:
node \
--experimental-loader unpkg \
--experimental-loader http-to-https \
--experimental-loader cache-buster
These are called in the following sequence: cache-buster
calls
http-to-https
which calls unpkg
.
Hooks#
Hooks are part of a chain, even if that chain consists of only one custom
(user-provided) hook and the default hook, which is always present. Hook
functions nest: each one must always return a plain object, and chaining happens
as a result of each function calling next<hookName>()
, which is a reference
to the subsequent loader's hook.
A hook that returns a value lacking a required property triggers an exception.
A hook that returns without calling next<hookName>()
and without returning
shortCircuit: true
also triggers an exception. These errors are to help
prevent unintentional breaks in the chain.
resolve(specifier, context, nextResolve)
#
The loaders API is being redesigned. This hook may disappear or its signature may change. Do not rely on the API described below.
specifier
<string>context
<Object>conditions
<string[]> Export conditions of the relevantpackage.json
importAssertions
<Object> An object whose key-value pairs represent the assertions for the module to importparentURL
<string> | <undefined> The module importing this one, or undefined if this is the Node.js entry point
nextResolve
<Function> The subsequentresolve
hook in the chain, or the Node.js defaultresolve
hook after the last user-suppliedresolve
hook- Returns: <Object>
format
<string> | <null> | <undefined> A hint to the load hook (it might be ignored)'builtin' | 'commonjs' | 'json' | 'module' | 'wasm'
importAssertions
<Object> | <undefined> The import assertions to use when caching the module (optional; if excluded the input will be used)shortCircuit
<undefined> | <boolean> A signal that this hook intends to terminate the chain ofresolve
hooks. Default:false
url
<string> The absolute URL to which this input resolves
The resolve
hook chain is responsible for telling Node.js where to find and
how to cache a given import
statement or expression. It can optionally return
its format (such as 'module'
) as a hint to the load
hook. If a format is
specified, the load
hook is ultimately responsible for providing the final
format
value (and it is free to ignore the hint provided by resolve
); if
resolve
provides a format
, a custom load
hook is required even if only to
pass the value to the Node.js default load
hook.
Import type assertions are part of the cache key for saving loaded modules into
the internal module cache. The resolve
hook is responsible for
returning an importAssertions
object if the module should be cached with
different assertions than were present in the source code.
The conditions
property in context
is an array of conditions for
package exports conditions that apply to this resolution
request. They can be used for looking up conditional mappings elsewhere or to
modify the list when calling the default resolution logic.
The current package exports conditions are always in
the context.conditions
array passed into the hook. To guarantee default
Node.js module specifier resolution behavior when calling defaultResolve
, the
context.conditions
array passed to it must include all elements of the
context.conditions
array originally passed into the resolve
hook.
export async function resolve(specifier, context, nextResolve) {
const { parentURL = null } = context;
if (Math.random() > 0.5) { // Some condition.
// For some or all specifiers, do some custom logic for resolving.
// Always return an object of the form {url: <string>}.
return {
shortCircuit: true,
url: parentURL ?
new URL(specifier, parentURL).href :
new URL(specifier).href,
};
}
if (Math.random() < 0.5) { // Another condition.
// When calling `defaultResolve`, the arguments can be modified. In this
// case it's adding another value for matching conditional exports.
return nextResolve(specifier, {
...context,
conditions: [...context.conditions, 'another-condition'],
});
}
// Defer to the next hook in the chain, which would be the
// Node.js default resolve if this is the last user-specified loader.
return nextResolve(specifier);
}
load(url, context, nextLoad)
#
The loaders API is being redesigned. This hook may disappear or its signature may change. Do not rely on the API described below.
In a previous version of this API, this was split across 3 separate, now deprecated, hooks (
getFormat
,getSource
, andtransformSource
).
url
<string> The URL returned by theresolve
chaincontext
<Object>conditions
<string[]> Export conditions of the relevantpackage.json
format
<string> | <null> | <undefined> The format optionally supplied by theresolve
hook chainimportAssertions
<Object>
nextLoad
<Function> The subsequentload
hook in the chain, or the Node.js defaultload
hook after the last user-suppliedload
hook- Returns: <Object>
format
<string>shortCircuit
<undefined> | <boolean> A signal that this hook intends to terminate the chain ofresolve
hooks. Default:false
source
<string> | <ArrayBuffer> | <TypedArray> The source for Node.js to evaluate
The load
hook provides a way to define a custom method of determining how
a URL should be interpreted, retrieved, and parsed. It is also in charge of
validating the import assertion.
The final value of format
must be one of the following:
format | Description | Acceptable types for source returned by load |
---|---|---|
'builtin' | Load a Node.js builtin module | Not applicable |
'commonjs' | Load a Node.js CommonJS module | Not applicable |
'json' | Load a JSON file | { string , ArrayBuffer , TypedArray } |
'module' | Load an ES module | { string , ArrayBuffer , TypedArray } |
'wasm' | Load a WebAssembly module | { ArrayBuffer , TypedArray } |
The value of source
is ignored for type 'builtin'
because currently it is
not possible to replace the value of a Node.js builtin (core) module. The value
of source
is ignored for type 'commonjs'
because the CommonJS module loader
does not provide a mechanism for the ES module loader to override the
CommonJS module return value. This limitation might be
overcome in the future.
Caveat: The ESM
load
hook and namespaced exports from CommonJS modules are incompatible. Attempting to use them together will result in an empty object from the import. This may be addressed in the future.
These types all correspond to classes defined in ECMAScript.
- The specific
ArrayBuffer
object is aSharedArrayBuffer
. - The specific
TypedArray
object is aUint8Array
.
If the source value of a text-based format (i.e., 'json'
, 'module'
)
is not a string, it is converted to a string using util.TextDecoder
.
The load
hook provides a way to define a custom method for retrieving the
source code of an ES module specifier. This would allow a loader to potentially
avoid reading files from disk. It could also be used to map an unrecognized
format to a supported one, for example yaml
to module
.
export async function load(url, context, nextLoad) {
const { format } = context;
if (Math.random() > 0.5) { // Some condition
/*
For some or all URLs, do some custom logic for retrieving the source.
Always return an object of the form {
format: <string>,
source: <string|buffer>,
}.
*/
return {
format,
shortCircuit: true,
source: '...',
};
}
// Defer to the next hook in the chain.
return nextLoad(url);
}
In a more advanced scenario, this can also be used to transform an unsupported source to a supported one (see Examples below).
globalPreload()
#
The loaders API is being redesigned. This hook may disappear or its signature may change. Do not rely on the API described below.
In a previous version of this API, this hook was named
getGlobalPreloadCode
.
context
<Object> Information to assist the preload codeport
<MessagePort>
- Returns: <string> Code to run before application startup
Sometimes it might be necessary to run some code inside of the same global scope that the application runs in. This hook allows the return of a string that is run as a sloppy-mode script on startup.
Similar to how CommonJS wrappers work, the code runs in an implicit function
scope. The only argument is a require
-like function that can be used to load
builtins like "fs": getBuiltin(request: string)
.
If the code needs more advanced require
features, it has to construct
its own require
using module.createRequire()
.
export function globalPreload(context) {
return `\
globalThis.someInjectedProperty = 42;
console.log('I just set some globals!');
const { createRequire } = getBuiltin('module');
const { cwd } = getBuiltin('process');
const require = createRequire(cwd() + '/<preload>');
// [...]
`;
}
In order to allow communication between the application and the loader, another
argument is provided to the preload code: port
. This is available as a
parameter to the loader hook and inside of the source text returned by the hook.
Some care must be taken in order to properly call port.ref()
and
port.unref()
to prevent a process from being in a state where it won't
close normally.
/**
* This example has the application context send a message to the loader
* and sends the message back to the application context
*/
export function globalPreload({ port }) {
port.onmessage = (evt) => {
port.postMessage(evt.data);
};
return `\
port.postMessage('console.log("I went to the Loader and back");');
port.onmessage = (evt) => {
eval(evt.data);
};
`;
}
Examples#
The various loader hooks can be used together to accomplish wide-ranging customizations of the Node.js code loading and evaluation behaviors.
HTTPS loader#
In current Node.js, specifiers starting with https://
are experimental (see
HTTPS and HTTP imports).
The loader below registers hooks to enable rudimentary support for such specifiers. While this may seem like a significant improvement to Node.js core functionality, there are substantial downsides to actually using this loader: performance is much slower than loading files from disk, there is no caching, and there is no security.
// https-loader.mjs
import { get } from 'node:https';
export function resolve(specifier, context, nextResolve) {
const { parentURL = null } = context;
// Normally Node.js would error on specifiers starting with 'https://', so
// this hook intercepts them and converts them into absolute URLs to be
// passed along to the later hooks below.
if (specifier.startsWith('https://')) {
return {
shortCircuit: true,
url: specifier,
};
} else if (parentURL && parentURL.startsWith('https://')) {
return {
shortCircuit: true,
url: new URL(specifier, parentURL).href,
};
}
// Let Node.js handle all other specifiers.
return nextResolve(specifier);
}
export function load(url, context, nextLoad) {
// For JavaScript to be loaded over the network, we need to fetch and
// return it.
if (url.startsWith('https://')) {
return new Promise((resolve, reject) => {
get(url, (res) => {
let data = '';
res.on('data', (chunk) => data += chunk);
res.on('end', () => resolve({
// This example assumes all network-provided JavaScript is ES module
// code.
format: 'module',
shortCircuit: true,
source: data,
}));
}).on('error', (err) => reject(err));
});
}
// Let Node.js handle all other URLs.
return nextLoad(url);
}
// main.mjs
import { VERSION } from 'https://coffeescript.org/browser-compiler-modern/coffeescript.js';
console.log(VERSION);
With the preceding loader, running
node --experimental-loader ./https-loader.mjs ./main.mjs
prints the current version of CoffeeScript per the module at the URL in
main.mjs
.
Transpiler loader#
Sources that are in formats Node.js doesn't understand can be converted into
JavaScript using the load
hook. Before that hook gets called,
however, a resolve
hook needs to tell Node.js not to
throw an error on unknown file types.
This is less performant than transpiling source files before running Node.js; a transpiler loader should only be used for development and testing purposes.
// coffeescript-loader.mjs
import { readFile } from 'node:fs/promises';
import { dirname, extname, resolve as resolvePath } from 'node:path';
import { cwd } from 'node:process';
import { fileURLToPath, pathToFileURL } from 'node:url';
import CoffeeScript from 'coffeescript';
const baseURL = pathToFileURL(`${cwd()}/`).href;
// CoffeeScript files end in .coffee, .litcoffee, or .coffee.md.
const extensionsRegex = /\.coffee$|\.litcoffee$|\.coffee\.md$/;
export async function resolve(specifier, context, nextResolve) {
if (extensionsRegex.test(specifier)) {
const { parentURL = baseURL } = context;
// Node.js normally errors on unknown file extensions, so return a URL for
// specifiers ending in the CoffeeScript file extensions.
return {
shortCircuit: true,
url: new URL(specifier, parentURL).href,
};
}
// Let Node.js handle all other specifiers.
return nextResolve(specifier);
}
export async function load(url, context, nextLoad) {
if (extensionsRegex.test(url)) {
// Now that we patched resolve to let CoffeeScript URLs through, we need to
// tell Node.js what format such URLs should be interpreted as. Because
// CoffeeScript transpiles into JavaScript, it should be one of the two
// JavaScript formats: 'commonjs' or 'module'.
// CoffeeScript files can be either CommonJS or ES modules, so we want any
// CoffeeScript file to be treated by Node.js the same as a .js file at the
// same location. To determine how Node.js would interpret an arbitrary .js
// file, search up the file system for the nearest parent package.json file
// and read its "type" field.
const format = await getPackageType(url);
// When a hook returns a format of 'commonjs', `source` is ignored.
// To handle CommonJS files, a handler needs to be registered with
// `require.extensions` in order to process the files with the CommonJS
// loader. Avoiding the need for a separate CommonJS handler is a future
// enhancement planned for ES module loaders.
if (format === 'commonjs') {
return {
format,
shortCircuit: true,
};
}
const { source: rawSource } = await nextLoad(url, { ...context, format });
// This hook converts CoffeeScript source code into JavaScript source code
// for all imported CoffeeScript files.
const transformedSource = coffeeCompile(rawSource.toString(), url);
return {
format,
shortCircuit: true,
source: transformedSource,
};
}
// Let Node.js handle all other URLs.
return nextLoad(url);
}
async function getPackageType(url) {
// `url` is only a file path during the first iteration when passed the
// resolved url from the load() hook
// an actual file path from load() will contain a file extension as it's
// required by the spec
// this simple truthy check for whether `url` contains a file extension will
// work for most projects but does not cover some edge-cases (such as
// extensionless files or a url ending in a trailing space)
const isFilePath = !!extname(url);
// If it is a file path, get the directory it's in
const dir = isFilePath ?
dirname(fileURLToPath(url)) :
url;
// Compose a file path to a package.json in the same directory,
// which may or may not exist
const packagePath = resolvePath(dir, 'package.json');
// Try to read the possibly nonexistent package.json
const type = await readFile(packagePath, { encoding: 'utf8' })
.then((filestring) => JSON.parse(filestring).type)
.catch((err) => {
if (err?.code !== 'ENOENT') console.error(err);
});
// Ff package.json existed and contained a `type` field with a value, voila
if (type) return type;
// Otherwise, (if not at the root) continue checking the next directory up
// If at the root, stop and return false
return dir.length > 1 && getPackageType(resolvePath(dir, '..'));
}
# main.coffee
import { scream } from './scream.coffee'
console.log scream 'hello, world'
import { version } from 'node:process'
console.log "Brought to you by Node.js version #{version}"
# scream.coffee
export scream = (str) -> str.toUpperCase()
With the preceding loader, running
node --experimental-loader ./coffeescript-loader.mjs main.coffee
causes main.coffee
to be turned into JavaScript after its source code is
loaded from disk but before Node.js executes it; and so on for any .coffee
,
.litcoffee
or .coffee.md
files referenced via import
statements of any
loaded file.
Resolution algorithm#
Features#
The resolver has the following properties:
- FileURL-based resolution as is used by ES modules
- Support for builtin module loading
- Relative and absolute URL resolution
- No default extensions
- No folder mains
- Bare specifier package resolution lookup through node_modules
Resolver algorithm#
The algorithm to load an ES module specifier is given through the ESM_RESOLVE method below. It returns the resolved URL for a module specifier relative to a parentURL.
The algorithm to determine the module format of a resolved URL is provided by ESM_FORMAT, which returns the unique module format for any file. The "module" format is returned for an ECMAScript Module, while the "commonjs" format is used to indicate loading through the legacy CommonJS loader. Additional formats such as "addon" can be extended in future updates.
In the following algorithms, all subroutine errors are propagated as errors of these top-level routines unless stated otherwise.
defaultConditions is the conditional environment name array,
["node", "import"]
.
The resolver can throw the following errors:
- Invalid Module Specifier: Module specifier is an invalid URL, package name or package subpath specifier.
- Invalid Package Configuration: package.json configuration is invalid or contains an invalid configuration.
- Invalid Package Target: Package exports or imports define a target module for the package that is an invalid type or string target.
- Package Path Not Exported: Package exports do not define or permit a target subpath in the package for the given module.
- Package Import Not Defined: Package imports do not define the specifier.
- Module Not Found: The package or module requested does not exist.
- Unsupported Directory Import: The resolved path corresponds to a directory, which is not a supported target for module imports.
Resolver Algorithm Specification#
ESM_RESOLVE(specifier, parentURL)
- Let resolved be undefined.
- If specifier is a valid URL, then
- Set resolved to the result of parsing and reserializing specifier as a URL.
- Otherwise, if specifier starts with "/", "./", or "../", then
- Set resolved to the URL resolution of specifier relative to parentURL.
- Otherwise, if specifier starts with "#", then
- Set resolved to the result of PACKAGE_IMPORTS_RESOLVE(specifier, parentURL, defaultConditions).
- Otherwise,
- Note: specifier is now a bare specifier.
- Set resolved the result of PACKAGE_RESOLVE(specifier, parentURL).
- Let format be undefined.
- If resolved is a "file:" URL, then
- If resolved contains any percent encodings of "/" or "\" ("%2F" and "%5C" respectively), then
- Throw an Invalid Module Specifier error.
- If the file at resolved is a directory, then
- Throw an Unsupported Directory Import error.
- If the file at resolved does not exist, then
- Throw a Module Not Found error.
- Set resolved to the real path of resolved, maintaining the same URL querystring and fragment components.
- Set format to the result of ESM_FILE_FORMAT(resolved).
- Otherwise,
- Set format the module format of the content type associated with the URL resolved.
- Load resolved as module format, format.
PACKAGE_RESOLVE(packageSpecifier, parentURL)
- Let packageName be undefined.
- If packageSpecifier is an empty string, then
- Throw an Invalid Module Specifier error.
- If packageSpecifier is a Node.js builtin module name, then
- Return the string "node:" concatenated with packageSpecifier.
- If packageSpecifier does not start with "@", then
- Set packageName to the substring of packageSpecifier until the first "/" separator or the end of the string.
- Otherwise,
- If packageSpecifier does not contain a "/" separator, then
- Throw an Invalid Module Specifier error.
- Set packageName to the substring of packageSpecifier until the second "/" separator or the end of the string.
- If packageName starts with "." or contains "\" or "%", then
- Throw an Invalid Module Specifier error.
- Let packageSubpath be "." concatenated with the substring of packageSpecifier from the position at the length of packageName.
- If packageSubpath ends in "/", then
- Throw an Invalid Module Specifier error.
- Let selfUrl be the result of PACKAGE_SELF_RESOLVE(packageName, packageSubpath, parentURL).
- If selfUrl is not undefined, return selfUrl.
- While parentURL is not the file system root,
- Let packageURL be the URL resolution of "node_modules/" concatenated with packageSpecifier, relative to parentURL.
- Set parentURL to the parent folder URL of parentURL.
- If the folder at packageURL does not exist, then
- Continue the next loop iteration.
- Let pjson be the result of READ_PACKAGE_JSON(packageURL).
- If pjson is not null and pjson.exports is not null or undefined, then
- Return the result of PACKAGE_EXPORTS_RESOLVE(packageURL, packageSubpath, pjson.exports, defaultConditions).
- Otherwise, if packageSubpath is equal to ".", then
- If pjson.main is a string, then
- Return the URL resolution of main in packageURL.
- Otherwise,
- Return the URL resolution of packageSubpath in packageURL.
- Throw a Module Not Found error.
PACKAGE_SELF_RESOLVE(packageName, packageSubpath, parentURL)
- Let packageURL be the result of LOOKUP_PACKAGE_SCOPE(parentURL).
- If packageURL is null, then
- Return undefined.
- Let pjson be the result of READ_PACKAGE_JSON(packageURL).
- If pjson is null or if pjson.exports is null or undefined, then
- Return undefined.
- If pjson.name is equal to packageName, then
- Return the result of PACKAGE_EXPORTS_RESOLVE(packageURL, packageSubpath, pjson.exports, defaultConditions).
- Otherwise, return undefined.
PACKAGE_EXPORTS_RESOLVE(packageURL, subpath, exports, conditions)
- If exports is an Object with both a key starting with "." and a key not starting with ".", throw an Invalid Package Configuration error.
- If subpath is equal to ".", then
- Let mainExport be undefined.
- If exports is a String or Array, or an Object containing no keys starting with ".", then
- Set mainExport to exports.
- Otherwise if exports is an Object containing a "." property, then
- Set mainExport to exports["."].
- If mainExport is not undefined, then
- Let resolved be the result of PACKAGE_TARGET_RESOLVE( packageURL, mainExport, null, false, conditions).
- If resolved is not null or undefined, return resolved.
- Otherwise, if exports is an Object and all keys of exports start with ".", then
- Let matchKey be the string "./" concatenated with subpath.
- Let resolved be the result of PACKAGE_IMPORTS_EXPORTS_RESOLVE( matchKey, exports, packageURL, false, conditions).
- If resolved is not null or undefined, return resolved.
- Throw a Package Path Not Exported error.
PACKAGE_IMPORTS_RESOLVE(specifier, parentURL, conditions)
- Assert: specifier begins with "#".
- If specifier is exactly equal to "#" or starts with "#/", then
- Throw an Invalid Module Specifier error.
- Let packageURL be the result of LOOKUP_PACKAGE_SCOPE(parentURL).
- If packageURL is not null, then
- Let pjson be the result of READ_PACKAGE_JSON(packageURL).
- If pjson.imports is a non-null Object, then
- Let resolved be the result of PACKAGE_IMPORTS_EXPORTS_RESOLVE( specifier, pjson.imports, packageURL, true, conditions).
- If resolved is not null or undefined, return resolved.
- Throw a Package Import Not Defined error.
PACKAGE_IMPORTS_EXPORTS_RESOLVE(matchKey, matchObj, packageURL, isImports, conditions)
- If matchKey is a key of matchObj and does not contain "*", then
- Let target be the value of matchObj[matchKey].
- Return the result of PACKAGE_TARGET_RESOLVE(packageURL, target, null, isImports, conditions).
- Let expansionKeys be the list of keys of matchObj containing only a single "*", sorted by the sorting function PATTERN_KEY_COMPARE which orders in descending order of specificity.
- For each key expansionKey in expansionKeys, do
- Let patternBase be the substring of expansionKey up to but excluding the first "*" character.
- If matchKey starts with but is not equal to patternBase, then
- Let patternTrailer be the substring of expansionKey from the index after the first "*" character.
- If patternTrailer has zero length, or if matchKey ends with patternTrailer and the length of matchKey is greater than or equal to the length of expansionKey, then
- Let target be the value of matchObj[expansionKey].
- Let patternMatch be the substring of matchKey starting at the index of the length of patternBase up to the length of matchKey minus the length of patternTrailer.
- Return the result of PACKAGE_TARGET_RESOLVE(packageURL, target, patternMatch, isImports, conditions).
- Return null.
PATTERN_KEY_COMPARE(keyA, keyB)
- Assert: keyA ends with "/" or contains only a single "*".
- Assert: keyB ends with "/" or contains only a single "*".
- Let baseLengthA be the index of "*" in keyA plus one, if keyA contains "*", or the length of keyA otherwise.
- Let baseLengthB be the index of "*" in keyB plus one, if keyB contains "*", or the length of keyB otherwise.
- If baseLengthA is greater than baseLengthB, return -1.
- If baseLengthB is greater than baseLengthA, return 1.
- If keyA does not contain "*", return 1.
- If keyB does not contain "*", return -1.
- If the length of keyA is greater than the length of keyB, return -1.
- If the length of keyB is greater than the length of keyA, return 1.
- Return 0.
PACKAGE_TARGET_RESOLVE(packageURL, target, patternMatch, isImports, conditions)
- If target is a String, then
- If target does not start with "./", then
- If isImports is false, or if target starts with "../" or "/", or if target is a valid URL, then
- Throw an Invalid Package Target error.
- If patternMatch is a String, then
- Return PACKAGE_RESOLVE(target with every instance of "*" replaced by patternMatch, packageURL + "/").
- Return PACKAGE_RESOLVE(target, packageURL + "/").
- If target split on "/" or "\" contains any "", ".", "..", or "node_modules" segments after the first "." segment, case insensitive and including percent encoded variants, throw an Invalid Package Target error.
- Let resolvedTarget be the URL resolution of the concatenation of packageURL and target.
- Assert: resolvedTarget is contained in packageURL.
- If patternMatch is null, then
- Return resolvedTarget.
- If patternMatch split on "/" or "\" contains any "", ".", "..", or "node_modules" segments, case insensitive and including percent encoded variants, throw an Invalid Module Specifier error.
- Return the URL resolution of resolvedTarget with every instance of "*" replaced with patternMatch.
- Otherwise, if target is a non-null Object, then
- If exports contains any index property keys, as defined in ECMA-262 6.1.7 Array Index, throw an Invalid Package Configuration error.
- For each property p of target, in object insertion order as,
- If p equals "default" or conditions contains an entry for p, then
- Let targetValue be the value of the p property in target.
- Let resolved be the result of PACKAGE_TARGET_RESOLVE( packageURL, targetValue, patternMatch, isImports, conditions).
- If resolved is equal to undefined, continue the loop.
- Return resolved.
- Return undefined.
- Otherwise, if target is an Array, then
- If _target.length is zero, return null.
- For each item targetValue in target, do
- Let resolved be the result of PACKAGE_TARGET_RESOLVE( packageURL, targetValue, patternMatch, isImports, conditions), continuing the loop on any Invalid Package Target error.
- If resolved is undefined, continue the loop.
- Return resolved.
- Return or throw the last fallback resolution null return or error.
- Otherwise, if target is null, return null.
- Otherwise throw an Invalid Package Target error.
ESM_FILE_FORMAT(url)
- Assert: url corresponds to an existing file.
- If url ends in ".mjs", then
- Return "module".
- If url ends in ".cjs", then
- Return "commonjs".
- If url ends in ".json", then
- Return "json".
- Let packageURL be the result of LOOKUP_PACKAGE_SCOPE(url).
- Let pjson be the result of READ_PACKAGE_JSON(packageURL).
- If pjson?.type exists and is "module", then
- If url ends in ".js", then
- Return "module".
- Throw an Unsupported File Extension error.
- Otherwise,
- Throw an Unsupported File Extension error.
LOOKUP_PACKAGE_SCOPE(url)
- Let scopeURL be url.
- While scopeURL is not the file system root,
- Set scopeURL to the parent URL of scopeURL.
- If scopeURL ends in a "node_modules" path segment, return null.
- Let pjsonURL be the resolution of "package.json" within scopeURL.
- if the file at pjsonURL exists, then
- Return scopeURL.
- Return null.
READ_PACKAGE_JSON(packageURL)
- Let pjsonURL be the resolution of "package.json" within packageURL.
- If the file at pjsonURL does not exist, then
- Return null.
- If the file at packageURL does not parse as valid JSON, then
- Throw an Invalid Package Configuration error.
- Return the parsed JSON source of the file at pjsonURL.
Customizing ESM specifier resolution algorithm#
The Loaders API provides a mechanism for customizing the ESM specifier resolution algorithm. An example loader that provides CommonJS-style resolution for ESM specifiers is commonjs-extension-resolution-loader.
Modules: node:module
API#
The Module
object#
Provides general utility methods when interacting with instances of
Module
, the module
variable often seen in CommonJS modules. Accessed
via import 'node:module'
or require('node:module')
.
module.builtinModules
#
A list of the names of all modules provided by Node.js. Can be used to verify if a module is maintained by a third party or not.
module
in this context isn't the same object that's provided
by the module wrapper. To access it, require the Module
module:
// module.mjs
// In an ECMAScript module
import { builtinModules as builtin } from 'node:module';
// module.cjs
// In a CommonJS module
const builtin = require('node:module').builtinModules;
module.createRequire(filename)
#
filename
<string> | <URL> Filename to be used to construct the require function. Must be a file URL object, file URL string, or absolute path string.- Returns: <require> Require function
import { createRequire } from 'node:module';
const require = createRequire(import.meta.url);
// sibling-module.js is a CommonJS module.
const siblingModule = require('./sibling-module');
module.isBuiltin(moduleName)
#
moduleName
<string> name of the module- Returns: <boolean> returns true if the module is builtin else returns false
import { isBuiltin } from 'node:module';
isBuiltin('node:fs'); // true
isBuiltin('fs'); // true
isBuiltin('wss'); // false
module.syncBuiltinESMExports()
#
The module.syncBuiltinESMExports()
method updates all the live bindings for
builtin ES Modules to match the properties of the CommonJS exports. It
does not add or remove exported names from the ES Modules.
const fs = require('node:fs');
const assert = require('node:assert');
const { syncBuiltinESMExports } = require('node:module');
fs.readFile = newAPI;
delete fs.readFileSync;
function newAPI() {
// ...
}
fs.newAPI = newAPI;
syncBuiltinESMExports();
import('node:fs').then((esmFS) => {
// It syncs the existing readFile property with the new value
assert.strictEqual(esmFS.readFile, newAPI);
// readFileSync has been deleted from the required fs
assert.strictEqual('readFileSync' in fs, false);
// syncBuiltinESMExports() does not remove readFileSync from esmFS
assert.strictEqual('readFileSync' in esmFS, true);
// syncBuiltinESMExports() does not add names
assert.strictEqual(esmFS.newAPI, undefined);
});
Source map v3 support#
Helpers for interacting with the source map cache. This cache is populated when source map parsing is enabled and source map include directives are found in a modules' footer.
To enable source map parsing, Node.js must be run with the flag
--enable-source-maps
, or with code coverage enabled by setting
NODE_V8_COVERAGE=dir
.
// module.mjs
// In an ECMAScript module
import { findSourceMap, SourceMap } from 'node:module';
// module.cjs
// In a CommonJS module
const { findSourceMap, SourceMap } = require('node:module');
module.findSourceMap(path)
#
path
<string>- Returns: <module.SourceMap> | <undefined> Returns
module.SourceMap
if a source map is found,undefined
otherwise.
path
is the resolved path for the file for which a corresponding source map
should be fetched.
Class: module.SourceMap
#
new SourceMap(payload)
#
payload
<Object>
Creates a new sourceMap
instance.
payload
is an object with keys matching the Source map v3 format:
file
: <string>version
: <number>sources
: <string[]>sourcesContent
: <string[]>names
: <string[]>mappings
: <string>sourceRoot
: <string>
sourceMap.payload
#
- Returns: <Object>
Getter for the payload used to construct the SourceMap
instance.
sourceMap.findEntry(lineNumber, columnNumber)
#
Given a line number and column number in the generated source file, returns an object representing the position in the original file. The object returned consists of the following keys:
Modules: Packages#
Introduction#
A package is a folder tree described by a package.json
file. The package
consists of the folder containing the package.json
file and all subfolders
until the next folder containing another package.json
file, or a folder
named node_modules
.
This page provides guidance for package authors writing package.json
files
along with a reference for the package.json
fields defined by Node.js.
Determining module system#
Node.js will treat the following as ES modules when passed to node
as the
initial input, or when referenced by import
statements or import()
expressions:
-
Files with an
.mjs
extension. -
Files with a
.js
extension when the nearest parentpackage.json
file contains a top-level"type"
field with a value of"module"
. -
Strings passed in as an argument to
--eval
, or piped tonode
viaSTDIN
, with the flag--input-type=module
.
Node.js will treat as CommonJS all other forms of input, such as .js
files
where the nearest parent package.json
file contains no top-level "type"
field, or string input without the flag --input-type
. This behavior is to
preserve backward compatibility. However, now that Node.js supports both
CommonJS and ES modules, it is best to be explicit whenever possible. Node.js
will treat the following as CommonJS when passed to node
as the initial input,
or when referenced by import
statements, import()
expressions, or
require()
expressions:
-
Files with a
.cjs
extension. -
Files with a
.js
extension when the nearest parentpackage.json
file contains a top-level field"type"
with a value of"commonjs"
. -
Strings passed in as an argument to
--eval
or--print
, or piped tonode
viaSTDIN
, with the flag--input-type=commonjs
.
Package authors should include the "type"
field, even in packages where
all sources are CommonJS. Being explicit about the type
of the package will
future-proof the package in case the default type of Node.js ever changes, and
it will also make things easier for build tools and loaders to determine how the
files in the package should be interpreted.
Modules loaders#
Node.js has two systems for resolving a specifier and loading modules.
There is the CommonJS module loader:
- It is fully synchronous.
- It is responsible for handling
require()
calls. - It is monkey patchable.
- It supports folders as modules.
- When resolving a specifier, if no exact match is found, it will try to add
extensions (
.js
,.json
, and finally.node
) and then attempt to resolve folders as modules. - It treats
.json
as JSON text files. .node
files are interpreted as compiled addon modules loaded withprocess.dlopen()
.- It treats all files that lack
.json
or.node
extensions as JavaScript text files. - It cannot be used to load ECMAScript modules (although it is possible to load ECMASCript modules from CommonJS modules). When used to load a JavaScript text file that is not an ECMAScript module, it loads it as a CommonJS module.
There is the ECMAScript module loader:
- It is asynchronous.
- It is responsible for handling
import
statements andimport()
expressions. - It is not monkey patchable, can be customized using loader hooks.
- It does not support folders as modules, directory indexes (e.g.
'./startup/index.js'
) must be fully specified. - It does no extension searching. A file extension must be provided when the specifier is a relative or absolute file URL.
- It can load JSON modules, but an import assertion is required.
- It accepts only
.js
,.mjs
, and.cjs
extensions for JavaScript text files. - It can be used to load JavaScript CommonJS modules. Such modules
are passed through the
cjs-module-lexer
to try to identify named exports, which are available if they can be determined through static analysis. Imported CommonJS modules have their URLs converted to absolute paths and are then loaded via the CommonJS module loader.
package.json
and file extensions#
Within a package, the package.json
"type"
field defines how
Node.js should interpret .js
files. If a package.json
file does not have a
"type"
field, .js
files are treated as CommonJS.
A package.json
"type"
value of "module"
tells Node.js to interpret .js
files within that package as using ES module syntax.
The "type"
field applies not only to initial entry points (node my-app.js
)
but also to files referenced by import
statements and import()
expressions.
// my-app.js, treated as an ES module because there is a package.json
// file in the same folder with "type": "module".
import './startup/init.js';
// Loaded as ES module since ./startup contains no package.json file,
// and therefore inherits the "type" value from one level up.
import 'commonjs-package';
// Loaded as CommonJS since ./node_modules/commonjs-package/package.json
// lacks a "type" field or contains "type": "commonjs".
import './node_modules/commonjs-package/index.js';
// Loaded as CommonJS since ./node_modules/commonjs-package/package.json
// lacks a "type" field or contains "type": "commonjs".
Files ending with .mjs
are always loaded as ES modules regardless of
the nearest parent package.json
.
Files ending with .cjs
are always loaded as CommonJS regardless of the
nearest parent package.json
.
import './legacy-file.cjs';
// Loaded as CommonJS since .cjs is always loaded as CommonJS.
import 'commonjs-package/src/index.mjs';
// Loaded as ES module since .mjs is always loaded as ES module.
The .mjs
and .cjs
extensions can be used to mix types within the same
package:
-
Within a
"type": "module"
package, Node.js can be instructed to interpret a particular file as CommonJS by naming it with a.cjs
extension (since both.js
and.mjs
files are treated as ES modules within a"module"
package). -
Within a
"type": "commonjs"
package, Node.js can be instructed to interpret a particular file as an ES module by naming it with an.mjs
extension (since both.js
and.cjs
files are treated as CommonJS within a"commonjs"
package).
--input-type
flag#
Strings passed in as an argument to --eval
(or -e
), or piped to node
via
STDIN
, are treated as ES modules when the --input-type=module
flag
is set.
node --input-type=module --eval "import { sep } from 'node:path'; console.log(sep);"
echo "import { sep } from 'node:path'; console.log(sep);" | node --input-type=module
For completeness there is also --input-type=commonjs
, for explicitly running
string input as CommonJS. This is the default behavior if --input-type
is
unspecified.
Determining package manager#
While all Node.js projects are expected to be installable by all package managers once published, their development teams are often required to use one specific package manager. To make this process easier, Node.js ships with a tool called Corepack that aims to make all package managers transparently available in your environment - provided you have Node.js installed.
By default Corepack won't enforce any specific package manager and will use
the generic "Last Known Good" versions associated with each Node.js release,
but you can improve this experience by setting the "packageManager"
field
in your project's package.json
.
Package entry points#
In a package's package.json
file, two fields can define entry points for a
package: "main"
and "exports"
. Both fields apply to both ES module
and CommonJS module entry points.
The "main"
field is supported in all versions of Node.js, but its
capabilities are limited: it only defines the main entry point of the package.
The "exports"
provides a modern alternative to "main"
allowing
multiple entry points to be defined, conditional entry resolution support
between environments, and preventing any other entry points besides those
defined in "exports"
. This encapsulation allows module authors to
clearly define the public interface for their package.
For new packages targeting the currently supported versions of Node.js, the
"exports"
field is recommended. For packages supporting Node.js 10 and
below, the "main"
field is required. If both "exports"
and
"main"
are defined, the "exports"
field takes precedence over
"main"
in supported versions of Node.js.
Conditional exports can be used within "exports"
to define different
package entry points per environment, including whether the package is
referenced via require
or via import
. For more information about supporting
both CommonJS and ES modules in a single package please consult
the dual CommonJS/ES module packages section.
Existing packages introducing the "exports"
field will prevent consumers
of the package from using any entry points that are not defined, including the
package.json
(e.g. require('your-package/package.json')
. This will
likely be a breaking change.
To make the introduction of "exports"
non-breaking, ensure that every
previously supported entry point is exported. It is best to explicitly specify
entry points so that the package's public API is well-defined. For example,
a project that previously exported main
, lib
,
feature
, and the package.json
could use the following package.exports
:
{
"name": "my-package",
"exports": {
".": "./lib/index.js",
"./lib": "./lib/index.js",
"./lib/index": "./lib/index.js",
"./lib/index.js": "./lib/index.js",
"./feature": "./feature/index.js",
"./feature/index": "./feature/index.js",
"./feature/index.js": "./feature/index.js",
"./package.json": "./package.json"
}
}
Alternatively a project could choose to export entire folders both with and without extensioned subpaths using export patterns:
{
"name": "my-package",
"exports": {
".": "./lib/index.js",
"./lib": "./lib/index.js",
"./lib/*": "./lib/*.js",
"./lib/*.js": "./lib/*.js",
"./feature": "./feature/index.js",
"./feature/*": "./feature/*.js",
"./feature/*.js": "./feature/*.js",
"./package.json": "./package.json"
}
}
With the above providing backwards-compatibility for any minor package versions, a future major change for the package can then properly restrict the exports to only the specific feature exports exposed:
{
"name": "my-package",
"exports": {
".": "./lib/index.js",
"./feature/*.js": "./feature/*.js",
"./feature/internal/*": null
}
}
Main entry point export#
When writing a new package, it is recommended to use the "exports"
field:
{
"exports": "./index.js"
}
When the "exports"
field is defined, all subpaths of the package are
encapsulated and no longer available to importers. For example,
require('pkg/subpath.js')
throws an ERR_PACKAGE_PATH_NOT_EXPORTED
error.
This encapsulation of exports provides more reliable guarantees
about package interfaces for tools and when handling semver upgrades for a
package. It is not a strong encapsulation since a direct require of any
absolute subpath of the package such as
require('/path/to/node_modules/pkg/subpath.js')
will still load subpath.js
.
All currently supported versions of Node.js and modern build tools support the
"exports"
field. For projects using an older version of Node.js or a related
build tool, compatibility can be achieved by including the "main"
field
alongside "exports"
pointing to the same module:
{
"main": "./index.js",
"exports": "./index.js"
}
Subpath exports#
When using the "exports"
field, custom subpaths can be defined along
with the main entry point by treating the main entry point as the
"."
subpath:
{
"exports": {
".": "./index.js",
"./submodule.js": "./src/submodule.js"
}
}
Now only the defined subpath in "exports"
can be imported by a consumer:
import submodule from 'es-module-package/submodule.js';
// Loads ./node_modules/es-module-package/src/submodule.js
While other subpaths will error:
import submodule from 'es-module-package/private-module.js';
// Throws ERR_PACKAGE_PATH_NOT_EXPORTED
Extensions in subpaths#
Package authors should provide either extensioned (import 'pkg/subpath.js'
) or
extensionless (import 'pkg/subpath'
) subpaths in their exports. This ensures
that there is only one subpath for each exported module so that all dependents
import the same consistent specifier, keeping the package contract clear for
consumers and simplifying package subpath completions.
Traditionally, packages tended to use the extensionless style, which has the benefits of readability and of masking the true path of the file within the package.
With import maps now providing a standard for package resolution in browsers and other JavaScript runtimes, using the extensionless style can result in bloated import map definitions. Explicit file extensions can avoid this issue by enabling the import map to utilize a packages folder mapping to map multiple subpaths where possible instead of a separate map entry per package subpath export. This also mirrors the requirement of using the full specifier path in relative and absolute import specifiers.
Exports sugar#
If the "."
export is the only export, the "exports"
field provides sugar
for this case being the direct "exports"
field value.
{
"exports": {
".": "./index.js"
}
}
can be written:
{
"exports": "./index.js"
}
Subpath imports#
In addition to the "exports"
field, there is a package "imports"
field
to create private mappings that only apply to import specifiers from within the
package itself.
Entries in the "imports"
field must always start with #
to ensure they are
disambiguated from external package specifiers.
For example, the imports field can be used to gain the benefits of conditional exports for internal modules:
// package.json
{
"imports": {
"#dep": {
"node": "dep-node-native",
"default": "./dep-polyfill.js"
}
},
"dependencies": {
"dep-node-native": "^1.0.0"
}
}
where import '#dep'
does not get the resolution of the external package
dep-node-native
(including its exports in turn), and instead gets the local
file ./dep-polyfill.js
relative to the package in other environments.
Unlike the "exports"
field, the "imports"
field permits mapping to external
packages.
The resolution rules for the imports field are otherwise analogous to the exports field.
Subpath patterns#
For packages with a small number of exports or imports, we recommend
explicitly listing each exports subpath entry. But for packages that have
large numbers of subpaths, this might cause package.json
bloat and
maintenance issues.
For these use cases, subpath export patterns can be used instead:
// ./node_modules/es-module-package/package.json
{
"exports": {
"./features/*.js": "./src/features/*.js"
},
"imports": {
"#internal/*.js": "./src/internal/*.js"
}
}
*
maps expose nested subpaths as it is a string replacement syntax
only.
All instances of *
on the right hand side will then be replaced with this
value, including if it contains any /
separators.
import featureX from 'es-module-package/features/x.js';
// Loads ./node_modules/es-module-package/src/features/x.js
import featureY from 'es-module-package/features/y/y.js';
// Loads ./node_modules/es-module-package/src/features/y/y.js
import internalZ from '#internal/z.js';
// Loads ./node_modules/es-module-package/src/internal/z.js
This is a direct static matching and replacement without any special handling
for file extensions. Including the "*.js"
on both sides of the mapping
restricts the exposed package exports to only JS files.
The property of exports being statically enumerable is maintained with exports
patterns since the individual exports for a package can be determined by
treating the right hand side target pattern as a **
glob against the list of
files within the package. Because node_modules
paths are forbidden in exports
targets, this expansion is dependent on only the files of the package itself.
To exclude private subfolders from patterns, null
targets can be used:
// ./node_modules/es-module-package/package.json
{
"exports": {
"./features/*.js": "./src/features/*.js",
"./features/private-internal/*": null
}
}
import featureInternal from 'es-module-package/features/private-internal/m.js';
// Throws: ERR_PACKAGE_PATH_NOT_EXPORTED
import featureX from 'es-module-package/features/x.js';
// Loads ./node_modules/es-module-package/src/features/x.js
Conditional exports#
Conditional exports provide a way to map to different paths depending on certain conditions. They are supported for both CommonJS and ES module imports.
For example, a package that wants to provide different ES module exports for
require()
and import
can be written:
// package.json
{
"exports": {
"import": "./index-module.js",
"require": "./index-require.cjs"
},
"type": "module"
}
Node.js implements the following conditions, listed in order from most specific to least specific as conditions should be defined:
"node-addons"
- similar to"node"
and matches for any Node.js environment. This condition can be used to provide an entry point which uses native C++ addons as opposed to an entry point which is more universal and doesn't rely on native addons. This condition can be disabled via the--no-addons
flag."node"
- matches for any Node.js environment. Can be a CommonJS or ES module file. In most cases explicitly calling out the Node.js platform is not necessary."import"
- matches when the package is loaded viaimport
orimport()
, or via any top-level import or resolve operation by the ECMAScript module loader. Applies regardless of the module format of the target file. Always mutually exclusive with"require"
."require"
- matches when the package is loaded viarequire()
. The referenced file should be loadable withrequire()
although the condition matches regardless of the module format of the target file. Expected formats include CommonJS, JSON, and native addons but not ES modules asrequire()
doesn't support them. Always mutually exclusive with"import"
."default"
- the generic fallback that always matches. Can be a CommonJS or ES module file. This condition should always come last.
Within the "exports"
object, key order is significant. During condition
matching, earlier entries have higher priority and take precedence over later
entries. The general rule is that conditions should be from most specific to
least specific in object order.
Using the "import"
and "require"
conditions can lead to some hazards,
which are further explained in the dual CommonJS/ES module packages section.
The "node-addons"
condition can be used to provide an entry point which
uses native C++ addons. However, this condition can be disabled via the
--no-addons
flag. When using "node-addons"
, it's recommended to treat
"default"
as an enhancement that provides a more universal entry point, e.g.
using WebAssembly instead of a native addon.
Conditional exports can also be extended to exports subpaths, for example:
{
"exports": {
".": "./index.js",
"./feature.js": {
"node": "./feature-node.js",
"default": "./feature.js"
}
}
}
Defines a package where require('pkg/feature.js')
and
import 'pkg/feature.js'
could provide different implementations between
Node.js and other JS environments.
When using environment branches, always include a "default"
condition where
possible. Providing a "default"
condition ensures that any unknown JS
environments are able to use this universal implementation, which helps avoid
these JS environments from having to pretend to be existing environments in
order to support packages with conditional exports. For this reason, using
"node"
and "default"
condition branches is usually preferable to using
"node"
and "browser"
condition branches.
Nested conditions#
In addition to direct mappings, Node.js also supports nested condition objects.
For example, to define a package that only has dual mode entry points for use in Node.js but not the browser:
{
"exports": {
"node": {
"import": "./feature-node.mjs",
"require": "./feature-node.cjs"
},
"default": "./feature.mjs"
}
}
Conditions continue to be matched in order as with flat conditions. If
a nested condition does not have any mapping it will continue checking
the remaining conditions of the parent condition. In this way nested
conditions behave analogously to nested JavaScript if
statements.
Resolving user conditions#
When running Node.js, custom user conditions can be added with the
--conditions
flag:
node --conditions=development index.js
which would then resolve the "development"
condition in package imports and
exports, while resolving the existing "node"
, "node-addons"
, "default"
,
"import"
, and "require"
conditions as appropriate.
Any number of custom conditions can be set with repeat flags.
Community Conditions Definitions#
Condition strings other than the "import"
, "require"
, "node"
,
"node-addons"
and "default"
conditions
implemented in Node.js core are ignored by default.
Other platforms may implement other conditions and user conditions can be
enabled in Node.js via the --conditions
/ -C
flag.
Since custom package conditions require clear definitions to ensure correct usage, a list of common known package conditions and their strict definitions is provided below to assist with ecosystem coordination.
"types"
- can be used by typing systems to resolve the typing file for the given export. This condition should always be included first."deno"
- indicates a variation for the Deno platform."browser"
- any web browser environment."react-native"
- will be matched by the React Native framework (all platforms). To target React Native for Web,"browser"
should be specified before this condition."development"
- can be used to define a development-only environment entry point, for example to provide additional debugging context such as better error messages when running in a development mode. Must always be mutually exclusive with"production"
."production"
- can be used to define a production environment entry point. Must always be mutually exclusive with"development"
.
New conditions definitions may be added to this list by creating a pull request to the Node.js documentation for this section. The requirements for listing a new condition definition here are that:
- The definition should be clear and unambiguous for all implementers.
- The use case for why the condition is needed should be clearly justified.
- There should exist sufficient existing implementation usage.
- The condition name should not conflict with another condition definition or condition in wide usage.
- The listing of the condition definition should provide a coordination benefit to the ecosystem that wouldn't otherwise be possible. For example, this would not necessarily be the case for company-specific or application-specific conditions.
The above definitions may be moved to a dedicated conditions registry in due course.
Self-referencing a package using its name#
Within a package, the values defined in the package's
package.json
"exports"
field can be referenced via the package's name.
For example, assuming the package.json
is:
// package.json
{
"name": "a-package",
"exports": {
".": "./index.mjs",
"./foo.js": "./foo.js"
}
}
Then any module in that package can reference an export in the package itself:
// ./a-module.mjs
import { something } from 'a-package'; // Imports "something" from ./index.mjs.
Self-referencing is available only if package.json
has "exports"
, and
will allow importing only what that "exports"
(in the package.json
)
allows. So the code below, given the previous package, will generate a runtime
error:
// ./another-module.mjs
// Imports "another" from ./m.mjs. Fails because
// the "package.json" "exports" field
// does not provide an export named "./m.mjs".
import { another } from 'a-package/m.mjs';
Self-referencing is also available when using require
, both in an ES module,
and in a CommonJS one. For example, this code will also work:
// ./a-module.js
const { something } = require('a-package/foo.js'); // Loads from ./foo.js.
Finally, self-referencing also works with scoped packages. For example, this code will also work:
// package.json
{
"name": "@my/package",
"exports": "./index.js"
}
// ./index.js
module.exports = 42;
// ./other.js
console.log(require('@my/package'));
$ node other.js
42
Dual CommonJS/ES module packages#
Prior to the introduction of support for ES modules in Node.js, it was a common
pattern for package authors to include both CommonJS and ES module JavaScript
sources in their package, with package.json
"main"
specifying the
CommonJS entry point and package.json
"module"
specifying the ES module
entry point.
This enabled Node.js to run the CommonJS entry point while build tools such as
bundlers used the ES module entry point, since Node.js ignored (and still
ignores) the top-level "module"
field.
Node.js can now run ES module entry points, and a package can contain both
CommonJS and ES module entry points (either via separate specifiers such as
'pkg'
and 'pkg/es-module'
, or both at the same specifier via Conditional
exports). Unlike in the scenario where "module"
is only used by bundlers,
or ES module files are transpiled into CommonJS on the fly before evaluation by
Node.js, the files referenced by the ES module entry point are evaluated as ES
modules.
Dual package hazard#
When an application is using a package that provides both CommonJS and ES module
sources, there is a risk of certain bugs if both versions of the package get
loaded. This potential comes from the fact that the pkgInstance
created by
const pkgInstance = require('pkg')
is not the same as the pkgInstance
created by import pkgInstance from 'pkg'
(or an alternative main path like
'pkg/module'
). This is the “dual package hazard,” where two versions of the
same package can be loaded within the same runtime environment. While it is
unlikely that an application or package would intentionally load both versions
directly, it is common for an application to load one version while a dependency
of the application loads the other version. This hazard can happen because
Node.js supports intermixing CommonJS and ES modules, and can lead to unexpected
behavior.
If the package main export is a constructor, an instanceof
comparison of
instances created by the two versions returns false
, and if the export is an
object, properties added to one (like pkgInstance.foo = 3
) are not present on
the other. This differs from how import
and require
statements work in
all-CommonJS or all-ES module environments, respectively, and therefore is
surprising to users. It also differs from the behavior users are familiar with
when using transpilation via tools like Babel or esm
.
Writing dual packages while avoiding or minimizing hazards#
First, the hazard described in the previous section occurs when a package
contains both CommonJS and ES module sources and both sources are provided for
use in Node.js, either via separate main entry points or exported paths. A
package might instead be written where any version of Node.js receives only
CommonJS sources, and any separate ES module sources the package might contain
are intended only for other environments such as browsers. Such a package
would be usable by any version of Node.js, since import
can refer to CommonJS
files; but it would not provide any of the advantages of using ES module syntax.
A package might also switch from CommonJS to ES module syntax in a breaking change version bump. This has the disadvantage that the newest version of the package would only be usable in ES module-supporting versions of Node.js.
Every pattern has tradeoffs, but there are two broad approaches that satisfy the following conditions:
- The package is usable via both
require
andimport
. - The package is usable in both current Node.js and older versions of Node.js that lack support for ES modules.
- The package main entry point, e.g.
'pkg'
can be used by bothrequire
to resolve to a CommonJS file and byimport
to resolve to an ES module file. (And likewise for exported paths, e.g.'pkg/feature'
.) - The package provides named exports, e.g.
import { name } from 'pkg'
rather thanimport pkg from 'pkg'; pkg.name
. - The package is potentially usable in other ES module environments such as browsers.
- The hazards described in the previous section are avoided or minimized.
Approach #1: Use an ES module wrapper#
Write the package in CommonJS or transpile ES module sources into CommonJS, and
create an ES module wrapper file that defines the named exports. Using
Conditional exports, the ES module wrapper is used for import
and the
CommonJS entry point for require
.
// ./node_modules/pkg/package.json
{
"type": "module",
"exports": {
"import": "./wrapper.mjs",
"require": "./index.cjs"
}
}
The preceding example uses explicit extensions .mjs
and .cjs
.
If your files use the .js
extension, "type": "module"
will cause such files
to be treated as ES modules, just as "type": "commonjs"
would cause them
to be treated as CommonJS.
See Enabling.
// ./node_modules/pkg/index.cjs
exports.name = 'value';
// ./node_modules/pkg/wrapper.mjs
import cjsModule from './index.cjs';
export const name = cjsModule.name;
In this example, the name
from import { name } from 'pkg'
is the same
singleton as the name
from const { name } = require('pkg')
. Therefore ===
returns true
when comparing the two name
s and the divergent specifier hazard
is avoided.
If the module is not simply a list of named exports, but rather contains a
unique function or object export like module.exports = function () { ... }
,
or if support in the wrapper for the import pkg from 'pkg'
pattern is desired,
then the wrapper would instead be written to export the default optionally
along with any named exports as well:
import cjsModule from './index.cjs';
export const name = cjsModule.name;
export default cjsModule;
This approach is appropriate for any of the following use cases:
- The package is currently written in CommonJS and the author would prefer not to refactor it into ES module syntax, but wishes to provide named exports for ES module consumers.
- The package has other packages that depend on it, and the end user might
install both this package and those other packages. For example a
utilities
package is used directly in an application, and autilities-plus
package adds a few more functions toutilities
. Because the wrapper exports underlying CommonJS files, it doesn't matter ifutilities-plus
is written in CommonJS or ES module syntax; it will work either way. - The package stores internal state, and the package author would prefer not to refactor the package to isolate its state management. See the next section.
A variant of this approach not requiring conditional exports for consumers could
be to add an export, e.g. "./module"
, to point to an all-ES module-syntax
version of the package. This could be used via import 'pkg/module'
by users
who are certain that the CommonJS version will not be loaded anywhere in the
application, such as by dependencies; or if the CommonJS version can be loaded
but doesn't affect the ES module version (for example, because the package is
stateless):
// ./node_modules/pkg/package.json
{
"type": "module",
"exports": {
".": "./index.cjs",
"./module": "./wrapper.mjs"
}
}
Approach #2: Isolate state#
A package.json
file can define the separate CommonJS and ES module entry
points directly:
// ./node_modules/pkg/package.json
{
"type": "module",
"exports": {
"import": "./index.mjs",
"require": "./index.cjs"
}
}
This can be done if both the CommonJS and ES module versions of the package are equivalent, for example because one is the transpiled output of the other; and the package's management of state is carefully isolated (or the package is stateless).
The reason that state is an issue is because both the CommonJS and ES module
versions of the package might get used within an application; for example, the
user's application code could import
the ES module version while a dependency
require
s the CommonJS version. If that were to occur, two copies of the
package would be loaded in memory and therefore two separate states would be
present. This would likely cause hard-to-troubleshoot bugs.
Aside from writing a stateless package (if JavaScript's Math
were a package,
for example, it would be stateless as all of its methods are static), there are
some ways to isolate state so that it's shared between the potentially loaded
CommonJS and ES module instances of the package:
-
If possible, contain all state within an instantiated object. JavaScript's
Date
, for example, needs to be instantiated to contain state; if it were a package, it would be used like this:import Date from 'date'; const someDate = new Date(); // someDate contains state; Date does not
The
new
keyword isn't required; a package's function can return a new object, or modify a passed-in object, to keep the state external to the package. -
Isolate the state in one or more CommonJS files that are shared between the CommonJS and ES module versions of the package. For example, if the CommonJS and ES module entry points are
index.cjs
andindex.mjs
, respectively:// ./node_modules/pkg/index.cjs const state = require('./state.cjs'); module.exports.state = state;
// ./node_modules/pkg/index.mjs import state from './state.cjs'; export { state, };
Even if
pkg
is used via bothrequire
andimport
in an application (for example, viaimport
in application code and viarequire
by a dependency) each reference ofpkg
will contain the same state; and modifying that state from either module system will apply to both.
Any plugins that attach to the package's singleton would need to separately attach to both the CommonJS and ES module singletons.
This approach is appropriate for any of the following use cases:
- The package is currently written in ES module syntax and the package author wants that version to be used wherever such syntax is supported.
- The package is stateless or its state can be isolated without too much difficulty.
- The package is unlikely to have other public packages that depend on it, or if it does, the package is stateless or has state that need not be shared between dependencies or with the overall application.
Even with isolated state, there is still the cost of possible extra code execution between the CommonJS and ES module versions of a package.
As with the previous approach, a variant of this approach not requiring
conditional exports for consumers could be to add an export, e.g.
"./module"
, to point to an all-ES module-syntax version of the package:
// ./node_modules/pkg/package.json
{
"type": "module",
"exports": {
".": "./index.cjs",
"./module": "./index.mjs"
}
}
Node.js package.json
field definitions#
This section describes the fields used by the Node.js runtime. Other tools (such as npm) use additional fields which are ignored by Node.js and not documented here.
The following fields in package.json
files are used in Node.js:
"name"
- Relevant when using named imports within a package. Also used by package managers as the name of the package."main"
- The default module when loading the package, if exports is not specified, and in versions of Node.js prior to the introduction of exports."packageManager"
- The package manager recommended when contributing to the package. Leveraged by the Corepack shims."type"
- The package type determining whether to load.js
files as CommonJS or ES modules."exports"
- Package exports and conditional exports. When present, limits which submodules can be loaded from within the package."imports"
- Package imports, for use by modules within the package itself.
"name"
#
- Type: <string>
{
"name": "package-name"
}
The "name"
field defines your package's name. Publishing to the
npm registry requires a name that satisfies
certain requirements.
The "name"
field can be used in addition to the "exports"
field to
self-reference a package using its name.
"main"
#
- Type: <string>
{
"main": "./index.js"
}
The "main"
field defines the entry point of a package when imported by name
via a node_modules
lookup. Its value is a path.
When a package has an "exports"
field, this will take precedence over the
"main"
field when importing the package by name.
It also defines the script that is used when the package directory is loaded
via require()
.
// This resolves to ./path/to/directory/index.js.
require('./path/to/directory');
"packageManager"
#
- Type: <string>
{
"packageManager": "<package manager name>@<version>"
}
The "packageManager"
field defines which package manager is expected to be
used when working on the current project. It can be set to any of the
supported package managers, and will ensure that your teams use the exact
same package manager versions without having to install anything else other than
Node.js.
This field is currently experimental and needs to be opted-in; check the Corepack page for details about the procedure.
"type"
#
- Type: <string>
The "type"
field defines the module format that Node.js uses for all
.js
files that have that package.json
file as their nearest parent.
Files ending with .js
are loaded as ES modules when the nearest parent
package.json
file contains a top-level field "type"
with a value of
"module"
.
The nearest parent package.json
is defined as the first package.json
found
when searching in the current folder, that folder's parent, and so on up
until a node_modules folder or the volume root is reached.
// package.json
{
"type": "module"
}
# In same folder as preceding package.json
node my-app.js # Runs as ES module
If the nearest parent package.json
lacks a "type"
field, or contains
"type": "commonjs"
, .js
files are treated as CommonJS. If the volume
root is reached and no package.json
is found, .js
files are treated as
CommonJS.
import
statements of .js
files are treated as ES modules if the nearest
parent package.json
contains "type": "module"
.
// my-app.js, part of the same example as above
import './startup.js'; // Loaded as ES module because of package.json
Regardless of the value of the "type"
field, .mjs
files are always treated
as ES modules and .cjs
files are always treated as CommonJS.
"exports"
#
- Type: <Object> | <string> | <string[]>
{
"exports": "./index.js"
}
The "exports"
field allows defining the entry points of a package when
imported by name loaded either via a node_modules
lookup or a
self-reference to its own name. It is supported in Node.js 12+ as an
alternative to the "main"
that can support defining subpath exports
and conditional exports while encapsulating internal unexported modules.
Conditional Exports can also be used within "exports"
to define different
package entry points per environment, including whether the package is
referenced via require
or via import
.
All paths defined in the "exports"
must be relative file URLs starting with
./
.
"imports"
#
- Type: <Object>
// package.json
{
"imports": {
"#dep": {
"node": "dep-node-native",
"default": "./dep-polyfill.js"
}
},
"dependencies": {
"dep-node-native": "^1.0.0"
}
}
Entries in the imports field must be strings starting with #
.
Package imports permit mapping to external packages.
This field defines subpath imports for the current package.
Net#
Source Code: lib/net.js
The node:net
module provides an asynchronous network API for creating stream-based
TCP or IPC servers (net.createServer()
) and clients
(net.createConnection()
).
It can be accessed using:
const net = require('node:net');
IPC support#
The node:net
module supports IPC with named pipes on Windows, and Unix domain
sockets on other operating systems.
Identifying paths for IPC connections#
net.connect()
, net.createConnection()
, server.listen()
, and
socket.connect()
take a path
parameter to identify IPC endpoints.
On Unix, the local domain is also known as the Unix domain. The path is a
file system pathname. It gets truncated to an OS-dependent length of
sizeof(sockaddr_un.sun_path) - 1
. Typical values are 107 bytes on Linux and
103 bytes on macOS. If a Node.js API abstraction creates the Unix domain socket,
it will unlink the Unix domain socket as well. For example,
net.createServer()
may create a Unix domain socket and
server.close()
will unlink it. But if a user creates the Unix domain
socket outside of these abstractions, the user will need to remove it. The same
applies when a Node.js API creates a Unix domain socket but the program then
crashes. In short, a Unix domain socket will be visible in the file system and
will persist until unlinked.
On Windows, the local domain is implemented using a named pipe. The path must
refer to an entry in \\?\pipe\
or \\.\pipe\
. Any characters are permitted,
but the latter may do some processing of pipe names, such as resolving ..
sequences. Despite how it might look, the pipe namespace is flat. Pipes will
not persist. They are removed when the last reference to them is closed.
Unlike Unix domain sockets, Windows will close and remove the pipe when the
owning process exits.
JavaScript string escaping requires paths to be specified with extra backslash escaping such as:
net.createServer().listen(
path.join('\\\\?\\pipe', process.cwd(), 'myctl'));
Class: net.BlockList
#
The BlockList
object can be used with some network APIs to specify rules for
disabling inbound or outbound access to specific IP addresses, IP ranges, or
IP subnets.
blockList.addAddress(address[, type])
#
address
<string> | <net.SocketAddress> An IPv4 or IPv6 address.type
<string> Either'ipv4'
or'ipv6'
. Default:'ipv4'
.
Adds a rule to block the given IP address.
blockList.addRange(start, end[, type])
#
start
<string> | <net.SocketAddress> The starting IPv4 or IPv6 address in the range.end
<string> | <net.SocketAddress> The ending IPv4 or IPv6 address in the range.type
<string> Either'ipv4'
or'ipv6'
. Default:'ipv4'
.
Adds a rule to block a range of IP addresses from start
(inclusive) to
end
(inclusive).
blockList.addSubnet(net, prefix[, type])
#
net
<string> | <net.SocketAddress> The network IPv4 or IPv6 address.prefix
<number> The number of CIDR prefix bits. For IPv4, this must be a value between0
and32
. For IPv6, this must be between0
and128
.type
<string> Either'ipv4'
or'ipv6'
. Default:'ipv4'
.
Adds a rule to block a range of IP addresses specified as a subnet mask.
blockList.check(address[, type])
#
address
<string> | <net.SocketAddress> The IP address to checktype
<string> Either'ipv4'
or'ipv6'
. Default:'ipv4'
.- Returns: <boolean>
Returns true
if the given IP address matches any of the rules added to the
BlockList
.
const blockList = new net.BlockList();
blockList.addAddress('123.123.123.123');
blockList.addRange('10.0.0.1', '10.0.0.10');
blockList.addSubnet('8592:757c:efae:4e45::', 64, 'ipv6');
console.log(blockList.check('123.123.123.123')); // Prints: true
console.log(blockList.check('10.0.0.3')); // Prints: true
console.log(blockList.check('222.111.111.222')); // Prints: false
// IPv6 notation for IPv4 addresses works:
console.log(blockList.check('::ffff:7b7b:7b7b', 'ipv6')); // Prints: true
console.log(blockList.check('::ffff:123.123.123.123', 'ipv6')); // Prints: true
blockList.rules
#
- Type: <string[]>
The list of rules added to the blocklist.
Class: net.SocketAddress
#
new net.SocketAddress([options])
#
options
<Object>
socketaddress.address
#
- Type <string>
socketaddress.family
#
- Type <string> Either
'ipv4'
or'ipv6'
.
socketaddress.flowlabel
#
- Type <number>
socketaddress.port
#
- Type <number>
Class: net.Server
#
- Extends: <EventEmitter>
This class is used to create a TCP or IPC server.
new net.Server([options][, connectionListener])
#
options
<Object> Seenet.createServer([options][, connectionListener])
.connectionListener
<Function> Automatically set as a listener for the'connection'
event.- Returns: <net.Server>
net.Server
is an EventEmitter
with the following events:
Event: 'close'
#
Emitted when the server closes. If connections exist, this event is not emitted until all connections are ended.
Event: 'connection'
#
- <net.Socket> The connection object
Emitted when a new connection is made. socket
is an instance of
net.Socket
.
Event: 'error'
#
Emitted when an error occurs. Unlike net.Socket
, the 'close'
event will not be emitted directly following this event unless
server.close()
is manually called. See the example in discussion of
server.listen()
.
Event: 'listening'
#
Emitted when the server has been bound after calling server.listen()
.
Event: 'drop'
#
When the number of connections reaches the threshold of server.maxConnections
,
the server will drop new connections and emit 'drop'
event instead. If it is a
TCP server, the argument is as follows, otherwise the argument is undefined
.
data
<Object> The argument passed to event listener.
server.address()
#
Returns the bound address
, the address family
name, and port
of the server
as reported by the operating system if listening on an IP socket
(useful to find which port was assigned when getting an OS-assigned address):
{ port: 12346, family: 'IPv4', address: '127.0.0.1' }
.
For a server listening on a pipe or Unix domain socket, the name is returned as a string.
const server = net.createServer((socket) => {
socket.end('goodbye\n');
}).on('error', (err) => {
// Handle errors here.
throw err;
});
// Grab an arbitrary unused port.
server.listen(() => {
console.log('opened server on', server.address());
});
server.address()
returns null
before the 'listening'
event has been
emitted or after calling server.close()
.
server.close([callback])
#
callback
<Function> Called when the server is closed.- Returns: <net.Server>
Stops the server from accepting new connections and keeps existing
connections. This function is asynchronous, the server is finally closed
when all connections are ended and the server emits a 'close'
event.
The optional callback
will be called once the 'close'
event occurs. Unlike
that event, it will be called with an Error
as its only argument if the server
was not open when it was closed.
server.getConnections(callback)
#
callback
<Function>- Returns: <net.Server>
Asynchronously get the number of concurrent connections on the server. Works when sockets were sent to forks.
Callback should take two arguments err
and count
.
server.listen()
#
Start a server listening for connections. A net.Server
can be a TCP or
an IPC server depending on what it listens to.
Possible signatures:
server.listen(handle[, backlog][, callback])
server.listen(options[, callback])
server.listen(path[, backlog][, callback])
for IPC serversserver.listen([port[, host[, backlog]]][, callback])
for TCP servers
This function is asynchronous. When the server starts listening, the
'listening'
event will be emitted. The last parameter callback
will be added as a listener for the 'listening'
event.
All listen()
methods can take a backlog
parameter to specify the maximum
length of the queue of pending connections. The actual length will be determined
by the OS through sysctl settings such as tcp_max_syn_backlog
and somaxconn
on Linux. The default value of this parameter is 511 (not 512).
All net.Socket
are set to SO_REUSEADDR
(see socket(7)
for
details).
The server.listen()
method can be called again if and only if there was an
error during the first server.listen()
call or server.close()
has been
called. Otherwise, an ERR_SERVER_ALREADY_LISTEN
error will be thrown.
One of the most common errors raised when listening is EADDRINUSE
.
This happens when another server is already listening on the requested
port
/path
/handle
. One way to handle this would be to retry
after a certain amount of time:
server.on('error', (e) => {
if (e.code === 'EADDRINUSE') {
console.error('Address in use, retrying...');
setTimeout(() => {
server.close();
server.listen(PORT, HOST);
}, 1000);
}
});
server.listen(handle[, backlog][, callback])
#
handle
<Object>backlog
<number> Common parameter ofserver.listen()
functionscallback
<Function>- Returns: <net.Server>
Start a server listening for connections on a given handle
that has
already been bound to a port, a Unix domain socket, or a Windows named pipe.
The handle
object can be either a server, a socket (anything with an
underlying _handle
member), or an object with an fd
member that is a
valid file descriptor.
Listening on a file descriptor is not supported on Windows.
server.listen(options[, callback])
#
options
<Object> Required. Supports the following properties:port
<number>host
<string>path
<string> Will be ignored ifport
is specified. See Identifying paths for IPC connections.backlog
<number> Common parameter ofserver.listen()
functions.exclusive
<boolean> Default:false
readableAll
<boolean> For IPC servers makes the pipe readable for all users. Default:false
.writableAll
<boolean> For IPC servers makes the pipe writable for all users. Default:false
.ipv6Only
<boolean> For TCP servers, settingipv6Only
totrue
will disable dual-stack support, i.e., binding to host::
won't make0.0.0.0
be bound. Default:false
.signal
<AbortSignal> An AbortSignal that may be used to close a listening server.
callback
<Function> functions.- Returns: <net.Server>
If port
is specified, it behaves the same as
server.listen([port[, host[, backlog]]][, callback])
.
Otherwise, if path
is specified, it behaves the same as
server.listen(path[, backlog][, callback])
.
If none of them is specified, an error will be thrown.
If exclusive
is false
(default), then cluster workers will use the same
underlying handle, allowing connection handling duties to be shared. When
exclusive
is true
, the handle is not shared, and attempted port sharing
results in an error. An example which listens on an exclusive port is
shown below.
server.listen({
host: 'localhost',
port: 80,
exclusive: true,
});
When exclusive
is true
and the underlying handle is shared, it is
possible that several workers query a handle with different backlogs.
In this case, the first backlog
passed to the master process will be used.
Starting an IPC server as root may cause the server path to be inaccessible for
unprivileged users. Using readableAll
and writableAll
will make the server
accessible for all users.
If the signal
option is enabled, calling .abort()
on the corresponding
AbortController
is similar to calling .close()
on the server:
const controller = new AbortController();
server.listen({
host: 'localhost',
port: 80,
signal: controller.signal,
});
// Later, when you want to close the server.
controller.abort();
server.listen(path[, backlog][, callback])
#
path
<string> Path the server should listen to. See Identifying paths for IPC connections.backlog
<number> Common parameter ofserver.listen()
functions.callback
<Function>.- Returns: <net.Server>
Start an IPC server listening for connections on the given path
.
server.listen([port[, host[, backlog]]][, callback])
#
port
<number>host
<string>backlog
<number> Common parameter ofserver.listen()
functions.callback
<Function>.- Returns: <net.Server>
Start a TCP server listening for connections on the given port
and host
.
If port
is omitted or is 0, the operating system will assign an arbitrary
unused port, which can be retrieved by using server.address().port
after the 'listening'
event has been emitted.
If host
is omitted, the server will accept connections on the
unspecified IPv6 address (::
) when IPv6 is available, or the
unspecified IPv4 address (0.0.0.0
) otherwise.
In most operating systems, listening to the unspecified IPv6 address (::
)
may cause the net.Server
to also listen on the unspecified IPv4 address
(0.0.0.0
).
server.listening
#
- <boolean> Indicates whether or not the server is listening for connections.
server.maxConnections
#
Set this property to reject connections when the server's connection count gets high.
It is not recommended to use this option once a socket has been sent to a child
with child_process.fork()
.
server.ref()
#
- Returns: <net.Server>
Opposite of unref()
, calling ref()
on a previously unref
ed server will
not let the program exit if it's the only server left (the default behavior).
If the server is ref
ed calling ref()
again will have no effect.
server.unref()
#
- Returns: <net.Server>
Calling unref()
on a server will allow the program to exit if this is the only
active server in the event system. If the server is already unref
ed calling
unref()
again will have no effect.
Class: net.Socket
#
- Extends: <stream.Duplex>
This class is an abstraction of a TCP socket or a streaming IPC endpoint
(uses named pipes on Windows, and Unix domain sockets otherwise). It is also
an EventEmitter
.
A net.Socket
can be created by the user and used directly to interact with
a server. For example, it is returned by net.createConnection()
,
so the user can use it to talk to the server.
It can also be created by Node.js and passed to the user when a connection
is received. For example, it is passed to the listeners of a
'connection'
event emitted on a net.Server
, so the user can use
it to interact with the client.
new net.Socket([options])
#
options
<Object> Available options are:fd
<number> If specified, wrap around an existing socket with the given file descriptor, otherwise a new socket will be created.allowHalfOpen
<boolean> If set tofalse
, then the socket will automatically end the writable side when the readable side ends. Seenet.createServer()
and the'end'
event for details. Default:false
.readable
<boolean> Allow reads on the socket when anfd
is passed, otherwise ignored. Default:false
.writable
<boolean> Allow writes on the socket when anfd
is passed, otherwise ignored. Default:false
.signal
<AbortSignal> An Abort signal that may be used to destroy the socket.
- Returns: <net.Socket>
Creates a new socket object.
The newly created socket can be either a TCP socket or a streaming IPC
endpoint, depending on what it connect()
to.
Event: 'close'
#
hadError
<boolean>true
if the socket had a transmission error.
Emitted once the socket is fully closed. The argument hadError
is a boolean
which says if the socket was closed due to a transmission error.
Event: 'connect'
#
Emitted when a socket connection is successfully established.
See net.createConnection()
.
Event: 'data'
#
Emitted when data is received. The argument data
will be a Buffer
or
String
. Encoding of data is set by socket.setEncoding()
.
The data will be lost if there is no listener when a Socket
emits a 'data'
event.
Event: 'drain'
#
Emitted when the write buffer becomes empty. Can be used to throttle uploads.
See also: the return values of socket.write()
.
Event: 'end'
#
Emitted when the other end of the socket signals the end of transmission, thus ending the readable side of the socket.
By default (allowHalfOpen
is false
) the socket will send an end of
transmission packet back and destroy its file descriptor once it has written out
its pending write queue. However, if allowHalfOpen
is set to true
, the
socket will not automatically end()
its writable side,
allowing the user to write arbitrary amounts of data. The user must call
end()
explicitly to close the connection (i.e. sending a
FIN packet back).
Event: 'error'
#
Emitted when an error occurs. The 'close'
event will be called directly
following this event.
Event: 'lookup'
#
Emitted after resolving the host name but before connecting. Not applicable to Unix sockets.
err
<Error> | <null> The error object. Seedns.lookup()
.address
<string> The IP address.family
<number> | <null> The address type. Seedns.lookup()
.host
<string> The host name.
Event: 'ready'
#
Emitted when a socket is ready to be used.
Triggered immediately after 'connect'
.
Event: 'timeout'
#
Emitted if the socket times out from inactivity. This is only to notify that the socket has been idle. The user must manually close the connection.
See also: socket.setTimeout()
.
socket.address()
#
- Returns: <Object>
Returns the bound address
, the address family
name and port
of the
socket as reported by the operating system:
{ port: 12346, family: 'IPv4', address: '127.0.0.1' }
socket.autoSelectFamilyAttemptedAddresses
#
This property is only present if the family autoselection algorithm is enabled in
socket.connect(options)
and it is an array of the addresses that have been attempted.
Each address is a string in the form of $IP:$PORT
. If the connection was successful,
then the last address is the one that the socket is currently connected to.
socket.bufferSize
#
writable.writableLength
instead.This property shows the number of characters buffered for writing. The buffer may contain strings whose length after encoding is not yet known. So this number is only an approximation of the number of bytes in the buffer.
net.Socket
has the property that socket.write()
always works. This is to
help users get up and running quickly. The computer cannot always keep up
with the amount of data that is written to a socket. The network connection
simply might be too slow. Node.js will internally queue up the data written to a
socket and send it out over the wire when it is possible.
The consequence of this internal buffering is that memory may grow.
Users who experience large or growing bufferSize
should attempt to
"throttle" the data flows in their program with
socket.pause()
and socket.resume()
.
socket.bytesRead
#
The amount of received bytes.
socket.bytesWritten
#
The amount of bytes sent.
socket.connect()
#
Initiate a connection on a given socket.
Possible signatures:
socket.connect(options[, connectListener])
socket.connect(path[, connectListener])
for IPC connections.socket.connect(port[, host][, connectListener])
for TCP connections.- Returns: <net.Socket> The socket itself.
This function is asynchronous. When the connection is established, the
'connect'
event will be emitted. If there is a problem connecting,
instead of a 'connect'
event, an 'error'
event will be emitted with
the error passed to the 'error'
listener.
The last parameter connectListener
, if supplied, will be added as a listener
for the 'connect'
event once.
This function should only be used for reconnecting a socket after
'close'
has been emitted or otherwise it may lead to undefined
behavior.
socket.connect(options[, connectListener])
#
options
<Object>connectListener
<Function> Common parameter ofsocket.connect()
methods. Will be added as a listener for the'connect'
event once.- Returns: <net.Socket> The socket itself.
Initiate a connection on a given socket. Normally this method is not needed,
the socket should be created and opened with net.createConnection()
. Use
this only when implementing a custom Socket.
For TCP connections, available options
are:
port
<number> Required. Port the socket should connect to.host
<string> Host the socket should connect to. Default:'localhost'
.localAddress
<string> Local address the socket should connect from.localPort
<number> Local port the socket should connect from.family
<number>: Version of IP stack. Must be4
,6
, or0
. The value0
indicates that both IPv4 and IPv6 addresses are allowed. Default:0
.hints
<number> Optionaldns.lookup()
hints.lookup
<Function> Custom lookup function. Default:dns.lookup()
.noDelay
<boolean> If set totrue
, it disables the use of Nagle's algorithm immediately after the socket is established. Default:false
.keepAlive
<boolean> If set totrue
, it enables keep-alive functionality on the socket immediately after the connection is established, similarly on what is done insocket.setKeepAlive([enable][, initialDelay])
. Default:false
.keepAliveInitialDelay
<number> If set to a positive number, it sets the initial delay before the first keepalive probe is sent on an idle socket.Default:0
.autoSelectFamily
<boolean>: If set totrue
, it enables a family autodetection algorithm that loosely implements section 5 of RFC 8305. Theall
option passed to lookup is set totrue
and the sockets attempts to connect to all obtained IPv6 and IPv4 addresses, in sequence, until a connection is established. The first returned AAAA address is tried first, then the first returned A address, then the second returned AAAA address and so on. Each connection attempt is given the amount of time specified by theautoSelectFamilyAttemptTimeout
option before timing out and trying the next address. Ignored if thefamily
option is not0
or iflocalAddress
is set. Connection errors are not emitted if at least one connection succeeds. Default: initiallyfalse
, but it can be changed at runtime usingnet.setDefaultAutoSelectFamily(value)
or via the command line option--enable-network-family-autoselection
.autoSelectFamilyAttemptTimeout
<number>: The amount of time in milliseconds to wait for a connection attempt to finish before trying the next address when using theautoSelectFamily
option. If set to a positive integer less than10
, then the value10
will be used instead. Default: initially250
, but it can be changed at runtime usingnet.setDefaultAutoSelectFamilyAttemptTimeout(value)
For IPC connections, available options
are:
path
<string> Required. Path the client should connect to. See Identifying paths for IPC connections. If provided, the TCP-specific options above are ignored.
For both types, available options
include:
onread
<Object> If specified, incoming data is stored in a singlebuffer
and passed to the suppliedcallback
when data arrives on the socket. This will cause the streaming functionality to not provide any data. The socket will emit events like'error'
,'end'
, and'close'
as usual. Methods likepause()
andresume()
will also behave as expected.buffer
<Buffer> | <Uint8Array> | <Function> Either a reusable chunk of memory to use for storing incoming data or a function that returns such.callback
<Function> This function is called for every chunk of incoming data. Two arguments are passed to it: the number of bytes written tobuffer
and a reference tobuffer
. Returnfalse
from this function to implicitlypause()
the socket. This function will be executed in the global context.
Following is an example of a client using the onread
option:
const net = require('node:net');
net.connect({
port: 80,
onread: {
// Reuses a 4KiB Buffer for every read from the socket.
buffer: Buffer.alloc(4 * 1024),
callback: function(nread, buf) {
// Received data is available in `buf` from 0 to `nread`.
console.log(buf.toString('utf8', 0, nread));
},
},
});
socket.connect(path[, connectListener])
#
path
<string> Path the client should connect to. See Identifying paths for IPC connections.connectListener
<Function> Common parameter ofsocket.connect()
methods. Will be added as a listener for the'connect'
event once.- Returns: <net.Socket> The socket itself.
Initiate an IPC connection on the given socket.
Alias to
socket.connect(options[, connectListener])
called with { path: path }
as options
.
socket.connect(port[, host][, connectListener])
#
port
<number> Port the client should connect to.host
<string> Host the client should connect to.connectListener
<Function> Common parameter ofsocket.connect()
methods. Will be added as a listener for the'connect'
event once.- Returns: <net.Socket> The socket itself.
Initiate a TCP connection on the given socket.
Alias to
socket.connect(options[, connectListener])
called with {port: port, host: host}
as options
.
socket.connecting
#
If true
,
socket.connect(options[, connectListener])
was
called and has not yet finished. It will stay true
until the socket becomes
connected, then it is set to false
and the 'connect'
event is emitted. Note
that the
socket.connect(options[, connectListener])
callback is a listener for the 'connect'
event.
socket.destroy([error])
#
error
<Object>- Returns: <net.Socket>
Ensures that no more I/O activity happens on this socket. Destroys the stream and closes the connection.
See writable.destroy()
for further details.
socket.destroyed
#
- <boolean> Indicates if the connection is destroyed or not. Once a connection is destroyed no further data can be transferred using it.
See writable.destroyed
for further details.
socket.destroySoon()
#
Destroys the socket after all data is written. If the 'finish'
event was
already emitted the socket is destroyed immediately. If the socket is still
writable it implicitly calls socket.end()
.
socket.end([data[, encoding]][, callback])
#
data
<string> | <Buffer> | <Uint8Array>encoding
<string> Only used when data isstring
. Default:'utf8'
.callback
<Function> Optional callback for when the socket is finished.- Returns: <net.Socket> The socket itself.
Half-closes the socket. i.e., it sends a FIN packet. It is possible the server will still send some data.
See writable.end()
for further details.
socket.localAddress
#
The string representation of the local IP address the remote client is
connecting on. For example, in a server listening on '0.0.0.0'
, if a client
connects on '192.168.1.1'
, the value of socket.localAddress
would be
'192.168.1.1'
.
socket.localPort
#
The numeric representation of the local port. For example, 80
or 21
.
socket.localFamily
#
The string representation of the local IP family. 'IPv4'
or 'IPv6'
.
socket.pause()
#
- Returns: <net.Socket> The socket itself.
Pauses the reading of data. That is, 'data'
events will not be emitted.
Useful to throttle back an upload.
socket.pending
#
This is true
if the socket is not connected yet, either because .connect()
has not yet been called or because it is still in the process of connecting
(see socket.connecting
).
socket.ref()
#
- Returns: <net.Socket> The socket itself.
Opposite of unref()
, calling ref()
on a previously unref
ed socket will
not let the program exit if it's the only socket left (the default behavior).
If the socket is ref
ed calling ref
again will have no effect.
socket.remoteAddress
#
The string representation of the remote IP address. For example,
'74.125.127.100'
or '2001:4860:a005::68'
. Value may be undefined
if
the socket is destroyed (for example, if the client disconnected).
socket.remoteFamily
#
The string representation of the remote IP family. 'IPv4'
or 'IPv6'
.
socket.remotePort
#
The numeric representation of the remote port. For example, 80
or 21
.
socket.resetAndDestroy()
#
- Returns: <net.Socket>
Close the TCP connection by sending an RST packet and destroy the stream.
If this TCP socket is in connecting status, it will send an RST packet and destroy this TCP socket once it is connected.
Otherwise, it will call socket.destroy
with an ERR_SOCKET_CLOSED
Error.
If this is not a TCP socket (for example, a pipe), calling this method will immediately throw an ERR_INVALID_HANDLE_TYPE
Error.
socket.resume()
#
- Returns: <net.Socket> The socket itself.
Resumes reading after a call to socket.pause()
.
socket.setEncoding([encoding])
#
encoding
<string>- Returns: <net.Socket> The socket itself.
Set the encoding for the socket as a Readable Stream. See
readable.setEncoding()
for more information.
socket.setKeepAlive([enable][, initialDelay])
#
enable
<boolean> Default:false
initialDelay
<number> Default:0
- Returns: <net.Socket> The socket itself.
Enable/disable keep-alive functionality, and optionally set the initial delay before the first keepalive probe is sent on an idle socket.
Set initialDelay
(in milliseconds) to set the delay between the last
data packet received and the first keepalive probe. Setting 0
for
initialDelay
will leave the value unchanged from the default
(or previous) setting.
Enabling the keep-alive functionality will set the following socket options:
SO_KEEPALIVE=1
TCP_KEEPIDLE=initialDelay
TCP_KEEPCNT=10
TCP_KEEPINTVL=1
socket.setNoDelay([noDelay])
#
noDelay
<boolean> Default:true
- Returns: <net.Socket> The socket itself.
Enable/disable the use of Nagle's algorithm.
When a TCP connection is created, it will have Nagle's algorithm enabled.
Nagle's algorithm delays data before it is sent via the network. It attempts to optimize throughput at the expense of latency.
Passing true
for noDelay
or not passing an argument will disable Nagle's
algorithm for the socket. Passing false
for noDelay
will enable Nagle's
algorithm.
socket.setTimeout(timeout[, callback])
#
timeout
<number>callback
<Function>- Returns: <net.Socket> The socket itself.
Sets the socket to timeout after timeout
milliseconds of inactivity on
the socket. By default net.Socket
do not have a timeout.
When an idle timeout is triggered the socket will receive a 'timeout'
event but the connection will not be severed. The user must manually call
socket.end()
or socket.destroy()
to end the connection.
socket.setTimeout(3000);
socket.on('timeout', () => {
console.log('socket timeout');
socket.end();
});
If timeout
is 0, then the existing idle timeout is disabled.
The optional callback
parameter will be added as a one-time listener for the
'timeout'
event.
socket.timeout
#
The socket timeout in milliseconds as set by socket.setTimeout()
.
It is undefined
if a timeout has not been set.
socket.unref()
#
- Returns: <net.Socket> The socket itself.
Calling unref()
on a socket will allow the program to exit if this is the only
active socket in the event system. If the socket is already unref
ed calling
unref()
again will have no effect.
socket.write(data[, encoding][, callback])
#
data
<string> | <Buffer> | <Uint8Array>encoding
<string> Only used when data isstring
. Default:utf8
.callback
<Function>- Returns: <boolean>
Sends data on the socket. The second parameter specifies the encoding in the case of a string. It defaults to UTF8 encoding.
Returns true
if the entire data was flushed successfully to the kernel
buffer. Returns false
if all or part of the data was queued in user memory.
'drain'
will be emitted when the buffer is again free.
The optional callback
parameter will be executed when the data is finally
written out, which may not be immediately.
See Writable
stream write()
method for more
information.
socket.readyState
#
This property represents the state of the connection as a string.
- If the stream is connecting
socket.readyState
isopening
. - If the stream is readable and writable, it is
open
. - If the stream is readable and not writable, it is
readOnly
. - If the stream is not readable and writable, it is
writeOnly
.
net.connect()
#
Aliases to
net.createConnection()
.
Possible signatures:
net.connect(options[, connectListener])
net.connect(path[, connectListener])
for IPC connections.net.connect(port[, host][, connectListener])
for TCP connections.
net.connect(options[, connectListener])
#
options
<Object>connectListener
<Function>- Returns: <net.Socket>
Alias to
net.createConnection(options[, connectListener])
.
net.connect(path[, connectListener])
#
path
<string>connectListener
<Function>- Returns: <net.Socket>
Alias to
net.createConnection(path[, connectListener])
.
net.connect(port[, host][, connectListener])
#
port
<number>host
<string>connectListener
<Function>- Returns: <net.Socket>
Alias to
net.createConnection(port[, host][, connectListener])
.
net.createConnection()
#
A factory function, which creates a new net.Socket
,
immediately initiates connection with socket.connect()
,
then returns the net.Socket
that starts the connection.
When the connection is established, a 'connect'
event will be emitted
on the returned socket. The last parameter connectListener
, if supplied,
will be added as a listener for the 'connect'
event once.
Possible signatures:
net.createConnection(options[, connectListener])
net.createConnection(path[, connectListener])
for IPC connections.net.createConnection(port[, host][, connectListener])
for TCP connections.
The net.connect()
function is an alias to this function.
net.createConnection(options[, connectListener])
#
options
<Object> Required. Will be passed to both thenew net.Socket([options])
call and thesocket.connect(options[, connectListener])
method.connectListener
<Function> Common parameter of thenet.createConnection()
functions. If supplied, will be added as a listener for the'connect'
event on the returned socket once.- Returns: <net.Socket> The newly created socket used to start the connection.
For available options, see
new net.Socket([options])
and socket.connect(options[, connectListener])
.
Additional options:
timeout
<number> If set, will be used to callsocket.setTimeout(timeout)
after the socket is created, but before it starts the connection.
Following is an example of a client of the echo server described
in the net.createServer()
section:
const net = require('node:net');
const client = net.createConnection({ port: 8124 }, () => {
// 'connect' listener.
console.log('connected to server!');
client.write('world!\r\n');
});
client.on('data', (data) => {
console.log(data.toString());
client.end();
});
client.on('end', () => {
console.log('disconnected from server');
});
To connect on the socket /tmp/echo.sock
:
const client = net.createConnection({ path: '/tmp/echo.sock' });
net.createConnection(path[, connectListener])
#
path
<string> Path the socket should connect to. Will be passed tosocket.connect(path[, connectListener])
. See Identifying paths for IPC connections.connectListener
<Function> Common parameter of thenet.createConnection()
functions, an "once" listener for the'connect'
event on the initiating socket. Will be passed tosocket.connect(path[, connectListener])
.- Returns: <net.Socket> The newly created socket used to start the connection.
Initiates an IPC connection.
This function creates a new net.Socket
with all options set to default,
immediately initiates connection with
socket.connect(path[, connectListener])
,
then returns the net.Socket
that starts the connection.
net.createConnection(port[, host][, connectListener])
#
port
<number> Port the socket should connect to. Will be passed tosocket.connect(port[, host][, connectListener])
.host
<string> Host the socket should connect to. Will be passed tosocket.connect(port[, host][, connectListener])
. Default:'localhost'
.connectListener
<Function> Common parameter of thenet.createConnection()
functions, an "once" listener for the'connect'
event on the initiating socket. Will be passed tosocket.connect(port[, host][, connectListener])
.- Returns: <net.Socket> The newly created socket used to start the connection.
Initiates a TCP connection.
This function creates a new net.Socket
with all options set to default,
immediately initiates connection with
socket.connect(port[, host][, connectListener])
,
then returns the net.Socket
that starts the connection.
net.createServer([options][, connectionListener])
#
-
options
<Object>allowHalfOpen
<boolean> If set tofalse
, then the socket will automatically end the writable side when the readable side ends. Default:false
.pauseOnConnect
<boolean> Indicates whether the socket should be paused on incoming connections. Default:false
.noDelay
<boolean> If set totrue
, it disables the use of Nagle's algorithm immediately after a new incoming connection is received. Default:false
.keepAlive
<boolean> If set totrue
, it enables keep-alive functionality on the socket immediately after a new incoming connection is received, similarly on what is done insocket.setKeepAlive([enable][, initialDelay])
. Default:false
.keepAliveInitialDelay
<number> If set to a positive number, it sets the initial delay before the first keepalive probe is sent on an idle socket.Default:0
.
-
connectionListener
<Function> Automatically set as a listener for the'connection'
event. -
Returns: <net.Server>
Creates a new TCP or IPC server.
If allowHalfOpen
is set to true
, when the other end of the socket
signals the end of transmission, the server will only send back the end of
transmission when socket.end()
is explicitly called. For example, in the
context of TCP, when a FIN packed is received, a FIN packed is sent
back only when socket.end()
is explicitly called. Until then the
connection is half-closed (non-readable but still writable). See 'end'
event and RFC 1122 (section 4.2.2.13) for more information.
If pauseOnConnect
is set to true
, then the socket associated with each
incoming connection will be paused, and no data will be read from its handle.
This allows connections to be passed between processes without any data being
read by the original process. To begin reading data from a paused socket, call
socket.resume()
.
The server can be a TCP server or an IPC server, depending on what it
listen()
to.
Here is an example of a TCP echo server which listens for connections on port 8124:
const net = require('node:net');
const server = net.createServer((c) => {
// 'connection' listener.
console.log('client connected');
c.on('end', () => {
console.log('client disconnected');
});
c.write('hello\r\n');
c.pipe(c);
});
server.on('error', (err) => {
throw err;
});
server.listen(8124, () => {
console.log('server bound');
});
Test this by using telnet
:
$ telnet localhost 8124
To listen on the socket /tmp/echo.sock
:
server.listen('/tmp/echo.sock', () => {
console.log('server bound');
});
Use nc
to connect to a Unix domain socket server:
$ nc -U /tmp/echo.sock
net.getDefaultAutoSelectFamily()
#
Gets the current default value of the autoSelectFamily
option of socket.connect(options)
.
- Returns: <boolean> The current default value of the
autoSelectFamily
option.
net.setDefaultAutoSelectFamily(value)
#
Sets the default value of the autoSelectFamily
option of socket.connect(options)
.
value
<boolean> The new default value. The initial default value isfalse
.
net.getDefaultAutoSelectFamilyAttemptTimeout()
#
Gets the current default value of the autoSelectFamilyAttemptTimeout
option of socket.connect(options)
.
- Returns: <number> The current default value of the
autoSelectFamilyAttemptTimeout
option.
net.setDefaultAutoSelectFamilyAttemptTimeout(value)
#
Sets the default value of the autoSelectFamilyAttemptTimeout
option of socket.connect(options)
.
value
<number> The new default value, which must be a positive number. If the number is less than10
, the value10
is used instead. The initial default value is250
.
net.isIP(input)
#
Returns 6
if input
is an IPv6 address. Returns 4
if input
is an IPv4
address in dot-decimal notation with no leading zeroes. Otherwise, returns
0
.
net.isIP('::1'); // returns 6
net.isIP('127.0.0.1'); // returns 4
net.isIP('127.000.000.001'); // returns 0
net.isIP('127.0.0.1/24'); // returns 0
net.isIP('fhqwhgads'); // returns 0
net.isIPv4(input)
#
Returns true
if input
is an IPv4 address in dot-decimal notation with no
leading zeroes. Otherwise, returns false
.
net.isIPv4('127.0.0.1'); // returns true
net.isIPv4('127.000.000.001'); // returns false
net.isIPv4('127.0.0.1/24'); // returns false
net.isIPv4('fhqwhgads'); // returns false
net.isIPv6(input)
#
Returns true
if input
is an IPv6 address. Otherwise, returns false
.
net.isIPv6('::1'); // returns true
net.isIPv6('fhqwhgads'); // returns false
OS#
Source Code: lib/os.js
The node:os
module provides operating system-related utility methods and
properties. It can be accessed using:
const os = require('node:os');
os.EOL
#
The operating system-specific end-of-line marker.
\n
on POSIX\r\n
on Windows
os.availableParallelism()
#
- Returns: <integer>
Returns an estimate of the default amount of parallelism a program should use. Always returns a value greater than zero.
This function is a small wrapper about libuv's uv_available_parallelism()
.
os.arch()
#
- Returns: <string>
Returns the operating system CPU architecture for which the Node.js binary was
compiled. Possible values are 'arm'
, 'arm64'
, 'ia32'
, 'mips'
,
'mipsel'
, 'ppc'
, 'ppc64'
, 's390'
, 's390x'
, and 'x64'
.
The return value is equivalent to process.arch
.
os.constants
#
Contains commonly used operating system-specific constants for error codes, process signals, and so on. The specific constants defined are described in OS constants.
os.cpus()
#
- Returns: <Object[]>
Returns an array of objects containing information about each logical CPU core.
The properties included on each object include:
model
<string>speed
<number> (in MHz)times
<Object>user
<number> The number of milliseconds the CPU has spent in user mode.nice
<number> The number of milliseconds the CPU has spent in nice mode.sys
<number> The number of milliseconds the CPU has spent in sys mode.idle
<number> The number of milliseconds the CPU has spent in idle mode.irq
<number> The number of milliseconds the CPU has spent in irq mode.
[
{
model: 'Intel(R) Core(TM) i7 CPU 860 @ 2.80GHz',
speed: 2926,
times: {
user: 252020,
nice: 0,
sys: 30340,
idle: 1070356870,
irq: 0,
},
},
{
model: 'Intel(R) Core(TM) i7 CPU 860 @ 2.80GHz',
speed: 2926,
times: {
user: 306960,
nice: 0,
sys: 26980,
idle: 1071569080,
irq: 0,
},
},
{
model: 'Intel(R) Core(TM) i7 CPU 860 @ 2.80GHz',
speed: 2926,
times: {
user: 248450,
nice: 0,
sys: 21750,
idle: 1070919370,
irq: 0,
},
},
{
model: 'Intel(R) Core(TM) i7 CPU 860 @ 2.80GHz',
speed: 2926,
times: {
user: 256880,
nice: 0,
sys: 19430,
idle: 1070905480,
irq: 20,
},
},
]
nice
values are POSIX-only. On Windows, the nice
values of all processors
are always 0.
os.cpus().length
should not be used to calculate the amount of parallelism
available to an application. Use
os.availableParallelism()
for this purpose.
os.devNull
#
The platform-specific file path of the null device.
\\.\nul
on Windows/dev/null
on POSIX
os.endianness()
#
- Returns: <string>
Returns a string identifying the endianness of the CPU for which the Node.js binary was compiled.
Possible values are 'BE'
for big endian and 'LE'
for little endian.
os.freemem()
#
- Returns: <integer>
Returns the amount of free system memory in bytes as an integer.
os.getPriority([pid])
#
Returns the scheduling priority for the process specified by pid
. If pid
is
not provided or is 0
, the priority of the current process is returned.
os.homedir()
#
- Returns: <string>
Returns the string path of the current user's home directory.
On POSIX, it uses the $HOME
environment variable if defined. Otherwise it
uses the effective UID to look up the user's home directory.
On Windows, it uses the USERPROFILE
environment variable if defined.
Otherwise it uses the path to the profile directory of the current user.
os.hostname()
#
- Returns: <string>
Returns the host name of the operating system as a string.
os.loadavg()
#
- Returns: <number[]>
Returns an array containing the 1, 5, and 15 minute load averages.
The load average is a measure of system activity calculated by the operating system and expressed as a fractional number.
The load average is a Unix-specific concept. On Windows, the return value is
always [0, 0, 0]
.
os.machine()
#
- Returns <string>
Returns the machine type as a string, such as arm
, arm64
, aarch64
,
mips
, mips64
, ppc64
, ppc64le
, s390
, s390x
, i386
, i686
, x86_64
.
On POSIX systems, the machine type is determined by calling
uname(3)
. On Windows, RtlGetVersion()
is used, and if it is not
available, GetVersionExW()
will be used. See
https://en.wikipedia.org/wiki/Uname#Examples for more information.
os.networkInterfaces()
#
- Returns: <Object>
Returns an object containing network interfaces that have been assigned a network address.
Each key on the returned object identifies a network interface. The associated value is an array of objects that each describe an assigned network address.
The properties available on the assigned network address object include:
address
<string> The assigned IPv4 or IPv6 addressnetmask
<string> The IPv4 or IPv6 network maskfamily
<string> EitherIPv4
orIPv6
mac
<string> The MAC address of the network interfaceinternal
<boolean>true
if the network interface is a loopback or similar interface that is not remotely accessible; otherwisefalse
scopeid
<number> The numeric IPv6 scope ID (only specified whenfamily
isIPv6
)cidr
<string> The assigned IPv4 or IPv6 address with the routing prefix in CIDR notation. If thenetmask
is invalid, this property is set tonull
.
{
lo: [
{
address: '127.0.0.1',
netmask: '255.0.0.0',
family: 'IPv4',
mac: '00:00:00:00:00:00',
internal: true,
cidr: '127.0.0.1/8'
},
{
address: '::1',
netmask: 'ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff',
family: 'IPv6',
mac: '00:00:00:00:00:00',
scopeid: 0,
internal: true,
cidr: '::1/128'
}
],
eth0: [
{
address: '192.168.1.108',
netmask: '255.255.255.0',
family: 'IPv4',
mac: '01:02:03:0a:0b:0c',
internal: false,
cidr: '192.168.1.108/24'
},
{
address: 'fe80::a00:27ff:fe4e:66a1',
netmask: 'ffff:ffff:ffff:ffff::',
family: 'IPv6',
mac: '01:02:03:0a:0b:0c',
scopeid: 1,
internal: false,
cidr: 'fe80::a00:27ff:fe4e:66a1/64'
}
]
}
os.platform()
#
- Returns: <string>
Returns a string identifying the operating system platform for which
the Node.js binary was compiled. The value is set at compile time.
Possible values are 'aix'
, 'darwin'
, 'freebsd'
,'linux'
,
'openbsd'
, 'sunos'
, and 'win32'
.
The return value is equivalent to process.platform
.
The value 'android'
may also be returned if Node.js is built on the Android
operating system. Android support is experimental.
os.release()
#
- Returns: <string>
Returns the operating system as a string.
On POSIX systems, the operating system release is determined by calling
uname(3)
. On Windows, GetVersionExW()
is used. See
https://en.wikipedia.org/wiki/Uname#Examples for more information.
os.setPriority([pid, ]priority)
#
pid
<integer> The process ID to set scheduling priority for. Default:0
.priority
<integer> The scheduling priority to assign to the process.
Attempts to set the scheduling priority for the process specified by pid
. If
pid
is not provided or is 0
, the process ID of the current process is used.
The priority
input must be an integer between -20
(high priority) and 19
(low priority). Due to differences between Unix priority levels and Windows
priority classes, priority
is mapped to one of six priority constants in
os.constants.priority
. When retrieving a process priority level, this range
mapping may cause the return value to be slightly different on Windows. To avoid
confusion, set priority
to one of the priority constants.
On Windows, setting priority to PRIORITY_HIGHEST
requires elevated user
privileges. Otherwise the set priority will be silently reduced to
PRIORITY_HIGH
.
os.tmpdir()
#
- Returns: <string>
Returns the operating system's default directory for temporary files as a string.
os.totalmem()
#
- Returns: <integer>
Returns the total amount of system memory in bytes as an integer.
os.type()
#
- Returns: <string>
Returns the operating system name as returned by uname(3)
. For example, it
returns 'Linux'
on Linux, 'Darwin'
on macOS, and 'Windows_NT'
on Windows.
See https://en.wikipedia.org/wiki/Uname#Examples for additional information
about the output of running uname(3)
on various operating systems.
os.uptime()
#
- Returns: <integer>
Returns the system uptime in number of seconds.
os.userInfo([options])
#
options
<Object>encoding
<string> Character encoding used to interpret resulting strings. Ifencoding
is set to'buffer'
, theusername
,shell
, andhomedir
values will beBuffer
instances. Default:'utf8'
.
- Returns: <Object>
Returns information about the currently effective user. On POSIX platforms,
this is typically a subset of the password file. The returned object includes
the username
, uid
, gid
, shell
, and homedir
. On Windows, the uid
and
gid
fields are -1
, and shell
is null
.
The value of homedir
returned by os.userInfo()
is provided by the operating
system. This differs from the result of os.homedir()
, which queries
environment variables for the home directory before falling back to the
operating system response.
Throws a SystemError
if a user has no username
or homedir
.
os.version()
#
- Returns <string>
Returns a string identifying the kernel version.
On POSIX systems, the operating system release is determined by calling
uname(3)
. On Windows, RtlGetVersion()
is used, and if it is not
available, GetVersionExW()
will be used. See
https://en.wikipedia.org/wiki/Uname#Examples for more information.
OS constants#
The following constants are exported by os.constants
.
Not all constants will be available on every operating system.
Signal constants#
The following signal constants are exported by os.constants.signals
.
Constant | Description |
---|---|
SIGHUP |
Sent to indicate when a controlling terminal is closed or a parent process exits. |
SIGINT |
Sent to indicate when a user wishes to interrupt a process (Ctrl+C). |
SIGQUIT |
Sent to indicate when a user wishes to terminate a process and perform a core dump. |
SIGILL |
Sent to a process to notify that it has attempted to perform an illegal, malformed, unknown, or privileged instruction. |
SIGTRAP |
Sent to a process when an exception has occurred. |
SIGABRT |
Sent to a process to request that it abort. |
SIGIOT |
Synonym for SIGABRT |
SIGBUS |
Sent to a process to notify that it has caused a bus error. |
SIGFPE |
Sent to a process to notify that it has performed an illegal arithmetic operation. |
SIGKILL |
Sent to a process to terminate it immediately. |
SIGUSR1 SIGUSR2 |
Sent to a process to identify user-defined conditions. |
SIGSEGV |
Sent to a process to notify of a segmentation fault. |
SIGPIPE |
Sent to a process when it has attempted to write to a disconnected pipe. |
SIGALRM |
Sent to a process when a system timer elapses. |
SIGTERM |
Sent to a process to request termination. |
SIGCHLD |
Sent to a process when a child process terminates. |
SIGSTKFLT |
Sent to a process to indicate a stack fault on a coprocessor. |
SIGCONT |
Sent to instruct the operating system to continue a paused process. |
SIGSTOP |
Sent to instruct the operating system to halt a process. |
SIGTSTP |
Sent to a process to request it to stop. |
SIGBREAK |
Sent to indicate when a user wishes to interrupt a process. |
SIGTTIN |
Sent to a process when it reads from the TTY while in the background. |
SIGTTOU |
Sent to a process when it writes to the TTY while in the background. |
SIGURG |
Sent to a process when a socket has urgent data to read. |
SIGXCPU |
Sent to a process when it has exceeded its limit on CPU usage. |
SIGXFSZ |
Sent to a process when it grows a file larger than the maximum allowed. |
SIGVTALRM |
Sent to a process when a virtual timer has elapsed. |
SIGPROF |
Sent to a process when a system timer has elapsed. |
SIGWINCH |
Sent to a process when the controlling terminal has changed its size. |
SIGIO |
Sent to a process when I/O is available. |
SIGPOLL |
Synonym for SIGIO |
SIGLOST |
Sent to a process when a file lock has been lost. |
SIGPWR |
Sent to a process to notify of a power failure. |
SIGINFO |
Synonym for SIGPWR |
SIGSYS |
Sent to a process to notify of a bad argument. |
SIGUNUSED |
Synonym for SIGSYS |
Error constants#
The following error constants are exported by os.constants.errno
.
POSIX error constants#
Constant | Description |
---|---|
E2BIG |
Indicates that the list of arguments is longer than expected. |
EACCES |
Indicates that the operation did not have sufficient permissions. |
EADDRINUSE |
Indicates that the network address is already in use. |
EADDRNOTAVAIL |
Indicates that the network address is currently unavailable for use. |
EAFNOSUPPORT |
Indicates that the network address family is not supported. |
EAGAIN |
Indicates that there is no data available and to try the operation again later. |
EALREADY |
Indicates that the socket already has a pending connection in progress. |
EBADF |
Indicates that a file descriptor is not valid. |
EBADMSG |
Indicates an invalid data message. |
EBUSY |
Indicates that a device or resource is busy. |
ECANCELED |
Indicates that an operation was canceled. |
ECHILD |
Indicates that there are no child processes. |
ECONNABORTED |
Indicates that the network connection has been aborted. |
ECONNREFUSED |
Indicates that the network connection has been refused. |
ECONNRESET |
Indicates that the network connection has been reset. |
EDEADLK |
Indicates that a resource deadlock has been avoided. |
EDESTADDRREQ |
Indicates that a destination address is required. |
EDOM |
Indicates that an argument is out of the domain of the function. |
EDQUOT |
Indicates that the disk quota has been exceeded. |
EEXIST |
Indicates that the file already exists. |
EFAULT |
Indicates an invalid pointer address. |
EFBIG |
Indicates that the file is too large. |
EHOSTUNREACH |
Indicates that the host is unreachable. |
EIDRM |
Indicates that the identifier has been removed. |
EILSEQ |
Indicates an illegal byte sequence. |
EINPROGRESS |
Indicates that an operation is already in progress. |
EINTR |
Indicates that a function call was interrupted. |
EINVAL |
Indicates that an invalid argument was provided. |
EIO |
Indicates an otherwise unspecified I/O error. |
EISCONN |
Indicates that the socket is connected. |
EISDIR |
Indicates that the path is a directory. |
ELOOP |
Indicates too many levels of symbolic links in a path. |
EMFILE |
Indicates that there are too many open files. |
EMLINK |
Indicates that there are too many hard links to a file. |
EMSGSIZE |
Indicates that the provided message is too long. |
EMULTIHOP |
Indicates that a multihop was attempted. |
ENAMETOOLONG |
Indicates that the filename is too long. |
ENETDOWN |
Indicates that the network is down. |
ENETRESET |
Indicates that the connection has been aborted by the network. |
ENETUNREACH |
Indicates that the network is unreachable. |
ENFILE |
Indicates too many open files in the system. |
ENOBUFS |
Indicates that no buffer space is available. |
ENODATA |
Indicates that no message is available on the stream head read queue. |
ENODEV |
Indicates that there is no such device. |
ENOENT |
Indicates that there is no such file or directory. |
ENOEXEC |
Indicates an exec format error. |
ENOLCK |
Indicates that there are no locks available. |
ENOLINK |
Indications that a link has been severed. |
ENOMEM |
Indicates that there is not enough space. |
ENOMSG |
Indicates that there is no message of the desired type. |
ENOPROTOOPT |
Indicates that a given protocol is not available. |
ENOSPC |
Indicates that there is no space available on the device. |
ENOSR |
Indicates that there are no stream resources available. |
ENOSTR |
Indicates that a given resource is not a stream. |
ENOSYS |
Indicates that a function has not been implemented. |
ENOTCONN |
Indicates that the socket is not connected. |
ENOTDIR |
Indicates that the path is not a directory. |
ENOTEMPTY |
Indicates that the directory is not empty. |
ENOTSOCK |
Indicates that the given item is not a socket. |
ENOTSUP |
Indicates that a given operation is not supported. |
ENOTTY |
Indicates an inappropriate I/O control operation. |
ENXIO |
Indicates no such device or address. |
EOPNOTSUPP |
Indicates that an operation is not supported on the socket. Although
ENOTSUP and EOPNOTSUPP have the same value
on Linux, according to POSIX.1 these error values should be distinct.) |
EOVERFLOW |
Indicates that a value is too large to be stored in a given data type. |
EPERM |
Indicates that the operation is not permitted. |
EPIPE |
Indicates a broken pipe. |
EPROTO |
Indicates a protocol error. |
EPROTONOSUPPORT |
Indicates that a protocol is not supported. |
EPROTOTYPE |
Indicates the wrong type of protocol for a socket. |
ERANGE |
Indicates that the results are too large. |
EROFS |
Indicates that the file system is read only. |
ESPIPE |
Indicates an invalid seek operation. |
ESRCH |
Indicates that there is no such process. |
ESTALE |
Indicates that the file handle is stale. |
ETIME |
Indicates an expired timer. |
ETIMEDOUT |
Indicates that the connection timed out. |
ETXTBSY |
Indicates that a text file is busy. |
EWOULDBLOCK |
Indicates that the operation would block. |
EXDEV |
Indicates an improper link. |
Windows-specific error constants#
The following error codes are specific to the Windows operating system.
Constant | Description |
---|---|
WSAEINTR |
Indicates an interrupted function call. |
WSAEBADF |
Indicates an invalid file handle. |
WSAEACCES |
Indicates insufficient permissions to complete the operation. |
WSAEFAULT |
Indicates an invalid pointer address. |
WSAEINVAL |
Indicates that an invalid argument was passed. |
WSAEMFILE |
Indicates that there are too many open files. |
WSAEWOULDBLOCK |
Indicates that a resource is temporarily unavailable. |
WSAEINPROGRESS |
Indicates that an operation is currently in progress. |
WSAEALREADY |
Indicates that an operation is already in progress. |
WSAENOTSOCK |
Indicates that the resource is not a socket. |
WSAEDESTADDRREQ |
Indicates that a destination address is required. |
WSAEMSGSIZE |
Indicates that the message size is too long. |
WSAEPROTOTYPE |
Indicates the wrong protocol type for the socket. |
WSAENOPROTOOPT |
Indicates a bad protocol option. |
WSAEPROTONOSUPPORT |
Indicates that the protocol is not supported. |
WSAESOCKTNOSUPPORT |
Indicates that the socket type is not supported. |
WSAEOPNOTSUPP |
Indicates that the operation is not supported. |
WSAEPFNOSUPPORT |
Indicates that the protocol family is not supported. |
WSAEAFNOSUPPORT |
Indicates that the address family is not supported. |
WSAEADDRINUSE |
Indicates that the network address is already in use. |
WSAEADDRNOTAVAIL |
Indicates that the network address is not available. |
WSAENETDOWN |
Indicates that the network is down. |
WSAENETUNREACH |
Indicates that the network is unreachable. |
WSAENETRESET |
Indicates that the network connection has been reset. |
WSAECONNABORTED |
Indicates that the connection has been aborted. |
WSAECONNRESET |
Indicates that the connection has been reset by the peer. |
WSAENOBUFS |
Indicates that there is no buffer space available. |
WSAEISCONN |
Indicates that the socket is already connected. |
WSAENOTCONN |
Indicates that the socket is not connected. |
WSAESHUTDOWN |
Indicates that data cannot be sent after the socket has been shutdown. |
WSAETOOMANYREFS |
Indicates that there are too many references. |
WSAETIMEDOUT |
Indicates that the connection has timed out. |
WSAECONNREFUSED |
Indicates that the connection has been refused. |
WSAELOOP |
Indicates that a name cannot be translated. |
WSAENAMETOOLONG |
Indicates that a name was too long. |
WSAEHOSTDOWN |
Indicates that a network host is down. |
WSAEHOSTUNREACH |
Indicates that there is no route to a network host. |
WSAENOTEMPTY |
Indicates that the directory is not empty. |
WSAEPROCLIM |
Indicates that there are too many processes. |
WSAEUSERS |
Indicates that the user quota has been exceeded. |
WSAEDQUOT |
Indicates that the disk quota has been exceeded. |
WSAESTALE |
Indicates a stale file handle reference. |
WSAEREMOTE |
Indicates that the item is remote. |
WSASYSNOTREADY |
Indicates that the network subsystem is not ready. |
WSAVERNOTSUPPORTED |
Indicates that the winsock.dll version is out of
range. |
WSANOTINITIALISED |
Indicates that successful WSAStartup has not yet been performed. |
WSAEDISCON |
Indicates that a graceful shutdown is in progress. |
WSAENOMORE |
Indicates that there are no more results. |
WSAECANCELLED |
Indicates that an operation has been canceled. |
WSAEINVALIDPROCTABLE |
Indicates that the procedure call table is invalid. |
WSAEINVALIDPROVIDER |
Indicates an invalid service provider. |
WSAEPROVIDERFAILEDINIT |
Indicates that the service provider failed to initialized. |
WSASYSCALLFAILURE |
Indicates a system call failure. |
WSASERVICE_NOT_FOUND |
Indicates that a service was not found. |
WSATYPE_NOT_FOUND |
Indicates that a class type was not found. |
WSA_E_NO_MORE |
Indicates that there are no more results. |
WSA_E_CANCELLED |
Indicates that the call was canceled. |
WSAEREFUSED |
Indicates that a database query was refused. |
dlopen constants#
If available on the operating system, the following constants
are exported in os.constants.dlopen
. See dlopen(3)
for detailed
information.
Constant | Description |
---|---|
RTLD_LAZY |
Perform lazy binding. Node.js sets this flag by default. |
RTLD_NOW |
Resolve all undefined symbols in the library before dlopen(3) returns. |
RTLD_GLOBAL |
Symbols defined by the library will be made available for symbol resolution of subsequently loaded libraries. |
RTLD_LOCAL |
The converse of RTLD_GLOBAL . This is the default behavior
if neither flag is specified. |
RTLD_DEEPBIND |
Make a self-contained library use its own symbols in preference to symbols from previously loaded libraries. |
Priority constants#
The following process scheduling constants are exported by
os.constants.priority
.
Constant | Description |
---|---|
PRIORITY_LOW |
The lowest process scheduling priority. This corresponds to
IDLE_PRIORITY_CLASS on Windows, and a nice value of
19 on all other platforms. |
PRIORITY_BELOW_NORMAL |
The process scheduling priority above PRIORITY_LOW and
below PRIORITY_NORMAL . This corresponds to
BELOW_NORMAL_PRIORITY_CLASS on Windows, and a nice value of
10 on all other platforms. |
PRIORITY_NORMAL |
The default process scheduling priority. This corresponds to
NORMAL_PRIORITY_CLASS on Windows, and a nice value of
0 on all other platforms. |
PRIORITY_ABOVE_NORMAL |
The process scheduling priority above PRIORITY_NORMAL and
below PRIORITY_HIGH . This corresponds to
ABOVE_NORMAL_PRIORITY_CLASS on Windows, and a nice value of
-7 on all other platforms. |
PRIORITY_HIGH |
The process scheduling priority above PRIORITY_ABOVE_NORMAL
and below PRIORITY_HIGHEST . This corresponds to
HIGH_PRIORITY_CLASS on Windows, and a nice value of
-14 on all other platforms. |
PRIORITY_HIGHEST |
The highest process scheduling priority. This corresponds to
REALTIME_PRIORITY_CLASS on Windows, and a nice value of
-20 on all other platforms. |
libuv constants#
Constant | Description |
---|---|
UV_UDP_REUSEADDR |
Path#
Source Code: lib/path.js
The node:path
module provides utilities for working with file and directory
paths. It can be accessed using:
const path = require('node:path');
Windows vs. POSIX#
The default operation of the node:path
module varies based on the operating
system on which a Node.js application is running. Specifically, when running on
a Windows operating system, the node:path
module will assume that
Windows-style paths are being used.
So using path.basename()
might yield different results on POSIX and Windows:
On POSIX:
path.basename('C:\\temp\\myfile.html');
// Returns: 'C:\\temp\\myfile.html'
On Windows:
path.basename('C:\\temp\\myfile.html');
// Returns: 'myfile.html'
To achieve consistent results when working with Windows file paths on any
operating system, use path.win32
:
On POSIX and Windows:
path.win32.basename('C:\\temp\\myfile.html');
// Returns: 'myfile.html'
To achieve consistent results when working with POSIX file paths on any
operating system, use path.posix
:
On POSIX and Windows:
path.posix.basename('/tmp/myfile.html');
// Returns: 'myfile.html'
On Windows Node.js follows the concept of per-drive working directory.
This behavior can be observed when using a drive path without a backslash. For
example, path.resolve('C:\\')
can potentially return a different result than
path.resolve('C:')
. For more information, see
this MSDN page.
path.basename(path[, suffix])
#
The path.basename()
method returns the last portion of a path
, similar to
the Unix basename
command. Trailing directory separators are
ignored.
path.basename('/foo/bar/baz/asdf/quux.html');
// Returns: 'quux.html'
path.basename('/foo/bar/baz/asdf/quux.html', '.html');
// Returns: 'quux'
Although Windows usually treats file names, including file extensions, in a
case-insensitive manner, this function does not. For example, C:\\foo.html
and
C:\\foo.HTML
refer to the same file, but basename
treats the extension as a
case-sensitive string:
path.win32.basename('C:\\foo.html', '.html');
// Returns: 'foo'
path.win32.basename('C:\\foo.HTML', '.html');
// Returns: 'foo.HTML'
A TypeError
is thrown if path
is not a string or if suffix
is given
and is not a string.
path.delimiter
#
Provides the platform-specific path delimiter:
;
for Windows:
for POSIX
For example, on POSIX:
console.log(process.env.PATH);
// Prints: '/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin'
process.env.PATH.split(path.delimiter);
// Returns: ['/usr/bin', '/bin', '/usr/sbin', '/sbin', '/usr/local/bin']
On Windows:
console.log(process.env.PATH);
// Prints: 'C:\Windows\system32;C:\Windows;C:\Program Files\node\'
process.env.PATH.split(path.delimiter);
// Returns ['C:\\Windows\\system32', 'C:\\Windows', 'C:\\Program Files\\node\\']
path.dirname(path)
#
The path.dirname()
method returns the directory name of a path
, similar to
the Unix dirname
command. Trailing directory separators are ignored, see
path.sep
.
path.dirname('/foo/bar/baz/asdf/quux');
// Returns: '/foo/bar/baz/asdf'
A TypeError
is thrown if path
is not a string.
path.extname(path)
#
The path.extname()
method returns the extension of the path
, from the last
occurrence of the .
(period) character to end of string in the last portion of
the path
. If there is no .
in the last portion of the path
, or if
there are no .
characters other than the first character of
the basename of path
(see path.basename()
) , an empty string is returned.
path.extname('index.html');
// Returns: '.html'
path.extname('index.coffee.md');
// Returns: '.md'
path.extname('index.');
// Returns: '.'
path.extname('index');
// Returns: ''
path.extname('.index');
// Returns: ''
path.extname('.index.md');
// Returns: '.md'
A TypeError
is thrown if path
is not a string.
path.format(pathObject)
#
The path.format()
method returns a path string from an object. This is the
opposite of path.parse()
.
When providing properties to the pathObject
remember that there are
combinations where one property has priority over another:
pathObject.root
is ignored ifpathObject.dir
is providedpathObject.ext
andpathObject.name
are ignored ifpathObject.base
exists
For example, on POSIX:
// If `dir`, `root` and `base` are provided,
// `${dir}${path.sep}${base}`
// will be returned. `root` is ignored.
path.format({
root: '/ignored',
dir: '/home/user/dir',
base: 'file.txt',
});
// Returns: '/home/user/dir/file.txt'
// `root` will be used if `dir` is not specified.
// If only `root` is provided or `dir` is equal to `root` then the
// platform separator will not be included. `ext` will be ignored.
path.format({
root: '/',
base: 'file.txt',
ext: 'ignored',
});
// Returns: '/file.txt'
// `name` + `ext` will be used if `base` is not specified.
path.format({
root: '/',
name: 'file',
ext: '.txt',
});
// Returns: '/file.txt'
// The dot will be added if it is not specified in `ext`.
path.format({
root: '/',
name: 'file',
ext: 'txt',
});
// Returns: '/file.txt'
On Windows:
path.format({
dir: 'C:\\path\\dir',
base: 'file.txt',
});
// Returns: 'C:\\path\\dir\\file.txt'
path.isAbsolute(path)
#
The path.isAbsolute()
method determines if path
is an absolute path.
If the given path
is a zero-length string, false
will be returned.
For example, on POSIX:
path.isAbsolute('/foo/bar'); // true
path.isAbsolute('/baz/..'); // true
path.isAbsolute('qux/'); // false
path.isAbsolute('.'); // false
On Windows:
path.isAbsolute('//server'); // true
path.isAbsolute('\\\\server'); // true
path.isAbsolute('C:/foo/..'); // true
path.isAbsolute('C:\\foo\\..'); // true
path.isAbsolute('bar\\baz'); // false
path.isAbsolute('bar/baz'); // false
path.isAbsolute('.'); // false
A TypeError
is thrown if path
is not a string.
path.join([...paths])
#
The path.join()
method joins all given path
segments together using the
platform-specific separator as a delimiter, then normalizes the resulting path.
Zero-length path
segments are ignored. If the joined path string is a
zero-length string then '.'
will be returned, representing the current
working directory.
path.join('/foo', 'bar', 'baz/asdf', 'quux', '..');
// Returns: '/foo/bar/baz/asdf'
path.join('foo', {}, 'bar');
// Throws 'TypeError: Path must be a string. Received {}'
A TypeError
is thrown if any of the path segments is not a string.
path.normalize(path)
#
The path.normalize()
method normalizes the given path
, resolving '..'
and
'.'
segments.
When multiple, sequential path segment separation characters are found (e.g.
/
on POSIX and either \
or /
on Windows), they are replaced by a single
instance of the platform-specific path segment separator (/
on POSIX and
\
on Windows). Trailing separators are preserved.
If the path
is a zero-length string, '.'
is returned, representing the
current working directory.
For example, on POSIX:
path.normalize('/foo/bar//baz/asdf/quux/..');
// Returns: '/foo/bar/baz/asdf'
On Windows:
path.normalize('C:\\temp\\\\foo\\bar\\..\\');
// Returns: 'C:\\temp\\foo\\'
Since Windows recognizes multiple path separators, both separators will be
replaced by instances of the Windows preferred separator (\
):
path.win32.normalize('C:////temp\\\\/\\/\\/foo/bar');
// Returns: 'C:\\temp\\foo\\bar'
A TypeError
is thrown if path
is not a string.
path.parse(path)
#
The path.parse()
method returns an object whose properties represent
significant elements of the path
. Trailing directory separators are ignored,
see path.sep
.
The returned object will have the following properties:
For example, on POSIX:
path.parse('/home/user/dir/file.txt');
// Returns:
// { root: '/',
// dir: '/home/user/dir',
// base: 'file.txt',
// ext: '.txt',
// name: 'file' }
┌─────────────────────┬────────────┐
│ dir │ base │
├──────┬ ├──────┬─────┤
│ root │ │ name │ ext │
" / home/user/dir / file .txt "
└──────┴──────────────┴──────┴─────┘
(All spaces in the "" line should be ignored. They are purely for formatting.)
On Windows:
path.parse('C:\\path\\dir\\file.txt');
// Returns:
// { root: 'C:\\',
// dir: 'C:\\path\\dir',
// base: 'file.txt',
// ext: '.txt',
// name: 'file' }
┌─────────────────────┬────────────┐
│ dir │ base │
├──────┬ ├──────┬─────┤
│ root │ │ name │ ext │
" C:\ path\dir \ file .txt "
└──────┴──────────────┴──────┴─────┘
(All spaces in the "" line should be ignored. They are purely for formatting.)
A TypeError
is thrown if path
is not a string.
path.posix
#
The path.posix
property provides access to POSIX specific implementations
of the path
methods.
The API is accessible via require('node:path').posix
or require('node:path/posix')
.
path.relative(from, to)
#
The path.relative()
method returns the relative path from from
to to
based
on the current working directory. If from
and to
each resolve to the same
path (after calling path.resolve()
on each), a zero-length string is returned.
If a zero-length string is passed as from
or to
, the current working
directory will be used instead of the zero-length strings.
For example, on POSIX:
path.relative('/data/orandea/test/aaa', '/data/orandea/impl/bbb');
// Returns: '../../impl/bbb'
On Windows:
path.relative('C:\\orandea\\test\\aaa', 'C:\\orandea\\impl\\bbb');
// Returns: '..\\..\\impl\\bbb'
A TypeError
is thrown if either from
or to
is not a string.
path.resolve([...paths])
#
The path.resolve()
method resolves a sequence of paths or path segments into
an absolute path.
The given sequence of paths is processed from right to left, with each
subsequent path
prepended until an absolute path is constructed.
For instance, given the sequence of path segments: /foo
, /bar
, baz
,
calling path.resolve('/foo', '/bar', 'baz')
would return /bar/baz
because 'baz'
is not an absolute path but '/bar' + '/' + 'baz'
is.
If, after processing all given path
segments, an absolute path has not yet
been generated, the current working directory is used.
The resulting path is normalized and trailing slashes are removed unless the path is resolved to the root directory.
Zero-length path
segments are ignored.
If no path
segments are passed, path.resolve()
will return the absolute path
of the current working directory.
path.resolve('/foo/bar', './baz');
// Returns: '/foo/bar/baz'
path.resolve('/foo/bar', '/tmp/file/');
// Returns: '/tmp/file'
path.resolve('wwwroot', 'static_files/png/', '../gif/image.gif');
// If the current working directory is /home/myself/node,
// this returns '/home/myself/node/wwwroot/static_files/gif/image.gif'
A TypeError
is thrown if any of the arguments is not a string.
path.sep
#
Provides the platform-specific path segment separator:
\
on Windows/
on POSIX
For example, on POSIX:
'foo/bar/baz'.split(path.sep);
// Returns: ['foo', 'bar', 'baz']
On Windows:
'foo\\bar\\baz'.split(path.sep);
// Returns: ['foo', 'bar', 'baz']
On Windows, both the forward slash (/
) and backward slash (\
) are accepted
as path segment separators; however, the path
methods only add backward
slashes (\
).
path.toNamespacedPath(path)
#
On Windows systems only, returns an equivalent namespace-prefixed path for
the given path
. If path
is not a string, path
will be returned without
modifications.
This method is meaningful only on Windows systems. On POSIX systems, the
method is non-operational and always returns path
without modifications.
path.win32
#
The path.win32
property provides access to Windows-specific implementations
of the path
methods.
The API is accessible via require('node:path').win32
or require('node:path/win32')
.
Performance measurement APIs#
Source Code: lib/perf_hooks.js
This module provides an implementation of a subset of the W3C Web Performance APIs as well as additional APIs for Node.js-specific performance measurements.
Node.js supports the following Web Performance APIs:
const { PerformanceObserver, performance } = require('node:perf_hooks');
const obs = new PerformanceObserver((items) => {
console.log(items.getEntries()[0].duration);
performance.clearMarks();
});
obs.observe({ type: 'measure' });
performance.measure('Start to Now');
performance.mark('A');
doSomeLongRunningProcess(() => {
performance.measure('A to Now', 'A');
performance.mark('B');
performance.measure('A to B', 'A', 'B');
});
perf_hooks.performance
#
An object that can be used to collect performance metrics from the current
Node.js instance. It is similar to window.performance
in browsers.
performance.clearMarks([name])
#
name
<string>
If name
is not provided, removes all PerformanceMark
objects from the
Performance Timeline. If name
is provided, removes only the named mark.
performance.clearMeasures([name])
#
name
<string>
If name
is not provided, removes all PerformanceMeasure
objects from the
Performance Timeline. If name
is provided, removes only the named measure.
performance.clearResourceTimings([name])
#
name
<string>
If name
is not provided, removes all PerformanceResourceTiming
objects from
the Resource Timeline. If name
is provided, removes only the named resource.
performance.eventLoopUtilization([utilization1[, utilization2]])
#
utilization1
<Object> The result of a previous call toeventLoopUtilization()
.utilization2
<Object> The result of a previous call toeventLoopUtilization()
prior toutilization1
.- Returns <Object>
The eventLoopUtilization()
method returns an object that contains the
cumulative duration of time the event loop has been both idle and active as a
high resolution milliseconds timer. The utilization
value is the calculated
Event Loop Utilization (ELU).
If bootstrapping has not yet finished on the main thread the properties have
the value of 0
. The ELU is immediately available on Worker threads since
bootstrap happens within the event loop.
Both utilization1
and utilization2
are optional parameters.
If utilization1
is passed, then the delta between the current call's active
and idle
times, as well as the corresponding utilization
value are
calculated and returned (similar to process.hrtime()
).
If utilization1
and utilization2
are both passed, then the delta is
calculated between the two arguments. This is a convenience option because,
unlike process.hrtime()
, calculating the ELU is more complex than a
single subtraction.
ELU is similar to CPU utilization, except that it only measures event loop
statistics and not CPU usage. It represents the percentage of time the event
loop has spent outside the event loop's event provider (e.g. epoll_wait
).
No other CPU idle time is taken into consideration. The following is an example
of how a mostly idle process will have a high ELU.
'use strict';
const { eventLoopUtilization } = require('node:perf_hooks').performance;
const { spawnSync } = require('node:child_process');
setImmediate(() => {
const elu = eventLoopUtilization();
spawnSync('sleep', ['5']);
console.log(eventLoopUtilization(elu).utilization);
});
Although the CPU is mostly idle while running this script, the value of
utilization
is 1
. This is because the call to
child_process.spawnSync()
blocks the event loop from proceeding.
Passing in a user-defined object instead of the result of a previous call to
eventLoopUtilization()
will lead to undefined behavior. The return values
are not guaranteed to reflect any correct state of the event loop.
performance.getEntries()
#
- Returns: <PerformanceEntry[]>
Returns a list of PerformanceEntry
objects in chronological order with
respect to performanceEntry.startTime
. If you are only interested in
performance entries of certain types or that have certain names, see
performance.getEntriesByType()
and performance.getEntriesByName()
.
performance.getEntriesByName(name[, type])
#
name
<string>type
<string>- Returns: <PerformanceEntry[]>
Returns a list of PerformanceEntry
objects in chronological order
with respect to performanceEntry.startTime
whose performanceEntry.name
is
equal to name
, and optionally, whose performanceEntry.entryType
is equal to
type
.
performance.getEntriesByType(type)
#
type
<string>- Returns: <PerformanceEntry[]>
Returns a list of PerformanceEntry
objects in chronological order
with respect to performanceEntry.startTime
whose performanceEntry.entryType
is equal to type
.
performance.mark(name[, options])
#
Creates a new PerformanceMark
entry in the Performance Timeline. A
PerformanceMark
is a subclass of PerformanceEntry
whose
performanceEntry.entryType
is always 'mark'
, and whose
performanceEntry.duration
is always 0
. Performance marks are used
to mark specific significant moments in the Performance Timeline.
The created PerformanceMark
entry is put in the global Performance Timeline
and can be queried with performance.getEntries
,
performance.getEntriesByName
, and performance.getEntriesByType
. When the
observation is performed, the entries should be cleared from the global
Performance Timeline manually with performance.clearMarks
.
performance.markResourceTiming(timingInfo, requestedUrl, initiatorType, global, cacheMode)
#
timingInfo
<Object> Fetch Timing InforequestedUrl
<string> The resource urlinitiatorType
<string> The initiator name, e.g: 'fetch'global
<Object>cacheMode
<string> The cache mode must be an empty string ('') or 'local'
This property is an extension by Node.js. It is not available in Web browsers.
Creates a new PerformanceResourceTiming
entry in the Resource Timeline. A
PerformanceResourceTiming
is a subclass of PerformanceEntry
whose
performanceEntry.entryType
is always 'resource'
. Performance resources
are used to mark moments in the Resource Timeline.
The created PerformanceMark
entry is put in the global Resource Timeline
and can be queried with performance.getEntries
,
performance.getEntriesByName
, and performance.getEntriesByType
. When the
observation is performed, the entries should be cleared from the global
Performance Timeline manually with performance.clearResourceTimings
.
performance.measure(name[, startMarkOrOptions[, endMark]])
#
name
<string>startMarkOrOptions
<string> | <Object> Optional.detail
<any> Additional optional detail to include with the measure.duration
<number> Duration between start and end times.end
<number> | <string> Timestamp to be used as the end time, or a string identifying a previously recorded mark.start
<number> | <string> Timestamp to be used as the start time, or a string identifying a previously recorded mark.
endMark
<string> Optional. Must be omitted ifstartMarkOrOptions
is an <Object>.
Creates a new PerformanceMeasure
entry in the Performance Timeline. A
PerformanceMeasure
is a subclass of PerformanceEntry
whose
performanceEntry.entryType
is always 'measure'
, and whose
performanceEntry.duration
measures the number of milliseconds elapsed since
startMark
and endMark
.
The startMark
argument may identify any existing PerformanceMark
in the
Performance Timeline, or may identify any of the timestamp properties
provided by the PerformanceNodeTiming
class. If the named startMark
does
not exist, an error is thrown.
The optional endMark
argument must identify any existing PerformanceMark
in the Performance Timeline or any of the timestamp properties provided by the
PerformanceNodeTiming
class. endMark
will be performance.now()
if no parameter is passed, otherwise if the named endMark
does not exist, an
error will be thrown.
The created PerformanceMeasure
entry is put in the global Performance Timeline
and can be queried with performance.getEntries
,
performance.getEntriesByName
, and performance.getEntriesByType
. When the
observation is performed, the entries should be cleared from the global
Performance Timeline manually with performance.clearMeasures
.
performance.nodeTiming
#
This property is an extension by Node.js. It is not available in Web browsers.
An instance of the PerformanceNodeTiming
class that provides performance
metrics for specific Node.js operational milestones.
performance.now()
#
- Returns: <number>
Returns the current high resolution millisecond timestamp, where 0 represents
the start of the current node
process.
performance.setResourceTimingBufferSize(maxSize)
#
Sets the global performance resource timing buffer size to the specified number of "resource" type performance entry objects.
By default the max buffer size is set to 250.
performance.timeOrigin
#
The timeOrigin
specifies the high resolution millisecond timestamp at
which the current node
process began, measured in Unix time.
performance.timerify(fn[, options])
#
fn
<Function>options
<Object>histogram
<RecordableHistogram> A histogram object created usingperf_hooks.createHistogram()
that will record runtime durations in nanoseconds.
This property is an extension by Node.js. It is not available in Web browsers.
Wraps a function within a new function that measures the running time of the
wrapped function. A PerformanceObserver
must be subscribed to the 'function'
event type in order for the timing details to be accessed.
const {
performance,
PerformanceObserver,
} = require('node:perf_hooks');
function someFunction() {
console.log('hello world');
}
const wrapped = performance.timerify(someFunction);
const obs = new PerformanceObserver((list) => {
console.log(list.getEntries()[0].duration);
performance.clearMarks();
performance.clearMeasures();
obs.disconnect();
});
obs.observe({ entryTypes: ['function'] });
// A performance timeline entry will be created
wrapped();
If the wrapped function returns a promise, a finally handler will be attached to the promise and the duration will be reported once the finally handler is invoked.
performance.toJSON()
#
An object which is JSON representation of the performance
object. It
is similar to window.performance.toJSON
in browsers.
Event: 'resourcetimingbufferfull'
#
The 'resourcetimingbufferfull'
event is fired when the global performance
resource timing buffer is full. Adjust resource timing buffer size with
performance.setResourceTimingBufferSize()
or clear the buffer with
performance.clearResourceTimings()
in the event listener to allow
more entries to be added to the performance timeline buffer.
Class: PerformanceEntry
#
The constructor of this class is not exposed to users directly.
performanceEntry.duration
#
The total number of milliseconds elapsed for this entry. This value will not be meaningful for all Performance Entry types.
performanceEntry.entryType
#
The type of the performance entry. It may be one of:
'node'
(Node.js only)'mark'
(available on the Web)'measure'
(available on the Web)'gc'
(Node.js only)'function'
(Node.js only)'http2'
(Node.js only)'http'
(Node.js only)
performanceEntry.name
#
The name of the performance entry.
performanceEntry.startTime
#
The high resolution millisecond timestamp marking the starting time of the Performance Entry.
Class: PerformanceMark
#
- Extends: <PerformanceEntry>
Exposes marks created via the Performance.mark()
method.
performanceMark.detail
#
Additional detail specified when creating with Performance.mark()
method.
Class: PerformanceMeasure
#
- Extends: <PerformanceEntry>
Exposes measures created via the Performance.measure()
method.
The constructor of this class is not exposed to users directly.
performanceMeasure.detail
#
Additional detail specified when creating with Performance.measure()
method.
Class: PerformanceNodeEntry
#
- Extends: <PerformanceEntry>
This class is an extension by Node.js. It is not available in Web browsers.
Provides detailed Node.js timing data.
The constructor of this class is not exposed to users directly.
performanceNodeEntry.detail
#
Additional detail specific to the entryType
.
performanceNodeEntry.flags
#
performanceNodeEntry.detail
instead.When performanceEntry.entryType
is equal to 'gc'
, the performance.flags
property contains additional information about garbage collection operation.
The value may be one of:
perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_NO
perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_CONSTRUCT_RETAINED
perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_FORCED
perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_SYNCHRONOUS_PHANTOM_PROCESSING
perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_ALL_AVAILABLE_GARBAGE
perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_ALL_EXTERNAL_MEMORY
perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_SCHEDULE_IDLE
performanceNodeEntry.kind
#
performanceNodeEntry.detail
instead.When performanceEntry.entryType
is equal to 'gc'
, the performance.kind
property identifies the type of garbage collection operation that occurred.
The value may be one of:
perf_hooks.constants.NODE_PERFORMANCE_GC_MAJOR
perf_hooks.constants.NODE_PERFORMANCE_GC_MINOR
perf_hooks.constants.NODE_PERFORMANCE_GC_INCREMENTAL
perf_hooks.constants.NODE_PERFORMANCE_GC_WEAKCB
Garbage Collection ('gc') Details#
When performanceEntry.type
is equal to 'gc'
, the
performanceNodeEntry.detail
property will be an <Object> with two properties:
kind
<number> One of:perf_hooks.constants.NODE_PERFORMANCE_GC_MAJOR
perf_hooks.constants.NODE_PERFORMANCE_GC_MINOR
perf_hooks.constants.NODE_PERFORMANCE_GC_INCREMENTAL
perf_hooks.constants.NODE_PERFORMANCE_GC_WEAKCB
flags
<number> One of:perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_NO
perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_CONSTRUCT_RETAINED
perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_FORCED
perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_SYNCHRONOUS_PHANTOM_PROCESSING
perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_ALL_AVAILABLE_GARBAGE
perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_ALL_EXTERNAL_MEMORY
perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_SCHEDULE_IDLE
HTTP ('http') Details#
When performanceEntry.type
is equal to 'http'
, the
performanceNodeEntry.detail
property will be an <Object> containing
additional information.
If performanceEntry.name
is equal to HttpClient
, the detail
will contain the following properties: req
, res
. And the req
property
will be an <Object> containing method
, url
, headers
, the res
property
will be an <Object> containing statusCode
, statusMessage
, headers
.
If performanceEntry.name
is equal to HttpRequest
, the detail
will contain the following properties: req
, res
. And the req
property
will be an <Object> containing method
, url
, headers
, the res
property
will be an <Object> containing statusCode
, statusMessage
, headers
.
This could add additional memory overhead and should only be used for diagnostic purposes, not left turned on in production by default.
HTTP/2 ('http2') Details#
When performanceEntry.type
is equal to 'http2'
, the
performanceNodeEntry.detail
property will be an <Object> containing
additional performance information.
If performanceEntry.name
is equal to Http2Stream
, the detail
will contain the following properties:
bytesRead
<number> The number ofDATA
frame bytes received for thisHttp2Stream
.bytesWritten
<number> The number ofDATA
frame bytes sent for thisHttp2Stream
.id
<number> The identifier of the associatedHttp2Stream
timeToFirstByte
<number> The number of milliseconds elapsed between thePerformanceEntry
startTime
and the reception of the firstDATA
frame.timeToFirstByteSent
<number> The number of milliseconds elapsed between thePerformanceEntry
startTime
and sending of the firstDATA
frame.timeToFirstHeader
<number> The number of milliseconds elapsed between thePerformanceEntry
startTime
and the reception of the first header.
If performanceEntry.name
is equal to Http2Session
, the detail
will
contain the following properties:
bytesRead
<number> The number of bytes received for thisHttp2Session
.bytesWritten
<number> The number of bytes sent for thisHttp2Session
.framesReceived
<number> The number of HTTP/2 frames received by theHttp2Session
.framesSent
<number> The number of HTTP/2 frames sent by theHttp2Session
.maxConcurrentStreams
<number> The maximum number of streams concurrently open during the lifetime of theHttp2Session
.pingRTT
<number> The number of milliseconds elapsed since the transmission of aPING
frame and the reception of its acknowledgment. Only present if aPING
frame has been sent on theHttp2Session
.streamAverageDuration
<number> The average duration (in milliseconds) for allHttp2Stream
instances.streamCount
<number> The number ofHttp2Stream
instances processed by theHttp2Session
.type
<string> Either'server'
or'client'
to identify the type ofHttp2Session
.
Timerify ('function') Details#
When performanceEntry.type
is equal to 'function'
, the
performanceNodeEntry.detail
property will be an <Array> listing
the input arguments to the timed function.
Net ('net') Details#
When performanceEntry.type
is equal to 'net'
, the
performanceNodeEntry.detail
property will be an <Object> containing
additional information.
If performanceEntry.name
is equal to connect
, the detail
will contain the following properties: host
, port
.
DNS ('dns') Details#
When performanceEntry.type
is equal to 'dns'
, the
performanceNodeEntry.detail
property will be an <Object> containing
additional information.
If performanceEntry.name
is equal to lookup
, the detail
will contain the following properties: hostname
, family
, hints
, verbatim
,
addresses
.
If performanceEntry.name
is equal to lookupService
, the detail
will
contain the following properties: host
, port
, hostname
, service
.
If performanceEntry.name
is equal to queryxxx
or getHostByAddr
, the detail
will
contain the following properties: host
, ttl
, result
. The value of result
is
same as the result of queryxxx
or getHostByAddr
.
Class: PerformanceNodeTiming
#
- Extends: <PerformanceEntry>
This property is an extension by Node.js. It is not available in Web browsers.
Provides timing details for Node.js itself. The constructor of this class is not exposed to users.
performanceNodeTiming.bootstrapComplete
#
The high resolution millisecond timestamp at which the Node.js process completed bootstrapping. If bootstrapping has not yet finished, the property has the value of -1.
performanceNodeTiming.environment
#
The high resolution millisecond timestamp at which the Node.js environment was initialized.
performanceNodeTiming.idleTime
#
The high resolution millisecond timestamp of the amount of time the event loop
has been idle within the event loop's event provider (e.g. epoll_wait
). This
does not take CPU usage into consideration. If the event loop has not yet
started (e.g., in the first tick of the main script), the property has the
value of 0.
performanceNodeTiming.loopExit
#
The high resolution millisecond timestamp at which the Node.js event loop
exited. If the event loop has not yet exited, the property has the value of -1.
It can only have a value of not -1 in a handler of the 'exit'
event.
performanceNodeTiming.loopStart
#
The high resolution millisecond timestamp at which the Node.js event loop started. If the event loop has not yet started (e.g., in the first tick of the main script), the property has the value of -1.
performanceNodeTiming.nodeStart
#
The high resolution millisecond timestamp at which the Node.js process was initialized.
performanceNodeTiming.v8Start
#
The high resolution millisecond timestamp at which the V8 platform was initialized.
Class: PerformanceResourceTiming
#
- Extends: <PerformanceEntry>
Provides detailed network timing data regarding the loading of an application's resources.
The constructor of this class is not exposed to users directly.
performanceResourceTiming.workerStart
#
The high resolution millisecond timestamp at immediately before dispatching
the fetch
request. If the resource is not intercepted by a worker the property
will always return 0.
performanceResourceTiming.redirectStart
#
The high resolution millisecond timestamp that represents the start time of the fetch which initiates the redirect.
performanceResourceTiming.redirectEnd
#
The high resolution millisecond timestamp that will be created immediately after receiving the last byte of the response of the last redirect.
performanceResourceTiming.fetchStart
#
The high resolution millisecond timestamp immediately before the Node.js starts to fetch the resource.
performanceResourceTiming.domainLookupStart
#
The high resolution millisecond timestamp immediately before the Node.js starts the domain name lookup for the resource.
performanceResourceTiming.domainLookupEnd
#
The high resolution millisecond timestamp representing the time immediately after the Node.js finished the domain name lookup for the resource.
performanceResourceTiming.connectStart
#
The high resolution millisecond timestamp representing the time immediately before Node.js starts to establish the connection to the server to retrieve the resource.
performanceResourceTiming.connectEnd
#
The high resolution millisecond timestamp representing the time immediately after Node.js finishes establishing the connection to the server to retrieve the resource.
performanceResourceTiming.secureConnectionStart
#
The high resolution millisecond timestamp representing the time immediately before Node.js starts the handshake process to secure the current connection.
performanceResourceTiming.requestStart
#
The high resolution millisecond timestamp representing the time immediately before Node.js receives the first byte of the response from the server.
performanceResourceTiming.responseEnd
#
The high resolution millisecond timestamp representing the time immediately after Node.js receives the last byte of the resource or immediately before the transport connection is closed, whichever comes first.
performanceResourceTiming.transferSize
#
A number representing the size (in octets) of the fetched resource. The size includes the response header fields plus the response payload body.
performanceResourceTiming.encodedBodySize
#
A number representing the size (in octets) received from the fetch (HTTP or cache), of the payload body, before removing any applied content-codings.
performanceResourceTiming.decodedBodySize
#
A number representing the size (in octets) received from the fetch (HTTP or cache), of the message body, after removing any applied content-codings.
performanceResourceTiming.toJSON()
#
Returns a object
that is the JSON representation of the
PerformanceResourceTiming
object
Class: PerformanceObserver
#
PerformanceObserver.supportedEntryTypes
#
Get supported types.
new PerformanceObserver(callback)
#
callback
<Function>list
<PerformanceObserverEntryList>observer
<PerformanceObserver>
PerformanceObserver
objects provide notifications when new
PerformanceEntry
instances have been added to the Performance Timeline.
const {
performance,
PerformanceObserver,
} = require('node:perf_hooks');
const obs = new PerformanceObserver((list, observer) => {
console.log(list.getEntries());
performance.clearMarks();
performance.clearMeasures();
observer.disconnect();
});
obs.observe({ entryTypes: ['mark'], buffered: true });
performance.mark('test');
Because PerformanceObserver
instances introduce their own additional
performance overhead, instances should not be left subscribed to notifications
indefinitely. Users should disconnect observers as soon as they are no
longer needed.
The callback
is invoked when a PerformanceObserver
is
notified about new PerformanceEntry
instances. The callback receives a
PerformanceObserverEntryList
instance and a reference to the
PerformanceObserver
.
performanceObserver.disconnect()
#
Disconnects the PerformanceObserver
instance from all notifications.
performanceObserver.observe(options)
#
options
<Object>type
<string> A single <PerformanceEntry> type. Must not be given ifentryTypes
is already specified.entryTypes
<string[]> An array of strings identifying the types of <PerformanceEntry> instances the observer is interested in. If not provided an error will be thrown.buffered
<boolean> If true, the observer callback is called with a list globalPerformanceEntry
buffered entries. If false, onlyPerformanceEntry
s created after the time point are sent to the observer callback. Default:false
.
Subscribes the <PerformanceObserver> instance to notifications of new
<PerformanceEntry> instances identified either by options.entryTypes
or options.type
:
const {
performance,
PerformanceObserver,
} = require('node:perf_hooks');
const obs = new PerformanceObserver((list, observer) => {
// Called once asynchronously. `list` contains three items.
});
obs.observe({ type: 'mark' });
for (let n = 0; n < 3; n++)
performance.mark(`test${n}`);
Class: PerformanceObserverEntryList
#
The PerformanceObserverEntryList
class is used to provide access to the
PerformanceEntry
instances passed to a PerformanceObserver
.
The constructor of this class is not exposed to users.
performanceObserverEntryList.getEntries()
#
- Returns: <PerformanceEntry[]>
Returns a list of PerformanceEntry
objects in chronological order
with respect to performanceEntry.startTime
.
const {
performance,
PerformanceObserver,
} = require('node:perf_hooks');
const obs = new PerformanceObserver((perfObserverList, observer) => {
console.log(perfObserverList.getEntries());
/**
* [
* PerformanceEntry {
* name: 'test',
* entryType: 'mark',
* startTime: 81.465639,
* duration: 0
* },
* PerformanceEntry {
* name: 'meow',
* entryType: 'mark',
* startTime: 81.860064,
* duration: 0
* }
* ]
*/
performance.clearMarks();
performance.clearMeasures();
observer.disconnect();
});
obs.observe({ type: 'mark' });
performance.mark('test');
performance.mark('meow');
performanceObserverEntryList.getEntriesByName(name[, type])
#
name
<string>type
<string>- Returns: <PerformanceEntry[]>
Returns a list of PerformanceEntry
objects in chronological order
with respect to performanceEntry.startTime
whose performanceEntry.name
is
equal to name
, and optionally, whose performanceEntry.entryType
is equal to
type
.
const {
performance,
PerformanceObserver,
} = require('node:perf_hooks');
const obs = new PerformanceObserver((perfObserverList, observer) => {
console.log(perfObserverList.getEntriesByName('meow'));
/**
* [
* PerformanceEntry {
* name: 'meow',
* entryType: 'mark',
* startTime: 98.545991,
* duration: 0
* }
* ]
*/
console.log(perfObserverList.getEntriesByName('nope')); // []
console.log(perfObserverList.getEntriesByName('test', 'mark'));
/**
* [
* PerformanceEntry {
* name: 'test',
* entryType: 'mark',
* startTime: 63.518931,
* duration: 0
* }
* ]
*/
console.log(perfObserverList.getEntriesByName('test', 'measure')); // []
performance.clearMarks();
performance.clearMeasures();
observer.disconnect();
});
obs.observe({ entryTypes: ['mark', 'measure'] });
performance.mark('test');
performance.mark('meow');
performanceObserverEntryList.getEntriesByType(type)
#
type
<string>- Returns: <PerformanceEntry[]>
Returns a list of PerformanceEntry
objects in chronological order
with respect to performanceEntry.startTime
whose performanceEntry.entryType
is equal to type
.
const {
performance,
PerformanceObserver,
} = require('node:perf_hooks');
const obs = new PerformanceObserver((perfObserverList, observer) => {
console.log(perfObserverList.getEntriesByType('mark'));
/**
* [
* PerformanceEntry {
* name: 'test',
* entryType: 'mark',
* startTime: 55.897834,
* duration: 0
* },
* PerformanceEntry {
* name: 'meow',
* entryType: 'mark',
* startTime: 56.350146,
* duration: 0
* }
* ]
*/
performance.clearMarks();
performance.clearMeasures();
observer.disconnect();
});
obs.observe({ type: 'mark' });
performance.mark('test');
performance.mark('meow');
perf_hooks.createHistogram([options])
#
options
<Object>lowest
<number> | <bigint> The lowest discernible value. Must be an integer value greater than 0. Default:1
.highest
<number> | <bigint> The highest recordable value. Must be an integer value that is equal to or greater than two timeslowest
. Default:Number.MAX_SAFE_INTEGER
.figures
<number> The number of accuracy digits. Must be a number between1
and5
. Default:3
.
- Returns <RecordableHistogram>
Returns a <RecordableHistogram>.
perf_hooks.monitorEventLoopDelay([options])
#
options
<Object>resolution
<number> The sampling rate in milliseconds. Must be greater than zero. Default:10
.
- Returns: <IntervalHistogram>
This property is an extension by Node.js. It is not available in Web browsers.
Creates an IntervalHistogram
object that samples and reports the event loop
delay over time. The delays will be reported in nanoseconds.
Using a timer to detect approximate event loop delay works because the execution of timers is tied specifically to the lifecycle of the libuv event loop. That is, a delay in the loop will cause a delay in the execution of the timer, and those delays are specifically what this API is intended to detect.
const { monitorEventLoopDelay } = require('node:perf_hooks');
const h = monitorEventLoopDelay({ resolution: 20 });
h.enable();
// Do something.
h.disable();
console.log(h.min);
console.log(h.max);
console.log(h.mean);
console.log(h.stddev);
console.log(h.percentiles);
console.log(h.percentile(50));
console.log(h.percentile(99));
Class: Histogram
#
histogram.count
#
The number of samples recorded by the histogram.
histogram.countBigInt
#
The number of samples recorded by the histogram.
histogram.exceeds
#
The number of times the event loop delay exceeded the maximum 1 hour event loop delay threshold.
histogram.exceedsBigInt
#
The number of times the event loop delay exceeded the maximum 1 hour event loop delay threshold.
histogram.max
#
The maximum recorded event loop delay.
histogram.maxBigInt
#
The maximum recorded event loop delay.
histogram.mean
#
The mean of the recorded event loop delays.
histogram.min
#
The minimum recorded event loop delay.
histogram.minBigInt
#
The minimum recorded event loop delay.
histogram.percentile(percentile)
#
Returns the value at the given percentile.
histogram.percentileBigInt(percentile)
#
Returns the value at the given percentile.
histogram.percentiles
#
Returns a Map
object detailing the accumulated percentile distribution.
histogram.percentilesBigInt
#
Returns a Map
object detailing the accumulated percentile distribution.
histogram.reset()
#
Resets the collected histogram data.
histogram.stddev
#
The standard deviation of the recorded event loop delays.
Class: IntervalHistogram extends Histogram
#
A Histogram
that is periodically updated on a given interval.
histogram.disable()
#
- Returns: <boolean>
Disables the update interval timer. Returns true
if the timer was
stopped, false
if it was already stopped.
histogram.enable()
#
- Returns: <boolean>
Enables the update interval timer. Returns true
if the timer was
started, false
if it was already started.
Cloning an IntervalHistogram
#
<IntervalHistogram> instances can be cloned via <MessagePort>. On the receiving
end, the histogram is cloned as a plain <Histogram> object that does not
implement the enable()
and disable()
methods.
Class: RecordableHistogram extends Histogram
#
histogram.add(other)
#
other
<RecordableHistogram>
Adds the values from other
to this histogram.
histogram.record(val)
#
histogram.recordDelta()
#
Calculates the amount of time (in nanoseconds) that has passed since the
previous call to recordDelta()
and records that amount in the histogram.
Examples#
Measuring the duration of async operations#
The following example uses the Async Hooks and Performance APIs to measure the actual duration of a Timeout operation (including the amount of time it took to execute the callback).
'use strict';
const async_hooks = require('node:async_hooks');
const {
performance,
PerformanceObserver,
} = require('node:perf_hooks');
const set = new Set();
const hook = async_hooks.createHook({
init(id, type) {
if (type === 'Timeout') {
performance.mark(`Timeout-${id}-Init`);
set.add(id);
}
},
destroy(id) {
if (set.has(id)) {
set.delete(id);
performance.mark(`Timeout-${id}-Destroy`);
performance.measure(`Timeout-${id}`,
`Timeout-${id}-Init`,
`Timeout-${id}-Destroy`);
}
},
});
hook.enable();
const obs = new PerformanceObserver((list, observer) => {
console.log(list.getEntries()[0]);
performance.clearMarks();
performance.clearMeasures();
observer.disconnect();
});
obs.observe({ entryTypes: ['measure'], buffered: true });
setTimeout(() => {}, 1000);
Measuring how long it takes to load dependencies#
The following example measures the duration of require()
operations to load
dependencies:
'use strict';
const {
performance,
PerformanceObserver,
} = require('node:perf_hooks');
const mod = require('node:module');
// Monkey patch the require function
mod.Module.prototype.require =
performance.timerify(mod.Module.prototype.require);
require = performance.timerify(require);
// Activate the observer
const obs = new PerformanceObserver((list) => {
const entries = list.getEntries();
entries.forEach((entry) => {
console.log(`require('${entry[0]}')`, entry.duration);
});
performance.clearMarks();
performance.clearMeasures();
obs.disconnect();
});
obs.observe({ entryTypes: ['function'], buffered: true });
require('some-module');
Measuring how long one HTTP round-trip takes#
The following example is used to trace the time spent by HTTP client
(OutgoingMessage
) and HTTP request (IncomingMessage
). For HTTP client,
it means the time interval between starting the request and receiving the
response, and for HTTP request, it means the time interval between receiving
the request and sending the response:
'use strict';
const { PerformanceObserver } = require('node:perf_hooks');
const http = require('node:http');
const obs = new PerformanceObserver((items) => {
items.getEntries().forEach((item) => {
console.log(item);
});
});
obs.observe({ entryTypes: ['http'] });
const PORT = 8080;
http.createServer((req, res) => {
res.end('ok');
}).listen(PORT, () => {
http.get(`http://127.0.0.1:${PORT}`);
});
Measuring how long the net.connect
(only for TCP) takes when the connection is successful#
'use strict';
const { PerformanceObserver } = require('node:perf_hooks');
const net = require('node:net');
const obs = new PerformanceObserver((items) => {
items.getEntries().forEach((item) => {
console.log(item);
});
});
obs.observe({ entryTypes: ['net'] });
const PORT = 8080;
net.createServer((socket) => {
socket.destroy();
}).listen(PORT, () => {
net.connect(PORT);
});
Measuring how long the DNS takes when the request is successful#
'use strict';
const { PerformanceObserver } = require('node:perf_hooks');
const dns = require('node:dns');
const obs = new PerformanceObserver((items) => {
items.getEntries().forEach((item) => {
console.log(item);
});
});
obs.observe({ entryTypes: ['dns'] });
dns.lookup('localhost', () => {});
dns.promises.resolve('localhost');
Permissions#
Permissions can be used to control what system resources the Node.js process has access to or what actions the process can take with those resources. Permissions can also control what modules can be accessed by other modules.
-
Module-based permissions control which files or URLs are available to other modules during application execution. This can be used to control what modules can be accessed by third-party dependencies, for example.
-
Process-based permissions control the Node.js process's access to resources. The resource can be entirely allowed or denied, or actions related to it can be controlled. For example, file system reads can be allowed while denying writes.
If you find a potential security vulnerability, please refer to our Security Policy.
Module-based permissions#
Policies#
Node.js contains experimental support for creating policies on loading code.
Policies are a security feature intended to allow guarantees about what code Node.js is able to load. The use of policies assumes safe practices for the policy files such as ensuring that policy files cannot be overwritten by the Node.js application by using file permissions.
A best practice would be to ensure that the policy manifest is read-only for the running Node.js application and that the file cannot be changed by the running Node.js application in any way. A typical setup would be to create the policy file as a different user id than the one running Node.js and granting read permissions to the user id running Node.js.
Enabling#
The --experimental-policy
flag can be used to enable features for policies
when loading modules.
Once this has been set, all modules must conform to a policy manifest file passed to the flag:
node --experimental-policy=policy.json app.js
The policy manifest will be used to enforce constraints on code loaded by Node.js.
To mitigate tampering with policy files on disk, an integrity for
the policy file itself may be provided via --policy-integrity
.
This allows running node
and asserting the policy file contents
even if the file is changed on disk.
node --experimental-policy=policy.json --policy-integrity="sha384-SggXRQHwCG8g+DktYYzxkXRIkTiEYWBHqev0xnpCxYlqMBufKZHAHQM3/boDaI/0" app.js
Features#
Error behavior#
When a policy check fails, Node.js by default will throw an error. It is possible to change the error behavior to one of a few possibilities by defining an "onerror" field in a policy manifest. The following values are available to change the behavior:
"exit"
: will exit the process immediately. No cleanup code will be allowed to run."log"
: will log the error at the site of the failure."throw"
: will throw a JS error at the site of the failure. This is the default.
{
"onerror": "log",
"resources": {
"./app/checked.js": {
"integrity": "sha384-SggXRQHwCG8g+DktYYzxkXRIkTiEYWBHqev0xnpCxYlqMBufKZHAHQM3/boDaI/0"
}
}
}
Integrity checks#
Policy files must use integrity checks with Subresource Integrity strings compatible with the browser integrity attribute associated with absolute URLs.
When using require()
or import
all resources involved in loading are checked
for integrity if a policy manifest has been specified. If a resource does not
match the integrity listed in the manifest, an error will be thrown.
An example policy file that would allow loading a file checked.js
:
{
"resources": {
"./app/checked.js": {
"integrity": "sha384-SggXRQHwCG8g+DktYYzxkXRIkTiEYWBHqev0xnpCxYlqMBufKZHAHQM3/boDaI/0"
}
}
}
Each resource listed in the policy manifest can be of one the following formats to determine its location:
- A relative-URL string to a resource from the manifest such as
./resource.js
,../resource.js
, or/resource.js
. - A complete URL string to a resource such as
file:///resource.js
.
When loading resources the entire URL must match including search parameters
and hash fragment. ./a.js?b
will not be used when attempting to load
./a.js
and vice versa.
To generate integrity strings, a script such as
node -e 'process.stdout.write("sha256-");process.stdin.pipe(crypto.createHash("sha256").setEncoding("base64")).pipe(process.stdout)' < FILE
can be used.
Integrity can be specified as the boolean value true
to accept any
body for the resource which can be useful for local development. It is not
recommended in production since it would allow unexpected alteration of
resources to be considered valid.
Dependency redirection#
An application may need to ship patched versions of modules or to prevent modules from allowing all modules access to all other modules. Redirection can be used by intercepting attempts to load the modules wishing to be replaced.
{
"resources": {
"./app/checked.js": {
"dependencies": {
"fs": true,
"os": "./app/node_modules/alt-os",
"http": { "import": true }
}
}
}
}
The dependencies are keyed by the requested specifier string and have values
of either true
, null
, a string pointing to a module to be resolved,
or a conditions object.
The specifier string does not perform any searching and must match exactly what
is provided to the require()
or import
except for a canonicalization step.
Therefore, multiple specifiers may be needed in the policy if it uses multiple
different strings to point to the same module (such as excluding the extension).
Specifier strings are canonicalized but not resolved prior to be used for
matching in order to have some compatibility with import maps, for example if a
resource file:///C:/app/server.js
was given the following redirection from a
policy located at file:///C:/app/policy.json
:
{
"resources": {
"file:///C:/app/utils.js": {
"dependencies": {
"./utils.js": "./utils-v2.js"
}
}
}
}
Any specifier used to load file:///C:/app/utils.js
would then be intercepted
and redirected to file:///C:/app/utils-v2.js
instead regardless of using an
absolute or relative specifier. However, if a specifier that is not an absolute
or relative URL string is used, it would not be intercepted. So, if an import
such as import('#utils')
was used, it would not be intercepted.
If the value of the redirection is true
, a "dependencies" field at the top of
the policy file will be used. If that field at the top of the policy file is
true
the default node searching algorithms are used to find the module.
If the value of the redirection is a string, it is resolved relative to the manifest and then immediately used without searching.
Any specifier string for which resolution is attempted and that is not listed in the dependencies results in an error according to the policy.
Redirection does not prevent access to APIs through means such as direct access
to require.cache
or through module.constructor
which allow access to
loading modules. Policy redirection only affects specifiers to require()
and
import
. Other means, such as to prevent undesired access to APIs through
variables, are necessary to lock down that path of loading modules.
A boolean value of true
for the dependencies map can be specified to allow a
module to load any specifier without redirection. This can be useful for local
development and may have some valid usage in production, but should be used
only with care after auditing a module to ensure its behavior is valid.
Similar to "exports"
in package.json
, dependencies can also be specified to
be objects containing conditions which branch how dependencies are loaded. In
the preceding example, "http"
is allowed when the "import"
condition is
part of loading it.
A value of null
for the resolved value causes the resolution to fail. This
can be used to ensure some kinds of dynamic access are explicitly prevented.
Unknown values for the resolved module location cause failures but are not guaranteed to be forward compatible.
Example: Patched dependency#
Redirected dependencies can provide attenuated or modified functionality as fits the application. For example, log data about timing of function durations by wrapping the original:
const original = require('fn');
module.exports = function fn(...args) {
console.time();
try {
return new.target ?
Reflect.construct(original, args) :
Reflect.apply(original, this, args);
} finally {
console.timeEnd();
}
};
Scopes#
Use the "scopes"
field of a manifest to set configuration for many resources
at once. The "scopes"
field works by matching resources by their segments.
If a scope or resource includes "cascade": true
, unknown specifiers will
be searched for in their containing scope. The containing scope for cascading
is found by recursively reducing the resource URL by removing segments for
special schemes, keeping trailing "/"
suffixes, and removing the query and
hash fragment. This leads to the eventual reduction of the URL to its origin.
If the URL is non-special the scope will be located by the URL's origin. If no
scope is found for the origin or in the case of opaque origins, a protocol
string can be used as a scope. If no scope is found for the URL's protocol, a
final empty string ""
scope will be used.
Note, blob:
URLs adopt their origin from the path they contain, and so a scope
of "blob:https://nodejs.org"
will have no effect since no URL can have an
origin of blob:https://nodejs.org
; URLs starting with
blob:https://nodejs.org/
will use https://nodejs.org
for its origin and
thus https:
for its protocol scope. For opaque origin blob:
URLs they will
have blob:
for their protocol scope since they do not adopt origins.
Example#
{
"scopes": {
"file:///C:/app/": {},
"file:": {},
"": {}
}
}
Given a file located at file:///C:/app/bin/main.js
, the following scopes would
be checked in order:
"file:///C:/app/bin/"
This determines the policy for all file based resources within
"file:///C:/app/bin/"
. This is not in the "scopes"
field of the policy and
would be skipped. Adding this scope to the policy would cause it to be used
prior to the "file:///C:/app/"
scope.
"file:///C:/app/"
This determines the policy for all file based resources within
"file:///C:/app/"
. This is in the "scopes"
field of the policy and it would
determine the policy for the resource at file:///C:/app/bin/main.js
. If the
scope has "cascade": true
, any unsatisfied queries about the resource would
delegate to the next relevant scope for file:///C:/app/bin/main.js
, "file:"
.
"file:///C:/"
This determines the policy for all file based resources within "file:///C:/"
.
This is not in the "scopes"
field of the policy and would be skipped. It would
not be used for file:///C:/app/bin/main.js
unless "file:///"
is set to
cascade or is not in the "scopes"
of the policy.
"file:///"
This determines the policy for all file based resources on the localhost
. This
is not in the "scopes"
field of the policy and would be skipped. It would not
be used for file:///C:/app/bin/main.js
unless "file:///"
is set to cascade
or is not in the "scopes"
of the policy.
"file:"
This determines the policy for all file based resources. It would not be used
for file:///C:/app/bin/main.js
unless "file:///"
is set to cascade or is not
in the "scopes"
of the policy.
""
This determines the policy for all resources. It would not be used for
file:///C:/app/bin/main.js
unless "file:"
is set to cascade.
Integrity using scopes#
Setting an integrity to true
on a scope will set the integrity for any
resource not found in the manifest to true
.
Setting an integrity to null
on a scope will set the integrity for any
resource not found in the manifest to fail matching.
Not including an integrity is the same as setting the integrity to null
.
"cascade"
for integrity checks will be ignored if "integrity"
is explicitly
set.
The following example allows loading any file:
{
"scopes": {
"file:": {
"integrity": true
}
}
}
Dependency redirection using scopes#
The following example, would allow access to fs
for all resources within
./app/
:
{
"resources": {
"./app/checked.js": {
"cascade": true,
"integrity": true
}
},
"scopes": {
"./app/": {
"dependencies": {
"fs": true
}
}
}
}
The following example, would allow access to fs
for all data:
resources:
{
"resources": {
"data:text/javascript,import('node:fs');": {
"cascade": true,
"integrity": true
}
},
"scopes": {
"data:": {
"dependencies": {
"fs": true
}
}
}
}
Example: import maps emulation#
Given an import map:
{
"imports": {
"react": "./app/node_modules/react/index.js"
},
"scopes": {
"./ssr/": {
"react": "./app/node_modules/server-side-react/index.js"
}
}
}
{
"dependencies": true,
"scopes": {
"": {
"cascade": true,
"dependencies": {
"react": "./app/node_modules/react/index.js"
}
},
"./ssr/": {
"cascade": true,
"dependencies": {
"react": "./app/node_modules/server-side-react/index.js"
}
}
}
}
Import maps assume you can get any resource by default. This means
"dependencies"
at the top level of the policy should be set to true
.
Policies require this to be opt-in since it enables all resources of the
application cross linkage which doesn't make sense for many scenarios. They also
assume any given scope has access to any scope above its allowed dependencies;
all scopes emulating import maps must set "cascade": true
.
Import maps only have a single top level scope for their "imports". So for
emulating "imports"
use the ""
scope. For emulating "scopes"
use the
"scopes"
in a similar manner to how "scopes"
works in import maps.
Caveats: Policies do not use string matching for various finding of scope. They
do URL traversals. This means things like blob:
and data:
URLs might not be
entirely interoperable between the two systems. For example import maps can
partially match a data:
or blob:
URL by partitioning the URL on a /
character, policies intentionally cannot. For blob:
URLs import map scopes do
not adopt the origin of the blob:
URL.
Additionally, import maps only work on import
so it may be desirable to add a
"import"
condition to all dependency mappings.
Process-based permissions#
Permission Model#
The Node.js Permission Model is a mechanism for restricting access to specific
resources during execution.
The API exists behind a flag --experimental-permission
which when enabled,
will restrict access to all available permissions.
The available permissions are documented by the --experimental-permission
flag.
When starting Node.js with --experimental-permission
,
the ability to access the file system, spawn processes, and
use node:worker_threads
will be restricted.
$ node --experimental-permission index.js
node:internal/modules/cjs/loader:171
const result = internalModuleStat(filename);
^
Error: Access to this API has been restricted
at stat (node:internal/modules/cjs/loader:171:18)
at Module._findPath (node:internal/modules/cjs/loader:627:16)
at resolveMainPath (node:internal/modules/run_main:19:25)
at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:76:24)
at node:internal/main/run_main_module:23:47 {
code: 'ERR_ACCESS_DENIED',
permission: 'FileSystemRead'
}
Allowing access to spawning a process and creating worker threads can be done
using the --allow-child-process
and --allow-worker
respectively.
Runtime API#
When enabling the Permission Model through the --experimental-permission
flag a new property permission
is added to the process
object.
This property contains two functions:
permission.deny(scope [,parameters])
#
API call to deny permissions at runtime (permission.deny()
)
process.permission.deny('fs'); // Deny permissions to ALL fs operations
// Deny permissions to ALL FileSystemWrite operations
process.permission.deny('fs.write');
// deny FileSystemWrite permissions to the protected-folder
process.permission.deny('fs.write', ['/home/rafaelgss/protected-folder']);
// Deny permissions to ALL FileSystemRead operations
process.permission.deny('fs.read');
// deny FileSystemRead permissions to the protected-folder
process.permission.deny('fs.read', ['/home/rafaelgss/protected-folder']);
permission.has(scope ,parameters)
#
API call to check permissions at runtime (permission.has()
)
process.permission.has('fs.write'); // true
process.permission.has('fs.write', '/home/rafaelgss/protected-folder'); // true
process.permission.deny('fs.write', '/home/rafaelgss/protected-folder');
process.permission.has('fs.write'); // true
process.permission.has('fs.write', '/home/rafaelgss/protected-folder'); // false
File System Permissions#
To allow access to the file system, use the --allow-fs-read
and
--allow-fs-write
flags:
$ node --experimental-permission --allow-fs-read=* --allow-fs-write=* index.js
Hello world!
(node:19836) ExperimentalWarning: Permission is an experimental feature
(Use `node --trace-warnings ...` to show where the warning was created)
The valid arguments for both flags are:
*
- To allow the all operations to given scope (read/write).- Paths delimited by comma (,) to manage reading/writing operations.
Example:
--allow-fs-read=*
- It will allow allFileSystemRead
operations.--allow-fs-write=*
- It will allow allFileSystemWrite
operations.--allow-fs-write=/tmp/
- It will allowFileSystemWrite
access to the/tmp/
folder.--allow-fs-read=/tmp/,/home/.gitignore
- It allowsFileSystemRead
access to the/tmp/
folder and the/home/.gitignore
path.
Wildcards are supported too:
--allow-fs-read:/home/test*
will allow read access to everything that matches the wildcard. e.g:/home/test/file1
or/home/test2
There are constraints you need to know before using this system:
- Native modules are restricted by default when using the Permission Model.
- Relative paths are not supported through the CLI (
--allow-fs-*
). The runtime API supports relative paths. - The model does not inherit to a child node process.
- The model does not inherit to a worker thread.
- When creating symlinks the target (first argument) should have read and write access.
- Permission changes are not retroactively applied to existing resources.
Consider the following snippet:
const fs = require('node:fs'); // Open a fd const fd = fs.openSync('./README.md', 'r'); // Then, deny access to all fs.read operations process.permission.deny('fs.read'); // This call will NOT fail and the file will be read const data = fs.readFileSync(fd);
Therefore, when possible, apply the permissions rules before any statement:
process.permission.deny('fs.read');
const fd = fs.openSync('./README.md', 'r');
// Error: Access to this API has been restricted
Process#
Source Code: lib/process.js
The process
object provides information about, and control over, the current
Node.js process.
import process from 'node:process';
const process = require('node:process');
Process events#
The process
object is an instance of EventEmitter
.
Event: 'beforeExit'
#
The 'beforeExit'
event is emitted when Node.js empties its event loop and has
no additional work to schedule. Normally, the Node.js process will exit when
there is no work scheduled, but a listener registered on the 'beforeExit'
event can make asynchronous calls, and thereby cause the Node.js process to
continue.
The listener callback function is invoked with the value of
process.exitCode
passed as the only argument.
The 'beforeExit'
event is not emitted for conditions causing explicit
termination, such as calling process.exit()
or uncaught exceptions.
The 'beforeExit'
should not be used as an alternative to the 'exit'
event
unless the intention is to schedule additional work.
import process from 'node:process';
process.on('beforeExit', (code) => {
console.log('Process beforeExit event with code: ', code);
});
process.on('exit', (code) => {
console.log('Process exit event with code: ', code);
});
console.log('This message is displayed first.');
// Prints:
// This message is displayed first.
// Process beforeExit event with code: 0
// Process exit event with code: 0
const process = require('node:process');
process.on('beforeExit', (code) => {
console.log('Process beforeExit event with code: ', code);
});
process.on('exit', (code) => {
console.log('Process exit event with code: ', code);
});
console.log('This message is displayed first.');
// Prints:
// This message is displayed first.
// Process beforeExit event with code: 0
// Process exit event with code: 0
Event: 'disconnect'
#
If the Node.js process is spawned with an IPC channel (see the Child Process
and Cluster documentation), the 'disconnect'
event will be emitted when
the IPC channel is closed.
Event: 'exit'
#
code
<integer>
The 'exit'
event is emitted when the Node.js process is about to exit as a
result of either:
- The
process.exit()
method being called explicitly; - The Node.js event loop no longer having any additional work to perform.
There is no way to prevent the exiting of the event loop at this point, and once
all 'exit'
listeners have finished running the Node.js process will terminate.
The listener callback function is invoked with the exit code specified either
by the process.exitCode
property, or the exitCode
argument passed to the
process.exit()
method.
import process from 'node:process';
process.on('exit', (code) => {
console.log(`About to exit with code: ${code}`);
});
const process = require('node:process');
process.on('exit', (code) => {
console.log(`About to exit with code: ${code}`);
});
Listener functions must only perform synchronous operations. The Node.js
process will exit immediately after calling the 'exit'
event listeners
causing any additional work still queued in the event loop to be abandoned.
In the following example, for instance, the timeout will never occur:
import process from 'node:process';
process.on('exit', (code) => {
setTimeout(() => {
console.log('This will not run');
}, 0);
});
const process = require('node:process');
process.on('exit', (code) => {
setTimeout(() => {
console.log('This will not run');
}, 0);
});
Event: 'message'
#
message
<Object> | <boolean> | <number> | <string> | <null> a parsed JSON object or a serializable primitive value.sendHandle
<net.Server> | <net.Socket> anet.Server
ornet.Socket
object, or undefined.
If the Node.js process is spawned with an IPC channel (see the Child Process
and Cluster documentation), the 'message'
event is emitted whenever a
message sent by a parent process using childprocess.send()
is received by
the child process.
The message goes through serialization and parsing. The resulting message might not be the same as what is originally sent.
If the serialization
option was set to advanced
used when spawning the
process, the message
argument can contain data that JSON is not able
to represent.
See Advanced serialization for child_process
for more details.
Event: 'multipleResolves'
#
type
<string> The resolution type. One of'resolve'
or'reject'
.promise
<Promise> The promise that resolved or rejected more than once.value
<any> The value with which the promise was either resolved or rejected after the original resolve.
The 'multipleResolves'
event is emitted whenever a Promise
has been either:
- Resolved more than once.
- Rejected more than once.
- Rejected after resolve.
- Resolved after reject.
This is useful for tracking potential errors in an application while using the
Promise
constructor, as multiple resolutions are silently swallowed. However,
the occurrence of this event does not necessarily indicate an error. For
example, Promise.race()
can trigger a 'multipleResolves'
event.
Because of the unreliability of the event in cases like the
Promise.race()
example above it has been deprecated.
import process from 'node:process';
process.on('multipleResolves', (type, promise, reason) => {
console.error(type, promise, reason);
setImmediate(() => process.exit(1));
});
async function main() {
try {
return await new Promise((resolve, reject) => {
resolve('First call');
resolve('Swallowed resolve');
reject(new Error('Swallowed reject'));
});
} catch {
throw new Error('Failed');
}
}
main().then(console.log);
// resolve: Promise { 'First call' } 'Swallowed resolve'
// reject: Promise { 'First call' } Error: Swallowed reject
// at Promise (*)
// at new Promise (<anonymous>)
// at main (*)
// First call
const process = require('node:process');
process.on('multipleResolves', (type, promise, reason) => {
console.error(type, promise, reason);
setImmediate(() => process.exit(1));
});
async function main() {
try {
return await new Promise((resolve, reject) => {
resolve('First call');
resolve('Swallowed resolve');
reject(new Error('Swallowed reject'));
});
} catch {
throw new Error('Failed');
}
}
main().then(console.log);
// resolve: Promise { 'First call' } 'Swallowed resolve'
// reject: Promise { 'First call' } Error: Swallowed reject
// at Promise (*)
// at new Promise (<anonymous>)
// at main (*)
// First call
Event: 'rejectionHandled'
#
promise
<Promise> The late handled promise.
The 'rejectionHandled'
event is emitted whenever a Promise
has been rejected
and an error handler was attached to it (using promise.catch()
, for
example) later than one turn of the Node.js event loop.
The Promise
object would have previously been emitted in an
'unhandledRejection'
event, but during the course of processing gained a
rejection handler.
There is no notion of a top level for a Promise
chain at which rejections can
always be handled. Being inherently asynchronous in nature, a Promise
rejection can be handled at a future point in time, possibly much later than
the event loop turn it takes for the 'unhandledRejection'
event to be emitted.
Another way of stating this is that, unlike in synchronous code where there is an ever-growing list of unhandled exceptions, with Promises there can be a growing-and-shrinking list of unhandled rejections.
In synchronous code, the 'uncaughtException'
event is emitted when the list of
unhandled exceptions grows.
In asynchronous code, the 'unhandledRejection'
event is emitted when the list
of unhandled rejections grows, and the 'rejectionHandled'
event is emitted
when the list of unhandled rejections shrinks.
import process from 'node:process';
const unhandledRejections = new Map();
process.on('unhandledRejection', (reason, promise) => {
unhandledRejections.set(promise, reason);
});
process.on('rejectionHandled', (promise) => {
unhandledRejections.delete(promise);
});
const process = require('node:process');
const unhandledRejections = new Map();
process.on('unhandledRejection', (reason, promise) => {
unhandledRejections.set(promise, reason);
});
process.on('rejectionHandled', (promise) => {
unhandledRejections.delete(promise);
});
In this example, the unhandledRejections
Map
will grow and shrink over time,
reflecting rejections that start unhandled and then become handled. It is
possible to record such errors in an error log, either periodically (which is
likely best for long-running application) or upon process exit (which is likely
most convenient for scripts).
Event: 'uncaughtException'
#
err
<Error> The uncaught exception.origin
<string> Indicates if the exception originates from an unhandled rejection or from a synchronous error. Can either be'uncaughtException'
or'unhandledRejection'
. The latter is used when an exception happens in aPromise
based async context (or if aPromise
is rejected) and--unhandled-rejections
flag set tostrict
orthrow
(which is the default) and the rejection is not handled, or when a rejection happens during the command line entry point's ES module static loading phase.
The 'uncaughtException'
event is emitted when an uncaught JavaScript
exception bubbles all the way back to the event loop. By default, Node.js
handles such exceptions by printing the stack trace to stderr
and exiting
with code 1, overriding any previously set process.exitCode
.
Adding a handler for the 'uncaughtException'
event overrides this default
behavior. Alternatively, change the process.exitCode
in the
'uncaughtException'
handler which will result in the process exiting with the
provided exit code. Otherwise, in the presence of such handler the process will
exit with 0.
import process from 'node:process';
process.on('uncaughtException', (err, origin) => {
fs.writeSync(
process.stderr.fd,
`Caught exception: ${err}\n` +
`Exception origin: ${origin}`,
);
});
setTimeout(() => {
console.log('This will still run.');
}, 500);
// Intentionally cause an exception, but don't catch it.
nonexistentFunc();
console.log('This will not run.');
const process = require('node:process');
process.on('uncaughtException', (err, origin) => {
fs.writeSync(
process.stderr.fd,
`Caught exception: ${err}\n` +
`Exception origin: ${origin}`,
);
});
setTimeout(() => {
console.log('This will still run.');
}, 500);
// Intentionally cause an exception, but don't catch it.
nonexistentFunc();
console.log('This will not run.');
It is possible to monitor 'uncaughtException'
events without overriding the
default behavior to exit the process by installing a
'uncaughtExceptionMonitor'
listener.
Warning: Using 'uncaughtException'
correctly#
'uncaughtException'
is a crude mechanism for exception handling
intended to be used only as a last resort. The event should not be used as
an equivalent to On Error Resume Next
. Unhandled exceptions inherently mean
that an application is in an undefined state. Attempting to resume application
code without properly recovering from the exception can cause additional
unforeseen and unpredictable issues.
Exceptions thrown from within the event handler will not be caught. Instead the process will exit with a non-zero exit code and the stack trace will be printed. This is to avoid infinite recursion.
Attempting to resume normally after an uncaught exception can be similar to pulling out the power cord when upgrading a computer. Nine out of ten times, nothing happens. But the tenth time, the system becomes corrupted.
The correct use of 'uncaughtException'
is to perform synchronous cleanup
of allocated resources (e.g. file descriptors, handles, etc) before shutting
down the process. It is not safe to resume normal operation after
'uncaughtException'
.
To restart a crashed application in a more reliable way, whether
'uncaughtException'
is emitted or not, an external monitor should be employed
in a separate process to detect application failures and recover or restart as
needed.
Event: 'uncaughtExceptionMonitor'
#
err
<Error> The uncaught exception.origin
<string> Indicates if the exception originates from an unhandled rejection or from synchronous errors. Can either be'uncaughtException'
or'unhandledRejection'
. The latter is used when an exception happens in aPromise
based async context (or if aPromise
is rejected) and--unhandled-rejections
flag set tostrict
orthrow
(which is the default) and the rejection is not handled, or when a rejection happens during the command line entry point's ES module static loading phase.
The 'uncaughtExceptionMonitor'
event is emitted before an
'uncaughtException'
event is emitted or a hook installed via
process.setUncaughtExceptionCaptureCallback()
is called.
Installing an 'uncaughtExceptionMonitor'
listener does not change the behavior
once an 'uncaughtException'
event is emitted. The process will
still crash if no 'uncaughtException'
listener is installed.
import process from 'node:process';
process.on('uncaughtExceptionMonitor', (err, origin) => {
MyMonitoringTool.logSync(err, origin);
});
// Intentionally cause an exception, but don't catch it.
nonexistentFunc();
// Still crashes Node.js
const process = require('node:process');
process.on('uncaughtExceptionMonitor', (err, origin) => {
MyMonitoringTool.logSync(err, origin);
});
// Intentionally cause an exception, but don't catch it.
nonexistentFunc();
// Still crashes Node.js
Event: 'unhandledRejection'
#
reason
<Error> | <any> The object with which the promise was rejected (typically anError
object).promise
<Promise> The rejected promise.
The 'unhandledRejection'
event is emitted whenever a Promise
is rejected and
no error handler is attached to the promise within a turn of the event loop.
When programming with Promises, exceptions are encapsulated as "rejected
promises". Rejections can be caught and handled using promise.catch()
and
are propagated through a Promise
chain. The 'unhandledRejection'
event is
useful for detecting and keeping track of promises that were rejected whose
rejections have not yet been handled.
import process from 'node:process';
process.on('unhandledRejection', (reason, promise) => {
console.log('Unhandled Rejection at:', promise, 'reason:', reason);
// Application specific logging, throwing an error, or other logic here
});
somePromise.then((res) => {
return reportToUser(JSON.pasre(res)); // Note the typo (`pasre`)
}); // No `.catch()` or `.then()`
const process = require('node:process');
process.on('unhandledRejection', (reason, promise) => {
console.log('Unhandled Rejection at:', promise, 'reason:', reason);
// Application specific logging, throwing an error, or other logic here
});
somePromise.then((res) => {
return reportToUser(JSON.pasre(res)); // Note the typo (`pasre`)
}); // No `.catch()` or `.then()`
The following will also trigger the 'unhandledRejection'
event to be
emitted:
import process from 'node:process';
function SomeResource() {
// Initially set the loaded status to a rejected promise
this.loaded = Promise.reject(new Error('Resource not yet loaded!'));
}
const resource = new SomeResource();
// no .catch or .then on resource.loaded for at least a turn
const process = require('node:process');
function SomeResource() {
// Initially set the loaded status to a rejected promise
this.loaded = Promise.reject(new Error('Resource not yet loaded!'));
}
const resource = new SomeResource();
// no .catch or .then on resource.loaded for at least a turn
In this example case, it is possible to track the rejection as a developer error
as would typically be the case for other 'unhandledRejection'
events. To
address such failures, a non-operational
.catch(() => { })
handler may be attached to
resource.loaded
, which would prevent the 'unhandledRejection'
event from
being emitted.
Event: 'warning'
#
warning
<Error> Key properties of the warning are:
The 'warning'
event is emitted whenever Node.js emits a process warning.
A process warning is similar to an error in that it describes exceptional conditions that are being brought to the user's attention. However, warnings are not part of the normal Node.js and JavaScript error handling flow. Node.js can emit warnings whenever it detects bad coding practices that could lead to sub-optimal application performance, bugs, or security vulnerabilities.
import process from 'node:process';
process.on('warning', (warning) => {
console.warn(warning.name); // Print the warning name
console.warn(warning.message); // Print the warning message
console.warn(warning.stack); // Print the stack trace
});
const process = require('node:process');
process.on('warning', (warning) => {
console.warn(warning.name); // Print the warning name
console.warn(warning.message); // Print the warning message
console.warn(warning.stack); // Print the stack trace
});
By default, Node.js will print process warnings to stderr
. The --no-warnings
command-line option can be used to suppress the default console output but the
'warning'
event will still be emitted by the process
object.
The following example illustrates the warning that is printed to stderr
when
too many listeners have been added to an event:
$ node
> events.defaultMaxListeners = 1;
> process.on('foo', () => {});
> process.on('foo', () => {});
> (node:38638) MaxListenersExceededWarning: Possible EventEmitter memory leak
detected. 2 foo listeners added. Use emitter.setMaxListeners() to increase limit
In contrast, the following example turns off the default warning output and
adds a custom handler to the 'warning'
event:
$ node --no-warnings
> const p = process.on('warning', (warning) => console.warn('Do not do that!'));
> events.defaultMaxListeners = 1;
> process.on('foo', () => {});
> process.on('foo', () => {});
> Do not do that!
The --trace-warnings
command-line option can be used to have the default
console output for warnings include the full stack trace of the warning.
Launching Node.js using the --throw-deprecation
command-line flag will
cause custom deprecation warnings to be thrown as exceptions.
Using the --trace-deprecation
command-line flag will cause the custom
deprecation to be printed to stderr
along with the stack trace.
Using the --no-deprecation
command-line flag will suppress all reporting
of the custom deprecation.
The *-deprecation
command-line flags only affect warnings that use the name
'DeprecationWarning'
.
Event: 'worker'
#
The 'worker'
event is emitted after a new <Worker> thread has been created.
Emitting custom warnings#
See the process.emitWarning()
method for issuing
custom or application-specific warnings.
Node.js warning names#
There are no strict guidelines for warning types (as identified by the name
property) emitted by Node.js. New types of warnings can be added at any time.
A few of the warning types that are most common include:
'DeprecationWarning'
- Indicates use of a deprecated Node.js API or feature. Such warnings must include a'code'
property identifying the deprecation code.'ExperimentalWarning'
- Indicates use of an experimental Node.js API or feature. Such features must be used with caution as they may change at any time and are not subject to the same strict semantic-versioning and long-term support policies as supported features.'MaxListenersExceededWarning'
- Indicates that too many listeners for a given event have been registered on either anEventEmitter
orEventTarget
. This is often an indication of a memory leak.'TimeoutOverflowWarning'
- Indicates that a numeric value that cannot fit within a 32-bit signed integer has been provided to either thesetTimeout()
orsetInterval()
functions.'UnsupportedWarning'
- Indicates use of an unsupported option or feature that will be ignored rather than treated as an error. One example is use of the HTTP response status message when using the HTTP/2 compatibility API.
Signal events#
Signal events will be emitted when the Node.js process receives a signal. Please
refer to signal(7)
for a listing of standard POSIX signal names such as
'SIGINT'
, 'SIGHUP'
, etc.
Signals are not available on Worker
threads.
The signal handler will receive the signal's name ('SIGINT'
,
'SIGTERM'
, etc.) as the first argument.
The name of each event will be the uppercase common name for the signal (e.g.
'SIGINT'
for SIGINT
signals).
import process from 'node:process';
// Begin reading from stdin so the process does not exit.
process.stdin.resume();
process.on('SIGINT', () => {
console.log('Received SIGINT. Press Control-D to exit.');
});
// Using a single function to handle multiple signals
function handle(signal) {
console.log(`Received ${signal}`);
}
process.on('SIGINT', handle);
process.on('SIGTERM', handle);
const process = require('node:process');
// Begin reading from stdin so the process does not exit.
process.stdin.resume();
process.on('SIGINT', () => {
console.log('Received SIGINT. Press Control-D to exit.');
});
// Using a single function to handle multiple signals
function handle(signal) {
console.log(`Received ${signal}`);
}
process.on('SIGINT', handle);
process.on('SIGTERM', handle);
'SIGUSR1'
is reserved by Node.js to start the debugger. It's possible to install a listener but doing so might interfere with the debugger.'SIGTERM'
and'SIGINT'
have default handlers on non-Windows platforms that reset the terminal mode before exiting with code128 + signal number
. If one of these signals has a listener installed, its default behavior will be removed (Node.js will no longer exit).'SIGPIPE'
is ignored by default. It can have a listener installed.'SIGHUP'
is generated on Windows when the console window is closed, and on other platforms under various similar conditions. Seesignal(7)
. It can have a listener installed, however Node.js will be unconditionally terminated by Windows about 10 seconds later. On non-Windows platforms, the default behavior ofSIGHUP
is to terminate Node.js, but once a listener has been installed its default behavior will be removed.'SIGTERM'
is not supported on Windows, it can be listened on.'SIGINT'
from the terminal is supported on all platforms, and can usually be generated with Ctrl+C (though this may be configurable). It is not generated when terminal raw mode is enabled and Ctrl+C is used.'SIGBREAK'
is delivered on Windows when Ctrl+Break is pressed. On non-Windows platforms, it can be listened on, but there is no way to send or generate it.'SIGWINCH'
is delivered when the console has been resized. On Windows, this will only happen on write to the console when the cursor is being moved, or when a readable tty is used in raw mode.'SIGKILL'
cannot have a listener installed, it will unconditionally terminate Node.js on all platforms.'SIGSTOP'
cannot have a listener installed.'SIGBUS'
,'SIGFPE'
,'SIGSEGV'
, and'SIGILL'
, when not raised artificially usingkill(2)
, inherently leave the process in a state from which it is not safe to call JS listeners. Doing so might cause the process to stop responding.0
can be sent to test for the existence of a process, it has no effect if the process exists, but will throw an error if the process does not exist.
Windows does not support signals so has no equivalent to termination by signal,
but Node.js offers some emulation with process.kill()
, and
subprocess.kill()
:
- Sending
SIGINT
,SIGTERM
, andSIGKILL
will cause the unconditional termination of the target process, and afterwards, subprocess will report that the process was terminated by signal. - Sending signal
0
can be used as a platform independent way to test for the existence of a process.
process.abort()
#
The process.abort()
method causes the Node.js process to exit immediately and
generate a core file.
This feature is not available in Worker
threads.
process.allowedNodeEnvironmentFlags
#
The process.allowedNodeEnvironmentFlags
property is a special,
read-only Set
of flags allowable within the NODE_OPTIONS
environment variable.
process.allowedNodeEnvironmentFlags
extends Set
, but overrides
Set.prototype.has
to recognize several different possible flag
representations. process.allowedNodeEnvironmentFlags.has()
will
return true
in the following cases:
- Flags may omit leading single (
-
) or double (--
) dashes; e.g.,inspect-brk
for--inspect-brk
, orr
for-r
. - Flags passed through to V8 (as listed in
--v8-options
) may replace one or more non-leading dashes for an underscore, or vice-versa; e.g.,--perf_basic_prof
,--perf-basic-prof
,--perf_basic-prof
, etc. - Flags may contain one or more equals (
=
) characters; all characters after and including the first equals will be ignored; e.g.,--stack-trace-limit=100
. - Flags must be allowable within
NODE_OPTIONS
.
When iterating over process.allowedNodeEnvironmentFlags
, flags will
appear only once; each will begin with one or more dashes. Flags
passed through to V8 will contain underscores instead of non-leading
dashes:
import { allowedNodeEnvironmentFlags } from 'node:process';
allowedNodeEnvironmentFlags.forEach((flag) => {
// -r
// --inspect-brk
// --abort_on_uncaught_exception
// ...
});
const { allowedNodeEnvironmentFlags } = require('node:process');
allowedNodeEnvironmentFlags.forEach((flag) => {
// -r
// --inspect-brk
// --abort_on_uncaught_exception
// ...
});
The methods add()
, clear()
, and delete()
of
process.allowedNodeEnvironmentFlags
do nothing, and will fail
silently.
If Node.js was compiled without NODE_OPTIONS
support (shown in
process.config
), process.allowedNodeEnvironmentFlags
will
contain what would have been allowable.
process.arch
#
The operating system CPU architecture for which the Node.js binary was compiled.
Possible values are: 'arm'
, 'arm64'
, 'ia32'
, 'mips'
,'mipsel'
, 'ppc'
,
'ppc64'
, 's390'
, 's390x'
, and 'x64'
.
import { arch } from 'node:process';
console.log(`This processor architecture is ${arch}`);
const { arch } = require('node:process');
console.log(`This processor architecture is ${arch}`);
process.argv
#
The process.argv
property returns an array containing the command-line
arguments passed when the Node.js process was launched. The first element will
be process.execPath
. See process.argv0
if access to the original value
of argv[0]
is needed. The second element will be the path to the JavaScript
file being executed. The remaining elements will be any additional command-line
arguments.
For example, assuming the following script for process-args.js
:
import { argv } from 'node:process';
// print process.argv
argv.forEach((val, index) => {
console.log(`${index}: ${val}`);
});
const { argv } = require('node:process');
// print process.argv
argv.forEach((val, index) => {
console.log(`${index}: ${val}`);
});
Launching the Node.js process as:
$ node process-args.js one two=three four
Would generate the output:
0: /usr/local/bin/node
1: /Users/mjr/work/node/process-args.js
2: one
3: two=three
4: four
process.argv0
#
The process.argv0
property stores a read-only copy of the original value of
argv[0]
passed when Node.js starts.
$ bash -c 'exec -a customArgv0 ./node'
> process.argv[0]
'/Volumes/code/external/node/out/Release/node'
> process.argv0
'customArgv0'
process.channel
#
If the Node.js process was spawned with an IPC channel (see the
Child Process documentation), the process.channel
property is a reference to the IPC channel. If no IPC channel exists, this
property is undefined
.
process.channel.ref()
#
This method makes the IPC channel keep the event loop of the process
running if .unref()
has been called before.
Typically, this is managed through the number of 'disconnect'
and 'message'
listeners on the process
object. However, this method can be used to
explicitly request a specific behavior.
process.channel.unref()
#
This method makes the IPC channel not keep the event loop of the process running, and lets it finish even while the channel is open.
Typically, this is managed through the number of 'disconnect'
and 'message'
listeners on the process
object. However, this method can be used to
explicitly request a specific behavior.
process.chdir(directory)
#
directory
<string>
The process.chdir()
method changes the current working directory of the
Node.js process or throws an exception if doing so fails (for instance, if
the specified directory
does not exist).
import { chdir, cwd } from 'node:process';
console.log(`Starting directory: ${cwd()}`);
try {
chdir('/tmp');
console.log(`New directory: ${cwd()}`);
} catch (err) {
console.error(`chdir: ${err}`);
}
const { chdir, cwd } = require('node:process');
console.log(`Starting directory: ${cwd()}`);
try {
chdir('/tmp');
console.log(`New directory: ${cwd()}`);
} catch (err) {
console.error(`chdir: ${err}`);
}
This feature is not available in Worker
threads.
process.config
#
The process.config
property returns a frozen Object
containing the
JavaScript representation of the configure options used to compile the current
Node.js executable. This is the same as the config.gypi
file that was produced
when running the ./configure
script.
An example of the possible output looks like:
{
target_defaults:
{ cflags: [],
default_configuration: 'Release',
defines: [],
include_dirs: [],
libraries: [] },
variables:
{
host_arch: 'x64',
napi_build_version: 5,
node_install_npm: 'true',
node_prefix: '',
node_shared_cares: 'false',
node_shared_http_parser: 'false',
node_shared_libuv: 'false',
node_shared_zlib: 'false',
node_use_openssl: 'true',
node_shared_openssl: 'false',
strict_aliasing: 'true',
target_arch: 'x64',
v8_use_snapshot: 1
}
}
process.connected
#
If the Node.js process is spawned with an IPC channel (see the Child Process
and Cluster documentation), the process.connected
property will return
true
so long as the IPC channel is connected and will return false
after
process.disconnect()
is called.
Once process.connected
is false
, it is no longer possible to send messages
over the IPC channel using process.send()
.
process.constrainedMemory()
#
Gets the amount of memory available to the process (in bytes) based on
limits imposed by the OS. If there is no such constraint, or the constraint
is unknown, undefined
is returned.
See uv_get_constrained_memory
for more
information.
process.cpuUsage([previousValue])
#
The process.cpuUsage()
method returns the user and system CPU time usage of
the current process, in an object with properties user
and system
, whose
values are microsecond values (millionth of a second). These values measure time
spent in user and system code respectively, and may end up being greater than
actual elapsed time if multiple CPU cores are performing work for this process.
The result of a previous call to process.cpuUsage()
can be passed as the
argument to the function, to get a diff reading.
import { cpuUsage } from 'node:process';
const startUsage = cpuUsage();
// { user: 38579, system: 6986 }
// spin the CPU for 500 milliseconds
const now = Date.now();
while (Date.now() - now < 500);
console.log(cpuUsage(startUsage));
// { user: 514883, system: 11226 }
const { cpuUsage } = require('node:process');
const startUsage = cpuUsage();
// { user: 38579, system: 6986 }
// spin the CPU for 500 milliseconds
const now = Date.now();
while (Date.now() - now < 500);
console.log(cpuUsage(startUsage));
// { user: 514883, system: 11226 }
process.cwd()
#
- Returns: <string>
The process.cwd()
method returns the current working directory of the Node.js
process.
import { cwd } from 'node:process';
console.log(`Current directory: ${cwd()}`);
const { cwd } = require('node:process');
console.log(`Current directory: ${cwd()}`);
process.debugPort
#
The port used by the Node.js debugger when enabled.
import process from 'node:process';
process.debugPort = 5858;
const process = require('node:process');
process.debugPort = 5858;
process.disconnect()
#
If the Node.js process is spawned with an IPC channel (see the Child Process
and Cluster documentation), the process.disconnect()
method will close the
IPC channel to the parent process, allowing the child process to exit gracefully
once there are no other connections keeping it alive.
The effect of calling process.disconnect()
is the same as calling
ChildProcess.disconnect()
from the parent process.
If the Node.js process was not spawned with an IPC channel,
process.disconnect()
will be undefined
.
process.dlopen(module, filename[, flags])
#
module
<Object>filename
<string>flags
<os.constants.dlopen> Default:os.constants.dlopen.RTLD_LAZY
The process.dlopen()
method allows dynamically loading shared objects. It is
primarily used by require()
to load C++ Addons, and should not be used
directly, except in special cases. In other words, require()
should be
preferred over process.dlopen()
unless there are specific reasons such as
custom dlopen flags or loading from ES modules.
The flags
argument is an integer that allows to specify dlopen
behavior. See the os.constants.dlopen
documentation for details.
An important requirement when calling process.dlopen()
is that the module
instance must be passed. Functions exported by the C++ Addon are then
accessible via module.exports
.
The example below shows how to load a C++ Addon, named local.node
,
that exports a foo
function. All the symbols are loaded before
the call returns, by passing the RTLD_NOW
constant. In this example
the constant is assumed to be available.
import { dlopen } from 'node:process';
import { constants } from 'node:os';
import { fileURLToPath } from 'node:url';
const module = { exports: {} };
dlopen(module, fileURLToPath(new URL('local.node', import.meta.url)),
constants.dlopen.RTLD_NOW);
module.exports.foo();
const { dlopen } = require('node:process');
const { constants } = require('node:os');
const { join } = require('node:path');
const module = { exports: {} };
dlopen(module, join(__dirname, 'local.node'), constants.dlopen.RTLD_NOW);
module.exports.foo();
process.emitWarning(warning[, options])
#
warning
<string> | <Error> The warning to emit.options
<Object>type
<string> Whenwarning
is aString
,type
is the name to use for the type of warning being emitted. Default:'Warning'
.code
<string> A unique identifier for the warning instance being emitted.ctor
<Function> Whenwarning
is aString
,ctor
is an optional function used to limit the generated stack trace. Default:process.emitWarning
.detail
<string> Additional text to include with the error.
The process.emitWarning()
method can be used to emit custom or application
specific process warnings. These can be listened for by adding a handler to the
'warning'
event.
import { emitWarning } from 'node:process';
// Emit a warning with a code and additional detail.
emitWarning('Something happened!', {
code: 'MY_WARNING',
detail: 'This is some additional information',
});
// Emits:
// (node:56338) [MY_WARNING] Warning: Something happened!
// This is some additional information
const { emitWarning } = require('node:process');
// Emit a warning with a code and additional detail.
emitWarning('Something happened!', {
code: 'MY_WARNING',
detail: 'This is some additional information',
});
// Emits:
// (node:56338) [MY_WARNING] Warning: Something happened!
// This is some additional information
In this example, an Error
object is generated internally by
process.emitWarning()
and passed through to the
'warning'
handler.
import process from 'node:process';
process.on('warning', (warning) => {
console.warn(warning.name); // 'Warning'
console.warn(warning.message); // 'Something happened!'
console.warn(warning.code); // 'MY_WARNING'
console.warn(warning.stack); // Stack trace
console.warn(warning.detail); // 'This is some additional information'
});
const process = require('node:process');
process.on('warning', (warning) => {
console.warn(warning.name); // 'Warning'
console.warn(warning.message); // 'Something happened!'
console.warn(warning.code); // 'MY_WARNING'
console.warn(warning.stack); // Stack trace
console.warn(warning.detail); // 'This is some additional information'
});
If warning
is passed as an Error
object, the options
argument is ignored.
process.emitWarning(warning[, type[, code]][, ctor])
#
warning
<string> | <Error> The warning to emit.type
<string> Whenwarning
is aString
,type
is the name to use for the type of warning being emitted. Default:'Warning'
.code
<string> A unique identifier for the warning instance being emitted.ctor
<Function> Whenwarning
is aString
,ctor
is an optional function used to limit the generated stack trace. Default:process.emitWarning
.
The process.emitWarning()
method can be used to emit custom or application
specific process warnings. These can be listened for by adding a handler to the
'warning'
event.
import { emitWarning } from 'node:process';
// Emit a warning using a string.
emitWarning('Something happened!');
// Emits: (node: 56338) Warning: Something happened!
const { emitWarning } = require('node:process');
// Emit a warning using a string.
emitWarning('Something happened!');
// Emits: (node: 56338) Warning: Something happened!
import { emitWarning } from 'node:process';
// Emit a warning using a string and a type.
emitWarning('Something Happened!', 'CustomWarning');
// Emits: (node:56338) CustomWarning: Something Happened!
const { emitWarning } = require('node:process');
// Emit a warning using a string and a type.
emitWarning('Something Happened!', 'CustomWarning');
// Emits: (node:56338) CustomWarning: Something Happened!
import { emitWarning } from 'node:process';
emitWarning('Something happened!', 'CustomWarning', 'WARN001');
// Emits: (node:56338) [WARN001] CustomWarning: Something happened!
const { emitWarning } = require('node:process');
process.emitWarning('Something happened!', 'CustomWarning', 'WARN001');
// Emits: (node:56338) [WARN001] CustomWarning: Something happened!
In each of the previous examples, an Error
object is generated internally by
process.emitWarning()
and passed through to the 'warning'
handler.
import process from 'node:process';
process.on('warning', (warning) => {
console.warn(warning.name);
console.warn(warning.message);
console.warn(warning.code);
console.warn(warning.stack);
});
const process = require('node:process');
process.on('warning', (warning) => {
console.warn(warning.name);
console.warn(warning.message);
console.warn(warning.code);
console.warn(warning.stack);
});
If warning
is passed as an Error
object, it will be passed through to the
'warning'
event handler unmodified (and the optional type
,
code
and ctor
arguments will be ignored):
import { emitWarning } from 'node:process';
// Emit a warning using an Error object.
const myWarning = new Error('Something happened!');
// Use the Error name property to specify the type name
myWarning.name = 'CustomWarning';
myWarning.code = 'WARN001';
emitWarning(myWarning);
// Emits: (node:56338) [WARN001] CustomWarning: Something happened!
const { emitWarning } = require('node:process');
// Emit a warning using an Error object.
const myWarning = new Error('Something happened!');
// Use the Error name property to specify the type name
myWarning.name = 'CustomWarning';
myWarning.code = 'WARN001';
emitWarning(myWarning);
// Emits: (node:56338) [WARN001] CustomWarning: Something happened!
A TypeError
is thrown if warning
is anything other than a string or Error
object.
While process warnings use Error
objects, the process warning
mechanism is not a replacement for normal error handling mechanisms.
The following additional handling is implemented if the warning type
is
'DeprecationWarning'
:
- If the
--throw-deprecation
command-line flag is used, the deprecation warning is thrown as an exception rather than being emitted as an event. - If the
--no-deprecation
command-line flag is used, the deprecation warning is suppressed. - If the
--trace-deprecation
command-line flag is used, the deprecation warning is printed tostderr
along with the full stack trace.
Avoiding duplicate warnings#
As a best practice, warnings should be emitted only once per process. To do
so, place the emitWarning()
behind a boolean.
import { emitWarning } from 'node:process';
function emitMyWarning() {
if (!emitMyWarning.warned) {
emitMyWarning.warned = true;
emitWarning('Only warn once!');
}
}
emitMyWarning();
// Emits: (node: 56339) Warning: Only warn once!
emitMyWarning();
// Emits nothing
const { emitWarning } = require('node:process');
function emitMyWarning() {
if (!emitMyWarning.warned) {
emitMyWarning.warned = true;
emitWarning('Only warn once!');
}
}
emitMyWarning();
// Emits: (node: 56339) Warning: Only warn once!
emitMyWarning();
// Emits nothing
process.env
#
The process.env
property returns an object containing the user environment.
See environ(7)
.
An example of this object looks like:
{
TERM: 'xterm-256color',
SHELL: '/usr/local/bin/bash',
USER: 'maciej',
PATH: '~/.bin/:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin',
PWD: '/Users/maciej',
EDITOR: 'vim',
SHLVL: '1',
HOME: '/Users/maciej',
LOGNAME: 'maciej',
_: '/usr/local/bin/node'
}
It is possible to modify this object, but such modifications will not be
reflected outside the Node.js process, or (unless explicitly requested)
to other Worker
threads.
In other words, the following example would not work:
$ node -e 'process.env.foo = "bar"' && echo $foo
While the following will:
import { env } from 'node:process';
env.foo = 'bar';
console.log(env.foo);
const { env } = require('node:process');
env.foo = 'bar';
console.log(env.foo);
Assigning a property on process.env
will implicitly convert the value
to a string. This behavior is deprecated. Future versions of Node.js may
throw an error when the value is not a string, number, or boolean.
import { env } from 'node:process';
env.test = null;
console.log(env.test);
// => 'null'
env.test = undefined;
console.log(env.test);
// => 'undefined'
const { env } = require('node:process');
env.test = null;
console.log(env.test);
// => 'null'
env.test = undefined;
console.log(env.test);
// => 'undefined'
Use delete
to delete a property from process.env
.
import { env } from 'node:process';
env.TEST = 1;
delete env.TEST;
console.log(env.TEST);
// => undefined
const { env } = require('node:process');
env.TEST = 1;
delete env.TEST;
console.log(env.TEST);
// => undefined
On Windows operating systems, environment variables are case-insensitive.
import { env } from 'node:process';
env.TEST = 1;
console.log(env.test);
// => 1
const { env } = require('node:process');
env.TEST = 1;
console.log(env.test);
// => 1
Unless explicitly specified when creating a Worker
instance,
each Worker
thread has its own copy of process.env
, based on its
parent thread's process.env
, or whatever was specified as the env
option
to the Worker
constructor. Changes to process.env
will not be visible
across Worker
threads, and only the main thread can make changes that
are visible to the operating system or to native add-ons.
process.execArgv
#
The process.execArgv
property returns the set of Node.js-specific command-line
options passed when the Node.js process was launched. These options do not
appear in the array returned by the process.argv
property, and do not
include the Node.js executable, the name of the script, or any options following
the script name. These options are useful in order to spawn child processes with
the same execution environment as the parent.
$ node --harmony script.js --version
Results in process.execArgv
:
['--harmony']
And process.argv
:
['/usr/local/bin/node', 'script.js', '--version']
Refer to Worker
constructor for the detailed behavior of worker
threads with this property.
process.execPath
#
The process.execPath
property returns the absolute pathname of the executable
that started the Node.js process. Symbolic links, if any, are resolved.
'/usr/local/bin/node'
process.exit([code])
#
code
<integer> | <string> | <null> | <undefined> The exit code. For string type, only integer strings (e.g.,'1') are allowed. Default:0
.
The process.exit()
method instructs Node.js to terminate the process
synchronously with an exit status of code
. If code
is omitted, exit uses
either the 'success' code 0
or the value of process.exitCode
if it has been
set. Node.js will not terminate until all the 'exit'
event listeners are
called.
To exit with a 'failure' code:
import { exit } from 'node:process';
exit(1);
const { exit } = require('node:process');
exit(1);
The shell that executed Node.js should see the exit code as 1
.
Calling process.exit()
will force the process to exit as quickly as possible
even if there are still asynchronous operations pending that have not yet
completed fully, including I/O operations to process.stdout
and
process.stderr
.
In most situations, it is not actually necessary to call process.exit()
explicitly. The Node.js process will exit on its own if there is no additional
work pending in the event loop. The process.exitCode
property can be set to
tell the process which exit code to use when the process exits gracefully.
For instance, the following example illustrates a misuse of the
process.exit()
method that could lead to data printed to stdout being
truncated and lost:
import { exit } from 'node:process';
// This is an example of what *not* to do:
if (someConditionNotMet()) {
printUsageToStdout();
exit(1);
}
const { exit } = require('node:process');
// This is an example of what *not* to do:
if (someConditionNotMet()) {
printUsageToStdout();
exit(1);
}
The reason this is problematic is because writes to process.stdout
in Node.js
are sometimes asynchronous and may occur over multiple ticks of the Node.js
event loop. Calling process.exit()
, however, forces the process to exit
before those additional writes to stdout
can be performed.
Rather than calling process.exit()
directly, the code should set the
process.exitCode
and allow the process to exit naturally by avoiding
scheduling any additional work for the event loop:
import process from 'node:process';
// How to properly set the exit code while letting
// the process exit gracefully.
if (someConditionNotMet()) {
printUsageToStdout();
process.exitCode = 1;
}
const process = require('node:process');
// How to properly set the exit code while letting
// the process exit gracefully.
if (someConditionNotMet()) {
printUsageToStdout();
process.exitCode = 1;
}
If it is necessary to terminate the Node.js process due to an error condition,
throwing an uncaught error and allowing the process to terminate accordingly
is safer than calling process.exit()
.
In Worker
threads, this function stops the current thread rather
than the current process.
process.exitCode
#
- <integer> | <string> | <null> | <undefined> The exit code. For string type, only
integer strings (e.g.,'1') are allowed. Default:
undefined
.
A number which will be the process exit code, when the process either
exits gracefully, or is exited via process.exit()
without specifying
a code.
Specifying a code to process.exit(code)
will override any
previous setting of process.exitCode
.
process.getActiveResourcesInfo()
#
- Returns: <string[]>
The process.getActiveResourcesInfo()
method returns an array of strings
containing the types of the active resources that are currently keeping the
event loop alive.
import { getActiveResourcesInfo } from 'node:process';
import { setTimeout } from 'node:timers';
console.log('Before:', getActiveResourcesInfo());
setTimeout(() => {}, 1000);
console.log('After:', getActiveResourcesInfo());
// Prints:
// Before: [ 'CloseReq', 'TTYWrap', 'TTYWrap', 'TTYWrap' ]
// After: [ 'CloseReq', 'TTYWrap', 'TTYWrap', 'TTYWrap', 'Timeout' ]
const { getActiveResourcesInfo } = require('node:process');
const { setTimeout } = require('node:timers');
console.log('Before:', getActiveResourcesInfo());
setTimeout(() => {}, 1000);
console.log('After:', getActiveResourcesInfo());
// Prints:
// Before: [ 'TTYWrap', 'TTYWrap', 'TTYWrap' ]
// After: [ 'TTYWrap', 'TTYWrap', 'TTYWrap', 'Timeout' ]
process.getegid()
#
The process.getegid()
method returns the numerical effective group identity
of the Node.js process. (See getegid(2)
.)
import process from 'node:process';
if (process.getegid) {
console.log(`Current gid: ${process.getegid()}`);
}
const process = require('node:process');
if (process.getegid) {
console.log(`Current gid: ${process.getegid()}`);
}
This function is only available on POSIX platforms (i.e. not Windows or Android).
process.geteuid()
#
- Returns: <Object>
The process.geteuid()
method returns the numerical effective user identity of
the process. (See geteuid(2)
.)
import process from 'node:process';
if (process.geteuid) {
console.log(`Current uid: ${process.geteuid()}`);
}
const process = require('node:process');
if (process.geteuid) {
console.log(`Current uid: ${process.geteuid()}`);
}
This function is only available on POSIX platforms (i.e. not Windows or Android).
process.getgid()
#
- Returns: <Object>
The process.getgid()
method returns the numerical group identity of the
process. (See getgid(2)
.)
import process from 'node:process';
if (process.getgid) {
console.log(`Current gid: ${process.getgid()}`);
}
const process = require('node:process');
if (process.getgid) {
console.log(`Current gid: ${process.getgid()}`);
}
This function is only available on POSIX platforms (i.e. not Windows or Android).
process.getgroups()
#
- Returns: <integer[]>
The process.getgroups()
method returns an array with the supplementary group
IDs. POSIX leaves it unspecified if the effective group ID is included but
Node.js ensures it always is.
import process from 'node:process';
if (process.getgroups) {
console.log(process.getgroups()); // [ 16, 21, 297 ]
}
const process = require('node:process');
if (process.getgroups) {
console.log(process.getgroups()); // [ 16, 21, 297 ]
}
This function is only available on POSIX platforms (i.e. not Windows or Android).
process.getuid()
#
- Returns: <integer>
The process.getuid()
method returns the numeric user identity of the process.
(See getuid(2)
.)
import process from 'node:process';
if (process.getuid) {
console.log(`Current uid: ${process.getuid()}`);
}
const process = require('node:process');
if (process.getuid) {
console.log(`Current uid: ${process.getuid()}`);
}
This function is only available on POSIX platforms (i.e. not Windows or Android).
process.hasUncaughtExceptionCaptureCallback()
#
- Returns: <boolean>
Indicates whether a callback has been set using
process.setUncaughtExceptionCaptureCallback()
.
process.hrtime([time])
#
process.hrtime.bigint()
instead.time
<integer[]> The result of a previous call toprocess.hrtime()
- Returns: <integer[]>
This is the legacy version of process.hrtime.bigint()
before bigint
was introduced in JavaScript.
The process.hrtime()
method returns the current high-resolution real time
in a [seconds, nanoseconds]
tuple Array
, where nanoseconds
is the
remaining part of the real time that can't be represented in second precision.
time
is an optional parameter that must be the result of a previous
process.hrtime()
call to diff with the current time. If the parameter
passed in is not a tuple Array
, a TypeError
will be thrown. Passing in a
user-defined array instead of the result of a previous call to
process.hrtime()
will lead to undefined behavior.
These times are relative to an arbitrary time in the past, and not related to the time of day and therefore not subject to clock drift. The primary use is for measuring performance between intervals:
import { hrtime } from 'node:process';
const NS_PER_SEC = 1e9;
const time = hrtime();
// [ 1800216, 25 ]
setTimeout(() => {
const diff = hrtime(time);
// [ 1, 552 ]
console.log(`Benchmark took ${diff[0] * NS_PER_SEC + diff[1]} nanoseconds`);
// Benchmark took 1000000552 nanoseconds
}, 1000);
const { hrtime } = require('node:process');
const NS_PER_SEC = 1e9;
const time = hrtime();
// [ 1800216, 25 ]
setTimeout(() => {
const diff = hrtime(time);
// [ 1, 552 ]
console.log(`Benchmark took ${diff[0] * NS_PER_SEC + diff[1]} nanoseconds`);
// Benchmark took 1000000552 nanoseconds
}, 1000);
process.hrtime.bigint()
#
- Returns: <bigint>
The bigint
version of the process.hrtime()
method returning the
current high-resolution real time in nanoseconds as a bigint
.
Unlike process.hrtime()
, it does not support an additional time
argument since the difference can just be computed directly
by subtraction of the two bigint
s.
import { hrtime } from 'node:process';
const start = hrtime.bigint();
// 191051479007711n
setTimeout(() => {
const end = hrtime.bigint();
// 191052633396993n
console.log(`Benchmark took ${end - start} nanoseconds`);
// Benchmark took 1154389282 nanoseconds
}, 1000);
const { hrtime } = require('node:process');
const start = hrtime.bigint();
// 191051479007711n
setTimeout(() => {
const end = hrtime.bigint();
// 191052633396993n
console.log(`Benchmark took ${end - start} nanoseconds`);
// Benchmark took 1154389282 nanoseconds
}, 1000);
process.initgroups(user, extraGroup)
#
user
<string> | <number> The user name or numeric identifier.extraGroup
<string> | <number> A group name or numeric identifier.
The process.initgroups()
method reads the /etc/group
file and initializes
the group access list, using all groups of which the user is a member. This is
a privileged operation that requires that the Node.js process either have root
access or the CAP_SETGID
capability.
Use care when dropping privileges:
import { getgroups, initgroups, setgid } from 'node:process';
console.log(getgroups()); // [ 0 ]
initgroups('nodeuser', 1000); // switch user
console.log(getgroups()); // [ 27, 30, 46, 1000, 0 ]
setgid(1000); // drop root gid
console.log(getgroups()); // [ 27, 30, 46, 1000 ]
const { getgroups, initgroups, setgid } = require('node:process');
console.log(getgroups()); // [ 0 ]
initgroups('nodeuser', 1000); // switch user
console.log(getgroups()); // [ 27, 30, 46, 1000, 0 ]
setgid(1000); // drop root gid
console.log(getgroups()); // [ 27, 30, 46, 1000 ]
This function is only available on POSIX platforms (i.e. not Windows or
Android).
This feature is not available in Worker
threads.
process.kill(pid[, signal])
#
pid
<number> A process IDsignal
<string> | <number> The signal to send, either as a string or number. Default:'SIGTERM'
.
The process.kill()
method sends the signal
to the process identified by
pid
.
Signal names are strings such as 'SIGINT'
or 'SIGHUP'
. See Signal Events
and kill(2)
for more information.
This method will throw an error if the target pid
does not exist. As a special
case, a signal of 0
can be used to test for the existence of a process.
Windows platforms will throw an error if the pid
is used to kill a process
group.
Even though the name of this function is process.kill()
, it is really just a
signal sender, like the kill
system call. The signal sent may do something
other than kill the target process.
import process, { kill } from 'node:process';
process.on('SIGHUP', () => {
console.log('Got SIGHUP signal.');
});
setTimeout(() => {
console.log('Exiting.');
process.exit(0);
}, 100);
kill(process.pid, 'SIGHUP');
const process = require('node:process');
process.on('SIGHUP', () => {
console.log('Got SIGHUP signal.');
});
setTimeout(() => {
console.log('Exiting.');
process.exit(0);
}, 100);
process.kill(process.pid, 'SIGHUP');
When SIGUSR1
is received by a Node.js process, Node.js will start the
debugger. See Signal Events.
process.mainModule
#
require.main
instead.The process.mainModule
property provides an alternative way of retrieving
require.main
. The difference is that if the main module changes at
runtime, require.main
may still refer to the original main module in
modules that were required before the change occurred. Generally, it's
safe to assume that the two refer to the same module.
As with require.main
, process.mainModule
will be undefined
if there
is no entry script.
process.memoryUsage()
#
- Returns: <Object>
Returns an object describing the memory usage of the Node.js process measured in bytes.
import { memoryUsage } from 'node:process';
console.log(memoryUsage());
// Prints:
// {
// rss: 4935680,
// heapTotal: 1826816,
// heapUsed: 650472,
// external: 49879,
// arrayBuffers: 9386
// }
const { memoryUsage } = require('node:process');
console.log(memoryUsage());
// Prints:
// {
// rss: 4935680,
// heapTotal: 1826816,
// heapUsed: 650472,
// external: 49879,
// arrayBuffers: 9386
// }
heapTotal
andheapUsed
refer to V8's memory usage.external
refers to the memory usage of C++ objects bound to JavaScript objects managed by V8.rss
, Resident Set Size, is the amount of space occupied in the main memory device (that is a subset of the total allocated memory) for the process, including all C++ and JavaScript objects and code.arrayBuffers
refers to memory allocated forArrayBuffer
s andSharedArrayBuffer
s, including all Node.jsBuffer
s. This is also included in theexternal
value. When Node.js is used as an embedded library, this value may be0
because allocations forArrayBuffer
s may not be tracked in that case.
When using Worker
threads, rss
will be a value that is valid for the
entire process, while the other fields will only refer to the current thread.
The process.memoryUsage()
method iterates over each page to gather
information about memory usage which might be slow depending on the
program memory allocations.
process.memoryUsage.rss()
#
- Returns: <integer>
The process.memoryUsage.rss()
method returns an integer representing the
Resident Set Size (RSS) in bytes.
The Resident Set Size, is the amount of space occupied in the main memory device (that is a subset of the total allocated memory) for the process, including all C++ and JavaScript objects and code.
This is the same value as the rss
property provided by process.memoryUsage()
but process.memoryUsage.rss()
is faster.
import { memoryUsage } from 'node:process';
console.log(memoryUsage.rss());
// 35655680
const { rss } = require('node:process');
console.log(memoryUsage.rss());
// 35655680
process.nextTick(callback[, ...args])
#
callback
<Function>...args
<any> Additional arguments to pass when invoking thecallback
process.nextTick()
adds callback
to the "next tick queue". This queue is
fully drained after the current operation on the JavaScript stack runs to
completion and before the event loop is allowed to continue. It's possible to
create an infinite loop if one were to recursively call process.nextTick()
.
See the Event Loop guide for more background.
import { nextTick } from 'node:process';
console.log('start');
nextTick(() => {
console.log('nextTick callback');
});
console.log('scheduled');
// Output:
// start
// scheduled
// nextTick callback
const { nextTick } = require('node:process');
console.log('start');
nextTick(() => {
console.log('nextTick callback');
});
console.log('scheduled');
// Output:
// start
// scheduled
// nextTick callback
This is important when developing APIs in order to give users the opportunity to assign event handlers after an object has been constructed but before any I/O has occurred:
import { nextTick } from 'node:process';
function MyThing(options) {
this.setupOptions(options);
nextTick(() => {
this.startDoingStuff();
});
}
const thing = new MyThing();
thing.getReadyForStuff();
// thing.startDoingStuff() gets called now, not before.
const { nextTick } = require('node:process');
function MyThing(options) {
this.setupOptions(options);
nextTick(() => {
this.startDoingStuff();
});
}
const thing = new MyThing();
thing.getReadyForStuff();
// thing.startDoingStuff() gets called now, not before.
It is very important for APIs to be either 100% synchronous or 100% asynchronous. Consider this example:
// WARNING! DO NOT USE! BAD UNSAFE HAZARD!
function maybeSync(arg, cb) {
if (arg) {
cb();
return;
}
fs.stat('file', cb);
}
This API is hazardous because in the following case:
const maybeTrue = Math.random() > 0.5;
maybeSync(maybeTrue, () => {
foo();
});
bar();
It is not clear whether foo()
or bar()
will be called first.
The following approach is much better:
import { nextTick } from 'node:process';
function definitelyAsync(arg, cb) {
if (arg) {
nextTick(cb);
return;
}
fs.stat('file', cb);
}
const { nextTick } = require('node:process');
function definitelyAsync(arg, cb) {
if (arg) {
nextTick(cb);
return;
}
fs.stat('file', cb);
}
When to use queueMicrotask()
vs. process.nextTick()
#
The queueMicrotask()
API is an alternative to process.nextTick()
that
also defers execution of a function using the same microtask queue used to
execute the then, catch, and finally handlers of resolved promises. Within
Node.js, every time the "next tick queue" is drained, the microtask queue
is drained immediately after.
import { nextTick } from 'node:process';
Promise.resolve().then(() => console.log(2));
queueMicrotask(() => console.log(3));
nextTick(() => console.log(1));
// Output:
// 1
// 2
// 3
const { nextTick } = require('node:process');
Promise.resolve().then(() => console.log(2));
queueMicrotask(() => console.log(3));
nextTick(() => console.log(1));
// Output:
// 1
// 2
// 3
For most userland use cases, the queueMicrotask()
API provides a portable
and reliable mechanism for deferring execution that works across multiple
JavaScript platform environments and should be favored over process.nextTick()
.
In simple scenarios, queueMicrotask()
can be a drop-in replacement for
process.nextTick()
.
console.log('start');
queueMicrotask(() => {
console.log('microtask callback');
});
console.log('scheduled');
// Output:
// start
// scheduled
// microtask callback
One note-worthy difference between the two APIs is that process.nextTick()
allows specifying additional values that will be passed as arguments to the
deferred function when it is called. Achieving the same result with
queueMicrotask()
requires using either a closure or a bound function:
function deferred(a, b) {
console.log('microtask', a + b);
}
console.log('start');
queueMicrotask(deferred.bind(undefined, 1, 2));
console.log('scheduled');
// Output:
// start
// scheduled
// microtask 3
There are minor differences in the way errors raised from within the next tick
queue and microtask queue are handled. Errors thrown within a queued microtask
callback should be handled within the queued callback when possible. If they are
not, the process.on('uncaughtException')
event handler can be used to capture
and handle the errors.
When in doubt, unless the specific capabilities of process.nextTick()
are
needed, use queueMicrotask()
.
process.noDeprecation
#
The process.noDeprecation
property indicates whether the --no-deprecation
flag is set on the current Node.js process. See the documentation for
the 'warning'
event and the
emitWarning()
method for more information about this
flag's behavior.
process.permission
#
This API is available through the --experimental-permission
flag.
process.permission
is an object whose methods are used to manage permissions
for the current process. Additional documentation is available in the
Permission Model.
process.permission.deny(scope[, reference])
#
Deny permissions at runtime.
The available scopes are:
fs
- All File Systemfs.read
- File System read operationsfs.write
- File System write operations
The reference has a meaning based on the provided scope. For example, the reference when the scope is File System means files and folders.
// Deny READ operations to the ./README.md file
process.permission.deny('fs.read', ['./README.md']);
// Deny ALL WRITE operations
process.permission.deny('fs.write');
process.permission.has(scope[, reference])
#
Verifies that the process is able to access the given scope and reference.
If no reference is provided, a global scope is assumed, for instance,
process.permission.has('fs.read')
will check if the process has ALL
file system read permissions.
The reference has a meaning based on the provided scope. For example, the reference when the scope is File System means files and folders.
The available scopes are:
fs
- All File Systemfs.read
- File System read operationsfs.write
- File System write operations
// Check if the process has permission to read the README file
process.permission.has('fs.read', './README.md');
// Check if the process has read permission operations
process.permission.has('fs.read');
process.pid
#
The process.pid
property returns the PID of the process.
import { pid } from 'node:process';
console.log(`This process is pid ${pid}`);
const { pid } = require('node:process');
console.log(`This process is pid ${pid}`);
process.platform
#
The process.platform
property returns a string identifying the operating
system platform for which the Node.js binary was compiled.
Currently possible values are:
'aix'
'darwin'
'freebsd'
'linux'
'openbsd'
'sunos'
'win32'
import { platform } from 'node:process';
console.log(`This platform is ${platform}`);
const { platform } = require('node:process');
console.log(`This platform is ${platform}`);
The value 'android'
may also be returned if the Node.js is built on the
Android operating system. However, Android support in Node.js
is experimental.
process.ppid
#
The process.ppid
property returns the PID of the parent of the
current process.
import { ppid } from 'node:process';
console.log(`The parent process is pid ${ppid}`);
const { ppid } = require('node:process');
console.log(`The parent process is pid ${ppid}`);
process.release
#
The process.release
property returns an Object
containing metadata related
to the current release, including URLs for the source tarball and headers-only
tarball.
process.release
contains the following properties:
name
<string> A value that will always be'node'
.sourceUrl
<string> an absolute URL pointing to a.tar.gz
file containing the source code of the current release.headersUrl
<string> an absolute URL pointing to a.tar.gz
file containing only the source header files for the current release. This file is significantly smaller than the full source file and can be used for compiling Node.js native add-ons.libUrl
<string> | <undefined> an absolute URL pointing to anode.lib
file matching the architecture and version of the current release. This file is used for compiling Node.js native add-ons. This property is only present on Windows builds of Node.js and will be missing on all other platforms.lts
<string> | <undefined> a string label identifying the LTS label for this release. This property only exists for LTS releases and isundefined
for all other release types, including Current releases. Valid values include the LTS Release code names (including those that are no longer supported).'Fermium'
for the 14.x LTS line beginning with 14.15.0.'Gallium'
for the 16.x LTS line beginning with 16.13.0.'Hydrogen'
for the 18.x LTS line beginning with 18.12.0. For other LTS Release code names, see Node.js Changelog Archive
{
name: 'node',
lts: 'Hydrogen',
sourceUrl: 'https://nodejs.org/download/release/v18.12.0/node-v18.12.0.tar.gz',
headersUrl: 'https://nodejs.org/download/release/v18.12.0/node-v18.12.0-headers.tar.gz',
libUrl: 'https://nodejs.org/download/release/v18.12.0/win-x64/node.lib'
}
In custom builds from non-release versions of the source tree, only the
name
property may be present. The additional properties should not be
relied upon to exist.
process.report
#
process.report
is an object whose methods are used to generate diagnostic
reports for the current process. Additional documentation is available in the
report documentation.
process.report.compact
#
Write reports in a compact format, single-line JSON, more easily consumable by log processing systems than the default multi-line format designed for human consumption.
import { report } from 'node:process';
console.log(`Reports are compact? ${report.compact}`);
const { report } = require('node:process');
console.log(`Reports are compact? ${report.compact}`);
process.report.directory
#
Directory where the report is written. The default value is the empty string, indicating that reports are written to the current working directory of the Node.js process.
import { report } from 'node:process';
console.log(`Report directory is ${report.directory}`);
const { report } = require('node:process');
console.log(`Report directory is ${report.directory}`);
process.report.filename
#
Filename where the report is written. If set to the empty string, the output filename will be comprised of a timestamp, PID, and sequence number. The default value is the empty string.
If the value of process.report.filename
is set to 'stdout'
or 'stderr'
,
the report is written to the stdout or stderr of the process respectively.
import { report } from 'node:process';
console.log(`Report filename is ${report.filename}`);
const { report } = require('node:process');
console.log(`Report filename is ${report.filename}`);
process.report.getReport([err])
#
Returns a JavaScript Object representation of a diagnostic report for the
running process. The report's JavaScript stack trace is taken from err
, if
present.
import { report } from 'node:process';
const data = report.getReport();
console.log(data.header.nodejsVersion);
// Similar to process.report.writeReport()
import fs from 'node:fs';
fs.writeFileSync('my-report.log', util.inspect(data), 'utf8');
const { report } = require('node:process');
const data = report.getReport();
console.log(data.header.nodejsVersion);
// Similar to process.report.writeReport()
const fs = require('node:fs');
fs.writeFileSync('my-report.log', util.inspect(data), 'utf8');
Additional documentation is available in the report documentation.
process.report.reportOnFatalError
#
If true
, a diagnostic report is generated on fatal errors, such as out of
memory errors or failed C++ assertions.
import { report } from 'node:process';
console.log(`Report on fatal error: ${report.reportOnFatalError}`);
const { report } = require('node:process');
console.log(`Report on fatal error: ${report.reportOnFatalError}`);
process.report.reportOnSignal
#
If true
, a diagnostic report is generated when the process receives the
signal specified by process.report.signal
.
import { report } from 'node:process';
console.log(`Report on signal: ${report.reportOnSignal}`);
const { report } = require('node:process');
console.log(`Report on signal: ${report.reportOnSignal}`);
process.report.reportOnUncaughtException
#
If true
, a diagnostic report is generated on uncaught exception.
import { report } from 'node:process';
console.log(`Report on exception: ${report.reportOnUncaughtException}`);
const { report } = require('node:process');
console.log(`Report on exception: ${report.reportOnUncaughtException}`);
process.report.signal
#
The signal used to trigger the creation of a diagnostic report. Defaults to
'SIGUSR2'
.
import { report } from 'node:process';
console.log(`Report signal: ${report.signal}`);
const { report } = require('node:process');
console.log(`Report signal: ${report.signal}`);
process.report.writeReport([filename][, err])
#
-
filename
<string> Name of the file where the report is written. This should be a relative path, that will be appended to the directory specified inprocess.report.directory
, or the current working directory of the Node.js process, if unspecified. -
err
<Error> A custom error used for reporting the JavaScript stack. -
Returns: <string> Returns the filename of the generated report.
Writes a diagnostic report to a file. If filename
is not provided, the default
filename includes the date, time, PID, and a sequence number. The report's
JavaScript stack trace is taken from err
, if present.
If the value of filename
is set to 'stdout'
or 'stderr'
, the report is
written to the stdout or stderr of the process respectively.
import { report } from 'node:process';
report.writeReport();
const { report } = require('node:process');
report.writeReport();
Additional documentation is available in the report documentation.
process.resourceUsage()
#
- Returns: <Object> the resource usage for the current process. All of these
values come from the
uv_getrusage
call which returns auv_rusage_t
struct.userCPUTime
<integer> maps toru_utime
computed in microseconds. It is the same value asprocess.cpuUsage().user
.systemCPUTime
<integer> maps toru_stime
computed in microseconds. It is the same value asprocess.cpuUsage().system
.maxRSS
<integer> maps toru_maxrss
which is the maximum resident set size used in kilobytes.sharedMemorySize
<integer> maps toru_ixrss
but is not supported by any platform.unsharedDataSize
<integer> maps toru_idrss
but is not supported by any platform.unsharedStackSize
<integer> maps toru_isrss
but is not supported by any platform.minorPageFault
<integer> maps toru_minflt
which is the number of minor page faults for the process, see this article for more details.majorPageFault
<integer> maps toru_majflt
which is the number of major page faults for the process, see this article for more details. This field is not supported on Windows.swappedOut
<integer> maps toru_nswap
but is not supported by any platform.fsRead
<integer> maps toru_inblock
which is the number of times the file system had to perform input.fsWrite
<integer> maps toru_oublock
which is the number of times the file system had to perform output.ipcSent
<integer> maps toru_msgsnd
but is not supported by any platform.ipcReceived
<integer> maps toru_msgrcv
but is not supported by any platform.signalsCount
<integer> maps toru_nsignals
but is not supported by any platform.voluntaryContextSwitches
<integer> maps toru_nvcsw
which is the number of times a CPU context switch resulted due to a process voluntarily giving up the processor before its time slice was completed (usually to await availability of a resource). This field is not supported on Windows.involuntaryContextSwitches
<integer> maps toru_nivcsw
which is the number of times a CPU context switch resulted due to a higher priority process becoming runnable or because the current process exceeded its time slice. This field is not supported on Windows.
import { resourceUsage } from 'node:process';
console.log(resourceUsage());
/*
Will output:
{
userCPUTime: 82872,
systemCPUTime: 4143,
maxRSS: 33164,
sharedMemorySize: 0,
unsharedDataSize: 0,
unsharedStackSize: 0,
minorPageFault: 2469,
majorPageFault: 0,
swappedOut: 0,
fsRead: 0,
fsWrite: 8,
ipcSent: 0,
ipcReceived: 0,
signalsCount: 0,
voluntaryContextSwitches: 79,
involuntaryContextSwitches: 1
}
*/
const { resourceUsage } = require('node:process');
console.log(resourceUsage());
/*
Will output:
{
userCPUTime: 82872,
systemCPUTime: 4143,
maxRSS: 33164,
sharedMemorySize: 0,
unsharedDataSize: 0,
unsharedStackSize: 0,
minorPageFault: 2469,
majorPageFault: 0,
swappedOut: 0,
fsRead: 0,
fsWrite: 8,
ipcSent: 0,
ipcReceived: 0,
signalsCount: 0,
voluntaryContextSwitches: 79,
involuntaryContextSwitches: 1
}
*/
process.send(message[, sendHandle[, options]][, callback])
#
message
<Object>sendHandle
<net.Server> | <net.Socket>options
<Object> used to parameterize the sending of certain types of handles.options
supports the following properties:keepOpen
<boolean> A value that can be used when passing instances ofnet.Socket
. Whentrue
, the socket is kept open in the sending process. Default:false
.
callback
<Function>- Returns: <boolean>
If Node.js is spawned with an IPC channel, the process.send()
method can be
used to send messages to the parent process. Messages will be received as a
'message'
event on the parent's ChildProcess
object.
If Node.js was not spawned with an IPC channel, process.send
will be
undefined
.
The message goes through serialization and parsing. The resulting message might not be the same as what is originally sent.
process.setegid(id)
#
The process.setegid()
method sets the effective group identity of the process.
(See setegid(2)
.) The id
can be passed as either a numeric ID or a group
name string. If a group name is specified, this method blocks while resolving
the associated a numeric ID.
import process from 'node:process';
if (process.getegid && process.setegid) {
console.log(`Current gid: ${process.getegid()}`);
try {
process.setegid(501);
console.log(`New gid: ${process.getegid()}`);
} catch (err) {
console.error(`Failed to set gid: ${err}`);
}
}
const process = require('node:process');
if (process.getegid && process.setegid) {
console.log(`Current gid: ${process.getegid()}`);
try {
process.setegid(501);
console.log(`New gid: ${process.getegid()}`);
} catch (err) {
console.error(`Failed to set gid: ${err}`);
}
}
This function is only available on POSIX platforms (i.e. not Windows or
Android).
This feature is not available in Worker
threads.
process.seteuid(id)
#
The process.seteuid()
method sets the effective user identity of the process.
(See seteuid(2)
.) The id
can be passed as either a numeric ID or a username
string. If a username is specified, the method blocks while resolving the
associated numeric ID.
import process from 'node:process';
if (process.geteuid && process.seteuid) {
console.log(`Current uid: ${process.geteuid()}`);
try {
process.seteuid(501);
console.log(`New uid: ${process.geteuid()}`);
} catch (err) {
console.error(`Failed to set uid: ${err}`);
}
}
const process = require('node:process');
if (process.geteuid && process.seteuid) {
console.log(`Current uid: ${process.geteuid()}`);
try {
process.seteuid(501);
console.log(`New uid: ${process.geteuid()}`);
} catch (err) {
console.error(`Failed to set uid: ${err}`);
}
}
This function is only available on POSIX platforms (i.e. not Windows or
Android).
This feature is not available in Worker
threads.
process.setgid(id)
#
The process.setgid()
method sets the group identity of the process. (See
setgid(2)
.) The id
can be passed as either a numeric ID or a group name
string. If a group name is specified, this method blocks while resolving the
associated numeric ID.
import process from 'node:process';
if (process.getgid && process.setgid) {
console.log(`Current gid: ${process.getgid()}`);
try {
process.setgid(501);
console.log(`New gid: ${process.getgid()}`);
} catch (err) {
console.error(`Failed to set gid: ${err}`);
}
}
const process = require('node:process');
if (process.getgid && process.setgid) {
console.log(`Current gid: ${process.getgid()}`);
try {
process.setgid(501);
console.log(`New gid: ${process.getgid()}`);
} catch (err) {
console.error(`Failed to set gid: ${err}`);
}
}
This function is only available on POSIX platforms (i.e. not Windows or
Android).
This feature is not available in Worker
threads.
process.setgroups(groups)
#
groups
<integer[]>
The process.setgroups()
method sets the supplementary group IDs for the
Node.js process. This is a privileged operation that requires the Node.js
process to have root
or the CAP_SETGID
capability.
The groups
array can contain numeric group IDs, group names, or both.
import process from 'node:process';
if (process.getgroups && process.setgroups) {
try {
process.setgroups([501]);
console.log(process.getgroups()); // new groups
} catch (err) {
console.error(`Failed to set groups: ${err}`);
}
}
const process = require('node:process');
if (process.getgroups && process.setgroups) {
try {
process.setgroups([501]);
console.log(process.getgroups()); // new groups
} catch (err) {
console.error(`Failed to set groups: ${err}`);
}
}
This function is only available on POSIX platforms (i.e. not Windows or
Android).
This feature is not available in Worker
threads.
process.setuid(id)
#
The process.setuid(id)
method sets the user identity of the process. (See
setuid(2)
.) The id
can be passed as either a numeric ID or a username string.
If a username is specified, the method blocks while resolving the associated
numeric ID.
import process from 'node:process';
if (process.getuid && process.setuid) {
console.log(`Current uid: ${process.getuid()}`);
try {
process.setuid(501);
console.log(`New uid: ${process.getuid()}`);
} catch (err) {
console.error(`Failed to set uid: ${err}`);
}
}
const process = require('node:process');
if (process.getuid && process.setuid) {
console.log(`Current uid: ${process.getuid()}`);
try {
process.setuid(501);
console.log(`New uid: ${process.getuid()}`);
} catch (err) {
console.error(`Failed to set uid: ${err}`);
}
}
This function is only available on POSIX platforms (i.e. not Windows or
Android).
This feature is not available in Worker
threads.
process.setSourceMapsEnabled(val)
#
val
<boolean>
This function enables or disables the Source Map v3 support for stack traces.
It provides same features as launching Node.js process with commandline options
--enable-source-maps
.
Only source maps in JavaScript files that are loaded after source maps has been enabled will be parsed and loaded.
process.setUncaughtExceptionCaptureCallback(fn)
#
fn
<Function> | <null>
The process.setUncaughtExceptionCaptureCallback()
function sets a function
that will be invoked when an uncaught exception occurs, which will receive the
exception value itself as its first argument.
If such a function is set, the 'uncaughtException'
event will
not be emitted. If --abort-on-uncaught-exception
was passed from the
command line or set through v8.setFlagsFromString()
, the process will
not abort. Actions configured to take place on exceptions such as report
generations will be affected too
To unset the capture function,
process.setUncaughtExceptionCaptureCallback(null)
may be used. Calling this
method with a non-null
argument while another capture function is set will
throw an error.
Using this function is mutually exclusive with using the deprecated
domain
built-in module.
process.stderr
#
The process.stderr
property returns a stream connected to
stderr
(fd 2
). It is a net.Socket
(which is a Duplex
stream) unless fd 2
refers to a file, in which case it is
a Writable stream.
process.stderr
differs from other Node.js streams in important ways. See
note on process I/O for more information.
process.stderr.fd
#
This property refers to the value of underlying file descriptor of
process.stderr
. The value is fixed at 2
. In Worker
threads,
this field does not exist.
process.stdin
#
The process.stdin
property returns a stream connected to
stdin
(fd 0
). It is a net.Socket
(which is a Duplex
stream) unless fd 0
refers to a file, in which case it is
a Readable stream.
For details of how to read from stdin
see readable.read()
.
As a Duplex stream, process.stdin
can also be used in "old" mode that
is compatible with scripts written for Node.js prior to v0.10.
For more information see Stream compatibility.
In "old" streams mode the stdin
stream is paused by default, so one
must call process.stdin.resume()
to read from it. Note also that calling
process.stdin.resume()
itself would switch stream to "old" mode.
process.stdin.fd
#
This property refers to the value of underlying file descriptor of
process.stdin
. The value is fixed at 0
. In Worker
threads,
this field does not exist.
process.stdout
#
The process.stdout
property returns a stream connected to
stdout
(fd 1
). It is a net.Socket
(which is a Duplex
stream) unless fd 1
refers to a file, in which case it is
a Writable stream.
For example, to copy process.stdin
to process.stdout
:
import { stdin, stdout } from 'node:process';
stdin.pipe(stdout);
const { stdin, stdout } = require('node:process');
stdin.pipe(stdout);
process.stdout
differs from other Node.js streams in important ways. See
note on process I/O for more information.
process.stdout.fd
#
This property refers to the value of underlying file descriptor of
process.stdout
. The value is fixed at 1
. In Worker
threads,
this field does not exist.
A note on process I/O#
process.stdout
and process.stderr
differ from other Node.js streams in
important ways:
- They are used internally by
console.log()
andconsole.error()
, respectively. - Writes may be synchronous depending on what the stream is connected to
and whether the system is Windows or POSIX:
- Files: synchronous on Windows and POSIX
- TTYs (Terminals): asynchronous on Windows, synchronous on POSIX
- Pipes (and sockets): synchronous on Windows, asynchronous on POSIX
These behaviors are partly for historical reasons, as changing them would create backward incompatibility, but they are also expected by some users.
Synchronous writes avoid problems such as output written with console.log()
or
console.error()
being unexpectedly interleaved, or not written at all if
process.exit()
is called before an asynchronous write completes. See
process.exit()
for more information.
Warning: Synchronous writes block the event loop until the write has completed. This can be near instantaneous in the case of output to a file, but under high system load, pipes that are not being read at the receiving end, or with slow terminals or file systems, it's possible for the event loop to be blocked often enough and long enough to have severe negative performance impacts. This may not be a problem when writing to an interactive terminal session, but consider this particularly careful when doing production logging to the process output streams.
To check if a stream is connected to a TTY context, check the isTTY
property.
For instance:
$ node -p "Boolean(process.stdin.isTTY)"
true
$ echo "foo" | node -p "Boolean(process.stdin.isTTY)"
false
$ node -p "Boolean(process.stdout.isTTY)"
true
$ node -p "Boolean(process.stdout.isTTY)" | cat
false
See the TTY documentation for more information.
process.throwDeprecation
#
The initial value of process.throwDeprecation
indicates whether the
--throw-deprecation
flag is set on the current Node.js process.
process.throwDeprecation
is mutable, so whether or not deprecation
warnings result in errors may be altered at runtime. See the
documentation for the 'warning'
event and the
emitWarning()
method for more information.
$ node --throw-deprecation -p "process.throwDeprecation"
true
$ node -p "process.throwDeprecation"
undefined
$ node
> process.emitWarning('test', 'DeprecationWarning');
undefined
> (node:26598) DeprecationWarning: test
> process.throwDeprecation = true;
true
> process.emitWarning('test', 'DeprecationWarning');
Thrown:
[DeprecationWarning: test] { name: 'DeprecationWarning' }
process.title
#
The process.title
property returns the current process title (i.e. returns
the current value of ps
). Assigning a new value to process.title
modifies
the current value of ps
.
When a new value is assigned, different platforms will impose different maximum
length restrictions on the title. Usually such restrictions are quite limited.
For instance, on Linux and macOS, process.title
is limited to the size of the
binary name plus the length of the command-line arguments because setting the
process.title
overwrites the argv
memory of the process. Node.js v0.8
allowed for longer process title strings by also overwriting the environ
memory but that was potentially insecure and confusing in some (rather obscure)
cases.
Assigning a value to process.title
might not result in an accurate label
within process manager applications such as macOS Activity Monitor or Windows
Services Manager.
process.traceDeprecation
#
The process.traceDeprecation
property indicates whether the
--trace-deprecation
flag is set on the current Node.js process. See the
documentation for the 'warning'
event and the
emitWarning()
method for more information about this
flag's behavior.
process.umask()
#
process.umask()
with no argument causes
the process-wide umask to be written twice. This introduces a race condition
between threads, and is a potential security vulnerability. There is no safe,
cross-platform alternative API.process.umask()
returns the Node.js process's file mode creation mask. Child
processes inherit the mask from the parent process.
process.umask(mask)
#
process.umask(mask)
sets the Node.js process's file mode creation mask. Child
processes inherit the mask from the parent process. Returns the previous mask.
import { umask } from 'node:process';
const newmask = 0o022;
const oldmask = umask(newmask);
console.log(
`Changed umask from ${oldmask.toString(8)} to ${newmask.toString(8)}`,
);
const { umask } = require('node:process');
const newmask = 0o022;
const oldmask = umask(newmask);
console.log(
`Changed umask from ${oldmask.toString(8)} to ${newmask.toString(8)}`,
);
In Worker
threads, process.umask(mask)
will throw an exception.
process.uptime()
#
- Returns: <number>
The process.uptime()
method returns the number of seconds the current Node.js
process has been running.
The return value includes fractions of a second. Use Math.floor()
to get whole
seconds.
process.version
#
The process.version
property contains the Node.js version string.
import { version } from 'node:process';
console.log(`Version: ${version}`);
// Version: v14.8.0
const { version } = require('node:process');
console.log(`Version: ${version}`);
// Version: v14.8.0
To get the version string without the prepended v, use
process.versions.node
.
process.versions
#
The process.versions
property returns an object listing the version strings of
Node.js and its dependencies. process.versions.modules
indicates the current
ABI version, which is increased whenever a C++ API changes. Node.js will refuse
to load modules that were compiled against a different module ABI version.
import { versions } from 'node:process';
console.log(versions);
const { versions } = require('node:process');
console.log(versions);
Will generate an object similar to:
{ node: '11.13.0',
v8: '7.0.276.38-node.18',
uv: '1.27.0',
zlib: '1.2.11',
brotli: '1.0.7',
ares: '1.15.0',
modules: '67',
nghttp2: '1.34.0',
napi: '4',
llhttp: '1.1.1',
openssl: '1.1.1b',
cldr: '34.0',
icu: '63.1',
tz: '2018e',
unicode: '11.0' }
Exit codes#
Node.js will normally exit with a 0
status code when no more async
operations are pending. The following status codes are used in other
cases:
1
Uncaught Fatal Exception: There was an uncaught exception, and it was not handled by a domain or an'uncaughtException'
event handler.2
: Unused (reserved by Bash for builtin misuse)3
Internal JavaScript Parse Error: The JavaScript source code internal in the Node.js bootstrapping process caused a parse error. This is extremely rare, and generally can only happen during development of Node.js itself.4
Internal JavaScript Evaluation Failure: The JavaScript source code internal in the Node.js bootstrapping process failed to return a function value when evaluated. This is extremely rare, and generally can only happen during development of Node.js itself.5
Fatal Error: There was a fatal unrecoverable error in V8. Typically a message will be printed to stderr with the prefixFATAL ERROR
.6
Non-function Internal Exception Handler: There was an uncaught exception, but the internal fatal exception handler function was somehow set to a non-function, and could not be called.7
Internal Exception Handler Run-Time Failure: There was an uncaught exception, and the internal fatal exception handler function itself threw an error while attempting to handle it. This can happen, for example, if an'uncaughtException'
ordomain.on('error')
handler throws an error.8
: Unused. In previous versions of Node.js, exit code 8 sometimes indicated an uncaught exception.9
Invalid Argument: Either an unknown option was specified, or an option requiring a value was provided without a value.10
Internal JavaScript Run-Time Failure: The JavaScript source code internal in the Node.js bootstrapping process threw an error when the bootstrapping function was called. This is extremely rare, and generally can only happen during development of Node.js itself.12
Invalid Debug Argument: The--inspect
and/or--inspect-brk
options were set, but the port number chosen was invalid or unavailable.13
Unfinished Top-Level Await:await
was used outside of a function in the top-level code, but the passedPromise
never resolved.14
Snapshot Failure: Node.js was started to build a V8 startup snapshot and it failed because certain requirements of the state of the application were not met.>128
Signal Exits: If Node.js receives a fatal signal such asSIGKILL
orSIGHUP
, then its exit code will be128
plus the value of the signal code. This is a standard POSIX practice, since exit codes are defined to be 7-bit integers, and signal exits set the high-order bit, and then contain the value of the signal code. For example, signalSIGABRT
has value6
, so the expected exit code will be128
+6
, or134
.
Punycode#
Source Code: lib/punycode.js
The version of the punycode module bundled in Node.js is being deprecated.
In a future major version of Node.js this module will be removed. Users
currently depending on the punycode
module should switch to using the
userland-provided Punycode.js module instead. For punycode-based URL
encoding, see url.domainToASCII
or, more generally, the
WHATWG URL API.
The punycode
module is a bundled version of the Punycode.js module. It
can be accessed using:
const punycode = require('punycode');
Punycode is a character encoding scheme defined by RFC 3492 that is
primarily intended for use in Internationalized Domain Names. Because host
names in URLs are limited to ASCII characters only, Domain Names that contain
non-ASCII characters must be converted into ASCII using the Punycode scheme.
For instance, the Japanese character that translates into the English word,
'example'
is '例'
. The Internationalized Domain Name, '例.com'
(equivalent
to 'example.com'
) is represented by Punycode as the ASCII string
'xn--fsq.com'
.
The punycode
module provides a simple implementation of the Punycode standard.
The punycode
module is a third-party dependency used by Node.js and
made available to developers as a convenience. Fixes or other modifications to
the module must be directed to the Punycode.js project.
punycode.decode(string)
#
string
<string>
The punycode.decode()
method converts a Punycode string of ASCII-only
characters to the equivalent string of Unicode codepoints.
punycode.decode('maana-pta'); // 'mañana'
punycode.decode('--dqo34k'); // '☃-⌘'
punycode.encode(string)
#
string
<string>
The punycode.encode()
method converts a string of Unicode codepoints to a
Punycode string of ASCII-only characters.
punycode.encode('mañana'); // 'maana-pta'
punycode.encode('☃-⌘'); // '--dqo34k'
punycode.toASCII(domain)
#
domain
<string>
The punycode.toASCII()
method converts a Unicode string representing an
Internationalized Domain Name to Punycode. Only the non-ASCII parts of the
domain name will be converted. Calling punycode.toASCII()
on a string that
already only contains ASCII characters will have no effect.
// encode domain names
punycode.toASCII('mañana.com'); // 'xn--maana-pta.com'
punycode.toASCII('☃-⌘.com'); // 'xn----dqo34k.com'
punycode.toASCII('example.com'); // 'example.com'
punycode.toUnicode(domain)
#
domain
<string>
The punycode.toUnicode()
method converts a string representing a domain name
containing Punycode encoded characters into Unicode. Only the Punycode
encoded parts of the domain name are be converted.
// decode domain names
punycode.toUnicode('xn--maana-pta.com'); // 'mañana.com'
punycode.toUnicode('xn----dqo34k.com'); // '☃-⌘.com'
punycode.toUnicode('example.com'); // 'example.com'
punycode.ucs2
#
punycode.ucs2.decode(string)
#
string
<string>
The punycode.ucs2.decode()
method returns an array containing the numeric
codepoint values of each Unicode symbol in the string.
punycode.ucs2.decode('abc'); // [0x61, 0x62, 0x63]
// surrogate pair for U+1D306 tetragram for centre:
punycode.ucs2.decode('\uD834\uDF06'); // [0x1D306]
punycode.ucs2.encode(codePoints)
#
codePoints
<integer[]>
The punycode.ucs2.encode()
method returns a string based on an array of
numeric code point values.
punycode.ucs2.encode([0x61, 0x62, 0x63]); // 'abc'
punycode.ucs2.encode([0x1D306]); // '\uD834\uDF06'
punycode.version
#
Returns a string identifying the current Punycode.js version number.
Query string#
Source Code: lib/querystring.js
The node:querystring
module provides utilities for parsing and formatting URL
query strings. It can be accessed using:
const querystring = require('node:querystring');
querystring
is more performant than <URLSearchParams> but is not a
standardized API. Use <URLSearchParams> when performance is not critical or
when compatibility with browser code is desirable.
querystring.decode()
#
The querystring.decode()
function is an alias for querystring.parse()
.
querystring.encode()
#
The querystring.encode()
function is an alias for querystring.stringify()
.
querystring.escape(str)
#
str
<string>
The querystring.escape()
method performs URL percent-encoding on the given
str
in a manner that is optimized for the specific requirements of URL
query strings.
The querystring.escape()
method is used by querystring.stringify()
and is
generally not expected to be used directly. It is exported primarily to allow
application code to provide a replacement percent-encoding implementation if
necessary by assigning querystring.escape
to an alternative function.
querystring.parse(str[, sep[, eq[, options]]])
#
str
<string> The URL query string to parsesep
<string> The substring used to delimit key and value pairs in the query string. Default:'&'
.eq
<string>. The substring used to delimit keys and values in the query string. Default:'='
.options
<Object>decodeURIComponent
<Function> The function to use when decoding percent-encoded characters in the query string. Default:querystring.unescape()
.maxKeys
<number> Specifies the maximum number of keys to parse. Specify0
to remove key counting limitations. Default:1000
.
The querystring.parse()
method parses a URL query string (str
) into a
collection of key and value pairs.
For example, the query string 'foo=bar&abc=xyz&abc=123'
is parsed into:
{
foo: 'bar',
abc: ['xyz', '123']
}
The object returned by the querystring.parse()
method does not
prototypically inherit from the JavaScript Object
. This means that typical
Object
methods such as obj.toString()
, obj.hasOwnProperty()
, and others
are not defined and will not work.
By default, percent-encoded characters within the query string will be assumed
to use UTF-8 encoding. If an alternative character encoding is used, then an
alternative decodeURIComponent
option will need to be specified:
// Assuming gbkDecodeURIComponent function already exists...
querystring.parse('w=%D6%D0%CE%C4&foo=bar', null, null,
{ decodeURIComponent: gbkDecodeURIComponent });
querystring.stringify(obj[, sep[, eq[, options]]])
#
obj
<Object> The object to serialize into a URL query stringsep
<string> The substring used to delimit key and value pairs in the query string. Default:'&'
.eq
<string>. The substring used to delimit keys and values in the query string. Default:'='
.options
encodeURIComponent
<Function> The function to use when converting URL-unsafe characters to percent-encoding in the query string. Default:querystring.escape()
.
The querystring.stringify()
method produces a URL query string from a
given obj
by iterating through the object's "own properties".
It serializes the following types of values passed in obj
:
<string> | <number> | <bigint> | <boolean> | <string[]> | <number[]> | <bigint[]> | <boolean[]>
The numeric values must be finite. Any other input values will be coerced to
empty strings.
querystring.stringify({ foo: 'bar', baz: ['qux', 'quux'], corge: '' });
// Returns 'foo=bar&baz=qux&baz=quux&corge='
querystring.stringify({ foo: 'bar', baz: 'qux' }, ';', ':');
// Returns 'foo:bar;baz:qux'
By default, characters requiring percent-encoding within the query string will
be encoded as UTF-8. If an alternative encoding is required, then an alternative
encodeURIComponent
option will need to be specified:
// Assuming gbkEncodeURIComponent function already exists,
querystring.stringify({ w: '中文', foo: 'bar' }, null, null,
{ encodeURIComponent: gbkEncodeURIComponent });
querystring.unescape(str)
#
str
<string>
The querystring.unescape()
method performs decoding of URL percent-encoded
characters on the given str
.
The querystring.unescape()
method is used by querystring.parse()
and is
generally not expected to be used directly. It is exported primarily to allow
application code to provide a replacement decoding implementation if
necessary by assigning querystring.unescape
to an alternative function.
By default, the querystring.unescape()
method will attempt to use the
JavaScript built-in decodeURIComponent()
method to decode. If that fails,
a safer equivalent that does not throw on malformed URLs will be used.
Readline#
Source Code: lib/readline.js
The node:readline
module provides an interface for reading data from a
Readable stream (such as process.stdin
) one line at a time.
To use the promise-based APIs:
import * as readline from 'node:readline/promises';
const readline = require('node:readline/promises');
To use the callback and sync APIs:
import * as readline from 'node:readline';
const readline = require('node:readline');
The following simple example illustrates the basic use of the node:readline
module.
import * as readline from 'node:readline/promises';
import { stdin as input, stdout as output } from 'node:process';
const rl = readline.createInterface({ input, output });
const answer = await rl.question('What do you think of Node.js? ');
console.log(`Thank you for your valuable feedback: ${answer}`);
rl.close();
const readline = require('node:readline');
const { stdin: input, stdout: output } = require('node:process');
const rl = readline.createInterface({ input, output });
rl.question('What do you think of Node.js? ', (answer) => {
// TODO: Log the answer in a database
console.log(`Thank you for your valuable feedback: ${answer}`);
rl.close();
});
Once this code is invoked, the Node.js application will not terminate until the
readline.Interface
is closed because the interface waits for data to be
received on the input
stream.
Class: InterfaceConstructor
#
- Extends: <EventEmitter>
Instances of the InterfaceConstructor
class are constructed using the
readlinePromises.createInterface()
or readline.createInterface()
method.
Every instance is associated with a single input
Readable stream and a
single output
Writable stream.
The output
stream is used to print prompts for user input that arrives on,
and is read from, the input
stream.
Event: 'close'
#
The 'close'
event is emitted when one of the following occur:
- The
rl.close()
method is called and theInterfaceConstructor
instance has relinquished control over theinput
andoutput
streams; - The
input
stream receives its'end'
event; - The
input
stream receives Ctrl+D to signal end-of-transmission (EOT); - The
input
stream receives Ctrl+C to signalSIGINT
and there is no'SIGINT'
event listener registered on theInterfaceConstructor
instance.
The listener function is called without passing any arguments.
The InterfaceConstructor
instance is finished once the 'close'
event is
emitted.
Event: 'line'
#
The 'line'
event is emitted whenever the input
stream receives an
end-of-line input (\n
, \r
, or \r\n
). This usually occurs when the user
presses Enter or Return.
The 'line'
event is also emitted if new data has been read from a stream and
that stream ends without a final end-of-line marker.
The listener function is called with a string containing the single line of received input.
rl.on('line', (input) => {
console.log(`Received: ${input}`);
});
Event: 'history'
#
The 'history'
event is emitted whenever the history array has changed.
The listener function is called with an array containing the history array.
It will reflect all changes, added lines and removed lines due to
historySize
and removeHistoryDuplicates
.
The primary purpose is to allow a listener to persist the history. It is also possible for the listener to change the history object. This could be useful to prevent certain lines to be added to the history, like a password.
rl.on('history', (history) => {
console.log(`Received: ${history}`);
});
Event: 'pause'
#
The 'pause'
event is emitted when one of the following occur:
- The
input
stream is paused. - The
input
stream is not paused and receives the'SIGCONT'
event. (See events'SIGTSTP'
and'SIGCONT'
.)
The listener function is called without passing any arguments.
rl.on('pause', () => {
console.log('Readline paused.');
});
Event: 'resume'
#
The 'resume'
event is emitted whenever the input
stream is resumed.
The listener function is called without passing any arguments.
rl.on('resume', () => {
console.log('Readline resumed.');
});
Event: 'SIGCONT'
#
The 'SIGCONT'
event is emitted when a Node.js process previously moved into
the background using Ctrl+Z (i.e. SIGTSTP
) is then
brought back to the foreground using fg(1p)
.
If the input
stream was paused before the SIGTSTP
request, this event will
not be emitted.
The listener function is invoked without passing any arguments.
rl.on('SIGCONT', () => {
// `prompt` will automatically resume the stream
rl.prompt();
});
The 'SIGCONT'
event is not supported on Windows.
Event: 'SIGINT'
#
The 'SIGINT'
event is emitted whenever the input
stream receives
a Ctrl+C input, known typically as SIGINT
. If there are no
'SIGINT'
event listeners registered when the input
stream receives a
SIGINT
, the 'pause'
event will be emitted.
The listener function is invoked without passing any arguments.
rl.on('SIGINT', () => {
rl.question('Are you sure you want to exit? ', (answer) => {
if (answer.match(/^y(es)?$/i)) rl.pause();
});
});
Event: 'SIGTSTP'
#
The 'SIGTSTP'
event is emitted when the input
stream receives
a Ctrl+Z input, typically known as SIGTSTP
. If there are
no 'SIGTSTP'
event listeners registered when the input
stream receives a
SIGTSTP
, the Node.js process will be sent to the background.
When the program is resumed using fg(1p)
, the 'pause'
and 'SIGCONT'
events
will be emitted. These can be used to resume the input
stream.
The 'pause'
and 'SIGCONT'
events will not be emitted if the input
was
paused before the process was sent to the background.
The listener function is invoked without passing any arguments.
rl.on('SIGTSTP', () => {
// This will override SIGTSTP and prevent the program from going to the
// background.
console.log('Caught SIGTSTP.');
});
The 'SIGTSTP'
event is not supported on Windows.
rl.close()
#
The rl.close()
method closes the InterfaceConstructor
instance and
relinquishes control over the input
and output
streams. When called,
the 'close'
event will be emitted.
Calling rl.close()
does not immediately stop other events (including 'line'
)
from being emitted by the InterfaceConstructor
instance.
rl.pause()
#
The rl.pause()
method pauses the input
stream, allowing it to be resumed
later if necessary.
Calling rl.pause()
does not immediately pause other events (including
'line'
) from being emitted by the InterfaceConstructor
instance.
rl.prompt([preserveCursor])
#
preserveCursor
<boolean> Iftrue
, prevents the cursor placement from being reset to0
.
The rl.prompt()
method writes the InterfaceConstructor
instances configured
prompt
to a new line in output
in order to provide a user with a new
location at which to provide input.
When called, rl.prompt()
will resume the input
stream if it has been
paused.
If the InterfaceConstructor
was created with output
set to null
or
undefined
the prompt is not written.
rl.resume()
#
The rl.resume()
method resumes the input
stream if it has been paused.
rl.setPrompt(prompt)
#
prompt
<string>
The rl.setPrompt()
method sets the prompt that will be written to output
whenever rl.prompt()
is called.
rl.getPrompt()
#
- Returns: <string> the current prompt string
The rl.getPrompt()
method returns the current prompt used by rl.prompt()
.
rl.write(data[, key])
#
The rl.write()
method will write either data
or a key sequence identified
by key
to the output
. The key
argument is supported only if output
is
a TTY text terminal. See TTY keybindings for a list of key
combinations.
If key
is specified, data
is ignored.
When called, rl.write()
will resume the input
stream if it has been
paused.
If the InterfaceConstructor
was created with output
set to null
or
undefined
the data
and key
are not written.
rl.write('Delete this!');
// Simulate Ctrl+U to delete the line written previously
rl.write(null, { ctrl: true, name: 'u' });
The rl.write()
method will write the data to the readline
Interface
's
input
as if it were provided by the user.
rl[Symbol.asyncIterator]()
#
- Returns: <AsyncIterator>
Create an AsyncIterator
object that iterates through each line in the input
stream as a string. This method allows asynchronous iteration of
InterfaceConstructor
objects through for await...of
loops.
Errors in the input stream are not forwarded.
If the loop is terminated with break
, throw
, or return
,
rl.close()
will be called. In other words, iterating over a
InterfaceConstructor
will always consume the input stream fully.
Performance is not on par with the traditional 'line'
event API. Use 'line'
instead for performance-sensitive applications.
async function processLineByLine() {
const rl = readline.createInterface({
// ...
});
for await (const line of rl) {
// Each line in the readline input will be successively available here as
// `line`.
}
}
readline.createInterface()
will start to consume the input stream once
invoked. Having asynchronous operations between interface creation and
asynchronous iteration may result in missed lines.
rl.line
#
The current input data being processed by node.
This can be used when collecting input from a TTY stream to retrieve the
current value that has been processed thus far, prior to the line
event
being emitted. Once the line
event has been emitted, this property will
be an empty string.
Be aware that modifying the value during the instance runtime may have
unintended consequences if rl.cursor
is not also controlled.
If not using a TTY stream for input, use the 'line'
event.
One possible use case would be as follows:
const values = ['lorem ipsum', 'dolor sit amet'];
const rl = readline.createInterface(process.stdin);
const showResults = debounce(() => {
console.log(
'\n',
values.filter((val) => val.startsWith(rl.line)).join(' '),
);
}, 300);
process.stdin.on('keypress', (c, k) => {
showResults();
});
rl.cursor
#
The cursor position relative to rl.line
.
This will track where the current cursor lands in the input string, when reading input from a TTY stream. The position of cursor determines the portion of the input string that will be modified as input is processed, as well as the column where the terminal caret will be rendered.
rl.getCursorPos()
#
- Returns: <Object>
Returns the real position of the cursor in relation to the input prompt + string. Long input (wrapping) strings, as well as multiple line prompts are included in the calculations.
Promises API#
Class: readlinePromises.Interface
#
- Extends: <readline.InterfaceConstructor>
Instances of the readlinePromises.Interface
class are constructed using the
readlinePromises.createInterface()
method. Every instance is associated with a
single input
Readable stream and a single output
Writable stream.
The output
stream is used to print prompts for user input that arrives on,
and is read from, the input
stream.
rl.question(query[, options])
#
query
<string> A statement or query to write tooutput
, prepended to the prompt.options
<Object>signal
<AbortSignal> Optionally allows thequestion()
to be canceled using anAbortSignal
.
- Returns: <Promise> A promise that is fulfilled with the user's
input in response to the
query
.
The rl.question()
method displays the query
by writing it to the output
,
waits for user input to be provided on input
, then invokes the callback
function passing the provided input as the first argument.
When called, rl.question()
will resume the input
stream if it has been
paused.
If the readlinePromises.Interface
was created with output
set to null
or
undefined
the query
is not written.
If the question is called after rl.close()
, it returns a rejected promise.
Example usage:
const answer = await rl.question('What is your favorite food? ');
console.log(`Oh, so your favorite food is ${answer}`);
Using an AbortSignal
to cancel a question.
const signal = AbortSignal.timeout(10_000);
signal.addEventListener('abort', () => {
console.log('The food question timed out');
}, { once: true });
const answer = await rl.question('What is your favorite food? ', { signal });
console.log(`Oh, so your favorite food is ${answer}`);
Class: readlinePromises.Readline
#
new readlinePromises.Readline(stream[, options])
#
stream
<stream.Writable> A TTY stream.options
<Object>autoCommit
<boolean> Iftrue
, no need to callrl.commit()
.
rl.clearLine(dir)
#
dir
<integer>-1
: to the left from cursor1
: to the right from cursor0
: the entire line
- Returns: this
The rl.clearLine()
method adds to the internal list of pending action an
action that clears current line of the associated stream
in a specified
direction identified by dir
.
Call rl.commit()
to see the effect of this method, unless autoCommit: true
was passed to the constructor.
rl.clearScreenDown()
#
- Returns: this
The rl.clearScreenDown()
method adds to the internal list of pending action an
action that clears the associated stream from the current position of the
cursor down.
Call rl.commit()
to see the effect of this method, unless autoCommit: true
was passed to the constructor.
rl.commit()
#
- Returns: <Promise>
The rl.commit()
method sends all the pending actions to the associated
stream
and clears the internal list of pending actions.
rl.cursorTo(x[, y])
#
The rl.cursorTo()
method adds to the internal list of pending action an action
that moves cursor to the specified position in the associated stream
.
Call rl.commit()
to see the effect of this method, unless autoCommit: true
was passed to the constructor.
rl.moveCursor(dx, dy)
#
The rl.moveCursor()
method adds to the internal list of pending action an
action that moves the cursor relative to its current position in the
associated stream
.
Call rl.commit()
to see the effect of this method, unless autoCommit: true
was passed to the constructor.
rl.rollback()
#
- Returns: this
The rl.rollback
methods clears the internal list of pending actions without
sending it to the associated stream
.
readlinePromises.createInterface(options)
#
options
<Object>input
<stream.Readable> The Readable stream to listen to. This option is required.output
<stream.Writable> The Writable stream to write readline data to.completer
<Function> An optional function used for Tab autocompletion.terminal
<boolean>true
if theinput
andoutput
streams should be treated like a TTY, and have ANSI/VT100 escape codes written to it. Default: checkingisTTY
on theoutput
stream upon instantiation.history
<string[]> Initial list of history lines. This option makes sense only ifterminal
is set totrue
by the user or by an internaloutput
check, otherwise the history caching mechanism is not initialized at all. Default:[]
.historySize
<number> Maximum number of history lines retained. To disable the history set this value to0
. This option makes sense only ifterminal
is set totrue
by the user or by an internaloutput
check, otherwise the history caching mechanism is not initialized at all. Default:30
.removeHistoryDuplicates
<boolean> Iftrue
, when a new input line added to the history list duplicates an older one, this removes the older line from the list. Default:false
.prompt
<string> The prompt string to use. Default:'> '
.crlfDelay
<number> If the delay between\r
and\n
exceedscrlfDelay
milliseconds, both\r
and\n
will be treated as separate end-of-line input.crlfDelay
will be coerced to a number no less than100
. It can be set toInfinity
, in which case\r
followed by\n
will always be considered a single newline (which may be reasonable for reading files with\r\n
line delimiter). Default:100
.escapeCodeTimeout
<number> The durationreadlinePromises
will wait for a character (when reading an ambiguous key sequence in milliseconds one that can both form a complete key sequence using the input read so far and can take additional input to complete a longer key sequence). Default:500
.tabSize
<integer> The number of spaces a tab is equal to (minimum 1). Default:8
.
- Returns: <readlinePromises.Interface>
The readlinePromises.createInterface()
method creates a new readlinePromises.Interface
instance.
const readlinePromises = require('node:readline/promises');
const rl = readlinePromises.createInterface({
input: process.stdin,
output: process.stdout,
});
Once the readlinePromises.Interface
instance is created, the most common case
is to listen for the 'line'
event:
rl.on('line', (line) => {
console.log(`Received: ${line}`);
});
If terminal
is true
for this instance then the output
stream will get
the best compatibility if it defines an output.columns
property and emits
a 'resize'
event on the output
if or when the columns ever change
(process.stdout
does this automatically when it is a TTY).
Use of the completer
function#
The completer
function takes the current line entered by the user
as an argument, and returns an Array
with 2 entries:
- An
Array
with matching entries for the completion. - The substring that was used for the matching.
For instance: [[substr1, substr2, ...], originalsubstring]
.
function completer(line) {
const completions = '.help .error .exit .quit .q'.split(' ');
const hits = completions.filter((c) => c.startsWith(line));
// Show all completions if none found
return [hits.length ? hits : completions, line];
}
The completer
function can also returns a <Promise>, or be asynchronous:
async function completer(linePartial) {
await someAsyncWork();
return [['123'], linePartial];
}
Callback API#
Class: readline.Interface
#
- Extends: <readline.InterfaceConstructor>
Instances of the readline.Interface
class are constructed using the
readline.createInterface()
method. Every instance is associated with a
single input
Readable stream and a single output
Writable stream.
The output
stream is used to print prompts for user input that arrives on,
and is read from, the input
stream.
rl.question(query[, options], callback)
#
query
<string> A statement or query to write tooutput
, prepended to the prompt.options
<Object>signal
<AbortSignal> Optionally allows thequestion()
to be canceled using anAbortController
.
callback
<Function> A callback function that is invoked with the user's input in response to thequery
.
The rl.question()
method displays the query
by writing it to the output
,
waits for user input to be provided on input
, then invokes the callback
function passing the provided input as the first argument.
When called, rl.question()
will resume the input
stream if it has been
paused.
If the readline.Interface
was created with output
set to null
or
undefined
the query
is not written.
The callback
function passed to rl.question()
does not follow the typical
pattern of accepting an Error
object or null
as the first argument.
The callback
is called with the provided answer as the only argument.
An error will be thrown if calling rl.question()
after rl.close()
.
Example usage:
rl.question('What is your favorite food? ', (answer) => {
console.log(`Oh, so your favorite food is ${answer}`);
});
Using an AbortController
to cancel a question.
const ac = new AbortController();
const signal = ac.signal;
rl.question('What is your favorite food? ', { signal }, (answer) => {
console.log(`Oh, so your favorite food is ${answer}`);
});
signal.addEventListener('abort', () => {
console.log('The food question timed out');
}, { once: true });
setTimeout(() => ac.abort(), 10000);
readline.clearLine(stream, dir[, callback])
#
stream
<stream.Writable>dir
<number>-1
: to the left from cursor1
: to the right from cursor0
: the entire line
callback
<Function> Invoked once the operation completes.- Returns: <boolean>
false
ifstream
wishes for the calling code to wait for the'drain'
event to be emitted before continuing to write additional data; otherwisetrue
.
The readline.clearLine()
method clears current line of given TTY stream
in a specified direction identified by dir
.
readline.clearScreenDown(stream[, callback])
#
stream
<stream.Writable>callback
<Function> Invoked once the operation completes.- Returns: <boolean>
false
ifstream
wishes for the calling code to wait for the'drain'
event to be emitted before continuing to write additional data; otherwisetrue
.
The readline.clearScreenDown()
method clears the given TTY stream from
the current position of the cursor down.
readline.createInterface(options)
#
options
<Object>input
<stream.Readable> The Readable stream to listen to. This option is required.output
<stream.Writable> The Writable stream to write readline data to.completer
<Function> An optional function used for Tab autocompletion.terminal
<boolean>true
if theinput
andoutput
streams should be treated like a TTY, and have ANSI/VT100 escape codes written to it. Default: checkingisTTY
on theoutput
stream upon instantiation.history
<string[]> Initial list of history lines. This option makes sense only ifterminal
is set totrue
by the user or by an internaloutput
check, otherwise the history caching mechanism is not initialized at all. Default:[]
.historySize
<number> Maximum number of history lines retained. To disable the history set this value to0
. This option makes sense only ifterminal
is set totrue
by the user or by an internaloutput
check, otherwise the history caching mechanism is not initialized at all. Default:30
.removeHistoryDuplicates
<boolean> Iftrue
, when a new input line added to the history list duplicates an older one, this removes the older line from the list. Default:false
.prompt
<string> The prompt string to use. Default:'> '
.crlfDelay
<number> If the delay between\r
and\n
exceedscrlfDelay
milliseconds, both\r
and\n
will be treated as separate end-of-line input.crlfDelay
will be coerced to a number no less than100
. It can be set toInfinity
, in which case\r
followed by\n
will always be considered a single newline (which may be reasonable for reading files with\r\n
line delimiter). Default:100
.escapeCodeTimeout
<number> The durationreadline
will wait for a character (when reading an ambiguous key sequence in milliseconds one that can both form a complete key sequence using the input read so far and can take additional input to complete a longer key sequence). Default:500
.tabSize
<integer> The number of spaces a tab is equal to (minimum 1). Default:8
.signal
<AbortSignal> Allows closing the interface using an AbortSignal. Aborting the signal will internally callclose
on the interface.
- Returns: <readline.Interface>
The readline.createInterface()
method creates a new readline.Interface
instance.
const readline = require('node:readline');
const rl = readline.createInterface({
input: process.stdin,
output: process.stdout,
});
Once the readline.Interface
instance is created, the most common case is to
listen for the 'line'
event:
rl.on('line', (line) => {
console.log(`Received: ${line}`);
});
If terminal
is true
for this instance then the output
stream will get
the best compatibility if it defines an output.columns
property and emits
a 'resize'
event on the output
if or when the columns ever change
(process.stdout
does this automatically when it is a TTY).
When creating a readline.Interface
using stdin
as input, the program
will not terminate until it receives an EOF character. To exit without
waiting for user input, call process.stdin.unref()
.
Use of the completer
function#
The completer
function takes the current line entered by the user
as an argument, and returns an Array
with 2 entries:
- An
Array
with matching entries for the completion. - The substring that was used for the matching.
For instance: [[substr1, substr2, ...], originalsubstring]
.
function completer(line) {
const completions = '.help .error .exit .quit .q'.split(' ');
const hits = completions.filter((c) => c.startsWith(line));
// Show all completions if none found
return [hits.length ? hits : completions, line];
}
The completer
function can be called asynchronously if it accepts two
arguments:
function completer(linePartial, callback) {
callback(null, [['123'], linePartial]);
}
readline.cursorTo(stream, x[, y][, callback])
#
stream
<stream.Writable>x
<number>y
<number>callback
<Function> Invoked once the operation completes.- Returns: <boolean>
false
ifstream
wishes for the calling code to wait for the'drain'
event to be emitted before continuing to write additional data; otherwisetrue
.
The readline.cursorTo()
method moves cursor to the specified position in a
given TTY stream
.
readline.moveCursor(stream, dx, dy[, callback])
#
stream
<stream.Writable>dx
<number>dy
<number>callback
<Function> Invoked once the operation completes.- Returns: <boolean>
false
ifstream
wishes for the calling code to wait for the'drain'
event to be emitted before continuing to write additional data; otherwisetrue
.
The readline.moveCursor()
method moves the cursor relative to its current
position in a given TTY stream
.
readline.emitKeypressEvents(stream[, interface])
#
stream
<stream.Readable>interface
<readline.InterfaceConstructor>
The readline.emitKeypressEvents()
method causes the given Readable
stream to begin emitting 'keypress'
events corresponding to received input.
Optionally, interface
specifies a readline.Interface
instance for which
autocompletion is disabled when copy-pasted input is detected.
If the stream
is a TTY, then it must be in raw mode.
This is automatically called by any readline instance on its input
if the
input
is a terminal. Closing the readline
instance does not stop
the input
from emitting 'keypress'
events.
readline.emitKeypressEvents(process.stdin);
if (process.stdin.isTTY)
process.stdin.setRawMode(true);
Example: Tiny CLI#
The following example illustrates the use of readline.Interface
class to
implement a small command-line interface:
const readline = require('node:readline');
const rl = readline.createInterface({
input: process.stdin,
output: process.stdout,
prompt: 'OHAI> ',
});
rl.prompt();
rl.on('line', (line) => {
switch (line.trim()) {
case 'hello':
console.log('world!');
break;
default:
console.log(`Say what? I might have heard '${line.trim()}'`);
break;
}
rl.prompt();
}).on('close', () => {
console.log('Have a great day!');
process.exit(0);
});
Example: Read file stream line-by-Line#
A common use case for readline
is to consume an input file one line at a
time. The easiest way to do so is leveraging the fs.ReadStream
API as
well as a for await...of
loop:
const fs = require('node:fs');
const readline = require('node:readline');
async function processLineByLine() {
const fileStream = fs.createReadStream('input.txt');
const rl = readline.createInterface({
input: fileStream,
crlfDelay: Infinity,
});
// Note: we use the crlfDelay option to recognize all instances of CR LF
// ('\r\n') in input.txt as a single line break.
for await (const line of rl) {
// Each line in input.txt will be successively available here as `line`.
console.log(`Line from file: ${line}`);
}
}
processLineByLine();
Alternatively, one could use the 'line'
event:
const fs = require('node:fs');
const readline = require('node:readline');
const rl = readline.createInterface({
input: fs.createReadStream('sample.txt'),
crlfDelay: Infinity,
});
rl.on('line', (line) => {
console.log(`Line from file: ${line}`);
});
Currently, for await...of
loop can be a bit slower. If async
/ await
flow and speed are both essential, a mixed approach can be applied:
const { once } = require('node:events');
const { createReadStream } = require('node:fs');
const { createInterface } = require('node:readline');
(async function processLineByLine() {
try {
const rl = createInterface({
input: createReadStream('big-file.txt'),
crlfDelay: Infinity,
});
rl.on('line', (line) => {
// Process the line.
});
await once(rl, 'close');
console.log('File processed.');
} catch (err) {
console.error(err);
}
})();
TTY keybindings#
Keybindings | Description | Notes |
---|---|---|
Ctrl+Shift+Backspace | Delete line left | Doesn't work on Linux, Mac and Windows |
Ctrl+Shift+Delete | Delete line right | Doesn't work on Mac |
Ctrl+C | Emit SIGINT or close the readline instance |
|
Ctrl+H | Delete left | |
Ctrl+D | Delete right or close the readline instance in case the current line is empty / EOF | Doesn't work on Windows |
Ctrl+U | Delete from the current position to the line start | |
Ctrl+K | Delete from the current position to the end of line | |
Ctrl+Y | Yank (Recall) the previously deleted text | Only works with text deleted by Ctrl+U or Ctrl+K |
Meta+Y | Cycle among previously deleted lines | Only available when the last keystroke is Ctrl+Y |
Ctrl+A | Go to start of line | |
Ctrl+E | Go to end of line | |
Ctrl+B | Back one character | |
Ctrl+F | Forward one character | |
Ctrl+L | Clear screen | |
Ctrl+N | Next history item | |
Ctrl+P | Previous history item | |
Ctrl+- | Undo previous change | Any keystroke that emits key code 0x1F will do this action.
In many terminals, for example xterm ,
this is bound to Ctrl+-. |
Ctrl+6 | Redo previous change | Many terminals don't have a default redo keystroke.
We choose key code 0x1E to perform redo.
In xterm , it is bound to Ctrl+6
by default. |
Ctrl+Z | Moves running process into background. Type
fg and press Enter
to return. |
Doesn't work on Windows |
Ctrl+W or Ctrl +Backspace | Delete backward to a word boundary | Ctrl+Backspace Doesn't work on Linux, Mac and Windows |
Ctrl+Delete | Delete forward to a word boundary | Doesn't work on Mac |
Ctrl+Left arrow or Meta+B | Word left | Ctrl+Left arrow Doesn't work on Mac |
Ctrl+Right arrow or Meta+F | Word right | Ctrl+Right arrow Doesn't work on Mac |
Meta+D or Meta +Delete | Delete word right | Meta+Delete Doesn't work on windows |
Meta+Backspace | Delete word left | Doesn't work on Mac |
REPL#
Source Code: lib/repl.js
The node:repl
module provides a Read-Eval-Print-Loop (REPL) implementation
that is available both as a standalone program or includible in other
applications. It can be accessed using:
const repl = require('node:repl');
Design and features#
The node:repl
module exports the repl.REPLServer
class. While running,
instances of repl.REPLServer
will accept individual lines of user input,
evaluate those according to a user-defined evaluation function, then output the
result. Input and output may be from stdin
and stdout
, respectively, or may
be connected to any Node.js stream.
Instances of repl.REPLServer
support automatic completion of inputs,
completion preview, simplistic Emacs-style line editing, multi-line inputs,
ZSH-like reverse-i-search, ZSH-like substring-based history search,
ANSI-styled output, saving and restoring current REPL session state, error
recovery, and customizable evaluation functions. Terminals that do not support
ANSI styles and Emacs-style line editing automatically fall back to a limited
feature set.
Commands and special keys#
The following special commands are supported by all REPL instances:
.break
: When in the process of inputting a multi-line expression, enter the.break
command (or press Ctrl+C) to abort further input or processing of that expression..clear
: Resets the REPLcontext
to an empty object and clears any multi-line expression being input..exit
: Close the I/O stream, causing the REPL to exit..help
: Show this list of special commands..save
: Save the current REPL session to a file:> .save ./file/to/save.js
.load
: Load a file into the current REPL session.> .load ./file/to/load.js
.editor
: Enter editor mode (Ctrl+D to finish, Ctrl+C to cancel).
> .editor
// Entering editor mode (^D to finish, ^C to cancel)
function welcome(name) {
return `Hello ${name}!`;
}
welcome('Node.js User');
// ^D
'Hello Node.js User!'
>
The following key combinations in the REPL have these special effects:
- Ctrl+C: When pressed once, has the same effect as the
.break
command. When pressed twice on a blank line, has the same effect as the.exit
command. - Ctrl+D: Has the same effect as the
.exit
command. - Tab: When pressed on a blank line, displays global and local (scope) variables. When pressed while entering other input, displays relevant autocompletion options.
For key bindings related to the reverse-i-search, see reverse-i-search
.
For all other key bindings, see TTY keybindings.
Default evaluation#
By default, all instances of repl.REPLServer
use an evaluation function
that evaluates JavaScript expressions and provides access to Node.js built-in
modules. This default behavior can be overridden by passing in an alternative
evaluation function when the repl.REPLServer
instance is created.
JavaScript expressions#
The default evaluator supports direct evaluation of JavaScript expressions:
> 1 + 1
2
> const m = 2
undefined
> m + 1
3
Unless otherwise scoped within blocks or functions, variables declared
either implicitly or using the const
, let
, or var
keywords
are declared at the global scope.
Global and local scope#
The default evaluator provides access to any variables that exist in the global
scope. It is possible to expose a variable to the REPL explicitly by assigning
it to the context
object associated with each REPLServer
:
const repl = require('node:repl');
const msg = 'message';
repl.start('> ').context.m = msg;
Properties in the context
object appear as local within the REPL:
$ node repl_test.js
> m
'message'
Context properties are not read-only by default. To specify read-only globals,
context properties must be defined using Object.defineProperty()
:
const repl = require('node:repl');
const msg = 'message';
const r = repl.start('> ');
Object.defineProperty(r.context, 'm', {
configurable: false,
enumerable: true,
value: msg,
});
Accessing core Node.js modules#
The default evaluator will automatically load Node.js core modules into the
REPL environment when used. For instance, unless otherwise declared as a
global or scoped variable, the input fs
will be evaluated on-demand as
global.fs = require('node:fs')
.
> fs.createReadStream('./some/file');
Global uncaught exceptions#
The REPL uses the domain
module to catch all uncaught exceptions for that
REPL session.
This use of the domain
module in the REPL has these side effects:
-
Uncaught exceptions only emit the
'uncaughtException'
event in the standalone REPL. Adding a listener for this event in a REPL within another Node.js program results inERR_INVALID_REPL_INPUT
.const r = repl.start(); r.write('process.on("uncaughtException", () => console.log("Foobar"));\n'); // Output stream includes: // TypeError [ERR_INVALID_REPL_INPUT]: Listeners for `uncaughtException` // cannot be used in the REPL r.close();
-
Trying to use
process.setUncaughtExceptionCaptureCallback()
throws anERR_DOMAIN_CANNOT_SET_UNCAUGHT_EXCEPTION_CAPTURE
error.
Assignment of the _
(underscore) variable#
The default evaluator will, by default, assign the result of the most recently
evaluated expression to the special variable _
(underscore).
Explicitly setting _
to a value will disable this behavior.
> [ 'a', 'b', 'c' ]
[ 'a', 'b', 'c' ]
> _.length
3
> _ += 1
Expression assignment to _ now disabled.
4
> 1 + 1
2
> _
4
Similarly, _error
will refer to the last seen error, if there was any.
Explicitly setting _error
to a value will disable this behavior.
> throw new Error('foo');
Uncaught Error: foo
> _error.message
'foo'
await
keyword#
Support for the await
keyword is enabled at the top level.
> await Promise.resolve(123)
123
> await Promise.reject(new Error('REPL await'))
Uncaught Error: REPL await
at REPL2:1:54
> const timeout = util.promisify(setTimeout);
undefined
> const old = Date.now(); await timeout(1000); console.log(Date.now() - old);
1002
undefined
One known limitation of using the await
keyword in the REPL is that
it will invalidate the lexical scoping of the const
and let
keywords.
For example:
> const m = await Promise.resolve(123)
undefined
> m
123
> const m = await Promise.resolve(234)
undefined
> m
234
--no-experimental-repl-await
shall disable top-level await in REPL.
Reverse-i-search#
The REPL supports bi-directional reverse-i-search similar to ZSH. It is triggered with Ctrl+R to search backward and Ctrl+S to search forwards.
Duplicated history entries will be skipped.
Entries are accepted as soon as any key is pressed that doesn't correspond with the reverse search. Cancelling is possible by pressing Esc or Ctrl+C.
Changing the direction immediately searches for the next entry in the expected direction from the current position on.
Custom evaluation functions#
When a new repl.REPLServer
is created, a custom evaluation function may be
provided. This can be used, for instance, to implement fully customized REPL
applications.
The following illustrates a hypothetical example of a REPL that performs translation of text from one language to another:
const repl = require('node:repl');
const { Translator } = require('translator');
const myTranslator = new Translator('en', 'fr');
function myEval(cmd, context, filename, callback) {
callback(null, myTranslator.translate(cmd));
}
repl.start({ prompt: '> ', eval: myEval });
Recoverable errors#
At the REPL prompt, pressing Enter sends the current line of input to
the eval
function. In order to support multi-line input, the eval
function
can return an instance of repl.Recoverable
to the provided callback function:
function myEval(cmd, context, filename, callback) {
let result;
try {
result = vm.runInThisContext(cmd);
} catch (e) {
if (isRecoverableError(e)) {
return callback(new repl.Recoverable(e));
}
}
callback(null, result);
}
function isRecoverableError(error) {
if (error.name === 'SyntaxError') {
return /^(Unexpected end of input|Unexpected token)/.test(error.message);
}
return false;
}
Customizing REPL output#
By default, repl.REPLServer
instances format output using the
util.inspect()
method before writing the output to the provided Writable
stream (process.stdout
by default). The showProxy
inspection option is set
to true by default and the colors
option is set to true depending on the
REPL's useColors
option.
The useColors
boolean option can be specified at construction to instruct the
default writer to use ANSI style codes to colorize the output from the
util.inspect()
method.
If the REPL is run as standalone program, it is also possible to change the
REPL's inspection defaults from inside the REPL by using the
inspect.replDefaults
property which mirrors the defaultOptions
from
util.inspect()
.
> util.inspect.replDefaults.compact = false;
false
> [1]
[
1
]
>
To fully customize the output of a repl.REPLServer
instance pass in a new
function for the writer
option on construction. The following example, for
instance, simply converts any input text to upper case:
const repl = require('node:repl');
const r = repl.start({ prompt: '> ', eval: myEval, writer: myWriter });
function myEval(cmd, context, filename, callback) {
callback(null, cmd);
}
function myWriter(output) {
return output.toUpperCase();
}
Class: REPLServer
#
options
<Object> | <string> Seerepl.start()
- Extends: <readline.Interface>
Instances of repl.REPLServer
are created using the repl.start()
method
or directly using the JavaScript new
keyword.
const repl = require('node:repl');
const options = { useColors: true };
const firstInstance = repl.start(options);
const secondInstance = new repl.REPLServer(options);
Event: 'exit'
#
The 'exit'
event is emitted when the REPL is exited either by receiving the
.exit
command as input, the user pressing Ctrl+C twice
to signal SIGINT
,
or by pressing Ctrl+D to signal 'end'
on the input
stream. The listener
callback is invoked without any arguments.
replServer.on('exit', () => {
console.log('Received "exit" event from repl!');
process.exit();
});
Event: 'reset'
#
The 'reset'
event is emitted when the REPL's context is reset. This occurs
whenever the .clear
command is received as input unless the REPL is using
the default evaluator and the repl.REPLServer
instance was created with the
useGlobal
option set to true
. The listener callback will be called with a
reference to the context
object as the only argument.
This can be used primarily to re-initialize REPL context to some pre-defined state:
const repl = require('node:repl');
function initializeContext(context) {
context.m = 'test';
}
const r = repl.start({ prompt: '> ' });
initializeContext(r.context);
r.on('reset', initializeContext);
When this code is executed, the global 'm'
variable can be modified but then
reset to its initial value using the .clear
command:
$ ./node example.js
> m
'test'
> m = 1
1
> m
1
> .clear
Clearing context...
> m
'test'
>
replServer.defineCommand(keyword, cmd)
#
keyword
<string> The command keyword (without a leading.
character).cmd
<Object> | <Function> The function to invoke when the command is processed.
The replServer.defineCommand()
method is used to add new .
-prefixed commands
to the REPL instance. Such commands are invoked by typing a .
followed by the
keyword
. The cmd
is either a Function
or an Object
with the following
properties:
help
<string> Help text to be displayed when.help
is entered (Optional).action
<Function> The function to execute, optionally accepting a single string argument.
The following example shows two new commands added to the REPL instance:
const repl = require('node:repl');
const replServer = repl.start({ prompt: '> ' });
replServer.defineCommand('sayhello', {
help: 'Say hello',
action(name) {
this.clearBufferedCommand();
console.log(`Hello, ${name}!`);
this.displayPrompt();
},
});
replServer.defineCommand('saybye', function saybye() {
console.log('Goodbye!');
this.close();
});
The new commands can then be used from within the REPL instance:
> .sayhello Node.js User
Hello, Node.js User!
> .saybye
Goodbye!
replServer.displayPrompt([preserveCursor])
#
preserveCursor
<boolean>
The replServer.displayPrompt()
method readies the REPL instance for input
from the user, printing the configured prompt
to a new line in the output
and resuming the input
to accept new input.
When multi-line input is being entered, an ellipsis is printed rather than the 'prompt'.
When preserveCursor
is true
, the cursor placement will not be reset to 0
.
The replServer.displayPrompt
method is primarily intended to be called from
within the action function for commands registered using the
replServer.defineCommand()
method.
replServer.clearBufferedCommand()
#
The replServer.clearBufferedCommand()
method clears any command that has been
buffered but not yet executed. This method is primarily intended to be
called from within the action function for commands registered using the
replServer.defineCommand()
method.
replServer.parseREPLKeyword(keyword[, rest])
#
keyword
<string> the potential keyword to parse and executerest
<any> any parameters to the keyword command- Returns: <boolean>
An internal method used to parse and execute REPLServer
keywords.
Returns true
if keyword
is a valid keyword, otherwise false
.
replServer.setupHistory(historyPath, callback)
#
historyPath
<string> the path to the history filecallback
<Function> called when history writes are ready or upon errorerr
<Error>repl
<repl.REPLServer>
Initializes a history log file for the REPL instance. When executing the Node.js binary and using the command-line REPL, a history file is initialized by default. However, this is not the case when creating a REPL programmatically. Use this method to initialize a history log file when working with REPL instances programmatically.
repl.builtinModules
#
A list of the names of all Node.js modules, e.g., 'http'
.
repl.start([options])
#
options
<Object> | <string>prompt
<string> The input prompt to display. Default:'> '
(with a trailing space).input
<stream.Readable> TheReadable
stream from which REPL input will be read. Default:process.stdin
.output
<stream.Writable> TheWritable
stream to which REPL output will be written. Default:process.stdout
.terminal
<boolean> Iftrue
, specifies that theoutput
should be treated as a TTY terminal. Default: checking the value of theisTTY
property on theoutput
stream upon instantiation.eval
<Function> The function to be used when evaluating each given line of input. Default: an async wrapper for the JavaScripteval()
function. Aneval
function can error withrepl.Recoverable
to indicate the input was incomplete and prompt for additional lines.useColors
<boolean> Iftrue
, specifies that the defaultwriter
function should include ANSI color styling to REPL output. If a customwriter
function is provided then this has no effect. Default: checking color support on theoutput
stream if the REPL instance'sterminal
value istrue
.useGlobal
<boolean> Iftrue
, specifies that the default evaluation function will use the JavaScriptglobal
as the context as opposed to creating a new separate context for the REPL instance. The node CLI REPL sets this value totrue
. Default:false
.ignoreUndefined
<boolean> Iftrue
, specifies that the default writer will not output the return value of a command if it evaluates toundefined
. Default:false
.writer
<Function> The function to invoke to format the output of each command before writing tooutput
. Default:util.inspect()
.completer
<Function> An optional function used for custom Tab auto completion. Seereadline.InterfaceCompleter
for an example.replMode
<symbol> A flag that specifies whether the default evaluator executes all JavaScript commands in strict mode or default (sloppy) mode. Acceptable values are:repl.REPL_MODE_SLOPPY
to evaluate expressions in sloppy mode.repl.REPL_MODE_STRICT
to evaluate expressions in strict mode. This is equivalent to prefacing every repl statement with'use strict'
.
breakEvalOnSigint
<boolean> Stop evaluating the current piece of code whenSIGINT
is received, such as when Ctrl+C is pressed. This cannot be used together with a customeval
function. Default:false
.preview
<boolean> Defines if the repl prints autocomplete and output previews or not. Default:true
with the default eval function andfalse
in case a custom eval function is used. Ifterminal
is falsy, then there are no previews and the value ofpreview
has no effect.
- Returns: <repl.REPLServer>
The repl.start()
method creates and starts a repl.REPLServer
instance.
If options
is a string, then it specifies the input prompt:
const repl = require('node:repl');
// a Unix style prompt
repl.start('$ ');
The Node.js REPL#
Node.js itself uses the node:repl
module to provide its own interactive
interface for executing JavaScript. This can be used by executing the Node.js
binary without passing any arguments (or by passing the -i
argument):
$ node
> const a = [1, 2, 3];
undefined
> a
[ 1, 2, 3 ]
> a.forEach((v) => {
... console.log(v);
... });
1
2
3
Environment variable options#
Various behaviors of the Node.js REPL can be customized using the following environment variables:
NODE_REPL_HISTORY
: When a valid path is given, persistent REPL history will be saved to the specified file rather than.node_repl_history
in the user's home directory. Setting this value to''
(an empty string) will disable persistent REPL history. Whitespace will be trimmed from the value. On Windows platforms environment variables with empty values are invalid so set this variable to one or more spaces to disable persistent REPL history.NODE_REPL_HISTORY_SIZE
: Controls how many lines of history will be persisted if history is available. Must be a positive number. Default:1000
.NODE_REPL_MODE
: May be either'sloppy'
or'strict'
. Default:'sloppy'
, which will allow non-strict mode code to be run.
Persistent history#
By default, the Node.js REPL will persist history between node
REPL sessions
by saving inputs to a .node_repl_history
file located in the user's home
directory. This can be disabled by setting the environment variable
NODE_REPL_HISTORY=''
.
Using the Node.js REPL with advanced line-editors#
For advanced line-editors, start Node.js with the environment variable
NODE_NO_READLINE=1
. This will start the main and debugger REPL in canonical
terminal settings, which will allow use with rlwrap
.
For example, the following can be added to a .bashrc
file:
alias node="env NODE_NO_READLINE=1 rlwrap node"
Starting multiple REPL instances against a single running instance#
It is possible to create and run multiple REPL instances against a single
running instance of Node.js that share a single global
object but have
separate I/O interfaces.
The following example, for instance, provides separate REPLs on stdin
, a Unix
socket, and a TCP socket:
const net = require('node:net');
const repl = require('node:repl');
let connections = 0;
repl.start({
prompt: 'Node.js via stdin> ',
input: process.stdin,
output: process.stdout,
});
net.createServer((socket) => {
connections += 1;
repl.start({
prompt: 'Node.js via Unix socket> ',
input: socket,
output: socket,
}).on('exit', () => {
socket.end();
});
}).listen('/tmp/node-repl-sock');
net.createServer((socket) => {
connections += 1;
repl.start({
prompt: 'Node.js via TCP socket> ',
input: socket,
output: socket,
}).on('exit', () => {
socket.end();
});
}).listen(5001);
Running this application from the command line will start a REPL on stdin.
Other REPL clients may connect through the Unix socket or TCP socket. telnet
,
for instance, is useful for connecting to TCP sockets, while socat
can be used
to connect to both Unix and TCP sockets.
By starting a REPL from a Unix socket-based server instead of stdin, it is possible to connect to a long-running Node.js process without restarting it.
For an example of running a "full-featured" (terminal
) REPL over
a net.Server
and net.Socket
instance, see:
https://gist.github.com/TooTallNate/2209310.
For an example of running a REPL instance over curl(1)
, see:
https://gist.github.com/TooTallNate/2053342.
Diagnostic report#
Delivers a JSON-formatted diagnostic summary, written to a file.
The report is intended for development, test, and production use, to capture and preserve information for problem determination. It includes JavaScript and native stack traces, heap statistics, platform information, resource usage etc. With the report option enabled, diagnostic reports can be triggered on unhandled exceptions, fatal errors and user signals, in addition to triggering programmatically through API calls.
A complete example report that was generated on an uncaught exception is provided below for reference.
{
"header": {
"reportVersion": 3,
"event": "exception",
"trigger": "Exception",
"filename": "report.20181221.005011.8974.0.001.json",
"dumpEventTime": "2018-12-21T00:50:11Z",
"dumpEventTimeStamp": "1545371411331",
"processId": 8974,
"cwd": "/home/nodeuser/project/node",
"commandLine": [
"/home/nodeuser/project/node/out/Release/node",
"--report-uncaught-exception",
"/home/nodeuser/project/node/test/report/test-exception.js",
"child"
],
"nodejsVersion": "v12.0.0-pre",
"glibcVersionRuntime": "2.17",
"glibcVersionCompiler": "2.17",
"wordSize": "64 bit",
"arch": "x64",
"platform": "linux",
"componentVersions": {
"node": "12.0.0-pre",
"v8": "7.1.302.28-node.5",
"uv": "1.24.1",
"zlib": "1.2.11",
"ares": "1.15.0",
"modules": "68",
"nghttp2": "1.34.0",
"napi": "3",
"llhttp": "1.0.1",
"openssl": "1.1.0j"
},
"release": {
"name": "node"
},
"osName": "Linux",
"osRelease": "3.10.0-862.el7.x86_64",
"osVersion": "#1 SMP Wed Mar 21 18:14:51 EDT 2018",
"osMachine": "x86_64",
"cpus": [
{
"model": "Intel(R) Core(TM) i7-6820HQ CPU @ 2.70GHz",
"speed": 2700,
"user": 88902660,
"nice": 0,
"sys": 50902570,
"idle": 241732220,
"irq": 0
},
{
"model": "Intel(R) Core(TM) i7-6820HQ CPU @ 2.70GHz",
"speed": 2700,
"user": 88902660,
"nice": 0,
"sys": 50902570,
"idle": 241732220,
"irq": 0
}
],
"networkInterfaces": [
{
"name": "en0",
"internal": false,
"mac": "13:10:de:ad:be:ef",
"address": "10.0.0.37",
"netmask": "255.255.255.0",
"family": "IPv4"
}
],
"host": "test_machine"
},
"javascriptStack": {
"message": "Error: *** test-exception.js: throwing uncaught Error",
"stack": [
"at myException (/home/nodeuser/project/node/test/report/test-exception.js:9:11)",
"at Object.<anonymous> (/home/nodeuser/project/node/test/report/test-exception.js:12:3)",
"at Module._compile (internal/modules/cjs/loader.js:718:30)",
"at Object.Module._extensions..js (internal/modules/cjs/loader.js:729:10)",
"at Module.load (internal/modules/cjs/loader.js:617:32)",
"at tryModuleLoad (internal/modules/cjs/loader.js:560:12)",
"at Function.Module._load (internal/modules/cjs/loader.js:552:3)",
"at Function.Module.runMain (internal/modules/cjs/loader.js:771:12)",
"at executeUserCode (internal/bootstrap/node.js:332:15)"
]
},
"nativeStack": [
{
"pc": "0x000055b57f07a9ef",
"symbol": "report::GetNodeReport(v8::Isolate*, node::Environment*, char const*, char const*, v8::Local<v8::String>, std::ostream&) [./node]"
},
{
"pc": "0x000055b57f07cf03",
"symbol": "report::GetReport(v8::FunctionCallbackInfo<v8::Value> const&) [./node]"
},
{
"pc": "0x000055b57f1bccfd",
"symbol": " [./node]"
},
{
"pc": "0x000055b57f1be048",
"symbol": "v8::internal::Builtin_HandleApiCall(int, v8::internal::Object**, v8::internal::Isolate*) [./node]"
},
{
"pc": "0x000055b57feeda0e",
"symbol": " [./node]"
}
],
"javascriptHeap": {
"totalMemory": 5660672,
"executableMemory": 524288,
"totalCommittedMemory": 5488640,
"availableMemory": 4341379928,
"totalGlobalHandlesMemory": 8192,
"usedGlobalHandlesMemory": 3136,
"usedMemory": 4816432,
"memoryLimit": 4345298944,
"mallocedMemory": 254128,
"externalMemory": 315644,
"peakMallocedMemory": 98752,
"nativeContextCount": 1,
"detachedContextCount": 0,
"doesZapGarbage": 0,
"heapSpaces": {
"read_only_space": {
"memorySize": 524288,
"committedMemory": 39208,
"capacity": 515584,
"used": 30504,
"available": 485080
},
"new_space": {
"memorySize": 2097152,
"committedMemory": 2019312,
"capacity": 1031168,
"used": 985496,
"available": 45672
},
"old_space": {
"memorySize": 2273280,
"committedMemory": 1769008,
"capacity": 1974640,
"used": 1725488,
"available": 249152
},
"code_space": {
"memorySize": 696320,
"committedMemory": 184896,
"capacity": 152128,
"used": 152128,
"available": 0
},
"map_space": {
"memorySize": 536576,
"committedMemory": 344928,
"capacity": 327520,
"used": 327520,
"available": 0
},
"large_object_space": {
"memorySize": 0,
"committedMemory": 0,
"capacity": 1520590336,
"used": 0,
"available": 1520590336
},
"new_large_object_space": {
"memorySize": 0,
"committedMemory": 0,
"capacity": 0,
"used": 0,
"available": 0
}
}
},
"resourceUsage": {
"rss": "35766272",
"free_memory": "1598337024",
"total_memory": "17179869184",
"available_memory": "1598337024",
"maxRss": "36624662528",
"constrained_memory": "36624662528",
"userCpuSeconds": 0.040072,
"kernelCpuSeconds": 0.016029,
"cpuConsumptionPercent": 5.6101,
"userCpuConsumptionPercent": 4.0072,
"kernelCpuConsumptionPercent": 1.6029,
"pageFaults": {
"IORequired": 0,
"IONotRequired": 4610
},
"fsActivity": {
"reads": 0,
"writes": 0
}
},
"uvthreadResourceUsage": {
"userCpuSeconds": 0.039843,
"kernelCpuSeconds": 0.015937,
"cpuConsumptionPercent": 5.578,
"userCpuConsumptionPercent": 3.9843,
"kernelCpuConsumptionPercent": 1.5937,
"fsActivity": {
"reads": 0,
"writes": 0
}
},
"libuv": [
{
"type": "async",
"is_active": true,
"is_referenced": false,
"address": "0x0000000102910900",
"details": ""
},
{
"type": "timer",
"is_active": false,
"is_referenced": false,
"address": "0x00007fff5fbfeab0",
"repeat": 0,
"firesInMsFromNow": 94403548320796,
"expired": true
},
{
"type": "check",
"is_active": true,
"is_referenced": false,
"address": "0x00007fff5fbfeb48"
},
{
"type": "idle",
"is_active": false,
"is_referenced": true,
"address": "0x00007fff5fbfebc0"
},
{
"type": "prepare",
"is_active": false,
"is_referenced": false,
"address": "0x00007fff5fbfec38"
},
{
"type": "check",
"is_active": false,
"is_referenced": false,
"address": "0x00007fff5fbfecb0"
},
{
"type": "async",
"is_active": true,
"is_referenced": false,
"address": "0x000000010188f2e0"
},
{
"type": "tty",
"is_active": false,
"is_referenced": true,
"address": "0x000055b581db0e18",
"width": 204,
"height": 55,
"fd": 17,
"writeQueueSize": 0,
"readable": true,
"writable": true
},
{
"type": "signal",
"is_active": true,
"is_referenced": false,
"address": "0x000055b581d80010",
"signum": 28,
"signal": "SIGWINCH"
},
{
"type": "tty",
"is_active": true,
"is_referenced": true,
"address": "0x000055b581df59f8",
"width": 204,
"height": 55,
"fd": 19,
"writeQueueSize": 0,
"readable": true,
"writable": true
},
{
"type": "loop",
"is_active": true,
"address": "0x000055fc7b2cb180",
"loopIdleTimeSeconds": 22644.8
}
],
"workers": [],
"environmentVariables": {
"REMOTEHOST": "REMOVED",
"MANPATH": "/opt/rh/devtoolset-3/root/usr/share/man:",
"XDG_SESSION_ID": "66126",
"HOSTNAME": "test_machine",
"HOST": "test_machine",
"TERM": "xterm-256color",
"SHELL": "/bin/csh",
"SSH_CLIENT": "REMOVED",
"PERL5LIB": "/opt/rh/devtoolset-3/root//usr/lib64/perl5/vendor_perl:/opt/rh/devtoolset-3/root/usr/lib/perl5:/opt/rh/devtoolset-3/root//usr/share/perl5/vendor_perl",
"OLDPWD": "/home/nodeuser/project/node/src",
"JAVACONFDIRS": "/opt/rh/devtoolset-3/root/etc/java:/etc/java",
"SSH_TTY": "/dev/pts/0",
"PCP_DIR": "/opt/rh/devtoolset-3/root",
"GROUP": "normaluser",
"USER": "nodeuser",
"LD_LIBRARY_PATH": "/opt/rh/devtoolset-3/root/usr/lib64:/opt/rh/devtoolset-3/root/usr/lib",
"HOSTTYPE": "x86_64-linux",
"XDG_CONFIG_DIRS": "/opt/rh/devtoolset-3/root/etc/xdg:/etc/xdg",
"MAIL": "/var/spool/mail/nodeuser",
"PATH": "/home/nodeuser/project/node:/opt/rh/devtoolset-3/root/usr/bin:/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin",
"PWD": "/home/nodeuser/project/node",
"LANG": "en_US.UTF-8",
"PS1": "\\u@\\h : \\[\\e[31m\\]\\w\\[\\e[m\\] > ",
"SHLVL": "2",
"HOME": "/home/nodeuser",
"OSTYPE": "linux",
"VENDOR": "unknown",
"PYTHONPATH": "/opt/rh/devtoolset-3/root/usr/lib64/python2.7/site-packages:/opt/rh/devtoolset-3/root/usr/lib/python2.7/site-packages",
"MACHTYPE": "x86_64",
"LOGNAME": "nodeuser",
"XDG_DATA_DIRS": "/opt/rh/devtoolset-3/root/usr/share:/usr/local/share:/usr/share",
"LESSOPEN": "||/usr/bin/lesspipe.sh %s",
"INFOPATH": "/opt/rh/devtoolset-3/root/usr/share/info",
"XDG_RUNTIME_DIR": "/run/user/50141",
"_": "./node"
},
"userLimits": {
"core_file_size_blocks": {
"soft": "",
"hard": "unlimited"
},
"data_seg_size_kbytes": {
"soft": "unlimited",
"hard": "unlimited"
},
"file_size_blocks": {
"soft": "unlimited",
"hard": "unlimited"
},
"max_locked_memory_bytes": {
"soft": "unlimited",
"hard": 65536
},
"max_memory_size_kbytes": {
"soft": "unlimited",
"hard": "unlimited"
},
"open_files": {
"soft": "unlimited",
"hard": 4096
},
"stack_size_bytes": {
"soft": "unlimited",
"hard": "unlimited"
},
"cpu_time_seconds": {
"soft": "unlimited",
"hard": "unlimited"
},
"max_user_processes": {
"soft": "unlimited",
"hard": 4127290
},
"virtual_memory_kbytes": {
"soft": "unlimited",
"hard": "unlimited"
}
},
"sharedObjects": [
"/lib64/libdl.so.2",
"/lib64/librt.so.1",
"/lib64/libstdc++.so.6",
"/lib64/libm.so.6",
"/lib64/libgcc_s.so.1",
"/lib64/libpthread.so.0",
"/lib64/libc.so.6",
"/lib64/ld-linux-x86-64.so.2"
]
}
Usage#
node --report-uncaught-exception --report-on-signal \
--report-on-fatalerror app.js
-
--report-uncaught-exception
Enables report to be generated on un-caught exceptions. Useful when inspecting JavaScript stack in conjunction with native stack and other runtime environment data. -
--report-on-signal
Enables report to be generated upon receiving the specified (or predefined) signal to the running Node.js process. (See below on how to modify the signal that triggers the report.) Default signal isSIGUSR2
. Useful when a report needs to be triggered from another program. Application monitors may leverage this feature to collect report at regular intervals and plot rich set of internal runtime data to their views.
Signal based report generation is not supported in Windows.
Under normal circumstances, there is no need to modify the report triggering
signal. However, if SIGUSR2
is already used for other purposes, then this
flag helps to change the signal for report generation and preserve the original
meaning of SIGUSR2
for the said purposes.
-
--report-on-fatalerror
Enables the report to be triggered on fatal errors (internal errors within the Node.js runtime, such as out of memory) that leads to termination of the application. Useful to inspect various diagnostic data elements such as heap, stack, event loop state, resource consumption etc. to reason about the fatal error. -
--report-compact
Write reports in a compact format, single-line JSON, more easily consumable by log processing systems than the default multi-line format designed for human consumption. -
--report-directory
Location at which the report will be generated. -
--report-filename
Name of the file to which the report will be written. -
--report-signal
Sets or resets the signal for report generation (not supported on Windows). Default signal isSIGUSR2
.
A report can also be triggered via an API call from a JavaScript application:
process.report.writeReport();
This function takes an optional additional argument filename
, which is
the name of a file into which the report is written.
process.report.writeReport('./foo.json');
This function takes an optional additional argument err
which is an Error
object that will be used as the context for the JavaScript stack printed in the
report. When using report to handle errors in a callback or an exception
handler, this allows the report to include the location of the original error as
well as where it was handled.
try {
process.chdir('/non-existent-path');
} catch (err) {
process.report.writeReport(err);
}
// Any other code
If both filename and error object are passed to writeReport()
the
error object must be the second parameter.
try {
process.chdir('/non-existent-path');
} catch (err) {
process.report.writeReport(filename, err);
}
// Any other code
The content of the diagnostic report can be returned as a JavaScript Object via an API call from a JavaScript application:
const report = process.report.getReport();
console.log(typeof report === 'object'); // true
// Similar to process.report.writeReport() output
console.log(JSON.stringify(report, null, 2));
This function takes an optional additional argument err
, which is an Error
object that will be used as the context for the JavaScript stack printed in the
report.
const report = process.report.getReport(new Error('custom error'));
console.log(typeof report === 'object'); // true
The API versions are useful when inspecting the runtime state from within the application, in expectation of self-adjusting the resource consumption, load balancing, monitoring etc.
The content of the report consists of a header section containing the event
type, date, time, PID, and Node.js version, sections containing JavaScript and
native stack traces, a section containing V8 heap information, a section
containing libuv
handle information, and an OS platform information section
showing CPU and memory usage and system limits. An example report can be
triggered using the Node.js REPL:
$ node
> process.report.writeReport();
Writing Node.js report to file: report.20181126.091102.8480.0.001.json
Node.js report completed
>
When a report is written, start and end messages are issued to stderr and the filename of the report is returned to the caller. The default filename includes the date, time, PID, and a sequence number. The sequence number helps in associating the report dump with the runtime state if generated multiple times for the same Node.js process.
Diagnostic report has an associated single-digit version number (report.header.reportVersion
),
uniquely representing the report format. The version number is bumped
when new key is added or removed, or the data type of a value is changed.
Report version definitions are consistent across LTS releases.
Configuration#
Additional runtime configuration of report generation is available via
the following properties of process.report
:
reportOnFatalError
triggers diagnostic reporting on fatal errors when true
.
Defaults to false
.
reportOnSignal
triggers diagnostic reporting on signal when true
. This is
not supported on Windows. Defaults to false
.
reportOnUncaughtException
triggers diagnostic reporting on uncaught exception
when true
. Defaults to false
.
signal
specifies the POSIX signal identifier that will be used
to intercept external triggers for report generation. Defaults to
'SIGUSR2'
.
filename
specifies the name of the output file in the file system.
Special meaning is attached to stdout
and stderr
. Usage of these
will result in report being written to the associated standard streams.
In cases where standard streams are used, the value in directory
is ignored.
URLs are not supported. Defaults to a composite filename that contains
timestamp, PID, and sequence number.
directory
specifies the file system directory where the report will be
written. URLs are not supported. Defaults to the current working directory of
the Node.js process.
// Trigger report only on uncaught exceptions.
process.report.reportOnFatalError = false;
process.report.reportOnSignal = false;
process.report.reportOnUncaughtException = true;
// Trigger report for both internal errors as well as external signal.
process.report.reportOnFatalError = true;
process.report.reportOnSignal = true;
process.report.reportOnUncaughtException = false;
// Change the default signal to 'SIGQUIT' and enable it.
process.report.reportOnFatalError = false;
process.report.reportOnUncaughtException = false;
process.report.reportOnSignal = true;
process.report.signal = 'SIGQUIT';
Configuration on module initialization is also available via environment variables:
NODE_OPTIONS="--report-uncaught-exception \
--report-on-fatalerror --report-on-signal \
--report-signal=SIGUSR2 --report-filename=./report.json \
--report-directory=/home/nodeuser"
Specific API documentation can be found under
process API documentation
section.
Interaction with workers#
Worker
threads can create reports in the same way that the main thread
does.
Reports will include information on any Workers that are children of the current
thread as part of the workers
section, with each Worker generating a report
in the standard report format.
The thread which is generating the report will wait for the reports from Worker threads to finish. However, the latency for this will usually be low, as both running JavaScript and the event loop are interrupted to generate the report.
Single executable applications#
Source Code: lib/internal/main/single_executable_application.js
This feature allows the distribution of a Node.js application conveniently to a system that does not have Node.js installed.
Node.js supports the creation of single executable applications by allowing
the injection of a JavaScript file into the node
binary. During start up, the
program checks if anything has been injected. If the script is found, it
executes its contents. Otherwise Node.js operates as it normally does.
The single executable application feature only supports running a single embedded CommonJS file.
A bundled JavaScript file can be turned into a single executable application
with any tool which can inject resources into the node
binary.
Here are the steps for creating a single executable application using one such tool, postject:
-
Create a JavaScript file:
$ echo 'console.log(`Hello, ${process.argv[2]}!`);' > hello.js
-
Create a copy of the
node
executable and name it according to your needs:$ cp $(command -v node) hello
-
Remove the signature of the binary:
- On macOS:
$ codesign --remove-signature hello
- On Windows (optional):
signtool can be used from the installed Windows SDK. If this step is skipped, ignore any signature-related warning from postject.
$ signtool remove /s hello
-
Inject the JavaScript file into the copied binary by running
postject
with the following options:hello
- The name of the copy of thenode
executable created in step 2.NODE_JS_CODE
- The name of the resource / note / section in the binary where the contents of the JavaScript file will be stored.hello.js
- The name of the JavaScript file created in step 1.--sentinel-fuse NODE_JS_FUSE_fce680ab2cc467b6e072b8b5df1996b2
- The fuse used by the Node.js project to detect if a file has been injected.--macho-segment-name NODE_JS
(only needed on macOS) - The name of the segment in the binary where the contents of the JavaScript file will be stored.
To summarize, here is the required command for each platform:
-
On systems other than macOS:
$ npx postject hello NODE_JS_CODE hello.js \ --sentinel-fuse NODE_JS_FUSE_fce680ab2cc467b6e072b8b5df1996b2
-
On macOS:
$ npx postject hello NODE_JS_CODE hello.js \ --sentinel-fuse NODE_JS_FUSE_fce680ab2cc467b6e072b8b5df1996b2 \ --macho-segment-name NODE_JS
-
Sign the binary:
- On macOS:
$ codesign --sign - hello
- On Windows (optional):
A certificate needs to be present for this to work. However, the unsigned binary would still be runnable.
$ signtool sign /fd SHA256 hello
-
Run the binary:
$ ./hello world Hello, world!
Notes#
require(id)
in the injected module is not file based#
require()
in the injected module is not the same as the require()
available to modules that are not injected. It also does not have any of the
properties that non-injected require()
has except require.main
. It
can only be used to load built-in modules. Attempting to load a module that can
only be found in the file system will throw an error.
Instead of relying on a file based require()
, users can bundle their
application into a standalone JavaScript file to inject into the executable.
This also ensures a more deterministic dependency graph.
However, if a file based require()
is still needed, that can also be achieved:
const { createRequire } = require('node:module');
require = createRequire(__filename);
__filename
and module.filename
in the injected module#
The values of __filename
and module.filename
in the injected module are
equal to process.execPath
.
__dirname
in the injected module#
The value of __dirname
in the injected module is equal to the directory name
of process.execPath
.
Single executable application creation process#
A tool aiming to create a single executable Node.js application must inject the contents of a JavaScript file into:
- a resource named
NODE_JS_CODE
if thenode
binary is a PE file - a section named
NODE_JS_CODE
in theNODE_JS
segment if thenode
binary is a Mach-O file - a note named
NODE_JS_CODE
if thenode
binary is an ELF file
Search the binary for the
NODE_JS_FUSE_fce680ab2cc467b6e072b8b5df1996b2:0
fuse string and flip the
last character to 1
to indicate that a resource has been injected.
Platform support#
Single-executable support is tested regularly on CI only on the following platforms:
- Windows
- macOS
- Linux (all distributions supported by Node.js except Alpine and all architectures supported by Node.js except s390x and ppc64)
This is due to a lack of better tools to generate single-executables that can be used to test this feature on other platforms.
Suggestions for other resource injection tools/workflows are welcomed. Please start a discussion at https://github.com/nodejs/single-executable/discussions to help us document them.
Stream[src]#
Source Code: lib/stream.js
A stream is an abstract interface for working with streaming data in Node.js.
The node:stream
module provides an API for implementing the stream interface.
There are many stream objects provided by Node.js. For instance, a
request to an HTTP server and process.stdout
are both stream instances.
Streams can be readable, writable, or both. All streams are instances of
EventEmitter
.
To access the node:stream
module:
const stream = require('node:stream');
The node:stream
module is useful for creating new types of stream instances.
It is usually not necessary to use the node:stream
module to consume streams.
Organization of this document#
This document contains two primary sections and a third section for notes. The first section explains how to use existing streams within an application. The second section explains how to create new types of streams.
Types of streams#
There are four fundamental stream types within Node.js:
Writable
: streams to which data can be written (for example,fs.createWriteStream()
).Readable
: streams from which data can be read (for example,fs.createReadStream()
).Duplex
: streams that are bothReadable
andWritable
(for example,net.Socket
).Transform
:Duplex
streams that can modify or transform the data as it is written and read (for example,zlib.createDeflate()
).
Additionally, this module includes the utility functions
stream.pipeline()
, stream.finished()
, stream.Readable.from()
and stream.addAbortSignal()
.
Streams Promises API#
The stream/promises
API provides an alternative set of asynchronous utility
functions for streams that return Promise
objects rather than using
callbacks. The API is accessible via require('node:stream/promises')
or require('node:stream').promises
.
stream.pipeline(source[, ...transforms], destination[, options])
#
stream.pipeline(streams[, options])
#
streams
<Stream[]> | <Iterable[]> | <AsyncIterable[]> | <Function[]>source
<Stream> | <Iterable> | <AsyncIterable> | <Function>- Returns: <Promise> | <AsyncIterable>
...transforms
<Stream> | <Function>source
<AsyncIterable>- Returns: <Promise> | <AsyncIterable>
destination
<Stream> | <Function>source
<AsyncIterable>- Returns: <Promise> | <AsyncIterable>
options
<Object>signal
<AbortSignal>end
<boolean>
- Returns: <Promise> Fulfills when the pipeline is complete.
const { pipeline } = require('node:stream/promises');
const fs = require('node:fs');
const zlib = require('node:zlib');
async function run() {
await pipeline(
fs.createReadStream('archive.tar'),
zlib.createGzip(),
fs.createWriteStream('archive.tar.gz'),
);
console.log('Pipeline succeeded.');
}
run().catch(console.error);
import { pipeline } from 'node:stream/promises';
import { createReadStream, createWriteStream } from 'node:fs';
import { createGzip } from 'node:zlib';
await pipeline(
createReadStream('archive.tar'),
createGzip(),
createWriteStream('archive.tar.gz'),
);
console.log('Pipeline succeeded.');
To use an AbortSignal
, pass it inside an options object, as the last argument.
When the signal is aborted, destroy
will be called on the underlying pipeline,
with an AbortError
.
const { pipeline } = require('node:stream/promises');
const fs = require('node:fs');
const zlib = require('node:zlib');
async function run() {
const ac = new AbortController();
const signal = ac.signal;
setImmediate(() => ac.abort());
await pipeline(
fs.createReadStream('archive.tar'),
zlib.createGzip(),
fs.createWriteStream('archive.tar.gz'),
{ signal },
);
}
run().catch(console.error); // AbortError
import { pipeline } from 'node:stream/promises';
import { createReadStream, createWriteStream } from 'node:fs';
import { createGzip } from 'node:zlib';
const ac = new AbortController();
const { signal } = ac;
setImmediate(() => ac.abort());
try {
await pipeline(
createReadStream('archive.tar'),
createGzip(),
createWriteStream('archive.tar.gz'),
{ signal },
);
} catch (err) {
console.error(err); // AbortError
}
The pipeline
API also supports async generators:
const { pipeline } = require('node:stream/promises');
const fs = require('node:fs');
async function run() {
await pipeline(
fs.createReadStream('lowercase.txt'),
async function* (source, { signal }) {
source.setEncoding('utf8'); // Work with strings rather than `Buffer`s.
for await (const chunk of source) {
yield await processChunk(chunk, { signal });
}
},
fs.createWriteStream('uppercase.txt'),
);
console.log('Pipeline succeeded.');
}
run().catch(console.error);
import { pipeline } from 'node:stream/promises';
import { createReadStream, createWriteStream } from 'node:fs';
await pipeline(
createReadStream('lowercase.txt'),
async function* (source, { signal }) {
source.setEncoding('utf8'); // Work with strings rather than `Buffer`s.
for await (const chunk of source) {
yield await processChunk(chunk, { signal });
}
},
createWriteStream('uppercase.txt'),
);
console.log('Pipeline succeeded.');
Remember to handle the signal
argument passed into the async generator.
Especially in the case where the async generator is the source for the
pipeline (i.e. first argument) or the pipeline will never complete.
const { pipeline } = require('node:stream/promises');
const fs = require('node:fs');
async function run() {
await pipeline(
async function* ({ signal }) {
await someLongRunningfn({ signal });
yield 'asd';
},
fs.createWriteStream('uppercase.txt'),
);
console.log('Pipeline succeeded.');
}
run().catch(console.error);
import { pipeline } from 'node:stream/promises';
import fs from 'node:fs';
await pipeline(
async function* ({ signal }) {
await someLongRunningfn({ signal });
yield 'asd';
},
fs.createWriteStream('uppercase.txt'),
);
console.log('Pipeline succeeded.');
The pipeline
API provides callback version:
stream.finished(stream[, options])
#
stream
<Stream>options
<Object>error
<boolean> | <undefined>readable
<boolean> | <undefined>writable
<boolean> | <undefined>signal
: <AbortSignal> | <undefined>
- Returns: <Promise> Fulfills when the stream is no longer readable or writable.
const { finished } = require('node:stream/promises');
const fs = require('node:fs');
const rs = fs.createReadStream('archive.tar');
async function run() {
await finished(rs);
console.log('Stream is done reading.');
}
run().catch(console.error);
rs.resume(); // Drain the stream.
import { finished } from 'node:stream/promises';
import { createReadStream } from 'node:fs';
const rs = createReadStream('archive.tar');
async function run() {
await finished(rs);
console.log('Stream is done reading.');
}
run().catch(console.error);
rs.resume(); // Drain the stream.
The finished
API provides callback version:
Object mode#
All streams created by Node.js APIs operate exclusively on strings and Buffer
(or Uint8Array
) objects. It is possible, however, for stream implementations
to work with other types of JavaScript values (with the exception of null
,
which serves a special purpose within streams). Such streams are considered to
operate in "object mode".
Stream instances are switched into object mode using the objectMode
option
when the stream is created. Attempting to switch an existing stream into
object mode is not safe.
Buffering#
Both Writable
and Readable
streams will store data in an internal
buffer.
The amount of data potentially buffered depends on the highWaterMark
option
passed into the stream's constructor. For normal streams, the highWaterMark
option specifies a total number of bytes. For streams operating
in object mode, the highWaterMark
specifies a total number of objects.
Data is buffered in Readable
streams when the implementation calls
stream.push(chunk)
. If the consumer of the Stream does not
call stream.read()
, the data will sit in the internal
queue until it is consumed.
Once the total size of the internal read buffer reaches the threshold specified
by highWaterMark
, the stream will temporarily stop reading data from the
underlying resource until the data currently buffered can be consumed (that is,
the stream will stop calling the internal readable._read()
method that is
used to fill the read buffer).
Data is buffered in Writable
streams when the
writable.write(chunk)
method is called repeatedly. While the
total size of the internal write buffer is below the threshold set by
highWaterMark
, calls to writable.write()
will return true
. Once
the size of the internal buffer reaches or exceeds the highWaterMark
, false
will be returned.
A key goal of the stream
API, particularly the stream.pipe()
method,
is to limit the buffering of data to acceptable levels such that sources and
destinations of differing speeds will not overwhelm the available memory.
The highWaterMark
option is a threshold, not a limit: it dictates the amount
of data that a stream buffers before it stops asking for more data. It does not
enforce a strict memory limitation in general. Specific stream implementations
may choose to enforce stricter limits but doing so is optional.
Because Duplex
and Transform
streams are both Readable
and
Writable
, each maintains two separate internal buffers used for reading and
writing, allowing each side to operate independently of the other while
maintaining an appropriate and efficient flow of data. For example,
net.Socket
instances are Duplex
streams whose Readable
side allows
consumption of data received from the socket and whose Writable
side allows
writing data to the socket. Because data may be written to the socket at a
faster or slower rate than data is received, each side should
operate (and buffer) independently of the other.
The mechanics of the internal buffering are an internal implementation detail
and may be changed at any time. However, for certain advanced implementations,
the internal buffers can be retrieved using writable.writableBuffer
or
readable.readableBuffer
. Use of these undocumented properties is discouraged.
API for stream consumers#
Almost all Node.js applications, no matter how simple, use streams in some manner. The following is an example of using streams in a Node.js application that implements an HTTP server:
const http = require('node:http');
const server = http.createServer((req, res) => {
// `req` is an http.IncomingMessage, which is a readable stream.
// `res` is an http.ServerResponse, which is a writable stream.
let body = '';
// Get the data as utf8 strings.
// If an encoding is not set, Buffer objects will be received.
req.setEncoding('utf8');
// Readable streams emit 'data' events once a listener is added.
req.on('data', (chunk) => {
body += chunk;
});
// The 'end' event indicates that the entire body has been received.
req.on('end', () => {
try {
const data = JSON.parse(body);
// Write back something interesting to the user:
res.write(typeof data);
res.end();
} catch (er) {
// uh oh! bad json!
res.statusCode = 400;
return res.end(`error: ${er.message}`);
}
});
});
server.listen(1337);
// $ curl localhost:1337 -d "{}"
// object
// $ curl localhost:1337 -d "\"foo\""
// string
// $ curl localhost:1337 -d "not json"
// error: Unexpected token 'o', "not json" is not valid JSON
Writable
streams (such as res
in the example) expose methods such as
write()
and end()
that are used to write data onto the stream.
Readable
streams use the EventEmitter
API for notifying application
code when data is available to be read off the stream. That available data can
be read from the stream in multiple ways.
Both Writable
and Readable
streams use the EventEmitter
API in
various ways to communicate the current state of the stream.
Duplex
and Transform
streams are both Writable
and
Readable
.
Applications that are either writing data to or consuming data from a stream
are not required to implement the stream interfaces directly and will generally
have no reason to call require('node:stream')
.
Developers wishing to implement new types of streams should refer to the section API for stream implementers.
Writable streams#
Writable streams are an abstraction for a destination to which data is written.
Examples of Writable
streams include:
- HTTP requests, on the client
- HTTP responses, on the server
- fs write streams
- zlib streams
- crypto streams
- TCP sockets
- child process stdin
process.stdout
,process.stderr
Some of these examples are actually Duplex
streams that implement the
Writable
interface.
All Writable
streams implement the interface defined by the
stream.Writable
class.
While specific instances of Writable
streams may differ in various ways,
all Writable
streams follow the same fundamental usage pattern as illustrated
in the example below:
const myStream = getWritableStreamSomehow();
myStream.write('some data');
myStream.write('some more data');
myStream.end('done writing data');
Class: stream.Writable
#
Event: 'close'
#
The 'close'
event is emitted when the stream and any of its underlying
resources (a file descriptor, for example) have been closed. The event indicates
that no more events will be emitted, and no further computation will occur.
A Writable
stream will always emit the 'close'
event if it is
created with the emitClose
option.
Event: 'drain'
#
If a call to stream.write(chunk)
returns false
, the
'drain'
event will be emitted when it is appropriate to resume writing data
to the stream.
// Write the data to the supplied writable stream one million times.
// Be attentive to back-pressure.
function writeOneMillionTimes(writer, data, encoding, callback) {
let i = 1000000;
write();
function write() {
let ok = true;
do {
i--;
if (i === 0) {
// Last time!
writer.write(data, encoding, callback);
} else {
// See if we should continue, or wait.
// Don't pass the callback, because we're not done yet.
ok = writer.write(data, encoding);
}
} while (i > 0 && ok);
if (i > 0) {
// Had to stop early!
// Write some more once it drains.
writer.once('drain', write);
}
}
}
Event: 'error'
#
The 'error'
event is emitted if an error occurred while writing or piping
data. The listener callback is passed a single Error
argument when called.
The stream is closed when the 'error'
event is emitted unless the
autoDestroy
option was set to false
when creating the
stream.
After 'error'
, no further events other than 'close'
should be emitted
(including 'error'
events).
Event: 'finish'
#
The 'finish'
event is emitted after the stream.end()
method
has been called, and all data has been flushed to the underlying system.
const writer = getWritableStreamSomehow();
for (let i = 0; i < 100; i++) {
writer.write(`hello, #${i}!\n`);
}
writer.on('finish', () => {
console.log('All writes are now complete.');
});
writer.end('This is the end\n');
Event: 'pipe'
#
src
<stream.Readable> source stream that is piping to this writable
The 'pipe'
event is emitted when the stream.pipe()
method is called on
a readable stream, adding this writable to its set of destinations.
const writer = getWritableStreamSomehow();
const reader = getReadableStreamSomehow();
writer.on('pipe', (src) => {
console.log('Something is piping into the writer.');
assert.equal(src, reader);
});
reader.pipe(writer);
Event: 'unpipe'
#
src
<stream.Readable> The source stream that unpiped this writable
The 'unpipe'
event is emitted when the stream.unpipe()
method is called
on a Readable
stream, removing this Writable
from its set of
destinations.
This is also emitted in case this Writable
stream emits an error when a
Readable
stream pipes into it.
const writer = getWritableStreamSomehow();
const reader = getReadableStreamSomehow();
writer.on('unpipe', (src) => {
console.log('Something has stopped piping into the writer.');
assert.equal(src, reader);
});
reader.pipe(writer);
reader.unpipe(writer);
writable.cork()
#
The writable.cork()
method forces all written data to be buffered in memory.
The buffered data will be flushed when either the stream.uncork()
or
stream.end()
methods are called.
The primary intent of writable.cork()
is to accommodate a situation in which
several small chunks are written to the stream in rapid succession. Instead of
immediately forwarding them to the underlying destination, writable.cork()
buffers all the chunks until writable.uncork()
is called, which will pass them
all to writable._writev()
, if present. This prevents a head-of-line blocking
situation where data is being buffered while waiting for the first small chunk
to be processed. However, use of writable.cork()
without implementing
writable._writev()
may have an adverse effect on throughput.
See also: writable.uncork()
, writable._writev()
.
writable.destroy([error])
#
Destroy the stream. Optionally emit an 'error'
event, and emit a 'close'
event (unless emitClose
is set to false
). After this call, the writable
stream has ended and subsequent calls to write()
or end()
will result in
an ERR_STREAM_DESTROYED
error.
This is a destructive and immediate way to destroy a stream. Previous calls to
write()
may not have drained, and may trigger an ERR_STREAM_DESTROYED
error.
Use end()
instead of destroy if data should flush before close, or wait for
the 'drain'
event before destroying the stream.
const { Writable } = require('node:stream');
const myStream = new Writable();
const fooErr = new Error('foo error');
myStream.destroy(fooErr);
myStream.on('error', (fooErr) => console.error(fooErr.message)); // foo error
const { Writable } = require('node:stream');
const myStream = new Writable();
myStream.destroy();
myStream.on('error', function wontHappen() {});
const { Writable } = require('node:stream');
const myStream = new Writable();
myStream.destroy();
myStream.write('foo', (error) => console.error(error.code));
// ERR_STREAM_DESTROYED
Once destroy()
has been called any further calls will be a no-op and no
further errors except from _destroy()
may be emitted as 'error'
.
Implementors should not override this method,
but instead implement writable._destroy()
.
writable.closed
#
Is true
after 'close'
has been emitted.
writable.destroyed
#
Is true
after writable.destroy()
has been called.
const { Writable } = require('node:stream');
const myStream = new Writable();
console.log(myStream.destroyed); // false
myStream.destroy();
console.log(myStream.destroyed); // true
writable.end([chunk[, encoding]][, callback])
#
chunk
<string> | <Buffer> | <Uint8Array> | <any> Optional data to write. For streams not operating in object mode,chunk
must be a string,Buffer
orUint8Array
. For object mode streams,chunk
may be any JavaScript value other thannull
.encoding
<string> The encoding ifchunk
is a stringcallback
<Function> Callback for when the stream is finished.- Returns: <this>
Calling the writable.end()
method signals that no more data will be written
to the Writable
. The optional chunk
and encoding
arguments allow one
final additional chunk of data to be written immediately before closing the
stream.
Calling the stream.write()
method after calling
stream.end()
will raise an error.
// Write 'hello, ' and then end with 'world!'.
const fs = require('node:fs');
const file = fs.createWriteStream('example.txt');
file.write('hello, ');
file.end('world!');
// Writing more now is not allowed!
writable.setDefaultEncoding(encoding)
#
The writable.setDefaultEncoding()
method sets the default encoding
for a
Writable
stream.
writable.uncork()
#
The writable.uncork()
method flushes all data buffered since
stream.cork()
was called.
When using writable.cork()
and writable.uncork()
to manage the buffering
of writes to a stream, defer calls to writable.uncork()
using
process.nextTick()
. Doing so allows batching of all
writable.write()
calls that occur within a given Node.js event loop phase.
stream.cork();
stream.write('some ');
stream.write('data ');
process.nextTick(() => stream.uncork());
If the writable.cork()
method is called multiple times on a stream, the
same number of calls to writable.uncork()
must be called to flush the buffered
data.
stream.cork();
stream.write('some ');
stream.cork();
stream.write('data ');
process.nextTick(() => {
stream.uncork();
// The data will not be flushed until uncork() is called a second time.
stream.uncork();
});
See also: writable.cork()
.
writable.writable
#
Is true
if it is safe to call writable.write()
, which means
the stream has not been destroyed, errored, or ended.
writable.writableAborted
#
Returns whether the stream was destroyed or errored before emitting 'finish'
.
writable.writableEnded
#
Is true
after writable.end()
has been called. This property
does not indicate whether the data has been flushed, for this use
writable.writableFinished
instead.
writable.writableCorked
#
Number of times writable.uncork()
needs to be
called in order to fully uncork the stream.
writable.errored
#
Returns error if the stream has been destroyed with an error.
writable.writableFinished
#
Is set to true
immediately before the 'finish'
event is emitted.
writable.writableHighWaterMark
#
Return the value of highWaterMark
passed when creating this Writable
.
writable.writableLength
#
This property contains the number of bytes (or objects) in the queue
ready to be written. The value provides introspection data regarding
the status of the highWaterMark
.
writable.writableNeedDrain
#
Is true
if the stream's buffer has been full and stream will emit 'drain'
.
writable.writableObjectMode
#
Getter for the property objectMode
of a given Writable
stream.
writable.write(chunk[, encoding][, callback])
#
chunk
<string> | <Buffer> | <Uint8Array> | <any> Optional data to write. For streams not operating in object mode,chunk
must be a string,Buffer
orUint8Array
. For object mode streams,chunk
may be any JavaScript value other thannull
.encoding
<string> | <null> The encoding, ifchunk
is a string. Default:'utf8'
callback
<Function> Callback for when this chunk of data is flushed.- Returns: <boolean>
false
if the stream wishes for the calling code to wait for the'drain'
event to be emitted before continuing to write additional data; otherwisetrue
.
The writable.write()
method writes some data to the stream, and calls the
supplied callback
once the data has been fully handled. If an error
occurs, the callback
will be called with the error as its
first argument. The callback
is called asynchronously and before 'error'
is
emitted.
The return value is true
if the internal buffer is less than the
highWaterMark
configured when the stream was created after admitting chunk
.
If false
is returned, further attempts to write data to the stream should
stop until the 'drain'
event is emitted.
While a stream is not draining, calls to write()
will buffer chunk
, and
return false. Once all currently buffered chunks are drained (accepted for
delivery by the operating system), the 'drain'
event will be emitted.
Once write()
returns false, do not write more chunks
until the 'drain'
event is emitted. While calling write()
on a stream that
is not draining is allowed, Node.js will buffer all written chunks until
maximum memory usage occurs, at which point it will abort unconditionally.
Even before it aborts, high memory usage will cause poor garbage collector
performance and high RSS (which is not typically released back to the system,
even after the memory is no longer required). Since TCP sockets may never
drain if the remote peer does not read the data, writing a socket that is
not draining may lead to a remotely exploitable vulnerability.
Writing data while the stream is not draining is particularly
problematic for a Transform
, because the Transform
streams are paused
by default until they are piped or a 'data'
or 'readable'
event handler
is added.
If the data to be written can be generated or fetched on demand, it is
recommended to encapsulate the logic into a Readable
and use
stream.pipe()
. However, if calling write()
is preferred, it is
possible to respect backpressure and avoid memory issues using the
'drain'
event:
function write(data, cb) {
if (!stream.write(data)) {
stream.once('drain', cb);
} else {
process.nextTick(cb);
}
}
// Wait for cb to be called before doing any other write.
write('hello', () => {
console.log('Write completed, do more writes now.');
});
A Writable
stream in object mode will always ignore the encoding
argument.
Readable streams#
Readable streams are an abstraction for a source from which data is consumed.
Examples of Readable
streams include:
- HTTP responses, on the client
- HTTP requests, on the server
- fs read streams
- zlib streams
- crypto streams
- TCP sockets
- child process stdout and stderr
process.stdin
All Readable
streams implement the interface defined by the
stream.Readable
class.
Two reading modes#
Readable
streams effectively operate in one of two modes: flowing and
paused. These modes are separate from object mode.
A Readable
stream can be in object mode or not, regardless of whether
it is in flowing mode or paused mode.
-
In flowing mode, data is read from the underlying system automatically and provided to an application as quickly as possible using events via the
EventEmitter
interface. -
In paused mode, the
stream.read()
method must be called explicitly to read chunks of data from the stream.
All Readable
streams begin in paused mode but can be switched to flowing
mode in one of the following ways:
- Adding a
'data'
event handler. - Calling the
stream.resume()
method. - Calling the
stream.pipe()
method to send the data to aWritable
.
The Readable
can switch back to paused mode using one of the following:
- If there are no pipe destinations, by calling the
stream.pause()
method. - If there are pipe destinations, by removing all pipe destinations.
Multiple pipe destinations may be removed by calling the
stream.unpipe()
method.
The important concept to remember is that a Readable
will not generate data
until a mechanism for either consuming or ignoring that data is provided. If
the consuming mechanism is disabled or taken away, the Readable
will attempt
to stop generating the data.
For backward compatibility reasons, removing 'data'
event handlers will
not automatically pause the stream. Also, if there are piped destinations,
then calling stream.pause()
will not guarantee that the
stream will remain paused once those destinations drain and ask for more data.
If a Readable
is switched into flowing mode and there are no consumers
available to handle the data, that data will be lost. This can occur, for
instance, when the readable.resume()
method is called without a listener
attached to the 'data'
event, or when a 'data'
event handler is removed
from the stream.
Adding a 'readable'
event handler automatically makes the stream
stop flowing, and the data has to be consumed via
readable.read()
. If the 'readable'
event handler is
removed, then the stream will start flowing again if there is a
'data'
event handler.
Three states#
The "two modes" of operation for a Readable
stream are a simplified
abstraction for the more complicated internal state management that is happening
within the Readable
stream implementation.
Specifically, at any given point in time, every Readable
is in one of three
possible states:
readable.readableFlowing === null
readable.readableFlowing === false
readable.readableFlowing === true
When readable.readableFlowing
is null
, no mechanism for consuming the
stream's data is provided. Therefore, the stream will not generate data.
While in this state, attaching a listener for the 'data'
event, calling the
readable.pipe()
method, or calling the readable.resume()
method will switch
readable.readableFlowing
to true
, causing the Readable
to begin actively
emitting events as data is generated.
Calling readable.pause()
, readable.unpipe()
, or receiving backpressure
will cause the readable.readableFlowing
to be set as false
,
temporarily halting the flowing of events but not halting the generation of
data. While in this state, attaching a listener for the 'data'
event
will not switch readable.readableFlowing
to true
.
const { PassThrough, Writable } = require('node:stream');
const pass = new PassThrough();
const writable = new Writable();
pass.pipe(writable);
pass.unpipe(writable);
// readableFlowing is now false.
pass.on('data', (chunk) => { console.log(chunk.toString()); });
// readableFlowing is still false.
pass.write('ok'); // Will not emit 'data'.
pass.resume(); // Must be called to make stream emit 'data'.
// readableFlowing is now true.
While readable.readableFlowing
is false
, data may be accumulating
within the stream's internal buffer.
Choose one API style#
The Readable
stream API evolved across multiple Node.js versions and provides
multiple methods of consuming stream data. In general, developers should choose
one of the methods of consuming data and should never use multiple methods
to consume data from a single stream. Specifically, using a combination
of on('data')
, on('readable')
, pipe()
, or async iterators could
lead to unintuitive behavior.
Class: stream.Readable
#
Event: 'close'
#
The 'close'
event is emitted when the stream and any of its underlying
resources (a file descriptor, for example) have been closed. The event indicates
that no more events will be emitted, and no further computation will occur.
A Readable
stream will always emit the 'close'
event if it is
created with the emitClose
option.
Event: 'data'
#
chunk
<Buffer> | <string> | <any> The chunk of data. For streams that are not operating in object mode, the chunk will be either a string orBuffer
. For streams that are in object mode, the chunk can be any JavaScript value other thannull
.
The 'data'
event is emitted whenever the stream is relinquishing ownership of
a chunk of data to a consumer. This may occur whenever the stream is switched
in flowing mode by calling readable.pipe()
, readable.resume()
, or by
attaching a listener callback to the 'data'
event. The 'data'
event will
also be emitted whenever the readable.read()
method is called and a chunk of
data is available to be returned.
Attaching a 'data'
event listener to a stream that has not been explicitly
paused will switch the stream into flowing mode. Data will then be passed as
soon as it is available.
The listener callback will be passed the chunk of data as a string if a default
encoding has been specified for the stream using the
readable.setEncoding()
method; otherwise the data will be passed as a
Buffer
.
const readable = getReadableStreamSomehow();
readable.on('data', (chunk) => {
console.log(`Received ${chunk.length} bytes of data.`);
});
Event: 'end'
#
The 'end'
event is emitted when there is no more data to be consumed from
the stream.
The 'end'
event will not be emitted unless the data is completely
consumed. This can be accomplished by switching the stream into flowing mode,
or by calling stream.read()
repeatedly until all data has been
consumed.
const readable = getReadableStreamSomehow();
readable.on('data', (chunk) => {
console.log(`Received ${chunk.length} bytes of data.`);
});
readable.on('end', () => {
console.log('There will be no more data.');
});
Event: 'error'
#
The 'error'
event may be emitted by a Readable
implementation at any time.
Typically, this may occur if the underlying stream is unable to generate data
due to an underlying internal failure, or when a stream implementation attempts
to push an invalid chunk of data.
The listener callback will be passed a single Error
object.
Event: 'pause'
#
The 'pause'
event is emitted when stream.pause()
is called
and readableFlowing
is not false
.
Event: 'readable'
#
The 'readable'
event is emitted when there is data available to be read from
the stream or when the end of the stream has been reached. Effectively, the
'readable'
event indicates that the stream has new information. If data is
available, stream.read()
will return that data.
const readable = getReadableStreamSomehow();
readable.on('readable', function() {
// There is some data to read now.
let data;
while ((data = this.read()) !== null) {
console.log(data);
}
});
If the end of the stream has been reached, calling
stream.read()
will return null
and trigger the 'end'
event. This is also true if there never was any data to be read. For instance,
in the following example, foo.txt
is an empty file:
const fs = require('node:fs');
const rr = fs.createReadStream('foo.txt');
rr.on('readable', () => {
console.log(`readable: ${rr.read()}`);
});
rr.on('end', () => {
console.log('end');
});
The output of running this script is:
$ node test.js
readable: null
end
In some cases, attaching a listener for the 'readable'
event will cause some
amount of data to be read into an internal buffer.
In general, the readable.pipe()
and 'data'
event mechanisms are easier to
understand than the 'readable'
event. However, handling 'readable'
might
result in increased throughput.
If both 'readable'
and 'data'
are used at the same time, 'readable'
takes precedence in controlling the flow, i.e. 'data'
will be emitted
only when stream.read()
is called. The
readableFlowing
property would become false
.
If there are 'data'
listeners when 'readable'
is removed, the stream
will start flowing, i.e. 'data'
events will be emitted without calling
.resume()
.
Event: 'resume'
#
The 'resume'
event is emitted when stream.resume()
is
called and readableFlowing
is not true
.
readable.destroy([error])
#
Destroy the stream. Optionally emit an 'error'
event, and emit a 'close'
event (unless emitClose
is set to false
). After this call, the readable
stream will release any internal resources and subsequent calls to push()
will be ignored.
Once destroy()
has been called any further calls will be a no-op and no
further errors except from _destroy()
may be emitted as 'error'
.
Implementors should not override this method, but instead implement
readable._destroy()
.
readable.closed
#
Is true
after 'close'
has been emitted.
readable.destroyed
#
Is true
after readable.destroy()
has been called.
readable.isPaused()
#
- Returns: <boolean>
The readable.isPaused()
method returns the current operating state of the
Readable
. This is used primarily by the mechanism that underlies the
readable.pipe()
method. In most typical cases, there will be no reason to
use this method directly.
const readable = new stream.Readable();
readable.isPaused(); // === false
readable.pause();
readable.isPaused(); // === true
readable.resume();
readable.isPaused(); // === false
readable.pause()
#
- Returns: <this>
The readable.pause()
method will cause a stream in flowing mode to stop
emitting 'data'
events, switching out of flowing mode. Any data that
becomes available will remain in the internal buffer.
const readable = getReadableStreamSomehow();
readable.on('data', (chunk) => {
console.log(`Received ${chunk.length} bytes of data.`);
readable.pause();
console.log('There will be no additional data for 1 second.');
setTimeout(() => {
console.log('Now data will start flowing again.');
readable.resume();
}, 1000);
});
The readable.pause()
method has no effect if there is a 'readable'
event listener.
readable.pipe(destination[, options])
#
destination
<stream.Writable> The destination for writing dataoptions
<Object> Pipe optionsend
<boolean> End the writer when the reader ends. Default:true
.
- Returns: <stream.Writable> The destination, allowing for a chain of pipes if
it is a
Duplex
or aTransform
stream
The readable.pipe()
method attaches a Writable
stream to the readable
,
causing it to switch automatically into flowing mode and push all of its data
to the attached Writable
. The flow of data will be automatically managed
so that the destination Writable
stream is not overwhelmed by a faster
Readable
stream.
The following example pipes all of the data from the readable
into a file
named file.txt
:
const fs = require('node:fs');
const readable = getReadableStreamSomehow();
const writable = fs.createWriteStream('file.txt');
// All the data from readable goes into 'file.txt'.
readable.pipe(writable);
It is possible to attach multiple Writable
streams to a single Readable
stream.
The readable.pipe()
method returns a reference to the destination stream
making it possible to set up chains of piped streams:
const fs = require('node:fs');
const zlib = require('node:zlib');
const r = fs.createReadStream('file.txt');
const z = zlib.createGzip();
const w = fs.createWriteStream('file.txt.gz');
r.pipe(z).pipe(w);
By default, stream.end()
is called on the destination Writable
stream when the source Readable
stream emits 'end'
, so that the
destination is no longer writable. To disable this default behavior, the end
option can be passed as false
, causing the destination stream to remain open:
reader.pipe(writer, { end: false });
reader.on('end', () => {
writer.end('Goodbye\n');
});
One important caveat is that if the Readable
stream emits an error during
processing, the Writable
destination is not closed automatically. If an
error occurs, it will be necessary to manually close each stream in order
to prevent memory leaks.
The process.stderr
and process.stdout
Writable
streams are never
closed until the Node.js process exits, regardless of the specified options.
readable.read([size])
#
size
<number> Optional argument to specify how much data to read.- Returns: <string> | <Buffer> | <null> | <any>
The readable.read()
method reads data out of the internal buffer and
returns it. If no data is available to be read, null
is returned. By default,
the data is returned as a Buffer
object unless an encoding has been
specified using the readable.setEncoding()
method or the stream is operating
in object mode.
The optional size
argument specifies a specific number of bytes to read. If
size
bytes are not available to be read, null
will be returned unless
the stream has ended, in which case all of the data remaining in the internal
buffer will be returned.
If the size
argument is not specified, all of the data contained in the
internal buffer will be returned.
The size
argument must be less than or equal to 1 GiB.
The readable.read()
method should only be called on Readable
streams
operating in paused mode. In flowing mode, readable.read()
is called
automatically until the internal buffer is fully drained.
const readable = getReadableStreamSomehow();
// 'readable' may be triggered multiple times as data is buffered in
readable.on('readable', () => {
let chunk;
console.log('Stream is readable (new data received in buffer)');
// Use a loop to make sure we read all currently available data
while (null !== (chunk = readable.read())) {
console.log(`Read ${chunk.length} bytes of data...`);
}
});
// 'end' will be triggered once when there is no more data available
readable.on('end', () => {
console.log('Reached end of stream.');
});
Each call to readable.read()
returns a chunk of data, or null
. The chunks
are not concatenated. A while
loop is necessary to consume all data
currently in the buffer. When reading a large file .read()
may return null
,
having consumed all buffered content so far, but there is still more data to
come not yet buffered. In this case a new 'readable'
event will be emitted
when there is more data in the buffer. Finally the 'end'
event will be
emitted when there is no more data to come.
Therefore to read a file's whole contents from a readable
, it is necessary
to collect chunks across multiple 'readable'
events:
const chunks = [];
readable.on('readable', () => {
let chunk;
while (null !== (chunk = readable.read())) {
chunks.push(chunk);
}
});
readable.on('end', () => {
const content = chunks.join('');
});
A Readable
stream in object mode will always return a single item from
a call to readable.read(size)
, regardless of the value of the
size
argument.
If the readable.read()
method returns a chunk of data, a 'data'
event will
also be emitted.
Calling stream.read([size])
after the 'end'
event has
been emitted will return null
. No runtime error will be raised.
readable.readable
#
Is true
if it is safe to call readable.read()
, which means
the stream has not been destroyed or emitted 'error'
or 'end'
.
readable.readableAborted
#
Returns whether the stream was destroyed or errored before emitting 'end'
.
readable.readableDidRead
#
Returns whether 'data'
has been emitted.
readable.readableEncoding
#
Getter for the property encoding
of a given Readable
stream. The encoding
property can be set using the readable.setEncoding()
method.
readable.readableEnded
#
Becomes true
when 'end'
event is emitted.
readable.errored
#
Returns error if the stream has been destroyed with an error.
readable.readableFlowing
#
This property reflects the current state of a Readable
stream as described
in the Three states section.
readable.readableHighWaterMark
#
Returns the value of highWaterMark
passed when creating this Readable
.
readable.readableLength
#
This property contains the number of bytes (or objects) in the queue
ready to be read. The value provides introspection data regarding
the status of the highWaterMark
.
readable.readableObjectMode
#
Getter for the property objectMode
of a given Readable
stream.
readable.resume()
#
- Returns: <this>
The readable.resume()
method causes an explicitly paused Readable
stream to
resume emitting 'data'
events, switching the stream into flowing mode.
The readable.resume()
method can be used to fully consume the data from a
stream without actually processing any of that data:
getReadableStreamSomehow()
.resume()
.on('end', () => {
console.log('Reached the end, but did not read anything.');
});
The readable.resume()
method has no effect if there is a 'readable'
event listener.
readable.setEncoding(encoding)
#
The readable.setEncoding()
method sets the character encoding for
data read from the Readable
stream.
By default, no encoding is assigned and stream data will be returned as
Buffer
objects. Setting an encoding causes the stream data
to be returned as strings of the specified encoding rather than as Buffer
objects. For instance, calling readable.setEncoding('utf8')
will cause the
output data to be interpreted as UTF-8 data, and passed as strings. Calling
readable.setEncoding('hex')
will cause the data to be encoded in hexadecimal
string format.
The Readable
stream will properly handle multi-byte characters delivered
through the stream that would otherwise become improperly decoded if simply
pulled from the stream as Buffer
objects.
const readable = getReadableStreamSomehow();
readable.setEncoding('utf8');
readable.on('data', (chunk) => {
assert.equal(typeof chunk, 'string');
console.log('Got %d characters of string data:', chunk.length);
});
readable.unpipe([destination])
#
destination
<stream.Writable> Optional specific stream to unpipe- Returns: <this>
The readable.unpipe()
method detaches a Writable
stream previously attached
using the stream.pipe()
method.
If the destination
is not specified, then all pipes are detached.
If the destination
is specified, but no pipe is set up for it, then
the method does nothing.
const fs = require('node:fs');
const readable = getReadableStreamSomehow();
const writable = fs.createWriteStream('file.txt');
// All the data from readable goes into 'file.txt',
// but only for the first second.
readable.pipe(writable);
setTimeout(() => {
console.log('Stop writing to file.txt.');
readable.unpipe(writable);
console.log('Manually close the file stream.');
writable.end();
}, 1000);
readable.unshift(chunk[, encoding])
#
chunk
<Buffer> | <Uint8Array> | <string> | <null> | <any> Chunk of data to unshift onto the read queue. For streams not operating in object mode,chunk
must be a string,Buffer
,Uint8Array
, ornull
. For object mode streams,chunk
may be any JavaScript value.encoding
<string> Encoding of string chunks. Must be a validBuffer
encoding, such as'utf8'
or'ascii'
.
Passing chunk
as null
signals the end of the stream (EOF) and behaves the
same as readable.push(null)
, after which no more data can be written. The EOF
signal is put at the end of the buffer and any buffered data will still be
flushed.
The readable.unshift()
method pushes a chunk of data back into the internal
buffer. This is useful in certain situations where a stream is being consumed by
code that needs to "un-consume" some amount of data that it has optimistically
pulled out of the source, so that the data can be passed on to some other party.
The stream.unshift(chunk)
method cannot be called after the 'end'
event
has been emitted or a runtime error will be thrown.
Developers using stream.unshift()
often should consider switching to
use of a Transform
stream instead. See the API for stream implementers
section for more information.
// Pull off a header delimited by \n\n.
// Use unshift() if we get too much.
// Call the callback with (error, header, stream).
const { StringDecoder } = require('node:string_decoder');
function parseHeader(stream, callback) {
stream.on('error', callback);
stream.on('readable', onReadable);
const decoder = new StringDecoder('utf8');
let header = '';
function onReadable() {
let chunk;
while (null !== (chunk = stream.read())) {
const str = decoder.write(chunk);
if (str.includes('\n\n')) {
// Found the header boundary.
const split = str.split(/\n\n/);
header += split.shift();
const remaining = split.join('\n\n');
const buf = Buffer.from(remaining, 'utf8');
stream.removeListener('error', callback);
// Remove the 'readable' listener before unshifting.
stream.removeListener('readable', onReadable);
if (buf.length)
stream.unshift(buf);
// Now the body of the message can be read from the stream.
callback(null, header, stream);
return;
}
// Still reading the header.
header += str;
}
}
}
Unlike stream.push(chunk)
, stream.unshift(chunk)
will not
end the reading process by resetting the internal reading state of the stream.
This can cause unexpected results if readable.unshift()
is called during a
read (i.e. from within a stream._read()
implementation on a
custom stream). Following the call to readable.unshift()
with an immediate
stream.push('')
will reset the reading state appropriately,
however it is best to simply avoid calling readable.unshift()
while in the
process of performing a read.
readable.wrap(stream)
#
Prior to Node.js 0.10, streams did not implement the entire node:stream
module API as it is currently defined. (See Compatibility for more
information.)
When using an older Node.js library that emits 'data'
events and has a
stream.pause()
method that is advisory only, the
readable.wrap()
method can be used to create a Readable
stream that uses
the old stream as its data source.
It will rarely be necessary to use readable.wrap()
but the method has been
provided as a convenience for interacting with older Node.js applications and
libraries.
const { OldReader } = require('./old-api-module.js');
const { Readable } = require('node:stream');
const oreader = new OldReader();
const myReader = new Readable().wrap(oreader);
myReader.on('readable', () => {
myReader.read(); // etc.
});
readable[Symbol.asyncIterator]()
#
- Returns: <AsyncIterator> to fully consume the stream.
const fs = require('node:fs');
async function print(readable) {
readable.setEncoding('utf8');
let data = '';
for await (const chunk of readable) {
data += chunk;
}
console.log(data);
}
print(fs.createReadStream('file')).catch(console.error);
If the loop terminates with a break
, return
, or a throw
, the stream will
be destroyed. In other terms, iterating over a stream will consume the stream
fully. The stream will be read in chunks of size equal to the highWaterMark
option. In the code example above, data will be in a single chunk if the file
has less then 64 KiB of data because no highWaterMark
option is provided to
fs.createReadStream()
.
readable.compose(stream[, options])
#
stream
<Stream> | <Iterable> | <AsyncIterable> | <Function>options
<Object>signal
<AbortSignal> allows destroying the stream if the signal is aborted.
- Returns: <Duplex> a stream composed with the stream
stream
.
import { Readable } from 'node:stream';
async function* splitToWords(source) {
for await (const chunk of source) {
const words = String(chunk).split(' ');
for (const word of words) {
yield word;
}
}
}
const wordsStream = Readable.from(['this is', 'compose as operator']).compose(splitToWords);
const words = await wordsStream.toArray();
console.log(words); // prints ['this', 'is', 'compose', 'as', 'operator']
See stream.compose
for more information.
readable.iterator([options])
#
options
<Object>destroyOnReturn
<boolean> When set tofalse
, callingreturn
on the async iterator, or exiting afor await...of
iteration using abreak
,return
, orthrow
will not destroy the stream. Default:true
.
- Returns: <AsyncIterator> to consume the stream.
The iterator created by this method gives users the option to cancel the
destruction of the stream if the for await...of
loop is exited by return
,
break
, or throw
, or if the iterator should destroy the stream if the stream
emitted an error during iteration.
const { Readable } = require('node:stream');
async function printIterator(readable) {
for await (const chunk of readable.iterator({ destroyOnReturn: false })) {
console.log(chunk); // 1
break;
}
console.log(readable.destroyed); // false
for await (const chunk of readable.iterator({ destroyOnReturn: false })) {
console.log(chunk); // Will print 2 and then 3
}
console.log(readable.destroyed); // True, stream was totally consumed
}
async function printSymbolAsyncIterator(readable) {
for await (const chunk of readable) {
console.log(chunk); // 1
break;
}
console.log(readable.destroyed); // true
}
async function showBoth() {
await printIterator(Readable.from([1, 2, 3]));
await printSymbolAsyncIterator(Readable.from([1, 2, 3]));
}
showBoth();
readable.map(fn[, options])
#
fn
<Function> | <AsyncFunction> a function to map over every chunk in the stream.data
<any> a chunk of data from the stream.options
<Object>signal
<AbortSignal> aborted if the stream is destroyed allowing to abort thefn
call early.
options
<Object>concurrency
<number> the maximum concurrent invocation offn
to call on the stream at once. Default:1
.signal
<AbortSignal> allows destroying the stream if the signal is aborted.
- Returns: <Readable> a stream mapped with the function
fn
.
This method allows mapping over the stream. The fn
function will be called
for every chunk in the stream. If the fn
function returns a promise - that
promise will be await
ed before being passed to the result stream.
import { Readable } from 'node:stream';
import { Resolver } from 'node:dns/promises';
// With a synchronous mapper.
for await (const chunk of Readable.from([1, 2, 3, 4]).map((x) => x * 2)) {
console.log(chunk); // 2, 4, 6, 8
}
// With an asynchronous mapper, making at most 2 queries at a time.
const resolver = new Resolver();
const dnsResults = Readable.from([
'nodejs.org',
'openjsf.org',
'www.linuxfoundation.org',
]).map((domain) => resolver.resolve4(domain), { concurrency: 2 });
for await (const result of dnsResults) {
console.log(result); // Logs the DNS result of resolver.resolve4.
}
readable.filter(fn[, options])
#
fn
<Function> | <AsyncFunction> a function to filter chunks from the stream.data
<any> a chunk of data from the stream.options
<Object>signal
<AbortSignal> aborted if the stream is destroyed allowing to abort thefn
call early.
options
<Object>concurrency
<number> the maximum concurrent invocation offn
to call on the stream at once. Default:1
.signal
<AbortSignal> allows destroying the stream if the signal is aborted.
- Returns: <Readable> a stream filtered with the predicate
fn
.
This method allows filtering the stream. For each chunk in the stream the fn
function will be called and if it returns a truthy value, the chunk will be
passed to the result stream. If the fn
function returns a promise - that
promise will be await
ed.
import { Readable } from 'node:stream';
import { Resolver } from 'node:dns/promises';
// With a synchronous predicate.
for await (const chunk of Readable.from([1, 2, 3, 4]).filter((x) => x > 2)) {
console.log(chunk); // 3, 4
}
// With an asynchronous predicate, making at most 2 queries at a time.
const resolver = new Resolver();
const dnsResults = Readable.from([
'nodejs.org',
'openjsf.org',
'www.linuxfoundation.org',
]).filter(async (domain) => {
const { address } = await resolver.resolve4(domain, { ttl: true });
return address.ttl > 60;
}, { concurrency: 2 });
for await (const result of dnsResults) {
// Logs domains with more than 60 seconds on the resolved dns record.
console.log(result);
}
readable.forEach(fn[, options])
#
fn
<Function> | <AsyncFunction> a function to call on each chunk of the stream.data
<any> a chunk of data from the stream.options
<Object>signal
<AbortSignal> aborted if the stream is destroyed allowing to abort thefn
call early.
options
<Object>concurrency
<number> the maximum concurrent invocation offn
to call on the stream at once. Default:1
.signal
<AbortSignal> allows destroying the stream if the signal is aborted.
- Returns: <Promise> a promise for when the stream has finished.
This method allows iterating a stream. For each chunk in the stream the
fn
function will be called. If the fn
function returns a promise - that
promise will be await
ed.
This method is different from for await...of
loops in that it can optionally
process chunks concurrently. In addition, a forEach
iteration can only be
stopped by having passed a signal
option and aborting the related
AbortController
while for await...of
can be stopped with break
or
return
. In either case the stream will be destroyed.
This method is different from listening to the 'data'
event in that it
uses the readable
event in the underlying machinary and can limit the
number of concurrent fn
calls.
import { Readable } from 'node:stream';
import { Resolver } from 'node:dns/promises';
// With a synchronous predicate.
for await (const chunk of Readable.from([1, 2, 3, 4]).filter((x) => x > 2)) {
console.log(chunk); // 3, 4
}
// With an asynchronous predicate, making at most 2 queries at a time.
const resolver = new Resolver();
const dnsResults = Readable.from([
'nodejs.org',
'openjsf.org',
'www.linuxfoundation.org',
]).map(async (domain) => {
const { address } = await resolver.resolve4(domain, { ttl: true });
return address;
}, { concurrency: 2 });
await dnsResults.forEach((result) => {
// Logs result, similar to `for await (const result of dnsResults)`
console.log(result);
});
console.log('done'); // Stream has finished
readable.toArray([options])
#
options
<Object>signal
<AbortSignal> allows cancelling the toArray operation if the signal is aborted.
- Returns: <Promise> a promise containing an array with the contents of the stream.
This method allows easily obtaining the contents of a stream.
As this method reads the entire stream into memory, it negates the benefits of streams. It's intended for interoperability and convenience, not as the primary way to consume streams.
import { Readable } from 'node:stream';
import { Resolver } from 'node:dns/promises';
await Readable.from([1, 2, 3, 4]).toArray(); // [1, 2, 3, 4]
// Make dns queries concurrently using .map and collect
// the results into an array using toArray
const dnsResults = await Readable.from([
'nodejs.org',
'openjsf.org',
'www.linuxfoundation.org',
]).map(async (domain) => {
const { address } = await resolver.resolve4(domain, { ttl: true });
return address;
}, { concurrency: 2 }).toArray();
readable.some(fn[, options])
#
fn
<Function> | <AsyncFunction> a function to call on each chunk of the stream.data
<any> a chunk of data from the stream.options
<Object>signal
<AbortSignal> aborted if the stream is destroyed allowing to abort thefn
call early.
options
<Object>concurrency
<number> the maximum concurrent invocation offn
to call on the stream at once. Default:1
.signal
<AbortSignal> allows destroying the stream if the signal is aborted.
- Returns: <Promise> a promise evaluating to
true
iffn
returned a truthy value for at least one of the chunks.
This method is similar to Array.prototype.some
and calls fn
on each chunk
in the stream until the awaited return value is true
(or any truthy value).
Once an fn
call on a chunk awaited return value is truthy, the stream is
destroyed and the promise is fulfilled with true
. If none of the fn
calls on the chunks return a truthy value, the promise is fulfilled with
false
.
import { Readable } from 'node:stream';
import { stat } from 'node:fs/promises';
// With a synchronous predicate.
await Readable.from([1, 2, 3, 4]).some((x) => x > 2); // true
await Readable.from([1, 2, 3, 4]).some((x) => x < 0); // false
// With an asynchronous predicate, making at most 2 file checks at a time.
const anyBigFile = await Readable.from([
'file1',
'file2',
'file3',
]).some(async (fileName) => {
const stats = await stat(fileName);
return stats.size > 1024 * 1024;
}, { concurrency: 2 });
console.log(anyBigFile); // `true` if any file in the list is bigger than 1MB
console.log('done'); // Stream has finished
readable.find(fn[, options])
#
fn
<Function> | <AsyncFunction> a function to call on each chunk of the stream.data
<any> a chunk of data from the stream.options
<Object>signal
<AbortSignal> aborted if the stream is destroyed allowing to abort thefn
call early.
options
<Object>concurrency
<number> the maximum concurrent invocation offn
to call on the stream at once. Default:1
.signal
<AbortSignal> allows destroying the stream if the signal is aborted.
- Returns: <Promise> a promise evaluating to the first chunk for which
fn
evaluated with a truthy value, orundefined
if no element was found.
This method is similar to Array.prototype.find
and calls fn
on each chunk
in the stream to find a chunk with a truthy value for fn
. Once an fn
call's
awaited return value is truthy, the stream is destroyed and the promise is
fulfilled with value for which fn
returned a truthy value. If all of the
fn
calls on the chunks return a falsy value, the promise is fulfilled with
undefined
.
import { Readable } from 'node:stream';
import { stat } from 'node:fs/promises';
// With a synchronous predicate.
await Readable.from([1, 2, 3, 4]).find((x) => x > 2); // 3
await Readable.from([1, 2, 3, 4]).find((x) => x > 0); // 1
await Readable.from([1, 2, 3, 4]).find((x) => x > 10); // undefined
// With an asynchronous predicate, making at most 2 file checks at a time.
const foundBigFile = await Readable.from([
'file1',
'file2',
'file3',
]).find(async (fileName) => {
const stats = await stat(fileName);
return stats.size > 1024 * 1024;
}, { concurrency: 2 });
console.log(foundBigFile); // File name of large file, if any file in the list is bigger than 1MB
console.log('done'); // Stream has finished
readable.every(fn[, options])
#
fn
<Function> | <AsyncFunction> a function to call on each chunk of the stream.data
<any> a chunk of data from the stream.options
<Object>signal
<AbortSignal> aborted if the stream is destroyed allowing to abort thefn
call early.
options
<Object>concurrency
<number> the maximum concurrent invocation offn
to call on the stream at once. Default:1
.signal
<AbortSignal> allows destroying the stream if the signal is aborted.
- Returns: <Promise> a promise evaluating to
true
iffn
returned a truthy value for all of the chunks.
This method is similar to Array.prototype.every
and calls fn
on each chunk
in the stream to check if all awaited return values are truthy value for fn
.
Once an fn
call on a chunk awaited return value is falsy, the stream is
destroyed and the promise is fulfilled with false
. If all of the fn
calls
on the chunks return a truthy value, the promise is fulfilled with true
.
import { Readable } from 'node:stream';
import { stat } from 'node:fs/promises';
// With a synchronous predicate.
await Readable.from([1, 2, 3, 4]).every((x) => x > 2); // false
await Readable.from([1, 2, 3, 4]).every((x) => x > 0); // true
// With an asynchronous predicate, making at most 2 file checks at a time.
const allBigFiles = await Readable.from([
'file1',
'file2',
'file3',
]).every(async (fileName) => {
const stats = await stat(fileName);
return stats.size > 1024 * 1024;
}, { concurrency: 2 });
// `true` if all files in the list are bigger than 1MiB
console.log(allBigFiles);
console.log('done'); // Stream has finished
readable.flatMap(fn[, options])
#
fn
<Function> | <AsyncGeneratorFunction> | <AsyncFunction> a function to map over every chunk in the stream.data
<any> a chunk of data from the stream.options
<Object>signal
<AbortSignal> aborted if the stream is destroyed allowing to abort thefn
call early.
options
<Object>concurrency
<number> the maximum concurrent invocation offn
to call on the stream at once. Default:1
.signal
<AbortSignal> allows destroying the stream if the signal is aborted.
- Returns: <Readable> a stream flat-mapped with the function
fn
.
This method returns a new stream by applying the given callback to each chunk of the stream and then flattening the result.
It is possible to return a stream or another iterable or async iterable from
fn
and the result streams will be merged (flattened) into the returned
stream.
import { Readable } from 'node:stream';
import { createReadStream } from 'node:fs';
// With a synchronous mapper.
for await (const chunk of Readable.from([1, 2, 3, 4]).flatMap((x) => [x, x])) {
console.log(chunk); // 1, 1, 2, 2, 3, 3, 4, 4
}
// With an asynchronous mapper, combine the contents of 4 files
const concatResult = Readable.from([
'./1.mjs',
'./2.mjs',
'./3.mjs',
'./4.mjs',
]).flatMap((fileName) => createReadStream(fileName));
for await (const result of concatResult) {
// This will contain the contents (all chunks) of all 4 files
console.log(result);
}
readable.drop(limit[, options])
#
limit
<number> the number of chunks to drop from the readable.options
<Object>signal
<AbortSignal> allows destroying the stream if the signal is aborted.
- Returns: <Readable> a stream with
limit
chunks dropped.
This method returns a new stream with the first limit
chunks dropped.
import { Readable } from 'node:stream';
await Readable.from([1, 2, 3, 4]).drop(2).toArray(); // [3, 4]
readable.take(limit[, options])
#
limit
<number> the number of chunks to take from the readable.options
<Object>signal
<AbortSignal> allows destroying the stream if the signal is aborted.
- Returns: <Readable> a stream with
limit
chunks taken.
This method returns a new stream with the first limit
chunks.
import { Readable } from 'node:stream';
await Readable.from([1, 2, 3, 4]).take(2).toArray(); // [1, 2]
readable.asIndexedPairs([options])
#
options
<Object>signal
<AbortSignal> allows destroying the stream if the signal is aborted.
- Returns: <Readable> a stream of indexed pairs.
This method returns a new stream with chunks of the underlying stream paired
with a counter in the form [index, chunk]
. The first index value is 0 and it
increases by 1 for each chunk produced.
import { Readable } from 'node:stream';
const pairs = await Readable.from(['a', 'b', 'c']).asIndexedPairs().toArray();
console.log(pairs); // [[0, 'a'], [1, 'b'], [2, 'c']]
readable.reduce(fn[, initial[, options]])
#
fn
<Function> | <AsyncFunction> a reducer function to call over every chunk in the stream.previous
<any> the value obtained from the last call tofn
or theinitial
value if specified or the first chunk of the stream otherwise.data
<any> a chunk of data from the stream.options
<Object>signal
<AbortSignal> aborted if the stream is destroyed allowing to abort thefn
call early.
initial
<any> the initial value to use in the reduction.options
<Object>signal
<AbortSignal> allows destroying the stream if the signal is aborted.
- Returns: <Promise> a promise for the final value of the reduction.
This method calls fn
on each chunk of the stream in order, passing it the
result from the calculation on the previous element. It returns a promise for
the final value of the reduction.
If no initial
value is supplied the first chunk of the stream is used as the
initial value. If the stream is empty, the promise is rejected with a
TypeError
with the ERR_INVALID_ARGS
code property.
import { Readable } from 'node:stream';
import { readdir, stat } from 'node:fs/promises';
import { join } from 'node:path';
const directoryPath = './src';
const filesInDir = await readdir(directoryPath);
const folderSize = await Readable.from(filesInDir)
.reduce(async (totalSize, file) => {
const { size } = await stat(join(directoryPath, file));
return totalSize + size;
}, 0);
console.log(folderSize);
The reducer function iterates the stream element-by-element which means that
there is no concurrency
parameter or parallelism. To perform a reduce
concurrently, you can extract the async function to readable.map
method.
import { Readable } from 'node:stream';
import { readdir, stat } from 'node:fs/promises';
import { join } from 'node:path';
const directoryPath = './src';
const filesInDir = await readdir(directoryPath);
const folderSize = await Readable.from(filesInDir)
.map((file) => stat(join(directoryPath, file)), { concurrency: 2 })
.reduce((totalSize, { size }) => totalSize + size, 0);
console.log(folderSize);
Duplex and transform streams#
Class: stream.Duplex
#
Duplex streams are streams that implement both the Readable
and
Writable
interfaces.
Examples of Duplex
streams include:
duplex.allowHalfOpen
#
If false
then the stream will automatically end the writable side when the
readable side ends. Set initially by the allowHalfOpen
constructor option,
which defaults to true
.
This can be changed manually to change the half-open behavior of an existing
Duplex
stream instance, but must be changed before the 'end'
event is
emitted.
Class: stream.Transform
#
Transform streams are Duplex
streams where the output is in some way
related to the input. Like all Duplex
streams, Transform
streams
implement both the Readable
and Writable
interfaces.
Examples of Transform
streams include:
transform.destroy([error])
#
Destroy the stream, and optionally emit an 'error'
event. After this call, the
transform stream would release any internal resources.
Implementors should not override this method, but instead implement
readable._destroy()
.
The default implementation of _destroy()
for Transform
also emit 'close'
unless emitClose
is set in false.
Once destroy()
has been called, any further calls will be a no-op and no
further errors except from _destroy()
may be emitted as 'error'
.
stream.finished(stream[, options], callback)
#
stream
<Stream> | <ReadableStream> | <WritableStream>
A readable and/or writable stream/webstream.
-
options
<Object>error
<boolean> If set tofalse
, then a call toemit('error', err)
is not treated as finished. Default:true
.readable
<boolean> When set tofalse
, the callback will be called when the stream ends even though the stream might still be readable. Default:true
.writable
<boolean> When set tofalse
, the callback will be called when the stream ends even though the stream might still be writable. Default:true
.signal
<AbortSignal> allows aborting the wait for the stream finish. The underlying stream will not be aborted if the signal is aborted. The callback will get called with anAbortError
. All registered listeners added by this function will also be removed.cleanup
<boolean> remove all registered stream listeners. Default:false
.
-
callback
<Function> A callback function that takes an optional error argument. -
Returns: <Function> A cleanup function which removes all registered listeners.
A function to get notified when a stream is no longer readable, writable or has experienced an error or a premature close event.
const { finished } = require('node:stream');
const fs = require('node:fs');
const rs = fs.createReadStream('archive.tar');
finished(rs, (err) => {
if (err) {
console.error('Stream failed.', err);
} else {
console.log('Stream is done reading.');
}
});
rs.resume(); // Drain the stream.
Especially useful in error handling scenarios where a stream is destroyed
prematurely (like an aborted HTTP request), and will not emit 'end'
or 'finish'
.
The finished
API provides promise version.
stream.finished()
leaves dangling event listeners (in particular
'error'
, 'end'
, 'finish'
and 'close'
) after callback
has been
invoked. The reason for this is so that unexpected 'error'
events (due to
incorrect stream implementations) do not cause unexpected crashes.
If this is unwanted behavior then the returned cleanup function needs to be
invoked in the callback:
const cleanup = finished(rs, (err) => {
cleanup();
// ...
});
stream.pipeline(source[, ...transforms], destination, callback)
#
stream.pipeline(streams, callback)
#
streams
<Stream[]> | <Iterable[]> | <AsyncIterable[]> | <Function[]> | <ReadableStream[]> | <WritableStream[]> | <TransformStream[]>source
<Stream> | <Iterable> | <AsyncIterable> | <Function> | <ReadableStream>- Returns: <Iterable> | <AsyncIterable>
...transforms
<Stream> | <Function> | <TransformStream>source
<AsyncIterable>- Returns: <AsyncIterable>
destination
<Stream> | <Function> | <WritableStream>source
<AsyncIterable>- Returns: <AsyncIterable> | <Promise>
callback
<Function> Called when the pipeline is fully done.err
<Error>val
Resolved value ofPromise
returned bydestination
.
- Returns: <Stream>
A module method to pipe between streams and generators forwarding errors and properly cleaning up and provide a callback when the pipeline is complete.
const { pipeline } = require('node:stream');
const fs = require('node:fs');
const zlib = require('node:zlib');
// Use the pipeline API to easily pipe a series of streams
// together and get notified when the pipeline is fully done.
// A pipeline to gzip a potentially huge tar file efficiently:
pipeline(
fs.createReadStream('archive.tar'),
zlib.createGzip(),
fs.createWriteStream('archive.tar.gz'),
(err) => {
if (err) {
console.error('Pipeline failed.', err);
} else {
console.log('Pipeline succeeded.');
}
},
);
The pipeline
API provides a promise version.
stream.pipeline()
will call stream.destroy(err)
on all streams except:
Readable
streams which have emitted'end'
or'close'
.Writable
streams which have emitted'finish'
or'close'
.
stream.pipeline()
leaves dangling event listeners on the streams
after the callback
has been invoked. In the case of reuse of streams after
failure, this can cause event listener leaks and swallowed errors. If the last
stream is readable, dangling event listeners will be removed so that the last
stream can be consumed later.
stream.pipeline()
closes all the streams when an error is raised.
The IncomingRequest
usage with pipeline
could lead to an unexpected behavior
once it would destroy the socket without sending the expected response.
See the example below:
const fs = require('node:fs');
const http = require('node:http');
const { pipeline } = require('node:stream');
const server = http.createServer((req, res) => {
const fileStream = fs.createReadStream('./fileNotExist.txt');
pipeline(fileStream, res, (err) => {
if (err) {
console.log(err); // No such file
// this message can't be sent once `pipeline` already destroyed the socket
return res.end('error!!!');
}
});
});
stream.compose(...streams)
#
stream.compose
is experimental.streams
<Stream[]> | <Iterable[]> | <AsyncIterable[]> | <Function[]> | <ReadableStream[]> | <WritableStream[]> | <TransformStream[]>- Returns: <stream.Duplex>
Combines two or more streams into a Duplex
stream that writes to the
first stream and reads from the last. Each provided stream is piped into
the next, using stream.pipeline
. If any of the streams error then all
are destroyed, including the outer Duplex
stream.
Because stream.compose
returns a new stream that in turn can (and
should) be piped into other streams, it enables composition. In contrast,
when passing streams to stream.pipeline
, typically the first stream is
a readable stream and the last a writable stream, forming a closed
circuit.
If passed a Function
it must be a factory method taking a source
Iterable
.
import { compose, Transform } from 'node:stream';
const removeSpaces = new Transform({
transform(chunk, encoding, callback) {
callback(null, String(chunk).replace(' ', ''));
},
});
async function* toUpper(source) {
for await (const chunk of source) {
yield String(chunk).toUpperCase();
}
}
let res = '';
for await (const buf of compose(removeSpaces, toUpper).end('hello world')) {
res += buf;
}
console.log(res); // prints 'HELLOWORLD'
stream.compose
can be used to convert async iterables, generators and
functions into streams.
AsyncIterable
converts into a readableDuplex
. Cannot yieldnull
.AsyncGeneratorFunction
converts into a readable/writable transformDuplex
. Must take a sourceAsyncIterable
as first parameter. Cannot yieldnull
.AsyncFunction
converts into a writableDuplex
. Must return eithernull
orundefined
.
import { compose } from 'node:stream';
import { finished } from 'node:stream/promises';
// Convert AsyncIterable into readable Duplex.
const s1 = compose(async function*() {
yield 'Hello';
yield 'World';
}());
// Convert AsyncGenerator into transform Duplex.
const s2 = compose(async function*(source) {
for await (const chunk of source) {
yield String(chunk).toUpperCase();
}
});
let res = '';
// Convert AsyncFunction into writable Duplex.
const s3 = compose(async function(source) {
for await (const chunk of source) {
res += chunk;
}
});
await finished(compose(s1, s2, s3));
console.log(res); // prints 'HELLOWORLD'
See readable.compose(stream)
for stream.compose
as operator.
stream.Readable.from(iterable[, options])
#
iterable
<Iterable> Object implementing theSymbol.asyncIterator
orSymbol.iterator
iterable protocol. Emits an 'error' event if a null value is passed.options
<Object> Options provided tonew stream.Readable([options])
. By default,Readable.from()
will setoptions.objectMode
totrue
, unless this is explicitly opted out by settingoptions.objectMode
tofalse
.- Returns: <stream.Readable>
A utility method for creating readable streams out of iterators.
const { Readable } = require('node:stream');
async function * generate() {
yield 'hello';
yield 'streams';
}
const readable = Readable.from(generate());
readable.on('data', (chunk) => {
console.log(chunk);
});
Calling Readable.from(string)
or Readable.from(buffer)
will not have
the strings or buffers be iterated to match the other streams semantics
for performance reasons.
If an Iterable
object containing promises is passed as an argument,
it might result in unhandled rejection.
const { Readable } = require('node:stream');
Readable.from([
new Promise((resolve) => setTimeout(resolve('1'), 1500)),
new Promise((_, reject) => setTimeout(reject(new Error('2')), 1000)), // Unhandled rejection
]);
stream.Readable.fromWeb(readableStream[, options])
#
readableStream
<ReadableStream>options
<Object>encoding
<string>highWaterMark
<number>objectMode
<boolean>signal
<AbortSignal>
- Returns: <stream.Readable>
stream.Readable.isDisturbed(stream)
#
stream
<stream.Readable> | <ReadableStream>- Returns:
boolean
Returns whether the stream has been read from or cancelled.
stream.isErrored(stream)
#
stream
<Readable> | <Writable> | <Duplex> | <WritableStream> | <ReadableStream>- Returns: <boolean>
Returns whether the stream has encountered an error.
stream.isReadable(stream)
#
stream
<Readable> | <Duplex> | <ReadableStream>- Returns: <boolean>
Returns whether the stream is readable.
stream.Readable.toWeb(streamReadable[, options])
#
streamReadable
<stream.Readable>options
<Object>strategy
<Object>highWaterMark
<number> The maximum internal queue size (of the createdReadableStream
) before backpressure is applied in reading from the givenstream.Readable
. If no value is provided, it will be taken from the givenstream.Readable
.size
<Function> A function that size of the given chunk of data. If no value is provided, the size will be1
for all the chunks.
- Returns: <ReadableStream>
stream.Writable.fromWeb(writableStream[, options])
#
writableStream
<WritableStream>options
<Object>decodeStrings
<boolean>highWaterMark
<number>objectMode
<boolean>signal
<AbortSignal>
- Returns: <stream.Writable>
stream.Writable.toWeb(streamWritable)
#
streamWritable
<stream.Writable>- Returns: <WritableStream>
stream.Duplex.from(src)
#
src
<Stream> | <Blob> | <ArrayBuffer> | <string> | <Iterable> | <AsyncIterable> | <AsyncGeneratorFunction> | <AsyncFunction> | <Promise> | <Object> | <ReadableStream> | <WritableStream>
A utility method for creating duplex streams.
Stream
converts writable stream into writableDuplex
and readable stream toDuplex
.Blob
converts into readableDuplex
.string
converts into readableDuplex
.ArrayBuffer
converts into readableDuplex
.AsyncIterable
converts into a readableDuplex
. Cannot yieldnull
.AsyncGeneratorFunction
converts into a readable/writable transformDuplex
. Must take a sourceAsyncIterable
as first parameter. Cannot yieldnull
.AsyncFunction
converts into a writableDuplex
. Must return eithernull
orundefined
Object ({ writable, readable })
convertsreadable
andwritable
intoStream
and then combines them intoDuplex
where theDuplex
will write to thewritable
and read from thereadable
.Promise
converts into readableDuplex
. Valuenull
is ignored.ReadableStream
converts into readableDuplex
.WritableStream
converts into writableDuplex
.- Returns: <stream.Duplex>
If an Iterable
object containing promises is passed as an argument,
it might result in unhandled rejection.
const { Duplex } = require('node:stream');
Duplex.from([
new Promise((resolve) => setTimeout(resolve('1'), 1500)),
new Promise((_, reject) => setTimeout(reject(new Error('2')), 1000)), // Unhandled rejection
]);
stream.Duplex.fromWeb(pair[, options])
#
pair
<Object>readable
<ReadableStream>writable
<WritableStream>
options
<Object>- Returns: <stream.Duplex>
import { Duplex } from 'node:stream';
import {
ReadableStream,
WritableStream,
} from 'node:stream/web';
const readable = new ReadableStream({
start(controller) {
controller.enqueue('world');
},
});
const writable = new WritableStream({
write(chunk) {
console.log('writable', chunk);
},
});
const pair = {
readable,
writable,
};
const duplex = Duplex.fromWeb(pair, { encoding: 'utf8', objectMode: true });
duplex.write('hello');
for await (const chunk of duplex) {
console.log('readable', chunk);
}
const { Duplex } = require('node:stream');
const {
ReadableStream,
WritableStream,
} = require('node:stream/web');
const readable = new ReadableStream({
start(controller) {
controller.enqueue('world');
},
});
const writable = new WritableStream({
write(chunk) {
console.log('writable', chunk);
},
});
const pair = {
readable,
writable,
};
const duplex = Duplex.fromWeb(pair, { encoding: 'utf8', objectMode: true });
duplex.write('hello');
duplex.once('readable', () => console.log('readable', duplex.read()));
stream.Duplex.toWeb(streamDuplex)
#
streamDuplex
<stream.Duplex>- Returns: <Object>
readable
<ReadableStream>writable
<WritableStream>
import { Duplex } from 'node:stream';
const duplex = Duplex({
objectMode: true,
read() {
this.push('world');
this.push(null);
},
write(chunk, encoding, callback) {
console.log('writable', chunk);
callback();
},
});
const { readable, writable } = Duplex.toWeb(duplex);
writable.getWriter().write('hello');
const { value } = await readable.getReader().read();
console.log('readable', value);
const { Duplex } = require('node:stream');
const duplex = Duplex({
objectMode: true,
read() {
this.push('world');
this.push(null);
},
write(chunk, encoding, callback) {
console.log('writable', chunk);
callback();
},
});
const { readable, writable } = Duplex.toWeb(duplex);
writable.getWriter().write('hello');
readable.getReader().read().then((result) => {
console.log('readable', result.value);
});
stream.addAbortSignal(signal, stream)
#
signal
<AbortSignal> A signal representing possible cancellationstream
<Stream> | <ReadableStream> | <WritableStream>
A stream to attach a signal to.
Attaches an AbortSignal to a readable or writeable stream. This lets code
control stream destruction using an AbortController
.
Calling abort
on the AbortController
corresponding to the passed
AbortSignal
will behave the same way as calling .destroy(new AbortError())
on the stream, and controller.error(new AbortError())
for webstreams.
const fs = require('node:fs');
const controller = new AbortController();
const read = addAbortSignal(
controller.signal,
fs.createReadStream(('object.json')),
);
// Later, abort the operation closing the stream
controller.abort();
Or using an AbortSignal
with a readable stream as an async iterable:
const controller = new AbortController();
setTimeout(() => controller.abort(), 10_000); // set a timeout
const stream = addAbortSignal(
controller.signal,
fs.createReadStream(('object.json')),
);
(async () => {
try {
for await (const chunk of stream) {
await process(chunk);
}
} catch (e) {
if (e.name === 'AbortError') {
// The operation was cancelled
} else {
throw e;
}
}
})();
Or using an AbortSignal
with a ReadableStream:
const controller = new AbortController();
const rs = new ReadableStream({
start(controller) {
controller.enqueue('hello');
controller.enqueue('world');
controller.close();
},
});
addAbortSignal(controller.signal, rs);
finished(rs, (err) => {
if (err) {
if (err.name === 'AbortError') {
// The operation was cancelled
}
}
});
const reader = rs.getReader();
reader.read().then(({ value, done }) => {
console.log(value); // hello
console.log(done); // false
controller.abort();
});
API for stream implementers#
The node:stream
module API has been designed to make it possible to easily
implement streams using JavaScript's prototypal inheritance model.
First, a stream developer would declare a new JavaScript class that extends one
of the four basic stream classes (stream.Writable
, stream.Readable
,
stream.Duplex
, or stream.Transform
), making sure they call the appropriate
parent class constructor:
const { Writable } = require('node:stream');
class MyWritable extends Writable {
constructor({ highWaterMark, ...options }) {
super({ highWaterMark });
// ...
}
}
When extending streams, keep in mind what options the user
can and should provide before forwarding these to the base constructor. For
example, if the implementation makes assumptions in regard to the
autoDestroy
and emitClose
options, do not allow the
user to override these. Be explicit about what
options are forwarded instead of implicitly forwarding all options.
The new stream class must then implement one or more specific methods, depending on the type of stream being created, as detailed in the chart below:
Use-case | Class | Method(s) to implement |
---|---|---|
Reading only | Readable | _read() |
Writing only | Writable | _write() , _writev() , _final() |
Reading and writing | Duplex | _read() , _write() , _writev() , _final() |
Operate on written data, then read the result | Transform | _transform() , _flush() , _final() |
The implementation code for a stream should never call the "public" methods of a stream that are intended for use by consumers (as described in the API for stream consumers section). Doing so may lead to adverse side effects in application code consuming the stream.
Avoid overriding public methods such as write()
, end()
, cork()
,
uncork()
, read()
and destroy()
, or emitting internal events such
as 'error'
, 'data'
, 'end'
, 'finish'
and 'close'
through .emit()
.
Doing so can break current and future stream invariants leading to behavior
and/or compatibility issues with other streams, stream utilities, and user
expectations.
Simplified construction#
For many simple cases, it is possible to create a stream without relying on
inheritance. This can be accomplished by directly creating instances of the
stream.Writable
, stream.Readable
, stream.Duplex
, or stream.Transform
objects and passing appropriate methods as constructor options.
const { Writable } = require('node:stream');
const myWritable = new Writable({
construct(callback) {
// Initialize state and load resources...
},
write(chunk, encoding, callback) {
// ...
},
destroy() {
// Free resources...
},
});
Implementing a writable stream#
The stream.Writable
class is extended to implement a Writable
stream.
Custom Writable
streams must call the new stream.Writable([options])
constructor and implement the writable._write()
and/or writable._writev()
method.
new stream.Writable([options])
#
options
<Object>highWaterMark
<number> Buffer level whenstream.write()
starts returningfalse
. Default:16384
(16 KiB), or16
forobjectMode
streams.decodeStrings
<boolean> Whether to encodestring
s passed tostream.write()
toBuffer
s (with the encoding specified in thestream.write()
call) before passing them tostream._write()
. Other types of data are not converted (i.e.Buffer
s are not decoded intostring
s). Setting to false will preventstring
s from being converted. Default:true
.defaultEncoding
<string> The default encoding that is used when no encoding is specified as an argument tostream.write()
. Default:'utf8'
.objectMode
<boolean> Whether or not thestream.write(anyObj)
is a valid operation. When set, it becomes possible to write JavaScript values other than string,Buffer
orUint8Array
if supported by the stream implementation. Default:false
.emitClose
<boolean> Whether or not the stream should emit'close'
after it has been destroyed. Default:true
.write
<Function> Implementation for thestream._write()
method.writev
<Function> Implementation for thestream._writev()
method.destroy
<Function> Implementation for thestream._destroy()
method.final
<Function> Implementation for thestream._final()
method.construct
<Function> Implementation for thestream._construct()
method.autoDestroy
<boolean> Whether this stream should automatically call.destroy()
on itself after ending. Default:true
.signal
<AbortSignal> A signal representing possible cancellation.
const { Writable } = require('node:stream');
class MyWritable extends Writable {
constructor(options) {
// Calls the stream.Writable() constructor.
super(options);
// ...
}
}
Or, when using pre-ES6 style constructors:
const { Writable } = require('node:stream');
const util = require('node:util');
function MyWritable(options) {
if (!(this instanceof MyWritable))
return new MyWritable(options);
Writable.call(this, options);
}
util.inherits(MyWritable, Writable);
Or, using the simplified constructor approach:
const { Writable } = require('node:stream');
const myWritable = new Writable({
write(chunk, encoding, callback) {
// ...
},
writev(chunks, callback) {
// ...
},
});
Calling abort
on the AbortController
corresponding to the passed
AbortSignal
will behave the same way as calling .destroy(new AbortError())
on the writeable stream.
const { Writable } = require('node:stream');
const controller = new AbortController();
const myWritable = new Writable({
write(chunk, encoding, callback) {
// ...
},
writev(chunks, callback) {
// ...
},
signal: controller.signal,
});
// Later, abort the operation closing the stream
controller.abort();
writable._construct(callback)
#
callback
<Function> Call this function (optionally with an error argument) when the stream has finished initializing.
The _construct()
method MUST NOT be called directly. It may be implemented
by child classes, and if so, will be called by the internal Writable
class methods only.
This optional function will be called in a tick after the stream constructor
has returned, delaying any _write()
, _final()
and _destroy()
calls until
callback
is called. This is useful to initialize state or asynchronously
initialize resources before the stream can be used.
const { Writable } = require('node:stream');
const fs = require('node:fs');
class WriteStream extends Writable {
constructor(filename) {
super();
this.filename = filename;
this.fd = null;
}
_construct(callback) {
fs.open(this.filename, (err, fd) => {
if (err) {
callback(err);
} else {
this.fd = fd;
callback();
}
});
}
_write(chunk, encoding, callback) {
fs.write(this.fd, chunk, callback);
}
_destroy(err, callback) {
if (this.fd) {
fs.close(this.fd, (er) => callback(er || err));
} else {
callback(err);
}
}
}
writable._write(chunk, encoding, callback)
#
chunk
<Buffer> | <string> | <any> TheBuffer
to be written, converted from thestring
passed tostream.write()
. If the stream'sdecodeStrings
option isfalse
or the stream is operating in object mode, the chunk will not be converted & will be whatever was passed tostream.write()
.encoding
<string> If the chunk is a string, thenencoding
is the character encoding of that string. If chunk is aBuffer
, or if the stream is operating in object mode,encoding
may be ignored.callback
<Function> Call this function (optionally with an error argument) when processing is complete for the supplied chunk.
All Writable
stream implementations must provide a
writable._write()
and/or
writable._writev()
method to send data to the underlying
resource.
Transform
streams provide their own implementation of the
writable._write()
.
This function MUST NOT be called by application code directly. It should be
implemented by child classes, and called by the internal Writable
class
methods only.
The callback
function must be called synchronously inside of
writable._write()
or asynchronously (i.e. different tick) to signal either
that the write completed successfully or failed with an error.
The first argument passed to the callback
must be the Error
object if the
call failed or null
if the write succeeded.
All calls to writable.write()
that occur between the time writable._write()
is called and the callback
is called will cause the written data to be
buffered. When the callback
is invoked, the stream might emit a 'drain'
event. If a stream implementation is capable of processing multiple chunks of
data at once, the writable._writev()
method should be implemented.
If the decodeStrings
property is explicitly set to false
in the constructor
options, then chunk
will remain the same object that is passed to .write()
,
and may be a string rather than a Buffer
. This is to support implementations
that have an optimized handling for certain string data encodings. In that case,
the encoding
argument will indicate the character encoding of the string.
Otherwise, the encoding
argument can be safely ignored.
The writable._write()
method is prefixed with an underscore because it is
internal to the class that defines it, and should never be called directly by
user programs.
writable._writev(chunks, callback)
#
chunks
<Object[]> The data to be written. The value is an array of <Object> that each represent a discrete chunk of data to write. The properties of these objects are:chunk
<Buffer> | <string> A buffer instance or string containing the data to be written. Thechunk
will be a string if theWritable
was created with thedecodeStrings
option set tofalse
and a string was passed towrite()
.encoding
<string> The character encoding of thechunk
. Ifchunk
is aBuffer
, theencoding
will be'buffer'
.
callback
<Function> A callback function (optionally with an error argument) to be invoked when processing is complete for the supplied chunks.
This function MUST NOT be called by application code directly. It should be
implemented by child classes, and called by the internal Writable
class
methods only.
The writable._writev()
method may be implemented in addition or alternatively
to writable._write()
in stream implementations that are capable of processing
multiple chunks of data at once. If implemented and if there is buffered data
from previous writes, _writev()
will be called instead of _write()
.
The writable._writev()
method is prefixed with an underscore because it is
internal to the class that defines it, and should never be called directly by
user programs.
writable._destroy(err, callback)
#
err
<Error> A possible error.callback
<Function> A callback function that takes an optional error argument.
The _destroy()
method is called by writable.destroy()
.
It can be overridden by child classes but it must not be called directly.
Furthermore, the callback
should not be mixed with async/await
once it is executed when a promise is resolved.
writable._final(callback)
#
callback
<Function> Call this function (optionally with an error argument) when finished writing any remaining data.
The _final()
method must not be called directly. It may be implemented
by child classes, and if so, will be called by the internal Writable
class methods only.
This optional function will be called before the stream closes, delaying the
'finish'
event until callback
is called. This is useful to close resources
or write buffered data before a stream ends.
Errors while writing#
Errors occurring during the processing of the writable._write()
,
writable._writev()
and writable._final()
methods must be propagated
by invoking the callback and passing the error as the first argument.
Throwing an Error
from within these methods or manually emitting an 'error'
event results in undefined behavior.
If a Readable
stream pipes into a Writable
stream when Writable
emits an
error, the Readable
stream will be unpiped.
const { Writable } = require('node:stream');
const myWritable = new Writable({
write(chunk, encoding, callback) {
if (chunk.toString().indexOf('a') >= 0) {
callback(new Error('chunk is invalid'));
} else {
callback();
}
},
});
An example writable stream#
The following illustrates a rather simplistic (and somewhat pointless) custom
Writable
stream implementation. While this specific Writable
stream instance
is not of any real particular usefulness, the example illustrates each of the
required elements of a custom Writable
stream instance:
const { Writable } = require('node:stream');
class MyWritable extends Writable {
_write(chunk, encoding, callback) {
if (chunk.toString().indexOf('a') >= 0) {
callback(new Error('chunk is invalid'));
} else {
callback();
}
}
}
Decoding buffers in a writable stream#
Decoding buffers is a common task, for instance, when using transformers whose
input is a string. This is not a trivial process when using multi-byte
characters encoding, such as UTF-8. The following example shows how to decode
multi-byte strings using StringDecoder
and Writable
.
const { Writable } = require('node:stream');
const { StringDecoder } = require('node:string_decoder');
class StringWritable extends Writable {
constructor(options) {
super(options);
this._decoder = new StringDecoder(options && options.defaultEncoding);
this.data = '';
}
_write(chunk, encoding, callback) {
if (encoding === 'buffer') {
chunk = this._decoder.write(chunk);
}
this.data += chunk;
callback();
}
_final(callback) {
this.data += this._decoder.end();
callback();
}
}
const euro = [[0xE2, 0x82], [0xAC]].map(Buffer.from);
const w = new StringWritable();
w.write('currency: ');
w.write(euro[0]);
w.end(euro[1]);
console.log(w.data); // currency: €
Implementing a readable stream#
The stream.Readable
class is extended to implement a Readable
stream.
Custom Readable
streams must call the new stream.Readable([options])
constructor and implement the readable._read()
method.
new stream.Readable([options])
#
options
<Object>highWaterMark
<number> The maximum number of bytes to store in the internal buffer before ceasing to read from the underlying resource. Default:16384
(16 KiB), or16
forobjectMode
streams.encoding
<string> If specified, then buffers will be decoded to strings using the specified encoding. Default:null
.objectMode
<boolean> Whether this stream should behave as a stream of objects. Meaning thatstream.read(n)
returns a single value instead of aBuffer
of sizen
. Default:false
.emitClose
<boolean> Whether or not the stream should emit'close'
after it has been destroyed. Default:true
.read
<Function> Implementation for thestream._read()
method.destroy
<Function> Implementation for thestream._destroy()
method.construct
<Function> Implementation for thestream._construct()
method.autoDestroy
<boolean> Whether this stream should automatically call.destroy()
on itself after ending. Default:true
.signal
<AbortSignal> A signal representing possible cancellation.
const { Readable } = require('node:stream');
class MyReadable extends Readable {
constructor(options) {
// Calls the stream.Readable(options) constructor.
super(options);
// ...
}
}
Or, when using pre-ES6 style constructors:
const { Readable } = require('node:stream');
const util = require('node:util');
function MyReadable(options) {
if (!(this instanceof MyReadable))
return new MyReadable(options);
Readable.call(this, options);
}
util.inherits(MyReadable, Readable);
Or, using the simplified constructor approach:
const { Readable } = require('node:stream');
const myReadable = new Readable({
read(size) {
// ...
},
});
Calling abort
on the AbortController
corresponding to the passed
AbortSignal
will behave the same way as calling .destroy(new AbortError())
on the readable created.
const { Readable } = require('node:stream');
const controller = new AbortController();
const read = new Readable({
read(size) {
// ...
},
signal: controller.signal,
});
// Later, abort the operation closing the stream
controller.abort();
readable._construct(callback)
#
callback
<Function> Call this function (optionally with an error argument) when the stream has finished initializing.
The _construct()
method MUST NOT be called directly. It may be implemented
by child classes, and if so, will be called by the internal Readable
class methods only.
This optional function will be scheduled in the next tick by the stream
constructor, delaying any _read()
and _destroy()
calls until callback
is
called. This is useful to initialize state or asynchronously initialize
resources before the stream can be used.
const { Readable } = require('node:stream');
const fs = require('node:fs');
class ReadStream extends Readable {
constructor(filename) {
super();
this.filename = filename;
this.fd = null;
}
_construct(callback) {
fs.open(this.filename, (err, fd) => {
if (err) {
callback(err);
} else {
this.fd = fd;
callback();
}
});
}
_read(n) {
const buf = Buffer.alloc(n);
fs.read(this.fd, buf, 0, n, null, (err, bytesRead) => {
if (err) {
this.destroy(err);
} else {
this.push(bytesRead > 0 ? buf.slice(0, bytesRead) : null);
}
});
}
_destroy(err, callback) {
if (this.fd) {
fs.close(this.fd, (er) => callback(er || err));
} else {
callback(err);
}
}
}
readable._read(size)
#
size
<number> Number of bytes to read asynchronously
This function MUST NOT be called by application code directly. It should be
implemented by child classes, and called by the internal Readable
class
methods only.
All Readable
stream implementations must provide an implementation of the
readable._read()
method to fetch data from the underlying resource.
When readable._read()
is called, if data is available from the resource,
the implementation should begin pushing that data into the read queue using the
this.push(dataChunk)
method. _read()
will be called again
after each call to this.push(dataChunk)
once the stream is
ready to accept more data. _read()
may continue reading from the resource and
pushing data until readable.push()
returns false
. Only when _read()
is
called again after it has stopped should it resume pushing additional data into
the queue.
Once the readable._read()
method has been called, it will not be called
again until more data is pushed through the readable.push()
method. Empty data such as empty buffers and strings will not cause
readable._read()
to be called.
The size
argument is advisory. For implementations where a "read" is a
single operation that returns data can use the size
argument to determine how
much data to fetch. Other implementations may ignore this argument and simply
provide data whenever it becomes available. There is no need to "wait" until
size
bytes are available before calling stream.push(chunk)
.
The readable._read()
method is prefixed with an underscore because it is
internal to the class that defines it, and should never be called directly by
user programs.
readable._destroy(err, callback)
#
err
<Error> A possible error.callback
<Function> A callback function that takes an optional error argument.
The _destroy()
method is called by readable.destroy()
.
It can be overridden by child classes but it must not be called directly.
readable.push(chunk[, encoding])
#
chunk
<Buffer> | <Uint8Array> | <string> | <null> | <any> Chunk of data to push into the read queue. For streams not operating in object mode,chunk
must be a string,Buffer
orUint8Array
. For object mode streams,chunk
may be any JavaScript value.encoding
<string> Encoding of string chunks. Must be a validBuffer
encoding, such as'utf8'
or'ascii'
.- Returns: <boolean>
true
if additional chunks of data may continue to be pushed;false
otherwise.
When chunk
is a Buffer
, Uint8Array
, or string
, the chunk
of data will
be added to the internal queue for users of the stream to consume.
Passing chunk
as null
signals the end of the stream (EOF), after which no
more data can be written.
When the Readable
is operating in paused mode, the data added with
readable.push()
can be read out by calling the
readable.read()
method when the 'readable'
event is
emitted.
When the Readable
is operating in flowing mode, the data added with
readable.push()
will be delivered by emitting a 'data'
event.
The readable.push()
method is designed to be as flexible as possible. For
example, when wrapping a lower-level source that provides some form of
pause/resume mechanism, and a data callback, the low-level source can be wrapped
by the custom Readable
instance:
// `_source` is an object with readStop() and readStart() methods,
// and an `ondata` member that gets called when it has data, and
// an `onend` member that gets called when the data is over.
class SourceWrapper extends Readable {
constructor(options) {
super(options);
this._source = getLowLevelSourceObject();
// Every time there's data, push it into the internal buffer.
this._source.ondata = (chunk) => {
// If push() returns false, then stop reading from source.
if (!this.push(chunk))
this._source.readStop();
};
// When the source ends, push the EOF-signaling `null` chunk.
this._source.onend = () => {
this.push(null);
};
}
// _read() will be called when the stream wants to pull more data in.
// The advisory size argument is ignored in this case.
_read(size) {
this._source.readStart();
}
}
The readable.push()
method is used to push the content
into the internal buffer. It can be driven by the readable._read()
method.
For streams not operating in object mode, if the chunk
parameter of
readable.push()
is undefined
, it will be treated as empty string or
buffer. See readable.push('')
for more information.
Errors while reading#
Errors occurring during processing of the readable._read()
must be
propagated through the readable.destroy(err)
method.
Throwing an Error
from within readable._read()
or manually emitting an
'error'
event results in undefined behavior.
const { Readable } = require('node:stream');
const myReadable = new Readable({
read(size) {
const err = checkSomeErrorCondition();
if (err) {
this.destroy(err);
} else {
// Do some work.
}
},
});
An example counting stream#
The following is a basic example of a Readable
stream that emits the numerals
from 1 to 1,000,000 in ascending order, and then ends.
const { Readable } = require('node:stream');
class Counter extends Readable {
constructor(opt) {
super(opt);
this._max = 1000000;
this._index = 1;
}
_read() {
const i = this._index++;
if (i > this._max)
this.push(null);
else {
const str = String(i);
const buf = Buffer.from(str, 'ascii');
this.push(buf);
}
}
}
Implementing a duplex stream#
A Duplex
stream is one that implements both Readable
and
Writable
, such as a TCP socket connection.
Because JavaScript does not have support for multiple inheritance, the
stream.Duplex
class is extended to implement a Duplex
stream (as opposed
to extending the stream.Readable
and stream.Writable
classes).
The stream.Duplex
class prototypically inherits from stream.Readable
and
parasitically from stream.Writable
, but instanceof
will work properly for
both base classes due to overriding Symbol.hasInstance
on
stream.Writable
.
Custom Duplex
streams must call the new stream.Duplex([options])
constructor and implement both the readable._read()
and
writable._write()
methods.
new stream.Duplex(options)
#
options
<Object> Passed to bothWritable
andReadable
constructors. Also has the following fields:allowHalfOpen
<boolean> If set tofalse
, then the stream will automatically end the writable side when the readable side ends. Default:true
.readable
<boolean> Sets whether theDuplex
should be readable. Default:true
.writable
<boolean> Sets whether theDuplex
should be writable. Default:true
.readableObjectMode
<boolean> SetsobjectMode
for readable side of the stream. Has no effect ifobjectMode
istrue
. Default:false
.writableObjectMode
<boolean> SetsobjectMode
for writable side of the stream. Has no effect ifobjectMode
istrue
. Default:false
.readableHighWaterMark
<number> SetshighWaterMark
for the readable side of the stream. Has no effect ifhighWaterMark
is provided.writableHighWaterMark
<number> SetshighWaterMark
for the writable side of the stream. Has no effect ifhighWaterMark
is provided.
const { Duplex } = require('node:stream');
class MyDuplex extends Duplex {
constructor(options) {
super(options);
// ...
}
}
Or, when using pre-ES6 style constructors:
const { Duplex } = require('node:stream');
const util = require('node:util');
function MyDuplex(options) {
if (!(this instanceof MyDuplex))
return new MyDuplex(options);
Duplex.call(this, options);
}
util.inherits(MyDuplex, Duplex);
Or, using the simplified constructor approach:
const { Duplex } = require('node:stream');
const myDuplex = new Duplex({
read(size) {
// ...
},
write(chunk, encoding, callback) {
// ...
},
});
When using pipeline:
const { Transform, pipeline } = require('node:stream');
const fs = require('node:fs');
pipeline(
fs.createReadStream('object.json')
.setEncoding('utf8'),
new Transform({
decodeStrings: false, // Accept string input rather than Buffers
construct(callback) {
this.data = '';
callback();
},
transform(chunk, encoding, callback) {
this.data += chunk;
callback();
},
flush(callback) {
try {
// Make sure is valid json.
JSON.parse(this.data);
this.push(this.data);
callback();
} catch (err) {
callback(err);
}
},
}),
fs.createWriteStream('valid-object.json'),
(err) => {
if (err) {
console.error('failed', err);
} else {
console.log('completed');
}
},
);
An example duplex stream#
The following illustrates a simple example of a Duplex
stream that wraps a
hypothetical lower-level source object to which data can be written, and
from which data can be read, albeit using an API that is not compatible with
Node.js streams.
The following illustrates a simple example of a Duplex
stream that buffers
incoming written data via the Writable
interface that is read back out
via the Readable
interface.
const { Duplex } = require('node:stream');
const kSource = Symbol('source');
class MyDuplex extends Duplex {
constructor(source, options) {
super(options);
this[kSource] = source;
}
_write(chunk, encoding, callback) {
// The underlying source only deals with strings.
if (Buffer.isBuffer(chunk))
chunk = chunk.toString();
this[kSource].writeSomeData(chunk);
callback();
}
_read(size) {
this[kSource].fetchSomeData(size, (data, encoding) => {
this.push(Buffer.from(data, encoding));
});
}
}
The most important aspect of a Duplex
stream is that the Readable
and
Writable
sides operate independently of one another despite co-existing within
a single object instance.
Object mode duplex streams#
For Duplex
streams, objectMode
can be set exclusively for either the
Readable
or Writable
side using the readableObjectMode
and
writableObjectMode
options respectively.
In the following example, for instance, a new Transform
stream (which is a
type of Duplex
stream) is created that has an object mode Writable
side
that accepts JavaScript numbers that are converted to hexadecimal strings on
the Readable
side.
const { Transform } = require('node:stream');
// All Transform streams are also Duplex Streams.
const myTransform = new Transform({
writableObjectMode: true,
transform(chunk, encoding, callback) {
// Coerce the chunk to a number if necessary.
chunk |= 0;
// Transform the chunk into something else.
const data = chunk.toString(16);
// Push the data onto the readable queue.
callback(null, '0'.repeat(data.length % 2) + data);
},
});
myTransform.setEncoding('ascii');
myTransform.on('data', (chunk) => console.log(chunk));
myTransform.write(1);
// Prints: 01
myTransform.write(10);
// Prints: 0a
myTransform.write(100);
// Prints: 64
Implementing a transform stream#
A Transform
stream is a Duplex
stream where the output is computed
in some way from the input. Examples include zlib streams or crypto
streams that compress, encrypt, or decrypt data.
There is no requirement that the output be the same size as the input, the same
number of chunks, or arrive at the same time. For example, a Hash
stream will
only ever have a single chunk of output which is provided when the input is
ended. A zlib
stream will produce output that is either much smaller or much
larger than its input.
The stream.Transform
class is extended to implement a Transform
stream.
The stream.Transform
class prototypically inherits from stream.Duplex
and
implements its own versions of the writable._write()
and
readable._read()
methods. Custom Transform
implementations must
implement the transform._transform()
method and may
also implement the transform._flush()
method.
Care must be taken when using Transform
streams in that data written to the
stream can cause the Writable
side of the stream to become paused if the
output on the Readable
side is not consumed.
new stream.Transform([options])
#
options
<Object> Passed to bothWritable
andReadable
constructors. Also has the following fields:transform
<Function> Implementation for thestream._transform()
method.flush
<Function> Implementation for thestream._flush()
method.
const { Transform } = require('node:stream');
class MyTransform extends Transform {
constructor(options) {
super(options);
// ...
}
}
Or, when using pre-ES6 style constructors:
const { Transform } = require('node:stream');
const util = require('node:util');
function MyTransform(options) {
if (!(this instanceof MyTransform))
return new MyTransform(options);
Transform.call(this, options);
}
util.inherits(MyTransform, Transform);
Or, using the simplified constructor approach:
const { Transform } = require('node:stream');
const myTransform = new Transform({
transform(chunk, encoding, callback) {
// ...
},
});
Event: 'end'
#
The 'end'
event is from the stream.Readable
class. The 'end'
event is
emitted after all data has been output, which occurs after the callback in
transform._flush()
has been called. In the case of an error,
'end'
should not be emitted.
Event: 'finish'
#
The 'finish'
event is from the stream.Writable
class. The 'finish'
event is emitted after stream.end()
is called and all chunks
have been processed by stream._transform()
. In the case
of an error, 'finish'
should not be emitted.
transform._flush(callback)
#
callback
<Function> A callback function (optionally with an error argument and data) to be called when remaining data has been flushed.
This function MUST NOT be called by application code directly. It should be
implemented by child classes, and called by the internal Readable
class
methods only.
In some cases, a transform operation may need to emit an additional bit of
data at the end of the stream. For example, a zlib
compression stream will
store an amount of internal state used to optimally compress the output. When
the stream ends, however, that additional data needs to be flushed so that the
compressed data will be complete.
Custom Transform
implementations may implement the transform._flush()
method. This will be called when there is no more written data to be consumed,
but before the 'end'
event is emitted signaling the end of the
Readable
stream.
Within the transform._flush()
implementation, the transform.push()
method
may be called zero or more times, as appropriate. The callback
function must
be called when the flush operation is complete.
The transform._flush()
method is prefixed with an underscore because it is
internal to the class that defines it, and should never be called directly by
user programs.
transform._transform(chunk, encoding, callback)
#
chunk
<Buffer> | <string> | <any> TheBuffer
to be transformed, converted from thestring
passed tostream.write()
. If the stream'sdecodeStrings
option isfalse
or the stream is operating in object mode, the chunk will not be converted & will be whatever was passed tostream.write()
.encoding
<string> If the chunk is a string, then this is the encoding type. If chunk is a buffer, then this is the special value'buffer'
. Ignore it in that case.callback
<Function> A callback function (optionally with an error argument and data) to be called after the suppliedchunk
has been processed.
This function MUST NOT be called by application code directly. It should be
implemented by child classes, and called by the internal Readable
class
methods only.
All Transform
stream implementations must provide a _transform()
method to accept input and produce output. The transform._transform()
implementation handles the bytes being written, computes an output, then passes
that output off to the readable portion using the transform.push()
method.
The transform.push()
method may be called zero or more times to generate
output from a single input chunk, depending on how much is to be output
as a result of the chunk.
It is possible that no output is generated from any given chunk of input data.
The callback
function must be called only when the current chunk is completely
consumed. The first argument passed to the callback
must be an Error
object
if an error occurred while processing the input or null
otherwise. If a second
argument is passed to the callback
, it will be forwarded on to the
transform.push()
method. In other words, the following are equivalent:
transform.prototype._transform = function(data, encoding, callback) {
this.push(data);
callback();
};
transform.prototype._transform = function(data, encoding, callback) {
callback(null, data);
};
The transform._transform()
method is prefixed with an underscore because it
is internal to the class that defines it, and should never be called directly by
user programs.
transform._transform()
is never called in parallel; streams implement a
queue mechanism, and to receive the next chunk, callback
must be
called, either synchronously or asynchronously.
Class: stream.PassThrough
#
The stream.PassThrough
class is a trivial implementation of a Transform
stream that simply passes the input bytes across to the output. Its purpose is
primarily for examples and testing, but there are some use cases where
stream.PassThrough
is useful as a building block for novel sorts of streams.
Additional notes#
Streams compatibility with async generators and async iterators#
With the support of async generators and iterators in JavaScript, async generators are effectively a first-class language-level stream construct at this point.
Some common interop cases of using Node.js streams with async generators and async iterators are provided below.
Consuming readable streams with async iterators#
(async function() {
for await (const chunk of readable) {
console.log(chunk);
}
})();
Async iterators register a permanent error handler on the stream to prevent any unhandled post-destroy errors.
Creating readable streams with async generators#
A Node.js readable stream can be created from an asynchronous generator using
the Readable.from()
utility method:
const { Readable } = require('node:stream');
const ac = new AbortController();
const signal = ac.signal;
async function * generate() {
yield 'a';
await someLongRunningFn({ signal });
yield 'b';
yield 'c';
}
const readable = Readable.from(generate());
readable.on('close', () => {
ac.abort();
});
readable.on('data', (chunk) => {
console.log(chunk);
});
Piping to writable streams from async iterators#
When writing to a writable stream from an async iterator, ensure correct
handling of backpressure and errors. stream.pipeline()
abstracts away
the handling of backpressure and backpressure-related errors:
const fs = require('node:fs');
const { pipeline } = require('node:stream');
const { pipeline: pipelinePromise } = require('node:stream/promises');
const writable = fs.createWriteStream('./file');
const ac = new AbortController();
const signal = ac.signal;
const iterator = createIterator({ signal });
// Callback Pattern
pipeline(iterator, writable, (err, value) => {
if (err) {
console.error(err);
} else {
console.log(value, 'value returned');
}
}).on('close', () => {
ac.abort();
});
// Promise Pattern
pipelinePromise(iterator, writable)
.then((value) => {
console.log(value, 'value returned');
})
.catch((err) => {
console.error(err);
ac.abort();
});
Compatibility with older Node.js versions#
Prior to Node.js 0.10, the Readable
stream interface was simpler, but also
less powerful and less useful.
- Rather than waiting for calls to the
stream.read()
method,'data'
events would begin emitting immediately. Applications that would need to perform some amount of work to decide how to handle data were required to store read data into buffers so the data would not be lost. - The
stream.pause()
method was advisory, rather than guaranteed. This meant that it was still necessary to be prepared to receive'data'
events even when the stream was in a paused state.
In Node.js 0.10, the Readable
class was added. For backward
compatibility with older Node.js programs, Readable
streams switch into
"flowing mode" when a 'data'
event handler is added, or when the
stream.resume()
method is called. The effect is that, even
when not using the new stream.read()
method and
'readable'
event, it is no longer necessary to worry about losing
'data'
chunks.
While most applications will continue to function normally, this introduces an edge case in the following conditions:
- No
'data'
event listener is added. - The
stream.resume()
method is never called. - The stream is not piped to any writable destination.
For example, consider the following code:
// WARNING! BROKEN!
net.createServer((socket) => {
// We add an 'end' listener, but never consume the data.
socket.on('end', () => {
// It will never get here.
socket.end('The message was received but was not processed.\n');
});
}).listen(1337);
Prior to Node.js 0.10, the incoming message data would be simply discarded. However, in Node.js 0.10 and beyond, the socket remains paused forever.
The workaround in this situation is to call the
stream.resume()
method to begin the flow of data:
// Workaround.
net.createServer((socket) => {
socket.on('end', () => {
socket.end('The message was received but was not processed.\n');
});
// Start the flow of data, discarding it.
socket.resume();
}).listen(1337);
In addition to new Readable
streams switching into flowing mode,
pre-0.10 style streams can be wrapped in a Readable
class using the
readable.wrap()
method.
readable.read(0)
#
There are some cases where it is necessary to trigger a refresh of the
underlying readable stream mechanisms, without actually consuming any
data. In such cases, it is possible to call readable.read(0)
, which will
always return null
.
If the internal read buffer is below the highWaterMark
, and the
stream is not currently reading, then calling stream.read(0)
will trigger
a low-level stream._read()
call.
While most applications will almost never need to do this, there are
situations within Node.js where this is done, particularly in the
Readable
stream class internals.
readable.push('')
#
Use of readable.push('')
is not recommended.
Pushing a zero-byte string, Buffer
, or Uint8Array
to a stream that is not in
object mode has an interesting side effect. Because it is a call to
readable.push()
, the call will end the reading process.
However, because the argument is an empty string, no data is added to the
readable buffer so there is nothing for a user to consume.
highWaterMark
discrepancy after calling readable.setEncoding()
#
The use of readable.setEncoding()
will change the behavior of how the
highWaterMark
operates in non-object mode.
Typically, the size of the current buffer is measured against the
highWaterMark
in bytes. However, after setEncoding()
is called, the
comparison function will begin to measure the buffer's size in characters.
This is not a problem in common cases with latin1
or ascii
. But it is
advised to be mindful about this behavior when working with strings that could
contain multi-byte characters.
String decoder#
Source Code: lib/string_decoder.js
The node:string_decoder
module provides an API for decoding Buffer
objects
into strings in a manner that preserves encoded multi-byte UTF-8 and UTF-16
characters. It can be accessed using:
const { StringDecoder } = require('node:string_decoder');
The following example shows the basic use of the StringDecoder
class.
const { StringDecoder } = require('node:string_decoder');
const decoder = new StringDecoder('utf8');
const cent = Buffer.from([0xC2, 0xA2]);
console.log(decoder.write(cent));
const euro = Buffer.from([0xE2, 0x82, 0xAC]);
console.log(decoder.write(euro));
When a Buffer
instance is written to the StringDecoder
instance, an
internal buffer is used to ensure that the decoded string does not contain
any incomplete multibyte characters. These are held in the buffer until the
next call to stringDecoder.write()
or until stringDecoder.end()
is called.
In the following example, the three UTF-8 encoded bytes of the European Euro
symbol (€
) are written over three separate operations:
const { StringDecoder } = require('node:string_decoder');
const decoder = new StringDecoder('utf8');
decoder.write(Buffer.from([0xE2]));
decoder.write(Buffer.from([0x82]));
console.log(decoder.end(Buffer.from([0xAC])));
Class: StringDecoder
#
new StringDecoder([encoding])
#
Creates a new StringDecoder
instance.
stringDecoder.end([buffer])
#
buffer
<Buffer> | <TypedArray> | <DataView> ABuffer
, orTypedArray
, orDataView
containing the bytes to decode.- Returns: <string>
Returns any remaining input stored in the internal buffer as a string. Bytes representing incomplete UTF-8 and UTF-16 characters will be replaced with substitution characters appropriate for the character encoding.
If the buffer
argument is provided, one final call to stringDecoder.write()
is performed before returning the remaining input.
After end()
is called, the stringDecoder
object can be reused for new input.
stringDecoder.write(buffer)
#
buffer
<Buffer> | <TypedArray> | <DataView> ABuffer
, orTypedArray
, orDataView
containing the bytes to decode.- Returns: <string>
Returns a decoded string, ensuring that any incomplete multibyte characters at
the end of the Buffer
, or TypedArray
, or DataView
are omitted from the
returned string and stored in an internal buffer for the next call to
stringDecoder.write()
or stringDecoder.end()
.
Test runner#
Source Code: lib/test.js
The node:test
module facilitates the creation of JavaScript tests.
To access it:
import test from 'node:test';
const test = require('node:test');
This module is only available under the node:
scheme. The following will not
work:
import test from 'test';
const test = require('test');
Tests created via the test
module consist of a single function that is
processed in one of three ways:
- A synchronous function that is considered failing if it throws an exception, and is considered passing otherwise.
- A function that returns a
Promise
that is considered failing if thePromise
rejects, and is considered passing if thePromise
resolves. - A function that receives a callback function. If the callback receives any
truthy value as its first argument, the test is considered failing. If a
falsy value is passed as the first argument to the callback, the test is
considered passing. If the test function receives a callback function and
also returns a
Promise
, the test will fail.
The following example illustrates how tests are written using the
test
module.
test('synchronous passing test', (t) => {
// This test passes because it does not throw an exception.
assert.strictEqual(1, 1);
});
test('synchronous failing test', (t) => {
// This test fails because it throws an exception.
assert.strictEqual(1, 2);
});
test('asynchronous passing test', async (t) => {
// This test passes because the Promise returned by the async
// function is not rejected.
assert.strictEqual(1, 1);
});
test('asynchronous failing test', async (t) => {
// This test fails because the Promise returned by the async
// function is rejected.
assert.strictEqual(1, 2);
});
test('failing test using Promises', (t) => {
// Promises can be used directly as well.
return new Promise((resolve, reject) => {
setImmediate(() => {
reject(new Error('this will cause the test to fail'));
});
});
});
test('callback passing test', (t, done) => {
// done() is the callback function. When the setImmediate() runs, it invokes
// done() with no arguments.
setImmediate(done);
});
test('callback failing test', (t, done) => {
// When the setImmediate() runs, done() is invoked with an Error object and
// the test fails.
setImmediate(() => {
done(new Error('callback failure'));
});
});
If any tests fail, the process exit code is set to 1
.
Subtests#
The test context's test()
method allows subtests to be created. This method
behaves identically to the top level test()
function. The following example
demonstrates the creation of a top level test with two subtests.
test('top level test', async (t) => {
await t.test('subtest 1', (t) => {
assert.strictEqual(1, 1);
});
await t.test('subtest 2', (t) => {
assert.strictEqual(2, 2);
});
});
In this example, await
is used to ensure that both subtests have completed.
This is necessary because parent tests do not wait for their subtests to
complete. Any subtests that are still outstanding when their parent finishes
are cancelled and treated as failures. Any subtest failures cause the parent
test to fail.
Skipping tests#
Individual tests can be skipped by passing the skip
option to the test, or by
calling the test context's skip()
method as shown in the
following example.
// The skip option is used, but no message is provided.
test('skip option', { skip: true }, (t) => {
// This code is never executed.
});
// The skip option is used, and a message is provided.
test('skip option with message', { skip: 'this is skipped' }, (t) => {
// This code is never executed.
});
test('skip() method', (t) => {
// Make sure to return here as well if the test contains additional logic.
t.skip();
});
test('skip() method with message', (t) => {
// Make sure to return here as well if the test contains additional logic.
t.skip('this is skipped');
});
describe
/it
syntax#
Running tests can also be done using describe
to declare a suite
and it
to declare a test.
A suite is used to organize and group related tests together.
it
is a shorthand for test()
.
describe('A thing', () => {
it('should work', () => {
assert.strictEqual(1, 1);
});
it('should be ok', () => {
assert.strictEqual(2, 2);
});
describe('a nested thing', () => {
it('should work', () => {
assert.strictEqual(3, 3);
});
});
});
describe
and it
are imported from the node:test
module.
import { describe, it } from 'node:test';
const { describe, it } = require('node:test');
only
tests#
If Node.js is started with the --test-only
command-line option, it is
possible to skip all top level tests except for a selected subset by passing
the only
option to the tests that should be run. When a test with the only
option set is run, all subtests are also run. The test context's runOnly()
method can be used to implement the same behavior at the subtest level.
// Assume Node.js is run with the --test-only command-line option.
// The 'only' option is set, so this test is run.
test('this test is run', { only: true }, async (t) => {
// Within this test, all subtests are run by default.
await t.test('running subtest');
// The test context can be updated to run subtests with the 'only' option.
t.runOnly(true);
await t.test('this subtest is now skipped');
await t.test('this subtest is run', { only: true });
// Switch the context back to execute all tests.
t.runOnly(false);
await t.test('this subtest is now run');
// Explicitly do not run these tests.
await t.test('skipped subtest 3', { only: false });
await t.test('skipped subtest 4', { skip: true });
});
// The 'only' option is not set, so this test is skipped.
test('this test is not run', () => {
// This code is not run.
throw new Error('fail');
});
Filtering tests by name#
The --test-name-pattern
command-line option can be used to only run tests
whose name matches the provided pattern. Test name patterns are interpreted as
JavaScript regular expressions. The --test-name-pattern
option can be
specified multiple times in order to run nested tests. For each test that is
executed, any corresponding test hooks, such as beforeEach()
, are also
run.
Given the following test file, starting Node.js with the
--test-name-pattern="test [1-3]"
option would cause the test runner to execute
test 1
, test 2
, and test 3
. If test 1
did not match the test name
pattern, then its subtests would not execute, despite matching the pattern. The
same set of tests could also be executed by passing --test-name-pattern
multiple times (e.g. --test-name-pattern="test 1"
,
--test-name-pattern="test 2"
, etc.).
test('test 1', async (t) => {
await t.test('test 2');
await t.test('test 3');
});
test('Test 4', async (t) => {
await t.test('Test 5');
await t.test('test 6');
});
Test name patterns can also be specified using regular expression literals. This
allows regular expression flags to be used. In the previous example, starting
Node.js with --test-name-pattern="/test [4-5]/i"
would match Test 4
and
Test 5
because the pattern is case-insensitive.
Test name patterns do not change the set of files that the test runner executes.
Extraneous asynchronous activity#
Once a test function finishes executing, the results are reported as quickly as possible while maintaining the order of the tests. However, it is possible for the test function to generate asynchronous activity that outlives the test itself. The test runner handles this type of activity, but does not delay the reporting of test results in order to accommodate it.
In the following example, a test completes with two setImmediate()
operations still outstanding. The first setImmediate()
attempts to create a
new subtest. Because the parent test has already finished and output its
results, the new subtest is immediately marked as failed, and reported later
to the <TestsStream>.
The second setImmediate()
creates an uncaughtException
event.
uncaughtException
and unhandledRejection
events originating from a completed
test are marked as failed by the test
module and reported as diagnostic
warnings at the top level by the <TestsStream>.
test('a test that creates asynchronous activity', (t) => {
setImmediate(() => {
t.test('subtest that is created too late', (t) => {
throw new Error('error1');
});
});
setImmediate(() => {
throw new Error('error2');
});
// The test finishes after this line.
});
Watch mode#
The Node.js test runner supports running in watch mode by passing the --watch
flag:
node --test --watch
In watch mode, the test runner will watch for changes to test files and their dependencies. When a change is detected, the test runner will rerun the tests affected by the change. The test runner will continue to run until the process is terminated.
Running tests from the command line#
The Node.js test runner can be invoked from the command line by passing the
--test
flag:
node --test
By default, Node.js will recursively search the current directory for JavaScript source files matching a specific naming convention. Matching files are executed as test files. More information on the expected test file naming convention and behavior can be found in the test runner execution model section.
Alternatively, one or more paths can be provided as the final argument(s) to the Node.js command, as shown below.
node --test test1.js test2.mjs custom_test_dir/
In this example, the test runner will execute the files test1.js
and
test2.mjs
. The test runner will also recursively search the
custom_test_dir/
directory for test files to execute.
Test runner execution model#
When searching for test files to execute, the test runner behaves as follows:
- Any files explicitly provided by the user are executed.
- If the user did not explicitly specify any paths, the current working directory is recursively searched for files as specified in the following steps.
node_modules
directories are skipped unless explicitly provided by the user.- If a directory named
test
is encountered, the test runner will search it recursively for all all.js
,.cjs
, and.mjs
files. All of these files are treated as test files, and do not need to match the specific naming convention detailed below. This is to accommodate projects that place all of their tests in a singletest
directory. - In all other directories,
.js
,.cjs
, and.mjs
files matching the following patterns are treated as test files:^test$
- Files whose basename is the string'test'
. Examples:test.js
,test.cjs
,test.mjs
.^test-.+
- Files whose basename starts with the string'test-'
followed by one or more characters. Examples:test-example.js
,test-another-example.mjs
..+[\.\-\_]test$
- Files whose basename ends with.test
,-test
, or_test
, preceded by one or more characters. Examples:example.test.js
,example-test.cjs
,example_test.mjs
.- Other file types understood by Node.js such as
.node
and.json
are not automatically executed by the test runner, but are supported if explicitly provided on the command line.
Each matching test file is executed in a separate child process. If the child
process finishes with an exit code of 0, the test is considered passing.
Otherwise, the test is considered to be a failure. Test files must be
executable by Node.js, but are not required to use the node:test
module
internally.
Collecting code coverage#
When Node.js is started with the --experimental-test-coverage
command-line flag, code coverage is collected and statistics are reported once
all tests have completed. If the NODE_V8_COVERAGE
environment variable is
used to specify a code coverage directory, the generated V8 coverage files are
written to that directory. Node.js core modules and files within
node_modules/
directories are not included in the coverage report. If
coverage is enabled, the coverage report is sent to any test reporters via
the 'test:coverage'
event.
Coverage can be disabled on a series of lines using the following comment syntax:
/* node:coverage disable */
if (anAlwaysFalseCondition) {
// Code in this branch will never be executed, but the lines are ignored for
// coverage purposes. All lines following the 'disable' comment are ignored
// until a corresponding 'enable' comment is encountered.
console.log('this is never executed');
}
/* node:coverage enable */
Coverage can also be disabled for a specified number of lines. After the specified number of lines, coverage will be automatically reenabled. If the number of lines is not explicitly provided, a single line is ignored.
/* node:coverage ignore next */
if (anAlwaysFalseCondition) { console.log('this is never executed'); }
/* node:coverage ignore next 3 */
if (anAlwaysFalseCondition) {
console.log('this is never executed');
}
The test runner's code coverage functionality has the following limitations, which will be addressed in a future Node.js release:
- Although coverage data is collected for child processes, this information is
not included in the coverage report. Because the command line test runner uses
child processes to execute test files, it cannot be used with
--experimental-test-coverage
. - Source maps are not supported.
- Excluding specific files or directories from the coverage report is not supported.
Mocking#
The node:test
module supports mocking during testing via a top-level mock
object. The following example creates a spy on a function that adds two numbers
together. The spy is then used to assert that the function was called as
expected.
import assert from 'node:assert';
import { mock, test } from 'node:test';
test('spies on a function', () => {
const sum = mock.fn((a, b) => {
return a + b;
});
assert.strictEqual(sum.mock.calls.length, 0);
assert.strictEqual(sum(3, 4), 7);
assert.strictEqual(sum.mock.calls.length, 1);
const call = sum.mock.calls[0];
assert.deepStrictEqual(call.arguments, [3, 4]);
assert.strictEqual(call.result, 7);
assert.strictEqual(call.error, undefined);
// Reset the globally tracked mocks.
mock.reset();
});
'use strict';
const assert = require('node:assert');
const { mock, test } = require('node:test');
test('spies on a function', () => {
const sum = mock.fn((a, b) => {
return a + b;
});
assert.strictEqual(sum.mock.calls.length, 0);
assert.strictEqual(sum(3, 4), 7);
assert.strictEqual(sum.mock.calls.length, 1);
const call = sum.mock.calls[0];
assert.deepStrictEqual(call.arguments, [3, 4]);
assert.strictEqual(call.result, 7);
assert.strictEqual(call.error, undefined);
// Reset the globally tracked mocks.
mock.reset();
});
The same mocking functionality is also exposed on the TestContext
object
of each test. The following example creates a spy on an object method using the
API exposed on the TestContext
. The benefit of mocking via the test context is
that the test runner will automatically restore all mocked functionality once
the test finishes.
test('spies on an object method', (t) => {
const number = {
value: 5,
add(a) {
return this.value + a;
},
};
t.mock.method(number, 'add');
assert.strictEqual(number.add.mock.calls.length, 0);
assert.strictEqual(number.add(3), 8);
assert.strictEqual(number.add.mock.calls.length, 1);
const call = number.add.mock.calls[0];
assert.deepStrictEqual(call.arguments, [3]);
assert.strictEqual(call.result, 8);
assert.strictEqual(call.target, undefined);
assert.strictEqual(call.this, number);
});
Test reporters#
The node:test
module supports passing --test-reporter
flags for the test runner to use a specific reporter.
The following built-reporters are supported:
-
tap
Thetap
reporter outputs the test results in the TAP format. -
spec
Thespec
reporter outputs the test results in a human-readable format. -
dot
Thedot
reporter outputs the test results in a compact format, where each passing test is represented by a.
, and each failing test is represented by aX
.
When stdout
is a TTY, the spec
reporter is used by default.
Otherwise, the tap
reporter is used by default.
The exact output of these reporters is subject to change between versions of Node.js, and should not be relied on programmatically. If programmatic access to the test runner's output is required, use the events emitted by the <TestsStream>.
The reporters are available via the node:test/reporters
module:
import { tap, spec, dot } from 'node:test/reporters';
const { tap, spec, dot } = require('node:test/reporters');
Custom reporters#
--test-reporter
can be used to specify a path to custom reporter.
A custom reporter is a module that exports a value
accepted by stream.compose.
Reporters should transform events emitted by a <TestsStream>
Example of a custom reporter using <stream.Transform>:
import { Transform } from 'node:stream';
const customReporter = new Transform({
writableObjectMode: true,
transform(event, encoding, callback) {
switch (event.type) {
case 'test:start':
callback(null, `test ${event.data.name} started`);
break;
case 'test:pass':
callback(null, `test ${event.data.name} passed`);
break;
case 'test:fail':
callback(null, `test ${event.data.name} failed`);
break;
case 'test:plan':
callback(null, 'test plan');
break;
case 'test:diagnostic':
callback(null, event.data.message);
break;
case 'test:coverage': {
const { totalLineCount } = event.data.summary.totals;
callback(null, `total line count: ${totalLineCount}\n`);
break;
}
}
},
});
export default customReporter;
const { Transform } = require('node:stream');
const customReporter = new Transform({
writableObjectMode: true,
transform(event, encoding, callback) {
switch (event.type) {
case 'test:start':
callback(null, `test ${event.data.name} started`);
break;
case 'test:pass':
callback(null, `test ${event.data.name} passed`);
break;
case 'test:fail':
callback(null, `test ${event.data.name} failed`);
break;
case 'test:plan':
callback(null, 'test plan');
break;
case 'test:diagnostic':
callback(null, event.data.message);
break;
case 'test:coverage': {
const { totalLineCount } = event.data.summary.totals;
callback(null, `total line count: ${totalLineCount}\n`);
break;
}
}
},
});
module.exports = customReporter;
Example of a custom reporter using a generator function:
export default async function * customReporter(source) {
for await (const event of source) {
switch (event.type) {
case 'test:start':
yield `test ${event.data.name} started\n`;
break;
case 'test:pass':
yield `test ${event.data.name} passed\n`;
break;
case 'test:fail':
yield `test ${event.data.name} failed\n`;
break;
case 'test:plan':
yield 'test plan';
break;
case 'test:diagnostic':
yield `${event.data.message}\n`;
break;
case 'test:coverage': {
const { totalLineCount } = event.data.summary.totals;
yield `total line count: ${totalLineCount}\n`;
break;
}
}
}
}
module.exports = async function * customReporter(source) {
for await (const event of source) {
switch (event.type) {
case 'test:start':
yield `test ${event.data.name} started\n`;
break;
case 'test:pass':
yield `test ${event.data.name} passed\n`;
break;
case 'test:fail':
yield `test ${event.data.name} failed\n`;
break;
case 'test:plan':
yield 'test plan\n';
break;
case 'test:diagnostic':
yield `${event.data.message}\n`;
break;
case 'test:coverage': {
const { totalLineCount } = event.data.summary.totals;
yield `total line count: ${totalLineCount}\n`;
break;
}
}
}
};
The value provided to --test-reporter
should be a string like one used in an
import()
in JavaScript code, or a value provided for --import
.
Multiple reporters#
The --test-reporter
flag can be specified multiple times to report test
results in several formats. In this situation
it is required to specify a destination for each reporter
using --test-reporter-destination
.
Destination can be stdout
, stderr
, or a file path.
Reporters and destinations are paired according
to the order they were specified.
In the following example, the spec
reporter will output to stdout
,
and the dot
reporter will output to file.txt
:
node --test-reporter=spec --test-reporter=dot --test-reporter-destination=stdout --test-reporter-destination=file.txt
When a single reporter is specified, the destination will default to stdout
,
unless a destination is explicitly provided.
run([options])
#
options
<Object> Configuration options for running tests. The following properties are supported:concurrency
<number> | <boolean> If a number is provided, then that many files would run in parallel. Iftrue
, it would runos.availableParallelism() - 1
test files in parallel. Iffalse
, it would only run one test file at a time. Default:false
.files
: <Array> An array containing the list of files to run. Default matching files from test runner execution model.setup
<Function> A function that accepts theTestsStream
instance and can be used to setup listeners before any tests are run. Default:undefined
.signal
<AbortSignal> Allows aborting an in-progress test execution.timeout
<number> A number of milliseconds the test execution will fail after. If unspecified, subtests inherit this value from their parent. Default:Infinity
.inspectPort
<number> | <Function> Sets inspector port of test child process. This can be a number, or a function that takes no arguments and returns a number. If a nullish value is provided, each process gets its own port, incremented from the primary'sprocess.debugPort
. Default:undefined
.
- Returns: <TestsStream>
import { tap } from 'node:test/reporters';
import process from 'node:process';
run({ files: [path.resolve('./tests/test.js')] })
.compose(tap)
.pipe(process.stdout);
const { tap } = require('node:test/reporters');
run({ files: [path.resolve('./tests/test.js')] })
.compose(tap)
.pipe(process.stdout);
test([name][, options][, fn])
#
name
<string> The name of the test, which is displayed when reporting test results. Default: Thename
property offn
, or'<anonymous>'
iffn
does not have a name.options
<Object> Configuration options for the test. The following properties are supported:concurrency
<number> | <boolean> If a number is provided, then that many tests would run in parallel. Iftrue
, it would runos.availableParallelism() - 1
tests in parallel. For subtests, it will beInfinity
tests in parallel. Iffalse
, it would only run one test at a time. If unspecified, subtests inherit this value from their parent. Default:false
.only
<boolean> If truthy, and the test context is configured to runonly
tests, then this test will be run. Otherwise, the test is skipped. Default:false
.signal
<AbortSignal> Allows aborting an in-progress test.skip
<boolean> | <string> If truthy, the test is skipped. If a string is provided, that string is displayed in the test results as the reason for skipping the test. Default:false
.todo
<boolean> | <string> If truthy, the test marked asTODO
. If a string is provided, that string is displayed in the test results as the reason why the test isTODO
. Default:false
.timeout
<number> A number of milliseconds the test will fail after. If unspecified, subtests inherit this value from their parent. Default:Infinity
.
fn
<Function> | <AsyncFunction> The function under test. The first argument to this function is aTestContext
object. If the test uses callbacks, the callback function is passed as the second argument. Default: A no-op function.- Returns: <Promise> Resolved with
undefined
once the test completes, or immediately if the test runs withindescribe()
.
The test()
function is the value imported from the test
module. Each
invocation of this function results in reporting the test to the <TestsStream>.
The TestContext
object passed to the fn
argument can be used to perform
actions related to the current test. Examples include skipping the test, adding
additional diagnostic information, or creating subtests.
test()
returns a Promise
that resolves once the test completes.
if test()
is called within a describe()
block, it resolve immediately.
The return value can usually be discarded for top level tests.
However, the return value from subtests should be used to prevent the parent
test from finishing first and cancelling the subtest
as shown in the following example.
test('top level test', async (t) => {
// The setTimeout() in the following subtest would cause it to outlive its
// parent test if 'await' is removed on the next line. Once the parent test
// completes, it will cancel any outstanding subtests.
await t.test('longer running subtest', async (t) => {
return new Promise((resolve, reject) => {
setTimeout(resolve, 1000);
});
});
});
The timeout
option can be used to fail the test if it takes longer than
timeout
milliseconds to complete. However, it is not a reliable mechanism for
canceling tests because a running test might block the application thread and
thus prevent the scheduled cancellation.
describe([name][, options][, fn])
#
name
<string> The name of the suite, which is displayed when reporting test results. Default: Thename
property offn
, or'<anonymous>'
iffn
does not have a name.options
<Object> Configuration options for the suite. supports the same options astest([name][, options][, fn])
.fn
<Function> | <AsyncFunction> The function under suite declaring all subtests and subsuites. The first argument to this function is aSuiteContext
object. Default: A no-op function.- Returns:
undefined
.
The describe()
function imported from the node:test
module. Each
invocation of this function results in the creation of a Subtest.
After invocation of top level describe
functions,
all top level tests and suites will execute.
describe.skip([name][, options][, fn])
#
Shorthand for skipping a suite, same as describe([name], { skip: true }[, fn])
.
describe.todo([name][, options][, fn])
#
Shorthand for marking a suite as TODO
, same as
describe([name], { todo: true }[, fn])
.
describe.only([name][, options][, fn])
#
Shorthand for marking a suite as only
, same as
describe([name], { only: true }[, fn])
.
it([name][, options][, fn])
#
Shorthand for test()
.
The it()
function is imported from the node:test
module.
it.skip([name][, options][, fn])
#
Shorthand for skipping a test,
same as it([name], { skip: true }[, fn])
.
it.todo([name][, options][, fn])
#
Shorthand for marking a test as TODO
,
same as it([name], { todo: true }[, fn])
.
it.only([name][, options][, fn])
#
Shorthand for marking a test as only
,
same as it([name], { only: true }[, fn])
.
before([fn][, options])
#
fn
<Function> | <AsyncFunction> The hook function. If the hook uses callbacks, the callback function is passed as the second argument. Default: A no-op function.options
<Object> Configuration options for the hook. The following properties are supported:signal
<AbortSignal> Allows aborting an in-progress hook.timeout
<number> A number of milliseconds the hook will fail after. If unspecified, subtests inherit this value from their parent. Default:Infinity
.
This function is used to create a hook running before running a suite.
describe('tests', async () => {
before(() => console.log('about to run some test'));
it('is a subtest', () => {
assert.ok('some relevant assertion here');
});
});
after([fn][, options])
#
fn
<Function> | <AsyncFunction> The hook function. If the hook uses callbacks, the callback function is passed as the second argument. Default: A no-op function.options
<Object> Configuration options for the hook. The following properties are supported:signal
<AbortSignal> Allows aborting an in-progress hook.timeout
<number> A number of milliseconds the hook will fail after. If unspecified, subtests inherit this value from their parent. Default:Infinity
.
This function is used to create a hook running after running a suite.
describe('tests', async () => {
after(() => console.log('finished running tests'));
it('is a subtest', () => {
assert.ok('some relevant assertion here');
});
});
beforeEach([fn][, options])
#
fn
<Function> | <AsyncFunction> The hook function. If the hook uses callbacks, the callback function is passed as the second argument. Default: A no-op function.options
<Object> Configuration options for the hook. The following properties are supported:signal
<AbortSignal> Allows aborting an in-progress hook.timeout
<number> A number of milliseconds the hook will fail after. If unspecified, subtests inherit this value from their parent. Default:Infinity
.
This function is used to create a hook running before each subtest of the current suite.
describe('tests', async () => {
beforeEach(() => console.log('about to run a test'));
it('is a subtest', () => {
assert.ok('some relevant assertion here');
});
});
afterEach([fn][, options])
#
fn
<Function> | <AsyncFunction> The hook function. If the hook uses callbacks, the callback function is passed as the second argument. Default: A no-op function.options
<Object> Configuration options for the hook. The following properties are supported:signal
<AbortSignal> Allows aborting an in-progress hook.timeout
<number> A number of milliseconds the hook will fail after. If unspecified, subtests inherit this value from their parent. Default:Infinity
.
This function is used to create a hook running after each subtest of the current test.
describe('tests', async () => {
afterEach(() => console.log('finished running a test'));
it('is a subtest', () => {
assert.ok('some relevant assertion here');
});
});
Class: MockFunctionContext
#
The MockFunctionContext
class is used to inspect or manipulate the behavior of
mocks created via the MockTracker
APIs.
ctx.calls
#
A getter that returns a copy of the internal array used to track calls to the mock. Each entry in the array is an object with the following properties.
arguments
<Array> An array of the arguments passed to the mock function.error
<any> If the mocked function threw then this property contains the thrown value. Default:undefined
.result
<any> The value returned by the mocked function.stack
<Error> AnError
object whose stack can be used to determine the callsite of the mocked function invocation.target
<Function> | <undefined> If the mocked function is a constructor, this field contains the class being constructed. Otherwise this will beundefined
.this
<any> The mocked function'sthis
value.
ctx.callCount()
#
- Returns: <integer> The number of times that this mock has been invoked.
This function returns the number of times that this mock has been invoked. This
function is more efficient than checking ctx.calls.length
because ctx.calls
is a getter that creates a copy of the internal call tracking array.
ctx.mockImplementation(implementation)
#
implementation
<Function> | <AsyncFunction> The function to be used as the mock's new implementation.
This function is used to change the behavior of an existing mock.
The following example creates a mock function using t.mock.fn()
, calls the
mock function, and then changes the mock implementation to a different function.
test('changes a mock behavior', (t) => {
let cnt = 0;
function addOne() {
cnt++;
return cnt;
}
function addTwo() {
cnt += 2;
return cnt;
}
const fn = t.mock.fn(addOne);
assert.strictEqual(fn(), 1);
fn.mock.mockImplementation(addTwo);
assert.strictEqual(fn(), 3);
assert.strictEqual(fn(), 5);
});
ctx.mockImplementationOnce(implementation[, onCall])
#
implementation
<Function> | <AsyncFunction> The function to be used as the mock's implementation for the invocation number specified byonCall
.onCall
<integer> The invocation number that will useimplementation
. If the specified invocation has already occurred then an exception is thrown. Default: The number of the next invocation.
This function is used to change the behavior of an existing mock for a single
invocation. Once invocation onCall
has occurred, the mock will revert to
whatever behavior it would have used had mockImplementationOnce()
not been
called.
The following example creates a mock function using t.mock.fn()
, calls the
mock function, changes the mock implementation to a different function for the
next invocation, and then resumes its previous behavior.
test('changes a mock behavior once', (t) => {
let cnt = 0;
function addOne() {
cnt++;
return cnt;
}
function addTwo() {
cnt += 2;
return cnt;
}
const fn = t.mock.fn(addOne);
assert.strictEqual(fn(), 1);
fn.mock.mockImplementationOnce(addTwo);
assert.strictEqual(fn(), 3);
assert.strictEqual(fn(), 4);
});
ctx.resetCalls()
#
Resets the call history of the mock function.
ctx.restore()
#
Resets the implementation of the mock function to its original behavior. The mock can still be used after calling this function.
Class: MockTracker
#
The MockTracker
class is used to manage mocking functionality. The test runner
module provides a top level mock
export which is a MockTracker
instance.
Each test also provides its own MockTracker
instance via the test context's
mock
property.
mock.fn([original[, implementation]][, options])
#
original
<Function> | <AsyncFunction> An optional function to create a mock on. Default: A no-op function.implementation
<Function> | <AsyncFunction> An optional function used as the mock implementation fororiginal
. This is useful for creating mocks that exhibit one behavior for a specified number of calls and then restore the behavior oforiginal
. Default: The function specified byoriginal
.options
<Object> Optional configuration options for the mock function. The following properties are supported:times
<integer> The number of times that the mock will use the behavior ofimplementation
. Once the mock function has been calledtimes
times, it will automatically restore the behavior oforiginal
. This value must be an integer greater than zero. Default:Infinity
.
- Returns: <Proxy> The mocked function. The mocked function contains a special
mock
property, which is an instance ofMockFunctionContext
, and can be used for inspecting and changing the behavior of the mocked function.
This function is used to create a mock function.
The following example creates a mock function that increments a counter by one
on each invocation. The times
option is used to modify the mock behavior such
that the first two invocations add two to the counter instead of one.
test('mocks a counting function', (t) => {
let cnt = 0;
function addOne() {
cnt++;
return cnt;
}
function addTwo() {
cnt += 2;
return cnt;
}
const fn = t.mock.fn(addOne, addTwo, { times: 2 });
assert.strictEqual(fn(), 2);
assert.strictEqual(fn(), 4);
assert.strictEqual(fn(), 5);
assert.strictEqual(fn(), 6);
});
mock.getter(object, methodName[, implementation][, options])
#
This function is syntax sugar for MockTracker.method
with options.getter
set to true
.
mock.method(object, methodName[, implementation][, options])
#
object
<Object> The object whose method is being mocked.methodName
<string> | <symbol> The identifier of the method onobject
to mock. Ifobject[methodName]
is not a function, an error is thrown.implementation
<Function> | <AsyncFunction> An optional function used as the mock implementation forobject[methodName]
. Default: The original method specified byobject[methodName]
.options
<Object> Optional configuration options for the mock method. The following properties are supported:getter
<boolean> Iftrue
,object[methodName]
is treated as a getter. This option cannot be used with thesetter
option. Default: false.setter
<boolean> Iftrue
,object[methodName]
is treated as a setter. This option cannot be used with thegetter
option. Default: false.times
<integer> The number of times that the mock will use the behavior ofimplementation
. Once the mocked method has been calledtimes
times, it will automatically restore the original behavior. This value must be an integer greater than zero. Default:Infinity
.
- Returns: <Proxy> The mocked method. The mocked method contains a special
mock
property, which is an instance ofMockFunctionContext
, and can be used for inspecting and changing the behavior of the mocked method.
This function is used to create a mock on an existing object method. The following example demonstrates how a mock is created on an existing object method.
test('spies on an object method', (t) => {
const number = {
value: 5,
subtract(a) {
return this.value - a;
},
};
t.mock.method(number, 'subtract');
assert.strictEqual(number.subtract.mock.calls.length, 0);
assert.strictEqual(number.subtract(3), 2);
assert.strictEqual(number.subtract.mock.calls.length, 1);
const call = number.subtract.mock.calls[0];
assert.deepStrictEqual(call.arguments, [3]);
assert.strictEqual(call.result, 2);
assert.strictEqual(call.error, undefined);
assert.strictEqual(call.target, undefined);
assert.strictEqual(call.this, number);
});
mock.reset()
#
This function restores the default behavior of all mocks that were previously
created by this MockTracker
and disassociates the mocks from the
MockTracker
instance. Once disassociated, the mocks can still be used, but the
MockTracker
instance can no longer be used to reset their behavior or
otherwise interact with them.
After each test completes, this function is called on the test context's
MockTracker
. If the global MockTracker
is used extensively, calling this
function manually is recommended.
mock.restoreAll()
#
This function restores the default behavior of all mocks that were previously
created by this MockTracker
. Unlike mock.reset()
, mock.restoreAll()
does
not disassociate the mocks from the MockTracker
instance.
mock.setter(object, methodName[, implementation][, options])
#
This function is syntax sugar for MockTracker.method
with options.setter
set to true
.
Class: TestsStream
#
- Extends <ReadableStream>
A successful call to run()
method will return a new <TestsStream>
object, streaming a series of events representing the execution of the tests.
TestsStream
will emit events, in the order of the tests definition
Event: 'test:coverage'
#
data
<Object>summary
<Object> An object containing the coverage report.files
<Array> An array of coverage reports for individual files. Each report is an object with the following schema:path
<string> The absolute path of the file.totalLineCount
<number> The total number of lines.totalBranchCount
<number> The total number of branches.totalFunctionCount
<number> The total number of functions.coveredLineCount
<number> The number of covered lines.coveredBranchCount
<number> The number of covered branches.coveredFunctionCount
<number> The number of covered functions.coveredLinePercent
<number> The percentage of lines covered.coveredBranchPercent
<number> The percentage of branches covered.coveredFunctionPercent
<number> The percentage of functions covered.uncoveredLineNumbers
<Array> An array of integers representing line numbers that are uncovered.
totals
<Object> An object containing a summary of coverage for all files.totalLineCount
<number> The total number of lines.totalBranchCount
<number> The total number of branches.totalFunctionCount
<number> The total number of functions.coveredLineCount
<number> The number of covered lines.coveredBranchCount
<number> The number of covered branches.coveredFunctionCount
<number> The number of covered functions.coveredLinePercent
<number> The percentage of lines covered.coveredBranchPercent
<number> The percentage of branches covered.coveredFunctionPercent
<number> The percentage of functions covered.
workingDirectory
<string> The working directory when code coverage began. This is useful for displaying relative path names in case the tests changed the working directory of the Node.js process.
nesting
<number> The nesting level of the test.
Emitted when code coverage is enabled and all tests have completed.
Event: 'test:diagnostic'
#
data
<Object>file
<string> | <undefined> The path of the test file, undefined if test is not ran through a file.message
<string> The diagnostic message.nesting
<number> The nesting level of the test.
Emitted when context.diagnostic
is called.
Event: 'test:fail'
#
data
<Object>details
<Object> Additional execution metadata.file
<string> | <undefined> The path of the test file, undefined if test is not ran through a file.name
<string> The test name.nesting
<number> The nesting level of the test.testNumber
<number> The ordinal number of the test.todo
<string> | <boolean> | <undefined> Present ifcontext.todo
is calledskip
<string> | <boolean> | <undefined> Present ifcontext.skip
is called
Emitted when a test fails.
Event: 'test:pass'
#
data
<Object>details
<Object> Additional execution metadata.duration
<number> The duration of the test in milliseconds.
file
<string> | <undefined> The path of the test file, undefined if test is not ran through a file.name
<string> The test name.nesting
<number> The nesting level of the test.testNumber
<number> The ordinal number of the test.todo
<string> | <boolean> | <undefined> Present ifcontext.todo
is calledskip
<string> | <boolean> | <undefined> Present ifcontext.skip
is called
Emitted when a test passes.
Event: 'test:plan'
#
data
<Object>file
<string> | <undefined> The path of the test file, undefined if test is not ran through a file.nesting
<number> The nesting level of the test.count
<number> The number of subtests that have ran.
Emitted when all subtests have completed for a given test.
Event: 'test:start'
#
data
<Object>file
<string> | <undefined> The path of the test file, undefined if test is not ran through a file.name
<string> The test name.nesting
<number> The nesting level of the test.
Emitted when a test starts.
Class: TestContext
#
An instance of TestContext
is passed to each test function in order to
interact with the test runner. However, the TestContext
constructor is not
exposed as part of the API.
context.beforeEach([fn][, options])
#
fn
<Function> | <AsyncFunction> The hook function. The first argument to this function is aTestContext
object. If the hook uses callbacks, the callback function is passed as the second argument. Default: A no-op function.options
<Object> Configuration options for the hook. The following properties are supported:signal
<AbortSignal> Allows aborting an in-progress hook.timeout
<number> A number of milliseconds the hook will fail after. If unspecified, subtests inherit this value from their parent. Default:Infinity
.
This function is used to create a hook running before each subtest of the current test.
test('top level test', async (t) => {
t.beforeEach((t) => t.diagnostic(`about to run ${t.name}`));
await t.test(
'This is a subtest',
(t) => {
assert.ok('some relevant assertion here');
},
);
});
context.after([fn][, options])
#
fn
<Function> | <AsyncFunction> The hook function. The first argument to this function is aTestContext
object. If the hook uses callbacks, the callback function is passed as the second argument. Default: A no-op function.options
<Object> Configuration options for the hook. The following properties are supported:signal
<AbortSignal> Allows aborting an in-progress hook.timeout
<number> A number of milliseconds the hook will fail after. If unspecified, subtests inherit this value from their parent. Default:Infinity
.
This function is used to create a hook that runs after the current test finishes.
test('top level test', async (t) => {
t.after((t) => t.diagnostic(`finished running ${t.name}`));
assert.ok('some relevant assertion here');
});
context.afterEach([fn][, options])
#
fn
<Function> | <AsyncFunction> The hook function. The first argument to this function is aTestContext
object. If the hook uses callbacks, the callback function is passed as the second argument. Default: A no-op function.options
<Object> Configuration options for the hook. The following properties are supported:signal
<AbortSignal> Allows aborting an in-progress hook.timeout
<number> A number of milliseconds the hook will fail after. If unspecified, subtests inherit this value from their parent. Default:Infinity
.
This function is used to create a hook running after each subtest of the current test.
test('top level test', async (t) => {
t.afterEach((t) => t.diagnostic(`finished running ${t.name}`));
await t.test(
'This is a subtest',
(t) => {
assert.ok('some relevant assertion here');
},
);
});
context.diagnostic(message)
#
message
<string> Message to be reported.
This function is used to write diagnostics to the output. Any diagnostic information is included at the end of the test's results. This function does not return a value.
test('top level test', (t) => {
t.diagnostic('A diagnostic message');
});
context.name
#
The name of the test.
context.runOnly(shouldRunOnlyTests)
#
shouldRunOnlyTests
<boolean> Whether or not to runonly
tests.
If shouldRunOnlyTests
is truthy, the test context will only run tests that
have the only
option set. Otherwise, all tests are run. If Node.js was not
started with the --test-only
command-line option, this function is a
no-op.
test('top level test', (t) => {
// The test context can be set to run subtests with the 'only' option.
t.runOnly(true);
return Promise.all([
t.test('this subtest is now skipped'),
t.test('this subtest is run', { only: true }),
]);
});
context.signal
#
- <AbortSignal> Can be used to abort test subtasks when the test has been aborted.
test('top level test', async (t) => {
await fetch('some/uri', { signal: t.signal });
});
context.skip([message])
#
message
<string> Optional skip message.
This function causes the test's output to indicate the test as skipped. If
message
is provided, it is included in the output. Calling skip()
does
not terminate execution of the test function. This function does not return a
value.
test('top level test', (t) => {
// Make sure to return here as well if the test contains additional logic.
t.skip('this is skipped');
});
context.todo([message])
#
message
<string> OptionalTODO
message.
This function adds a TODO
directive to the test's output. If message
is
provided, it is included in the output. Calling todo()
does not terminate
execution of the test function. This function does not return a value.
test('top level test', (t) => {
// This test is marked as `TODO`
t.todo('this is a todo');
});
context.test([name][, options][, fn])
#
name
<string> The name of the subtest, which is displayed when reporting test results. Default: Thename
property offn
, or'<anonymous>'
iffn
does not have a name.options
<Object> Configuration options for the subtest. The following properties are supported:concurrency
<number> | <boolean> | <null> If a number is provided, then that many tests would run in parallel. Iftrue
, it would run all subtests in parallel. Iffalse
, it would only run one test at a time. If unspecified, subtests inherit this value from their parent. Default:null
.only
<boolean> If truthy, and the test context is configured to runonly
tests, then this test will be run. Otherwise, the test is skipped. Default:false
.signal
<AbortSignal> Allows aborting an in-progress test.skip
<boolean> | <string> If truthy, the test is skipped. If a string is provided, that string is displayed in the test results as the reason for skipping the test. Default:false
.todo
<boolean> | <string> If truthy, the test marked asTODO
. If a string is provided, that string is displayed in the test results as the reason why the test isTODO
. Default:false
.timeout
<number> A number of milliseconds the test will fail after. If unspecified, subtests inherit this value from their parent. Default:Infinity
.
fn
<Function> | <AsyncFunction> The function under test. The first argument to this function is aTestContext
object. If the test uses callbacks, the callback function is passed as the second argument. Default: A no-op function.- Returns: <Promise> Resolved with
undefined
once the test completes.
This function is used to create subtests under the current test. This function
behaves in the same fashion as the top level test()
function.
test('top level test', async (t) => {
await t.test(
'This is a subtest',
{ only: false, skip: false, concurrency: 1, todo: false },
(t) => {
assert.ok('some relevant assertion here');
},
);
});
Class: SuiteContext
#
An instance of SuiteContext
is passed to each suite function in order to
interact with the test runner. However, the SuiteContext
constructor is not
exposed as part of the API.
context.name
#
The name of the suite.
context.signal
#
- <AbortSignal> Can be used to abort test subtasks when the test has been aborted.
Timers#
Source Code: lib/timers.js
The timer
module exposes a global API for scheduling functions to
be called at some future period of time. Because the timer functions are
globals, there is no need to call require('node:timers')
to use the API.
The timer functions within Node.js implement a similar API as the timers API provided by Web Browsers but use a different internal implementation that is built around the Node.js Event Loop.
Class: Immediate
#
This object is created internally and is returned from setImmediate()
. It
can be passed to clearImmediate()
in order to cancel the scheduled
actions.
By default, when an immediate is scheduled, the Node.js event loop will continue
running as long as the immediate is active. The Immediate
object returned by
setImmediate()
exports both immediate.ref()
and immediate.unref()
functions that can be used to control this default behavior.
immediate.hasRef()
#
- Returns: <boolean>
If true, the Immediate
object will keep the Node.js event loop active.
immediate.ref()
#
- Returns: <Immediate> a reference to
immediate
When called, requests that the Node.js event loop not exit so long as the
Immediate
is active. Calling immediate.ref()
multiple times will have no
effect.
By default, all Immediate
objects are "ref'ed", making it normally unnecessary
to call immediate.ref()
unless immediate.unref()
had been called previously.
immediate.unref()
#
- Returns: <Immediate> a reference to
immediate
When called, the active Immediate
object will not require the Node.js event
loop to remain active. If there is no other activity keeping the event loop
running, the process may exit before the Immediate
object's callback is
invoked. Calling immediate.unref()
multiple times will have no effect.
Class: Timeout
#
This object is created internally and is returned from setTimeout()
and
setInterval()
. It can be passed to either clearTimeout()
or
clearInterval()
in order to cancel the scheduled actions.
By default, when a timer is scheduled using either setTimeout()
or
setInterval()
, the Node.js event loop will continue running as long as the
timer is active. Each of the Timeout
objects returned by these functions
export both timeout.ref()
and timeout.unref()
functions that can be used to
control this default behavior.
timeout.close()
#
clearTimeout()
instead.- Returns: <Timeout> a reference to
timeout
Cancels the timeout.
timeout.hasRef()
#
- Returns: <boolean>
If true, the Timeout
object will keep the Node.js event loop active.
timeout.ref()
#
- Returns: <Timeout> a reference to
timeout
When called, requests that the Node.js event loop not exit so long as the
Timeout
is active. Calling timeout.ref()
multiple times will have no effect.
By default, all Timeout
objects are "ref'ed", making it normally unnecessary
to call timeout.ref()
unless timeout.unref()
had been called previously.
timeout.refresh()
#
- Returns: <Timeout> a reference to
timeout
Sets the timer's start time to the current time, and reschedules the timer to call its callback at the previously specified duration adjusted to the current time. This is useful for refreshing a timer without allocating a new JavaScript object.
Using this on a timer that has already called its callback will reactivate the timer.
timeout.unref()
#
- Returns: <Timeout> a reference to
timeout
When called, the active Timeout
object will not require the Node.js event loop
to remain active. If there is no other activity keeping the event loop running,
the process may exit before the Timeout
object's callback is invoked. Calling
timeout.unref()
multiple times will have no effect.
timeout[Symbol.toPrimitive]()
#
- Returns: <integer> a number that can be used to reference this
timeout
Coerce a Timeout
to a primitive. The primitive can be used to
clear the Timeout
. The primitive can only be used in the
same thread where the timeout was created. Therefore, to use it
across worker_threads
it must first be passed to the correct
thread. This allows enhanced compatibility with browser
setTimeout()
and setInterval()
implementations.
Scheduling timers#
A timer in Node.js is an internal construct that calls a given function after a certain period of time. When a timer's function is called varies depending on which method was used to create the timer and what other work the Node.js event loop is doing.
setImmediate(callback[, ...args])
#
callback
<Function> The function to call at the end of this turn of the Node.js Event Loop...args
<any> Optional arguments to pass when thecallback
is called.- Returns: <Immediate> for use with
clearImmediate()
Schedules the "immediate" execution of the callback
after I/O events'
callbacks.
When multiple calls to setImmediate()
are made, the callback
functions are
queued for execution in the order in which they are created. The entire callback
queue is processed every event loop iteration. If an immediate timer is queued
from inside an executing callback, that timer will not be triggered until the
next event loop iteration.
If callback
is not a function, a TypeError
will be thrown.
This method has a custom variant for promises that is available using
timersPromises.setImmediate()
.
setInterval(callback[, delay[, ...args]])
#
callback
<Function> The function to call when the timer elapses.delay
<number> The number of milliseconds to wait before calling thecallback
. Default:1
....args
<any> Optional arguments to pass when thecallback
is called.- Returns: <Timeout> for use with
clearInterval()
Schedules repeated execution of callback
every delay
milliseconds.
When delay
is larger than 2147483647
or less than 1
, the delay
will be
set to 1
. Non-integer delays are truncated to an integer.
If callback
is not a function, a TypeError
will be thrown.
This method has a custom variant for promises that is available using
timersPromises.setInterval()
.
setTimeout(callback[, delay[, ...args]])
#
callback
<Function> The function to call when the timer elapses.delay
<number> The number of milliseconds to wait before calling thecallback
. Default:1
....args
<any> Optional arguments to pass when thecallback
is called.- Returns: <Timeout> for use with
clearTimeout()
Schedules execution of a one-time callback
after delay
milliseconds.
The callback
will likely not be invoked in precisely delay
milliseconds.
Node.js makes no guarantees about the exact timing of when callbacks will fire,
nor of their ordering. The callback will be called as close as possible to the
time specified.
When delay
is larger than 2147483647
or less than 1
, the delay
will be set to 1
. Non-integer delays are truncated to an integer.
If callback
is not a function, a TypeError
will be thrown.
This method has a custom variant for promises that is available using
timersPromises.setTimeout()
.
Cancelling timers#
The setImmediate()
, setInterval()
, and setTimeout()
methods
each return objects that represent the scheduled timers. These can be used to
cancel the timer and prevent it from triggering.
For the promisified variants of setImmediate()
and setTimeout()
,
an AbortController
may be used to cancel the timer. When canceled, the
returned Promises will be rejected with an 'AbortError'
.
For setImmediate()
:
const { setImmediate: setImmediatePromise } = require('node:timers/promises');
const ac = new AbortController();
const signal = ac.signal;
setImmediatePromise('foobar', { signal })
.then(console.log)
.catch((err) => {
if (err.name === 'AbortError')
console.error('The immediate was aborted');
});
ac.abort();
For setTimeout()
:
const { setTimeout: setTimeoutPromise } = require('node:timers/promises');
const ac = new AbortController();
const signal = ac.signal;
setTimeoutPromise(1000, 'foobar', { signal })
.then(console.log)
.catch((err) => {
if (err.name === 'AbortError')
console.error('The timeout was aborted');
});
ac.abort();
clearImmediate(immediate)
#
immediate
<Immediate> AnImmediate
object as returned bysetImmediate()
.
Cancels an Immediate
object created by setImmediate()
.
clearInterval(timeout)
#
timeout
<Timeout> | <string> | <number> ATimeout
object as returned bysetInterval()
or the primitive of theTimeout
object as a string or a number.
Cancels a Timeout
object created by setInterval()
.
clearTimeout(timeout)
#
timeout
<Timeout> | <string> | <number> ATimeout
object as returned bysetTimeout()
or the primitive of theTimeout
object as a string or a number.
Cancels a Timeout
object created by setTimeout()
.
Timers Promises API#
The timers/promises
API provides an alternative set of timer functions
that return Promise
objects. The API is accessible via
require('node:timers/promises')
.
import {
setTimeout,
setImmediate,
setInterval,
} from 'timers/promises';
const {
setTimeout,
setImmediate,
setInterval,
} = require('node:timers/promises');
timersPromises.setTimeout([delay[, value[, options]]])
#
delay
<number> The number of milliseconds to wait before fulfilling the promise. Default:1
.value
<any> A value with which the promise is fulfilled.options
<Object>ref
<boolean> Set tofalse
to indicate that the scheduledTimeout
should not require the Node.js event loop to remain active. Default:true
.signal
<AbortSignal> An optionalAbortSignal
that can be used to cancel the scheduledTimeout
.
import {
setTimeout,
} from 'timers/promises';
const res = await setTimeout(100, 'result');
console.log(res); // Prints 'result'
const {
setTimeout,
} = require('node:timers/promises');
setTimeout(100, 'result').then((res) => {
console.log(res); // Prints 'result'
});
timersPromises.setImmediate([value[, options]])
#
value
<any> A value with which the promise is fulfilled.options
<Object>ref
<boolean> Set tofalse
to indicate that the scheduledImmediate
should not require the Node.js event loop to remain active. Default:true
.signal
<AbortSignal> An optionalAbortSignal
that can be used to cancel the scheduledImmediate
.
import {
setImmediate,
} from 'timers/promises';
const res = await setImmediate('result');
console.log(res); // Prints 'result'
const {
setImmediate,
} = require('node:timers/promises');
setImmediate('result').then((res) => {
console.log(res); // Prints 'result'
});
timersPromises.setInterval([delay[, value[, options]]])
#
Returns an async iterator that generates values in an interval of delay
ms.
If ref
is true
, you need to call next()
of async iterator explicitly
or implicitly to keep the event loop alive.
delay
<number> The number of milliseconds to wait between iterations. Default:1
.value
<any> A value with which the iterator returns.options
<Object>ref
<boolean> Set tofalse
to indicate that the scheduledTimeout
between iterations should not require the Node.js event loop to remain active. Default:true
.signal
<AbortSignal> An optionalAbortSignal
that can be used to cancel the scheduledTimeout
between operations.
import {
setInterval,
} from 'timers/promises';
const interval = 100;
for await (const startTime of setInterval(interval, Date.now())) {
const now = Date.now();
console.log(now);
if ((now - startTime) > 1000)
break;
}
console.log(Date.now());
const {
setInterval,
} = require('node:timers/promises');
const interval = 100;
(async function() {
for await (const startTime of setInterval(interval, Date.now())) {
const now = Date.now();
console.log(now);
if ((now - startTime) > 1000)
break;
}
console.log(Date.now());
})();
timersPromises.scheduler.wait(delay[, options])
#
delay
<number> The number of milliseconds to wait before resolving the promise.options
<Object>signal
<AbortSignal> An optionalAbortSignal
that can be used to cancel waiting.
- Returns: <Promise>
An experimental API defined by the Scheduling APIs draft specification being developed as a standard Web Platform API.
Calling timersPromises.scheduler.wait(delay, options)
is roughly equivalent
to calling timersPromises.setTimeout(delay, undefined, options)
except that
the ref
option is not supported.
import { scheduler } from 'node:timers/promises';
await scheduler.wait(1000); // Wait one second before continuing
timersPromises.scheduler.yield()
#
- Returns: <Promise>
An experimental API defined by the Scheduling APIs draft specification being developed as a standard Web Platform API.
Calling timersPromises.scheduler.yield()
is equivalent to calling
timersPromises.setImmediate()
with no arguments.
TLS (SSL)#
Source Code: lib/tls.js
The node:tls
module provides an implementation of the Transport Layer Security
(TLS) and Secure Socket Layer (SSL) protocols that is built on top of OpenSSL.
The module can be accessed using:
const tls = require('node:tls');
Determining if crypto support is unavailable#
It is possible for Node.js to be built without including support for the
node:crypto
module. In such cases, attempting to import
from tls
or
calling require('node:tls')
will result in an error being thrown.
When using CommonJS, the error thrown can be caught using try/catch:
let tls;
try {
tls = require('node:tls');
} catch (err) {
console.error('tls support is disabled!');
}
When using the lexical ESM import
keyword, the error can only be
caught if a handler for process.on('uncaughtException')
is registered
before any attempt to load the module is made (using, for instance,
a preload module).
When using ESM, if there is a chance that the code may be run on a build
of Node.js where crypto support is not enabled, consider using the
import()
function instead of the lexical import
keyword:
let tls;
try {
tls = await import('node:tls');
} catch (err) {
console.error('tls support is disabled!');
}
TLS/SSL concepts#
TLS/SSL is a set of protocols that rely on a public key infrastructure (PKI) to enable secure communication between a client and a server. For most common cases, each server must have a private key.
Private keys can be generated in multiple ways. The example below illustrates use of the OpenSSL command-line interface to generate a 2048-bit RSA private key:
openssl genrsa -out ryans-key.pem 2048
With TLS/SSL, all servers (and some clients) must have a certificate. Certificates are public keys that correspond to a private key, and that are digitally signed either by a Certificate Authority or by the owner of the private key (such certificates are referred to as "self-signed"). The first step to obtaining a certificate is to create a Certificate Signing Request (CSR) file.
The OpenSSL command-line interface can be used to generate a CSR for a private key:
openssl req -new -sha256 -key ryans-key.pem -out ryans-csr.pem
Once the CSR file is generated, it can either be sent to a Certificate Authority for signing or used to generate a self-signed certificate.
Creating a self-signed certificate using the OpenSSL command-line interface is illustrated in the example below:
openssl x509 -req -in ryans-csr.pem -signkey ryans-key.pem -out ryans-cert.pem
Once the certificate is generated, it can be used to generate a .pfx
or
.p12
file:
openssl pkcs12 -export -in ryans-cert.pem -inkey ryans-key.pem \
-certfile ca-cert.pem -out ryans.pfx
Where:
in
: is the signed certificateinkey
: is the associated private keycertfile
: is a concatenation of all Certificate Authority (CA) certs into a single file, e.g.cat ca1-cert.pem ca2-cert.pem > ca-cert.pem
Perfect forward secrecy#
The term forward secrecy or perfect forward secrecy describes a feature of key-agreement (i.e., key-exchange) methods. That is, the server and client keys are used to negotiate new temporary keys that are used specifically and only for the current communication session. Practically, this means that even if the server's private key is compromised, communication can only be decrypted by eavesdroppers if the attacker manages to obtain the key-pair specifically generated for the session.
Perfect forward secrecy is achieved by randomly generating a key pair for key-agreement on every TLS/SSL handshake (in contrast to using the same key for all sessions). Methods implementing this technique are called "ephemeral".
Currently two methods are commonly used to achieve perfect forward secrecy (note the character "E" appended to the traditional abbreviations):
- ECDHE: An ephemeral version of the Elliptic Curve Diffie-Hellman key-agreement protocol.
- DHE: An ephemeral version of the Diffie-Hellman key-agreement protocol.
Perfect forward secrecy using ECDHE is enabled by default. The ecdhCurve
option can be used when creating a TLS server to customize the list of supported
ECDH curves to use. See tls.createServer()
for more info.
DHE is disabled by default but can be enabled alongside ECDHE by setting the
dhparam
option to 'auto'
. Custom DHE parameters are also supported but
discouraged in favor of automatically selected, well-known parameters.
Perfect forward secrecy was optional up to TLSv1.2. As of TLSv1.3, (EC)DHE is always used (with the exception of PSK-only connections).
ALPN and SNI#
ALPN (Application-Layer Protocol Negotiation Extension) and SNI (Server Name Indication) are TLS handshake extensions:
- ALPN: Allows the use of one TLS server for multiple protocols (HTTP, HTTP/2)
- SNI: Allows the use of one TLS server for multiple hostnames with different certificates.
Pre-shared keys#
TLS-PSK support is available as an alternative to normal certificate-based authentication. It uses a pre-shared key instead of certificates to authenticate a TLS connection, providing mutual authentication. TLS-PSK and public key infrastructure are not mutually exclusive. Clients and servers can accommodate both, choosing either of them during the normal cipher negotiation step.
TLS-PSK is only a good choice where means exist to securely share a key with every connecting machine, so it does not replace the public key infrastructure (PKI) for the majority of TLS uses. The TLS-PSK implementation in OpenSSL has seen many security flaws in recent years, mostly because it is used only by a minority of applications. Please consider all alternative solutions before switching to PSK ciphers. Upon generating PSK it is of critical importance to use sufficient entropy as discussed in RFC 4086. Deriving a shared secret from a password or other low-entropy sources is not secure.
PSK ciphers are disabled by default, and using TLS-PSK thus requires explicitly
specifying a cipher suite with the ciphers
option. The list of available
ciphers can be retrieved via openssl ciphers -v 'PSK'
. All TLS 1.3
ciphers are eligible for PSK but currently only those that use SHA256 digest are
supported they can be retrieved via openssl ciphers -v -s -tls1_3 -psk
.
According to the RFC 4279, PSK identities up to 128 bytes in length and PSKs up to 64 bytes in length must be supported. As of OpenSSL 1.1.0 maximum identity size is 128 bytes, and maximum PSK length is 256 bytes.
The current implementation doesn't support asynchronous PSK callbacks due to the limitations of the underlying OpenSSL API.
Client-initiated renegotiation attack mitigation#
The TLS protocol allows clients to renegotiate certain aspects of the TLS session. Unfortunately, session renegotiation requires a disproportionate amount of server-side resources, making it a potential vector for denial-of-service attacks.
To mitigate the risk, renegotiation is limited to three times every ten minutes.
An 'error'
event is emitted on the tls.TLSSocket
instance when this
threshold is exceeded. The limits are configurable:
tls.CLIENT_RENEG_LIMIT
<number> Specifies the number of renegotiation requests. Default:3
.tls.CLIENT_RENEG_WINDOW
<number> Specifies the time renegotiation window in seconds. Default:600
(10 minutes).
The default renegotiation limits should not be modified without a full understanding of the implications and risks.
TLSv1.3 does not support renegotiation.
Session resumption#
Establishing a TLS session can be relatively slow. The process can be sped up by saving and later reusing the session state. There are several mechanisms to do so, discussed here from oldest to newest (and preferred).
Session identifiers#
Servers generate a unique ID for new connections and send it to the client. Clients and servers save the session state. When reconnecting, clients send the ID of their saved session state and if the server also has the state for that ID, it can agree to use it. Otherwise, the server will create a new session. See RFC 2246 for more information, page 23 and 30.
Resumption using session identifiers is supported by most web browsers when making HTTPS requests.
For Node.js, clients wait for the 'session'
event to get the session data,
and provide the data to the session
option of a subsequent tls.connect()
to reuse the session. Servers must
implement handlers for the 'newSession'
and 'resumeSession'
events
to save and restore the session data using the session ID as the lookup key to
reuse sessions. To reuse sessions across load balancers or cluster workers,
servers must use a shared session cache (such as Redis) in their session
handlers.
Session tickets#
The servers encrypt the entire session state and send it to the client as a "ticket". When reconnecting, the state is sent to the server in the initial connection. This mechanism avoids the need for a server-side session cache. If the server doesn't use the ticket, for any reason (failure to decrypt it, it's too old, etc.), it will create a new session and send a new ticket. See RFC 5077 for more information.
Resumption using session tickets is becoming commonly supported by many web browsers when making HTTPS requests.
For Node.js, clients use the same APIs for resumption with session identifiers
as for resumption with session tickets. For debugging, if
tls.TLSSocket.getTLSTicket()
returns a value, the session data contains a
ticket, otherwise it contains client-side session state.
With TLSv1.3, be aware that multiple tickets may be sent by the server,
resulting in multiple 'session'
events, see 'session'
for more
information.
Single process servers need no specific implementation to use session tickets. To use session tickets across server restarts or load balancers, servers must all have the same ticket keys. There are three 16-byte keys internally, but the tls API exposes them as a single 48-byte buffer for convenience.
It's possible to get the ticket keys by calling server.getTicketKeys()
on
one server instance and then distribute them, but it is more reasonable to
securely generate 48 bytes of secure random data and set them with the
ticketKeys
option of tls.createServer()
. The keys should be regularly
regenerated and server's keys can be reset with
server.setTicketKeys()
.
Session ticket keys are cryptographic keys, and they must be stored securely. With TLS 1.2 and below, if they are compromised all sessions that used tickets encrypted with them can be decrypted. They should not be stored on disk, and they should be regenerated regularly.
If clients advertise support for tickets, the server will send them. The
server can disable tickets by supplying
require('node:constants').SSL_OP_NO_TICKET
in secureOptions
.
Both session identifiers and session tickets timeout, causing the server to
create new sessions. The timeout can be configured with the sessionTimeout
option of tls.createServer()
.
For all the mechanisms, when resumption fails, servers will create new sessions.
Since failing to resume the session does not cause TLS/HTTPS connection
failures, it is easy to not notice unnecessarily poor TLS performance. The
OpenSSL CLI can be used to verify that servers are resuming sessions. Use the
-reconnect
option to openssl s_client
, for example:
$ openssl s_client -connect localhost:443 -reconnect
Read through the debug output. The first connection should say "New", for example:
New, TLSv1.2, Cipher is ECDHE-RSA-AES128-GCM-SHA256
Subsequent connections should say "Reused", for example:
Reused, TLSv1.2, Cipher is ECDHE-RSA-AES128-GCM-SHA256
Modifying the default TLS cipher suite#
Node.js is built with a default suite of enabled and disabled TLS ciphers. This default cipher list can be configured when building Node.js to allow distributions to provide their own default list.
The following command can be used to show the default cipher suite:
node -p crypto.constants.defaultCoreCipherList | tr ':' '\n'
TLS_AES_256_GCM_SHA384
TLS_CHACHA20_POLY1305_SHA256
TLS_AES_128_GCM_SHA256
ECDHE-RSA-AES128-GCM-SHA256
ECDHE-ECDSA-AES128-GCM-SHA256
ECDHE-RSA-AES256-GCM-SHA384
ECDHE-ECDSA-AES256-GCM-SHA384
DHE-RSA-AES128-GCM-SHA256
ECDHE-RSA-AES128-SHA256
DHE-RSA-AES128-SHA256
ECDHE-RSA-AES256-SHA384
DHE-RSA-AES256-SHA384
ECDHE-RSA-AES256-SHA256
DHE-RSA-AES256-SHA256
HIGH
!aNULL
!eNULL
!EXPORT
!DES
!RC4
!MD5
!PSK
!SRP
!CAMELLIA
This default can be replaced entirely using the --tls-cipher-list
command-line switch (directly, or via the NODE_OPTIONS
environment
variable). For instance, the following makes ECDHE-RSA-AES128-GCM-SHA256:!RC4
the default TLS cipher suite:
node --tls-cipher-list='ECDHE-RSA-AES128-GCM-SHA256:!RC4' server.js
export NODE_OPTIONS=--tls-cipher-list='ECDHE-RSA-AES128-GCM-SHA256:!RC4'
node server.js
To verify, use the following command to show the set cipher list, note the
difference between defaultCoreCipherList
and defaultCipherList
:
node --tls-cipher-list='ECDHE-RSA-AES128-GCM-SHA256:!RC4' -p crypto.constants.defaultCipherList | tr ':' '\n'
ECDHE-RSA-AES128-GCM-SHA256
!RC4
i.e. the defaultCoreCipherList
list is set at compilation time and the
defaultCipherList
is set at runtime.
To modify the default cipher suites from within the runtime, modify the
tls.DEFAULT_CIPHERS
variable, this must be performed before listening on any
sockets, it will not affect sockets already opened. For example:
// Remove Obsolete CBC Ciphers and RSA Key Exchange based Ciphers as they don't provide Forward Secrecy
tls.DEFAULT_CIPHERS +=
':!ECDHE-RSA-AES128-SHA:!ECDHE-RSA-AES128-SHA256:!ECDHE-RSA-AES256-SHA:!ECDHE-RSA-AES256-SHA384' +
':!ECDHE-ECDSA-AES128-SHA:!ECDHE-ECDSA-AES128-SHA256:!ECDHE-ECDSA-AES256-SHA:!ECDHE-ECDSA-AES256-SHA384' +
':!kRSA';
The default can also be replaced on a per client or server basis using the
ciphers
option from tls.createSecureContext()
, which is also available
in tls.createServer()
, tls.connect()
, and when creating new
tls.TLSSocket
s.
The ciphers list can contain a mixture of TLSv1.3 cipher suite names, the ones
that start with 'TLS_'
, and specifications for TLSv1.2 and below cipher
suites. The TLSv1.2 ciphers support a legacy specification format, consult
the OpenSSL cipher list format documentation for details, but those
specifications do not apply to TLSv1.3 ciphers. The TLSv1.3 suites can only
be enabled by including their full name in the cipher list. They cannot, for
example, be enabled or disabled by using the legacy TLSv1.2 'EECDH'
or
'!EECDH'
specification.
Despite the relative order of TLSv1.3 and TLSv1.2 cipher suites, the TLSv1.3 protocol is significantly more secure than TLSv1.2, and will always be chosen over TLSv1.2 if the handshake indicates it is supported, and if any TLSv1.3 cipher suites are enabled.
The default cipher suite included within Node.js has been carefully
selected to reflect current security best practices and risk mitigation.
Changing the default cipher suite can have a significant impact on the security
of an application. The --tls-cipher-list
switch and ciphers
option should by
used only if absolutely necessary.
The default cipher suite prefers GCM ciphers for Chrome's 'modern cryptography' setting and also prefers ECDHE and DHE ciphers for perfect forward secrecy, while offering some backward compatibility.
Old clients that rely on insecure and deprecated RC4 or DES-based ciphers (like Internet Explorer 6) cannot complete the handshaking process with the default configuration. If these clients must be supported, the TLS recommendations may offer a compatible cipher suite. For more details on the format, see the OpenSSL cipher list format documentation.
There are only five TLSv1.3 cipher suites:
'TLS_AES_256_GCM_SHA384'
'TLS_CHACHA20_POLY1305_SHA256'
'TLS_AES_128_GCM_SHA256'
'TLS_AES_128_CCM_SHA256'
'TLS_AES_128_CCM_8_SHA256'
The first three are enabled by default. The two CCM
-based suites are supported
by TLSv1.3 because they may be more performant on constrained systems, but they
are not enabled by default since they offer less security.
X509 certificate error codes#
Multiple functions can fail due to certificate errors that are reported by
OpenSSL. In such a case, the function provides an <Error> via its callback that
has the property code
which can take one of the following values:
'UNABLE_TO_GET_ISSUER_CERT'
: Unable to get issuer certificate.'UNABLE_TO_GET_CRL'
: Unable to get certificate CRL.'UNABLE_TO_DECRYPT_CERT_SIGNATURE'
: Unable to decrypt certificate's signature.'UNABLE_TO_DECRYPT_CRL_SIGNATURE'
: Unable to decrypt CRL's signature.'UNABLE_TO_DECODE_ISSUER_PUBLIC_KEY'
: Unable to decode issuer public key.'CERT_SIGNATURE_FAILURE'
: Certificate signature failure.'CRL_SIGNATURE_FAILURE'
: CRL signature failure.'CERT_NOT_YET_VALID'
: Certificate is not yet valid.'CERT_HAS_EXPIRED'
: Certificate has expired.'CRL_NOT_YET_VALID'
: CRL is not yet valid.'CRL_HAS_EXPIRED'
: CRL has expired.'ERROR_IN_CERT_NOT_BEFORE_FIELD'
: Format error in certificate's notBefore field.'ERROR_IN_CERT_NOT_AFTER_FIELD'
: Format error in certificate's notAfter field.'ERROR_IN_CRL_LAST_UPDATE_FIELD'
: Format error in CRL's lastUpdate field.'ERROR_IN_CRL_NEXT_UPDATE_FIELD'
: Format error in CRL's nextUpdate field.'OUT_OF_MEM'
: Out of memory.'DEPTH_ZERO_SELF_SIGNED_CERT'
: Self signed certificate.'SELF_SIGNED_CERT_IN_CHAIN'
: Self signed certificate in certificate chain.'UNABLE_TO_GET_ISSUER_CERT_LOCALLY'
: Unable to get local issuer certificate.'UNABLE_TO_VERIFY_LEAF_SIGNATURE'
: Unable to verify the first certificate.'CERT_CHAIN_TOO_LONG'
: Certificate chain too long.'CERT_REVOKED'
: Certificate revoked.'INVALID_CA'
: Invalid CA certificate.'PATH_LENGTH_EXCEEDED'
: Path length constraint exceeded.'INVALID_PURPOSE'
: Unsupported certificate purpose.'CERT_UNTRUSTED'
: Certificate not trusted.'CERT_REJECTED'
: Certificate rejected.'HOSTNAME_MISMATCH'
: Hostname mismatch.
Class: tls.CryptoStream
#
tls.TLSSocket
instead.The tls.CryptoStream
class represents a stream of encrypted data. This class
is deprecated and should no longer be used.
cryptoStream.bytesWritten
#
The cryptoStream.bytesWritten
property returns the total number of bytes
written to the underlying socket including the bytes required for the
implementation of the TLS protocol.
Class: tls.SecurePair
#
tls.TLSSocket
instead.Returned by tls.createSecurePair()
.
Event: 'secure'
#
The 'secure'
event is emitted by the SecurePair
object once a secure
connection has been established.
As with checking for the server
'secureConnection'
event, pair.cleartext.authorized
should be inspected to confirm whether the
certificate used is properly authorized.
Class: tls.Server
#
- Extends: <net.Server>
Accepts encrypted connections using TLS or SSL.
Event: 'connection'
#
socket
<stream.Duplex>
This event is emitted when a new TCP stream is established, before the TLS
handshake begins. socket
is typically an object of type net.Socket
but
will not receive events unlike the socket created from the net.Server
'connection'
event. Usually users will not want to access this event.
This event can also be explicitly emitted by users to inject connections
into the TLS server. In that case, any Duplex
stream can be passed.
Event: 'keylog'
#
line
<Buffer> Line of ASCII text, in NSSSSLKEYLOGFILE
format.tlsSocket
<tls.TLSSocket> Thetls.TLSSocket
instance on which it was generated.
The keylog
event is emitted when key material is generated or received by
a connection to this server (typically before handshake has completed, but not
necessarily). This keying material can be stored for debugging, as it allows
captured TLS traffic to be decrypted. It may be emitted multiple times for
each socket.
A typical use case is to append received lines to a common text file, which is later used by software (such as Wireshark) to decrypt the traffic:
const logFile = fs.createWriteStream('/tmp/ssl-keys.log', { flags: 'a' });
// ...
server.on('keylog', (line, tlsSocket) => {
if (tlsSocket.remoteAddress !== '...')
return; // Only log keys for a particular IP
logFile.write(line);
});
Event: 'newSession'
#
The 'newSession'
event is emitted upon creation of a new TLS session. This may
be used to store sessions in external storage. The data should be provided to
the 'resumeSession'
callback.
The listener callback is passed three arguments when called:
sessionId
<Buffer> The TLS session identifiersessionData
<Buffer> The TLS session datacallback
<Function> A callback function taking no arguments that must be invoked in order for data to be sent or received over the secure connection.
Listening for this event will have an effect only on connections established after the addition of the event listener.
Event: 'OCSPRequest'
#
The 'OCSPRequest'
event is emitted when the client sends a certificate status
request. The listener callback is passed three arguments when called:
certificate
<Buffer> The server certificateissuer
<Buffer> The issuer's certificatecallback
<Function> A callback function that must be invoked to provide the results of the OCSP request.
The server's current certificate can be parsed to obtain the OCSP URL
and certificate ID; after obtaining an OCSP response, callback(null, resp)
is
then invoked, where resp
is a Buffer
instance containing the OCSP response.
Both certificate
and issuer
are Buffer
DER-representations of the
primary and issuer's certificates. These can be used to obtain the OCSP
certificate ID and OCSP endpoint URL.
Alternatively, callback(null, null)
may be called, indicating that there was
no OCSP response.
Calling callback(err)
will result in a socket.destroy(err)
call.
The typical flow of an OCSP request is as follows:
- Client connects to the server and sends an
'OCSPRequest'
(via the status info extension in ClientHello). - Server receives the request and emits the
'OCSPRequest'
event, calling the listener if registered. - Server extracts the OCSP URL from either the
certificate
orissuer
and performs an OCSP request to the CA. - Server receives
'OCSPResponse'
from the CA and sends it back to the client via thecallback
argument - Client validates the response and either destroys the socket or performs a handshake.
The issuer
can be null
if the certificate is either self-signed or the
issuer is not in the root certificates list. (An issuer may be provided
via the ca
option when establishing the TLS connection.)
Listening for this event will have an effect only on connections established after the addition of the event listener.
An npm module like asn1.js may be used to parse the certificates.
Event: 'resumeSession'
#
The 'resumeSession'
event is emitted when the client requests to resume a
previous TLS session. The listener callback is passed two arguments when
called:
sessionId
<Buffer> The TLS session identifiercallback
<Function> A callback function to be called when the prior session has been recovered:callback([err[, sessionData]])
The event listener should perform a lookup in external storage for the
sessionData
saved by the 'newSession'
event handler using the given
sessionId
. If found, call callback(null, sessionData)
to resume the session.
If not found, the session cannot be resumed. callback()
must be called
without sessionData
so that the handshake can continue and a new session can
be created. It is possible to call callback(err)
to terminate the incoming
connection and destroy the socket.
Listening for this event will have an effect only on connections established after the addition of the event listener.
The following illustrates resuming a TLS session:
const tlsSessionStore = {};
server.on('newSession', (id, data, cb) => {
tlsSessionStore[id.toString('hex')] = data;
cb();
});
server.on('resumeSession', (id, cb) => {
cb(null, tlsSessionStore[id.toString('hex')] || null);
});
Event: 'secureConnection'
#
The 'secureConnection'
event is emitted after the handshaking process for a
new connection has successfully completed. The listener callback is passed a
single argument when called:
tlsSocket
<tls.TLSSocket> The established TLS socket.
The tlsSocket.authorized
property is a boolean
indicating whether the
client has been verified by one of the supplied Certificate Authorities for the
server. If tlsSocket.authorized
is false
, then socket.authorizationError
is set to describe how authorization failed. Depending on the settings
of the TLS server, unauthorized connections may still be accepted.
The tlsSocket.alpnProtocol
property is a string that contains the selected
ALPN protocol. When ALPN has no selected protocol because the client or the
server did not send an ALPN extension, tlsSocket.alpnProtocol
equals false
.
The tlsSocket.servername
property is a string containing the server name
requested via SNI.
Event: 'tlsClientError'
#
The 'tlsClientError'
event is emitted when an error occurs before a secure
connection is established. The listener callback is passed two arguments when
called:
exception
<Error> TheError
object describing the errortlsSocket
<tls.TLSSocket> Thetls.TLSSocket
instance from which the error originated.
server.addContext(hostname, context)
#
hostname
<string> A SNI host name or wildcard (e.g.'*'
)context
<Object> An object containing any of the possible properties from thetls.createSecureContext()
options
arguments (e.g.key
,cert
,ca
, etc).
The server.addContext()
method adds a secure context that will be used if
the client request's SNI name matches the supplied hostname
(or wildcard).
When there are multiple matching contexts, the most recently added one is used.
server.address()
#
- Returns: <Object>
Returns the bound address, the address family name, and port of the
server as reported by the operating system. See net.Server.address()
for
more information.
server.close([callback])
#
callback
<Function> A listener callback that will be registered to listen for the server instance's'close'
event.- Returns: <tls.Server>
The server.close()
method stops the server from accepting new connections.
This function operates asynchronously. The 'close'
event will be emitted
when the server has no more open connections.
server.getTicketKeys()
#
- Returns: <Buffer> A 48-byte buffer containing the session ticket keys.
Returns the session ticket keys.
See Session Resumption for more information.
server.listen()
#
Starts the server listening for encrypted connections.
This method is identical to server.listen()
from net.Server
.
server.setSecureContext(options)
#
options
<Object> An object containing any of the possible properties from thetls.createSecureContext()
options
arguments (e.g.key
,cert
,ca
, etc).
The server.setSecureContext()
method replaces the secure context of an
existing server. Existing connections to the server are not interrupted.
server.setTicketKeys(keys)
#
keys
<Buffer> | <TypedArray> | <DataView> A 48-byte buffer containing the session ticket keys.
Sets the session ticket keys.
Changes to the ticket keys are effective only for future server connections. Existing or currently pending server connections will use the previous keys.
See Session Resumption for more information.
Class: tls.TLSSocket
#
- Extends: <net.Socket>
Performs transparent encryption of written data and all required TLS negotiation.
Instances of tls.TLSSocket
implement the duplex Stream interface.
Methods that return TLS connection metadata (e.g.
tls.TLSSocket.getPeerCertificate()
) will only return data while the
connection is open.
new tls.TLSSocket(socket[, options])
#
socket
<net.Socket> | <stream.Duplex> On the server side, anyDuplex
stream. On the client side, any instance ofnet.Socket
(for genericDuplex
stream support on the client side,tls.connect()
must be used).options
<Object>enableTrace
: Seetls.createServer()
isServer
: The SSL/TLS protocol is asymmetrical, TLSSockets must know if they are to behave as a server or a client. Iftrue
the TLS socket will be instantiated as a server. Default:false
.server
<net.Server> Anet.Server
instance.requestCert
: Whether to authenticate the remote peer by requesting a certificate. Clients always request a server certificate. Servers (isServer
is true) may setrequestCert
to true to request a client certificate.rejectUnauthorized
: Seetls.createServer()
ALPNProtocols
: Seetls.createServer()
SNICallback
: Seetls.createServer()
session
<Buffer> ABuffer
instance containing a TLS session.requestOCSP
<boolean> Iftrue
, specifies that the OCSP status request extension will be added to the client hello and an'OCSPResponse'
event will be emitted on the socket before establishing a secure communicationsecureContext
: TLS context object created withtls.createSecureContext()
. If asecureContext
is not provided, one will be created by passing the entireoptions
object totls.createSecureContext()
.- ...:
tls.createSecureContext()
options that are used if thesecureContext
option is missing. Otherwise, they are ignored.
Construct a new tls.TLSSocket
object from an existing TCP socket.
Event: 'keylog'
#
line
<Buffer> Line of ASCII text, in NSSSSLKEYLOGFILE
format.
The keylog
event is emitted on a tls.TLSSocket
when key material
is generated or received by the socket. This keying material can be stored
for debugging, as it allows captured TLS traffic to be decrypted. It may
be emitted multiple times, before or after the handshake completes.
A typical use case is to append received lines to a common text file, which is later used by software (such as Wireshark) to decrypt the traffic:
const logFile = fs.createWriteStream('/tmp/ssl-keys.log', { flags: 'a' });
// ...
tlsSocket.on('keylog', (line) => logFile.write(line));
Event: 'OCSPResponse'
#
The 'OCSPResponse'
event is emitted if the requestOCSP
option was set
when the tls.TLSSocket
was created and an OCSP response has been received.
The listener callback is passed a single argument when called:
response
<Buffer> The server's OCSP response
Typically, the response
is a digitally signed object from the server's CA that
contains information about server's certificate revocation status.
Event: 'secureConnect'
#
The 'secureConnect'
event is emitted after the handshaking process for a new
connection has successfully completed. The listener callback will be called
regardless of whether or not the server's certificate has been authorized. It
is the client's responsibility to check the tlsSocket.authorized
property to
determine if the server certificate was signed by one of the specified CAs. If
tlsSocket.authorized === false
, then the error can be found by examining the
tlsSocket.authorizationError
property. If ALPN was used, the
tlsSocket.alpnProtocol
property can be checked to determine the negotiated
protocol.
The 'secureConnect'
event is not emitted when a <tls.TLSSocket> is created
using the new tls.TLSSocket()
constructor.
Event: 'session'
#
session
<Buffer>
The 'session'
event is emitted on a client tls.TLSSocket
when a new session
or TLS ticket is available. This may or may not be before the handshake is
complete, depending on the TLS protocol version that was negotiated. The event
is not emitted on the server, or if a new session was not created, for example,
when the connection was resumed. For some TLS protocol versions the event may be
emitted multiple times, in which case all the sessions can be used for
resumption.
On the client, the session
can be provided to the session
option of
tls.connect()
to resume the connection.
See Session Resumption for more information.
For TLSv1.2 and below, tls.TLSSocket.getSession()
can be called once
the handshake is complete. For TLSv1.3, only ticket-based resumption is allowed
by the protocol, multiple tickets are sent, and the tickets aren't sent until
after the handshake completes. So it is necessary to wait for the
'session'
event to get a resumable session. Applications
should use the 'session'
event instead of getSession()
to ensure
they will work for all TLS versions. Applications that only expect to
get or use one session should listen for this event only once:
tlsSocket.once('session', (session) => {
// The session can be used immediately or later.
tls.connect({
session: session,
// Other connect options...
});
});
tlsSocket.address()
#
- Returns: <Object>
Returns the bound address
, the address family
name, and port
of the
underlying socket as reported by the operating system:
{ port: 12346, family: 'IPv4', address: '127.0.0.1' }
.
tlsSocket.authorizationError
#
Returns the reason why the peer's certificate was not been verified. This
property is set only when tlsSocket.authorized === false
.
tlsSocket.authorized
#
This property is true
if the peer certificate was signed by one of the CAs
specified when creating the tls.TLSSocket
instance, otherwise false
.
tlsSocket.disableRenegotiation()
#
Disables TLS renegotiation for this TLSSocket
instance. Once called, attempts
to renegotiate will trigger an 'error'
event on the TLSSocket
.
tlsSocket.enableTrace()
#
When enabled, TLS packet trace information is written to stderr
. This can be
used to debug TLS connection problems.
The format of the output is identical to the output of
openssl s_client -trace
or openssl s_server -trace
. While it is produced by
OpenSSL's SSL_trace()
function, the format is undocumented, can change
without notice, and should not be relied on.
tlsSocket.encrypted
#
Always returns true
. This may be used to distinguish TLS sockets from regular
net.Socket
instances.
tlsSocket.exportKeyingMaterial(length, label[, context])
#
-
length
<number> number of bytes to retrieve from keying material -
label
<string> an application specific label, typically this will be a value from the IANA Exporter Label Registry. -
context
<Buffer> Optionally provide a context. -
Returns: <Buffer> requested bytes of the keying material
Keying material is used for validations to prevent different kind of attacks in network protocols, for example in the specifications of IEEE 802.1X.
Example
const keyingMaterial = tlsSocket.exportKeyingMaterial(
128,
'client finished');
/*
Example return value of keyingMaterial:
<Buffer 76 26 af 99 c5 56 8e 42 09 91 ef 9f 93 cb ad 6c 7b 65 f8 53 f1 d8 d9
12 5a 33 b8 b5 25 df 7b 37 9f e0 e2 4f b8 67 83 a3 2f cd 5d 41 42 4c 91
74 ef 2c ... 78 more bytes>
*/
See the OpenSSL SSL_export_keying_material
documentation for more
information.
tlsSocket.getCertificate()
#
- Returns: <Object>
Returns an object representing the local certificate. The returned object has some properties corresponding to the fields of the certificate.
See tls.TLSSocket.getPeerCertificate()
for an example of the certificate
structure.
If there is no local certificate, an empty object will be returned. If the
socket has been destroyed, null
will be returned.
tlsSocket.getCipher()
#
- Returns: <Object>
name
<string> OpenSSL name for the cipher suite.standardName
<string> IETF name for the cipher suite.version
<string> The minimum TLS protocol version supported by this cipher suite. For the actual negotiated protocol, seetls.TLSSocket.getProtocol()
.
Returns an object containing information on the negotiated cipher suite.
For example, a TLSv1.2 protocol with AES256-SHA cipher:
{
"name": "AES256-SHA",
"standardName": "TLS_RSA_WITH_AES_256_CBC_SHA",
"version": "SSLv3"
}
See SSL_CIPHER_get_name for more information.
tlsSocket.getEphemeralKeyInfo()
#
- Returns: <Object>
Returns an object representing the type, name, and size of parameter of
an ephemeral key exchange in perfect forward secrecy on a client
connection. It returns an empty object when the key exchange is not
ephemeral. As this is only supported on a client socket; null
is returned
if called on a server socket. The supported types are 'DH'
and 'ECDH'
. The
name
property is available only when type is 'ECDH'
.
For example: { type: 'ECDH', name: 'prime256v1', size: 256 }
.
tlsSocket.getFinished()
#
- Returns: <Buffer> | <undefined> The latest
Finished
message that has been sent to the socket as part of a SSL/TLS handshake, orundefined
if noFinished
message has been sent yet.
As the Finished
messages are message digests of the complete handshake
(with a total of 192 bits for TLS 1.0 and more for SSL 3.0), they can
be used for external authentication procedures when the authentication
provided by SSL/TLS is not desired or is not enough.
Corresponds to the SSL_get_finished
routine in OpenSSL and may be used
to implement the tls-unique
channel binding from RFC 5929.
tlsSocket.getPeerCertificate([detailed])
#
detailed
<boolean> Include the full certificate chain iftrue
, otherwise include just the peer's certificate.- Returns: <Object> A certificate object.
Returns an object representing the peer's certificate. If the peer does not
provide a certificate, an empty object will be returned. If the socket has been
destroyed, null
will be returned.
If the full certificate chain was requested, each certificate will include an
issuerCertificate
property containing an object representing its issuer's
certificate.
Certificate object#
A certificate object has properties corresponding to the fields of the certificate.
ca
<boolean>true
if a Certificate Authority (CA),false
otherwise.raw
<Buffer> The DER encoded X.509 certificate data.subject
<Object> The certificate subject, described in terms of Country (C
), StateOrProvince (ST
), Locality (L
), Organization (O
), OrganizationalUnit (OU
), and CommonName (CN
). The CommonName is typically a DNS name with TLS certificates. Example:{C: 'UK', ST: 'BC', L: 'Metro', O: 'Node Fans', OU: 'Docs', CN: 'example.com'}
.issuer
<Object> The certificate issuer, described in the same terms as thesubject
.valid_from
<string> The date-time the certificate is valid from.valid_to
<string> The date-time the certificate is valid to.serialNumber
<string> The certificate serial number, as a hex string. Example:'B9B0D332A1AA5635'
.fingerprint
<string> The SHA-1 digest of the DER encoded certificate. It is returned as a:
separated hexadecimal string. Example:'2A:7A:C2:DD:...'
.fingerprint256
<string> The SHA-256 digest of the DER encoded certificate. It is returned as a:
separated hexadecimal string. Example:'2A:7A:C2:DD:...'
.fingerprint512
<string> The SHA-512 digest of the DER encoded certificate. It is returned as a:
separated hexadecimal string. Example:'2A:7A:C2:DD:...'
.ext_key_usage
<Array> (Optional) The extended key usage, a set of OIDs.subjectaltname
<string> (Optional) A string containing concatenated names for the subject, an alternative to thesubject
names.infoAccess
<Array> (Optional) An array describing the AuthorityInfoAccess, used with OCSP.issuerCertificate
<Object> (Optional) The issuer certificate object. For self-signed certificates, this may be a circular reference.
The certificate may contain information about the public key, depending on the key type.
For RSA keys, the following properties may be defined:
bits
<number> The RSA bit size. Example:1024
.exponent
<string> The RSA exponent, as a string in hexadecimal number notation. Example:'0x010001'
.modulus
<string> The RSA modulus, as a hexadecimal string. Example:'B56CE45CB7...'
.pubkey
<Buffer> The public key.
For EC keys, the following properties may be defined:
pubkey
<Buffer> The public key.bits
<number> The key size in bits. Example:256
.asn1Curve
<string> (Optional) The ASN.1 name of the OID of the elliptic curve. Well-known curves are identified by an OID. While it is unusual, it is possible that the curve is identified by its mathematical properties, in which case it will not have an OID. Example:'prime256v1'
.nistCurve
<string> (Optional) The NIST name for the elliptic curve, if it has one (not all well-known curves have been assigned names by NIST). Example:'P-256'
.
Example certificate:
{ subject:
{ OU: [ 'Domain Control Validated', 'PositiveSSL Wildcard' ],
CN: '*.nodejs.org' },
issuer:
{ C: 'GB',
ST: 'Greater Manchester',
L: 'Salford',
O: 'COMODO CA Limited',
CN: 'COMODO RSA Domain Validation Secure Server CA' },
subjectaltname: 'DNS:*.nodejs.org, DNS:nodejs.org',
infoAccess:
{ 'CA Issuers - URI':
[ 'http://crt.comodoca.com/COMODORSADomainValidationSecureServerCA.crt' ],
'OCSP - URI': [ 'http://ocsp.comodoca.com' ] },
modulus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
exponent: '0x10001',
pubkey: <Buffer ... >,
valid_from: 'Aug 14 00:00:00 2017 GMT',
valid_to: 'Nov 20 23:59:59 2019 GMT',
fingerprint: '01:02:59:D9:C3:D2:0D:08:F7:82:4E:44:A4:B4:53:C5:E2:3A:87:4D',
fingerprint256: '69:AE:1A:6A:D4:3D:C6:C1:1B:EA:C6:23:DE:BA:2A:14:62:62:93:5C:7A:EA:06:41:9B:0B:BC:87:CE:48:4E:02',
fingerprint512: '19:2B:3E:C3:B3:5B:32:E8:AE:BB:78:97:27:E4:BA:6C:39:C9:92:79:4F:31:46:39:E2:70:E5:5F:89:42:17:C9:E8:64:CA:FF:BB:72:56:73:6E:28:8A:92:7E:A3:2A:15:8B:C2:E0:45:CA:C3:BC:EA:40:52:EC:CA:A2:68:CB:32',
ext_key_usage: [ '1.3.6.1.5.5.7.3.1', '1.3.6.1.5.5.7.3.2' ],
serialNumber: '66593D57F20CBC573E433381B5FEC280',
raw: <Buffer ... > }
tlsSocket.getPeerFinished()
#
- Returns: <Buffer> | <undefined> The latest
Finished
message that is expected or has actually been received from the socket as part of a SSL/TLS handshake, orundefined
if there is noFinished
message so far.
As the Finished
messages are message digests of the complete handshake
(with a total of 192 bits for TLS 1.0 and more for SSL 3.0), they can
be used for external authentication procedures when the authentication
provided by SSL/TLS is not desired or is not enough.
Corresponds to the SSL_get_peer_finished
routine in OpenSSL and may be used
to implement the tls-unique
channel binding from RFC 5929.
tlsSocket.getPeerX509Certificate()
#
- Returns: <X509Certificate>
Returns the peer certificate as an <X509Certificate> object.
If there is no peer certificate, or the socket has been destroyed,
undefined
will be returned.
tlsSocket.getProtocol()
#
Returns a string containing the negotiated SSL/TLS protocol version of the
current connection. The value 'unknown'
will be returned for connected
sockets that have not completed the handshaking process. The value null
will
be returned for server sockets or disconnected client sockets.
Protocol versions are:
'SSLv3'
'TLSv1'
'TLSv1.1'
'TLSv1.2'
'TLSv1.3'
See the OpenSSL SSL_get_version
documentation for more information.
tlsSocket.getSession()
#
Returns the TLS session data or undefined
if no session was
negotiated. On the client, the data can be provided to the session
option of
tls.connect()
to resume the connection. On the server, it may be useful
for debugging.
See Session Resumption for more information.
Note: getSession()
works only for TLSv1.2 and below. For TLSv1.3, applications
must use the 'session'
event (it also works for TLSv1.2 and below).
tlsSocket.getSharedSigalgs()
#
- Returns: <Array> List of signature algorithms shared between the server and the client in the order of decreasing preference.
See SSL_get_shared_sigalgs for more information.
tlsSocket.getTLSTicket()
#
For a client, returns the TLS session ticket if one is available, or
undefined
. For a server, always returns undefined
.
It may be useful for debugging.
See Session Resumption for more information.
tlsSocket.getX509Certificate()
#
- Returns: <X509Certificate>
Returns the local certificate as an <X509Certificate> object.
If there is no local certificate, or the socket has been destroyed,
undefined
will be returned.
tlsSocket.isSessionReused()
#
- Returns: <boolean>
true
if the session was reused,false
otherwise.
See Session Resumption for more information.
tlsSocket.localAddress
#
Returns the string representation of the local IP address.
tlsSocket.localPort
#
Returns the numeric representation of the local port.
tlsSocket.remoteAddress
#
Returns the string representation of the remote IP address. For example,
'74.125.127.100'
or '2001:4860:a005::68'
.
tlsSocket.remoteFamily
#
Returns the string representation of the remote IP family. 'IPv4'
or 'IPv6'
.
tlsSocket.remotePort
#
Returns the numeric representation of the remote port. For example, 443
.
tlsSocket.renegotiate(options, callback)
#
-
options
<Object>rejectUnauthorized
<boolean> If notfalse
, the server certificate is verified against the list of supplied CAs. An'error'
event is emitted if verification fails;err.code
contains the OpenSSL error code. Default:true
.requestCert
-
callback
<Function> Ifrenegotiate()
returnedtrue
, callback is attached once to the'secure'
event. Ifrenegotiate()
returnedfalse
,callback
will be called in the next tick with an error, unless thetlsSocket
has been destroyed, in which casecallback
will not be called at all. -
Returns: <boolean>
true
if renegotiation was initiated,false
otherwise.
The tlsSocket.renegotiate()
method initiates a TLS renegotiation process.
Upon completion, the callback
function will be passed a single argument
that is either an Error
(if the request failed) or null
.
This method can be used to request a peer's certificate after the secure connection has been established.
When running as the server, the socket will be destroyed with an error after
handshakeTimeout
timeout.
For TLSv1.3, renegotiation cannot be initiated, it is not supported by the protocol.
tlsSocket.setMaxSendFragment(size)
#
size
<number> The maximum TLS fragment size. The maximum value is16384
. Default:16384
.- Returns: <boolean>
The tlsSocket.setMaxSendFragment()
method sets the maximum TLS fragment size.
Returns true
if setting the limit succeeded; false
otherwise.
Smaller fragment sizes decrease the buffering latency on the client: larger fragments are buffered by the TLS layer until the entire fragment is received and its integrity is verified; large fragments can span multiple roundtrips and their processing can be delayed due to packet loss or reordering. However, smaller fragments add extra TLS framing bytes and CPU overhead, which may decrease overall server throughput.
tls.checkServerIdentity(hostname, cert)
#
hostname
<string> The host name or IP address to verify the certificate against.cert
<Object> A certificate object representing the peer's certificate.- Returns: <Error> | <undefined>
Verifies the certificate cert
is issued to hostname
.
Returns <Error> object, populating it with reason
, host
, and cert
on
failure. On success, returns <undefined>.
This function is intended to be used in combination with the
checkServerIdentity
option that can be passed to tls.connect()
and as
such operates on a certificate object. For other purposes, consider using
x509.checkHost()
instead.
This function can be overwritten by providing an alternative function as the
options.checkServerIdentity
option that is passed to tls.connect()
. The
overwriting function can call tls.checkServerIdentity()
of course, to augment
the checks done with additional verification.
This function is only called if the certificate passed all other checks, such as
being issued by trusted CA (options.ca
).
Earlier versions of Node.js incorrectly accepted certificates for a given
hostname
if a matching uniformResourceIdentifier
subject alternative name
was present (see CVE-2021-44531). Applications that wish to accept
uniformResourceIdentifier
subject alternative names can use a custom
options.checkServerIdentity
function that implements the desired behavior.
tls.connect(options[, callback])
#
options
<Object>-
enableTrace
: Seetls.createServer()
-
host
<string> Host the client should connect to. Default:'localhost'
. -
port
<number> Port the client should connect to. -
path
<string> Creates Unix socket connection to path. If this option is specified,host
andport
are ignored. -
socket
<stream.Duplex> Establish secure connection on a given socket rather than creating a new socket. Typically, this is an instance ofnet.Socket
, but anyDuplex
stream is allowed. If this option is specified,path
,host
, andport
are ignored, except for certificate validation. Usually, a socket is already connected when passed totls.connect()
, but it can be connected later. Connection/disconnection/destruction ofsocket
is the user's responsibility; callingtls.connect()
will not causenet.connect()
to be called. -
allowHalfOpen
<boolean> If set tofalse
, then the socket will automatically end the writable side when the readable side ends. If thesocket
option is set, this option has no effect. See theallowHalfOpen
option ofnet.Socket
for details. Default:false
. -
rejectUnauthorized
<boolean> If notfalse
, the server certificate is verified against the list of supplied CAs. An'error'
event is emitted if verification fails;err.code
contains the OpenSSL error code. Default:true
. -
pskCallback
<Function>- hint: <string> optional message sent from the server to help client
decide which identity to use during negotiation.
Always
null
if TLS 1.3 is used. - Returns: <Object> in the form
{ psk: <Buffer|TypedArray|DataView>, identity: <string> }
ornull
to stop the negotiation process.psk
must be compatible with the selected cipher's digest.identity
must use UTF-8 encoding.
When negotiating TLS-PSK (pre-shared keys), this function is called with optional identity
hint
provided by the server ornull
in case of TLS 1.3 wherehint
was removed. It will be necessary to provide a customtls.checkServerIdentity()
for the connection as the default one will try to check host name/IP of the server against the certificate but that's not applicable for PSK because there won't be a certificate present. More information can be found in the RFC 4279. - hint: <string> optional message sent from the server to help client
decide which identity to use during negotiation.
Always
-
ALPNProtocols
: <string[]> | <Buffer[]> | <TypedArray[]> | <DataView[]> | <Buffer> | <TypedArray> | <DataView> An array of strings,Buffer
s,TypedArray
s, orDataView
s, or a singleBuffer
,TypedArray
, orDataView
containing the supported ALPN protocols.Buffer
s should have the format[len][name][len][name]...
e.g.'\x08http/1.1\x08http/1.0'
, where thelen
byte is the length of the next protocol name. Passing an array is usually much simpler, e.g.['http/1.1', 'http/1.0']
. Protocols earlier in the list have higher preference than those later. -
servername
: <string> Server name for the SNI (Server Name Indication) TLS extension. It is the name of the host being connected to, and must be a host name, and not an IP address. It can be used by a multi-homed server to choose the correct certificate to present to the client, see theSNICallback
option totls.createServer()
. -
checkServerIdentity(servername, cert)
<Function> A callback function to be used (instead of the builtintls.checkServerIdentity()
function) when checking the server's host name (or the providedservername
when explicitly set) against the certificate. This should return an <Error> if verification fails. The method should returnundefined
if theservername
andcert
are verified. -
session
<Buffer> ABuffer
instance, containing TLS session. -
minDHSize
<number> Minimum size of the DH parameter in bits to accept a TLS connection. When a server offers a DH parameter with a size less thanminDHSize
, the TLS connection is destroyed and an error is thrown. Default:1024
. -
highWaterMark
: <number> Consistent with the readable streamhighWaterMark
parameter. Default:16 * 1024
. -
secureContext
: TLS context object created withtls.createSecureContext()
. If asecureContext
is not provided, one will be created by passing the entireoptions
object totls.createSecureContext()
. -
onread
<Object> If thesocket
option is missing, incoming data is stored in a singlebuffer
and passed to the suppliedcallback
when data arrives on the socket, otherwise the option is ignored. See theonread
option ofnet.Socket
for details. -
...:
tls.createSecureContext()
options that are used if thesecureContext
option is missing, otherwise they are ignored. -
...: Any
socket.connect()
option not already listed.
-
callback
<Function>- Returns: <tls.TLSSocket>
The callback
function, if specified, will be added as a listener for the
'secureConnect'
event.
tls.connect()
returns a tls.TLSSocket
object.
Unlike the https
API, tls.connect()
does not enable the
SNI (Server Name Indication) extension by default, which may cause some
servers to return an incorrect certificate or reject the connection
altogether. To enable SNI, set the servername
option in addition
to host
.
The following illustrates a client for the echo server example from
tls.createServer()
:
// Assumes an echo server that is listening on port 8000.
const tls = require('node:tls');
const fs = require('node:fs');
const options = {
// Necessary only if the server requires client certificate authentication.
key: fs.readFileSync('client-key.pem'),
cert: fs.readFileSync('client-cert.pem'),
// Necessary only if the server uses a self-signed certificate.
ca: [ fs.readFileSync('server-cert.pem') ],
// Necessary only if the server's cert isn't for "localhost".
checkServerIdentity: () => { return null; },
};
const socket = tls.connect(8000, options, () => {
console.log('client connected',
socket.authorized ? 'authorized' : 'unauthorized');
process.stdin.pipe(socket);
process.stdin.resume();
});
socket.setEncoding('utf8');
socket.on('data', (data) => {
console.log(data);
});
socket.on('end', () => {
console.log('server ends connection');
});
tls.connect(path[, options][, callback])
#
path
<string> Default value foroptions.path
.options
<Object> Seetls.connect()
.callback
<Function> Seetls.connect()
.- Returns: <tls.TLSSocket>
Same as tls.connect()
except that path
can be provided
as an argument instead of an option.
A path option, if specified, will take precedence over the path argument.
tls.connect(port[, host][, options][, callback])
#
port
<number> Default value foroptions.port
.host
<string> Default value foroptions.host
.options
<Object> Seetls.connect()
.callback
<Function> Seetls.connect()
.- Returns: <tls.TLSSocket>
Same as tls.connect()
except that port
and host
can be provided
as arguments instead of options.
A port or host option, if specified, will take precedence over any port or host argument.
tls.createSecureContext([options])
#
options
<Object>ca
<string> | <string[]> | <Buffer> | <Buffer[]> Optionally override the trusted CA certificates. Default is to trust the well-known CAs curated by Mozilla. Mozilla's CAs are completely replaced when CAs are explicitly specified using this option. The value can be a string orBuffer
, or anArray
of strings and/orBuffer
s. Any string orBuffer
can contain multiple PEM CAs concatenated together. The peer's certificate must be chainable to a CA trusted by the server for the connection to be authenticated. When using certificates that are not chainable to a well-known CA, the certificate's CA must be explicitly specified as a trusted or the connection will fail to authenticate. If the peer uses a certificate that doesn't match or chain to one of the default CAs, use theca
option to provide a CA certificate that the peer's certificate can match or chain to. For self-signed certificates, the certificate is its own CA, and must be provided. For PEM encoded certificates, supported types are "TRUSTED CERTIFICATE", "X509 CERTIFICATE", and "CERTIFICATE". See alsotls.rootCertificates
.cert
<string> | <string[]> | <Buffer> | <Buffer[]> Cert chains in PEM format. One cert chain should be provided per private key. Each cert chain should consist of the PEM formatted certificate for a provided privatekey
, followed by the PEM formatted intermediate certificates (if any), in order, and not including the root CA (the root CA must be pre-known to the peer, seeca
). When providing multiple cert chains, they do not have to be in the same order as their private keys inkey
. If the intermediate certificates are not provided, the peer will not be able to validate the certificate, and the handshake will fail.sigalgs
<string> Colon-separated list of supported signature algorithms. The list can contain digest algorithms (SHA256
,MD5
etc.), public key algorithms (RSA-PSS
,ECDSA
etc.), combination of both (e.g 'RSA+SHA384') or TLS v1.3 scheme names (e.g.rsa_pss_pss_sha512
). See OpenSSL man pages for more info.ciphers
<string> Cipher suite specification, replacing the default. For more information, see Modifying the default TLS cipher suite. Permitted ciphers can be obtained viatls.getCiphers()
. Cipher names must be uppercased in order for OpenSSL to accept them.clientCertEngine
<string> Name of an OpenSSL engine which can provide the client certificate.crl
<string> | <string[]> | <Buffer> | <Buffer[]> PEM formatted CRLs (Certificate Revocation Lists).dhparam
<string> | <Buffer>'auto'
or custom Diffie-Hellman parameters, required for non-ECDHE perfect forward secrecy. If omitted or invalid, the parameters are silently discarded and DHE ciphers will not be available. ECDHE-based perfect forward secrecy will still be available.ecdhCurve
<string> A string describing a named curve or a colon separated list of curve NIDs or names, for exampleP-521:P-384:P-256
, to use for ECDH key agreement. Set toauto
to select the curve automatically. Usecrypto.getCurves()
to obtain a list of available curve names. On recent releases,openssl ecparam -list_curves
will also display the name and description of each available elliptic curve. Default:tls.DEFAULT_ECDH_CURVE
.honorCipherOrder
<boolean> Attempt to use the server's cipher suite preferences instead of the client's. Whentrue
, causesSSL_OP_CIPHER_SERVER_PREFERENCE
to be set insecureOptions
, see OpenSSL Options for more information.key
<string> | <string[]> | <Buffer> | <Buffer[]> | <Object[]> Private keys in PEM format. PEM allows the option of private keys being encrypted. Encrypted keys will be decrypted withoptions.passphrase
. Multiple keys using different algorithms can be provided either as an array of unencrypted key strings or buffers, or an array of objects in the form{pem: <string|buffer>[, passphrase: <string>]}
. The object form can only occur in an array.object.passphrase
is optional. Encrypted keys will be decrypted withobject.passphrase
if provided, oroptions.passphrase
if it is not.privateKeyEngine
<string> Name of an OpenSSL engine to get private key from. Should be used together withprivateKeyIdentifier
.privateKeyIdentifier
<string> Identifier of a private key managed by an OpenSSL engine. Should be used together withprivateKeyEngine
. Should not be set together withkey
, because both options define a private key in different ways.maxVersion
<string> Optionally set the maximum TLS version to allow. One of'TLSv1.3'
,'TLSv1.2'
,'TLSv1.1'
, or'TLSv1'
. Cannot be specified along with thesecureProtocol
option; use one or the other. Default:tls.DEFAULT_MAX_VERSION
.minVersion
<string> Optionally set the minimum TLS version to allow. One of'TLSv1.3'
,'TLSv1.2'
,'TLSv1.1'
, or'TLSv1'
. Cannot be specified along with thesecureProtocol
option; use one or the other. Avoid setting to less than TLSv1.2, but it may be required for interoperability. Default:tls.DEFAULT_MIN_VERSION
.passphrase
<string> Shared passphrase used for a single private key and/or a PFX.pfx
<string> | <string[]> | <Buffer> | <Buffer[]> | <Object[]> PFX or PKCS12 encoded private key and certificate chain.pfx
is an alternative to providingkey
andcert
individually. PFX is usually encrypted, if it is,passphrase
will be used to decrypt it. Multiple PFX can be provided either as an array of unencrypted PFX buffers, or an array of objects in the form{buf: <string|buffer>[, passphrase: <string>]}
. The object form can only occur in an array.object.passphrase
is optional. Encrypted PFX will be decrypted withobject.passphrase
if provided, oroptions.passphrase
if it is not.secureOptions
<number> Optionally affect the OpenSSL protocol behavior, which is not usually necessary. This should be used carefully if at all! Value is a numeric bitmask of theSSL_OP_*
options from OpenSSL Options.secureProtocol
<string> Legacy mechanism to select the TLS protocol version to use, it does not support independent control of the minimum and maximum version, and does not support limiting the protocol to TLSv1.3. UseminVersion
andmaxVersion
instead. The possible values are listed as SSL_METHODS, use the function names as strings. For example, use'TLSv1_1_method'
to force TLS version 1.1, or'TLS_method'
to allow any TLS protocol version up to TLSv1.3. It is not recommended to use TLS versions less than 1.2, but it may be required for interoperability. Default: none, seeminVersion
.sessionIdContext
<string> Opaque identifier used by servers to ensure session state is not shared between applications. Unused by clients.ticketKeys
: <Buffer> 48-bytes of cryptographically strong pseudorandom data. See Session Resumption for more information.sessionTimeout
<number> The number of seconds after which a TLS session created by the server will no longer be resumable. See Session Resumption for more information. Default:300
.
tls.createServer()
sets the default value of the honorCipherOrder
option
to true
, other APIs that create secure contexts leave it unset.
tls.createServer()
uses a 128 bit truncated SHA1 hash value generated
from process.argv
as the default value of the sessionIdContext
option, other
APIs that create secure contexts have no default value.
The tls.createSecureContext()
method creates a SecureContext
object. It is
usable as an argument to several tls
APIs, such as server.addContext()
,
but has no public methods. The tls.Server
constructor and the
tls.createServer()
method do not support the secureContext
option.
A key is required for ciphers that use certificates. Either key
or
pfx
can be used to provide it.
If the ca
option is not given, then Node.js will default to using
Mozilla's publicly trusted list of CAs.
Custom DHE parameters are discouraged in favor of the new dhparam: 'auto'
option. When set to 'auto'
, well-known DHE parameters of sufficient strength
will be selected automatically. Otherwise, if necessary, openssl dhparam
can
be used to create custom parameters. The key length must be greater than or
equal to 1024 bits or else an error will be thrown. Although 1024 bits is
permissible, use 2048 bits or larger for stronger security.
tls.createSecurePair([context][, isServer][, requestCert][, rejectUnauthorized][, options])
#
tls.TLSSocket
instead.context
<Object> A secure context object as returned bytls.createSecureContext()
isServer
<boolean>true
to specify that this TLS connection should be opened as a server.requestCert
<boolean>true
to specify whether a server should request a certificate from a connecting client. Only applies whenisServer
istrue
.rejectUnauthorized
<boolean> If notfalse
a server automatically reject clients with invalid certificates. Only applies whenisServer
istrue
.options
enableTrace
: Seetls.createServer()
secureContext
: A TLS context object fromtls.createSecureContext()
isServer
: Iftrue
the TLS socket will be instantiated in server-mode. Default:false
.server
<net.Server> Anet.Server
instancerequestCert
: Seetls.createServer()
rejectUnauthorized
: Seetls.createServer()
ALPNProtocols
: Seetls.createServer()
SNICallback
: Seetls.createServer()
session
<Buffer> ABuffer
instance containing a TLS session.requestOCSP
<boolean> Iftrue
, specifies that the OCSP status request extension will be added to the client hello and an'OCSPResponse'
event will be emitted on the socket before establishing a secure communication.
Creates a new secure pair object with two streams, one of which reads and writes the encrypted data and the other of which reads and writes the cleartext data. Generally, the encrypted stream is piped to/from an incoming encrypted data stream and the cleartext one is used as a replacement for the initial encrypted stream.
tls.createSecurePair()
returns a tls.SecurePair
object with cleartext
and
encrypted
stream properties.
Using cleartext
has the same API as tls.TLSSocket
.
The tls.createSecurePair()
method is now deprecated in favor of
tls.TLSSocket()
. For example, the code:
pair = tls.createSecurePair(/* ... */);
pair.encrypted.pipe(socket);
socket.pipe(pair.encrypted);
can be replaced by:
secureSocket = tls.TLSSocket(socket, options);
where secureSocket
has the same API as pair.cleartext
.
tls.createServer([options][, secureConnectionListener])
#
options
<Object>-
ALPNProtocols
: <string[]> | <Buffer[]> | <TypedArray[]> | <DataView[]> | <Buffer> | <TypedArray> | <DataView> An array of strings,Buffer
s,TypedArray
s, orDataView
s, or a singleBuffer
,TypedArray
, orDataView
containing the supported ALPN protocols.Buffer
s should have the format[len][name][len][name]...
e.g.0x05hello0x05world
, where the first byte is the length of the next protocol name. Passing an array is usually much simpler, e.g.['hello', 'world']
. (Protocols should be ordered by their priority.) -
clientCertEngine
<string> Name of an OpenSSL engine which can provide the client certificate. -
enableTrace
<boolean> Iftrue
,tls.TLSSocket.enableTrace()
will be called on new connections. Tracing can be enabled after the secure connection is established, but this option must be used to trace the secure connection setup. Default:false
. -
handshakeTimeout
<number> Abort the connection if the SSL/TLS handshake does not finish in the specified number of milliseconds. A'tlsClientError'
is emitted on thetls.Server
object whenever a handshake times out. Default:120000
(120 seconds). -
rejectUnauthorized
<boolean> If notfalse
the server will reject any connection which is not authorized with the list of supplied CAs. This option only has an effect ifrequestCert
istrue
. Default:true
. -
requestCert
<boolean> Iftrue
the server will request a certificate from clients that connect and attempt to verify that certificate. Default:false
. -
sessionTimeout
<number> The number of seconds after which a TLS session created by the server will no longer be resumable. See Session Resumption for more information. Default:300
. -
SNICallback(servername, callback)
<Function> A function that will be called if the client supports SNI TLS extension. Two arguments will be passed when called:servername
andcallback
.callback
is an error-first callback that takes two optional arguments:error
andctx
.ctx
, if provided, is aSecureContext
instance.tls.createSecureContext()
can be used to get a properSecureContext
. Ifcallback
is called with a falsyctx
argument, the default secure context of the server will be used. IfSNICallback
wasn't provided the default callback with high-level API will be used (see below). -
ticketKeys
: <Buffer> 48-bytes of cryptographically strong pseudorandom data. See Session Resumption for more information. -
pskCallback
<Function>- socket: <tls.TLSSocket> the server
tls.TLSSocket
instance for this connection. - identity: <string> identity parameter sent from the client.
- Returns: <Buffer> | <TypedArray> | <DataView> pre-shared key that must either be
a buffer or
null
to stop the negotiation process. Returned PSK must be compatible with the selected cipher's digest.
When negotiating TLS-PSK (pre-shared keys), this function is called with the identity provided by the client. If the return value is
null
the negotiation process will stop and an "unknown_psk_identity" alert message will be sent to the other party. If the server wishes to hide the fact that the PSK identity was not known, the callback must provide some random data aspsk
to make the connection fail with "decrypt_error" before negotiation is finished. PSK ciphers are disabled by default, and using TLS-PSK thus requires explicitly specifying a cipher suite with theciphers
option. More information can be found in the RFC 4279. - socket: <tls.TLSSocket> the server
-
pskIdentityHint
<string> optional hint to send to a client to help with selecting the identity during TLS-PSK negotiation. Will be ignored in TLS 1.3. Upon failing to set pskIdentityHint'tlsClientError'
will be emitted with'ERR_TLS_PSK_SET_IDENTIY_HINT_FAILED'
code. -
...: Any
tls.createSecureContext()
option can be provided. For servers, the identity options (pfx
,key
/cert
, orpskCallback
) are usually required. -
...: Any
net.createServer()
option can be provided.
-
secureConnectionListener
<Function>- Returns: <tls.Server>
Creates a new tls.Server
. The secureConnectionListener
, if provided, is
automatically set as a listener for the 'secureConnection'
event.
The ticketKeys
options is automatically shared between node:cluster
module
workers.
The following illustrates a simple echo server:
const tls = require('node:tls');
const fs = require('node:fs');
const options = {
key: fs.readFileSync('server-key.pem'),
cert: fs.readFileSync('server-cert.pem'),
// This is necessary only if using client certificate authentication.
requestCert: true,
// This is necessary only if the client uses a self-signed certificate.
ca: [ fs.readFileSync('client-cert.pem') ],
};
const server = tls.createServer(options, (socket) => {
console.log('server connected',
socket.authorized ? 'authorized' : 'unauthorized');
socket.write('welcome!\n');
socket.setEncoding('utf8');
socket.pipe(socket);
});
server.listen(8000, () => {
console.log('server bound');
});
The server can be tested by connecting to it using the example client from
tls.connect()
.
tls.getCiphers()
#
- Returns: <string[]>
Returns an array with the names of the supported TLS ciphers. The names are
lower-case for historical reasons, but must be uppercased to be used in
the ciphers
option of tls.createSecureContext()
.
Not all supported ciphers are enabled by default. See Modifying the default TLS cipher suite.
Cipher names that start with 'tls_'
are for TLSv1.3, all the others are for
TLSv1.2 and below.
console.log(tls.getCiphers()); // ['aes128-gcm-sha256', 'aes128-sha', ...]
tls.rootCertificates
#
An immutable array of strings representing the root certificates (in PEM format) from the bundled Mozilla CA store as supplied by the current Node.js version.
The bundled CA store, as supplied by Node.js, is a snapshot of Mozilla CA store that is fixed at release time. It is identical on all supported platforms.
tls.DEFAULT_ECDH_CURVE
#
The default curve name to use for ECDH key agreement in a tls server. The
default value is 'auto'
. See tls.createSecureContext()
for further
information.
tls.DEFAULT_MAX_VERSION
#
- <string> The default value of the
maxVersion
option oftls.createSecureContext()
. It can be assigned any of the supported TLS protocol versions,'TLSv1.3'
,'TLSv1.2'
,'TLSv1.1'
, or'TLSv1'
. Default:'TLSv1.3'
, unless changed using CLI options. Using--tls-max-v1.2
sets the default to'TLSv1.2'
. Using--tls-max-v1.3
sets the default to'TLSv1.3'
. If multiple of the options are provided, the highest maximum is used.
tls.DEFAULT_MIN_VERSION
#
- <string> The default value of the
minVersion
option oftls.createSecureContext()
. It can be assigned any of the supported TLS protocol versions,'TLSv1.3'
,'TLSv1.2'
,'TLSv1.1'
, or'TLSv1'
. Default:'TLSv1.2'
, unless changed using CLI options. Using--tls-min-v1.0
sets the default to'TLSv1'
. Using--tls-min-v1.1
sets the default to'TLSv1.1'
. Using--tls-min-v1.3
sets the default to'TLSv1.3'
. If multiple of the options are provided, the lowest minimum is used.
tls.DEFAULT_CIPHERS
#
- <string> The default value of the
ciphers
option oftls.createSecureContext()
. It can be assigned any of the supported OpenSSL ciphers. Defaults to the content ofcrypto.constants.defaultCoreCipherList
, unless changed using CLI options using--tls-default-ciphers
.
Trace events#
Source Code: lib/trace_events.js
The node:trace_events
module provides a mechanism to centralize tracing
information generated by V8, Node.js core, and userspace code.
Tracing can be enabled with the --trace-event-categories
command-line flag
or by using the node:trace_events
module. The --trace-event-categories
flag
accepts a list of comma-separated category names.
The available categories are:
node
: An empty placeholder.node.async_hooks
: Enables capture of detailedasync_hooks
trace data. Theasync_hooks
events have a uniqueasyncId
and a specialtriggerId
triggerAsyncId
property.node.bootstrap
: Enables capture of Node.js bootstrap milestones.node.console
: Enables capture ofconsole.time()
andconsole.count()
output.node.threadpoolwork.sync
: Enables capture of trace data for threadpool synchronous operations, such asblob
,zlib
,crypto
andnode_api
.node.threadpoolwork.async
: Enables capture of trace data for threadpool asynchronous operations, such asblob
,zlib
,crypto
andnode_api
.node.dns.native
: Enables capture of trace data for DNS queries.node.net.native
: Enables capture of trace data for network.node.environment
: Enables capture of Node.js Environment milestones.node.fs.sync
: Enables capture of trace data for file system sync methods.node.fs_dir.sync
: Enables capture of trace data for file system sync directory methods.node.fs.async
: Enables capture of trace data for file system async methods.node.fs_dir.async
: Enables capture of trace data for file system async directory methods.node.perf
: Enables capture of Performance API measurements.node.perf.usertiming
: Enables capture of only Performance API User Timing measures and marks.node.perf.timerify
: Enables capture of only Performance API timerify measurements.
node.promises.rejections
: Enables capture of trace data tracking the number of unhandled Promise rejections and handled-after-rejections.node.vm.script
: Enables capture of trace data for thenode:vm
module'srunInNewContext()
,runInContext()
, andrunInThisContext()
methods.v8
: The V8 events are GC, compiling, and execution related.node.http
: Enables capture of trace data for http request / response.
By default the node
, node.async_hooks
, and v8
categories are enabled.
node --trace-event-categories v8,node,node.async_hooks server.js
Prior versions of Node.js required the use of the --trace-events-enabled
flag to enable trace events. This requirement has been removed. However, the
--trace-events-enabled
flag may still be used and will enable the
node
, node.async_hooks
, and v8
trace event categories by default.
node --trace-events-enabled
# is equivalent to
node --trace-event-categories v8,node,node.async_hooks
Alternatively, trace events may be enabled using the node:trace_events
module:
const trace_events = require('node:trace_events');
const tracing = trace_events.createTracing({ categories: ['node.perf'] });
tracing.enable(); // Enable trace event capture for the 'node.perf' category
// do work
tracing.disable(); // Disable trace event capture for the 'node.perf' category
Running Node.js with tracing enabled will produce log files that can be opened
in the chrome://tracing
tab of Chrome.
The logging file is by default called node_trace.${rotation}.log
, where
${rotation}
is an incrementing log-rotation id. The filepath pattern can
be specified with --trace-event-file-pattern
that accepts a template
string that supports ${rotation}
and ${pid}
:
node --trace-event-categories v8 --trace-event-file-pattern '${pid}-${rotation}.log' server.js
To guarantee that the log file is properly generated after signal events like
SIGINT
, SIGTERM
, or SIGBREAK
, make sure to have the appropriate handlers
in your code, such as:
process.on('SIGINT', function onSigint() {
console.info('Received SIGINT.');
process.exit(130); // Or applicable exit code depending on OS and signal
});
The tracing system uses the same time source
as the one used by process.hrtime()
.
However the trace-event timestamps are expressed in microseconds,
unlike process.hrtime()
which returns nanoseconds.
The features from this module are not available in Worker
threads.
The node:trace_events
module#
Tracing
object#
The Tracing
object is used to enable or disable tracing for sets of
categories. Instances are created using the trace_events.createTracing()
method.
When created, the Tracing
object is disabled. Calling the
tracing.enable()
method adds the categories to the set of enabled trace event
categories. Calling tracing.disable()
will remove the categories from the
set of enabled trace event categories.
tracing.categories
#
A comma-separated list of the trace event categories covered by this
Tracing
object.
tracing.disable()
#
Disables this Tracing
object.
Only trace event categories not covered by other enabled Tracing
objects
and not specified by the --trace-event-categories
flag will be disabled.
const trace_events = require('node:trace_events');
const t1 = trace_events.createTracing({ categories: ['node', 'v8'] });
const t2 = trace_events.createTracing({ categories: ['node.perf', 'node'] });
t1.enable();
t2.enable();
// Prints 'node,node.perf,v8'
console.log(trace_events.getEnabledCategories());
t2.disable(); // Will only disable emission of the 'node.perf' category
// Prints 'node,v8'
console.log(trace_events.getEnabledCategories());
tracing.enable()
#
Enables this Tracing
object for the set of categories covered by the
Tracing
object.
tracing.enabled
#
- <boolean>
true
only if theTracing
object has been enabled.
trace_events.createTracing(options)
#
options
<Object>categories
<string[]> An array of trace category names. Values included in the array are coerced to a string when possible. An error will be thrown if the value cannot be coerced.
- Returns: <Tracing>.
Creates and returns a Tracing
object for the given set of categories
.
const trace_events = require('node:trace_events');
const categories = ['node.perf', 'node.async_hooks'];
const tracing = trace_events.createTracing({ categories });
tracing.enable();
// do stuff
tracing.disable();
trace_events.getEnabledCategories()
#
- Returns: <string>
Returns a comma-separated list of all currently-enabled trace event
categories. The current set of enabled trace event categories is determined
by the union of all currently-enabled Tracing
objects and any categories
enabled using the --trace-event-categories
flag.
Given the file test.js
below, the command
node --trace-event-categories node.perf test.js
will print
'node.async_hooks,node.perf'
to the console.
const trace_events = require('node:trace_events');
const t1 = trace_events.createTracing({ categories: ['node.async_hooks'] });
const t2 = trace_events.createTracing({ categories: ['node.perf'] });
const t3 = trace_events.createTracing({ categories: ['v8'] });
t1.enable();
t2.enable();
console.log(trace_events.getEnabledCategories());
Examples#
Collect trace events data by inspector#
'use strict';
const { Session } = require('inspector');
const session = new Session();
session.connect();
function post(message, data) {
return new Promise((resolve, reject) => {
session.post(message, data, (err, result) => {
if (err)
reject(new Error(JSON.stringify(err)));
else
resolve(result);
});
});
}
async function collect() {
const data = [];
session.on('NodeTracing.dataCollected', (chunk) => data.push(chunk));
session.on('NodeTracing.tracingComplete', () => {
// done
});
const traceConfig = { includedCategories: ['v8'] };
await post('NodeTracing.start', { traceConfig });
// do something
setTimeout(() => {
post('NodeTracing.stop').then(() => {
session.disconnect();
console.log(data);
});
}, 1000);
}
collect();
TTY#
Source Code: lib/tty.js
The node:tty
module provides the tty.ReadStream
and tty.WriteStream
classes. In most cases, it will not be necessary or possible to use this module
directly. However, it can be accessed using:
const tty = require('node:tty');
When Node.js detects that it is being run with a text terminal ("TTY")
attached, process.stdin
will, by default, be initialized as an instance of
tty.ReadStream
and both process.stdout
and process.stderr
will, by
default, be instances of tty.WriteStream
. The preferred method of determining
whether Node.js is being run within a TTY context is to check that the value of
the process.stdout.isTTY
property is true
:
$ node -p -e "Boolean(process.stdout.isTTY)"
true
$ node -p -e "Boolean(process.stdout.isTTY)" | cat
false
In most cases, there should be little to no reason for an application to
manually create instances of the tty.ReadStream
and tty.WriteStream
classes.
Class: tty.ReadStream
#
- Extends: <net.Socket>
Represents the readable side of a TTY. In normal circumstances
process.stdin
will be the only tty.ReadStream
instance in a Node.js
process and there should be no reason to create additional instances.
readStream.isRaw
#
A boolean
that is true
if the TTY is currently configured to operate as a
raw device. Defaults to false
.
readStream.isTTY
#
A boolean
that is always true
for tty.ReadStream
instances.
readStream.setRawMode(mode)
#
mode
<boolean> Iftrue
, configures thetty.ReadStream
to operate as a raw device. Iffalse
, configures thetty.ReadStream
to operate in its default mode. ThereadStream.isRaw
property will be set to the resulting mode.- Returns: <this> The read stream instance.
Allows configuration of tty.ReadStream
so that it operates as a raw device.
When in raw mode, input is always available character-by-character, not
including modifiers. Additionally, all special processing of characters by the
terminal is disabled, including echoing input
characters. Ctrl+C will no longer cause a SIGINT
when
in this mode.
Class: tty.WriteStream
#
- Extends: <net.Socket>
Represents the writable side of a TTY. In normal circumstances,
process.stdout
and process.stderr
will be the only
tty.WriteStream
instances created for a Node.js process and there
should be no reason to create additional instances.
Event: 'resize'
#
The 'resize'
event is emitted whenever either of the writeStream.columns
or writeStream.rows
properties have changed. No arguments are passed to the
listener callback when called.
process.stdout.on('resize', () => {
console.log('screen size has changed!');
console.log(`${process.stdout.columns}x${process.stdout.rows}`);
});
writeStream.clearLine(dir[, callback])
#
dir
<number>-1
: to the left from cursor1
: to the right from cursor0
: the entire line
callback
<Function> Invoked once the operation completes.- Returns: <boolean>
false
if the stream wishes for the calling code to wait for the'drain'
event to be emitted before continuing to write additional data; otherwisetrue
.
writeStream.clearLine()
clears the current line of this WriteStream
in a
direction identified by dir
.
writeStream.clearScreenDown([callback])
#
callback
<Function> Invoked once the operation completes.- Returns: <boolean>
false
if the stream wishes for the calling code to wait for the'drain'
event to be emitted before continuing to write additional data; otherwisetrue
.
writeStream.clearScreenDown()
clears this WriteStream
from the current
cursor down.
writeStream.columns
#
A number
specifying the number of columns the TTY currently has. This property
is updated whenever the 'resize'
event is emitted.
writeStream.cursorTo(x[, y][, callback])
#
x
<number>y
<number>callback
<Function> Invoked once the operation completes.- Returns: <boolean>
false
if the stream wishes for the calling code to wait for the'drain'
event to be emitted before continuing to write additional data; otherwisetrue
.
writeStream.cursorTo()
moves this WriteStream
's cursor to the specified
position.
writeStream.getColorDepth([env])
#
env
<Object> An object containing the environment variables to check. This enables simulating the usage of a specific terminal. Default:process.env
.- Returns: <number>
Returns:
1
for 2,4
for 16,8
for 256,24
for 16,777,216 colors supported.
Use this to determine what colors the terminal supports. Due to the nature of
colors in terminals it is possible to either have false positives or false
negatives. It depends on process information and the environment variables that
may lie about what terminal is used.
It is possible to pass in an env
object to simulate the usage of a specific
terminal. This can be useful to check how specific environment settings behave.
To enforce a specific color support, use one of the below environment settings.
- 2 colors:
FORCE_COLOR = 0
(Disables colors) - 16 colors:
FORCE_COLOR = 1
- 256 colors:
FORCE_COLOR = 2
- 16,777,216 colors:
FORCE_COLOR = 3
Disabling color support is also possible by using the NO_COLOR
and
NODE_DISABLE_COLORS
environment variables.
writeStream.getWindowSize()
#
- Returns: <number[]>
writeStream.getWindowSize()
returns the size of the TTY
corresponding to this WriteStream
. The array is of the type
[numColumns, numRows]
where numColumns
and numRows
represent the number
of columns and rows in the corresponding TTY.
writeStream.hasColors([count][, env])
#
count
<integer> The number of colors that are requested (minimum 2). Default: 16.env
<Object> An object containing the environment variables to check. This enables simulating the usage of a specific terminal. Default:process.env
.- Returns: <boolean>
Returns true
if the writeStream
supports at least as many colors as provided
in count
. Minimum support is 2 (black and white).
This has the same false positives and negatives as described in
writeStream.getColorDepth()
.
process.stdout.hasColors();
// Returns true or false depending on if `stdout` supports at least 16 colors.
process.stdout.hasColors(256);
// Returns true or false depending on if `stdout` supports at least 256 colors.
process.stdout.hasColors({ TMUX: '1' });
// Returns true.
process.stdout.hasColors(2 ** 24, { TMUX: '1' });
// Returns false (the environment setting pretends to support 2 ** 8 colors).
writeStream.isTTY
#
A boolean
that is always true
.
writeStream.moveCursor(dx, dy[, callback])
#
dx
<number>dy
<number>callback
<Function> Invoked once the operation completes.- Returns: <boolean>
false
if the stream wishes for the calling code to wait for the'drain'
event to be emitted before continuing to write additional data; otherwisetrue
.
writeStream.moveCursor()
moves this WriteStream
's cursor relative to its
current position.
writeStream.rows
#
A number
specifying the number of rows the TTY currently has. This property
is updated whenever the 'resize'
event is emitted.
tty.isatty(fd)
#
The tty.isatty()
method returns true
if the given fd
is associated with
a TTY and false
if it is not, including whenever fd
is not a non-negative
integer.
UDP/datagram sockets#
Source Code: lib/dgram.js
The node:dgram
module provides an implementation of UDP datagram sockets.
import dgram from 'node:dgram';
const server = dgram.createSocket('udp4');
server.on('error', (err) => {
console.error(`server error:\n${err.stack}`);
server.close();
});
server.on('message', (msg, rinfo) => {
console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
});
server.on('listening', () => {
const address = server.address();
console.log(`server listening ${address.address}:${address.port}`);
});
server.bind(41234);
// Prints: server listening 0.0.0.0:41234
const dgram = require('node:dgram');
const server = dgram.createSocket('udp4');
server.on('error', (err) => {
console.error(`server error:\n${err.stack}`);
server.close();
});
server.on('message', (msg, rinfo) => {
console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
});
server.on('listening', () => {
const address = server.address();
console.log(`server listening ${address.address}:${address.port}`);
});
server.bind(41234);
// Prints: server listening 0.0.0.0:41234
Class: dgram.Socket
#
- Extends: <EventEmitter>
Encapsulates the datagram functionality.
New instances of dgram.Socket
are created using dgram.createSocket()
.
The new
keyword is not to be used to create dgram.Socket
instances.
Event: 'close'
#
The 'close'
event is emitted after a socket is closed with close()
.
Once triggered, no new 'message'
events will be emitted on this socket.
Event: 'connect'
#
The 'connect'
event is emitted after a socket is associated to a remote
address as a result of a successful connect()
call.
Event: 'error'
#
exception
<Error>
The 'error'
event is emitted whenever any error occurs. The event handler
function is passed a single Error
object.
Event: 'listening'
#
The 'listening'
event is emitted once the dgram.Socket
is addressable and
can receive data. This happens either explicitly with socket.bind()
or
implicitly the first time data is sent using socket.send()
.
Until the dgram.Socket
is listening, the underlying system resources do not
exist and calls such as socket.address()
and socket.setTTL()
will fail.
Event: 'message'
#
The 'message'
event is emitted when a new datagram is available on a socket.
The event handler function is passed two arguments: msg
and rinfo
.
If the source address of the incoming packet is an IPv6 link-local
address, the interface name is added to the address
. For
example, a packet received on the en0
interface might have the
address field set to 'fe80::2618:1234:ab11:3b9c%en0'
, where '%en0'
is the interface name as a zone ID suffix.
socket.addMembership(multicastAddress[, multicastInterface])
#
Tells the kernel to join a multicast group at the given multicastAddress
and
multicastInterface
using the IP_ADD_MEMBERSHIP
socket option. If the
multicastInterface
argument is not specified, the operating system will choose
one interface and will add membership to it. To add membership to every
available interface, call addMembership
multiple times, once per interface.
When called on an unbound socket, this method will implicitly bind to a random port, listening on all interfaces.
When sharing a UDP socket across multiple cluster
workers, the
socket.addMembership()
function must be called only once or an
EADDRINUSE
error will occur:
import cluster from 'node:cluster';
import dgram from 'node:dgram';
if (cluster.isPrimary) {
cluster.fork(); // Works ok.
cluster.fork(); // Fails with EADDRINUSE.
} else {
const s = dgram.createSocket('udp4');
s.bind(1234, () => {
s.addMembership('224.0.0.114');
});
}
const cluster = require('node:cluster');
const dgram = require('node:dgram');
if (cluster.isPrimary) {
cluster.fork(); // Works ok.
cluster.fork(); // Fails with EADDRINUSE.
} else {
const s = dgram.createSocket('udp4');
s.bind(1234, () => {
s.addMembership('224.0.0.114');
});
}
socket.addSourceSpecificMembership(sourceAddress, groupAddress[, multicastInterface])
#
Tells the kernel to join a source-specific multicast channel at the given
sourceAddress
and groupAddress
, using the multicastInterface
with the
IP_ADD_SOURCE_MEMBERSHIP
socket option. If the multicastInterface
argument
is not specified, the operating system will choose one interface and will add
membership to it. To add membership to every available interface, call
socket.addSourceSpecificMembership()
multiple times, once per interface.
When called on an unbound socket, this method will implicitly bind to a random port, listening on all interfaces.
socket.address()
#
- Returns: <Object>
Returns an object containing the address information for a socket.
For UDP sockets, this object will contain address
, family
, and port
properties.
This method throws EBADF
if called on an unbound socket.
socket.bind([port][, address][, callback])
#
port
<integer>address
<string>callback
<Function> with no parameters. Called when binding is complete.
For UDP sockets, causes the dgram.Socket
to listen for datagram
messages on a named port
and optional address
. If port
is not
specified or is 0
, the operating system will attempt to bind to a
random port. If address
is not specified, the operating system will
attempt to listen on all addresses. Once binding is complete, a
'listening'
event is emitted and the optional callback
function is
called.
Specifying both a 'listening'
event listener and passing a
callback
to the socket.bind()
method is not harmful but not very
useful.
A bound datagram socket keeps the Node.js process running to receive datagram messages.
If binding fails, an 'error'
event is generated. In rare case (e.g.
attempting to bind with a closed socket), an Error
may be thrown.
Example of a UDP server listening on port 41234:
import dgram from 'node:dgram';
const server = dgram.createSocket('udp4');
server.on('error', (err) => {
console.error(`server error:\n${err.stack}`);
server.close();
});
server.on('message', (msg, rinfo) => {
console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
});
server.on('listening', () => {
const address = server.address();
console.log(`server listening ${address.address}:${address.port}`);
});
server.bind(41234);
// Prints: server listening 0.0.0.0:41234
const dgram = require('node:dgram');
const server = dgram.createSocket('udp4');
server.on('error', (err) => {
console.error(`server error:\n${err.stack}`);
server.close();
});
server.on('message', (msg, rinfo) => {
console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
});
server.on('listening', () => {
const address = server.address();
console.log(`server listening ${address.address}:${address.port}`);
});
server.bind(41234);
// Prints: server listening 0.0.0.0:41234
socket.bind(options[, callback])
#
options
<Object> Required. Supports the following properties:callback
<Function>
For UDP sockets, causes the dgram.Socket
to listen for datagram
messages on a named port
and optional address
that are passed as
properties of an options
object passed as the first argument. If
port
is not specified or is 0
, the operating system will attempt
to bind to a random port. If address
is not specified, the operating
system will attempt to listen on all addresses. Once binding is
complete, a 'listening'
event is emitted and the optional callback
function is called.
The options
object may contain a fd
property. When a fd
greater
than 0
is set, it will wrap around an existing socket with the given
file descriptor. In this case, the properties of port
and address
will be ignored.
Specifying both a 'listening'
event listener and passing a
callback
to the socket.bind()
method is not harmful but not very
useful.
The options
object may contain an additional exclusive
property that is
used when using dgram.Socket
objects with the cluster
module. When
exclusive
is set to false
(the default), cluster workers will use the same
underlying socket handle allowing connection handling duties to be shared.
When exclusive
is true
, however, the handle is not shared and attempted
port sharing results in an error.
A bound datagram socket keeps the Node.js process running to receive datagram messages.
If binding fails, an 'error'
event is generated. In rare case (e.g.
attempting to bind with a closed socket), an Error
may be thrown.
An example socket listening on an exclusive port is shown below.
socket.bind({
address: 'localhost',
port: 8000,
exclusive: true,
});
socket.close([callback])
#
callback
<Function> Called when the socket has been closed.
Close the underlying socket and stop listening for data on it. If a callback is
provided, it is added as a listener for the 'close'
event.
socket.connect(port[, address][, callback])
#
port
<integer>address
<string>callback
<Function> Called when the connection is completed or on error.
Associates the dgram.Socket
to a remote address and port. Every
message sent by this handle is automatically sent to that destination. Also,
the socket will only receive messages from that remote peer.
Trying to call connect()
on an already connected socket will result
in an ERR_SOCKET_DGRAM_IS_CONNECTED
exception. If address
is not
provided, '127.0.0.1'
(for udp4
sockets) or '::1'
(for udp6
sockets)
will be used by default. Once the connection is complete, a 'connect'
event
is emitted and the optional callback
function is called. In case of failure,
the callback
is called or, failing this, an 'error'
event is emitted.
socket.disconnect()
#
A synchronous function that disassociates a connected dgram.Socket
from
its remote address. Trying to call disconnect()
on an unbound or already
disconnected socket will result in an ERR_SOCKET_DGRAM_NOT_CONNECTED
exception.
socket.dropMembership(multicastAddress[, multicastInterface])
#
Instructs the kernel to leave a multicast group at multicastAddress
using the
IP_DROP_MEMBERSHIP
socket option. This method is automatically called by the
kernel when the socket is closed or the process terminates, so most apps will
never have reason to call this.
If multicastInterface
is not specified, the operating system will attempt to
drop membership on all valid interfaces.
socket.dropSourceSpecificMembership(sourceAddress, groupAddress[, multicastInterface])
#
Instructs the kernel to leave a source-specific multicast channel at the given
sourceAddress
and groupAddress
using the IP_DROP_SOURCE_MEMBERSHIP
socket option. This method is automatically called by the kernel when the
socket is closed or the process terminates, so most apps will never have
reason to call this.
If multicastInterface
is not specified, the operating system will attempt to
drop membership on all valid interfaces.
socket.getRecvBufferSize()
#
- Returns: <number> the
SO_RCVBUF
socket receive buffer size in bytes.
This method throws ERR_SOCKET_BUFFER_SIZE
if called on an unbound socket.
socket.getSendBufferSize()
#
- Returns: <number> the
SO_SNDBUF
socket send buffer size in bytes.
This method throws ERR_SOCKET_BUFFER_SIZE
if called on an unbound socket.
socket.getSendQueueSize()
#
- Returns: <number> Number of bytes queued for sending.
socket.getSendQueueCount()
#
- Returns: <number> Number of send requests currently in the queue awaiting to be processed.
socket.ref()
#
- Returns: <dgram.Socket>
By default, binding a socket will cause it to block the Node.js process from
exiting as long as the socket is open. The socket.unref()
method can be used
to exclude the socket from the reference counting that keeps the Node.js
process active. The socket.ref()
method adds the socket back to the reference
counting and restores the default behavior.
Calling socket.ref()
multiples times will have no additional effect.
The socket.ref()
method returns a reference to the socket so calls can be
chained.
socket.remoteAddress()
#
- Returns: <Object>
Returns an object containing the address
, family
, and port
of the remote
endpoint. This method throws an ERR_SOCKET_DGRAM_NOT_CONNECTED
exception
if the socket is not connected.
socket.send(msg[, offset, length][, port][, address][, callback])
#
msg
<Buffer> | <TypedArray> | <DataView> | <string> | <Array> Message to be sent.offset
<integer> Offset in the buffer where the message starts.length
<integer> Number of bytes in the message.port
<integer> Destination port.address
<string> Destination host name or IP address.callback
<Function> Called when the message has been sent.
Broadcasts a datagram on the socket.
For connectionless sockets, the destination port
and address
must be
specified. Connected sockets, on the other hand, will use their associated
remote endpoint, so the port
and address
arguments must not be set.
The msg
argument contains the message to be sent.
Depending on its type, different behavior can apply. If msg
is a Buffer
,
any TypedArray
or a DataView
,
the offset
and length
specify the offset within the Buffer
where the
message begins and the number of bytes in the message, respectively.
If msg
is a String
, then it is automatically converted to a Buffer
with 'utf8'
encoding. With messages that
contain multi-byte characters, offset
and length
will be calculated with
respect to byte length and not the character position.
If msg
is an array, offset
and length
must not be specified.
The address
argument is a string. If the value of address
is a host name,
DNS will be used to resolve the address of the host. If address
is not
provided or otherwise nullish, '127.0.0.1'
(for udp4
sockets) or '::1'
(for udp6
sockets) will be used by default.
If the socket has not been previously bound with a call to bind
, the socket
is assigned a random port number and is bound to the "all interfaces" address
('0.0.0.0'
for udp4
sockets, '::0'
for udp6
sockets.)
An optional callback
function may be specified to as a way of reporting
DNS errors or for determining when it is safe to reuse the buf
object.
DNS lookups delay the time to send for at least one tick of the
Node.js event loop.
The only way to know for sure that the datagram has been sent is by using a
callback
. If an error occurs and a callback
is given, the error will be
passed as the first argument to the callback
. If a callback
is not given,
the error is emitted as an 'error'
event on the socket
object.
Offset and length are optional but both must be set if either are used.
They are supported only when the first argument is a Buffer
, a TypedArray
,
or a DataView
.
This method throws ERR_SOCKET_BAD_PORT
if called on an unbound socket.
Example of sending a UDP packet to a port on localhost
;
import dgram from 'node:dgram';
import { Buffer } from 'node:buffer';
const message = Buffer.from('Some bytes');
const client = dgram.createSocket('udp4');
client.send(message, 41234, 'localhost', (err) => {
client.close();
});
const dgram = require('node:dgram');
const { Buffer } = require('node:buffer');
const message = Buffer.from('Some bytes');
const client = dgram.createSocket('udp4');
client.send(message, 41234, 'localhost', (err) => {
client.close();
});
Example of sending a UDP packet composed of multiple buffers to a port on
127.0.0.1
;
import dgram from 'node:dgram';
import { Buffer } from 'node:buffer';
const buf1 = Buffer.from('Some ');
const buf2 = Buffer.from('bytes');
const client = dgram.createSocket('udp4');
client.send([buf1, buf2], 41234, (err) => {
client.close();
});
const dgram = require('node:dgram');
const { Buffer } = require('node:buffer');
const buf1 = Buffer.from('Some ');
const buf2 = Buffer.from('bytes');
const client = dgram.createSocket('udp4');
client.send([buf1, buf2], 41234, (err) => {
client.close();
});
Sending multiple buffers might be faster or slower depending on the application and operating system. Run benchmarks to determine the optimal strategy on a case-by-case basis. Generally speaking, however, sending multiple buffers is faster.
Example of sending a UDP packet using a socket connected to a port on
localhost
:
import dgram from 'node:dgram';
import { Buffer } from 'node:buffer';
const message = Buffer.from('Some bytes');
const client = dgram.createSocket('udp4');
client.connect(41234, 'localhost', (err) => {
client.send(message, (err) => {
client.close();
});
});
const dgram = require('node:dgram');
const { Buffer } = require('node:buffer');
const message = Buffer.from('Some bytes');
const client = dgram.createSocket('udp4');
client.connect(41234, 'localhost', (err) => {
client.send(message, (err) => {
client.close();
});
});
Note about UDP datagram size#
The maximum size of an IPv4/v6 datagram depends on the MTU
(Maximum Transmission Unit) and on the Payload Length
field size.
-
The
Payload Length
field is 16 bits wide, which means that a normal payload cannot exceed 64K octets including the internet header and data (65,507 bytes = 65,535 − 8 bytes UDP header − 20 bytes IP header); this is generally true for loopback interfaces, but such long datagram messages are impractical for most hosts and networks. -
The
MTU
is the largest size a given link layer technology can support for datagram messages. For any link, IPv4 mandates a minimumMTU
of 68 octets, while the recommendedMTU
for IPv4 is 576 (typically recommended as theMTU
for dial-up type applications), whether they arrive whole or in fragments.For IPv6, the minimum
MTU
is 1280 octets. However, the mandatory minimum fragment reassembly buffer size is 1500 octets. The value of 68 octets is very small, since most current link layer technologies, like Ethernet, have a minimumMTU
of 1500.
It is impossible to know in advance the MTU of each link through which
a packet might travel. Sending a datagram greater than the receiver MTU
will
not work because the packet will get silently dropped without informing the
source that the data did not reach its intended recipient.
socket.setBroadcast(flag)
#
flag
<boolean>
Sets or clears the SO_BROADCAST
socket option. When set to true
, UDP
packets may be sent to a local interface's broadcast address.
This method throws EBADF
if called on an unbound socket.
socket.setMulticastInterface(multicastInterface)
#
multicastInterface
<string>
All references to scope in this section are referring to
IPv6 Zone Indices, which are defined by RFC 4007. In string form, an IP
with a scope index is written as 'IP%scope'
where scope is an interface name
or interface number.
Sets the default outgoing multicast interface of the socket to a chosen
interface or back to system interface selection. The multicastInterface
must
be a valid string representation of an IP from the socket's family.
For IPv4 sockets, this should be the IP configured for the desired physical interface. All packets sent to multicast on the socket will be sent on the interface determined by the most recent successful use of this call.
For IPv6 sockets, multicastInterface
should include a scope to indicate the
interface as in the examples that follow. In IPv6, individual send
calls can
also use explicit scope in addresses, so only packets sent to a multicast
address without specifying an explicit scope are affected by the most recent
successful use of this call.
This method throws EBADF
if called on an unbound socket.
Example: IPv6 outgoing multicast interface#
On most systems, where scope format uses the interface name:
const socket = dgram.createSocket('udp6');
socket.bind(1234, () => {
socket.setMulticastInterface('::%eth1');
});
On Windows, where scope format uses an interface number:
const socket = dgram.createSocket('udp6');
socket.bind(1234, () => {
socket.setMulticastInterface('::%2');
});
Example: IPv4 outgoing multicast interface#
All systems use an IP of the host on the desired physical interface:
const socket = dgram.createSocket('udp4');
socket.bind(1234, () => {
socket.setMulticastInterface('10.0.0.2');
});
Call results#
A call on a socket that is not ready to send or no longer open may throw a Not
running Error
.
If multicastInterface
can not be parsed into an IP then an EINVAL
System Error
is thrown.
On IPv4, if multicastInterface
is a valid address but does not match any
interface, or if the address does not match the family then
a System Error
such as EADDRNOTAVAIL
or EPROTONOSUP
is thrown.
On IPv6, most errors with specifying or omitting scope will result in the socket continuing to use (or returning to) the system's default interface selection.
A socket's address family's ANY address (IPv4 '0.0.0.0'
or IPv6 '::'
) can be
used to return control of the sockets default outgoing interface to the system
for future multicast packets.
socket.setMulticastLoopback(flag)
#
flag
<boolean>
Sets or clears the IP_MULTICAST_LOOP
socket option. When set to true
,
multicast packets will also be received on the local interface.
This method throws EBADF
if called on an unbound socket.
socket.setMulticastTTL(ttl)
#
ttl
<integer>
Sets the IP_MULTICAST_TTL
socket option. While TTL generally stands for
"Time to Live", in this context it specifies the number of IP hops that a
packet is allowed to travel through, specifically for multicast traffic. Each
router or gateway that forwards a packet decrements the TTL. If the TTL is
decremented to 0 by a router, it will not be forwarded.
The ttl
argument may be between 0 and 255. The default on most systems is 1
.
This method throws EBADF
if called on an unbound socket.
socket.setRecvBufferSize(size)
#
size
<integer>
Sets the SO_RCVBUF
socket option. Sets the maximum socket receive buffer
in bytes.
This method throws ERR_SOCKET_BUFFER_SIZE
if called on an unbound socket.
socket.setSendBufferSize(size)
#
size
<integer>
Sets the SO_SNDBUF
socket option. Sets the maximum socket send buffer
in bytes.
This method throws ERR_SOCKET_BUFFER_SIZE
if called on an unbound socket.
socket.setTTL(ttl)
#
ttl
<integer>
Sets the IP_TTL
socket option. While TTL generally stands for "Time to Live",
in this context it specifies the number of IP hops that a packet is allowed to
travel through. Each router or gateway that forwards a packet decrements the
TTL. If the TTL is decremented to 0 by a router, it will not be forwarded.
Changing TTL values is typically done for network probes or when multicasting.
The ttl
argument may be between 1 and 255. The default on most systems
is 64.
This method throws EBADF
if called on an unbound socket.
socket.unref()
#
- Returns: <dgram.Socket>
By default, binding a socket will cause it to block the Node.js process from
exiting as long as the socket is open. The socket.unref()
method can be used
to exclude the socket from the reference counting that keeps the Node.js
process active, allowing the process to exit even if the socket is still
listening.
Calling socket.unref()
multiple times will have no addition effect.
The socket.unref()
method returns a reference to the socket so calls can be
chained.
node:dgram
module functions#
dgram.createSocket(options[, callback])
#
options
<Object> Available options are:type
<string> The family of socket. Must be either'udp4'
or'udp6'
. Required.reuseAddr
<boolean> Whentrue
socket.bind()
will reuse the address, even if another process has already bound a socket on it. Default:false
.ipv6Only
<boolean> Settingipv6Only
totrue
will disable dual-stack support, i.e., binding to address::
won't make0.0.0.0
be bound. Default:false
.recvBufferSize
<number> Sets theSO_RCVBUF
socket value.sendBufferSize
<number> Sets theSO_SNDBUF
socket value.lookup
<Function> Custom lookup function. Default:dns.lookup()
.signal
<AbortSignal> An AbortSignal that may be used to close a socket.
callback
<Function> Attached as a listener for'message'
events. Optional.- Returns: <dgram.Socket>
Creates a dgram.Socket
object. Once the socket is created, calling
socket.bind()
will instruct the socket to begin listening for datagram
messages. When address
and port
are not passed to socket.bind()
the
method will bind the socket to the "all interfaces" address on a random port
(it does the right thing for both udp4
and udp6
sockets). The bound address
and port can be retrieved using socket.address().address
and
socket.address().port
.
If the signal
option is enabled, calling .abort()
on the corresponding
AbortController
is similar to calling .close()
on the socket:
const controller = new AbortController();
const { signal } = controller;
const server = dgram.createSocket({ type: 'udp4', signal });
server.on('message', (msg, rinfo) => {
console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
});
// Later, when you want to close the server.
controller.abort();
dgram.createSocket(type[, callback])
#
type
<string> Either'udp4'
or'udp6'
.callback
<Function> Attached as a listener to'message'
events.- Returns: <dgram.Socket>
Creates a dgram.Socket
object of the specified type
.
Once the socket is created, calling socket.bind()
will instruct the
socket to begin listening for datagram messages. When address
and port
are
not passed to socket.bind()
the method will bind the socket to the "all
interfaces" address on a random port (it does the right thing for both udp4
and udp6
sockets). The bound address and port can be retrieved using
socket.address().address
and socket.address().port
.
URL#
Source Code: lib/url.js
The node:url
module provides utilities for URL resolution and parsing. It can
be accessed using:
import url from 'node:url';
const url = require('node:url');
URL strings and URL objects#
A URL string is a structured string containing multiple meaningful components. When parsed, a URL object is returned containing properties for each of these components.
The node:url
module provides two APIs for working with URLs: a legacy API that
is Node.js specific, and a newer API that implements the same
WHATWG URL Standard used by web browsers.
A comparison between the WHATWG and legacy APIs is provided below. Above the URL
'https://user:[email protected]:8080/p/a/t/h?query=string#hash'
, properties
of an object returned by the legacy url.parse()
are shown. Below it are
properties of a WHATWG URL
object.
WHATWG URL's origin
property includes protocol
and host
, but not
username
or password
.
┌────────────────────────────────────────────────────────────────────────────────────────────────┐
│ href │
├──────────┬──┬─────────────────────┬────────────────────────┬───────────────────────────┬───────┤
│ protocol │ │ auth │ host │ path │ hash │
│ │ │ ├─────────────────┬──────┼──────────┬────────────────┤ │
│ │ │ │ hostname │ port │ pathname │ search │ │
│ │ │ │ │ │ ├─┬──────────────┤ │
│ │ │ │ │ │ │ │ query │ │
" https: // user : pass @ sub.example.com : 8080 /p/a/t/h ? query=string #hash "
│ │ │ │ │ hostname │ port │ │ │ │
│ │ │ │ ├─────────────────┴──────┤ │ │ │
│ protocol │ │ username │ password │ host │ │ │ │
├──────────┴──┼──────────┴──────────┼────────────────────────┤ │ │ │
│ origin │ │ origin │ pathname │ search │ hash │
├─────────────┴─────────────────────┴────────────────────────┴──────────┴────────────────┴───────┤
│ href │
└────────────────────────────────────────────────────────────────────────────────────────────────┘
(All spaces in the "" line should be ignored. They are purely for formatting.)
Parsing the URL string using the WHATWG API:
const myURL =
new URL('https://user:[email protected]:8080/p/a/t/h?query=string#hash');
Parsing the URL string using the legacy API:
import url from 'node:url';
const myURL =
url.parse('https://user:[email protected]:8080/p/a/t/h?query=string#hash');
const url = require('node:url');
const myURL =
url.parse('https://user:[email protected]:8080/p/a/t/h?query=string#hash');
Constructing a URL from component parts and getting the constructed string#
It is possible to construct a WHATWG URL from component parts using either the property setters or a template literal string:
const myURL = new URL('https://example.org');
myURL.pathname = '/a/b/c';
myURL.search = '?d=e';
myURL.hash = '#fgh';
const pathname = '/a/b/c';
const search = '?d=e';
const hash = '#fgh';
const myURL = new URL(`https://example.org${pathname}${search}${hash}`);
To get the constructed URL string, use the href
property accessor:
console.log(myURL.href);
The WHATWG URL API#
Class: URL
#
Browser-compatible URL
class, implemented by following the WHATWG URL
Standard. Examples of parsed URLs may be found in the Standard itself.
The URL
class is also available on the global object.
In accordance with browser conventions, all properties of URL
objects
are implemented as getters and setters on the class prototype, rather than as
data properties on the object itself. Thus, unlike legacy urlObject
s,
using the delete
keyword on any properties of URL
objects (e.g. delete myURL.protocol
, delete myURL.pathname
, etc) has no effect but will still
return true
.
new URL(input[, base])
#
input
<string> The absolute or relative input URL to parse. Ifinput
is relative, thenbase
is required. Ifinput
is absolute, thebase
is ignored. Ifinput
is not a string, it is converted to a string first.base
<string> The base URL to resolve against if theinput
is not absolute. Ifbase
is not a string, it is converted to a string first.
Creates a new URL
object by parsing the input
relative to the base
. If
base
is passed as a string, it will be parsed equivalent to new URL(base)
.
const myURL = new URL('/foo', 'https://example.org/');
// https://example.org/foo
The URL constructor is accessible as a property on the global object. It can also be imported from the built-in url module:
import { URL } from 'node:url';
console.log(URL === globalThis.URL); // Prints 'true'.
console.log(URL === require('node:url').URL); // Prints 'true'.
A TypeError
will be thrown if the input
or base
are not valid URLs. Note
that an effort will be made to coerce the given values into strings. For
instance:
const myURL = new URL({ toString: () => 'https://example.org/' });
// https://example.org/
Unicode characters appearing within the host name of input
will be
automatically converted to ASCII using the Punycode algorithm.
const myURL = new URL('https://測試');
// https://xn--g6w251d/
This feature is only available if the node
executable was compiled with
ICU enabled. If not, the domain names are passed through unchanged.
In cases where it is not known in advance if input
is an absolute URL
and a base
is provided, it is advised to validate that the origin
of
the URL
object is what is expected.
let myURL = new URL('http://Example.com/', 'https://example.org/');
// http://example.com/
myURL = new URL('https://Example.com/', 'https://example.org/');
// https://example.com/
myURL = new URL('foo://Example.com/', 'https://example.org/');
// foo://Example.com/
myURL = new URL('http:Example.com/', 'https://example.org/');
// http://example.com/
myURL = new URL('https:Example.com/', 'https://example.org/');
// https://example.org/Example.com/
myURL = new URL('foo:Example.com/', 'https://example.org/');
// foo:Example.com/
url.hash
#
Gets and sets the fragment portion of the URL.
const myURL = new URL('https://example.org/foo#bar');
console.log(myURL.hash);
// Prints #bar
myURL.hash = 'baz';
console.log(myURL.href);
// Prints https://example.org/foo#baz
Invalid URL characters included in the value assigned to the hash
property
are percent-encoded. The selection of which characters to
percent-encode may vary somewhat from what the url.parse()
and
url.format()
methods would produce.
url.host
#
Gets and sets the host portion of the URL.
const myURL = new URL('https://example.org:81/foo');
console.log(myURL.host);
// Prints example.org:81
myURL.host = 'example.com:82';
console.log(myURL.href);
// Prints https://example.com:82/foo
Invalid host values assigned to the host
property are ignored.
url.hostname
#
Gets and sets the host name portion of the URL. The key difference between
url.host
and url.hostname
is that url.hostname
does not include the
port.
const myURL = new URL('https://example.org:81/foo');
console.log(myURL.hostname);
// Prints example.org
// Setting the hostname does not change the port
myURL.hostname = 'example.com';
console.log(myURL.href);
// Prints https://example.com:81/foo
// Use myURL.host to change the hostname and port
myURL.host = 'example.org:82';
console.log(myURL.href);
// Prints https://example.org:82/foo
Invalid host name values assigned to the hostname
property are ignored.
url.href
#
Gets and sets the serialized URL.
const myURL = new URL('https://example.org/foo');
console.log(myURL.href);
// Prints https://example.org/foo
myURL.href = 'https://example.com/bar';
console.log(myURL.href);
// Prints https://example.com/bar
Getting the value of the href
property is equivalent to calling
url.toString()
.
Setting the value of this property to a new value is equivalent to creating a
new URL
object using new URL(value)
. Each of the URL
object's properties will be modified.
If the value assigned to the href
property is not a valid URL, a TypeError
will be thrown.
url.origin
#
Gets the read-only serialization of the URL's origin.
const myURL = new URL('https://example.org/foo/bar?baz');
console.log(myURL.origin);
// Prints https://example.org
const idnURL = new URL('https://測試');
console.log(idnURL.origin);
// Prints https://xn--g6w251d
console.log(idnURL.hostname);
// Prints xn--g6w251d
url.password
#
Gets and sets the password portion of the URL.
const myURL = new URL('https://abc:[email protected]');
console.log(myURL.password);
// Prints xyz
myURL.password = '123';
console.log(myURL.href);
// Prints https://abc:[email protected]/
Invalid URL characters included in the value assigned to the password
property
are percent-encoded. The selection of which characters to
percent-encode may vary somewhat from what the url.parse()
and
url.format()
methods would produce.
url.pathname
#
Gets and sets the path portion of the URL.
const myURL = new URL('https://example.org/abc/xyz?123');
console.log(myURL.pathname);
// Prints /abc/xyz
myURL.pathname = '/abcdef';
console.log(myURL.href);
// Prints https://example.org/abcdef?123
Invalid URL characters included in the value assigned to the pathname
property are percent-encoded. The selection of which characters
to percent-encode may vary somewhat from what the url.parse()
and
url.format()
methods would produce.
url.port
#
Gets and sets the port portion of the URL.
The port value may be a number or a string containing a number in the range
0
to 65535
(inclusive). Setting the value to the default port of the
URL
objects given protocol
will result in the port
value becoming
the empty string (''
).
The port value can be an empty string in which case the port depends on the protocol/scheme:
protocol | port |
---|---|
"ftp" | 21 |
"file" | |
"http" | 80 |
"https" | 443 |
"ws" | 80 |
"wss" | 443 |
Upon assigning a value to the port, the value will first be converted to a
string using .toString()
.
If that string is invalid but it begins with a number, the leading number is
assigned to port
.
If the number lies outside the range denoted above, it is ignored.
const myURL = new URL('https://example.org:8888');
console.log(myURL.port);
// Prints 8888
// Default ports are automatically transformed to the empty string
// (HTTPS protocol's default port is 443)
myURL.port = '443';
console.log(myURL.port);
// Prints the empty string
console.log(myURL.href);
// Prints https://example.org/
myURL.port = 1234;
console.log(myURL.port);
// Prints 1234
console.log(myURL.href);
// Prints https://example.org:1234/
// Completely invalid port strings are ignored
myURL.port = 'abcd';
console.log(myURL.port);
// Prints 1234
// Leading numbers are treated as a port number
myURL.port = '5678abcd';
console.log(myURL.port);
// Prints 5678
// Non-integers are truncated
myURL.port = 1234.5678;
console.log(myURL.port);
// Prints 1234
// Out-of-range numbers which are not represented in scientific notation
// will be ignored.
myURL.port = 1e10; // 10000000000, will be range-checked as described below
console.log(myURL.port);
// Prints 1234
Numbers which contain a decimal point, such as floating-point numbers or numbers in scientific notation, are not an exception to this rule. Leading numbers up to the decimal point will be set as the URL's port, assuming they are valid:
myURL.port = 4.567e21;
console.log(myURL.port);
// Prints 4 (because it is the leading number in the string '4.567e21')
url.protocol
#
Gets and sets the protocol portion of the URL.
const myURL = new URL('https://example.org');
console.log(myURL.protocol);
// Prints https:
myURL.protocol = 'ftp';
console.log(myURL.href);
// Prints ftp://example.org/
Invalid URL protocol values assigned to the protocol
property are ignored.
Special schemes#
The WHATWG URL Standard considers a handful of URL protocol schemes to be
special in terms of how they are parsed and serialized. When a URL is
parsed using one of these special protocols, the url.protocol
property
may be changed to another special protocol but cannot be changed to a
non-special protocol, and vice versa.
For instance, changing from http
to https
works:
const u = new URL('http://example.org');
u.protocol = 'https';
console.log(u.href);
// https://example.org/
However, changing from http
to a hypothetical fish
protocol does not
because the new protocol is not special.
const u = new URL('http://example.org');
u.protocol = 'fish';
console.log(u.href);
// http://example.org/
Likewise, changing from a non-special protocol to a special protocol is also not permitted:
const u = new URL('fish://example.org');
u.protocol = 'http';
console.log(u.href);
// fish://example.org
According to the WHATWG URL Standard, special protocol schemes are ftp
,
file
, http
, https
, ws
, and wss
.
url.search
#
Gets and sets the serialized query portion of the URL.
const myURL = new URL('https://example.org/abc?123');
console.log(myURL.search);
// Prints ?123
myURL.search = 'abc=xyz';
console.log(myURL.href);
// Prints https://example.org/abc?abc=xyz
Any invalid URL characters appearing in the value assigned the search
property will be percent-encoded. The selection of which
characters to percent-encode may vary somewhat from what the url.parse()
and url.format()
methods would produce.
url.searchParams
#
Gets the URLSearchParams
object representing the query parameters of the
URL. This property is read-only but the URLSearchParams
object it provides
can be used to mutate the URL instance; to replace the entirety of query
parameters of the URL, use the url.search
setter. See
URLSearchParams
documentation for details.
Use care when using .searchParams
to modify the URL
because,
per the WHATWG specification, the URLSearchParams
object uses
different rules to determine which characters to percent-encode. For
instance, the URL
object will not percent encode the ASCII tilde (~
)
character, while URLSearchParams
will always encode it:
const myURL = new URL('https://example.org/abc?foo=~bar');
console.log(myURL.search); // prints ?foo=~bar
// Modify the URL via searchParams...
myURL.searchParams.sort();
console.log(myURL.search); // prints ?foo=%7Ebar
url.username
#
Gets and sets the username portion of the URL.
const myURL = new URL('https://abc:[email protected]');
console.log(myURL.username);
// Prints abc
myURL.username = '123';
console.log(myURL.href);
// Prints https://123:[email protected]/
Any invalid URL characters appearing in the value assigned the username
property will be percent-encoded. The selection of which
characters to percent-encode may vary somewhat from what the url.parse()
and url.format()
methods would produce.
url.toString()
#
- Returns: <string>
The toString()
method on the URL
object returns the serialized URL. The
value returned is equivalent to that of url.href
and url.toJSON()
.
url.toJSON()
#
- Returns: <string>
The toJSON()
method on the URL
object returns the serialized URL. The
value returned is equivalent to that of url.href
and
url.toString()
.
This method is automatically called when an URL
object is serialized
with JSON.stringify()
.
const myURLs = [
new URL('https://www.example.com'),
new URL('https://test.example.org'),
];
console.log(JSON.stringify(myURLs));
// Prints ["https://www.example.com/","https://test.example.org/"]
URL.createObjectURL(blob)
#
Creates a 'blob:nodedata:...'
URL string that represents the given <Blob>
object and can be used to retrieve the Blob
later.
const {
Blob,
resolveObjectURL,
} = require('node:buffer');
const blob = new Blob(['hello']);
const id = URL.createObjectURL(blob);
// later...
const otherBlob = resolveObjectURL(id);
console.log(otherBlob.size);
The data stored by the registered <Blob> will be retained in memory until
URL.revokeObjectURL()
is called to remove it.
Blob
objects are registered within the current thread. If using Worker
Threads, Blob
objects registered within one Worker will not be available
to other workers or the main thread.
URL.revokeObjectURL(id)
#
id
<string> A'blob:nodedata:...
URL string returned by a prior call toURL.createObjectURL()
.
Removes the stored <Blob> identified by the given ID. Attempting to revoke a ID that isn't registered will silently fail.
URL.canParse(input[, base])
#
input
<string> The absolute or relative input URL to parse. Ifinput
is relative, thenbase
is required. Ifinput
is absolute, thebase
is ignored. Ifinput
is not a string, it is converted to a string first.base
<string> The base URL to resolve against if theinput
is not absolute. Ifbase
is not a string, it is converted to a string first.- Returns: <boolean>
Checks if an input
relative to the base
can be parsed to a URL
.
const isValid = URL.canParse('/foo', 'https://example.org/'); // true
const isNotValid = URL.canParse('/foo'); // false
Class: URLSearchParams
#
The URLSearchParams
API provides read and write access to the query of a
URL
. The URLSearchParams
class can also be used standalone with one of the
four following constructors.
The URLSearchParams
class is also available on the global object.
The WHATWG URLSearchParams
interface and the querystring
module have
similar purpose, but the purpose of the querystring
module is more
general, as it allows the customization of delimiter characters (&
and =
).
On the other hand, this API is designed purely for URL query strings.
const myURL = new URL('https://example.org/?abc=123');
console.log(myURL.searchParams.get('abc'));
// Prints 123
myURL.searchParams.append('abc', 'xyz');
console.log(myURL.href);
// Prints https://example.org/?abc=123&abc=xyz
myURL.searchParams.delete('abc');
myURL.searchParams.set('a', 'b');
console.log(myURL.href);
// Prints https://example.org/?a=b
const newSearchParams = new URLSearchParams(myURL.searchParams);
// The above is equivalent to
// const newSearchParams = new URLSearchParams(myURL.search);
newSearchParams.append('a', 'c');
console.log(myURL.href);
// Prints https://example.org/?a=b
console.log(newSearchParams.toString());
// Prints a=b&a=c
// newSearchParams.toString() is implicitly called
myURL.search = newSearchParams;
console.log(myURL.href);
// Prints https://example.org/?a=b&a=c
newSearchParams.delete('a');
console.log(myURL.href);
// Prints https://example.org/?a=b&a=c
new URLSearchParams()
#
Instantiate a new empty URLSearchParams
object.
new URLSearchParams(string)
#
string
<string> A query string
Parse the string
as a query string, and use it to instantiate a new
URLSearchParams
object. A leading '?'
, if present, is ignored.
let params;
params = new URLSearchParams('user=abc&query=xyz');
console.log(params.get('user'));
// Prints 'abc'
console.log(params.toString());
// Prints 'user=abc&query=xyz'
params = new URLSearchParams('?user=abc&query=xyz');
console.log(params.toString());
// Prints 'user=abc&query=xyz'
new URLSearchParams(obj)
#
obj
<Object> An object representing a collection of key-value pairs
Instantiate a new URLSearchParams
object with a query hash map. The key and
value of each property of obj
are always coerced to strings.
Unlike querystring
module, duplicate keys in the form of array values are
not allowed. Arrays are stringified using array.toString()
, which simply
joins all array elements with commas.
const params = new URLSearchParams({
user: 'abc',
query: ['first', 'second'],
});
console.log(params.getAll('query'));
// Prints [ 'first,second' ]
console.log(params.toString());
// Prints 'user=abc&query=first%2Csecond'
new URLSearchParams(iterable)
#
iterable
<Iterable> An iterable object whose elements are key-value pairs
Instantiate a new URLSearchParams
object with an iterable map in a way that
is similar to Map
's constructor. iterable
can be an Array
or any
iterable object. That means iterable
can be another URLSearchParams
, in
which case the constructor will simply create a clone of the provided
URLSearchParams
. Elements of iterable
are key-value pairs, and can
themselves be any iterable object.
Duplicate keys are allowed.
let params;
// Using an array
params = new URLSearchParams([
['user', 'abc'],
['query', 'first'],
['query', 'second'],
]);
console.log(params.toString());
// Prints 'user=abc&query=first&query=second'
// Using a Map object
const map = new Map();
map.set('user', 'abc');
map.set('query', 'xyz');
params = new URLSearchParams(map);
console.log(params.toString());
// Prints 'user=abc&query=xyz'
// Using a generator function
function* getQueryPairs() {
yield ['user', 'abc'];
yield ['query', 'first'];
yield ['query', 'second'];
}
params = new URLSearchParams(getQueryPairs());
console.log(params.toString());
// Prints 'user=abc&query=first&query=second'
// Each key-value pair must have exactly two elements
new URLSearchParams([
['user', 'abc', 'error'],
]);
// Throws TypeError [ERR_INVALID_TUPLE]:
// Each query pair must be an iterable [name, value] tuple
urlSearchParams.append(name, value)
#
Append a new name-value pair to the query string.
urlSearchParams.delete(name)
#
name
<string>
Remove all name-value pairs whose name is name
.
urlSearchParams.entries()
#
- Returns: <Iterator>
Returns an ES6 Iterator
over each of the name-value pairs in the query.
Each item of the iterator is a JavaScript Array
. The first item of the Array
is the name
, the second item of the Array
is the value
.
Alias for urlSearchParams[@@iterator]()
.
urlSearchParams.forEach(fn[, thisArg])
#
fn
<Function> Invoked for each name-value pair in the querythisArg
<Object> To be used asthis
value for whenfn
is called
Iterates over each name-value pair in the query and invokes the given function.
const myURL = new URL('https://example.org/?a=b&c=d');
myURL.searchParams.forEach((value, name, searchParams) => {
console.log(name, value, myURL.searchParams === searchParams);
});
// Prints:
// a b true
// c d true
urlSearchParams.get(name)
#
Returns the value of the first name-value pair whose name is name
. If there
are no such pairs, null
is returned.
urlSearchParams.getAll(name)
#
name
<string>- Returns: <string[]>
Returns the values of all name-value pairs whose name is name
. If there are
no such pairs, an empty array is returned.
urlSearchParams.has(name)
#
Returns true
if there is at least one name-value pair whose name is name
.
urlSearchParams.keys()
#
- Returns: <Iterator>
Returns an ES6 Iterator
over the names of each name-value pair.
const params = new URLSearchParams('foo=bar&foo=baz');
for (const name of params.keys()) {
console.log(name);
}
// Prints:
// foo
// foo
urlSearchParams.set(name, value)
#
Sets the value in the URLSearchParams
object associated with name
to
value
. If there are any pre-existing name-value pairs whose names are name
,
set the first such pair's value to value
and remove all others. If not,
append the name-value pair to the query string.
const params = new URLSearchParams();
params.append('foo', 'bar');
params.append('foo', 'baz');
params.append('abc', 'def');
console.log(params.toString());
// Prints foo=bar&foo=baz&abc=def
params.set('foo', 'def');
params.set('xyz', 'opq');
console.log(params.toString());
// Prints foo=def&abc=def&xyz=opq
urlSearchParams.size
#
The total number of parameter entries.
urlSearchParams.sort()
#
Sort all existing name-value pairs in-place by their names. Sorting is done with a stable sorting algorithm, so relative order between name-value pairs with the same name is preserved.
This method can be used, in particular, to increase cache hits.
const params = new URLSearchParams('query[]=abc&type=search&query[]=123');
params.sort();
console.log(params.toString());
// Prints query%5B%5D=abc&query%5B%5D=123&type=search
urlSearchParams.toString()
#
- Returns: <string>
Returns the search parameters serialized as a string, with characters percent-encoded where necessary.
urlSearchParams.values()
#
- Returns: <Iterator>
Returns an ES6 Iterator
over the values of each name-value pair.
urlSearchParams[Symbol.iterator]()
#
- Returns: <Iterator>
Returns an ES6 Iterator
over each of the name-value pairs in the query string.
Each item of the iterator is a JavaScript Array
. The first item of the Array
is the name
, the second item of the Array
is the value
.
Alias for urlSearchParams.entries()
.
const params = new URLSearchParams('foo=bar&xyz=baz');
for (const [name, value] of params) {
console.log(name, value);
}
// Prints:
// foo bar
// xyz baz
url.domainToASCII(domain)
#
Returns the Punycode ASCII serialization of the domain
. If domain
is an
invalid domain, the empty string is returned.
It performs the inverse operation to url.domainToUnicode()
.
This feature is only available if the node
executable was compiled with
ICU enabled. If not, the domain names are passed through unchanged.
import url from 'node:url';
console.log(url.domainToASCII('español.com'));
// Prints xn--espaol-zwa.com
console.log(url.domainToASCII('中文.com'));
// Prints xn--fiq228c.com
console.log(url.domainToASCII('xn--iñvalid.com'));
// Prints an empty string
const url = require('node:url');
console.log(url.domainToASCII('español.com'));
// Prints xn--espaol-zwa.com
console.log(url.domainToASCII('中文.com'));
// Prints xn--fiq228c.com
console.log(url.domainToASCII('xn--iñvalid.com'));
// Prints an empty string
url.domainToUnicode(domain)
#
Returns the Unicode serialization of the domain
. If domain
is an invalid
domain, the empty string is returned.
It performs the inverse operation to url.domainToASCII()
.
This feature is only available if the node
executable was compiled with
ICU enabled. If not, the domain names are passed through unchanged.
import url from 'node:url';
console.log(url.domainToUnicode('xn--espaol-zwa.com'));
// Prints español.com
console.log(url.domainToUnicode('xn--fiq228c.com'));
// Prints 中文.com
console.log(url.domainToUnicode('xn--iñvalid.com'));
// Prints an empty string
const url = require('node:url');
console.log(url.domainToUnicode('xn--espaol-zwa.com'));
// Prints español.com
console.log(url.domainToUnicode('xn--fiq228c.com'));
// Prints 中文.com
console.log(url.domainToUnicode('xn--iñvalid.com'));
// Prints an empty string
url.fileURLToPath(url)
#
url
<URL> | <string> The file URL string or URL object to convert to a path.- Returns: <string> The fully-resolved platform-specific Node.js file path.
This function ensures the correct decodings of percent-encoded characters as well as ensuring a cross-platform valid absolute path string.
import { fileURLToPath } from 'node:url';
const __filename = fileURLToPath(import.meta.url);
new URL('file:///C:/path/').pathname; // Incorrect: /C:/path/
fileURLToPath('file:///C:/path/'); // Correct: C:\path\ (Windows)
new URL('file://nas/foo.txt').pathname; // Incorrect: /foo.txt
fileURLToPath('file://nas/foo.txt'); // Correct: \\nas\foo.txt (Windows)
new URL('file:///你好.txt').pathname; // Incorrect: /%E4%BD%A0%E5%A5%BD.txt
fileURLToPath('file:///你好.txt'); // Correct: /你好.txt (POSIX)
new URL('file:///hello world').pathname; // Incorrect: /hello%20world
fileURLToPath('file:///hello world'); // Correct: /hello world (POSIX)
const { fileURLToPath } = require('node:url');
new URL('file:///C:/path/').pathname; // Incorrect: /C:/path/
fileURLToPath('file:///C:/path/'); // Correct: C:\path\ (Windows)
new URL('file://nas/foo.txt').pathname; // Incorrect: /foo.txt
fileURLToPath('file://nas/foo.txt'); // Correct: \\nas\foo.txt (Windows)
new URL('file:///你好.txt').pathname; // Incorrect: /%E4%BD%A0%E5%A5%BD.txt
fileURLToPath('file:///你好.txt'); // Correct: /你好.txt (POSIX)
new URL('file:///hello world').pathname; // Incorrect: /hello%20world
fileURLToPath('file:///hello world'); // Correct: /hello world (POSIX)
url.format(URL[, options])
#
URL
<URL> A WHATWG URL objectoptions
<Object>auth
<boolean>true
if the serialized URL string should include the username and password,false
otherwise. Default:true
.fragment
<boolean>true
if the serialized URL string should include the fragment,false
otherwise. Default:true
.search
<boolean>true
if the serialized URL string should include the search query,false
otherwise. Default:true
.unicode
<boolean>true
if Unicode characters appearing in the host component of the URL string should be encoded directly as opposed to being Punycode encoded. Default:false
.
- Returns: <string>
Returns a customizable serialization of a URL String
representation of a
WHATWG URL object.
The URL object has both a toString()
method and href
property that return
string serializations of the URL. These are not, however, customizable in
any way. The url.format(URL[, options])
method allows for basic customization
of the output.
import url from 'node:url';
const myURL = new URL('https://a:b@測試?abc#foo');
console.log(myURL.href);
// Prints https://a:b@xn--g6w251d/?abc#foo
console.log(myURL.toString());
// Prints https://a:b@xn--g6w251d/?abc#foo
console.log(url.format(myURL, { fragment: false, unicode: true, auth: false }));
// Prints 'https://測試/?abc'
const url = require('node:url');
const myURL = new URL('https://a:b@測試?abc#foo');
console.log(myURL.href);
// Prints https://a:b@xn--g6w251d/?abc#foo
console.log(myURL.toString());
// Prints https://a:b@xn--g6w251d/?abc#foo
console.log(url.format(myURL, { fragment: false, unicode: true, auth: false }));
// Prints 'https://測試/?abc'
url.pathToFileURL(path)
#
This function ensures that path
is resolved absolutely, and that the URL
control characters are correctly encoded when converting into a File URL.
import { pathToFileURL } from 'node:url';
new URL('/foo#1', 'file:'); // Incorrect: file:///foo#1
pathToFileURL('/foo#1'); // Correct: file:///foo%231 (POSIX)
new URL('/some/path%.c', 'file:'); // Incorrect: file:///some/path%.c
pathToFileURL('/some/path%.c'); // Correct: file:///some/path%25.c (POSIX)
const { pathToFileURL } = require('node:url');
new URL(__filename); // Incorrect: throws (POSIX)
new URL(__filename); // Incorrect: C:\... (Windows)
pathToFileURL(__filename); // Correct: file:///... (POSIX)
pathToFileURL(__filename); // Correct: file:///C:/... (Windows)
new URL('/foo#1', 'file:'); // Incorrect: file:///foo#1
pathToFileURL('/foo#1'); // Correct: file:///foo%231 (POSIX)
new URL('/some/path%.c', 'file:'); // Incorrect: file:///some/path%.c
pathToFileURL('/some/path%.c'); // Correct: file:///some/path%25.c (POSIX)
url.urlToHttpOptions(url)
#
url
<URL> The WHATWG URL object to convert to an options object.- Returns: <Object> Options object
protocol
<string> Protocol to use.hostname
<string> A domain name or IP address of the server to issue the request to.hash
<string> The fragment portion of the URL.search
<string> The serialized query portion of the URL.pathname
<string> The path portion of the URL.path
<string> Request path. Should include query string if any. E.G.'/index.html?page=12'
. An exception is thrown when the request path contains illegal characters. Currently, only spaces are rejected but that may change in the future.href
<string> The serialized URL.port
<number> Port of remote server.auth
<string> Basic authentication i.e.'user:password'
to compute an Authorization header.
This utility function converts a URL object into an ordinary options object as
expected by the http.request()
and https.request()
APIs.
import { urlToHttpOptions } from 'node:url';
const myURL = new URL('https://a:b@測試?abc#foo');
console.log(urlToHttpOptions(myURL));
/*
{
protocol: 'https:',
hostname: 'xn--g6w251d',
hash: '#foo',
search: '?abc',
pathname: '/',
path: '/?abc',
href: 'https://a:b@xn--g6w251d/?abc#foo',
auth: 'a:b'
}
*/
const { urlToHttpOptions } = require('node:url');
const myURL = new URL('https://a:b@測試?abc#foo');
console.log(urlToHttpOptions(myURL));
/*
{
protocol: 'https:',
hostname: 'xn--g6w251d',
hash: '#foo',
search: '?abc',
pathname: '/',
path: '/?abc',
href: 'https://a:b@xn--g6w251d/?abc#foo',
auth: 'a:b'
}
*/
Legacy URL API#
Legacy urlObject
#
The legacy urlObject
(require('node:url').Url
or
import { Url } from 'node:url'
) is
created and returned by the url.parse()
function.
urlObject.auth
#
The auth
property is the username and password portion of the URL, also
referred to as userinfo. This string subset follows the protocol
and
double slashes (if present) and precedes the host
component, delimited by @
.
The string is either the username, or it is the username and password separated
by :
.
For example: 'user:pass'
.
urlObject.hash
#
The hash
property is the fragment identifier portion of the URL including the
leading #
character.
For example: '#hash'
.
urlObject.host
#
The host
property is the full lower-cased host portion of the URL, including
the port
if specified.
For example: 'sub.example.com:8080'
.
urlObject.hostname
#
The hostname
property is the lower-cased host name portion of the host
component without the port
included.
For example: 'sub.example.com'
.
urlObject.href
#
The href
property is the full URL string that was parsed with both the
protocol
and host
components converted to lower-case.
For example: 'http://user:[email protected]:8080/p/a/t/h?query=string#hash'
.
urlObject.path
#
The path
property is a concatenation of the pathname
and search
components.
For example: '/p/a/t/h?query=string'
.
No decoding of the path
is performed.
urlObject.pathname
#
The pathname
property consists of the entire path section of the URL. This
is everything following the host
(including the port
) and before the start
of the query
or hash
components, delimited by either the ASCII question
mark (?
) or hash (#
) characters.
For example: '/p/a/t/h'
.
No decoding of the path string is performed.
urlObject.port
#
The port
property is the numeric port portion of the host
component.
For example: '8080'
.
urlObject.protocol
#
The protocol
property identifies the URL's lower-cased protocol scheme.
For example: 'http:'
.
urlObject.query
#
The query
property is either the query string without the leading ASCII
question mark (?
), or an object returned by the querystring
module's
parse()
method. Whether the query
property is a string or object is
determined by the parseQueryString
argument passed to url.parse()
.
For example: 'query=string'
or {'query': 'string'}
.
If returned as a string, no decoding of the query string is performed. If returned as an object, both keys and values are decoded.
urlObject.search
#
The search
property consists of the entire "query string" portion of the
URL, including the leading ASCII question mark (?
) character.
For example: '?query=string'
.
No decoding of the query string is performed.
urlObject.slashes
#
The slashes
property is a boolean
with a value of true
if two ASCII
forward-slash characters (/
) are required following the colon in the
protocol
.
url.format(urlObject)
#
urlObject
<Object> | <string> A URL object (as returned byurl.parse()
or constructed otherwise). If a string, it is converted to an object by passing it tourl.parse()
.
The url.format()
method returns a formatted URL string derived from
urlObject
.
const url = require('node:url');
url.format({
protocol: 'https',
hostname: 'example.com',
pathname: '/some/path',
query: {
page: 1,
format: 'json',
},
});
// => 'https://example.com/some/path?page=1&format=json'
If urlObject
is not an object or a string, url.format()
will throw a
TypeError
.
The formatting process operates as follows:
- A new empty string
result
is created. - If
urlObject.protocol
is a string, it is appended as-is toresult
. - Otherwise, if
urlObject.protocol
is notundefined
and is not a string, anError
is thrown. - For all string values of
urlObject.protocol
that do not end with an ASCII colon (:
) character, the literal string:
will be appended toresult
. - If either of the following conditions is true, then the literal string
//
will be appended toresult
:urlObject.slashes
property is true;urlObject.protocol
begins withhttp
,https
,ftp
,gopher
, orfile
;
- If the value of the
urlObject.auth
property is truthy, and eitherurlObject.host
orurlObject.hostname
are notundefined
, the value ofurlObject.auth
will be coerced into a string and appended toresult
followed by the literal string@
. - If the
urlObject.host
property isundefined
then:- If the
urlObject.hostname
is a string, it is appended toresult
. - Otherwise, if
urlObject.hostname
is notundefined
and is not a string, anError
is thrown. - If the
urlObject.port
property value is truthy, andurlObject.hostname
is notundefined
:- The literal string
:
is appended toresult
, and - The value of
urlObject.port
is coerced to a string and appended toresult
.
- The literal string
- If the
- Otherwise, if the
urlObject.host
property value is truthy, the value ofurlObject.host
is coerced to a string and appended toresult
. - If the
urlObject.pathname
property is a string that is not an empty string:- If the
urlObject.pathname
does not start with an ASCII forward slash (/
), then the literal string'/'
is appended toresult
. - The value of
urlObject.pathname
is appended toresult
.
- If the
- Otherwise, if
urlObject.pathname
is notundefined
and is not a string, anError
is thrown. - If the
urlObject.search
property isundefined
and if theurlObject.query
property is anObject
, the literal string?
is appended toresult
followed by the output of calling thequerystring
module'sstringify()
method passing the value ofurlObject.query
. - Otherwise, if
urlObject.search
is a string:- If the value of
urlObject.search
does not start with the ASCII question mark (?
) character, the literal string?
is appended toresult
. - The value of
urlObject.search
is appended toresult
.
- If the value of
- Otherwise, if
urlObject.search
is notundefined
and is not a string, anError
is thrown. - If the
urlObject.hash
property is a string:- If the value of
urlObject.hash
does not start with the ASCII hash (#
) character, the literal string#
is appended toresult
. - The value of
urlObject.hash
is appended toresult
.
- If the value of
- Otherwise, if the
urlObject.hash
property is notundefined
and is not a string, anError
is thrown. result
is returned.
url.parse(urlString[, parseQueryString[, slashesDenoteHost]])
#
urlString
<string> The URL string to parse.parseQueryString
<boolean> Iftrue
, thequery
property will always be set to an object returned by thequerystring
module'sparse()
method. Iffalse
, thequery
property on the returned URL object will be an unparsed, undecoded string. Default:false
.slashesDenoteHost
<boolean> Iftrue
, the first token after the literal string//
and preceding the next/
will be interpreted as thehost
. For instance, given//foo/bar
, the result would be{host: 'foo', pathname: '/bar'}
rather than{pathname: '//foo/bar'}
. Default:false
.
The url.parse()
method takes a URL string, parses it, and returns a URL
object.
A TypeError
is thrown if urlString
is not a string.
A URIError
is thrown if the auth
property is present but cannot be decoded.
url.parse()
uses a lenient, non-standard algorithm for parsing URL
strings. It is prone to security issues such as host name spoofing
and incorrect handling of usernames and passwords. Do not use with untrusted
input. CVEs are not issued for url.parse()
vulnerabilities. Use the
WHATWG URL API instead.
url.resolve(from, to)
#
The url.resolve()
method resolves a target URL relative to a base URL in a
manner similar to that of a web browser resolving an anchor tag.
const url = require('node:url');
url.resolve('/one/two/three', 'four'); // '/one/two/four'
url.resolve('http://example.com/', '/one'); // 'http://example.com/one'
url.resolve('http://example.com/one', '/two'); // 'http://example.com/two'
To achieve the same result using the WHATWG URL API:
function resolve(from, to) {
const resolvedUrl = new URL(to, new URL(from, 'resolve://'));
if (resolvedUrl.protocol === 'resolve:') {
// `from` is a relative URL.
const { pathname, search, hash } = resolvedUrl;
return pathname + search + hash;
}
return resolvedUrl.toString();
}
resolve('/one/two/three', 'four'); // '/one/two/four'
resolve('http://example.com/', '/one'); // 'http://example.com/one'
resolve('http://example.com/one', '/two'); // 'http://example.com/two'
Percent-encoding in URLs#
URLs are permitted to only contain a certain range of characters. Any character falling outside of that range must be encoded. How such characters are encoded, and which characters to encode depends entirely on where the character is located within the structure of the URL.
Legacy API#
Within the Legacy API, spaces (' '
) and the following characters will be
automatically escaped in the properties of URL objects:
< > " ` \r \n \t { } | \ ^ '
For example, the ASCII space character (' '
) is encoded as %20
. The ASCII
forward slash (/
) character is encoded as %3C
.
WHATWG API#
The WHATWG URL Standard uses a more selective and fine grained approach to selecting encoded characters than that used by the Legacy API.
The WHATWG algorithm defines four "percent-encode sets" that describe ranges of characters that must be percent-encoded:
-
The C0 control percent-encode set includes code points in range U+0000 to U+001F (inclusive) and all code points greater than U+007E.
-
The fragment percent-encode set includes the C0 control percent-encode set and code points U+0020, U+0022, U+003C, U+003E, and U+0060.
-
The path percent-encode set includes the C0 control percent-encode set and code points U+0020, U+0022, U+0023, U+003C, U+003E, U+003F, U+0060, U+007B, and U+007D.
-
The userinfo encode set includes the path percent-encode set and code points U+002F, U+003A, U+003B, U+003D, U+0040, U+005B, U+005C, U+005D, U+005E, and U+007C.
The userinfo percent-encode set is used exclusively for username and passwords encoded within the URL. The path percent-encode set is used for the path of most URLs. The fragment percent-encode set is used for URL fragments. The C0 control percent-encode set is used for host and path under certain specific conditions, in addition to all other cases.
When non-ASCII characters appear within a host name, the host name is encoded using the Punycode algorithm. Note, however, that a host name may contain both Punycode encoded and percent-encoded characters:
const myURL = new URL('https://%CF%80.example.com/foo');
console.log(myURL.href);
// Prints https://xn--1xa.example.com/foo
console.log(myURL.origin);
// Prints https://xn--1xa.example.com
Util#
Source Code: lib/util.js
The node:util
module supports the needs of Node.js internal APIs. Many of the
utilities are useful for application and module developers as well. To access
it:
const util = require('node:util');
util.callbackify(original)
#
original
<Function> Anasync
function- Returns: <Function> a callback style function
Takes an async
function (or a function that returns a Promise
) and returns a
function following the error-first callback style, i.e. taking
an (err, value) => ...
callback as the last argument. In the callback, the
first argument will be the rejection reason (or null
if the Promise
resolved), and the second argument will be the resolved value.
const util = require('node:util');
async function fn() {
return 'hello world';
}
const callbackFunction = util.callbackify(fn);
callbackFunction((err, ret) => {
if (err) throw err;
console.log(ret);
});
Will print:
hello world
The callback is executed asynchronously, and will have a limited stack trace.
If the callback throws, the process will emit an 'uncaughtException'
event, and if not handled will exit.
Since null
has a special meaning as the first argument to a callback, if a
wrapped function rejects a Promise
with a falsy value as a reason, the value
is wrapped in an Error
with the original value stored in a field named
reason
.
function fn() {
return Promise.reject(null);
}
const callbackFunction = util.callbackify(fn);
callbackFunction((err, ret) => {
// When the Promise was rejected with `null` it is wrapped with an Error and
// the original value is stored in `reason`.
err && Object.hasOwn(err, 'reason') && err.reason === null; // true
});
util.debuglog(section[, callback])
#
section
<string> A string identifying the portion of the application for which thedebuglog
function is being created.callback
<Function> A callback invoked the first time the logging function is called with a function argument that is a more optimized logging function.- Returns: <Function> The logging function
The util.debuglog()
method is used to create a function that conditionally
writes debug messages to stderr
based on the existence of the NODE_DEBUG
environment variable. If the section
name appears within the value of that
environment variable, then the returned function operates similar to
console.error()
. If not, then the returned function is a no-op.
const util = require('node:util');
const debuglog = util.debuglog('foo');
debuglog('hello from foo [%d]', 123);
If this program is run with NODE_DEBUG=foo
in the environment, then
it will output something like:
FOO 3245: hello from foo [123]
where 3245
is the process id. If it is not run with that
environment variable set, then it will not print anything.
The section
supports wildcard also:
const util = require('node:util');
const debuglog = util.debuglog('foo-bar');
debuglog('hi there, it\'s foo-bar [%d]', 2333);
if it is run with NODE_DEBUG=foo*
in the environment, then it will output
something like:
FOO-BAR 3257: hi there, it's foo-bar [2333]
Multiple comma-separated section
names may be specified in the NODE_DEBUG
environment variable: NODE_DEBUG=fs,net,tls
.
The optional callback
argument can be used to replace the logging function
with a different function that doesn't have any initialization or
unnecessary wrapping.
const util = require('node:util');
let debuglog = util.debuglog('internals', (debug) => {
// Replace with a logging function that optimizes out
// testing if the section is enabled
debuglog = debug;
});
debuglog().enabled
#
The util.debuglog().enabled
getter is used to create a test that can be used
in conditionals based on the existence of the NODE_DEBUG
environment variable.
If the section
name appears within the value of that environment variable,
then the returned value will be true
. If not, then the returned value will be
false
.
const util = require('node:util');
const enabled = util.debuglog('foo').enabled;
if (enabled) {
console.log('hello from foo [%d]', 123);
}
If this program is run with NODE_DEBUG=foo
in the environment, then it will
output something like:
hello from foo [123]
util.debug(section)
#
Alias for util.debuglog
. Usage allows for readability of that doesn't imply
logging when only using util.debuglog().enabled
.
util.deprecate(fn, msg[, code])
#
fn
<Function> The function that is being deprecated.msg
<string> A warning message to display when the deprecated function is invoked.code
<string> A deprecation code. See the list of deprecated APIs for a list of codes.- Returns: <Function> The deprecated function wrapped to emit a warning.
The util.deprecate()
method wraps fn
(which may be a function or class) in
such a way that it is marked as deprecated.
const util = require('node:util');
exports.obsoleteFunction = util.deprecate(() => {
// Do something here.
}, 'obsoleteFunction() is deprecated. Use newShinyFunction() instead.');
When called, util.deprecate()
will return a function that will emit a
DeprecationWarning
using the 'warning'
event. The warning will
be emitted and printed to stderr
the first time the returned function is
called. After the warning is emitted, the wrapped function is called without
emitting a warning.
If the same optional code
is supplied in multiple calls to util.deprecate()
,
the warning will be emitted only once for that code
.
const util = require('node:util');
const fn1 = util.deprecate(someFunction, someMessage, 'DEP0001');
const fn2 = util.deprecate(someOtherFunction, someOtherMessage, 'DEP0001');
fn1(); // Emits a deprecation warning with code DEP0001
fn2(); // Does not emit a deprecation warning because it has the same code
If either the --no-deprecation
or --no-warnings
command-line flags are
used, or if the process.noDeprecation
property is set to true
prior to
the first deprecation warning, the util.deprecate()
method does nothing.
If the --trace-deprecation
or --trace-warnings
command-line flags are set,
or the process.traceDeprecation
property is set to true
, a warning and a
stack trace are printed to stderr
the first time the deprecated function is
called.
If the --throw-deprecation
command-line flag is set, or the
process.throwDeprecation
property is set to true
, then an exception will be
thrown when the deprecated function is called.
The --throw-deprecation
command-line flag and process.throwDeprecation
property take precedence over --trace-deprecation
and
process.traceDeprecation
.
util.format(format[, ...args])
#
format
<string> Aprintf
-like format string.
The util.format()
method returns a formatted string using the first argument
as a printf
-like format string which can contain zero or more format
specifiers. Each specifier is replaced with the converted value from the
corresponding argument. Supported specifiers are:
%s
:String
will be used to convert all values exceptBigInt
,Object
and-0
.BigInt
values will be represented with ann
and Objects that have no user definedtoString
function are inspected usingutil.inspect()
with options{ depth: 0, colors: false, compact: 3 }
.%d
:Number
will be used to convert all values exceptBigInt
andSymbol
.%i
:parseInt(value, 10)
is used for all values exceptBigInt
andSymbol
.%f
:parseFloat(value)
is used for all values expectSymbol
.%j
: JSON. Replaced with the string'[Circular]'
if the argument contains circular references.%o
:Object
. A string representation of an object with generic JavaScript object formatting. Similar toutil.inspect()
with options{ showHidden: true, showProxy: true }
. This will show the full object including non-enumerable properties and proxies.%O
:Object
. A string representation of an object with generic JavaScript object formatting. Similar toutil.inspect()
without options. This will show the full object not including non-enumerable properties and proxies.%c
:CSS
. This specifier is ignored and will skip any CSS passed in.%%
: single percent sign ('%'
). This does not consume an argument.- Returns: <string> The formatted string
If a specifier does not have a corresponding argument, it is not replaced:
util.format('%s:%s', 'foo');
// Returns: 'foo:%s'
Values that are not part of the format string are formatted using
util.inspect()
if their type is not string
.
If there are more arguments passed to the util.format()
method than the
number of specifiers, the extra arguments are concatenated to the returned
string, separated by spaces:
util.format('%s:%s', 'foo', 'bar', 'baz');
// Returns: 'foo:bar baz'
If the first argument does not contain a valid format specifier, util.format()
returns a string that is the concatenation of all arguments separated by spaces:
util.format(1, 2, 3);
// Returns: '1 2 3'
If only one argument is passed to util.format()
, it is returned as it is
without any formatting:
util.format('%% %s');
// Returns: '%% %s'
util.format()
is a synchronous method that is intended as a debugging tool.
Some input values can have a significant performance overhead that can block the
event loop. Use this function with care and never in a hot code path.
util.formatWithOptions(inspectOptions, format[, ...args])
#
This function is identical to util.format()
, except in that it takes
an inspectOptions
argument which specifies options that are passed along to
util.inspect()
.
util.formatWithOptions({ colors: true }, 'See object %O', { foo: 42 });
// Returns 'See object { foo: 42 }', where `42` is colored as a number
// when printed to a terminal.
util.getSystemErrorName(err)
#
Returns the string name for a numeric error code that comes from a Node.js API. The mapping between error codes and error names is platform-dependent. See Common System Errors for the names of common errors.
fs.access('file/that/does/not/exist', (err) => {
const name = util.getSystemErrorName(err.errno);
console.error(name); // ENOENT
});
util.getSystemErrorMap()
#
- Returns: <Map>
Returns a Map of all system error codes available from the Node.js API. The mapping between error codes and error names is platform-dependent. See Common System Errors for the names of common errors.
fs.access('file/that/does/not/exist', (err) => {
const errorMap = util.getSystemErrorMap();
const name = errorMap.get(err.errno);
console.error(name); // ENOENT
});
util.inherits(constructor, superConstructor)
#
extends
keyword instead.constructor
<Function>superConstructor
<Function>
Usage of util.inherits()
is discouraged. Please use the ES6 class
and
extends
keywords to get language level inheritance support. Also note
that the two styles are semantically incompatible.
Inherit the prototype methods from one constructor into another. The
prototype of constructor
will be set to a new object created from
superConstructor
.
This mainly adds some input validation on top of
Object.setPrototypeOf(constructor.prototype, superConstructor.prototype)
.
As an additional convenience, superConstructor
will be accessible
through the constructor.super_
property.
const util = require('node:util');
const EventEmitter = require('node:events');
function MyStream() {
EventEmitter.call(this);
}
util.inherits(MyStream, EventEmitter);
MyStream.prototype.write = function(data) {
this.emit('data', data);
};
const stream = new MyStream();
console.log(stream instanceof EventEmitter); // true
console.log(MyStream.super_ === EventEmitter); // true
stream.on('data', (data) => {
console.log(`Received data: "${data}"`);
});
stream.write('It works!'); // Received data: "It works!"
ES6 example using class
and extends
:
const EventEmitter = require('node:events');
class MyStream extends EventEmitter {
write(data) {
this.emit('data', data);
}
}
const stream = new MyStream();
stream.on('data', (data) => {
console.log(`Received data: "${data}"`);
});
stream.write('With ES6');
util.inspect(object[, options])
#
util.inspect(object[, showHidden[, depth[, colors]]])
object
<any> Any JavaScript primitive orObject
.options
<Object>showHidden
<boolean> Iftrue
,object
's non-enumerable symbols and properties are included in the formatted result.WeakMap
andWeakSet
entries are also included as well as user defined prototype properties (excluding method properties). Default:false
.depth
<number> Specifies the number of times to recurse while formattingobject
. This is useful for inspecting large objects. To recurse up to the maximum call stack size passInfinity
ornull
. Default:2
.colors
<boolean> Iftrue
, the output is styled with ANSI color codes. Colors are customizable. See Customizingutil.inspect
colors. Default:false
.customInspect
<boolean> Iffalse
,[util.inspect.custom](depth, opts, inspect)
functions are not invoked. Default:true
.showProxy
<boolean> Iftrue
,Proxy
inspection includes thetarget
andhandler
objects. Default:false
.maxArrayLength
<integer> Specifies the maximum number ofArray
,TypedArray
,Map
,Set
,WeakMap
, andWeakSet
elements to include when formatting. Set tonull
orInfinity
to show all elements. Set to0
or negative to show no elements. Default:100
.maxStringLength
<integer> Specifies the maximum number of characters to include when formatting. Set tonull
orInfinity
to show all elements. Set to0
or negative to show no characters. Default:10000
.breakLength
<integer> The length at which input values are split across multiple lines. Set toInfinity
to format the input as a single line (in combination withcompact
set totrue
or any number >=1
). Default:80
.compact
<boolean> | <integer> Setting this tofalse
causes each object key to be displayed on a new line. It will break on new lines in text that is longer thanbreakLength
. If set to a number, the mostn
inner elements are united on a single line as long as all properties fit intobreakLength
. Short array elements are also grouped together. For more information, see the example below. Default:3
.sorted
<boolean> | <Function> If set totrue
or a function, all properties of an object, andSet
andMap
entries are sorted in the resulting string. If set totrue
the default sort is used. If set to a function, it is used as a compare function.getters
<boolean> | <string> If set totrue
, getters are inspected. If set to'get'
, only getters without a corresponding setter are inspected. If set to'set'
, only getters with a corresponding setter are inspected. This might cause side effects depending on the getter function. Default:false
.numericSeparator
<boolean> If set totrue
, an underscore is used to separate every three digits in all bigints and numbers. Default:false
.
- Returns: <string> The representation of
object
.
The util.inspect()
method returns a string representation of object
that is
intended for debugging. The output of util.inspect
may change at any time
and should not be depended upon programmatically. Additional options
may be
passed that alter the result.
util.inspect()
will use the constructor's name and/or @@toStringTag
to make
an identifiable tag for an inspected value.
class Foo {
get [Symbol.toStringTag]() {
return 'bar';
}
}
class Bar {}
const baz = Object.create(null, { [Symbol.toStringTag]: { value: 'foo' } });
util.inspect(new Foo()); // 'Foo [bar] {}'
util.inspect(new Bar()); // 'Bar {}'
util.inspect(baz); // '[foo] {}'
Circular references point to their anchor by using a reference index:
const { inspect } = require('node:util');
const obj = {};
obj.a = [obj];
obj.b = {};
obj.b.inner = obj.b;
obj.b.obj = obj;
console.log(inspect(obj));
// <ref *1> {
// a: [ [Circular *1] ],
// b: <ref *2> { inner: [Circular *2], obj: [Circular *1] }
// }
The following example inspects all properties of the util
object:
const util = require('node:util');
console.log(util.inspect(util, { showHidden: true, depth: null }));
The following example highlights the effect of the compact
option:
const util = require('node:util');
const o = {
a: [1, 2, [[
'Lorem ipsum dolor sit amet,\nconsectetur adipiscing elit, sed do ' +
'eiusmod \ntempor incididunt ut labore et dolore magna aliqua.',
'test',
'foo']], 4],
b: new Map([['za', 1], ['zb', 'test']]),
};
console.log(util.inspect(o, { compact: true, depth: 5, breakLength: 80 }));
// { a:
// [ 1,
// 2,
// [ [ 'Lorem ipsum dolor sit amet,\nconsectetur [...]', // A long line
// 'test',
// 'foo' ] ],
// 4 ],
// b: Map(2) { 'za' => 1, 'zb' => 'test' } }
// Setting `compact` to false or an integer creates more reader friendly output.
console.log(util.inspect(o, { compact: false, depth: 5, breakLength: 80 }));
// {
// a: [
// 1,
// 2,
// [
// [
// 'Lorem ipsum dolor sit amet,\n' +
// 'consectetur adipiscing elit, sed do eiusmod \n' +
// 'tempor incididunt ut labore et dolore magna aliqua.',
// 'test',
// 'foo'
// ]
// ],
// 4
// ],
// b: Map(2) {
// 'za' => 1,
// 'zb' => 'test'
// }
// }
// Setting `breakLength` to e.g. 150 will print the "Lorem ipsum" text in a
// single line.
The showHidden
option allows WeakMap
and WeakSet
entries to be
inspected. If there are more entries than maxArrayLength
, there is no
guarantee which entries are displayed. That means retrieving the same
WeakSet
entries twice may result in different output. Furthermore, entries
with no remaining strong references may be garbage collected at any time.
const { inspect } = require('node:util');
const obj = { a: 1 };
const obj2 = { b: 2 };
const weakSet = new WeakSet([obj, obj2]);
console.log(inspect(weakSet, { showHidden: true }));
// WeakSet { { a: 1 }, { b: 2 } }
The sorted
option ensures that an object's property insertion order does not
impact the result of util.inspect()
.
const { inspect } = require('node:util');
const assert = require('node:assert');
const o1 = {
b: [2, 3, 1],
a: '`a` comes before `b`',
c: new Set([2, 3, 1]),
};
console.log(inspect(o1, { sorted: true }));
// { a: '`a` comes before `b`', b: [ 2, 3, 1 ], c: Set(3) { 1, 2, 3 } }
console.log(inspect(o1, { sorted: (a, b) => b.localeCompare(a) }));
// { c: Set(3) { 3, 2, 1 }, b: [ 2, 3, 1 ], a: '`a` comes before `b`' }
const o2 = {
c: new Set([2, 1, 3]),
a: '`a` comes before `b`',
b: [2, 3, 1],
};
assert.strict.equal(
inspect(o1, { sorted: true }),
inspect(o2, { sorted: true }),
);
The numericSeparator
option adds an underscore every three digits to all
numbers.
const { inspect } = require('node:util');
const thousand = 1_000;
const million = 1_000_000;
const bigNumber = 123_456_789n;
const bigDecimal = 1_234.123_45;
console.log(thousand, million, bigNumber, bigDecimal);
// 1_000 1_000_000 123_456_789n 1_234.123_45
util.inspect()
is a synchronous method intended for debugging. Its maximum
output length is approximately 128 MiB. Inputs that result in longer output will
be truncated.
Customizing util.inspect
colors#
Color output (if enabled) of util.inspect
is customizable globally
via the util.inspect.styles
and util.inspect.colors
properties.
util.inspect.styles
is a map associating a style name to a color from
util.inspect.colors
.
The default styles and associated colors are:
bigint
:yellow
boolean
:yellow
date
:magenta
module
:underline
name
: (no styling)null
:bold
number
:yellow
regexp
:red
special
:cyan
(e.g.,Proxies
)string
:green
symbol
:green
undefined
:grey
Color styling uses ANSI control codes that may not be supported on all
terminals. To verify color support use tty.hasColors()
.
Predefined control codes are listed below (grouped as "Modifiers", "Foreground colors", and "Background colors").
Modifiers#
Modifier support varies throughout different terminals. They will mostly be ignored, if not supported.
reset
- Resets all (color) modifiers to their defaults- bold - Make text bold
- italic - Make text italic
- underline - Make text underlined
strikethrough- Puts a horizontal line through the center of the text (Alias:strikeThrough
,crossedout
,crossedOut
)hidden
- Prints the text, but makes it invisible (Alias: conceal)- dim - Decreased color intensity (Alias:
faint
) - overlined - Make text overlined
- blink - Hides and shows the text in an interval
- inverse - Swap foreground and
background colors (Alias:
swapcolors
,swapColors
) - doubleunderline - Make text
double underlined (Alias:
doubleUnderline
) - framed - Draw a frame around the text
Foreground colors#
black
red
green
yellow
blue
magenta
cyan
white
gray
(alias:grey
,blackBright
)redBright
greenBright
yellowBright
blueBright
magentaBright
cyanBright
whiteBright
Background colors#
bgBlack
bgRed
bgGreen
bgYellow
bgBlue
bgMagenta
bgCyan
bgWhite
bgGray
(alias:bgGrey
,bgBlackBright
)bgRedBright
bgGreenBright
bgYellowBright
bgBlueBright
bgMagentaBright
bgCyanBright
bgWhiteBright
Custom inspection functions on objects#
Objects may also define their own
[util.inspect.custom](depth, opts, inspect)
function,
which util.inspect()
will invoke and use the result of when inspecting
the object.
const util = require('node:util');
class Box {
constructor(value) {
this.value = value;
}
[util.inspect.custom](depth, options, inspect) {
if (depth < 0) {
return options.stylize('[Box]', 'special');
}
const newOptions = Object.assign({}, options, {
depth: options.depth === null ? null : options.depth - 1,
});
// Five space padding because that's the size of "Box< ".
const padding = ' '.repeat(5);
const inner = inspect(this.value, newOptions)
.replace(/\n/g, `\n${padding}`);
return `${options.stylize('Box', 'special')}< ${inner} >`;
}
}
const box = new Box(true);
util.inspect(box);
// Returns: "Box< true >"
Custom [util.inspect.custom](depth, opts, inspect)
functions typically return
a string but may return a value of any type that will be formatted accordingly
by util.inspect()
.
const util = require('node:util');
const obj = { foo: 'this will not show up in the inspect() output' };
obj[util.inspect.custom] = (depth) => {
return { bar: 'baz' };
};
util.inspect(obj);
// Returns: "{ bar: 'baz' }"
util.inspect.custom
#
- <symbol> that can be used to declare custom inspect functions.
In addition to being accessible through util.inspect.custom
, this
symbol is registered globally and can be
accessed in any environment as Symbol.for('nodejs.util.inspect.custom')
.
Using this allows code to be written in a portable fashion, so that the custom
inspect function is used in an Node.js environment and ignored in the browser.
The util.inspect()
function itself is passed as third argument to the custom
inspect function to allow further portability.
const customInspectSymbol = Symbol.for('nodejs.util.inspect.custom');
class Password {
constructor(value) {
this.value = value;
}
toString() {
return 'xxxxxxxx';
}
[customInspectSymbol](depth, inspectOptions, inspect) {
return `Password <${this.toString()}>`;
}
}
const password = new Password('r0sebud');
console.log(password);
// Prints Password <xxxxxxxx>
See Custom inspection functions on Objects for more details.
util.inspect.defaultOptions
#
The defaultOptions
value allows customization of the default options used by
util.inspect
. This is useful for functions like console.log
or
util.format
which implicitly call into util.inspect
. It shall be set to an
object containing one or more valid util.inspect()
options. Setting
option properties directly is also supported.
const util = require('node:util');
const arr = Array(101).fill(0);
console.log(arr); // Logs the truncated array
util.inspect.defaultOptions.maxArrayLength = null;
console.log(arr); // logs the full array
util.isDeepStrictEqual(val1, val2)
#
Returns true
if there is deep strict equality between val1
and val2
.
Otherwise, returns false
.
See assert.deepStrictEqual()
for more information about deep strict
equality.
Class: util.MIMEType
#
An implementation of the MIMEType class.
In accordance with browser conventions, all properties of MIMEType
objects
are implemented as getters and setters on the class prototype, rather than as
data properties on the object itself.
A MIME string is a structured string containing multiple meaningful
components. When parsed, a MIMEType
object is returned containing
properties for each of these components.
Constructor: new MIMEType(input)
#
input
<string> The input MIME to parse
Creates a new MIMEType
object by parsing the input
.
import { MIMEType } from 'node:util';
const myMIME = new MIMEType('text/plain');
const { MIMEType } = require('node:util');
const myMIME = new MIMEType('text/plain');
A TypeError
will be thrown if the input
is not a valid MIME. Note
that an effort will be made to coerce the given values into strings. For
instance:
import { MIMEType } from 'node:util';
const myMIME = new MIMEType({ toString: () => 'text/plain' });
console.log(String(myMIME));
// Prints: text/plain
const { MIMEType } = require('node:util');
const myMIME = new MIMEType({ toString: () => 'text/plain' });
console.log(String(myMIME));
// Prints: text/plain
mime.type
#
Gets and sets the type portion of the MIME.
import { MIMEType } from 'node:util';
const myMIME = new MIMEType('text/javascript');
console.log(myMIME.type);
// Prints: text
myMIME.type = 'application';
console.log(myMIME.type);
// Prints: application
console.log(String(myMIME));
// Prints: application/javascript
const { MIMEType } = require('node:util');
const myMIME = new MIMEType('text/javascript');
console.log(myMIME.type);
// Prints: text
myMIME.type = 'application';
console.log(myMIME.type);
// Prints: application
console.log(String(myMIME));
// Prints: application/javascript
mime.subtype
#
Gets and sets the subtype portion of the MIME.
import { MIMEType } from 'node:util';
const myMIME = new MIMEType('text/ecmascript');
console.log(myMIME.subtype);
// Prints: ecmascript
myMIME.subtype = 'javascript';
console.log(myMIME.subtype);
// Prints: javascript
console.log(String(myMIME));
// Prints: text/javascript
const { MIMEType } = require('node:util');
const myMIME = new MIMEType('text/ecmascript');
console.log(myMIME.subtype);
// Prints: ecmascript
myMIME.subtype = 'javascript';
console.log(myMIME.subtype);
// Prints: javascript
console.log(String(myMIME));
// Prints: text/javascript
mime.essence
#
Gets the essence of the MIME. This property is read only.
Use mime.type
or mime.subtype
to alter the MIME.
import { MIMEType } from 'node:util';
const myMIME = new MIMEType('text/javascript;key=value');
console.log(myMIME.essence);
// Prints: text/javascript
myMIME.type = 'application';
console.log(myMIME.essence);
// Prints: application/javascript
console.log(String(myMIME));
// Prints: application/javascript;key=value
const { MIMEType } = require('node:util');
const myMIME = new MIMEType('text/javascript;key=value');
console.log(myMIME.essence);
// Prints: text/javascript
myMIME.type = 'application';
console.log(myMIME.essence);
// Prints: application/javascript
console.log(String(myMIME));
// Prints: application/javascript;key=value
mime.params
#
Gets the MIMEParams
object representing the
parameters of the MIME. This property is read-only. See
MIMEParams
documentation for details.
mime.toString()
#
- Returns: <string>
The toString()
method on the MIMEType
object returns the serialized MIME.
Because of the need for standard compliance, this method does not allow users to customize the serialization process of the MIME.
mime.toJSON()
#
- Returns: <string>
Alias for mime.toString()
.
This method is automatically called when an MIMEType
object is serialized
with JSON.stringify()
.
import { MIMEType } from 'node:util';
const myMIMES = [
new MIMEType('image/png'),
new MIMEType('image/gif'),
];
console.log(JSON.stringify(myMIMES));
// Prints: ["image/png", "image/gif"]
const { MIMEType } = require('node:util');
const myMIMES = [
new MIMEType('image/png'),
new MIMEType('image/gif'),
];
console.log(JSON.stringify(myMIMES));
// Prints: ["image/png", "image/gif"]
Class: util.MIMEParams
#
The MIMEParams
API provides read and write access to the parameters of a
MIMEType
.
Constructor: new MIMEParams()
#
Creates a new MIMEParams
object by with empty parameters
import { MIMEParams } from 'node:util';
const myParams = new MIMEParams();
const { MIMEParams } = require('node:util');
const myParams = new MIMEParams();
mimeParams.delete(name)
#
name
<string>
Remove all name-value pairs whose name is name
.
mimeParams.entries()
#
- Returns: <Iterator>
Returns an iterator over each of the name-value pairs in the parameters.
Each item of the iterator is a JavaScript Array
. The first item of the array
is the name
, the second item of the array is the value
.
mimeParams.get(name)
#
Returns the value of the first name-value pair whose name is name
. If there
are no such pairs, null
is returned.
mimeParams.has(name)
#
Returns true
if there is at least one name-value pair whose name is name
.
mimeParams.keys()
#
- Returns: <Iterator>
Returns an iterator over the names of each name-value pair.
import { MIMEType } from 'node:util';
const { params } = new MIMEType('text/plain;foo=0;bar=1');
for (const name of params.keys()) {
console.log(name);
}
// Prints:
// foo
// bar
const { MIMEType } = require('node:util');
const { params } = new MIMEType('text/plain;foo=0;bar=1');
for (const name of params.keys()) {
console.log(name);
}
// Prints:
// foo
// bar
mimeParams.set(name, value)
#
Sets the value in the MIMEParams
object associated with name
to
value
. If there are any pre-existing name-value pairs whose names are name
,
set the first such pair's value to value
.
import { MIMEType } from 'node:util';
const { params } = new MIMEType('text/plain;foo=0;bar=1');
params.set('foo', 'def');
params.set('baz', 'xyz');
console.log(params.toString());
// Prints: foo=def&bar=1&baz=xyz
const { MIMEType } = require('node:util');
const { params } = new MIMEType('text/plain;foo=0;bar=1');
params.set('foo', 'def');
params.set('baz', 'xyz');
console.log(params.toString());
// Prints: foo=def&bar=1&baz=xyz
mimeParams.values()
#
- Returns: <Iterator>
Returns an iterator over the values of each name-value pair.
mimeParams[@@iterator]()
#
- Returns: <Iterator>
Alias for mimeParams.entries()
.
import { MIMEType } from 'node:util';
const { params } = new MIMEType('text/plain;foo=bar;xyz=baz');
for (const [name, value] of params) {
console.log(name, value);
}
// Prints:
// foo bar
// xyz baz
const { MIMEType } = require('node:util');
const { params } = new MIMEType('text/plain;foo=bar;xyz=baz');
for (const [name, value] of params) {
console.log(name, value);
}
// Prints:
// foo bar
// xyz baz
util.parseArgs([config])
#
-
config
<Object> Used to provide arguments for parsing and to configure the parser.config
supports the following properties:args
<string[]> array of argument strings. Default:process.argv
withexecPath
andfilename
removed.options
<Object> Used to describe arguments known to the parser. Keys ofoptions
are the long names of options and values are an <Object> accepting the following properties:type
<string> Type of argument, which must be eitherboolean
orstring
.multiple
<boolean> Whether this option can be provided multiple times. Iftrue
, all values will be collected in an array. Iffalse
, values for the option are last-wins. Default:false
.short
<string> A single character alias for the option.default
<string> | <boolean> | <string[]> | <boolean[]> The default option value when it is not set by args. It must be of the same type as thetype
property. Whenmultiple
istrue
, it must be an array.
strict
<boolean> Should an error be thrown when unknown arguments are encountered, or when arguments are passed that do not match thetype
configured inoptions
. Default:true
.allowPositionals
<boolean> Whether this command accepts positional arguments. Default:false
ifstrict
istrue
, otherwisetrue
.tokens
<boolean> Return the parsed tokens. This is useful for extending the built-in behavior, from adding additional checks through to reprocessing the tokens in different ways. Default:false
.
-
Returns: <Object> The parsed command line arguments:
values
<Object> A mapping of parsed option names with their <string> or <boolean> values.positionals
<string[]> Positional arguments.tokens
<Object[]> | <undefined> See parseArgs tokens section. Only returned ifconfig
includestokens: true
.
Provides a higher level API for command-line argument parsing than interacting
with process.argv
directly. Takes a specification for the expected arguments
and returns a structured object with the parsed options and positionals.
import { parseArgs } from 'node:util';
const args = ['-f', '--bar', 'b'];
const options = {
foo: {
type: 'boolean',
short: 'f',
},
bar: {
type: 'string',
},
};
const {
values,
positionals,
} = parseArgs({ args, options });
console.log(values, positionals);
// Prints: [Object: null prototype] { foo: true, bar: 'b' } []
const { parseArgs } = require('node:util');
const args = ['-f', '--bar', 'b'];
const options = {
foo: {
type: 'boolean',
short: 'f',
},
bar: {
type: 'string',
},
};
const {
values,
positionals,
} = parseArgs({ args, options });
console.log(values, positionals);
// Prints: [Object: null prototype] { foo: true, bar: 'b' } []
parseArgs
tokens
#
Detailed parse information is available for adding custom behaviours by
specifying tokens: true
in the configuration.
The returned tokens have properties describing:
- all tokens
- option tokens
name
<string> Long name of option.rawName
<string> How option used in args, like-f
of--foo
.value
<string> | <undefined> Option value specified in args. Undefined for boolean options.inlineValue
<boolean> | <undefined> Whether option value specified inline, like--foo=bar
.
- positional tokens
value
<string> The value of the positional argument in args (i.e.args[index]
).
- option-terminator token
The returned tokens are in the order encountered in the input args. Options
that appear more than once in args produce a token for each use. Short option
groups like -xy
expand to a token for each option. So -xxx
produces
three tokens.
For example to use the returned tokens to add support for a negated option
like --no-color
, the tokens can be reprocessed to change the value stored
for the negated option.
import { parseArgs } from 'node:util';
const options = {
'color': { type: 'boolean' },
'no-color': { type: 'boolean' },
'logfile': { type: 'string' },
'no-logfile': { type: 'boolean' },
};
const { values, tokens } = parseArgs({ options, tokens: true });
// Reprocess the option tokens and overwrite the returned values.
tokens
.filter((token) => token.kind === 'option')
.forEach((token) => {
if (token.name.startsWith('no-')) {
// Store foo:false for --no-foo
const positiveName = token.name.slice(3);
values[positiveName] = false;
delete values[token.name];
} else {
// Resave value so last one wins if both --foo and --no-foo.
values[token.name] = token.value ?? true;
}
});
const color = values.color;
const logfile = values.logfile ?? 'default.log';
console.log({ logfile, color });
const { parseArgs } = require('node:util');
const options = {
'color': { type: 'boolean' },
'no-color': { type: 'boolean' },
'logfile': { type: 'string' },
'no-logfile': { type: 'boolean' },
};
const { values, tokens } = parseArgs({ options, tokens: true });
// Reprocess the option tokens and overwrite the returned values.
tokens
.filter((token) => token.kind === 'option')
.forEach((token) => {
if (token.name.startsWith('no-')) {
// Store foo:false for --no-foo
const positiveName = token.name.slice(3);
values[positiveName] = false;
delete values[token.name];
} else {
// Resave value so last one wins if both --foo and --no-foo.
values[token.name] = token.value ?? true;
}
});
const color = values.color;
const logfile = values.logfile ?? 'default.log';
console.log({ logfile, color });
Example usage showing negated options, and when an option is used multiple ways then last one wins.
$ node negate.js
{ logfile: 'default.log', color: undefined }
$ node negate.js --no-logfile --no-color
{ logfile: false, color: false }
$ node negate.js --logfile=test.log --color
{ logfile: 'test.log', color: true }
$ node negate.js --no-logfile --logfile=test.log --color --no-color
{ logfile: 'test.log', color: false }
util.promisify(original)
#
original
<Function>- Returns: <Function>
Takes a function following the common error-first callback style, i.e. taking
an (err, value) => ...
callback as the last argument, and returns a version
that returns promises.
const util = require('node:util');
const fs = require('node:fs');
const stat = util.promisify(fs.stat);
stat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});
Or, equivalently using async function
s:
const util = require('node:util');
const fs = require('node:fs');
const stat = util.promisify(fs.stat);
async function callStat() {
const stats = await stat('.');
console.log(`This directory is owned by ${stats.uid}`);
}
If there is an original[util.promisify.custom]
property present, promisify
will return its value, see Custom promisified functions.
promisify()
assumes that original
is a function taking a callback as its
final argument in all cases. If original
is not a function, promisify()
will throw an error. If original
is a function but its last argument is not
an error-first callback, it will still be passed an error-first
callback as its last argument.
Using promisify()
on class methods or other methods that use this
may not
work as expected unless handled specially:
const util = require('node:util');
class Foo {
constructor() {
this.a = 42;
}
bar(callback) {
callback(null, this.a);
}
}
const foo = new Foo();
const naiveBar = util.promisify(foo.bar);
// TypeError: Cannot read property 'a' of undefined
// naiveBar().then(a => console.log(a));
naiveBar.call(foo).then((a) => console.log(a)); // '42'
const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'
Custom promisified functions#
Using the util.promisify.custom
symbol one can override the return value of
util.promisify()
:
const util = require('node:util');
function doSomething(foo, callback) {
// ...
}
doSomething[util.promisify.custom] = (foo) => {
return getPromiseSomehow();
};
const promisified = util.promisify(doSomething);
console.log(promisified === doSomething[util.promisify.custom]);
// prints 'true'
This can be useful for cases where the original function does not follow the standard format of taking an error-first callback as the last argument.
For example, with a function that takes in
(foo, onSuccessCallback, onErrorCallback)
:
doSomething[util.promisify.custom] = (foo) => {
return new Promise((resolve, reject) => {
doSomething(foo, resolve, reject);
});
};
If promisify.custom
is defined but is not a function, promisify()
will
throw an error.
util.promisify.custom
#
- <symbol> that can be used to declare custom promisified variants of functions, see Custom promisified functions.
In addition to being accessible through util.promisify.custom
, this
symbol is registered globally and can be
accessed in any environment as Symbol.for('nodejs.util.promisify.custom')
.
For example, with a function that takes in
(foo, onSuccessCallback, onErrorCallback)
:
const kCustomPromisifiedSymbol = Symbol.for('nodejs.util.promisify.custom');
doSomething[kCustomPromisifiedSymbol] = (foo) => {
return new Promise((resolve, reject) => {
doSomething(foo, resolve, reject);
});
};
util.stripVTControlCharacters(str)
#
Returns str
with any ANSI escape codes removed.
console.log(util.stripVTControlCharacters('\u001B[4mvalue\u001B[0m'));
// Prints "value"
Class: util.TextDecoder
#
An implementation of the WHATWG Encoding Standard TextDecoder
API.
const decoder = new TextDecoder();
const u8arr = new Uint8Array([72, 101, 108, 108, 111]);
console.log(decoder.decode(u8arr)); // Hello
WHATWG supported encodings#
Per the WHATWG Encoding Standard, the encodings supported by the
TextDecoder
API are outlined in the tables below. For each encoding,
one or more aliases may be used.
Different Node.js build configurations support different sets of encodings. (see Internationalization)
Encodings supported by default (with full ICU data)#
Encoding | Aliases |
---|---|
'ibm866' | '866' , 'cp866' , 'csibm866' |
'iso-8859-2' | 'csisolatin2' , 'iso-ir-101' , 'iso8859-2' , 'iso88592' , 'iso_8859-2' , 'iso_8859-2:1987' , 'l2' , 'latin2' |
'iso-8859-3' | 'csisolatin3' , 'iso-ir-109' , 'iso8859-3' , 'iso88593' , 'iso_8859-3' , 'iso_8859-3:1988' , 'l3' , 'latin3' |
'iso-8859-4' | 'csisolatin4' , 'iso-ir-110' , 'iso8859-4' , 'iso88594' , 'iso_8859-4' , 'iso_8859-4:1988' , 'l4' , 'latin4' |
'iso-8859-5' | 'csisolatincyrillic' , 'cyrillic' , 'iso-ir-144' , 'iso8859-5' , 'iso88595' , 'iso_8859-5' , 'iso_8859-5:1988' |
'iso-8859-6' | 'arabic' , 'asmo-708' , 'csiso88596e' , 'csiso88596i' , 'csisolatinarabic' , 'ecma-114' , 'iso-8859-6-e' , 'iso-8859-6-i' , 'iso-ir-127' , 'iso8859-6' , 'iso88596' , 'iso_8859-6' , 'iso_8859-6:1987' |
'iso-8859-7' | 'csisolatingreek' , 'ecma-118' , 'elot_928' , 'greek' , 'greek8' , 'iso-ir-126' , 'iso8859-7' , 'iso88597' , 'iso_8859-7' , 'iso_8859-7:1987' , 'sun_eu_greek' |
'iso-8859-8' | 'csiso88598e' , 'csisolatinhebrew' , 'hebrew' , 'iso-8859-8-e' , 'iso-ir-138' , 'iso8859-8' , 'iso88598' , 'iso_8859-8' , 'iso_8859-8:1988' , 'visual' |
'iso-8859-8-i' | 'csiso88598i' , 'logical' |
'iso-8859-10' | 'csisolatin6' , 'iso-ir-157' , 'iso8859-10' , 'iso885910' , 'l6' , 'latin6' |
'iso-8859-13' | 'iso8859-13' , 'iso885913' |
'iso-8859-14' | 'iso8859-14' , 'iso885914' |
'iso-8859-15' | 'csisolatin9' , 'iso8859-15' , 'iso885915' , 'iso_8859-15' , 'l9' |
'koi8-r' | 'cskoi8r' , 'koi' , 'koi8' , 'koi8_r' |
'koi8-u' | 'koi8-ru' |
'macintosh' | 'csmacintosh' , 'mac' , 'x-mac-roman' |
'windows-874' | 'dos-874' , 'iso-8859-11' , 'iso8859-11' , 'iso885911' , 'tis-620' |
'windows-1250' | 'cp1250' , 'x-cp1250' |
'windows-1251' | 'cp1251' , 'x-cp1251' |
'windows-1252' | 'ansi_x3.4-1968' , 'ascii' , 'cp1252' , 'cp819' , 'csisolatin1' , 'ibm819' , 'iso-8859-1' , 'iso-ir-100' , 'iso8859-1' , 'iso88591' , 'iso_8859-1' , 'iso_8859-1:1987' , 'l1' , 'latin1' , 'us-ascii' , 'x-cp1252' |
'windows-1253' | 'cp1253' , 'x-cp1253' |
'windows-1254' | 'cp1254' , 'csisolatin5' , 'iso-8859-9' , 'iso-ir-148' , 'iso8859-9' , 'iso88599' , 'iso_8859-9' , 'iso_8859-9:1989' , 'l5' , 'latin5' , 'x-cp1254' |
'windows-1255' | 'cp1255' , 'x-cp1255' |
'windows-1256' | 'cp1256' , 'x-cp1256' |
'windows-1257' | 'cp1257' , 'x-cp1257' |
'windows-1258' | 'cp1258' , 'x-cp1258' |
'x-mac-cyrillic' | 'x-mac-ukrainian' |
'gbk' | 'chinese' , 'csgb2312' , 'csiso58gb231280' , 'gb2312' , 'gb_2312' , 'gb_2312-80' , 'iso-ir-58' , 'x-gbk' |
'gb18030' | |
'big5' | 'big5-hkscs' , 'cn-big5' , 'csbig5' , 'x-x-big5' |
'euc-jp' | 'cseucpkdfmtjapanese' , 'x-euc-jp' |
'iso-2022-jp' | 'csiso2022jp' |
'shift_jis' | 'csshiftjis' , 'ms932' , 'ms_kanji' , 'shift-jis' , 'sjis' , 'windows-31j' , 'x-sjis' |
'euc-kr' | 'cseuckr' , 'csksc56011987' , 'iso-ir-149' , 'korean' , 'ks_c_5601-1987' , 'ks_c_5601-1989' , 'ksc5601' , 'ksc_5601' , 'windows-949' |
Encodings supported when Node.js is built with the small-icu
option#
Encoding | Aliases |
---|---|
'utf-8' | 'unicode-1-1-utf-8' , 'utf8' |
'utf-16le' | 'utf-16' |
'utf-16be' |
Encodings supported when ICU is disabled#
Encoding | Aliases |
---|---|
'utf-8' | 'unicode-1-1-utf-8' , 'utf8' |
'utf-16le' | 'utf-16' |
The 'iso-8859-16'
encoding listed in the WHATWG Encoding Standard
is not supported.
new TextDecoder([encoding[, options]])
#
encoding
<string> Identifies theencoding
that thisTextDecoder
instance supports. Default:'utf-8'
.options
<Object>fatal
<boolean>true
if decoding failures are fatal. This option is not supported when ICU is disabled (see Internationalization). Default:false
.ignoreBOM
<boolean> Whentrue
, theTextDecoder
will include the byte order mark in the decoded result. Whenfalse
, the byte order mark will be removed from the output. This option is only used whenencoding
is'utf-8'
,'utf-16be'
, or'utf-16le'
. Default:false
.
Creates a new TextDecoder
instance. The encoding
may specify one of the
supported encodings or an alias.
The TextDecoder
class is also available on the global object.
textDecoder.decode([input[, options]])
#
input
<ArrayBuffer> | <DataView> | <TypedArray> AnArrayBuffer
,DataView
, orTypedArray
instance containing the encoded data.options
<Object>stream
<boolean>true
if additional chunks of data are expected. Default:false
.
- Returns: <string>
Decodes the input
and returns a string. If options.stream
is true
, any
incomplete byte sequences occurring at the end of the input
are buffered
internally and emitted after the next call to textDecoder.decode()
.
If textDecoder.fatal
is true
, decoding errors that occur will result in a
TypeError
being thrown.
textDecoder.encoding
#
The encoding supported by the TextDecoder
instance.
textDecoder.fatal
#
The value will be true
if decoding errors result in a TypeError
being
thrown.
textDecoder.ignoreBOM
#
The value will be true
if the decoding result will include the byte order
mark.
Class: util.TextEncoder
#
An implementation of the WHATWG Encoding Standard TextEncoder
API. All
instances of TextEncoder
only support UTF-8 encoding.
const encoder = new TextEncoder();
const uint8array = encoder.encode('this is some data');
The TextEncoder
class is also available on the global object.
textEncoder.encode([input])
#
input
<string> The text to encode. Default: an empty string.- Returns: <Uint8Array>
UTF-8 encodes the input
string and returns a Uint8Array
containing the
encoded bytes.
textEncoder.encodeInto(src, dest)
#
src
<string> The text to encode.dest
<Uint8Array> The array to hold the encode result.- Returns: <Object>
UTF-8 encodes the src
string to the dest
Uint8Array and returns an object
containing the read Unicode code units and written UTF-8 bytes.
const encoder = new TextEncoder();
const src = 'this is some data';
const dest = new Uint8Array(10);
const { read, written } = encoder.encodeInto(src, dest);
textEncoder.encoding
#
The encoding supported by the TextEncoder
instance. Always set to 'utf-8'
.
util.toUSVString(string)
#
string
<string>
Returns the string
after replacing any surrogate code points
(or equivalently, any unpaired surrogate code units) with the
Unicode "replacement character" U+FFFD.
util.transferableAbortController()
#
Creates and returns an <AbortController> instance whose <AbortSignal> is marked
as transferable and can be used with structuredClone()
or postMessage()
.
util.transferableAbortSignal(signal)
#
signal
<AbortSignal>- Returns: <AbortSignal>
Marks the given <AbortSignal> as transferable so that it can be used with
structuredClone()
and postMessage()
.
const signal = transferableAbortSignal(AbortSignal.timeout(100));
const channel = new MessageChannel();
channel.port2.postMessage(signal, [signal]);
util.aborted(signal, resource)
#
signal
<AbortSignal>resource
<Object> Any non-null entity, reference to which is held weakly.- Returns: <Promise>
Listens to abort event on the provided signal
and
returns a promise that is fulfilled when the signal
is
aborted. If the passed resource
is garbage collected before the signal
is
aborted, the returned promise shall remain pending indefinitely.
const { aborted } = require('node:util');
const dependent = obtainSomethingAbortable();
aborted(dependent.signal, dependent).then(() => {
// Do something when dependent is aborted.
});
dependent.on('event', () => {
dependent.abort();
});
import { aborted } from 'node:util';
const dependent = obtainSomethingAbortable();
aborted(dependent.signal, dependent).then(() => {
// Do something when dependent is aborted.
});
dependent.on('event', () => {
dependent.abort();
});
util.types
#
util.types
provides type checks for different kinds of built-in objects.
Unlike instanceof
or Object.prototype.toString.call(value)
, these checks do
not inspect properties of the object that are accessible from JavaScript (like
their prototype), and usually have the overhead of calling into C++.
The result generally does not make any guarantees about what kinds of properties or behavior a value exposes in JavaScript. They are primarily useful for addon developers who prefer to do type checking in JavaScript.
The API is accessible via require('node:util').types
or require('node:util/types')
.
util.types.isAnyArrayBuffer(value)
#
Returns true
if the value is a built-in ArrayBuffer
or
SharedArrayBuffer
instance.
See also util.types.isArrayBuffer()
and
util.types.isSharedArrayBuffer()
.
util.types.isAnyArrayBuffer(new ArrayBuffer()); // Returns true
util.types.isAnyArrayBuffer(new SharedArrayBuffer()); // Returns true
util.types.isArrayBufferView(value)
#
Returns true
if the value is an instance of one of the ArrayBuffer
views, such as typed array objects or DataView
. Equivalent to
ArrayBuffer.isView()
.
util.types.isArrayBufferView(new Int8Array()); // true
util.types.isArrayBufferView(Buffer.from('hello world')); // true
util.types.isArrayBufferView(new DataView(new ArrayBuffer(16))); // true
util.types.isArrayBufferView(new ArrayBuffer()); // false
util.types.isArgumentsObject(value)
#
Returns true
if the value is an arguments
object.
function foo() {
util.types.isArgumentsObject(arguments); // Returns true
}
util.types.isArrayBuffer(value)
#
Returns true
if the value is a built-in ArrayBuffer
instance.
This does not include SharedArrayBuffer
instances. Usually, it is
desirable to test for both; See util.types.isAnyArrayBuffer()
for that.
util.types.isArrayBuffer(new ArrayBuffer()); // Returns true
util.types.isArrayBuffer(new SharedArrayBuffer()); // Returns false
util.types.isAsyncFunction(value)
#
Returns true
if the value is an async function.
This only reports back what the JavaScript engine is seeing;
in particular, the return value may not match the original source code if
a transpilation tool was used.
util.types.isAsyncFunction(function foo() {}); // Returns false
util.types.isAsyncFunction(async function foo() {}); // Returns true
util.types.isBigInt64Array(value)
#
Returns true
if the value is a BigInt64Array
instance.
util.types.isBigInt64Array(new BigInt64Array()); // Returns true
util.types.isBigInt64Array(new BigUint64Array()); // Returns false
util.types.isBigUint64Array(value)
#
Returns true
if the value is a BigUint64Array
instance.
util.types.isBigUint64Array(new BigInt64Array()); // Returns false
util.types.isBigUint64Array(new BigUint64Array()); // Returns true
util.types.isBooleanObject(value)
#
Returns true
if the value is a boolean object, e.g. created
by new Boolean()
.
util.types.isBooleanObject(false); // Returns false
util.types.isBooleanObject(true); // Returns false
util.types.isBooleanObject(new Boolean(false)); // Returns true
util.types.isBooleanObject(new Boolean(true)); // Returns true
util.types.isBooleanObject(Boolean(false)); // Returns false
util.types.isBooleanObject(Boolean(true)); // Returns false
util.types.isBoxedPrimitive(value)
#
Returns true
if the value is any boxed primitive object, e.g. created
by new Boolean()
, new String()
or Object(Symbol())
.
For example:
util.types.isBoxedPrimitive(false); // Returns false
util.types.isBoxedPrimitive(new Boolean(false)); // Returns true
util.types.isBoxedPrimitive(Symbol('foo')); // Returns false
util.types.isBoxedPrimitive(Object(Symbol('foo'))); // Returns true
util.types.isBoxedPrimitive(Object(BigInt(5))); // Returns true
util.types.isCryptoKey(value)
#
Returns true
if value
is a <CryptoKey>, false
otherwise.
util.types.isDataView(value)
#
Returns true
if the value is a built-in DataView
instance.
const ab = new ArrayBuffer(20);
util.types.isDataView(new DataView(ab)); // Returns true
util.types.isDataView(new Float64Array()); // Returns false
See also ArrayBuffer.isView()
.
util.types.isDate(value)
#
Returns true
if the value is a built-in Date
instance.
util.types.isDate(new Date()); // Returns true
util.types.isExternal(value)
#
Returns true
if the value is a native External
value.
A native External
value is a special type of object that contains a
raw C++ pointer (void*
) for access from native code, and has no other
properties. Such objects are created either by Node.js internals or native
addons. In JavaScript, they are frozen objects with a
null
prototype.
#include <js_native_api.h>
#include <stdlib.h>
napi_value result;
static napi_value MyNapi(napi_env env, napi_callback_info info) {
int* raw = (int*) malloc(1024);
napi_status status = napi_create_external(env, (void*) raw, NULL, NULL, &result);
if (status != napi_ok) {
napi_throw_error(env, NULL, "napi_create_external failed");
return NULL;
}
return result;
}
...
DECLARE_NAPI_PROPERTY("myNapi", MyNapi)
...
const native = require('napi_addon.node');
const data = native.myNapi();
util.types.isExternal(data); // returns true
util.types.isExternal(0); // returns false
util.types.isExternal(new String('foo')); // returns false
For further information on napi_create_external
, refer to
napi_create_external()
.
util.types.isFloat32Array(value)
#
Returns true
if the value is a built-in Float32Array
instance.
util.types.isFloat32Array(new ArrayBuffer()); // Returns false
util.types.isFloat32Array(new Float32Array()); // Returns true
util.types.isFloat32Array(new Float64Array()); // Returns false
util.types.isFloat64Array(value)
#
Returns true
if the value is a built-in Float64Array
instance.
util.types.isFloat64Array(new ArrayBuffer()); // Returns false
util.types.isFloat64Array(new Uint8Array()); // Returns false
util.types.isFloat64Array(new Float64Array()); // Returns true
util.types.isGeneratorFunction(value)
#
Returns true
if the value is a generator function.
This only reports back what the JavaScript engine is seeing;
in particular, the return value may not match the original source code if
a transpilation tool was used.
util.types.isGeneratorFunction(function foo() {}); // Returns false
util.types.isGeneratorFunction(function* foo() {}); // Returns true
util.types.isGeneratorObject(value)
#
Returns true
if the value is a generator object as returned from a
built-in generator function.
This only reports back what the JavaScript engine is seeing;
in particular, the return value may not match the original source code if
a transpilation tool was used.
function* foo() {}
const generator = foo();
util.types.isGeneratorObject(generator); // Returns true
util.types.isInt8Array(value)
#
Returns true
if the value is a built-in Int8Array
instance.
util.types.isInt8Array(new ArrayBuffer()); // Returns false
util.types.isInt8Array(new Int8Array()); // Returns true
util.types.isInt8Array(new Float64Array()); // Returns false
util.types.isInt16Array(value)
#
Returns true
if the value is a built-in Int16Array
instance.
util.types.isInt16Array(new ArrayBuffer()); // Returns false
util.types.isInt16Array(new Int16Array()); // Returns true
util.types.isInt16Array(new Float64Array()); // Returns false
util.types.isInt32Array(value)
#
Returns true
if the value is a built-in Int32Array
instance.
util.types.isInt32Array(new ArrayBuffer()); // Returns false
util.types.isInt32Array(new Int32Array()); // Returns true
util.types.isInt32Array(new Float64Array()); // Returns false
util.types.isKeyObject(value)
#
Returns true
if value
is a <KeyObject>, false
otherwise.
util.types.isMap(value)
#
Returns true
if the value is a built-in Map
instance.
util.types.isMap(new Map()); // Returns true
util.types.isMapIterator(value)
#
Returns true
if the value is an iterator returned for a built-in
Map
instance.
const map = new Map();
util.types.isMapIterator(map.keys()); // Returns true
util.types.isMapIterator(map.values()); // Returns true
util.types.isMapIterator(map.entries()); // Returns true
util.types.isMapIterator(map[Symbol.iterator]()); // Returns true
util.types.isModuleNamespaceObject(value)
#
Returns true
if the value is an instance of a Module Namespace Object.
import * as ns from './a.js';
util.types.isModuleNamespaceObject(ns); // Returns true
util.types.isNativeError(value)
#
Returns true
if the value was returned by the constructor of a
built-in Error
type.
console.log(util.types.isNativeError(new Error())); // true
console.log(util.types.isNativeError(new TypeError())); // true
console.log(util.types.isNativeError(new RangeError())); // true
Subclasses of the native error types are also native errors:
class MyError extends Error {}
console.log(util.types.isNativeError(new MyError())); // true
A value being instanceof
a native error class is not equivalent to isNativeError()
returning true
for that value. isNativeError()
returns true
for errors
which come from a different realm while instanceof Error
returns false
for these errors:
const vm = require('node:vm');
const context = vm.createContext({});
const myError = vm.runInContext('new Error', context);
console.log(util.types.isNativeError(myError)); // true
console.log(myError instanceof Error); // false
Conversely, isNativeError()
returns false
for all objects which were not
returned by the constructor of a native error. That includes values
which are instanceof
native errors:
const myError = { __proto__: Error.prototype };
console.log(util.types.isNativeError(myError)); // false
console.log(myError instanceof Error); // true
util.types.isNumberObject(value)
#
Returns true
if the value is a number object, e.g. created
by new Number()
.
util.types.isNumberObject(0); // Returns false
util.types.isNumberObject(new Number(0)); // Returns true
util.types.isPromise(value)
#
Returns true
if the value is a built-in Promise
.
util.types.isPromise(Promise.resolve(42)); // Returns true
util.types.isProxy(value)
#
Returns true
if the value is a Proxy
instance.
const target = {};
const proxy = new Proxy(target, {});
util.types.isProxy(target); // Returns false
util.types.isProxy(proxy); // Returns true
util.types.isRegExp(value)
#
Returns true
if the value is a regular expression object.
util.types.isRegExp(/abc/); // Returns true
util.types.isRegExp(new RegExp('abc')); // Returns true
util.types.isSet(value)
#
Returns true
if the value is a built-in Set
instance.
util.types.isSet(new Set()); // Returns true
util.types.isSetIterator(value)
#
Returns true
if the value is an iterator returned for a built-in
Set
instance.
const set = new Set();
util.types.isSetIterator(set.keys()); // Returns true
util.types.isSetIterator(set.values()); // Returns true
util.types.isSetIterator(set.entries()); // Returns true
util.types.isSetIterator(set[Symbol.iterator]()); // Returns true
util.types.isSharedArrayBuffer(value)
#
Returns true
if the value is a built-in SharedArrayBuffer
instance.
This does not include ArrayBuffer
instances. Usually, it is
desirable to test for both; See util.types.isAnyArrayBuffer()
for that.
util.types.isSharedArrayBuffer(new ArrayBuffer()); // Returns false
util.types.isSharedArrayBuffer(new SharedArrayBuffer()); // Returns true
util.types.isStringObject(value)
#
Returns true
if the value is a string object, e.g. created
by new String()
.
util.types.isStringObject('foo'); // Returns false
util.types.isStringObject(new String('foo')); // Returns true
util.types.isSymbolObject(value)
#
Returns true
if the value is a symbol object, created
by calling Object()
on a Symbol
primitive.
const symbol = Symbol('foo');
util.types.isSymbolObject(symbol); // Returns false
util.types.isSymbolObject(Object(symbol)); // Returns true
util.types.isTypedArray(value)
#
Returns true
if the value is a built-in TypedArray
instance.
util.types.isTypedArray(new ArrayBuffer()); // Returns false
util.types.isTypedArray(new Uint8Array()); // Returns true
util.types.isTypedArray(new Float64Array()); // Returns true
See also ArrayBuffer.isView()
.
util.types.isUint8Array(value)
#
Returns true
if the value is a built-in Uint8Array
instance.
util.types.isUint8Array(new ArrayBuffer()); // Returns false
util.types.isUint8Array(new Uint8Array()); // Returns true
util.types.isUint8Array(new Float64Array()); // Returns false
util.types.isUint8ClampedArray(value)
#
Returns true
if the value is a built-in Uint8ClampedArray
instance.
util.types.isUint8ClampedArray(new ArrayBuffer()); // Returns false
util.types.isUint8ClampedArray(new Uint8ClampedArray()); // Returns true
util.types.isUint8ClampedArray(new Float64Array()); // Returns false
util.types.isUint16Array(value)
#
Returns true
if the value is a built-in Uint16Array
instance.
util.types.isUint16Array(new ArrayBuffer()); // Returns false
util.types.isUint16Array(new Uint16Array()); // Returns true
util.types.isUint16Array(new Float64Array()); // Returns false
util.types.isUint32Array(value)
#
Returns true
if the value is a built-in Uint32Array
instance.
util.types.isUint32Array(new ArrayBuffer()); // Returns false
util.types.isUint32Array(new Uint32Array()); // Returns true
util.types.isUint32Array(new Float64Array()); // Returns false
util.types.isWeakMap(value)
#
Returns true
if the value is a built-in WeakMap
instance.
util.types.isWeakMap(new WeakMap()); // Returns true
util.types.isWeakSet(value)
#
Returns true
if the value is a built-in WeakSet
instance.
util.types.isWeakSet(new WeakSet()); // Returns true
util.types.isWebAssemblyCompiledModule(value)
#
value instanceof WebAssembly.Module
instead.Returns true
if the value is a built-in WebAssembly.Module
instance.
const module = new WebAssembly.Module(wasmBuffer);
util.types.isWebAssemblyCompiledModule(module); // Returns true
Deprecated APIs#
The following APIs are deprecated and should no longer be used. Existing applications and modules should be updated to find alternative approaches.
util._extend(target, source)
#
Object.assign()
instead.The util._extend()
method was never intended to be used outside of internal
Node.js modules. The community found and used it anyway.
It is deprecated and should not be used in new code. JavaScript comes with very
similar built-in functionality through Object.assign()
.
util.isArray(object)
#
Array.isArray()
instead.Alias for Array.isArray()
.
Returns true
if the given object
is an Array
. Otherwise, returns false
.
const util = require('node:util');
util.isArray([]);
// Returns: true
util.isArray(new Array());
// Returns: true
util.isArray({});
// Returns: false
util.isBoolean(object)
#
typeof value === 'boolean'
instead.Returns true
if the given object
is a Boolean
. Otherwise, returns false
.
const util = require('node:util');
util.isBoolean(1);
// Returns: false
util.isBoolean(0);
// Returns: false
util.isBoolean(false);
// Returns: true
util.isBuffer(object)
#
Buffer.isBuffer()
instead.Returns true
if the given object
is a Buffer
. Otherwise, returns false
.
const util = require('node:util');
util.isBuffer({ length: 0 });
// Returns: false
util.isBuffer([]);
// Returns: false
util.isBuffer(Buffer.from('hello world'));
// Returns: true
util.isDate(object)
#
util.types.isDate()
instead.Returns true
if the given object
is a Date
. Otherwise, returns false
.
const util = require('node:util');
util.isDate(new Date());
// Returns: true
util.isDate(Date());
// false (without 'new' returns a String)
util.isDate({});
// Returns: false
util.isError(object)
#
util.types.isNativeError()
instead.Returns true
if the given object
is an Error
. Otherwise, returns
false
.
const util = require('node:util');
util.isError(new Error());
// Returns: true
util.isError(new TypeError());
// Returns: true
util.isError({ name: 'Error', message: 'an error occurred' });
// Returns: false
This method relies on Object.prototype.toString()
behavior. It is
possible to obtain an incorrect result when the object
argument manipulates
@@toStringTag
.
const util = require('node:util');
const obj = { name: 'Error', message: 'an error occurred' };
util.isError(obj);
// Returns: false
obj[Symbol.toStringTag] = 'Error';
util.isError(obj);
// Returns: true
util.isFunction(object)
#
typeof value === 'function'
instead.Returns true
if the given object
is a Function
. Otherwise, returns
false
.
const util = require('node:util');
function Foo() {}
const Bar = () => {};
util.isFunction({});
// Returns: false
util.isFunction(Foo);
// Returns: true
util.isFunction(Bar);
// Returns: true
util.isNull(object)
#
value === null
instead.Returns true
if the given object
is strictly null
. Otherwise, returns
false
.
const util = require('node:util');
util.isNull(0);
// Returns: false
util.isNull(undefined);
// Returns: false
util.isNull(null);
// Returns: true
util.isNullOrUndefined(object)
#
value === undefined || value === null
instead.Returns true
if the given object
is null
or undefined
. Otherwise,
returns false
.
const util = require('node:util');
util.isNullOrUndefined(0);
// Returns: false
util.isNullOrUndefined(undefined);
// Returns: true
util.isNullOrUndefined(null);
// Returns: true
util.isNumber(object)
#
typeof value === 'number'
instead.Returns true
if the given object
is a Number
. Otherwise, returns false
.
const util = require('node:util');
util.isNumber(false);
// Returns: false
util.isNumber(Infinity);
// Returns: true
util.isNumber(0);
// Returns: true
util.isNumber(NaN);
// Returns: true
util.isObject(object)
#
value !== null && typeof value === 'object'
instead.Returns true
if the given object
is strictly an Object
and not a
Function
(even though functions are objects in JavaScript).
Otherwise, returns false
.
const util = require('node:util');
util.isObject(5);
// Returns: false
util.isObject(null);
// Returns: false
util.isObject({});
// Returns: true
util.isObject(() => {});
// Returns: false
util.isPrimitive(object)
#
(typeof value !== 'object' && typeof value !== 'function') || value === null
instead.Returns true
if the given object
is a primitive type. Otherwise, returns
false
.
const util = require('node:util');
util.isPrimitive(5);
// Returns: true
util.isPrimitive('foo');
// Returns: true
util.isPrimitive(false);
// Returns: true
util.isPrimitive(null);
// Returns: true
util.isPrimitive(undefined);
// Returns: true
util.isPrimitive({});
// Returns: false
util.isPrimitive(() => {});
// Returns: false
util.isPrimitive(/^$/);
// Returns: false
util.isPrimitive(new Date());
// Returns: false
util.isRegExp(object)
#
Returns true
if the given object
is a RegExp
. Otherwise, returns false
.
const util = require('node:util');
util.isRegExp(/some regexp/);
// Returns: true
util.isRegExp(new RegExp('another regexp'));
// Returns: true
util.isRegExp({});
// Returns: false
util.isString(object)
#
typeof value === 'string'
instead.Returns true
if the given object
is a string
. Otherwise, returns false
.
const util = require('node:util');
util.isString('');
// Returns: true
util.isString('foo');
// Returns: true
util.isString(String('foo'));
// Returns: true
util.isString(5);
// Returns: false
util.isSymbol(object)
#
typeof value === 'symbol'
instead.Returns true
if the given object
is a Symbol
. Otherwise, returns false
.
const util = require('node:util');
util.isSymbol(5);
// Returns: false
util.isSymbol('foo');
// Returns: false
util.isSymbol(Symbol('foo'));
// Returns: true
util.isUndefined(object)
#
value === undefined
instead.Returns true
if the given object
is undefined
. Otherwise, returns false
.
const util = require('node:util');
const foo = undefined;
util.isUndefined(5);
// Returns: false
util.isUndefined(foo);
// Returns: true
util.isUndefined(null);
// Returns: false
util.log(string)
#
string
<string>
The util.log()
method prints the given string
to stdout
with an included
timestamp.
const util = require('node:util');
util.log('Timestamped message.');
V8#
Source Code: lib/v8.js
The node:v8
module exposes APIs that are specific to the version of V8
built into the Node.js binary. It can be accessed using:
const v8 = require('node:v8');
v8.cachedDataVersionTag()
#
- Returns: <integer>
Returns an integer representing a version tag derived from the V8 version,
command-line flags, and detected CPU features. This is useful for determining
whether a vm.Script
cachedData
buffer is compatible with this instance
of V8.
console.log(v8.cachedDataVersionTag()); // 3947234607
// The value returned by v8.cachedDataVersionTag() is derived from the V8
// version, command-line flags, and detected CPU features. Test that the value
// does indeed update when flags are toggled.
v8.setFlagsFromString('--allow_natives_syntax');
console.log(v8.cachedDataVersionTag()); // 183726201
v8.getHeapCodeStatistics()
#
- Returns: <Object>
Get statistics about code and its metadata in the heap, see V8
GetHeapCodeAndMetadataStatistics
API. Returns an object with the
following properties:
code_and_metadata_size
<number>bytecode_and_metadata_size
<number>external_script_source_size
<number>cpu_profiler_metadata_size
<number>
{
code_and_metadata_size: 212208,
bytecode_and_metadata_size: 161368,
external_script_source_size: 1410794,
cpu_profiler_metadata_size: 0,
}
v8.getHeapSnapshot([options])
#
-
options
<Object> -
Returns: <stream.Readable> A Readable containing the V8 heap snapshot.
Generates a snapshot of the current V8 heap and returns a Readable Stream that may be used to read the JSON serialized representation. This JSON stream format is intended to be used with tools such as Chrome DevTools. The JSON schema is undocumented and specific to the V8 engine. Therefore, the schema may change from one version of V8 to the next.
Creating a heap snapshot requires memory about twice the size of the heap at the time the snapshot is created. This results in the risk of OOM killers terminating the process.
Generating a snapshot is a synchronous operation which blocks the event loop for a duration depending on the heap size.
// Print heap snapshot to the console
const v8 = require('node:v8');
const stream = v8.getHeapSnapshot();
stream.pipe(process.stdout);
v8.getHeapSpaceStatistics()
#
- Returns: <Object[]>
Returns statistics about the V8 heap spaces, i.e. the segments which make up
the V8 heap. Neither the ordering of heap spaces, nor the availability of a
heap space can be guaranteed as the statistics are provided via the V8
GetHeapSpaceStatistics
function and may change from one V8 version to the
next.
The value returned is an array of objects containing the following properties:
space_name
<string>space_size
<number>space_used_size
<number>space_available_size
<number>physical_space_size
<number>
[
{
"space_name": "new_space",
"space_size": 2063872,
"space_used_size": 951112,
"space_available_size": 80824,
"physical_space_size": 2063872
},
{
"space_name": "old_space",
"space_size": 3090560,
"space_used_size": 2493792,
"space_available_size": 0,
"physical_space_size": 3090560
},
{
"space_name": "code_space",
"space_size": 1260160,
"space_used_size": 644256,
"space_available_size": 960,
"physical_space_size": 1260160
},
{
"space_name": "map_space",
"space_size": 1094160,
"space_used_size": 201608,
"space_available_size": 0,
"physical_space_size": 1094160
},
{
"space_name": "large_object_space",
"space_size": 0,
"space_used_size": 0,
"space_available_size": 1490980608,
"physical_space_size": 0
}
]
v8.getHeapStatistics()
#
- Returns: <Object>
Returns an object with the following properties:
total_heap_size
<number>total_heap_size_executable
<number>total_physical_size
<number>total_available_size
<number>used_heap_size
<number>heap_size_limit
<number>malloced_memory
<number>peak_malloced_memory
<number>does_zap_garbage
<number>number_of_native_contexts
<number>number_of_detached_contexts
<number>total_global_handles_size
<number>used_global_handles_size
<number>external_memory
<number>
does_zap_garbage
is a 0/1 boolean, which signifies whether the
--zap_code_space
option is enabled or not. This makes V8 overwrite heap
garbage with a bit pattern. The RSS footprint (resident set size) gets bigger
because it continuously touches all heap pages and that makes them less likely
to get swapped out by the operating system.
number_of_native_contexts
The value of native_context is the number of the
top-level contexts currently active. Increase of this number over time indicates
a memory leak.
number_of_detached_contexts
The value of detached_context is the number
of contexts that were detached and not yet garbage collected. This number
being non-zero indicates a potential memory leak.
total_global_handles_size
The value of total_global_handles_size is the
total memory size of V8 global handles.
used_global_handles_size
The value of used_global_handles_size is the
used memory size of V8 global handles.
external_memory
The value of external_memory is the memory size of array
buffers and external strings.
{
total_heap_size: 7326976,
total_heap_size_executable: 4194304,
total_physical_size: 7326976,
total_available_size: 1152656,
used_heap_size: 3476208,
heap_size_limit: 1535115264,
malloced_memory: 16384,
peak_malloced_memory: 1127496,
does_zap_garbage: 0,
number_of_native_contexts: 1,
number_of_detached_contexts: 0,
total_global_handles_size: 8192,
used_global_handles_size: 3296,
external_memory: 318824
}
v8.setFlagsFromString(flags)
#
flags
<string>
The v8.setFlagsFromString()
method can be used to programmatically set
V8 command-line flags. This method should be used with care. Changing settings
after the VM has started may result in unpredictable behavior, including
crashes and data loss; or it may simply do nothing.
The V8 options available for a version of Node.js may be determined by running
node --v8-options
.
Usage:
// Print GC events to stdout for one minute.
const v8 = require('node:v8');
v8.setFlagsFromString('--trace_gc');
setTimeout(() => { v8.setFlagsFromString('--notrace_gc'); }, 60e3);
v8.stopCoverage()
#
The v8.stopCoverage()
method allows the user to stop the coverage collection
started by NODE_V8_COVERAGE
, so that V8 can release the execution count
records and optimize code. This can be used in conjunction with
v8.takeCoverage()
if the user wants to collect the coverage on demand.
v8.takeCoverage()
#
The v8.takeCoverage()
method allows the user to write the coverage started by
NODE_V8_COVERAGE
to disk on demand. This method can be invoked multiple
times during the lifetime of the process. Each time the execution counter will
be reset and a new coverage report will be written to the directory specified
by NODE_V8_COVERAGE
.
When the process is about to exit, one last coverage will still be written to
disk unless v8.stopCoverage()
is invoked before the process exits.
v8.writeHeapSnapshot([filename[,options]])
#
filename
<string> The file path where the V8 heap snapshot is to be saved. If not specified, a file name with the pattern'Heap-${yyyymmdd}-${hhmmss}-${pid}-${thread_id}.heapsnapshot'
will be generated, where{pid}
will be the PID of the Node.js process,{thread_id}
will be0
whenwriteHeapSnapshot()
is called from the main Node.js thread or the id of a worker thread.options
<Object>- Returns: <string> The filename where the snapshot was saved.
Generates a snapshot of the current V8 heap and writes it to a JSON file. This file is intended to be used with tools such as Chrome DevTools. The JSON schema is undocumented and specific to the V8 engine, and may change from one version of V8 to the next.
A heap snapshot is specific to a single V8 isolate. When using worker threads, a heap snapshot generated from the main thread will not contain any information about the workers, and vice versa.
Creating a heap snapshot requires memory about twice the size of the heap at the time the snapshot is created. This results in the risk of OOM killers terminating the process.
Generating a snapshot is a synchronous operation which blocks the event loop for a duration depending on the heap size.
const { writeHeapSnapshot } = require('node:v8');
const {
Worker,
isMainThread,
parentPort,
} = require('node:worker_threads');
if (isMainThread) {
const worker = new Worker(__filename);
worker.once('message', (filename) => {
console.log(`worker heapdump: ${filename}`);
// Now get a heapdump for the main thread.
console.log(`main thread heapdump: ${writeHeapSnapshot()}`);
});
// Tell the worker to create a heapdump.
worker.postMessage('heapdump');
} else {
parentPort.once('message', (message) => {
if (message === 'heapdump') {
// Generate a heapdump for the worker
// and return the filename to the parent.
parentPort.postMessage(writeHeapSnapshot());
}
});
}
v8.setHeapSnapshotNearHeapLimit(limit)
#
limit
<integer>
The API is a no-op if --heapsnapshot-near-heap-limit
is already set from the
command line or the API is called more than once. limit
must be a positive
integer. See --heapsnapshot-near-heap-limit
for more information.
Serialization API#
The serialization API provides means of serializing JavaScript values in a way that is compatible with the HTML structured clone algorithm.
The format is backward-compatible (i.e. safe to store to disk). Equal JavaScript values may result in different serialized output.
v8.serialize(value)
#
Uses a DefaultSerializer
to serialize value
into a buffer.
ERR_BUFFER_TOO_LARGE
will be thrown when trying to
serialize a huge object which requires buffer
larger than buffer.constants.MAX_LENGTH
.
v8.deserialize(buffer)
#
buffer
<Buffer> | <TypedArray> | <DataView> A buffer returned byserialize()
.
Uses a DefaultDeserializer
with default options to read a JS value
from a buffer.
Class: v8.Serializer
#
new Serializer()
#
Creates a new Serializer
object.
serializer.writeHeader()
#
Writes out a header, which includes the serialization format version.
serializer.writeValue(value)
#
value
<any>
Serializes a JavaScript value and adds the serialized representation to the internal buffer.
This throws an error if value
cannot be serialized.
serializer.releaseBuffer()
#
- Returns: <Buffer>
Returns the stored internal buffer. This serializer should not be used once the buffer is released. Calling this method results in undefined behavior if a previous write has failed.
serializer.transferArrayBuffer(id, arrayBuffer)
#
id
<integer> A 32-bit unsigned integer.arrayBuffer
<ArrayBuffer> AnArrayBuffer
instance.
Marks an ArrayBuffer
as having its contents transferred out of band.
Pass the corresponding ArrayBuffer
in the deserializing context to
deserializer.transferArrayBuffer()
.
serializer.writeUint32(value)
#
value
<integer>
Write a raw 32-bit unsigned integer.
For use inside of a custom serializer._writeHostObject()
.
serializer.writeUint64(hi, lo)
#
Write a raw 64-bit unsigned integer, split into high and low 32-bit parts.
For use inside of a custom serializer._writeHostObject()
.
serializer.writeDouble(value)
#
value
<number>
Write a JS number
value.
For use inside of a custom serializer._writeHostObject()
.
serializer.writeRawBytes(buffer)
#
buffer
<Buffer> | <TypedArray> | <DataView>
Write raw bytes into the serializer's internal buffer. The deserializer
will require a way to compute the length of the buffer.
For use inside of a custom serializer._writeHostObject()
.
serializer._writeHostObject(object)
#
object
<Object>
This method is called to write some kind of host object, i.e. an object created
by native C++ bindings. If it is not possible to serialize object
, a suitable
exception should be thrown.
This method is not present on the Serializer
class itself but can be provided
by subclasses.
serializer._getDataCloneError(message)
#
message
<string>
This method is called to generate error objects that will be thrown when an object can not be cloned.
This method defaults to the Error
constructor and can be overridden on
subclasses.
serializer._getSharedArrayBufferId(sharedArrayBuffer)
#
sharedArrayBuffer
<SharedArrayBuffer>
This method is called when the serializer is going to serialize a
SharedArrayBuffer
object. It must return an unsigned 32-bit integer ID for
the object, using the same ID if this SharedArrayBuffer
has already been
serialized. When deserializing, this ID will be passed to
deserializer.transferArrayBuffer()
.
If the object cannot be serialized, an exception should be thrown.
This method is not present on the Serializer
class itself but can be provided
by subclasses.
serializer._setTreatArrayBufferViewsAsHostObjects(flag)
#
flag
<boolean> Default:false
Indicate whether to treat TypedArray
and DataView
objects as
host objects, i.e. pass them to serializer._writeHostObject()
.
Class: v8.Deserializer
#
new Deserializer(buffer)
#
buffer
<Buffer> | <TypedArray> | <DataView> A buffer returned byserializer.releaseBuffer()
.
Creates a new Deserializer
object.
deserializer.readHeader()
#
Reads and validates a header (including the format version).
May, for example, reject an invalid or unsupported wire format. In that case,
an Error
is thrown.
deserializer.readValue()
#
Deserializes a JavaScript value from the buffer and returns it.
deserializer.transferArrayBuffer(id, arrayBuffer)
#
id
<integer> A 32-bit unsigned integer.arrayBuffer
<ArrayBuffer> | <SharedArrayBuffer> AnArrayBuffer
instance.
Marks an ArrayBuffer
as having its contents transferred out of band.
Pass the corresponding ArrayBuffer
in the serializing context to
serializer.transferArrayBuffer()
(or return the id
from
serializer._getSharedArrayBufferId()
in the case of SharedArrayBuffer
s).
deserializer.getWireFormatVersion()
#
- Returns: <integer>
Reads the underlying wire format version. Likely mostly to be useful to
legacy code reading old wire format versions. May not be called before
.readHeader()
.
deserializer.readUint32()
#
- Returns: <integer>
Read a raw 32-bit unsigned integer and return it.
For use inside of a custom deserializer._readHostObject()
.
deserializer.readUint64()
#
- Returns: <integer[]>
Read a raw 64-bit unsigned integer and return it as an array [hi, lo]
with two 32-bit unsigned integer entries.
For use inside of a custom deserializer._readHostObject()
.
deserializer.readDouble()
#
- Returns: <number>
Read a JS number
value.
For use inside of a custom deserializer._readHostObject()
.
deserializer.readRawBytes(length)
#
Read raw bytes from the deserializer's internal buffer. The length
parameter
must correspond to the length of the buffer that was passed to
serializer.writeRawBytes()
.
For use inside of a custom deserializer._readHostObject()
.
deserializer._readHostObject()
#
This method is called to read some kind of host object, i.e. an object that is created by native C++ bindings. If it is not possible to deserialize the data, a suitable exception should be thrown.
This method is not present on the Deserializer
class itself but can be
provided by subclasses.
Class: v8.DefaultSerializer
#
A subclass of Serializer
that serializes TypedArray
(in particular Buffer
) and DataView
objects as host objects, and only
stores the part of their underlying ArrayBuffer
s that they are referring to.
Class: v8.DefaultDeserializer
#
A subclass of Deserializer
corresponding to the format written by
DefaultSerializer
.
Promise hooks#
The promiseHooks
interface can be used to track promise lifecycle events.
To track all async activity, see async_hooks
which internally uses this
module to produce promise lifecycle events in addition to events for other
async resources. For request context management, see AsyncLocalStorage
.
import { promiseHooks } from 'node:v8';
// There are four lifecycle events produced by promises:
// The `init` event represents the creation of a promise. This could be a
// direct creation such as with `new Promise(...)` or a continuation such
// as `then()` or `catch()`. It also happens whenever an async function is
// called or does an `await`. If a continuation promise is created, the
// `parent` will be the promise it is a continuation from.
function init(promise, parent) {
console.log('a promise was created', { promise, parent });
}
// The `settled` event happens when a promise receives a resolution or
// rejection value. This may happen synchronously such as when using
// `Promise.resolve()` on non-promise input.
function settled(promise) {
console.log('a promise resolved or rejected', { promise });
}
// The `before` event runs immediately before a `then()` or `catch()` handler
// runs or an `await` resumes execution.
function before(promise) {
console.log('a promise is about to call a then handler', { promise });
}
// The `after` event runs immediately after a `then()` handler runs or when
// an `await` begins after resuming from another.
function after(promise) {
console.log('a promise is done calling a then handler', { promise });
}
// Lifecycle hooks may be started and stopped individually
const stopWatchingInits = promiseHooks.onInit(init);
const stopWatchingSettleds = promiseHooks.onSettled(settled);
const stopWatchingBefores = promiseHooks.onBefore(before);
const stopWatchingAfters = promiseHooks.onAfter(after);
// Or they may be started and stopped in groups
const stopHookSet = promiseHooks.createHook({
init,
settled,
before,
after,
});
// To stop a hook, call the function returned at its creation.
stopWatchingInits();
stopWatchingSettleds();
stopWatchingBefores();
stopWatchingAfters();
stopHookSet();
promiseHooks.onInit(init)
#
init
<Function> Theinit
callback to call when a promise is created.- Returns: <Function> Call to stop the hook.
The init
hook must be a plain function. Providing an async function will
throw as it would produce an infinite microtask loop.
import { promiseHooks } from 'node:v8';
const stop = promiseHooks.onInit((promise, parent) => {});
const { promiseHooks } = require('node:v8');
const stop = promiseHooks.onInit((promise, parent) => {});
promiseHooks.onSettled(settled)
#
settled
<Function> Thesettled
callback to call when a promise is resolved or rejected.- Returns: <Function> Call to stop the hook.
The settled
hook must be a plain function. Providing an async function will
throw as it would produce an infinite microtask loop.
import { promiseHooks } from 'node:v8';
const stop = promiseHooks.onSettled((promise) => {});
const { promiseHooks } = require('node:v8');
const stop = promiseHooks.onSettled((promise) => {});
promiseHooks.onBefore(before)
#
before
<Function> Thebefore
callback to call before a promise continuation executes.- Returns: <Function> Call to stop the hook.
The before
hook must be a plain function. Providing an async function will
throw as it would produce an infinite microtask loop.
import { promiseHooks } from 'node:v8';
const stop = promiseHooks.onBefore((promise) => {});
const { promiseHooks } = require('node:v8');
const stop = promiseHooks.onBefore((promise) => {});
promiseHooks.onAfter(after)
#
after
<Function> Theafter
callback to call after a promise continuation executes.- Returns: <Function> Call to stop the hook.
The after
hook must be a plain function. Providing an async function will
throw as it would produce an infinite microtask loop.
import { promiseHooks } from 'node:v8';
const stop = promiseHooks.onAfter((promise) => {});
const { promiseHooks } = require('node:v8');
const stop = promiseHooks.onAfter((promise) => {});
promiseHooks.createHook(callbacks)
#
callbacks
<Object> The Hook Callbacks to registerinit
<Function> Theinit
callback.before
<Function> Thebefore
callback.after
<Function> Theafter
callback.settled
<Function> Thesettled
callback.
- Returns: <Function> Used for disabling hooks
The hook callbacks must be plain functions. Providing async functions will throw as it would produce an infinite microtask loop.
Registers functions to be called for different lifetime events of each promise.
The callbacks init()
/before()
/after()
/settled()
are called for the
respective events during a promise's lifetime.
All callbacks are optional. For example, if only promise creation needs to
be tracked, then only the init
callback needs to be passed. The
specifics of all functions that can be passed to callbacks
is in the
Hook Callbacks section.
import { promiseHooks } from 'node:v8';
const stopAll = promiseHooks.createHook({
init(promise, parent) {},
});
const { promiseHooks } = require('node:v8');
const stopAll = promiseHooks.createHook({
init(promise, parent) {},
});
Hook callbacks#
Key events in the lifetime of a promise have been categorized into four areas: creation of a promise, before/after a continuation handler is called or around an await, and when the promise resolves or rejects.
While these hooks are similar to those of async_hooks
they lack a
destroy
hook. Other types of async resources typically represent sockets or
file descriptors which have a distinct "closed" state to express the destroy
lifecycle event while promises remain usable for as long as code can still
reach them. Garbage collection tracking is used to make promises fit into the
async_hooks
event model, however this tracking is very expensive and they may
not necessarily ever even be garbage collected.
Because promises are asynchronous resources whose lifecycle is tracked
via the promise hooks mechanism, the init()
, before()
, after()
, and
settled()
callbacks must not be async functions as they create more
promises which would produce an infinite loop.
While this API is used to feed promise events into async_hooks
, the
ordering between the two is undefined. Both APIs are multi-tenant
and therefore could produce events in any order relative to each other.
init(promise, parent)
#
promise
<Promise> The promise being created.parent
<Promise> The promise continued from, if applicable.
Called when a promise is constructed. This does not mean that corresponding
before
/after
events will occur, only that the possibility exists. This will
happen if a promise is created without ever getting a continuation.
before(promise)
#
promise
<Promise>
Called before a promise continuation executes. This can be in the form of
then()
, catch()
, or finally()
handlers or an await
resuming.
The before
callback will be called 0 to N times. The before
callback
will typically be called 0 times if no continuation was ever made for the
promise. The before
callback may be called many times in the case where
many continuations have been made from the same promise.
after(promise)
#
promise
<Promise>
Called immediately after a promise continuation executes. This may be after a
then()
, catch()
, or finally()
handler or before an await
after another
await
.
settled(promise)
#
promise
<Promise>
Called when the promise receives a resolution or rejection value. This may
occur synchronously in the case of Promise.resolve()
or Promise.reject()
.
Startup Snapshot API#
The v8.startupSnapshot
interface can be used to add serialization and
deserialization hooks for custom startup snapshots. Currently the startup
snapshots can only be built into the Node.js binary from source.
$ cd /path/to/node
$ ./configure --node-snapshot-main=entry.js
$ make node
# This binary contains the result of the execution of entry.js
$ out/Release/node
In the example above, entry.js
can use methods from the v8.startupSnapshot
interface to specify how to save information for custom objects in the snapshot
during serialization and how the information can be used to synchronize these
objects during deserialization of the snapshot. For example, if the entry.js
contains the following script:
'use strict';
const fs = require('node:fs');
const zlib = require('node:zlib');
const path = require('node:path');
const assert = require('node:assert');
const {
isBuildingSnapshot,
addSerializeCallback,
addDeserializeCallback,
setDeserializeMainFunction,
} = require('node:v8').startupSnapshot;
const filePath = path.resolve(__dirname, '../x1024.txt');
const storage = {};
assert(isBuildingSnapshot());
addSerializeCallback(({ filePath }) => {
storage[filePath] = zlib.gzipSync(fs.readFileSync(filePath));
}, { filePath });
addDeserializeCallback(({ filePath }) => {
storage[filePath] = zlib.gunzipSync(storage[filePath]);
}, { filePath });
setDeserializeMainFunction(({ filePath }) => {
console.log(storage[filePath].toString());
}, { filePath });
The resulted binary will simply print the data deserialized from the snapshot during start up:
$ out/Release/node
# Prints content of ./test/fixtures/x1024.txt
Currently the API is only available to a Node.js instance launched from the default snapshot, that is, the application deserialized from a user-land snapshot cannot use these APIs again.
v8.startupSnapshot.addSerializeCallback(callback[, data])
#
callback
<Function> Callback to be invoked before serialization.data
<any> Optional data that will be passed to thecallback
when it gets called.
Add a callback that will be called when the Node.js instance is about to get serialized into a snapshot and exit. This can be used to release resources that should not or cannot be serialized or to convert user data into a form more suitable for serialization.
v8.startupSnapshot.addDeserializeCallback(callback[, data])
#
callback
<Function> Callback to be invoked after the snapshot is deserialized.data
<any> Optional data that will be passed to thecallback
when it gets called.
Add a callback that will be called when the Node.js instance is deserialized
from a snapshot. The callback
and the data
(if provided) will be
serialized into the snapshot, they can be used to re-initialize the state
of the application or to re-acquire resources that the application needs
when the application is restarted from the snapshot.
v8.startupSnapshot.setDeserializeMainFunction(callback[, data])
#
callback
<Function> Callback to be invoked as the entry point after the snapshot is deserialized.data
<any> Optional data that will be passed to thecallback
when it gets called.
This sets the entry point of the Node.js application when it is deserialized from a snapshot. This can be called only once in the snapshot building script. If called, the deserialized application no longer needs an additional entry point script to start up and will simply invoke the callback along with the deserialized data (if provided), otherwise an entry point script still needs to be provided to the deserialized application.
v8.startupSnapshot.isBuildingSnapshot()
#
- Returns: <boolean>
Returns true if the Node.js instance is run to build a snapshot.
Class: v8.GCProfiler
#
This API collects GC data in current thread.
new v8.GCProfiler()
#
Create a new instance of the v8.GCProfiler
class.
profiler.start()
#
Start collecting GC data.
profiler.stop()
#
Stop collecting GC data and return an object.The content of object is as follows.
{
"version": 1,
"startTime": 1674059033862,
"statistics": [
{
"gcType": "Scavenge",
"beforeGC": {
"heapStatistics": {
"totalHeapSize": 5005312,
"totalHeapSizeExecutable": 524288,
"totalPhysicalSize": 5226496,
"totalAvailableSize": 4341325216,
"totalGlobalHandlesSize": 8192,
"usedGlobalHandlesSize": 2112,
"usedHeapSize": 4883840,
"heapSizeLimit": 4345298944,
"mallocedMemory": 254128,
"externalMemory": 225138,
"peakMallocedMemory": 181760
},
"heapSpaceStatistics": [
{
"spaceName": "read_only_space",
"spaceSize": 0,
"spaceUsedSize": 0,
"spaceAvailableSize": 0,
"physicalSpaceSize": 0
}
]
},
"cost": 1574.14,
"afterGC": {
"heapStatistics": {
"totalHeapSize": 6053888,
"totalHeapSizeExecutable": 524288,
"totalPhysicalSize": 5500928,
"totalAvailableSize": 4341101384,
"totalGlobalHandlesSize": 8192,
"usedGlobalHandlesSize": 2112,
"usedHeapSize": 4059096,
"heapSizeLimit": 4345298944,
"mallocedMemory": 254128,
"externalMemory": 225138,
"peakMallocedMemory": 181760
},
"heapSpaceStatistics": [
{
"spaceName": "read_only_space",
"spaceSize": 0,
"spaceUsedSize": 0,
"spaceAvailableSize": 0,
"physicalSpaceSize": 0
}
]
}
}
],
"endTime": 1674059036865
}
Here's an example.
const { GCProfiler } = require('v8');
const profiler = new GCProfiler();
profiler.start();
setTimeout(() => {
console.log(profiler.stop());
}, 1000);
VM (executing JavaScript)#
Source Code: lib/vm.js
The node:vm
module enables compiling and running code within V8 Virtual
Machine contexts.
The node:vm
module is not a security
mechanism. Do not use it to run untrusted code.
JavaScript code can be compiled and run immediately or compiled, saved, and run later.
A common use case is to run the code in a different V8 Context. This means invoked code has a different global object than the invoking code.
One can provide the context by contextifying an object. The invoked code treats any property in the context like a global variable. Any changes to global variables caused by the invoked code are reflected in the context object.
const vm = require('node:vm');
const x = 1;
const context = { x: 2 };
vm.createContext(context); // Contextify the object.
const code = 'x += 40; var y = 17;';
// `x` and `y` are global variables in the context.
// Initially, x has the value 2 because that is the value of context.x.
vm.runInContext(code, context);
console.log(context.x); // 42
console.log(context.y); // 17
console.log(x); // 1; y is not defined.
Class: vm.Script
#
Instances of the vm.Script
class contain precompiled scripts that can be
executed in specific contexts.
new vm.Script(code[, options])
#
code
<string> The JavaScript code to compile.options
<Object> | <string>filename
<string> Specifies the filename used in stack traces produced by this script. Default:'evalmachine.<anonymous>'
.lineOffset
<number> Specifies the line number offset that is displayed in stack traces produced by this script. Default:0
.columnOffset
<number> Specifies the first-line column number offset that is displayed in stack traces produced by this script. Default:0
.cachedData
<Buffer> | <TypedArray> | <DataView> Provides an optionalBuffer
orTypedArray
, orDataView
with V8's code cache data for the supplied source. When supplied, thecachedDataRejected
value will be set to eithertrue
orfalse
depending on acceptance of the data by V8.produceCachedData
<boolean> Whentrue
and nocachedData
is present, V8 will attempt to produce code cache data forcode
. Upon success, aBuffer
with V8's code cache data will be produced and stored in thecachedData
property of the returnedvm.Script
instance. ThecachedDataProduced
value will be set to eithertrue
orfalse
depending on whether code cache data is produced successfully. This option is deprecated in favor ofscript.createCachedData()
. Default:false
.importModuleDynamically
<Function> Called during evaluation of this module whenimport()
is called. If this option is not specified, calls toimport()
will reject withERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING
. This option is part of the experimental modules API. We do not recommend using it in a production environment.specifier
<string> specifier passed toimport()
script
<vm.Script>importAssertions
<Object> The"assert"
value passed to theoptionsExpression
optional parameter, or an empty object if no value was provided.- Returns: <Module Namespace Object> | <vm.Module> Returning a
vm.Module
is recommended in order to take advantage of error tracking, and to avoid issues with namespaces that containthen
function exports.
If options
is a string, then it specifies the filename.
Creating a new vm.Script
object compiles code
but does not run it. The
compiled vm.Script
can be run later multiple times. The code
is not bound to
any global object; rather, it is bound before each run, just for that run.
script.cachedDataRejected
#
When cachedData
is supplied to create the vm.Script
, this value will be set
to either true
or false
depending on acceptance of the data by V8.
Otherwise the value is undefined
.
script.createCachedData()
#
- Returns: <Buffer>
Creates a code cache that can be used with the Script
constructor's
cachedData
option. Returns a Buffer
. This method may be called at any
time and any number of times.
The code cache of the Script
doesn't contain any JavaScript observable
states. The code cache is safe to be saved along side the script source and
used to construct new Script
instances multiple times.
Functions in the Script
source can be marked as lazily compiled and they are
not compiled at construction of the Script
. These functions are going to be
compiled when they are invoked the first time. The code cache serializes the
metadata that V8 currently knows about the Script
that it can use to speed up
future compilations.
const script = new vm.Script(`
function add(a, b) {
return a + b;
}
const x = add(1, 2);
`);
const cacheWithoutAdd = script.createCachedData();
// In `cacheWithoutAdd` the function `add()` is marked for full compilation
// upon invocation.
script.runInThisContext();
const cacheWithAdd = script.createCachedData();
// `cacheWithAdd` contains fully compiled function `add()`.
script.runInContext(contextifiedObject[, options])
#
contextifiedObject
<Object> A contextified object as returned by thevm.createContext()
method.options
<Object>displayErrors
<boolean> Whentrue
, if anError
occurs while compiling thecode
, the line of code causing the error is attached to the stack trace. Default:true
.timeout
<integer> Specifies the number of milliseconds to executecode
before terminating execution. If execution is terminated, anError
will be thrown. This value must be a strictly positive integer.breakOnSigint
<boolean> Iftrue
, receivingSIGINT
(Ctrl+C) will terminate execution and throw anError
. Existing handlers for the event that have been attached viaprocess.on('SIGINT')
are disabled during script execution, but continue to work after that. Default:false
.
- Returns: <any> the result of the very last statement executed in the script.
Runs the compiled code contained by the vm.Script
object within the given
contextifiedObject
and returns the result. Running code does not have access
to local scope.
The following example compiles code that increments a global variable, sets
the value of another global variable, then execute the code multiple times.
The globals are contained in the context
object.
const vm = require('node:vm');
const context = {
animal: 'cat',
count: 2,
};
const script = new vm.Script('count += 1; name = "kitty";');
vm.createContext(context);
for (let i = 0; i < 10; ++i) {
script.runInContext(context);
}
console.log(context);
// Prints: { animal: 'cat', count: 12, name: 'kitty' }
Using the timeout
or breakOnSigint
options will result in new event loops
and corresponding threads being started, which have a non-zero performance
overhead.
script.runInNewContext([contextObject[, options]])
#
contextObject
<Object> An object that will be contextified. Ifundefined
, a new object will be created.options
<Object>displayErrors
<boolean> Whentrue
, if anError
occurs while compiling thecode
, the line of code causing the error is attached to the stack trace. Default:true
.timeout
<integer> Specifies the number of milliseconds to executecode
before terminating execution. If execution is terminated, anError
will be thrown. This value must be a strictly positive integer.breakOnSigint
<boolean> Iftrue
, receivingSIGINT
(Ctrl+C) will terminate execution and throw anError
. Existing handlers for the event that have been attached viaprocess.on('SIGINT')
are disabled during script execution, but continue to work after that. Default:false
.contextName
<string> Human-readable name of the newly created context. Default:'VM Context i'
, wherei
is an ascending numerical index of the created context.contextOrigin
<string> Origin corresponding to the newly created context for display purposes. The origin should be formatted like a URL, but with only the scheme, host, and port (if necessary), like the value of theurl.origin
property of aURL
object. Most notably, this string should omit the trailing slash, as that denotes a path. Default:''
.contextCodeGeneration
<Object>microtaskMode
<string> If set toafterEvaluate
, microtasks (tasks scheduled throughPromise
s andasync function
s) will be run immediately after the script has run. They are included in thetimeout
andbreakOnSigint
scopes in that case.
- Returns: <any> the result of the very last statement executed in the script.
First contextifies the given contextObject
, runs the compiled code contained
by the vm.Script
object within the created context, and returns the result.
Running code does not have access to local scope.
The following example compiles code that sets a global variable, then executes
the code multiple times in different contexts. The globals are set on and
contained within each individual context
.
const vm = require('node:vm');
const script = new vm.Script('globalVar = "set"');
const contexts = [{}, {}, {}];
contexts.forEach((context) => {
script.runInNewContext(context);
});
console.log(contexts);
// Prints: [{ globalVar: 'set' }, { globalVar: 'set' }, { globalVar: 'set' }]
script.runInThisContext([options])
#
options
<Object>displayErrors
<boolean> Whentrue
, if anError
occurs while compiling thecode
, the line of code causing the error is attached to the stack trace. Default:true
.timeout
<integer> Specifies the number of milliseconds to executecode
before terminating execution. If execution is terminated, anError
will be thrown. This value must be a strictly positive integer.breakOnSigint
<boolean> Iftrue
, receivingSIGINT
(Ctrl+C) will terminate execution and throw anError
. Existing handlers for the event that have been attached viaprocess.on('SIGINT')
are disabled during script execution, but continue to work after that. Default:false
.
- Returns: <any> the result of the very last statement executed in the script.
Runs the compiled code contained by the vm.Script
within the context of the
current global
object. Running code does not have access to local scope, but
does have access to the current global
object.
The following example compiles code that increments a global
variable then
executes that code multiple times:
const vm = require('node:vm');
global.globalVar = 0;
const script = new vm.Script('globalVar += 1', { filename: 'myfile.vm' });
for (let i = 0; i < 1000; ++i) {
script.runInThisContext();
}
console.log(globalVar);
// 1000
script.sourceMapURL
#
When the script is compiled from a source that contains a source map magic comment, this property will be set to the URL of the source map.
import vm from 'node:vm';
const script = new vm.Script(`
function myFunc() {}
//# sourceMappingURL=sourcemap.json
`);
console.log(script.sourceMapURL);
// Prints: sourcemap.json
const vm = require('node:vm');
const script = new vm.Script(`
function myFunc() {}
//# sourceMappingURL=sourcemap.json
`);
console.log(script.sourceMapURL);
// Prints: sourcemap.json
Class: vm.Module
#
This feature is only available with the --experimental-vm-modules
command
flag enabled.
The vm.Module
class provides a low-level interface for using
ECMAScript modules in VM contexts. It is the counterpart of the vm.Script
class that closely mirrors Module Records as defined in the ECMAScript
specification.
Unlike vm.Script
however, every vm.Module
object is bound to a context from
its creation. Operations on vm.Module
objects are intrinsically asynchronous,
in contrast with the synchronous nature of vm.Script
objects. The use of
'async' functions can help with manipulating vm.Module
objects.
Using a vm.Module
object requires three distinct steps: creation/parsing,
linking, and evaluation. These three steps are illustrated in the following
example.
This implementation lies at a lower level than the ECMAScript Module loader. There is also no way to interact with the Loader yet, though support is planned.
import vm from 'node:vm';
const contextifiedObject = vm.createContext({
secret: 42,
print: console.log,
});
// Step 1
//
// Create a Module by constructing a new `vm.SourceTextModule` object. This
// parses the provided source text, throwing a `SyntaxError` if anything goes
// wrong. By default, a Module is created in the top context. But here, we
// specify `contextifiedObject` as the context this Module belongs to.
//
// Here, we attempt to obtain the default export from the module "foo", and
// put it into local binding "secret".
const bar = new vm.SourceTextModule(`
import s from 'foo';
s;
print(s);
`, { context: contextifiedObject });
// Step 2
//
// "Link" the imported dependencies of this Module to it.
//
// The provided linking callback (the "linker") accepts two arguments: the
// parent module (`bar` in this case) and the string that is the specifier of
// the imported module. The callback is expected to return a Module that
// corresponds to the provided specifier, with certain requirements documented
// in `module.link()`.
//
// If linking has not started for the returned Module, the same linker
// callback will be called on the returned Module.
//
// Even top-level Modules without dependencies must be explicitly linked. The
// callback provided would never be called, however.
//
// The link() method returns a Promise that will be resolved when all the
// Promises returned by the linker resolve.
//
// Note: This is a contrived example in that the linker function creates a new
// "foo" module every time it is called. In a full-fledged module system, a
// cache would probably be used to avoid duplicated modules.
async function linker(specifier, referencingModule) {
if (specifier === 'foo') {
return new vm.SourceTextModule(`
// The "secret" variable refers to the global variable we added to
// "contextifiedObject" when creating the context.
export default secret;
`, { context: referencingModule.context });
// Using `contextifiedObject` instead of `referencingModule.context`
// here would work as well.
}
throw new Error(`Unable to resolve dependency: ${specifier}`);
}
await bar.link(linker);
// Step 3
//
// Evaluate the Module. The evaluate() method returns a promise which will
// resolve after the module has finished evaluating.
// Prints 42.
await bar.evaluate();
const vm = require('node:vm');
const contextifiedObject = vm.createContext({
secret: 42,
print: console.log,
});
(async () => {
// Step 1
//
// Create a Module by constructing a new `vm.SourceTextModule` object. This
// parses the provided source text, throwing a `SyntaxError` if anything goes
// wrong. By default, a Module is created in the top context. But here, we
// specify `contextifiedObject` as the context this Module belongs to.
//
// Here, we attempt to obtain the default export from the module "foo", and
// put it into local binding "secret".
const bar = new vm.SourceTextModule(`
import s from 'foo';
s;
print(s);
`, { context: contextifiedObject });
// Step 2
//
// "Link" the imported dependencies of this Module to it.
//
// The provided linking callback (the "linker") accepts two arguments: the
// parent module (`bar` in this case) and the string that is the specifier of
// the imported module. The callback is expected to return a Module that
// corresponds to the provided specifier, with certain requirements documented
// in `module.link()`.
//
// If linking has not started for the returned Module, the same linker
// callback will be called on the returned Module.
//
// Even top-level Modules without dependencies must be explicitly linked. The
// callback provided would never be called, however.
//
// The link() method returns a Promise that will be resolved when all the
// Promises returned by the linker resolve.
//
// Note: This is a contrived example in that the linker function creates a new
// "foo" module every time it is called. In a full-fledged module system, a
// cache would probably be used to avoid duplicated modules.
async function linker(specifier, referencingModule) {
if (specifier === 'foo') {
return new vm.SourceTextModule(`
// The "secret" variable refers to the global variable we added to
// "contextifiedObject" when creating the context.
export default secret;
`, { context: referencingModule.context });
// Using `contextifiedObject` instead of `referencingModule.context`
// here would work as well.
}
throw new Error(`Unable to resolve dependency: ${specifier}`);
}
await bar.link(linker);
// Step 3
//
// Evaluate the Module. The evaluate() method returns a promise which will
// resolve after the module has finished evaluating.
// Prints 42.
await bar.evaluate();
})();
module.dependencySpecifiers
#
The specifiers of all dependencies of this module. The returned array is frozen to disallow any changes to it.
Corresponds to the [[RequestedModules]]
field of Cyclic Module Records in
the ECMAScript specification.
module.error
#
If the module.status
is 'errored'
, this property contains the exception
thrown by the module during evaluation. If the status is anything else,
accessing this property will result in a thrown exception.
The value undefined
cannot be used for cases where there is not a thrown
exception due to possible ambiguity with throw undefined;
.
Corresponds to the [[EvaluationError]]
field of Cyclic Module Records
in the ECMAScript specification.
module.evaluate([options])
#
options
<Object>timeout
<integer> Specifies the number of milliseconds to evaluate before terminating execution. If execution is interrupted, anError
will be thrown. This value must be a strictly positive integer.breakOnSigint
<boolean> Iftrue
, receivingSIGINT
(Ctrl+C) will terminate execution and throw anError
. Existing handlers for the event that have been attached viaprocess.on('SIGINT')
are disabled during script execution, but continue to work after that. Default:false
.
- Returns: <Promise> Fulfills with
undefined
upon success.
Evaluate the module.
This must be called after the module has been linked; otherwise it will reject.
It could be called also when the module has already been evaluated, in which
case it will either do nothing if the initial evaluation ended in success
(module.status
is 'evaluated'
) or it will re-throw the exception that the
initial evaluation resulted in (module.status
is 'errored'
).
This method cannot be called while the module is being evaluated
(module.status
is 'evaluating'
).
Corresponds to the Evaluate() concrete method field of Cyclic Module Records in the ECMAScript specification.
module.identifier
#
The identifier of the current module, as set in the constructor.
module.link(linker)
#
linker
<Function>-
specifier
<string> The specifier of the requested module:import foo from 'foo'; // ^^^^^ the module specifier
-
referencingModule
<vm.Module> TheModule
objectlink()
is called on. -
extra
<Object>assert
<Object> The data from the assertion:
Per ECMA-262, hosts are expected to ignore assertions that they do not support, as opposed to, for example, triggering an error if an unsupported assertion is present.import foo from 'foo' assert { name: 'value' }; // ^^^^^^^^^^^^^^^^^ the assertion
-
Returns: <vm.Module> | <Promise>
-
- Returns: <Promise>
Link module dependencies. This method must be called before evaluation, and can only be called once per module.
The function is expected to return a Module
object or a Promise
that
eventually resolves to a Module
object. The returned Module
must satisfy the
following two invariants:
- It must belong to the same context as the parent
Module
. - Its
status
must not be'errored'
.
If the returned Module
's status
is 'unlinked'
, this method will be
recursively called on the returned Module
with the same provided linker
function.
link()
returns a Promise
that will either get resolved when all linking
instances resolve to a valid Module
, or rejected if the linker function either
throws an exception or returns an invalid Module
.
The linker function roughly corresponds to the implementation-defined HostResolveImportedModule abstract operation in the ECMAScript specification, with a few key differences:
- The linker function is allowed to be asynchronous while HostResolveImportedModule is synchronous.
The actual HostResolveImportedModule implementation used during module linking is one that returns the modules linked during linking. Since at that point all modules would have been fully linked already, the HostResolveImportedModule implementation is fully synchronous per specification.
Corresponds to the Link() concrete method field of Cyclic Module Records in the ECMAScript specification.
module.namespace
#
The namespace object of the module. This is only available after linking
(module.link()
) has completed.
Corresponds to the GetModuleNamespace abstract operation in the ECMAScript specification.
module.status
#
The current status of the module. Will be one of:
-
'unlinked'
:module.link()
has not yet been called. -
'linking'
:module.link()
has been called, but not all Promises returned by the linker function have been resolved yet. -
'linked'
: The module has been linked successfully, and all of its dependencies are linked, butmodule.evaluate()
has not yet been called. -
'evaluating'
: The module is being evaluated through amodule.evaluate()
on itself or a parent module. -
'evaluated'
: The module has been successfully evaluated. -
'errored'
: The module has been evaluated, but an exception was thrown.
Other than 'errored'
, this status string corresponds to the specification's
Cyclic Module Record's [[Status]]
field. 'errored'
corresponds to
'evaluated'
in the specification, but with [[EvaluationError]]
set to a
value that is not undefined
.
Class: vm.SourceTextModule
#
This feature is only available with the --experimental-vm-modules
command
flag enabled.
- Extends: <vm.Module>
The vm.SourceTextModule
class provides the Source Text Module Record as
defined in the ECMAScript specification.
new vm.SourceTextModule(code[, options])
#
code
<string> JavaScript Module code to parseoptions
identifier
<string> String used in stack traces. Default:'vm:module(i)'
wherei
is a context-specific ascending index.cachedData
<Buffer> | <TypedArray> | <DataView> Provides an optionalBuffer
orTypedArray
, orDataView
with V8's code cache data for the supplied source. Thecode
must be the same as the module from which thiscachedData
was created.context
<Object> The contextified object as returned by thevm.createContext()
method, to compile and evaluate thisModule
in.lineOffset
<integer> Specifies the line number offset that is displayed in stack traces produced by thisModule
. Default:0
.columnOffset
<integer> Specifies the first-line column number offset that is displayed in stack traces produced by thisModule
. Default:0
.initializeImportMeta
<Function> Called during evaluation of thisModule
to initialize theimport.meta
.meta
<import.meta>module
<vm.SourceTextModule>
importModuleDynamically
<Function> Called during evaluation of this module whenimport()
is called. If this option is not specified, calls toimport()
will reject withERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING
.specifier
<string> specifier passed toimport()
module
<vm.Module>importAssertions
<Object> The"assert"
value passed to theoptionsExpression
optional parameter, or an empty object if no value was provided.- Returns: <Module Namespace Object> | <vm.Module> Returning a
vm.Module
is recommended in order to take advantage of error tracking, and to avoid issues with namespaces that containthen
function exports.
Creates a new SourceTextModule
instance.
Properties assigned to the import.meta
object that are objects may
allow the module to access information outside the specified context
. Use
vm.runInContext()
to create objects in a specific context.
import vm from 'node:vm';
const contextifiedObject = vm.createContext({ secret: 42 });
const module = new vm.SourceTextModule(
'Object.getPrototypeOf(import.meta.prop).secret = secret;',
{
initializeImportMeta(meta) {
// Note: this object is created in the top context. As such,
// Object.getPrototypeOf(import.meta.prop) points to the
// Object.prototype in the top context rather than that in
// the contextified object.
meta.prop = {};
},
});
// Since module has no dependencies, the linker function will never be called.
await module.link(() => {});
await module.evaluate();
// Now, Object.prototype.secret will be equal to 42.
//
// To fix this problem, replace
// meta.prop = {};
// above with
// meta.prop = vm.runInContext('{}', contextifiedObject);
const vm = require('node:vm');
const contextifiedObject = vm.createContext({ secret: 42 });
(async () => {
const module = new vm.SourceTextModule(
'Object.getPrototypeOf(import.meta.prop).secret = secret;',
{
initializeImportMeta(meta) {
// Note: this object is created in the top context. As such,
// Object.getPrototypeOf(import.meta.prop) points to the
// Object.prototype in the top context rather than that in
// the contextified object.
meta.prop = {};
},
});
// Since module has no dependencies, the linker function will never be called.
await module.link(() => {});
await module.evaluate();
// Now, Object.prototype.secret will be equal to 42.
//
// To fix this problem, replace
// meta.prop = {};
// above with
// meta.prop = vm.runInContext('{}', contextifiedObject);
})();
sourceTextModule.createCachedData()
#
- Returns: <Buffer>
Creates a code cache that can be used with the SourceTextModule
constructor's
cachedData
option. Returns a Buffer
. This method may be called any number
of times before the module has been evaluated.
The code cache of the SourceTextModule
doesn't contain any JavaScript
observable states. The code cache is safe to be saved along side the script
source and used to construct new SourceTextModule
instances multiple times.
Functions in the SourceTextModule
source can be marked as lazily compiled
and they are not compiled at construction of the SourceTextModule
. These
functions are going to be compiled when they are invoked the first time. The
code cache serializes the metadata that V8 currently knows about the
SourceTextModule
that it can use to speed up future compilations.
// Create an initial module
const module = new vm.SourceTextModule('const a = 1;');
// Create cached data from this module
const cachedData = module.createCachedData();
// Create a new module using the cached data. The code must be the same.
const module2 = new vm.SourceTextModule('const a = 1;', { cachedData });
Class: vm.SyntheticModule
#
This feature is only available with the --experimental-vm-modules
command
flag enabled.
- Extends: <vm.Module>
The vm.SyntheticModule
class provides the Synthetic Module Record as
defined in the WebIDL specification. The purpose of synthetic modules is to
provide a generic interface for exposing non-JavaScript sources to ECMAScript
module graphs.
const vm = require('node:vm');
const source = '{ "a": 1 }';
const module = new vm.SyntheticModule(['default'], function() {
const obj = JSON.parse(source);
this.setExport('default', obj);
});
// Use `module` in linking...
new vm.SyntheticModule(exportNames, evaluateCallback[, options])
#
exportNames
<string[]> Array of names that will be exported from the module.evaluateCallback
<Function> Called when the module is evaluated.options
identifier
<string> String used in stack traces. Default:'vm:module(i)'
wherei
is a context-specific ascending index.context
<Object> The contextified object as returned by thevm.createContext()
method, to compile and evaluate thisModule
in.
Creates a new SyntheticModule
instance.
Objects assigned to the exports of this instance may allow importers of
the module to access information outside the specified context
. Use
vm.runInContext()
to create objects in a specific context.
syntheticModule.setExport(name, value)
#
This method is used after the module is linked to set the values of exports. If
it is called before the module is linked, an ERR_VM_MODULE_STATUS
error
will be thrown.
import vm from 'node:vm';
const m = new vm.SyntheticModule(['x'], () => {
m.setExport('x', 1);
});
await m.link(() => {});
await m.evaluate();
assert.strictEqual(m.namespace.x, 1);
const vm = require('node:vm');
(async () => {
const m = new vm.SyntheticModule(['x'], () => {
m.setExport('x', 1);
});
await m.link(() => {});
await m.evaluate();
assert.strictEqual(m.namespace.x, 1);
})();
vm.compileFunction(code[, params[, options]])
#
code
<string> The body of the function to compile.params
<string[]> An array of strings containing all parameters for the function.options
<Object>filename
<string> Specifies the filename used in stack traces produced by this script. Default:''
.lineOffset
<number> Specifies the line number offset that is displayed in stack traces produced by this script. Default:0
.columnOffset
<number> Specifies the first-line column number offset that is displayed in stack traces produced by this script. Default:0
.cachedData
<Buffer> | <TypedArray> | <DataView> Provides an optionalBuffer
orTypedArray
, orDataView
with V8's code cache data for the supplied source.produceCachedData
<boolean> Specifies whether to produce new cache data. Default:false
.parsingContext
<Object> The contextified object in which the said function should be compiled in.contextExtensions
<Object[]> An array containing a collection of context extensions (objects wrapping the current scope) to be applied while compiling. Default:[]
.importModuleDynamically
<Function> Called during evaluation of this module whenimport()
is called. If this option is not specified, calls toimport()
will reject withERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING
. This option is part of the experimental modules API, and should not be considered stable.specifier
<string> specifier passed toimport()
function
<Function>importAssertions
<Object> The"assert"
value passed to theoptionsExpression
optional parameter, or an empty object if no value was provided.- Returns: <Module Namespace Object> | <vm.Module> Returning a
vm.Module
is recommended in order to take advantage of error tracking, and to avoid issues with namespaces that containthen
function exports.
- Returns: <Function>
Compiles the given code into the provided context (if no context is
supplied, the current context is used), and returns it wrapped inside a
function with the given params
.
vm.createContext([contextObject[, options]])
#
contextObject
<Object>options
<Object>name
<string> Human-readable name of the newly created context. Default:'VM Context i'
, wherei
is an ascending numerical index of the created context.origin
<string> Origin corresponding to the newly created context for display purposes. The origin should be formatted like a URL, but with only the scheme, host, and port (if necessary), like the value of theurl.origin
property of aURL
object. Most notably, this string should omit the trailing slash, as that denotes a path. Default:''
.codeGeneration
<Object>microtaskMode
<string> If set toafterEvaluate
, microtasks (tasks scheduled throughPromise
s andasync function
s) will be run immediately after a script has run throughscript.runInContext()
. They are included in thetimeout
andbreakOnSigint
scopes in that case.
- Returns: <Object> contextified object.
If given a contextObject
, the vm.createContext()
method will prepare
that object so that it can be used in calls to
vm.runInContext()
or script.runInContext()
. Inside such scripts,
the contextObject
will be the global object, retaining all of its existing
properties but also having the built-in objects and functions any standard
global object has. Outside of scripts run by the vm module, global variables
will remain unchanged.
const vm = require('node:vm');
global.globalVar = 3;
const context = { globalVar: 1 };
vm.createContext(context);
vm.runInContext('globalVar *= 2;', context);
console.log(context);
// Prints: { globalVar: 2 }
console.log(global.globalVar);
// Prints: 3
If contextObject
is omitted (or passed explicitly as undefined
), a new,
empty contextified object will be returned.
The vm.createContext()
method is primarily useful for creating a single
context that can be used to run multiple scripts. For instance, if emulating a
web browser, the method can be used to create a single context representing a
window's global object, then run all <script>
tags together within that
context.
The provided name
and origin
of the context are made visible through the
Inspector API.
vm.isContext(object)
#
Returns true
if the given object
object has been contextified using
vm.createContext()
.
vm.measureMemory([options])
#
Measure the memory known to V8 and used by all contexts known to the current V8 isolate, or the main context.
options
<Object> Optional.mode
<string> Either'summary'
or'detailed'
. In summary mode, only the memory measured for the main context will be returned. In detailed mode, the memory measured for all contexts known to the current V8 isolate will be returned. Default:'summary'
execution
<string> Either'default'
or'eager'
. With default execution, the promise will not resolve until after the next scheduled garbage collection starts, which may take a while (or never if the program exits before the next GC). With eager execution, the GC will be started right away to measure the memory. Default:'default'
- Returns: <Promise> If the memory is successfully measured the promise will resolve with an object containing information about the memory usage.
The format of the object that the returned Promise may resolve with is specific to the V8 engine and may change from one version of V8 to the next.
The returned result is different from the statistics returned by
v8.getHeapSpaceStatistics()
in that vm.measureMemory()
measure the
memory reachable by each V8 specific contexts in the current instance of
the V8 engine, while the result of v8.getHeapSpaceStatistics()
measure
the memory occupied by each heap space in the current V8 instance.
const vm = require('node:vm');
// Measure the memory used by the main context.
vm.measureMemory({ mode: 'summary' })
// This is the same as vm.measureMemory()
.then((result) => {
// The current format is:
// {
// total: {
// jsMemoryEstimate: 2418479, jsMemoryRange: [ 2418479, 2745799 ]
// }
// }
console.log(result);
});
const context = vm.createContext({ a: 1 });
vm.measureMemory({ mode: 'detailed', execution: 'eager' })
.then((result) => {
// Reference the context here so that it won't be GC'ed
// until the measurement is complete.
console.log(context.a);
// {
// total: {
// jsMemoryEstimate: 2574732,
// jsMemoryRange: [ 2574732, 2904372 ]
// },
// current: {
// jsMemoryEstimate: 2438996,
// jsMemoryRange: [ 2438996, 2768636 ]
// },
// other: [
// {
// jsMemoryEstimate: 135736,
// jsMemoryRange: [ 135736, 465376 ]
// }
// ]
// }
console.log(result);
});
vm.runInContext(code, contextifiedObject[, options])
#
code
<string> The JavaScript code to compile and run.contextifiedObject
<Object> The contextified object that will be used as theglobal
when thecode
is compiled and run.options
<Object> | <string>filename
<string> Specifies the filename used in stack traces produced by this script. Default:'evalmachine.<anonymous>'
.lineOffset
<number> Specifies the line number offset that is displayed in stack traces produced by this script. Default:0
.columnOffset
<number> Specifies the first-line column number offset that is displayed in stack traces produced by this script. Default:0
.displayErrors
<boolean> Whentrue
, if anError
occurs while compiling thecode
, the line of code causing the error is attached to the stack trace. Default:true
.timeout
<integer> Specifies the number of milliseconds to executecode
before terminating execution. If execution is terminated, anError
will be thrown. This value must be a strictly positive integer.breakOnSigint
<boolean> Iftrue
, receivingSIGINT
(Ctrl+C) will terminate execution and throw anError
. Existing handlers for the event that have been attached viaprocess.on('SIGINT')
are disabled during script execution, but continue to work after that. Default:false
.cachedData
<Buffer> | <TypedArray> | <DataView> Provides an optionalBuffer
orTypedArray
, orDataView
with V8's code cache data for the supplied source.importModuleDynamically
<Function> Called during evaluation of this module whenimport()
is called. If this option is not specified, calls toimport()
will reject withERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING
. This option is part of the experimental modules API. We do not recommend using it in a production environment.specifier
<string> specifier passed toimport()
script
<vm.Script>importAssertions
<Object> The"assert"
value passed to theoptionsExpression
optional parameter, or an empty object if no value was provided.- Returns: <Module Namespace Object> | <vm.Module> Returning a
vm.Module
is recommended in order to take advantage of error tracking, and to avoid issues with namespaces that containthen
function exports.
- Returns: <any> the result of the very last statement executed in the script.
The vm.runInContext()
method compiles code
, runs it within the context of
the contextifiedObject
, then returns the result. Running code does not have
access to the local scope. The contextifiedObject
object must have been
previously contextified using the vm.createContext()
method.
If options
is a string, then it specifies the filename.
The following example compiles and executes different scripts using a single contextified object:
const vm = require('node:vm');
const contextObject = { globalVar: 1 };
vm.createContext(contextObject);
for (let i = 0; i < 10; ++i) {
vm.runInContext('globalVar *= 2;', contextObject);
}
console.log(contextObject);
// Prints: { globalVar: 1024 }
vm.runInNewContext(code[, contextObject[, options]])
#
code
<string> The JavaScript code to compile and run.contextObject
<Object> An object that will be contextified. Ifundefined
, a new object will be created.options
<Object> | <string>filename
<string> Specifies the filename used in stack traces produced by this script. Default:'evalmachine.<anonymous>'
.lineOffset
<number> Specifies the line number offset that is displayed in stack traces produced by this script. Default:0
.columnOffset
<number> Specifies the first-line column number offset that is displayed in stack traces produced by this script. Default:0
.displayErrors
<boolean> Whentrue
, if anError
occurs while compiling thecode
, the line of code causing the error is attached to the stack trace. Default:true
.timeout
<integer> Specifies the number of milliseconds to executecode
before terminating execution. If execution is terminated, anError
will be thrown. This value must be a strictly positive integer.breakOnSigint
<boolean> Iftrue
, receivingSIGINT
(Ctrl+C) will terminate execution and throw anError
. Existing handlers for the event that have been attached viaprocess.on('SIGINT')
are disabled during script execution, but continue to work after that. Default:false
.contextName
<string> Human-readable name of the newly created context. Default:'VM Context i'
, wherei
is an ascending numerical index of the created context.contextOrigin
<string> Origin corresponding to the newly created context for display purposes. The origin should be formatted like a URL, but with only the scheme, host, and port (if necessary), like the value of theurl.origin
property of aURL
object. Most notably, this string should omit the trailing slash, as that denotes a path. Default:''
.contextCodeGeneration
<Object>cachedData
<Buffer> | <TypedArray> | <DataView> Provides an optionalBuffer
orTypedArray
, orDataView
with V8's code cache data for the supplied source.importModuleDynamically
<Function> Called during evaluation of this module whenimport()
is called. If this option is not specified, calls toimport()
will reject withERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING
. This option is part of the experimental modules API. We do not recommend using it in a production environment.specifier
<string> specifier passed toimport()
script
<vm.Script>importAssertions
<Object> The"assert"
value passed to theoptionsExpression
optional parameter, or an empty object if no value was provided.- Returns: <Module Namespace Object> | <vm.Module> Returning a
vm.Module
is recommended in order to take advantage of error tracking, and to avoid issues with namespaces that containthen
function exports.
microtaskMode
<string> If set toafterEvaluate
, microtasks (tasks scheduled throughPromise
s andasync function
s) will be run immediately after the script has run. They are included in thetimeout
andbreakOnSigint
scopes in that case.
- Returns: <any> the result of the very last statement executed in the script.
The vm.runInNewContext()
first contextifies the given contextObject
(or
creates a new contextObject
if passed as undefined
), compiles the code
,
runs it within the created context, then returns the result. Running code
does not have access to the local scope.
If options
is a string, then it specifies the filename.
The following example compiles and executes code that increments a global
variable and sets a new one. These globals are contained in the contextObject
.
const vm = require('node:vm');
const contextObject = {
animal: 'cat',
count: 2,
};
vm.runInNewContext('count += 1; name = "kitty"', contextObject);
console.log(contextObject);
// Prints: { animal: 'cat', count: 3, name: 'kitty' }
vm.runInThisContext(code[, options])
#
code
<string> The JavaScript code to compile and run.options
<Object> | <string>filename
<string> Specifies the filename used in stack traces produced by this script. Default:'evalmachine.<anonymous>'
.lineOffset
<number> Specifies the line number offset that is displayed in stack traces produced by this script. Default:0
.columnOffset
<number> Specifies the first-line column number offset that is displayed in stack traces produced by this script. Default:0
.displayErrors
<boolean> Whentrue
, if anError
occurs while compiling thecode
, the line of code causing the error is attached to the stack trace. Default:true
.timeout
<integer> Specifies the number of milliseconds to executecode
before terminating execution. If execution is terminated, anError
will be thrown. This value must be a strictly positive integer.breakOnSigint
<boolean> Iftrue
, receivingSIGINT
(Ctrl+C) will terminate execution and throw anError
. Existing handlers for the event that have been attached viaprocess.on('SIGINT')
are disabled during script execution, but continue to work after that. Default:false
.cachedData
<Buffer> | <TypedArray> | <DataView> Provides an optionalBuffer
orTypedArray
, orDataView
with V8's code cache data for the supplied source.importModuleDynamically
<Function> Called during evaluation of this module whenimport()
is called. If this option is not specified, calls toimport()
will reject withERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING
. This option is part of the experimental modules API. We do not recommend using it in a production environment.specifier
<string> specifier passed toimport()
script
<vm.Script>importAssertions
<Object> The"assert"
value passed to theoptionsExpression
optional parameter, or an empty object if no value was provided.- Returns: <Module Namespace Object> | <vm.Module> Returning a
vm.Module
is recommended in order to take advantage of error tracking, and to avoid issues with namespaces that containthen
function exports.
- Returns: <any> the result of the very last statement executed in the script.
vm.runInThisContext()
compiles code
, runs it within the context of the
current global
and returns the result. Running code does not have access to
local scope, but does have access to the current global
object.
If options
is a string, then it specifies the filename.
The following example illustrates using both vm.runInThisContext()
and
the JavaScript eval()
function to run the same code:
const vm = require('node:vm');
let localVar = 'initial value';
const vmResult = vm.runInThisContext('localVar = "vm";');
console.log(`vmResult: '${vmResult}', localVar: '${localVar}'`);
// Prints: vmResult: 'vm', localVar: 'initial value'
const evalResult = eval('localVar = "eval";');
console.log(`evalResult: '${evalResult}', localVar: '${localVar}'`);
// Prints: evalResult: 'eval', localVar: 'eval'
Because vm.runInThisContext()
does not have access to the local scope,
localVar
is unchanged. In contrast, eval()
does have access to the
local scope, so the value localVar
is changed. In this way
vm.runInThisContext()
is much like an indirect eval()
call, e.g.
(0,eval)('code')
.
Example: Running an HTTP server within a VM#
When using either script.runInThisContext()
or
vm.runInThisContext()
, the code is executed within the current V8 global
context. The code passed to this VM context will have its own isolated scope.
In order to run a simple web server using the node:http
module the code passed
to the context must either call require('node:http')
on its own, or have a
reference to the node:http
module passed to it. For instance:
'use strict';
const vm = require('node:vm');
const code = `
((require) => {
const http = require('node:http');
http.createServer((request, response) => {
response.writeHead(200, { 'Content-Type': 'text/plain' });
response.end('Hello World\\n');
}).listen(8124);
console.log('Server running at http://127.0.0.1:8124/');
})`;
vm.runInThisContext(code)(require);
The require()
in the above case shares the state with the context it is
passed from. This may introduce risks when untrusted code is executed, e.g.
altering objects in the context in unwanted ways.
What does it mean to "contextify" an object?#
All JavaScript executed within Node.js runs within the scope of a "context". According to the V8 Embedder's Guide:
In V8, a context is an execution environment that allows separate, unrelated, JavaScript applications to run in a single instance of V8. You must explicitly specify the context in which you want any JavaScript code to be run.
When the method vm.createContext()
is called, the contextObject
argument
(or a newly-created object if contextObject
is undefined
) is associated
internally with a new instance of a V8 Context. This V8 Context provides the
code
run using the node:vm
module's methods with an isolated global
environment within which it can operate. The process of creating the V8 Context
and associating it with the contextObject
is what this document refers to as
"contextifying" the object.
Timeout interactions with asynchronous tasks and Promises#
Promise
s and async function
s can schedule tasks run by the JavaScript
engine asynchronously. By default, these tasks are run after all JavaScript
functions on the current stack are done executing.
This allows escaping the functionality of the timeout
and
breakOnSigint
options.
For example, the following code executed by vm.runInNewContext()
with a
timeout of 5 milliseconds schedules an infinite loop to run after a promise
resolves. The scheduled loop is never interrupted by the timeout:
const vm = require('node:vm');
function loop() {
console.log('entering loop');
while (1) console.log(Date.now());
}
vm.runInNewContext(
'Promise.resolve().then(() => loop());',
{ loop, console },
{ timeout: 5 },
);
// This is printed *before* 'entering loop' (!)
console.log('done executing');
This can be addressed by passing microtaskMode: 'afterEvaluate'
to the code
that creates the Context
:
const vm = require('node:vm');
function loop() {
while (1) console.log(Date.now());
}
vm.runInNewContext(
'Promise.resolve().then(() => loop());',
{ loop, console },
{ timeout: 5, microtaskMode: 'afterEvaluate' },
);
In this case, the microtask scheduled through promise.then()
will be run
before returning from vm.runInNewContext()
, and will be interrupted
by the timeout
functionality. This applies only to code running in a
vm.Context
, so e.g. vm.runInThisContext()
does not take this option.
Promise callbacks are entered into the microtask queue of the context in which
they were created. For example, if () => loop()
is replaced with just loop
in the above example, then loop
will be pushed into the global microtask
queue, because it is a function from the outer (main) context, and thus will
also be able to escape the timeout.
If asynchronous scheduling functions such as process.nextTick()
,
queueMicrotask()
, setTimeout()
, setImmediate()
, etc. are made available
inside a vm.Context
, functions passed to them will be added to global queues,
which are shared by all contexts. Therefore, callbacks passed to those functions
are not controllable through the timeout either.
WebAssembly System Interface (WASI)#
Source Code: lib/wasi.js
The WASI API provides an implementation of the WebAssembly System Interface specification. WASI gives sandboxed WebAssembly applications access to the underlying operating system via a collection of POSIX-like functions.
import { readFile } from 'node:fs/promises';
import { WASI } from 'wasi';
import { argv, env } from 'node:process';
const wasi = new WASI({
version: 'preview1',
args: argv,
env,
preopens: {
'/sandbox': '/some/real/path/that/wasm/can/access',
},
});
const wasm = await WebAssembly.compile(
await readFile(new URL('./demo.wasm', import.meta.url)),
);
const instance = await WebAssembly.instantiate(wasm, wasi.getImportObject());
wasi.start(instance);
'use strict';
const { readFile } = require('node:fs/promises');
const { WASI } = require('wasi');
const { argv, env } = require('node:process');
const { join } = require('node:path');
const wasi = new WASI({
version: 'preview1',
args: argv,
env,
preopens: {
'/sandbox': '/some/real/path/that/wasm/can/access',
},
});
(async () => {
const wasm = await WebAssembly.compile(
await readFile(join(__dirname, 'demo.wasm')),
);
const instance = await WebAssembly.instantiate(wasm, wasi.getImportObject());
wasi.start(instance);
})();
To run the above example, create a new WebAssembly text format file named
demo.wat
:
(module
;; Import the required fd_write WASI function which will write the given io vectors to stdout
;; The function signature for fd_write is:
;; (File Descriptor, *iovs, iovs_len, nwritten) -> Returns number of bytes written
(import "wasi_snapshot_preview1" "fd_write" (func $fd_write (param i32 i32 i32 i32) (result i32)))
(memory 1)
(export "memory" (memory 0))
;; Write 'hello world\n' to memory at an offset of 8 bytes
;; Note the trailing newline which is required for the text to appear
(data (i32.const 8) "hello world\n")
(func $main (export "_start")
;; Creating a new io vector within linear memory
(i32.store (i32.const 0) (i32.const 8)) ;; iov.iov_base - This is a pointer to the start of the 'hello world\n' string
(i32.store (i32.const 4) (i32.const 12)) ;; iov.iov_len - The length of the 'hello world\n' string
(call $fd_write
(i32.const 1) ;; file_descriptor - 1 for stdout
(i32.const 0) ;; *iovs - The pointer to the iov array, which is stored at memory location 0
(i32.const 1) ;; iovs_len - We're printing 1 string stored in an iov - so one.
(i32.const 20) ;; nwritten - A place in memory to store the number of bytes written
)
drop ;; Discard the number of bytes written from the top of the stack
)
)
Use wabt to compile .wat
to .wasm
$ wat2wasm demo.wat
The --experimental-wasi-unstable-preview1
CLI argument is needed for this
example to run.
Class: WASI
#
The WASI
class provides the WASI system call API and additional convenience
methods for working with WASI-based applications. Each WASI
instance
represents a distinct sandbox environment. For security purposes, each WASI
instance must have its command-line arguments, environment variables, and
sandbox directory structure configured explicitly.
new WASI([options])
#
options
<Object>args
<Array> An array of strings that the WebAssembly application will see as command-line arguments. The first argument is the virtual path to the WASI command itself. Default:[]
.env
<Object> An object similar toprocess.env
that the WebAssembly application will see as its environment. Default:{}
.preopens
<Object> This object represents the WebAssembly application's sandbox directory structure. The string keys ofpreopens
are treated as directories within the sandbox. The corresponding values inpreopens
are the real paths to those directories on the host machine.returnOnExit
<boolean> By default, WASI applications terminate the Node.js process via the__wasi_proc_exit()
function. Setting this option totrue
causeswasi.start()
to return the exit code rather than terminate the process. Default:false
.stdin
<integer> The file descriptor used as standard input in the WebAssembly application. Default:0
.stdout
<integer> The file descriptor used as standard output in the WebAssembly application. Default:1
.stderr
<integer> The file descriptor used as standard error in the WebAssembly application. Default:2
.version
<string> The version of WASI requested. Currently the only supported versions areunstable
andpreview1
. Default:preview1
.
wasi.getImportObject()
#
Return an import object that can be passed to WebAssembly.instantiate()
if
no other WASM imports are needed beyond those provided by WASI.
If version unstable
was passed into the constructor it will return:
{ wasi_unstable: wasi.wasiImport }
If version preview1
was passed into the constructor or no version was
specified it will return:
{ wasi_snapshot_preview1: wasi.wasiImport }
wasi.start(instance)
#
instance
<WebAssembly.Instance>
Attempt to begin execution of instance
as a WASI command by invoking its
_start()
export. If instance
does not contain a _start()
export, or if
instance
contains an _initialize()
export, then an exception is thrown.
start()
requires that instance
exports a WebAssembly.Memory
named
memory
. If instance
does not have a memory
export an exception is thrown.
If start()
is called more than once, an exception is thrown.
wasi.initialize(instance)
#
instance
<WebAssembly.Instance>
Attempt to initialize instance
as a WASI reactor by invoking its
_initialize()
export, if it is present. If instance
contains a _start()
export, then an exception is thrown.
initialize()
requires that instance
exports a WebAssembly.Memory
named
memory
. If instance
does not have a memory
export an exception is thrown.
If initialize()
is called more than once, an exception is thrown.
wasi.wasiImport
#
wasiImport
is an object that implements the WASI system call API. This object
should be passed as the wasi_snapshot_preview1
import during the instantiation
of a WebAssembly.Instance
.
Web Crypto API#
Node.js provides an implementation of the standard Web Crypto API.
Use globalThis.crypto
or require('node:crypto').webcrypto
to access this
module.
const { subtle } = globalThis.crypto;
(async function() {
const key = await subtle.generateKey({
name: 'HMAC',
hash: 'SHA-256',
length: 256,
}, true, ['sign', 'verify']);
const enc = new TextEncoder();
const message = enc.encode('I love cupcakes');
const digest = await subtle.sign({
name: 'HMAC',
}, key, message);
})();
Examples#
Generating keys#
The <SubtleCrypto> class can be used to generate symmetric (secret) keys or asymmetric key pairs (public key and private key).
AES keys#
const { subtle } = globalThis.crypto;
async function generateAesKey(length = 256) {
const key = await subtle.generateKey({
name: 'AES-CBC',
length,
}, true, ['encrypt', 'decrypt']);
return key;
}
ECDSA key pairs#
const { subtle } = globalThis.crypto;
async function generateEcKey(namedCurve = 'P-521') {
const {
publicKey,
privateKey,
} = await subtle.generateKey({
name: 'ECDSA',
namedCurve,
}, true, ['sign', 'verify']);
return { publicKey, privateKey };
}
Ed25519/Ed448/X25519/X448 key pairs#
const { subtle } = globalThis.crypto;
async function generateEd25519Key() {
return subtle.generateKey({
name: 'Ed25519',
}, true, ['sign', 'verify']);
}
async function generateX25519Key() {
return subtle.generateKey({
name: 'X25519',
}, true, ['deriveKey']);
}
HMAC keys#
const { subtle } = globalThis.crypto;
async function generateHmacKey(hash = 'SHA-256') {
const key = await subtle.generateKey({
name: 'HMAC',
hash,
}, true, ['sign', 'verify']);
return key;
}
RSA key pairs#
const { subtle } = globalThis.crypto;
const publicExponent = new Uint8Array([1, 0, 1]);
async function generateRsaKey(modulusLength = 2048, hash = 'SHA-256') {
const {
publicKey,
privateKey,
} = await subtle.generateKey({
name: 'RSASSA-PKCS1-v1_5',
modulusLength,
publicExponent,
hash,
}, true, ['sign', 'verify']);
return { publicKey, privateKey };
}
Encryption and decryption#
const crypto = globalThis.crypto;
async function aesEncrypt(plaintext) {
const ec = new TextEncoder();
const key = await generateAesKey();
const iv = crypto.getRandomValues(new Uint8Array(16));
const ciphertext = await crypto.subtle.encrypt({
name: 'AES-CBC',
iv,
}, key, ec.encode(plaintext));
return {
key,
iv,
ciphertext,
};
}
async function aesDecrypt(ciphertext, key, iv) {
const dec = new TextDecoder();
const plaintext = await crypto.subtle.decrypt({
name: 'AES-CBC',
iv,
}, key, ciphertext);
return dec.decode(plaintext);
}
Exporting and importing keys#
const { subtle } = globalThis.crypto;
async function generateAndExportHmacKey(format = 'jwk', hash = 'SHA-512') {
const key = await subtle.generateKey({
name: 'HMAC',
hash,
}, true, ['sign', 'verify']);
return subtle.exportKey(format, key);
}
async function importHmacKey(keyData, format = 'jwk', hash = 'SHA-512') {
const key = await subtle.importKey(format, keyData, {
name: 'HMAC',
hash,
}, true, ['sign', 'verify']);
return key;
}
Wrapping and unwrapping keys#
const { subtle } = globalThis.crypto;
async function generateAndWrapHmacKey(format = 'jwk', hash = 'SHA-512') {
const [
key,
wrappingKey,
] = await Promise.all([
subtle.generateKey({
name: 'HMAC', hash,
}, true, ['sign', 'verify']),
subtle.generateKey({
name: 'AES-KW',
length: 256,
}, true, ['wrapKey', 'unwrapKey']),
]);
const wrappedKey = await subtle.wrapKey(format, key, wrappingKey, 'AES-KW');
return { wrappedKey, wrappingKey };
}
async function unwrapHmacKey(
wrappedKey,
wrappingKey,
format = 'jwk',
hash = 'SHA-512') {
const key = await subtle.unwrapKey(
format,
wrappedKey,
wrappingKey,
'AES-KW',
{ name: 'HMAC', hash },
true,
['sign', 'verify']);
return key;
}
Sign and verify#
const { subtle } = globalThis.crypto;
async function sign(key, data) {
const ec = new TextEncoder();
const signature =
await subtle.sign('RSASSA-PKCS1-v1_5', key, ec.encode(data));
return signature;
}
async function verify(key, signature, data) {
const ec = new TextEncoder();
const verified =
await subtle.verify(
'RSASSA-PKCS1-v1_5',
key,
signature,
ec.encode(data));
return verified;
}
Deriving bits and keys#
const { subtle } = globalThis.crypto;
async function pbkdf2(pass, salt, iterations = 1000, length = 256) {
const ec = new TextEncoder();
const key = await subtle.importKey(
'raw',
ec.encode(pass),
'PBKDF2',
false,
['deriveBits']);
const bits = await subtle.deriveBits({
name: 'PBKDF2',
hash: 'SHA-512',
salt: ec.encode(salt),
iterations,
}, key, length);
return bits;
}
async function pbkdf2Key(pass, salt, iterations = 1000, length = 256) {
const ec = new TextEncoder();
const keyMaterial = await subtle.importKey(
'raw',
ec.encode(pass),
'PBKDF2',
false,
['deriveKey']);
const key = await subtle.deriveKey({
name: 'PBKDF2',
hash: 'SHA-512',
salt: ec.encode(salt),
iterations,
}, keyMaterial, {
name: 'AES-GCM',
length: 256,
}, true, ['encrypt', 'decrypt']);
return key;
}
Digest#
const { subtle } = globalThis.crypto;
async function digest(data, algorithm = 'SHA-512') {
const ec = new TextEncoder();
const digest = await subtle.digest(algorithm, ec.encode(data));
return digest;
}
Algorithm matrix#
The table details the algorithms supported by the Node.js Web Crypto API implementation and the APIs supported for each:
Algorithm | generateKey | exportKey | importKey | encrypt | decrypt | wrapKey | unwrapKey | deriveBits | deriveKey | sign | verify | digest |
---|---|---|---|---|---|---|---|---|---|---|---|---|
'RSASSA-PKCS1-v1_5' | ✔ | ✔ | ✔ | ✔ | ✔ | |||||||
'RSA-PSS' | ✔ | ✔ | ✔ | ✔ | ✔ | |||||||
'RSA-OAEP' | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | |||||
'ECDSA' | ✔ | ✔ | ✔ | ✔ | ✔ | |||||||
'Ed25519' 1 | ✔ | ✔ | ✔ | ✔ | ✔ | |||||||
'Ed448' 1 | ✔ | ✔ | ✔ | ✔ | ✔ | |||||||
'ECDH' | ✔ | ✔ | ✔ | ✔ | ✔ | |||||||
'X25519' 1 | ✔ | ✔ | ✔ | ✔ | ✔ | |||||||
'X448' 1 | ✔ | ✔ | ✔ | ✔ | ✔ | |||||||
'AES-CTR' | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | |||||
'AES-CBC' | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | |||||
'AES-GCM' | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | |||||
'AES-KW' | ✔ | ✔ | ✔ | ✔ | ✔ | |||||||
'HMAC' | ✔ | ✔ | ✔ | ✔ | ✔ | |||||||
'HKDF' | ✔ | ✔ | ✔ | ✔ | ||||||||
'PBKDF2' | ✔ | ✔ | ✔ | ✔ | ||||||||
'SHA-1' | ✔ | |||||||||||
'SHA-256' | ✔ | |||||||||||
'SHA-384' | ✔ | |||||||||||
'SHA-512' | ✔ |
Class: Crypto
#
globalThis.crypto
is an instance of the Crypto
class. Crypto
is a singleton that provides access to the remainder of the
crypto API.
crypto.subtle
#
- Type: <SubtleCrypto>
Provides access to the SubtleCrypto
API.
crypto.getRandomValues(typedArray)
#
typedArray
<Buffer> | <TypedArray>- Returns: <Buffer> | <TypedArray>
Generates cryptographically strong random values. The given typedArray
is
filled with random values, and a reference to typedArray
is returned.
The given typedArray
must be an integer-based instance of <TypedArray>,
i.e. Float32Array
and Float64Array
are not accepted.
An error will be thrown if the given typedArray
is larger than 65,536 bytes.
crypto.randomUUID()
#
- Returns: <string>
Generates a random RFC 4122 version 4 UUID. The UUID is generated using a cryptographic pseudorandom number generator.
Class: CryptoKey
#
cryptoKey.algorithm
#
An object detailing the algorithm for which the key can be used along with additional algorithm-specific parameters.
Read-only.
cryptoKey.extractable
#
- Type: <boolean>
When true
, the <CryptoKey> can be extracted using either
subtleCrypto.exportKey()
or subtleCrypto.wrapKey()
.
Read-only.
cryptoKey.type
#
- Type: <string> One of
'secret'
,'private'
, or'public'
.
A string identifying whether the key is a symmetric ('secret'
) or
asymmetric ('private'
or 'public'
) key.
cryptoKey.usages
#
- Type: <string[]>
An array of strings identifying the operations for which the key may be used.
The possible usages are:
'encrypt'
- The key may be used to encrypt data.'decrypt'
- The key may be used to decrypt data.'sign'
- The key may be used to generate digital signatures.'verify'
- The key may be used to verify digital signatures.'deriveKey'
- The key may be used to derive a new key.'deriveBits'
- The key may be used to derive bits.'wrapKey'
- The key may be used to wrap another key.'unwrapKey'
- The key may be used to unwrap another key.
Valid key usages depend on the key algorithm (identified by
cryptokey.algorithm.name
).
Key Type | 'encrypt' | 'decrypt' | 'sign' | 'verify' | 'deriveKey' | 'deriveBits' | 'wrapKey' | 'unwrapKey' |
---|---|---|---|---|---|---|---|---|
'AES-CBC' | ✔ | ✔ | ✔ | ✔ | ||||
'AES-CTR' | ✔ | ✔ | ✔ | ✔ | ||||
'AES-GCM' | ✔ | ✔ | ✔ | ✔ | ||||
'AES-KW' | ✔ | ✔ | ||||||
'ECDH' | ✔ | ✔ | ||||||
'X25519' 1 | ✔ | ✔ | ||||||
'X448' 1 | ✔ | ✔ | ||||||
'ECDSA' | ✔ | ✔ | ||||||
'Ed25519' 1 | ✔ | ✔ | ||||||
'Ed448' 1 | ✔ | ✔ | ||||||
'HDKF' | ✔ | ✔ | ||||||
'HMAC' | ✔ | ✔ | ||||||
'PBKDF2' | ✔ | ✔ | ||||||
'RSA-OAEP' | ✔ | ✔ | ✔ | ✔ | ||||
'RSA-PSS' | ✔ | ✔ | ||||||
'RSASSA-PKCS1-v1_5' | ✔ | ✔ |
Class: CryptoKeyPair
#
The CryptoKeyPair
is a simple dictionary object with publicKey
and
privateKey
properties, representing an asymmetric key pair.
cryptoKeyPair.privateKey
#
- Type: <CryptoKey> A <CryptoKey> whose
type
will be'private'
.
cryptoKeyPair.publicKey
#
- Type: <CryptoKey> A <CryptoKey> whose
type
will be'public'
.
Class: SubtleCrypto
#
subtle.decrypt(algorithm, key, data)
#
algorithm
: <RsaOaepParams> | <AesCtrParams> | <AesCbcParams> | <AesGcmParams>key
: <CryptoKey>data
: <ArrayBuffer> | <TypedArray> | <DataView> | <Buffer>- Returns: <Promise> containing <ArrayBuffer>
Using the method and parameters specified in algorithm
and the keying
material provided by key
, subtle.decrypt()
attempts to decipher the
provided data
. If successful, the returned promise will be resolved with
an <ArrayBuffer> containing the plaintext result.
The algorithms currently supported include:
'RSA-OAEP'
'AES-CTR'
'AES-CBC'
'AES-GCM
'
subtle.deriveBits(algorithm, baseKey, length)
#
algorithm
: <AlgorithmIdentifier> | <EcdhKeyDeriveParams> | <HkdfParams> | <Pbkdf2Params>baseKey
: <CryptoKey>length
: <number> | <null>- Returns: <Promise> containing <ArrayBuffer>
Using the method and parameters specified in algorithm
and the keying
material provided by baseKey
, subtle.deriveBits()
attempts to generate
length
bits.
The Node.js implementation requires that when length
is a
number it must be multiple of 8
.
When length
is null
the maximum number of bits for a given algorithm is
generated. This is allowed for the 'ECDH'
, 'X25519'
, and 'X448'
algorithms.
If successful, the returned promise will be resolved with an <ArrayBuffer> containing the generated data.
The algorithms currently supported include:
subtle.deriveKey(algorithm, baseKey, derivedKeyAlgorithm, extractable, keyUsages)
#
algorithm
: <AlgorithmIdentifier> | <EcdhKeyDeriveParams> | <HkdfParams> | <Pbkdf2Params>baseKey
: <CryptoKey>derivedKeyAlgorithm
: <HmacKeyGenParams> | <AesKeyGenParams>extractable
: <boolean>keyUsages
: <string[]> See Key usages.- Returns: <Promise> containing <CryptoKey>
Using the method and parameters specified in algorithm
, and the keying
material provided by baseKey
, subtle.deriveKey()
attempts to generate
a new <CryptoKey> based on the method and parameters in derivedKeyAlgorithm
.
Calling subtle.deriveKey()
is equivalent to calling subtle.deriveBits()
to
generate raw keying material, then passing the result into the
subtle.importKey()
method using the deriveKeyAlgorithm
, extractable
, and
keyUsages
parameters as input.
The algorithms currently supported include:
subtle.digest(algorithm, data)
#
algorithm
: <string> | <Object>data
: <ArrayBuffer> | <TypedArray> | <DataView> | <Buffer>- Returns: <Promise> containing <ArrayBuffer>
Using the method identified by algorithm
, subtle.digest()
attempts to
generate a digest of data
. If successful, the returned promise is resolved
with an <ArrayBuffer> containing the computed digest.
If algorithm
is provided as a <string>, it must be one of:
'SHA-1'
'SHA-256'
'SHA-384'
'SHA-512'
If algorithm
is provided as an <Object>, it must have a name
property
whose value is one of the above.
subtle.encrypt(algorithm, key, data)
#
algorithm
: <RsaOaepParams> | <AesCtrParams> | <AesCbcParams> | <AesGcmParams>key
: <CryptoKey>- Returns: <Promise> containing <ArrayBuffer>
Using the method and parameters specified by algorithm
and the keying
material provided by key
, subtle.encrypt()
attempts to encipher data
.
If successful, the returned promise is resolved with an <ArrayBuffer>
containing the encrypted result.
The algorithms currently supported include:
'RSA-OAEP'
'AES-CTR'
'AES-CBC'
'AES-GCM
'
subtle.exportKey(format, key)
#
format
: <string> Must be one of'raw'
,'pkcs8'
,'spki'
, or'jwk'
.key
: <CryptoKey>- Returns: <Promise> containing <ArrayBuffer> | <Object>.
Exports the given key into the specified format, if supported.
If the <CryptoKey> is not extractable, the returned promise will reject.
When format
is either 'pkcs8'
or 'spki'
and the export is successful,
the returned promise will be resolved with an <ArrayBuffer> containing the
exported key data.
When format
is 'jwk'
and the export is successful, the returned promise
will be resolved with a JavaScript object conforming to the JSON Web Key
specification.
Key Type | 'spki' | 'pkcs8' | 'jwk' | 'raw' |
---|---|---|---|---|
'AES-CBC' | ✔ | ✔ | ||
'AES-CTR' | ✔ | ✔ | ||
'AES-GCM' | ✔ | ✔ | ||
'AES-KW' | ✔ | ✔ | ||
'ECDH' | ✔ | ✔ | ✔ | ✔ |
'ECDSA' | ✔ | ✔ | ✔ | ✔ |
'Ed25519' 1 | ✔ | ✔ | ✔ | ✔ |
'Ed448' 1 | ✔ | ✔ | ✔ | ✔ |
'HDKF' | ||||
'HMAC' | ✔ | ✔ | ||
'PBKDF2' | ||||
'RSA-OAEP' | ✔ | ✔ | ✔ | |
'RSA-PSS' | ✔ | ✔ | ✔ | |
'RSASSA-PKCS1-v1_5' | ✔ | ✔ | ✔ |
subtle.generateKey(algorithm, extractable, keyUsages)
#
algorithm
: <AlgorithmIdentifier> | <RsaHashedKeyGenParams> | <EcKeyGenParams> | <HmacKeyGenParams> | <AesKeyGenParams>
extractable
: <boolean>keyUsages
: <string[]> See Key usages.- Returns: <Promise> containing <CryptoKey> | <CryptoKeyPair>
Using the method and parameters provided in algorithm
, subtle.generateKey()
attempts to generate new keying material. Depending the method used, the method
may generate either a single <CryptoKey> or a <CryptoKeyPair>.
The <CryptoKeyPair> (public and private key) generating algorithms supported include:
The <CryptoKey> (secret key) generating algorithms supported include:
'HMAC'
'AES-CTR'
'AES-CBC'
'AES-GCM'
'AES-KW'
subtle.importKey(format, keyData, algorithm, extractable, keyUsages)
#
format
: <string> Must be one of'raw'
,'pkcs8'
,'spki'
, or'jwk'
.keyData
: <ArrayBuffer> | <TypedArray> | <DataView> | <Buffer> | <Object>
algorithm
: <AlgorithmIdentifier> | <RsaHashedImportParams> | <EcKeyImportParams> | <HmacImportParams>
extractable
: <boolean>keyUsages
: <string[]> See Key usages.- Returns: <Promise> containing <CryptoKey>
The subtle.importKey()
method attempts to interpret the provided keyData
as the given format
to create a <CryptoKey> instance using the provided
algorithm
, extractable
, and keyUsages
arguments. If the import is
successful, the returned promise will be resolved with the created <CryptoKey>.
If importing a 'PBKDF2'
key, extractable
must be false
.
The algorithms currently supported include:
Key Type | 'spki' | 'pkcs8' | 'jwk' | 'raw' |
---|---|---|---|---|
'AES-CBC' | ✔ | ✔ | ||
'AES-CTR' | ✔ | ✔ | ||
'AES-GCM' | ✔ | ✔ | ||
'AES-KW' | ✔ | ✔ | ||
'ECDH' | ✔ | ✔ | ✔ | ✔ |
'X25519' 1 | ✔ | ✔ | ✔ | ✔ |
'X448' 1 | ✔ | ✔ | ✔ | ✔ |
'ECDSA' | ✔ | ✔ | ✔ | ✔ |
'Ed25519' 1 | ✔ | ✔ | ✔ | ✔ |
'Ed448' 1 | ✔ | ✔ | ✔ | ✔ |
'HDKF' | ✔ | |||
'HMAC' | ✔ | ✔ | ||
'PBKDF2' | ✔ | |||
'RSA-OAEP' | ✔ | ✔ | ✔ | |
'RSA-PSS' | ✔ | ✔ | ✔ | |
'RSASSA-PKCS1-v1_5' | ✔ | ✔ | ✔ |
subtle.sign(algorithm, key, data)
#
algorithm
: <AlgorithmIdentifier> | <RsaPssParams> | <EcdsaParams> | <Ed448Params>key
: <CryptoKey>data
: <ArrayBuffer> | <TypedArray> | <DataView> | <Buffer>- Returns: <Promise> containing <ArrayBuffer>
Using the method and parameters given by algorithm
and the keying material
provided by key
, subtle.sign()
attempts to generate a cryptographic
signature of data
. If successful, the returned promise is resolved with
an <ArrayBuffer> containing the generated signature.
The algorithms currently supported include:
subtle.unwrapKey(format, wrappedKey, unwrappingKey, unwrapAlgo, unwrappedKeyAlgo, extractable, keyUsages)
#
format
: <string> Must be one of'raw'
,'pkcs8'
,'spki'
, or'jwk'
.wrappedKey
: <ArrayBuffer> | <TypedArray> | <DataView> | <Buffer>unwrappingKey
: <CryptoKey>
unwrapAlgo
: <AlgorithmIdentifier> | <RsaOaepParams> | <AesCtrParams> | <AesCbcParams> | <AesGcmParams>unwrappedKeyAlgo
: <AlgorithmIdentifier> | <RsaHashedImportParams> | <EcKeyImportParams> | <HmacImportParams>
extractable
: <boolean>keyUsages
: <string[]> See Key usages.- Returns: <Promise> containing <CryptoKey>
In cryptography, "wrapping a key" refers to exporting and then encrypting the
keying material. The subtle.unwrapKey()
method attempts to decrypt a wrapped
key and create a <CryptoKey> instance. It is equivalent to calling
subtle.decrypt()
first on the encrypted key data (using the wrappedKey
,
unwrapAlgo
, and unwrappingKey
arguments as input) then passing the results
in to the subtle.importKey()
method using the unwrappedKeyAlgo
,
extractable
, and keyUsages
arguments as inputs. If successful, the returned
promise is resolved with a <CryptoKey> object.
The wrapping algorithms currently supported include:
'RSA-OAEP'
'AES-CTR'
'AES-CBC'
'AES-GCM'
'AES-KW'
The unwrapped key algorithms supported include:
'RSASSA-PKCS1-v1_5'
'RSA-PSS'
'RSA-OAEP'
'ECDSA'
'Ed25519'
1'Ed448'
1'ECDH'
'X25519'
1'X448'
1'HMAC'
'AES-CTR'
'AES-CBC'
'AES-GCM'
'AES-KW'
subtle.verify(algorithm, key, signature, data)
#
algorithm
: <AlgorithmIdentifier> | <RsaPssParams> | <EcdsaParams> | <Ed448Params>key
: <CryptoKey>signature
: <ArrayBuffer> | <TypedArray> | <DataView> | <Buffer>data
: <ArrayBuffer> | <TypedArray> | <DataView> | <Buffer>- Returns: <Promise> containing <boolean>
Using the method and parameters given in algorithm
and the keying material
provided by key
, subtle.verify()
attempts to verify that signature
is
a valid cryptographic signature of data
. The returned promise is resolved
with either true
or false
.
The algorithms currently supported include:
subtle.wrapKey(format, key, wrappingKey, wrapAlgo)
#
format
: <string> Must be one of'raw'
,'pkcs8'
,'spki'
, or'jwk'
.key
: <CryptoKey>wrappingKey
: <CryptoKey>wrapAlgo
: <AlgorithmIdentifier> | <RsaOaepParams> | <AesCtrParams> | <AesCbcParams> | <AesGcmParams>- Returns: <Promise> containing <ArrayBuffer>
In cryptography, "wrapping a key" refers to exporting and then encrypting the
keying material. The subtle.wrapKey()
method exports the keying material into
the format identified by format
, then encrypts it using the method and
parameters specified by wrapAlgo
and the keying material provided by
wrappingKey
. It is the equivalent to calling subtle.exportKey()
using
format
and key
as the arguments, then passing the result to the
subtle.encrypt()
method using wrappingKey
and wrapAlgo
as inputs. If
successful, the returned promise will be resolved with an <ArrayBuffer>
containing the encrypted key data.
The wrapping algorithms currently supported include:
'RSA-OAEP'
'AES-CTR'
'AES-CBC'
'AES-GCM'
'AES-KW'
Algorithm parameters#
The algorithm parameter objects define the methods and parameters used by the various <SubtleCrypto> methods. While described here as "classes", they are simple JavaScript dictionary objects.
Class: AlgorithmIdentifier
#
algorithmIdentifier.name
#
- Type: <string>
Class: AesCbcParams
#
aesCbcParams.iv
#
- Type: <ArrayBuffer> | <TypedArray> | <DataView> | <Buffer>
Provides the initialization vector. It must be exactly 16-bytes in length and should be unpredictable and cryptographically random.
aesCbcParams.name
#
- Type: <string> Must be
'AES-CBC'
.
Class: AesCtrParams
#
aesCtrParams.counter
#
- Type: <ArrayBuffer> | <TypedArray> | <DataView> | <Buffer>
The initial value of the counter block. This must be exactly 16 bytes long.
The AES-CTR
method uses the rightmost length
bits of the block as the
counter and the remaining bits as the nonce.
aesCtrParams.length
#
- Type: <number> The number of bits in the
aesCtrParams.counter
that are to be used as the counter.
aesCtrParams.name
#
- Type: <string> Must be
'AES-CTR'
.
Class: AesGcmParams
#
aesGcmParams.additionalData
#
- Type: <ArrayBuffer> | <TypedArray> | <DataView> | <Buffer> | <undefined>
With the AES-GCM method, the additionalData
is extra input that is not
encrypted but is included in the authentication of the data. The use of
additionalData
is optional.
aesGcmParams.iv
#
- Type: <ArrayBuffer> | <TypedArray> | <DataView> | <Buffer>
The initialization vector must be unique for every encryption operation using a given key.
Ideally, this is a deterministic 12-byte value that is computed in such a way that it is guaranteed to be unique across all invocations that use the same key. Alternatively, the initialization vector may consist of at least 12 cryptographically random bytes. For more information on constructing initialization vectors for AES-GCM, refer to Section 8 of NIST SP 800-38D.
aesGcmParams.name
#
- Type: <string> Must be
'AES-GCM'
.
aesGcmParams.tagLength
#
- Type: <number> The size in bits of the generated authentication tag.
This values must be one of
32
,64
,96
,104
,112
,120
, or128
. Default:128
.
Class: AesKeyGenParams
#
aesKeyGenParams.length
#
- Type: <number>
The length of the AES key to be generated. This must be either 128
, 192
,
or 256
.
aesKeyGenParams.name
#
- Type: <string> Must be one of
'AES-CBC'
,'AES-CTR'
,'AES-GCM'
, or'AES-KW'
Class: EcdhKeyDeriveParams
#
ecdhKeyDeriveParams.name
#
- Type: <string> Must be
'ECDH'
,'X25519'
, or'X448'
.
ecdhKeyDeriveParams.public
#
- Type: <CryptoKey>
ECDH key derivation operates by taking as input one parties private key and
another parties public key -- using both to generate a common shared secret.
The ecdhKeyDeriveParams.public
property is set to the other parties public
key.
Class: EcdsaParams
#
ecdsaParams.hash
#
If represented as a <string>, the value must be one of:
'SHA-1'
'SHA-256'
'SHA-384'
'SHA-512'
If represented as an <Object>, the object must have a name
property
whose value is one of the above listed values.
ecdsaParams.name
#
- Type: <string> Must be
'ECDSA'
.
Class: EcKeyGenParams
#
ecKeyGenParams.name
#
- Type: <string> Must be one of
'ECDSA'
or'ECDH'
.
ecKeyGenParams.namedCurve
#
- Type: <string> Must be one of
'P-256'
,'P-384'
,'P-521'
.
Class: EcKeyImportParams
#
ecKeyImportParams.name
#
- Type: <string> Must be one of
'ECDSA'
or'ECDH'
.
ecKeyImportParams.namedCurve
#
- Type: <string> Must be one of
'P-256'
,'P-384'
,'P-521'
.
Class: Ed448Params
#
ed448Params.name
#
- Type: <string> Must be
'Ed448'
.
ed448Params.context
#
- Type: <ArrayBuffer> | <TypedArray> | <DataView> | <Buffer> | <undefined>
The context
member represents the optional context data to associate with
the message.
The Node.js Web Crypto API implementation only supports zero-length context
which is equivalent to not providing context at all.
Class: HkdfParams
#
hkdfParams.hash
#
If represented as a <string>, the value must be one of:
'SHA-1'
'SHA-256'
'SHA-384'
'SHA-512'
If represented as an <Object>, the object must have a name
property
whose value is one of the above listed values.
hkdfParams.info
#
- Type: <ArrayBuffer> | <TypedArray> | <DataView> | <Buffer>
Provides application-specific contextual input to the HKDF algorithm. This can be zero-length but must be provided.
hkdfParams.name
#
- Type: <string> Must be
'HKDF'
.
hkdfParams.salt
#
- Type: <ArrayBuffer> | <TypedArray> | <DataView> | <Buffer>
The salt value significantly improves the strength of the HKDF algorithm.
It should be random or pseudorandom and should be the same length as the
output of the digest function (for instance, if using 'SHA-256'
as the
digest, the salt should be 256-bits of random data).
Class: HmacImportParams
#
hmacImportParams.hash
#
If represented as a <string>, the value must be one of:
'SHA-1'
'SHA-256'
'SHA-384'
'SHA-512'
If represented as an <Object>, the object must have a name
property
whose value is one of the above listed values.
hmacImportParams.length
#
- Type: <number>
The optional number of bits in the HMAC key. This is optional and should be omitted for most cases.
hmacImportParams.name
#
- Type: <string> Must be
'HMAC'
.
Class: HmacKeyGenParams
#
hmacKeyGenParams.hash
#
If represented as a <string>, the value must be one of:
'SHA-1'
'SHA-256'
'SHA-384'
'SHA-512'
If represented as an <Object>, the object must have a name
property
whose value is one of the above listed values.
hmacKeyGenParams.length
#
- Type: <number>
The number of bits to generate for the HMAC key. If omitted, the length will be determined by the hash algorithm used. This is optional and should be omitted for most cases.
hmacKeyGenParams.name
#
- Type: <string> Must be
'HMAC'
.
Class: Pbkdf2Params
#
pbkdb2Params.hash
#
If represented as a <string>, the value must be one of:
'SHA-1'
'SHA-256'
'SHA-384'
'SHA-512'
If represented as an <Object>, the object must have a name
property
whose value is one of the above listed values.
pbkdf2Params.iterations
#
- Type: <number>
The number of iterations the PBKDF2 algorithm should make when deriving bits.
pbkdf2Params.name
#
- Type: <string> Must be
'PBKDF2'
.
pbkdf2Params.salt
#
- Type: <ArrayBuffer> | <TypedArray> | <DataView> | <Buffer>
Should be at least 16 random or pseudorandom bytes.
Class: RsaHashedImportParams
#
rsaHashedImportParams.hash
#
If represented as a <string>, the value must be one of:
'SHA-1'
'SHA-256'
'SHA-384'
'SHA-512'
If represented as an <Object>, the object must have a name
property
whose value is one of the above listed values.
rsaHashedImportParams.name
#
- Type: <string> Must be one of
'RSASSA-PKCS1-v1_5'
,'RSA-PSS'
, or'RSA-OAEP'
.
Class: RsaHashedKeyGenParams
#
rsaHashedKeyGenParams.hash
#
If represented as a <string>, the value must be one of:
'SHA-1'
'SHA-256'
'SHA-384'
'SHA-512'
If represented as an <Object>, the object must have a name
property
whose value is one of the above listed values.
rsaHashedKeyGenParams.modulusLength
#
- Type: <number>
The length in bits of the RSA modulus. As a best practice, this should be
at least 2048
.
rsaHashedKeyGenParams.name
#
- Type: <string> Must be one of
'RSASSA-PKCS1-v1_5'
,'RSA-PSS'
, or'RSA-OAEP'
.
rsaHashedKeyGenParams.publicExponent
#
- Type: <Uint8Array>
The RSA public exponent. This must be a <Uint8Array> containing a big-endian,
unsigned integer that must fit within 32-bits. The <Uint8Array> may contain an
arbitrary number of leading zero-bits. The value must be a prime number. Unless
there is reason to use a different value, use new Uint8Array([1, 0, 1])
(65537) as the public exponent.
Class: RsaOaepParams
#
rsaOaepParams.label
#
- Type: <ArrayBuffer> | <TypedArray> | <DataView> | <Buffer>
An additional collection of bytes that will not be encrypted, but will be bound to the generated ciphertext.
The rsaOaepParams.label
parameter is optional.
rsaOaepParams.name
#
- Type: <string> must be
'RSA-OAEP'
.
Class: RsaPssParams
#
rsaPssParams.name
#
- Type: <string> Must be
'RSA-PSS'
.
rsaPssParams.saltLength
#
- Type: <number>
The length (in bytes) of the random salt to use.
Footnotes
Web Streams API#
An implementation of the WHATWG Streams Standard.
Overview#
The WHATWG Streams Standard (or "web streams") defines an API for handling streaming data. It is similar to the Node.js Streams API but emerged later and has become the "standard" API for streaming data across many JavaScript environments.
There are three primary types of objects:
ReadableStream
- Represents a source of streaming data.WritableStream
- Represents a destination for streaming data.TransformStream
- Represents an algorithm for transforming streaming data.
Example ReadableStream
#
This example creates a simple ReadableStream
that pushes the current
performance.now()
timestamp once every second forever. An async iterable
is used to read the data from the stream.
import {
ReadableStream,
} from 'node:stream/web';
import {
setInterval as every,
} from 'node:timers/promises';
import {
performance,
} from 'node:perf_hooks';
const SECOND = 1000;
const stream = new ReadableStream({
async start(controller) {
for await (const _ of every(SECOND))
controller.enqueue(performance.now());
},
});
for await (const value of stream)
console.log(value);
const {
ReadableStream,
} = require('node:stream/web');
const {
setInterval: every,
} = require('node:timers/promises');
const {
performance,
} = require('node:perf_hooks');
const SECOND = 1000;
const stream = new ReadableStream({
async start(controller) {
for await (const _ of every(SECOND))
controller.enqueue(performance.now());
},
});
(async () => {
for await (const value of stream)
console.log(value);
})();
API#
Class: ReadableStream
#
new ReadableStream([underlyingSource [, strategy]])
#
underlyingSource
<Object>start
<Function> A user-defined function that is invoked immediately when theReadableStream
is created.controller
<ReadableStreamDefaultController> | <ReadableByteStreamController>- Returns:
undefined
or a promise fulfilled withundefined
.
pull
<Function> A user-defined function that is called repeatedly when theReadableStream
internal queue is not full. The operation may be sync or async. If async, the function will not be called again until the previously returned promise is fulfilled.controller
<ReadableStreamDefaultController> | <ReadableByteStreamController>- Returns: A promise fulfilled with
undefined
.
cancel
<Function> A user-defined function that is called when theReadableStream
is canceled.reason
<any>- Returns: A promise fulfilled with
undefined
.
type
<string> Must be'bytes'
orundefined
.autoAllocateChunkSize
<number> Used only whentype
is equal to'bytes'
.
strategy
<Object>highWaterMark
<number> The maximum internal queue size before backpressure is applied.size
<Function> A user-defined function used to identify the size of each chunk of data.
readableStream.locked
#
- Type: <boolean> Set to
true
if there is an active reader for this <ReadableStream>.
The readableStream.locked
property is false
by default, and is
switched to true
while there is an active reader consuming the
stream's data.
readableStream.cancel([reason])
#
reason
<any>- Returns: A promise fulfilled with
undefined
once cancelation has been completed.
readableStream.getReader([options])
#
options
<Object>mode
<string>'byob'
orundefined
- Returns: <ReadableStreamDefaultReader> | <ReadableStreamBYOBReader>
import { ReadableStream } from 'node:stream/web';
const stream = new ReadableStream();
const reader = stream.getReader();
console.log(await reader.read());
const { ReadableStream } = require('node:stream/web');
const stream = new ReadableStream();
const reader = stream.getReader();
reader.read().then(console.log);
Causes the readableStream.locked
to be true
.
readableStream.pipeThrough(transform[, options])
#
transform
<Object>readable
<ReadableStream> TheReadableStream
to whichtransform.writable
will push the potentially modified data is receives from thisReadableStream
.writable
<WritableStream> TheWritableStream
to which thisReadableStream
's data will be written.
options
<Object>preventAbort
<boolean> Whentrue
, errors in thisReadableStream
will not causetransform.writable
to be aborted.preventCancel
<boolean> Whentrue
, errors in the destinationtransform.writable
do not cause thisReadableStream
to be canceled.preventClose
<boolean> Whentrue
, closing thisReadableStream
does not causetransform.writable
to be closed.signal
<AbortSignal> Allows the transfer of data to be canceled using an <AbortController>.
- Returns: <ReadableStream> From
transform.readable
.
Connects this <ReadableStream> to the pair of <ReadableStream> and
<WritableStream> provided in the transform
argument such that the
data from this <ReadableStream> is written in to transform.writable
,
possibly transformed, then pushed to transform.readable
. Once the
pipeline is configured, transform.readable
is returned.
Causes the readableStream.locked
to be true
while the pipe operation
is active.
import {
ReadableStream,
TransformStream,
} from 'node:stream/web';
const stream = new ReadableStream({
start(controller) {
controller.enqueue('a');
},
});
const transform = new TransformStream({
transform(chunk, controller) {
controller.enqueue(chunk.toUpperCase());
},
});
const transformedStream = stream.pipeThrough(transform);
for await (const chunk of transformedStream)
console.log(chunk);
const {
ReadableStream,
TransformStream,
} = require('node:stream/web');
const stream = new ReadableStream({
start(controller) {
controller.enqueue('a');
},
});
const transform = new TransformStream({
transform(chunk, controller) {
controller.enqueue(chunk.toUpperCase());
},
});
const transformedStream = stream.pipeThrough(transform);
(async () => {
for await (const chunk of transformedStream)
console.log(chunk);
})();
readableStream.pipeTo(destination[, options])
#
destination
<WritableStream> A <WritableStream> to which thisReadableStream
's data will be written.options
<Object>preventAbort
<boolean> Whentrue
, errors in thisReadableStream
will not causedestination
to be aborted.preventCancel
<boolean> Whentrue
, errors in thedestination
will not cause thisReadableStream
to be canceled.preventClose
<boolean> Whentrue
, closing thisReadableStream
does not causedestination
to be closed.signal
<AbortSignal> Allows the transfer of data to be canceled using an <AbortController>.
- Returns: A promise fulfilled with
undefined
Causes the readableStream.locked
to be true
while the pipe operation
is active.
readableStream.tee()
#
- Returns: <ReadableStream[]>
Returns a pair of new <ReadableStream> instances to which this
ReadableStream
's data will be forwarded. Each will receive the
same data.
Causes the readableStream.locked
to be true
.
readableStream.values([options])
#
options
<Object>preventCancel
<boolean> Whentrue
, prevents the <ReadableStream> from being closed when the async iterator abruptly terminates. Default:false
.
Creates and returns an async iterator usable for consuming this
ReadableStream
's data.
Causes the readableStream.locked
to be true
while the async iterator
is active.
import { Buffer } from 'node:buffer';
const stream = new ReadableStream(getSomeSource());
for await (const chunk of stream.values({ preventCancel: true }))
console.log(Buffer.from(chunk).toString());
Async Iteration#
The <ReadableStream> object supports the async iterator protocol using
for await
syntax.
import { Buffer } from 'node:buffer';
const stream = new ReadableStream(getSomeSource());
for await (const chunk of stream)
console.log(Buffer.from(chunk).toString());
The async iterator will consume the <ReadableStream> until it terminates.
By default, if the async iterator exits early (via either a break
,
return
, or a throw
), the <ReadableStream> will be closed. To prevent
automatic closing of the <ReadableStream>, use the readableStream.values()
method to acquire the async iterator and set the preventCancel
option to
true
.
The <ReadableStream> must not be locked (that is, it must not have an existing active reader). During the async iteration, the <ReadableStream> will be locked.
Transferring with postMessage()
#
A <ReadableStream> instance can be transferred using a <MessagePort>.
const stream = new ReadableStream(getReadableSourceSomehow());
const { port1, port2 } = new MessageChannel();
port1.onmessage = ({ data }) => {
data.getReader().read().then((chunk) => {
console.log(chunk);
});
};
port2.postMessage(stream, [stream]);
Class: ReadableStreamDefaultReader
#
By default, calling readableStream.getReader()
with no arguments
will return an instance of ReadableStreamDefaultReader
. The default
reader treats the chunks of data passed through the stream as opaque
values, which allows the <ReadableStream> to work with generally any
JavaScript value.
new ReadableStreamDefaultReader(stream)
#
stream
<ReadableStream>
Creates a new <ReadableStreamDefaultReader> that is locked to the given <ReadableStream>.
readableStreamDefaultReader.cancel([reason])
#
reason
<any>- Returns: A promise fulfilled with
undefined
.
Cancels the <ReadableStream> and returns a promise that is fulfilled when the underlying stream has been canceled.
readableStreamDefaultReader.closed
#
- Type: <Promise> Fulfilled with
undefined
when the associated <ReadableStream> is closed or rejected if the stream errors or the reader's lock is released before the stream finishes closing.
readableStreamDefaultReader.read()
#
- Returns: A promise fulfilled with an object:
value
<ArrayBuffer>done
<boolean>
Requests the next chunk of data from the underlying <ReadableStream> and returns a promise that is fulfilled with the data once it is available.
readableStreamDefaultReader.releaseLock()
#
Releases this reader's lock on the underlying <ReadableStream>.
Class: ReadableStreamBYOBReader
#
The ReadableStreamBYOBReader
is an alternative consumer for
byte-oriented <ReadableStream>s (those that are created with
underlyingSource.type
set equal to 'bytes'
when the
ReadableStream
was created).
The BYOB
is short for "bring your own buffer". This is a
pattern that allows for more efficient reading of byte-oriented
data that avoids extraneous copying.
import {
open,
} from 'node:fs/promises';
import {
ReadableStream,
} from 'node:stream/web';
import { Buffer } from 'node:buffer';
class Source {
type = 'bytes';
autoAllocateChunkSize = 1024;
async start(controller) {
this.file = await open(new URL(import.meta.url));
this.controller = controller;
}
async pull(controller) {
const view = controller.byobRequest?.view;
const {
bytesRead,
} = await this.file.read({
buffer: view,
offset: view.byteOffset,
length: view.byteLength,
});
if (bytesRead === 0) {
await this.file.close();
this.controller.close();
}
controller.byobRequest.respond(bytesRead);
}
}
const stream = new ReadableStream(new Source());
async function read(stream) {
const reader = stream.getReader({ mode: 'byob' });
const chunks = [];
let result;
do {
result = await reader.read(Buffer.alloc(100));
if (result.value !== undefined)
chunks.push(Buffer.from(result.value));
} while (!result.done);
return Buffer.concat(chunks);
}
const data = await read(stream);
console.log(Buffer.from(data).toString());
new ReadableStreamBYOBReader(stream)
#
stream
<ReadableStream>
Creates a new ReadableStreamBYOBReader
that is locked to the
given <ReadableStream>.
readableStreamBYOBReader.cancel([reason])
#
reason
<any>- Returns: A promise fulfilled with
undefined
.
Cancels the <ReadableStream> and returns a promise that is fulfilled when the underlying stream has been canceled.
readableStreamBYOBReader.closed
#
- Type: <Promise> Fulfilled with
undefined
when the associated <ReadableStream> is closed or rejected if the stream errors or the reader's lock is released before the stream finishes closing.
readableStreamBYOBReader.read(view)
#
view
<Buffer> | <TypedArray> | <DataView>- Returns: A promise fulfilled with an object:
value
<ArrayBuffer>done
<boolean>
Requests the next chunk of data from the underlying <ReadableStream> and returns a promise that is fulfilled with the data once it is available.
Do not pass a pooled <Buffer> object instance in to this method.
Pooled Buffer
objects are created using Buffer.allocUnsafe()
,
or Buffer.from()
, or are often returned by various node:fs
module
callbacks. These types of Buffer
s use a shared underlying
<ArrayBuffer> object that contains all of the data from all of
the pooled Buffer
instances. When a Buffer
, <TypedArray>,
or <DataView> is passed in to readableStreamBYOBReader.read()
,
the view's underlying ArrayBuffer
is detached, invalidating
all existing views that may exist on that ArrayBuffer
. This
can have disastrous consequences for your application.
readableStreamBYOBReader.releaseLock()
#
Releases this reader's lock on the underlying <ReadableStream>.
Class: ReadableStreamDefaultController
#
Every <ReadableStream> has a controller that is responsible for
the internal state and management of the stream's queue. The
ReadableStreamDefaultController
is the default controller
implementation for ReadableStream
s that are not byte-oriented.
readableStreamDefaultController.close()
#
Closes the <ReadableStream> to which this controller is associated.
readableStreamDefaultController.desiredSize
#
- Type: <number>
Returns the amount of data remaining to fill the <ReadableStream>'s queue.
readableStreamDefaultController.enqueue([chunk])
#
chunk
<any>
Appends a new chunk of data to the <ReadableStream>'s queue.
readableStreamDefaultController.error([error])
#
error
<any>
Signals an error that causes the <ReadableStream> to error and close.
Class: ReadableByteStreamController
#
Every <ReadableStream> has a controller that is responsible for
the internal state and management of the stream's queue. The
ReadableByteStreamController
is for byte-oriented ReadableStream
s.
readableByteStreamController.byobRequest
#
readableByteStreamController.close()
#
Closes the <ReadableStream> to which this controller is associated.
readableByteStreamController.desiredSize
#
- Type: <number>
Returns the amount of data remaining to fill the <ReadableStream>'s queue.
readableByteStreamController.enqueue(chunk)
#
chunk
: <Buffer> | <TypedArray> | <DataView>
Appends a new chunk of data to the <ReadableStream>'s queue.
readableByteStreamController.error([error])
#
error
<any>
Signals an error that causes the <ReadableStream> to error and close.
Class: ReadableStreamBYOBRequest
#
When using ReadableByteStreamController
in byte-oriented
streams, and when using the ReadableStreamBYOBReader
,
the readableByteStreamController.byobRequest
property
provides access to a ReadableStreamBYOBRequest
instance
that represents the current read request. The object
is used to gain access to the ArrayBuffer
/TypedArray
that has been provided for the read request to fill,
and provides methods for signaling that the data has
been provided.
readableStreamBYOBRequest.respond(bytesWritten)
#
bytesWritten
<number>
Signals that a bytesWritten
number of bytes have been written
to readableStreamBYOBRequest.view
.
readableStreamBYOBRequest.respondWithNewView(view)
#
view
<Buffer> | <TypedArray> | <DataView>
Signals that the request has been fulfilled with bytes written
to a new Buffer
, TypedArray
, or DataView
.
readableStreamBYOBRequest.view
#
- Type: <Buffer> | <TypedArray> | <DataView>
Class: WritableStream
#
The WritableStream
is a destination to which stream data is sent.
import {
WritableStream,
} from 'node:stream/web';
const stream = new WritableStream({
write(chunk) {
console.log(chunk);
},
});
await stream.getWriter().write('Hello World');
new WritableStream([underlyingSink[, strategy]])
#
underlyingSink
<Object>start
<Function> A user-defined function that is invoked immediately when theWritableStream
is created.controller
<WritableStreamDefaultController>- Returns:
undefined
or a promise fulfilled withundefined
.
write
<Function> A user-defined function that is invoked when a chunk of data has been written to theWritableStream
.chunk
<any>controller
<WritableStreamDefaultController>- Returns: A promise fulfilled with
undefined
.
close
<Function> A user-defined function that is called when theWritableStream
is closed.- Returns: A promise fulfilled with
undefined
.
- Returns: A promise fulfilled with
abort
<Function> A user-defined function that is called to abruptly close theWritableStream
.reason
<any>- Returns: A promise fulfilled with
undefined
.
type
<any> Thetype
option is reserved for future use and must be undefined.
strategy
<Object>highWaterMark
<number> The maximum internal queue size before backpressure is applied.size
<Function> A user-defined function used to identify the size of each chunk of data.
writableStream.abort([reason])
#
reason
<any>- Returns: A promise fulfilled with
undefined
.
Abruptly terminates the WritableStream
. All queued writes will be
canceled with their associated promises rejected.
writableStream.close()
#
- Returns: A promise fulfilled with
undefined
.
Closes the WritableStream
when no additional writes are expected.
writableStream.getWriter()
#
- Returns: <WritableStreamDefaultWriter>
Creates and creates a new writer instance that can be used to write
data into the WritableStream
.
writableStream.locked
#
- Type: <boolean>
The writableStream.locked
property is false
by default, and is
switched to true
while there is an active writer attached to this
WritableStream
.
Transferring with postMessage()#
A <WritableStream> instance can be transferred using a <MessagePort>.
const stream = new WritableStream(getWritableSinkSomehow());
const { port1, port2 } = new MessageChannel();
port1.onmessage = ({ data }) => {
data.getWriter().write('hello');
};
port2.postMessage(stream, [stream]);
Class: WritableStreamDefaultWriter
#
new WritableStreamDefaultWriter(stream)
#
stream
<WritableStream>
Creates a new WritableStreamDefaultWriter
that is locked to the given
WritableStream
.
writableStreamDefaultWriter.abort([reason])
#
reason
<any>- Returns: A promise fulfilled with
undefined
.
Abruptly terminates the WritableStream
. All queued writes will be
canceled with their associated promises rejected.
writableStreamDefaultWriter.close()
#
- Returns: A promise fulfilled with
undefined
.
Closes the WritableStream
when no additional writes are expected.
writableStreamDefaultWriter.closed
#
- Type: <Promise> Fulfilled with
undefined
when the associated <WritableStream> is closed or rejected if the stream errors or the writer's lock is released before the stream finishes closing.
writableStreamDefaultWriter.desiredSize
#
- Type: <number>
The amount of data required to fill the <WritableStream>'s queue.
writableStreamDefaultWriter.ready
#
- type: A promise that is fulfilled with
undefined
when the writer is ready to be used.
writableStreamDefaultWriter.releaseLock()
#
Releases this writer's lock on the underlying <ReadableStream>.
writableStreamDefaultWriter.write([chunk])
#
chunk
: <any>- Returns: A promise fulfilled with
undefined
.
Appends a new chunk of data to the <WritableStream>'s queue.
Class: WritableStreamDefaultController
#
The WritableStreamDefaultController
manage's the <WritableStream>'s
internal state.
writableStreamDefaultController.error([error])
#
error
<any>
Called by user-code to signal that an error has occurred while processing
the WritableStream
data. When called, the <WritableStream> will be aborted,
with currently pending writes canceled.
writableStreamDefaultController.signal
#
- Type: <AbortSignal> An
AbortSignal
that can be used to cancel pending write or close operations when a <WritableStream> is aborted.
Class: TransformStream
#
A TransformStream
consists of a <ReadableStream> and a <WritableStream> that
are connected such that the data written to the WritableStream
is received,
and potentially transformed, before being pushed into the ReadableStream
's
queue.
import {
TransformStream,
} from 'node:stream/web';
const transform = new TransformStream({
transform(chunk, controller) {
controller.enqueue(chunk.toUpperCase());
},
});
await Promise.all([
transform.writable.getWriter().write('A'),
transform.readable.getReader().read(),
]);
new TransformStream([transformer[, writableStrategy[, readableStrategy]]])
#
transformer
<Object>start
<Function> A user-defined function that is invoked immediately when theTransformStream
is created.controller
<TransformStreamDefaultController>- Returns:
undefined
or a promise fulfilled withundefined
transform
<Function> A user-defined function that receives, and potentially modifies, a chunk of data written totransformStream.writable
, before forwarding that on totransformStream.readable
.chunk
<any>controller
<TransformStreamDefaultController>- Returns: A promise fulfilled with
undefined
.
flush
<Function> A user-defined function that is called immediately before the writable side of theTransformStream
is closed, signaling the end of the transformation process.controller
<TransformStreamDefaultController>- Returns: A promise fulfilled with
undefined
.
readableType
<any> thereadableType
option is reserved for future use and must beundefined
.writableType
<any> thewritableType
option is reserved for future use and must beundefined
.
writableStrategy
<Object>highWaterMark
<number> The maximum internal queue size before backpressure is applied.size
<Function> A user-defined function used to identify the size of each chunk of data.
readableStrategy
<Object>highWaterMark
<number> The maximum internal queue size before backpressure is applied.size
<Function> A user-defined function used to identify the size of each chunk of data.
transformStream.readable
#
- Type: <ReadableStream>
transformStream.writable
#
- Type: <WritableStream>
Transferring with postMessage()#
A <TransformStream> instance can be transferred using a <MessagePort>.
const stream = new TransformStream();
const { port1, port2 } = new MessageChannel();
port1.onmessage = ({ data }) => {
const { writable, readable } = data;
// ...
};
port2.postMessage(stream, [stream]);
Class: TransformStreamDefaultController
#
The TransformStreamDefaultController
manages the internal state
of the TransformStream
.
transformStreamDefaultController.desiredSize
#
- Type: <number>
The amount of data required to fill the readable side's queue.
transformStreamDefaultController.enqueue([chunk])
#
chunk
<any>
Appends a chunk of data to the readable side's queue.
transformStreamDefaultController.error([reason])
#
reason
<any>
Signals to both the readable and writable side that an error has occurred while processing the transform data, causing both sides to be abruptly closed.
transformStreamDefaultController.terminate()
#
Closes the readable side of the transport and causes the writable side to be abruptly closed with an error.
Class: ByteLengthQueuingStrategy
#
new ByteLengthQueuingStrategy(options)
#
byteLengthQueuingStrategy.highWaterMark
#
- Type: <number>
byteLengthQueuingStrategy.size
#
- Type: <Function>
Class: CountQueuingStrategy
#
new CountQueuingStrategy(options)
#
countQueuingStrategy.highWaterMark
#
- Type: <number>
countQueuingStrategy.size
#
- Type: <Function>
Class: TextEncoderStream
#
new TextEncoderStream()
#
Creates a new TextEncoderStream
instance.
textEncoderStream.encoding
#
- Type: <string>
The encoding supported by the TextEncoderStream
instance.
textEncoderStream.readable
#
- Type: <ReadableStream>
textEncoderStream.writable
#
- Type: <WritableStream>
Class: TextDecoderStream
#
new TextDecoderStream([encoding[, options]])
#
encoding
<string> Identifies theencoding
that thisTextDecoder
instance supports. Default:'utf-8'
.options
<Object>fatal
<boolean>true
if decoding failures are fatal.ignoreBOM
<boolean> Whentrue
, theTextDecoderStream
will include the byte order mark in the decoded result. Whenfalse
, the byte order mark will be removed from the output. This option is only used whenencoding
is'utf-8'
,'utf-16be'
, or'utf-16le'
. Default:false
.
Creates a new TextDecoderStream
instance.
textDecoderStream.encoding
#
- Type: <string>
The encoding supported by the TextDecoderStream
instance.
textDecoderStream.fatal
#
- Type: <boolean>
The value will be true
if decoding errors result in a TypeError
being
thrown.
textDecoderStream.ignoreBOM
#
- Type: <boolean>
The value will be true
if the decoding result will include the byte order
mark.
textDecoderStream.readable
#
- Type: <ReadableStream>
textDecoderStream.writable
#
- Type: <WritableStream>
Class: CompressionStream
#
new CompressionStream(format)
#
format
<string> One of either'deflate'
or'gzip'
.
compressionStream.readable
#
- Type: <ReadableStream>
compressionStream.writable
#
- Type: <WritableStream>
Class: DecompressionStream
#
new DecompressionStream(format)
#
format
<string> One of either'deflate'
or'gzip'
.
decompressionStream.readable
#
- Type: <ReadableStream>
decompressionStream.writable
#
- Type: <WritableStream>
Utility Consumers#
The utility consumer functions provide common options for consuming streams.
They are accessed using:
import {
arrayBuffer,
blob,
buffer,
json,
text,
} from 'node:stream/consumers';
const {
arrayBuffer,
blob,
buffer,
json,
text,
} = require('node:stream/consumers');
streamConsumers.arrayBuffer(stream)
#
stream
<ReadableStream> | <stream.Readable> | <AsyncIterator>- Returns: <Promise> Fulfills with an
ArrayBuffer
containing the full contents of the stream.
import { buffer as arrayBuffer } from 'node:stream/consumers';
import { Readable } from 'node:stream';
import { TextEncoder } from 'node:util';
const encoder = new TextEncoder();
const dataArray = encoder.encode('hello world from consumers!');
const readable = Readable.from(dataArray);
const data = await arrayBuffer(readable);
console.log(`from readable: ${data.byteLength}`);
const { arrayBuffer } = require('node:stream/consumers');
const { Readable } = require('node:stream');
const { TextEncoder } = require('node:util');
const encoder = new TextEncoder();
const dataArray = encoder.encode('hello world from consumers!');
const readable = Readable.from(dataArray);
arrayBuffer(readable).then((data) => {
console.log(`from readable: ${data.byteLength}`);
});
streamConsumers.blob(stream)
#
stream
<ReadableStream> | <stream.Readable> | <AsyncIterator>- Returns: <Promise> Fulfills with a <Blob> containing the full contents of the stream.
import { blob } from 'node:stream/consumers';
const dataBlob = new Blob(['hello world from consumers!']);
const readable = dataBlob.stream();
const data = await blob(readable);
console.log(`from readable: ${data.size}`);
const { blob } = require('node:stream/consumers');
const dataBlob = new Blob(['hello world from consumers!']);
const readable = dataBlob.stream();
blob(readable).then((data) => {
console.log(`from readable: ${data.size}`);
});
streamConsumers.buffer(stream)
#
stream
<ReadableStream> | <stream.Readable> | <AsyncIterator>- Returns: <Promise> Fulfills with a <Buffer> containing the full contents of the stream.
import { buffer } from 'node:stream/consumers';
import { Readable } from 'node:stream';
import { Buffer } from 'node:buffer';
const dataBuffer = Buffer.from('hello world from consumers!');
const readable = Readable.from(dataBuffer);
const data = await buffer(readable);
console.log(`from readable: ${data.length}`);
const { buffer } = require('node:stream/consumers');
const { Readable } = require('node:stream');
const { Buffer } = require('node:buffer');
const dataBuffer = Buffer.from('hello world from consumers!');
const readable = Readable.from(dataBuffer);
buffer(readable).then((data) => {
console.log(`from readable: ${data.length}`);
});
streamConsumers.json(stream)
#
stream
<ReadableStream> | <stream.Readable> | <AsyncIterator>- Returns: <Promise> Fulfills with the contents of the stream parsed as a
UTF-8 encoded string that is then passed through
JSON.parse()
.
import { json } from 'node:stream/consumers';
import { Readable } from 'node:stream';
const items = Array.from(
{
length: 100,
},
() => ({
message: 'hello world from consumers!',
}),
);
const readable = Readable.from(JSON.stringify(items));
const data = await json(readable);
console.log(`from readable: ${data.length}`);
const { json } = require('node:stream/consumers');
const { Readable } = require('node:stream');
const items = Array.from(
{
length: 100,
},
() => ({
message: 'hello world from consumers!',
}),
);
const readable = Readable.from(JSON.stringify(items));
json(readable).then((data) => {
console.log(`from readable: ${data.length}`);
});
streamConsumers.text(stream)
#
stream
<ReadableStream> | <stream.Readable> | <AsyncIterator>- Returns: <Promise> Fulfills with the contents of the stream parsed as a UTF-8 encoded string.
import { text } from 'node:stream/consumers';
import { Readable } from 'node:stream';
const readable = Readable.from('Hello world from consumers!');
const data = await text(readable);
console.log(`from readable: ${data.length}`);
const { text } = require('node:stream/consumers');
const { Readable } = require('node:stream');
const readable = Readable.from('Hello world from consumers!');
text(readable).then((data) => {
console.log(`from readable: ${data.length}`);
});
Worker threads#
Source Code: lib/worker_threads.js
The node:worker_threads
module enables the use of threads that execute
JavaScript in parallel. To access it:
const worker = require('node:worker_threads');
Workers (threads) are useful for performing CPU-intensive JavaScript operations. They do not help much with I/O-intensive work. The Node.js built-in asynchronous I/O operations are more efficient than Workers can be.
Unlike child_process
or cluster
, worker_threads
can share memory. They do
so by transferring ArrayBuffer
instances or sharing SharedArrayBuffer
instances.
const {
Worker, isMainThread, parentPort, workerData,
} = require('node:worker_threads');
if (isMainThread) {
module.exports = function parseJSAsync(script) {
return new Promise((resolve, reject) => {
const worker = new Worker(__filename, {
workerData: script,
});
worker.on('message', resolve);
worker.on('error', reject);
worker.on('exit', (code) => {
if (code !== 0)
reject(new Error(`Worker stopped with exit code ${code}`));
});
});
};
} else {
const { parse } = require('some-js-parsing-library');
const script = workerData;
parentPort.postMessage(parse(script));
}
The above example spawns a Worker thread for each parseJSAsync()
call. In
practice, use a pool of Workers for these kinds of tasks. Otherwise, the
overhead of creating Workers would likely exceed their benefit.
When implementing a worker pool, use the AsyncResource
API to inform
diagnostic tools (e.g. to provide asynchronous stack traces) about the
correlation between tasks and their outcomes. See
"Using AsyncResource
for a Worker
thread pool"
in the async_hooks
documentation for an example implementation.
Worker threads inherit non-process-specific options by default. Refer to
Worker constructor options
to know how to customize worker thread options,
specifically argv
and execArgv
options.
worker.getEnvironmentData(key)
#
Within a worker thread, worker.getEnvironmentData()
returns a clone
of data passed to the spawning thread's worker.setEnvironmentData()
.
Every new Worker
receives its own copy of the environment data
automatically.
const {
Worker,
isMainThread,
setEnvironmentData,
getEnvironmentData,
} = require('node:worker_threads');
if (isMainThread) {
setEnvironmentData('Hello', 'World!');
const worker = new Worker(__filename);
} else {
console.log(getEnvironmentData('Hello')); // Prints 'World!'.
}
worker.isMainThread
#
Is true
if this code is not running inside of a Worker
thread.
const { Worker, isMainThread } = require('node:worker_threads');
if (isMainThread) {
// This re-loads the current file inside a Worker instance.
new Worker(__filename);
} else {
console.log('Inside Worker!');
console.log(isMainThread); // Prints 'false'.
}
worker.markAsUntransferable(object)
#
Mark an object as not transferable. If object
occurs in the transfer list of
a port.postMessage()
call, it is ignored.
In particular, this makes sense for objects that can be cloned, rather than
transferred, and which are used by other objects on the sending side.
For example, Node.js marks the ArrayBuffer
s it uses for its
Buffer
pool with this.
This operation cannot be undone.
const { MessageChannel, markAsUntransferable } = require('node:worker_threads');
const pooledBuffer = new ArrayBuffer(8);
const typedArray1 = new Uint8Array(pooledBuffer);
const typedArray2 = new Float64Array(pooledBuffer);
markAsUntransferable(pooledBuffer);
const { port1 } = new MessageChannel();
port1.postMessage(typedArray1, [ typedArray1.buffer ]);
// The following line prints the contents of typedArray1 -- it still owns
// its memory and has been cloned, not transferred. Without
// `markAsUntransferable()`, this would print an empty Uint8Array.
// typedArray2 is intact as well.
console.log(typedArray1);
console.log(typedArray2);
There is no equivalent to this API in browsers.
worker.moveMessagePortToContext(port, contextifiedSandbox)
#
-
port
<MessagePort> The message port to transfer. -
contextifiedSandbox
<Object> A contextified object as returned by thevm.createContext()
method. -
Returns: <MessagePort>
Transfer a MessagePort
to a different vm
Context. The original port
object is rendered unusable, and the returned MessagePort
instance
takes its place.
The returned MessagePort
is an object in the target context and
inherits from its global Object
class. Objects passed to the
port.onmessage()
listener are also created in the target context
and inherit from its global Object
class.
However, the created MessagePort
no longer inherits from
EventTarget
, and only port.onmessage()
can be used to receive
events using it.
worker.parentPort
#
If this thread is a Worker
, this is a MessagePort
allowing communication with the parent thread. Messages sent using
parentPort.postMessage()
are available in the parent thread
using worker.on('message')
, and messages sent from the parent thread
using worker.postMessage()
are available in this thread using
parentPort.on('message')
.
const { Worker, isMainThread, parentPort } = require('node:worker_threads');
if (isMainThread) {
const worker = new Worker(__filename);
worker.once('message', (message) => {
console.log(message); // Prints 'Hello, world!'.
});
worker.postMessage('Hello, world!');
} else {
// When a message from the parent thread is received, send it back:
parentPort.once('message', (message) => {
parentPort.postMessage(message);
});
}
worker.receiveMessageOnPort(port)
#
-
port
<MessagePort> | <BroadcastChannel> -
Returns: <Object> | <undefined>
Receive a single message from a given MessagePort
. If no message is available,
undefined
is returned, otherwise an object with a single message
property
that contains the message payload, corresponding to the oldest message in the
MessagePort
's queue.
const { MessageChannel, receiveMessageOnPort } = require('node:worker_threads');
const { port1, port2 } = new MessageChannel();
port1.postMessage({ hello: 'world' });
console.log(receiveMessageOnPort(port2));
// Prints: { message: { hello: 'world' } }
console.log(receiveMessageOnPort(port2));
// Prints: undefined
When this function is used, no 'message'
event is emitted and the
onmessage
listener is not invoked.
worker.resourceLimits
#
Provides the set of JS engine resource constraints inside this Worker thread.
If the resourceLimits
option was passed to the Worker
constructor,
this matches its values.
If this is used in the main thread, its value is an empty object.
worker.SHARE_ENV
#
A special value that can be passed as the env
option of the Worker
constructor, to indicate that the current thread and the Worker thread should
share read and write access to the same set of environment variables.
const { Worker, SHARE_ENV } = require('node:worker_threads');
new Worker('process.env.SET_IN_WORKER = "foo"', { eval: true, env: SHARE_ENV })
.on('exit', () => {
console.log(process.env.SET_IN_WORKER); // Prints 'foo'.
});
worker.setEnvironmentData(key[, value])
#
key
<any> Any arbitrary, cloneable JavaScript value that can be used as a <Map> key.value
<any> Any arbitrary, cloneable JavaScript value that will be cloned and passed automatically to all newWorker
instances. Ifvalue
is passed asundefined
, any previously set value for thekey
will be deleted.
The worker.setEnvironmentData()
API sets the content of
worker.getEnvironmentData()
in the current thread and all new Worker
instances spawned from the current context.
worker.threadId
#
An integer identifier for the current thread. On the corresponding worker object
(if there is any), it is available as worker.threadId
.
This value is unique for each Worker
instance inside a single process.
worker.workerData
#
An arbitrary JavaScript value that contains a clone of the data passed
to this thread's Worker
constructor.
The data is cloned as if using postMessage()
,
according to the HTML structured clone algorithm.
const { Worker, isMainThread, workerData } = require('node:worker_threads');
if (isMainThread) {
const worker = new Worker(__filename, { workerData: 'Hello, world!' });
} else {
console.log(workerData); // Prints 'Hello, world!'.
}
Class: BroadcastChannel extends EventTarget
#
Instances of BroadcastChannel
allow asynchronous one-to-many communication
with all other BroadcastChannel
instances bound to the same channel name.
'use strict';
const {
isMainThread,
BroadcastChannel,
Worker,
} = require('node:worker_threads');
const bc = new BroadcastChannel('hello');
if (isMainThread) {
let c = 0;
bc.onmessage = (event) => {
console.log(event.data);
if (++c === 10) bc.close();
};
for (let n = 0; n < 10; n++)
new Worker(__filename);
} else {
bc.postMessage('hello from every worker');
bc.close();
}
new BroadcastChannel(name)
#
name
<any> The name of the channel to connect to. Any JavaScript value that can be converted to a string using`${name}`
is permitted.
broadcastChannel.close()
#
Closes the BroadcastChannel
connection.
broadcastChannel.onmessage
#
- Type: <Function> Invoked with a single
MessageEvent
argument when a message is received.
broadcastChannel.onmessageerror
#
- Type: <Function> Invoked with a received message cannot be deserialized.
broadcastChannel.postMessage(message)
#
message
<any> Any cloneable JavaScript value.
broadcastChannel.ref()
#
Opposite of unref()
. Calling ref()
on a previously unref()
ed
BroadcastChannel does not let the program exit if it's the only active handle
left (the default behavior). If the port is ref()
ed, calling ref()
again
has no effect.
broadcastChannel.unref()
#
Calling unref()
on a BroadcastChannel allows the thread to exit if this
is the only active handle in the event system. If the BroadcastChannel is
already unref()
ed calling unref()
again has no effect.
Class: MessageChannel
#
Instances of the worker.MessageChannel
class represent an asynchronous,
two-way communications channel.
The MessageChannel
has no methods of its own. new MessageChannel()
yields an object with port1
and port2
properties, which refer to linked
MessagePort
instances.
const { MessageChannel } = require('node:worker_threads');
const { port1, port2 } = new MessageChannel();
port1.on('message', (message) => console.log('received', message));
port2.postMessage({ foo: 'bar' });
// Prints: received { foo: 'bar' } from the `port1.on('message')` listener
Class: MessagePort
#
- Extends: <EventTarget>
Instances of the worker.MessagePort
class represent one end of an
asynchronous, two-way communications channel. It can be used to transfer
structured data, memory regions and other MessagePort
s between different
Worker
s.
This implementation matches browser MessagePort
s.
Event: 'close'
#
The 'close'
event is emitted once either side of the channel has been
disconnected.
const { MessageChannel } = require('node:worker_threads');
const { port1, port2 } = new MessageChannel();
// Prints:
// foobar
// closed!
port2.on('message', (message) => console.log(message));
port2.on('close', () => console.log('closed!'));
port1.postMessage('foobar');
port1.close();
Event: 'message'
#
value
<any> The transmitted value
The 'message'
event is emitted for any incoming message, containing the cloned
input of port.postMessage()
.
Listeners on this event receive a clone of the value
parameter as passed
to postMessage()
and no further arguments.
Event: 'messageerror'
#
error
<Error> An Error object
The 'messageerror'
event is emitted when deserializing a message failed.
Currently, this event is emitted when there is an error occurring while
instantiating the posted JS object on the receiving end. Such situations
are rare, but can happen, for instance, when certain Node.js API objects
are received in a vm.Context
(where Node.js APIs are currently
unavailable).
port.close()
#
Disables further sending of messages on either side of the connection.
This method can be called when no further communication will happen over this
MessagePort
.
The 'close'
event is emitted on both MessagePort
instances that
are part of the channel.
port.postMessage(value[, transferList])
#
value
<any>transferList
<Object[]>
Sends a JavaScript value to the receiving side of this channel.
value
is transferred in a way which is compatible with
the HTML structured clone algorithm.
In particular, the significant differences to JSON
are:
value
may contain circular references.value
may contain instances of builtin JS types such asRegExp
s,BigInt
s,Map
s,Set
s, etc.value
may contain typed arrays, both usingArrayBuffer
s andSharedArrayBuffer
s.value
may containWebAssembly.Module
instances.value
may not contain native (C++-backed) objects other than:
const { MessageChannel } = require('node:worker_threads');
const { port1, port2 } = new MessageChannel();
port1.on('message', (message) => console.log(message));
const circularData = {};
circularData.foo = circularData;
// Prints: { foo: [Circular] }
port2.postMessage(circularData);
transferList
may be a list of ArrayBuffer
, MessagePort
, and
FileHandle
objects.
After transferring, they are not usable on the sending side of the channel
anymore (even if they are not contained in value
). Unlike with
child processes, transferring handles such as network sockets is currently
not supported.
If value
contains SharedArrayBuffer
instances, those are accessible
from either thread. They cannot be listed in transferList
.
value
may still contain ArrayBuffer
instances that are not in
transferList
; in that case, the underlying memory is copied rather than moved.
const { MessageChannel } = require('node:worker_threads');
const { port1, port2 } = new MessageChannel();
port1.on('message', (message) => console.log(message));
const uint8Array = new Uint8Array([ 1, 2, 3, 4 ]);
// This posts a copy of `uint8Array`:
port2.postMessage(uint8Array);
// This does not copy data, but renders `uint8Array` unusable:
port2.postMessage(uint8Array, [ uint8Array.buffer ]);
// The memory for the `sharedUint8Array` is accessible from both the
// original and the copy received by `.on('message')`:
const sharedUint8Array = new Uint8Array(new SharedArrayBuffer(4));
port2.postMessage(sharedUint8Array);
// This transfers a freshly created message port to the receiver.
// This can be used, for example, to create communication channels between
// multiple `Worker` threads that are children of the same parent thread.
const otherChannel = new MessageChannel();
port2.postMessage({ port: otherChannel.port1 }, [ otherChannel.port1 ]);
The message object is cloned immediately, and can be modified after posting without having side effects.
For more information on the serialization and deserialization mechanisms
behind this API, see the serialization API of the node:v8
module.
Considerations when transferring TypedArrays and Buffers#
All TypedArray
and Buffer
instances are views over an underlying
ArrayBuffer
. That is, it is the ArrayBuffer
that actually stores
the raw data while the TypedArray
and Buffer
objects provide a
way of viewing and manipulating the data. It is possible and common
for multiple views to be created over the same ArrayBuffer
instance.
Great care must be taken when using a transfer list to transfer an
ArrayBuffer
as doing so causes all TypedArray
and Buffer
instances that share that same ArrayBuffer
to become unusable.
const ab = new ArrayBuffer(10);
const u1 = new Uint8Array(ab);
const u2 = new Uint16Array(ab);
console.log(u2.length); // prints 5
port.postMessage(u1, [u1.buffer]);
console.log(u2.length); // prints 0
For Buffer
instances, specifically, whether the underlying
ArrayBuffer
can be transferred or cloned depends entirely on how
instances were created, which often cannot be reliably determined.
An ArrayBuffer
can be marked with markAsUntransferable()
to indicate
that it should always be cloned and never transferred.
Depending on how a Buffer
instance was created, it may or may
not own its underlying ArrayBuffer
. An ArrayBuffer
must not
be transferred unless it is known that the Buffer
instance
owns it. In particular, for Buffer
s created from the internal
Buffer
pool (using, for instance Buffer.from()
or Buffer.allocUnsafe()
),
transferring them is not possible and they are always cloned,
which sends a copy of the entire Buffer
pool.
This behavior may come with unintended higher memory
usage and possible security concerns.
See Buffer.allocUnsafe()
for more details on Buffer
pooling.
The ArrayBuffer
s for Buffer
instances created using
Buffer.alloc()
or Buffer.allocUnsafeSlow()
can always be
transferred but doing so renders all other existing views of
those ArrayBuffer
s unusable.
Considerations when cloning objects with prototypes, classes, and accessors#
Because object cloning uses the HTML structured clone algorithm,
non-enumerable properties, property accessors, and object prototypes are
not preserved. In particular, Buffer
objects will be read as
plain Uint8Array
s on the receiving side, and instances of JavaScript
classes will be cloned as plain JavaScript objects.
const b = Symbol('b');
class Foo {
#a = 1;
constructor() {
this[b] = 2;
this.c = 3;
}
get d() { return 4; }
}
const { port1, port2 } = new MessageChannel();
port1.onmessage = ({ data }) => console.log(data);
port2.postMessage(new Foo());
// Prints: { c: 3 }
This limitation extends to many built-in objects, such as the global URL
object:
const { port1, port2 } = new MessageChannel();
port1.onmessage = ({ data }) => console.log(data);
port2.postMessage(new URL('https://example.org'));
// Prints: { }
port.hasRef()
#
- Returns: <boolean>
If true, the MessagePort
object will keep the Node.js event loop active.
port.ref()
#
Opposite of unref()
. Calling ref()
on a previously unref()
ed port does
not let the program exit if it's the only active handle left (the default
behavior). If the port is ref()
ed, calling ref()
again has no effect.
If listeners are attached or removed using .on('message')
, the port
is ref()
ed and unref()
ed automatically depending on whether
listeners for the event exist.
port.start()
#
Starts receiving messages on this MessagePort
. When using this port
as an event emitter, this is called automatically once 'message'
listeners are attached.
This method exists for parity with the Web MessagePort
API. In Node.js,
it is only useful for ignoring messages when no event listener is present.
Node.js also diverges in its handling of .onmessage
. Setting it
automatically calls .start()
, but unsetting it lets messages queue up
until a new handler is set or the port is discarded.
port.unref()
#
Calling unref()
on a port allows the thread to exit if this is the only
active handle in the event system. If the port is already unref()
ed calling
unref()
again has no effect.
If listeners are attached or removed using .on('message')
, the port is
ref()
ed and unref()
ed automatically depending on whether
listeners for the event exist.
Class: Worker
#
- Extends: <EventEmitter>
The Worker
class represents an independent JavaScript execution thread.
Most Node.js APIs are available inside of it.
Notable differences inside a Worker environment are:
- The
process.stdin
,process.stdout
, andprocess.stderr
streams may be redirected by the parent thread. - The
require('node:worker_threads').isMainThread
property is set tofalse
. - The
require('node:worker_threads').parentPort
message port is available. process.exit()
does not stop the whole program, just the single thread, andprocess.abort()
is not available.process.chdir()
andprocess
methods that set group or user ids are not available.process.env
is a copy of the parent thread's environment variables, unless otherwise specified. Changes to one copy are not visible in other threads, and are not visible to native add-ons (unlessworker.SHARE_ENV
is passed as theenv
option to theWorker
constructor).process.title
cannot be modified.- Signals are not delivered through
process.on('...')
. - Execution may stop at any point as a result of
worker.terminate()
being invoked. - IPC channels from parent processes are not accessible.
- The
trace_events
module is not supported. - Native add-ons can only be loaded from multiple threads if they fulfill certain conditions.
Creating Worker
instances inside of other Worker
s is possible.
Like Web Workers and the node:cluster
module, two-way communication
can be achieved through inter-thread message passing. Internally, a Worker
has
a built-in pair of MessagePort
s that are already associated with each
other when the Worker
is created. While the MessagePort
object on the parent
side is not directly exposed, its functionalities are exposed through
worker.postMessage()
and the worker.on('message')
event
on the Worker
object for the parent thread.
To create custom messaging channels (which is encouraged over using the default
global channel because it facilitates separation of concerns), users can create
a MessageChannel
object on either thread and pass one of the
MessagePort
s on that MessageChannel
to the other thread through a
pre-existing channel, such as the global one.
See port.postMessage()
for more information on how messages are passed,
and what kind of JavaScript values can be successfully transported through
the thread barrier.
const assert = require('node:assert');
const {
Worker, MessageChannel, MessagePort, isMainThread, parentPort,
} = require('node:worker_threads');
if (isMainThread) {
const worker = new Worker(__filename);
const subChannel = new MessageChannel();
worker.postMessage({ hereIsYourPort: subChannel.port1 }, [subChannel.port1]);
subChannel.port2.on('message', (value) => {
console.log('received:', value);
});
} else {
parentPort.once('message', (value) => {
assert(value.hereIsYourPort instanceof MessagePort);
value.hereIsYourPort.postMessage('the worker is sending this');
value.hereIsYourPort.close();
});
}
new Worker(filename[, options])
#
filename
<string> | <URL> The path to the Worker's main script or module. Must be either an absolute path or a relative path (i.e. relative to the current working directory) starting with./
or../
, or a WHATWGURL
object usingfile:
ordata:
protocol. When using adata:
URL, the data is interpreted based on MIME type using the ECMAScript module loader. Ifoptions.eval
istrue
, this is a string containing JavaScript code rather than a path.options
<Object>argv
<any[]> List of arguments which would be stringified and appended toprocess.argv
in the worker. This is mostly similar to theworkerData
but the values are available on the globalprocess.argv
as if they were passed as CLI options to the script.env
<Object> If set, specifies the initial value ofprocess.env
inside the Worker thread. As a special value,worker.SHARE_ENV
may be used to specify that the parent thread and the child thread should share their environment variables; in that case, changes to one thread'sprocess.env
object affect the other thread as well. Default:process.env
.eval
<boolean> Iftrue
and the first argument is astring
, interpret the first argument to the constructor as a script that is executed once the worker is online.execArgv
<string[]> List of node CLI options passed to the worker. V8 options (such as--max-old-space-size
) and options that affect the process (such as--title
) are not supported. If set, this is provided asprocess.execArgv
inside the worker. By default, options are inherited from the parent thread.stdin
<boolean> If this is set totrue
, thenworker.stdin
provides a writable stream whose contents appear asprocess.stdin
inside the Worker. By default, no data is provided.stdout
<boolean> If this is set totrue
, thenworker.stdout
is not automatically piped through toprocess.stdout
in the parent.stderr
<boolean> If this is set totrue
, thenworker.stderr
is not automatically piped through toprocess.stderr
in the parent.workerData
<any> Any JavaScript value that is cloned and made available asrequire('node:worker_threads').workerData
. The cloning occurs as described in the HTML structured clone algorithm, and an error is thrown if the object cannot be cloned (e.g. because it containsfunction
s).trackUnmanagedFds
<boolean> If this is set totrue
, then the Worker tracks raw file descriptors managed throughfs.open()
andfs.close()
, and closes them when the Worker exits, similar to other resources like network sockets or file descriptors managed through theFileHandle
API. This option is automatically inherited by all nestedWorker
s. Default:true
.transferList
<Object[]> If one or moreMessagePort
-like objects are passed inworkerData
, atransferList
is required for those items orERR_MISSING_MESSAGE_PORT_IN_TRANSFER_LIST
is thrown. Seeport.postMessage()
for more information.resourceLimits
<Object> An optional set of resource limits for the new JS engine instance. Reaching these limits leads to termination of theWorker
instance. These limits only affect the JS engine, and no external data, including noArrayBuffer
s. Even if these limits are set, the process may still abort if it encounters a global out-of-memory situation.maxOldGenerationSizeMb
<number> The maximum size of the main heap in MB. If the command-line argument--max-old-space-size
is set, it overrides this setting.maxYoungGenerationSizeMb
<number> The maximum size of a heap space for recently created objects. If the command-line argument--max-semi-space-size
is set, it overrides this setting.codeRangeSizeMb
<number> The size of a pre-allocated memory range used for generated code.stackSizeMb
<number> The default maximum stack size for the thread. Small values may lead to unusable Worker instances. Default:4
.
name
<string> An optionalname
to be appended to the worker title for debuggin/identification purposes, making the final title as[worker ${id}] ${name}
. Default:''
.
Event: 'error'
#
err
<Error>
The 'error'
event is emitted if the worker thread throws an uncaught
exception. In that case, the worker is terminated.
Event: 'exit'
#
exitCode
<integer>
The 'exit'
event is emitted once the worker has stopped. If the worker
exited by calling process.exit()
, the exitCode
parameter is the
passed exit code. If the worker was terminated, the exitCode
parameter is
1
.
This is the final event emitted by any Worker
instance.
Event: 'message'
#
value
<any> The transmitted value
The 'message'
event is emitted when the worker thread has invoked
require('node:worker_threads').parentPort.postMessage()
.
See the port.on('message')
event for more details.
All messages sent from the worker thread are emitted before the
'exit'
event is emitted on the Worker
object.
Event: 'messageerror'
#
error
<Error> An Error object
The 'messageerror'
event is emitted when deserializing a message failed.
Event: 'online'
#
The 'online'
event is emitted when the worker thread has started executing
JavaScript code.
worker.getHeapSnapshot([options])
#
Returns a readable stream for a V8 snapshot of the current state of the Worker.
See v8.getHeapSnapshot()
for more details.
If the Worker thread is no longer running, which may occur before the
'exit'
event is emitted, the returned Promise
is rejected
immediately with an ERR_WORKER_NOT_RUNNING
error.
worker.performance
#
An object that can be used to query performance information from a worker
instance. Similar to perf_hooks.performance
.
performance.eventLoopUtilization([utilization1[, utilization2]])
#
utilization1
<Object> The result of a previous call toeventLoopUtilization()
.utilization2
<Object> The result of a previous call toeventLoopUtilization()
prior toutilization1
.- Returns <Object>
The same call as perf_hooks
eventLoopUtilization()
, except the values
of the worker instance are returned.
One difference is that, unlike the main thread, bootstrapping within a worker is done within the event loop. So the event loop utilization is immediately available once the worker's script begins execution.
An idle
time that does not increase does not indicate that the worker is
stuck in bootstrap. The following examples shows how the worker's entire
lifetime never accumulates any idle
time, but is still be able to process
messages.
const { Worker, isMainThread, parentPort } = require('node:worker_threads');
if (isMainThread) {
const worker = new Worker(__filename);
setInterval(() => {
worker.postMessage('hi');
console.log(worker.performance.eventLoopUtilization());
}, 100).unref();
return;
}
parentPort.on('message', () => console.log('msg')).unref();
(function r(n) {
if (--n < 0) return;
const t = Date.now();
while (Date.now() - t < 300);
setImmediate(r, n);
})(10);
The event loop utilization of a worker is available only after the 'online'
event emitted, and if called before this, or after the 'exit'
event, then all properties have the value of 0
.
worker.postMessage(value[, transferList])
#
value
<any>transferList
<Object[]>
Send a message to the worker that is received via
require('node:worker_threads').parentPort.on('message')
.
See port.postMessage()
for more details.
worker.ref()
#
Opposite of unref()
, calling ref()
on a previously unref()
ed worker does
not let the program exit if it's the only active handle left (the default
behavior). If the worker is ref()
ed, calling ref()
again has
no effect.
worker.resourceLimits
#
Provides the set of JS engine resource constraints for this Worker thread.
If the resourceLimits
option was passed to the Worker
constructor,
this matches its values.
If the worker has stopped, the return value is an empty object.
worker.stderr
#
This is a readable stream which contains data written to process.stderr
inside the worker thread. If stderr: true
was not passed to the
Worker
constructor, then data is piped to the parent thread's
process.stderr
stream.
worker.stdin
#
If stdin: true
was passed to the Worker
constructor, this is a
writable stream. The data written to this stream will be made available in
the worker thread as process.stdin
.
worker.stdout
#
This is a readable stream which contains data written to process.stdout
inside the worker thread. If stdout: true
was not passed to the
Worker
constructor, then data is piped to the parent thread's
process.stdout
stream.
worker.terminate()
#
- Returns: <Promise>
Stop all JavaScript execution in the worker thread as soon as possible.
Returns a Promise for the exit code that is fulfilled when the
'exit'
event is emitted.
worker.threadId
#
An integer identifier for the referenced thread. Inside the worker thread,
it is available as require('node:worker_threads').threadId
.
This value is unique for each Worker
instance inside a single process.
worker.unref()
#
Calling unref()
on a worker allows the thread to exit if this is the only
active handle in the event system. If the worker is already unref()
ed calling
unref()
again has no effect.
Notes#
Synchronous blocking of stdio#
Worker
s utilize message passing via <MessagePort> to implement interactions
with stdio
. This means that stdio
output originating from a Worker
can
get blocked by synchronous code on the receiving end that is blocking the
Node.js event loop.
import {
Worker,
isMainThread,
} from 'worker_threads';
if (isMainThread) {
new Worker(new URL(import.meta.url));
for (let n = 0; n < 1e10; n++) {
// Looping to simulate work.
}
} else {
// This output will be blocked by the for loop in the main thread.
console.log('foo');
}
'use strict';
const {
Worker,
isMainThread,
} = require('node:worker_threads');
if (isMainThread) {
new Worker(__filename);
for (let n = 0; n < 1e10; n++) {
// Looping to simulate work.
}
} else {
// This output will be blocked by the for loop in the main thread.
console.log('foo');
}
Launching worker threads from preload scripts#
Take care when launching worker threads from preload scripts (scripts loaded
and run using the -r
command line flag). Unless the execArgv
option is
explicitly set, new Worker threads automatically inherit the command line flags
from the running process and will preload the same preload scripts as the main
thread. If the preload script unconditionally launches a worker thread, every
thread spawned will spawn another until the application crashes.
Zlib#
Source Code: lib/zlib.js
The node:zlib
module provides compression functionality implemented using
Gzip, Deflate/Inflate, and Brotli.
To access it:
const zlib = require('node:zlib');
Compression and decompression are built around the Node.js Streams API.
Compressing or decompressing a stream (such as a file) can be accomplished by
piping the source stream through a zlib
Transform
stream into a destination
stream:
const { createGzip } = require('node:zlib');
const { pipeline } = require('node:stream');
const {
createReadStream,
createWriteStream,
} = require('node:fs');
const gzip = createGzip();
const source = createReadStream('input.txt');
const destination = createWriteStream('input.txt.gz');
pipeline(source, gzip, destination, (err) => {
if (err) {
console.error('An error occurred:', err);
process.exitCode = 1;
}
});
// Or, Promisified
const { promisify } = require('node:util');
const pipe = promisify(pipeline);
async function do_gzip(input, output) {
const gzip = createGzip();
const source = createReadStream(input);
const destination = createWriteStream(output);
await pipe(source, gzip, destination);
}
do_gzip('input.txt', 'input.txt.gz')
.catch((err) => {
console.error('An error occurred:', err);
process.exitCode = 1;
});
It is also possible to compress or decompress data in a single step:
const { deflate, unzip } = require('node:zlib');
const input = '.................................';
deflate(input, (err, buffer) => {
if (err) {
console.error('An error occurred:', err);
process.exitCode = 1;
}
console.log(buffer.toString('base64'));
});
const buffer = Buffer.from('eJzT0yMAAGTvBe8=', 'base64');
unzip(buffer, (err, buffer) => {
if (err) {
console.error('An error occurred:', err);
process.exitCode = 1;
}
console.log(buffer.toString());
});
// Or, Promisified
const { promisify } = require('node:util');
const do_unzip = promisify(unzip);
do_unzip(buffer)
.then((buf) => console.log(buf.toString()))
.catch((err) => {
console.error('An error occurred:', err);
process.exitCode = 1;
});
Threadpool usage and performance considerations#
All zlib
APIs, except those that are explicitly synchronous, use the Node.js
internal threadpool. This can lead to surprising effects and performance
limitations in some applications.
Creating and using a large number of zlib objects simultaneously can cause significant memory fragmentation.
const zlib = require('node:zlib');
const payload = Buffer.from('This is some data');
// WARNING: DO NOT DO THIS!
for (let i = 0; i < 30000; ++i) {
zlib.deflate(payload, (err, buffer) => {});
}
In the preceding example, 30,000 deflate instances are created concurrently. Because of how some operating systems handle memory allocation and deallocation, this may lead to significant memory fragmentation.
It is strongly recommended that the results of compression operations be cached to avoid duplication of effort.
Compressing HTTP requests and responses#
The node:zlib
module can be used to implement support for the gzip
, deflate
and br
content-encoding mechanisms defined by
HTTP.
The HTTP Accept-Encoding
header is used within an HTTP request to identify
the compression encodings accepted by the client. The Content-Encoding
header is used to identify the compression encodings actually applied to a
message.
The examples given below are drastically simplified to show the basic concept.
Using zlib
encoding can be expensive, and the results ought to be cached.
See Memory usage tuning for more information on the speed/memory/compression
tradeoffs involved in zlib
usage.
// Client request example
const zlib = require('node:zlib');
const http = require('node:http');
const fs = require('node:fs');
const { pipeline } = require('node:stream');
const request = http.get({ host: 'example.com',
path: '/',
port: 80,
headers: { 'Accept-Encoding': 'br,gzip,deflate' } });
request.on('response', (response) => {
const output = fs.createWriteStream('example.com_index.html');
const onError = (err) => {
if (err) {
console.error('An error occurred:', err);
process.exitCode = 1;
}
};
switch (response.headers['content-encoding']) {
case 'br':
pipeline(response, zlib.createBrotliDecompress(), output, onError);
break;
// Or, just use zlib.createUnzip() to handle both of the following cases:
case 'gzip':
pipeline(response, zlib.createGunzip(), output, onError);
break;
case 'deflate':
pipeline(response, zlib.createInflate(), output, onError);
break;
default:
pipeline(response, output, onError);
break;
}
});
// server example
// Running a gzip operation on every request is quite expensive.
// It would be much more efficient to cache the compressed buffer.
const zlib = require('node:zlib');
const http = require('node:http');
const fs = require('node:fs');
const { pipeline } = require('node:stream');
http.createServer((request, response) => {
const raw = fs.createReadStream('index.html');
// Store both a compressed and an uncompressed version of the resource.
response.setHeader('Vary', 'Accept-Encoding');
let acceptEncoding = request.headers['accept-encoding'];
if (!acceptEncoding) {
acceptEncoding = '';
}
const onError = (err) => {
if (err) {
// If an error occurs, there's not much we can do because
// the server has already sent the 200 response code and
// some amount of data has already been sent to the client.
// The best we can do is terminate the response immediately
// and log the error.
response.end();
console.error('An error occurred:', err);
}
};
// Note: This is not a conformant accept-encoding parser.
// See https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.3
if (/\bdeflate\b/.test(acceptEncoding)) {
response.writeHead(200, { 'Content-Encoding': 'deflate' });
pipeline(raw, zlib.createDeflate(), response, onError);
} else if (/\bgzip\b/.test(acceptEncoding)) {
response.writeHead(200, { 'Content-Encoding': 'gzip' });
pipeline(raw, zlib.createGzip(), response, onError);
} else if (/\bbr\b/.test(acceptEncoding)) {
response.writeHead(200, { 'Content-Encoding': 'br' });
pipeline(raw, zlib.createBrotliCompress(), response, onError);
} else {
response.writeHead(200, {});
pipeline(raw, response, onError);
}
}).listen(1337);
By default, the zlib
methods will throw an error when decompressing
truncated data. However, if it is known that the data is incomplete, or
the desire is to inspect only the beginning of a compressed file, it is
possible to suppress the default error handling by changing the flushing
method that is used to decompress the last chunk of input data:
// This is a truncated version of the buffer from the above examples
const buffer = Buffer.from('eJzT0yMA', 'base64');
zlib.unzip(
buffer,
// For Brotli, the equivalent is zlib.constants.BROTLI_OPERATION_FLUSH.
{ finishFlush: zlib.constants.Z_SYNC_FLUSH },
(err, buffer) => {
if (err) {
console.error('An error occurred:', err);
process.exitCode = 1;
}
console.log(buffer.toString());
});
This will not change the behavior in other error-throwing situations, e.g. when the input data has an invalid format. Using this method, it will not be possible to determine whether the input ended prematurely or lacks the integrity checks, making it necessary to manually check that the decompressed result is valid.
Memory usage tuning#
For zlib-based streams#
From zlib/zconf.h
, modified for Node.js usage:
The memory requirements for deflate are (in bytes):
(1 << (windowBits + 2)) + (1 << (memLevel + 9))
That is: 128K for windowBits
= 15 + 128K for memLevel
= 8
(default values) plus a few kilobytes for small objects.
For example, to reduce the default memory requirements from 256K to 128K, the options should be set to:
const options = { windowBits: 14, memLevel: 7 };
This will, however, generally degrade compression.
The memory requirements for inflate are (in bytes) 1 << windowBits
.
That is, 32K for windowBits
= 15 (default value) plus a few kilobytes
for small objects.
This is in addition to a single internal output slab buffer of size
chunkSize
, which defaults to 16K.
The speed of zlib
compression is affected most dramatically by the
level
setting. A higher level will result in better compression, but
will take longer to complete. A lower level will result in less
compression, but will be much faster.
In general, greater memory usage options will mean that Node.js has to make
fewer calls to zlib
because it will be able to process more data on
each write
operation. So, this is another factor that affects the
speed, at the cost of memory usage.
For Brotli-based streams#
There are equivalents to the zlib options for Brotli-based streams, although these options have different ranges than the zlib ones:
- zlib's
level
option matches Brotli'sBROTLI_PARAM_QUALITY
option. - zlib's
windowBits
option matches Brotli'sBROTLI_PARAM_LGWIN
option.
See below for more details on Brotli-specific options.
Flushing#
Calling .flush()
on a compression stream will make zlib
return as much
output as currently possible. This may come at the cost of degraded compression
quality, but can be useful when data needs to be available as soon as possible.
In the following example, flush()
is used to write a compressed partial
HTTP response to the client:
const zlib = require('node:zlib');
const http = require('node:http');
const { pipeline } = require('node:stream');
http.createServer((request, response) => {
// For the sake of simplicity, the Accept-Encoding checks are omitted.
response.writeHead(200, { 'content-encoding': 'gzip' });
const output = zlib.createGzip();
let i;
pipeline(output, response, (err) => {
if (err) {
// If an error occurs, there's not much we can do because
// the server has already sent the 200 response code and
// some amount of data has already been sent to the client.
// The best we can do is terminate the response immediately
// and log the error.
clearInterval(i);
response.end();
console.error('An error occurred:', err);
}
});
i = setInterval(() => {
output.write(`The current time is ${Date()}\n`, () => {
// The data has been passed to zlib, but the compression algorithm may
// have decided to buffer the data for more efficient compression.
// Calling .flush() will make the data available as soon as the client
// is ready to receive it.
output.flush();
});
}, 1000);
}).listen(1337);
Constants#
zlib constants#
All of the constants defined in zlib.h
are also defined on
require('node:zlib').constants
. In the normal course of operations, it will
not be necessary to use these constants. They are documented so that their
presence is not surprising. This section is taken almost directly from the
zlib documentation.
Previously, the constants were available directly from require('node:zlib')
,
for instance zlib.Z_NO_FLUSH
. Accessing the constants directly from the module
is currently still possible but is deprecated.
Allowed flush values.
zlib.constants.Z_NO_FLUSH
zlib.constants.Z_PARTIAL_FLUSH
zlib.constants.Z_SYNC_FLUSH
zlib.constants.Z_FULL_FLUSH
zlib.constants.Z_FINISH
zlib.constants.Z_BLOCK
zlib.constants.Z_TREES
Return codes for the compression/decompression functions. Negative values are errors, positive values are used for special but normal events.
zlib.constants.Z_OK
zlib.constants.Z_STREAM_END
zlib.constants.Z_NEED_DICT
zlib.constants.Z_ERRNO
zlib.constants.Z_STREAM_ERROR
zlib.constants.Z_DATA_ERROR
zlib.constants.Z_MEM_ERROR
zlib.constants.Z_BUF_ERROR
zlib.constants.Z_VERSION_ERROR
Compression levels.
zlib.constants.Z_NO_COMPRESSION
zlib.constants.Z_BEST_SPEED
zlib.constants.Z_BEST_COMPRESSION
zlib.constants.Z_DEFAULT_COMPRESSION
Compression strategy.
zlib.constants.Z_FILTERED
zlib.constants.Z_HUFFMAN_ONLY
zlib.constants.Z_RLE
zlib.constants.Z_FIXED
zlib.constants.Z_DEFAULT_STRATEGY
Brotli constants#
There are several options and other constants available for Brotli-based streams:
Flush operations#
The following values are valid flush operations for Brotli-based streams:
zlib.constants.BROTLI_OPERATION_PROCESS
(default for all operations)zlib.constants.BROTLI_OPERATION_FLUSH
(default when calling.flush()
)zlib.constants.BROTLI_OPERATION_FINISH
(default for the last chunk)zlib.constants.BROTLI_OPERATION_EMIT_METADATA
- This particular operation may be hard to use in a Node.js context, as the streaming layer makes it hard to know which data will end up in this frame. Also, there is currently no way to consume this data through the Node.js API.
Compressor options#
There are several options that can be set on Brotli encoders, affecting
compression efficiency and speed. Both the keys and the values can be accessed
as properties of the zlib.constants
object.
The most important options are:
BROTLI_PARAM_MODE
BROTLI_MODE_GENERIC
(default)BROTLI_MODE_TEXT
, adjusted for UTF-8 textBROTLI_MODE_FONT
, adjusted for WOFF 2.0 fonts
BROTLI_PARAM_QUALITY
- Ranges from
BROTLI_MIN_QUALITY
toBROTLI_MAX_QUALITY
, with a default ofBROTLI_DEFAULT_QUALITY
.
- Ranges from
BROTLI_PARAM_SIZE_HINT
- Integer value representing the expected input size;
defaults to
0
for an unknown input size.
- Integer value representing the expected input size;
defaults to
The following flags can be set for advanced control over the compression algorithm and memory usage tuning:
BROTLI_PARAM_LGWIN
- Ranges from
BROTLI_MIN_WINDOW_BITS
toBROTLI_MAX_WINDOW_BITS
, with a default ofBROTLI_DEFAULT_WINDOW
, or up toBROTLI_LARGE_MAX_WINDOW_BITS
if theBROTLI_PARAM_LARGE_WINDOW
flag is set.
- Ranges from
BROTLI_PARAM_LGBLOCK
- Ranges from
BROTLI_MIN_INPUT_BLOCK_BITS
toBROTLI_MAX_INPUT_BLOCK_BITS
.
- Ranges from
BROTLI_PARAM_DISABLE_LITERAL_CONTEXT_MODELING
- Boolean flag that decreases compression ratio in favour of decompression speed.
BROTLI_PARAM_LARGE_WINDOW
- Boolean flag enabling “Large Window Brotli” mode (not compatible with the Brotli format as standardized in RFC 7932).
BROTLI_PARAM_NPOSTFIX
- Ranges from
0
toBROTLI_MAX_NPOSTFIX
.
- Ranges from
BROTLI_PARAM_NDIRECT
- Ranges from
0
to15 << NPOSTFIX
in steps of1 << NPOSTFIX
.
- Ranges from
Decompressor options#
These advanced options are available for controlling decompression:
BROTLI_DECODER_PARAM_DISABLE_RING_BUFFER_REALLOCATION
- Boolean flag that affects internal memory allocation patterns.
BROTLI_DECODER_PARAM_LARGE_WINDOW
- Boolean flag enabling “Large Window Brotli” mode (not compatible with the Brotli format as standardized in RFC 7932).
Class: Options
#
Each zlib-based class takes an options
object. No options are required.
Some options are only relevant when compressing and are ignored by the decompression classes.
flush
<integer> Default:zlib.constants.Z_NO_FLUSH
finishFlush
<integer> Default:zlib.constants.Z_FINISH
chunkSize
<integer> Default:16 * 1024
windowBits
<integer>level
<integer> (compression only)memLevel
<integer> (compression only)strategy
<integer> (compression only)dictionary
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> (deflate/inflate only, empty dictionary by default)info
<boolean> (Iftrue
, returns an object withbuffer
andengine
.)maxOutputLength
<integer> Limits output size when using convenience methods. Default:buffer.kMaxLength
See the deflateInit2
and inflateInit2
documentation for more
information.
Class: BrotliOptions
#
Each Brotli-based class takes an options
object. All options are optional.
flush
<integer> Default:zlib.constants.BROTLI_OPERATION_PROCESS
finishFlush
<integer> Default:zlib.constants.BROTLI_OPERATION_FINISH
chunkSize
<integer> Default:16 * 1024
params
<Object> Key-value object containing indexed Brotli parameters.maxOutputLength
<integer> Limits output size when using convenience methods. Default:buffer.kMaxLength
For example:
const stream = zlib.createBrotliCompress({
chunkSize: 32 * 1024,
params: {
[zlib.constants.BROTLI_PARAM_MODE]: zlib.constants.BROTLI_MODE_TEXT,
[zlib.constants.BROTLI_PARAM_QUALITY]: 4,
[zlib.constants.BROTLI_PARAM_SIZE_HINT]: fs.statSync(inputFile).size,
},
});
Class: zlib.BrotliCompress
#
Compress data using the Brotli algorithm.
Class: zlib.BrotliDecompress
#
Decompress data using the Brotli algorithm.
Class: zlib.Deflate
#
Compress data using deflate.
Class: zlib.DeflateRaw
#
Compress data using deflate, and do not append a zlib
header.
Class: zlib.Gunzip
#
Decompress a gzip stream.
Class: zlib.Gzip
#
Compress data using gzip.
Class: zlib.Inflate
#
Decompress a deflate stream.
Class: zlib.InflateRaw
#
Decompress a raw deflate stream.
Class: zlib.Unzip
#
Decompress either a Gzip- or Deflate-compressed stream by auto-detecting the header.
Class: zlib.ZlibBase
#
Not exported by the node:zlib
module. It is documented here because it is the
base class of the compressor/decompressor classes.
This class inherits from stream.Transform
, allowing node:zlib
objects to
be used in pipes and similar stream operations.
zlib.bytesRead
#
zlib.bytesWritten
instead.Deprecated alias for zlib.bytesWritten
. This original name was chosen
because it also made sense to interpret the value as the number of bytes
read by the engine, but is inconsistent with other streams in Node.js that
expose values under these names.
zlib.bytesWritten
#
The zlib.bytesWritten
property specifies the number of bytes written to
the engine, before the bytes are processed (compressed or decompressed,
as appropriate for the derived class).
zlib.close([callback])
#
callback
<Function>
Close the underlying handle.
zlib.flush([kind, ]callback)
#
kind
Default:zlib.constants.Z_FULL_FLUSH
for zlib-based streams,zlib.constants.BROTLI_OPERATION_FLUSH
for Brotli-based streams.callback
<Function>
Flush pending data. Don't call this frivolously, premature flushes negatively impact the effectiveness of the compression algorithm.
Calling this only flushes data from the internal zlib
state, and does not
perform flushing of any kind on the streams level. Rather, it behaves like a
normal call to .write()
, i.e. it will be queued up behind other pending
writes and will only produce output when data is being read from the stream.
zlib.params(level, strategy, callback)
#
level
<integer>strategy
<integer>callback
<Function>
This function is only available for zlib-based streams, i.e. not Brotli.
Dynamically update the compression level and compression strategy. Only applicable to deflate algorithm.
zlib.reset()
#
Reset the compressor/decompressor to factory defaults. Only applicable to the inflate and deflate algorithms.
zlib.constants
#
Provides an object enumerating Zlib-related constants.
zlib.createBrotliCompress([options])
#
options
<brotli options>
Creates and returns a new BrotliCompress
object.
zlib.createBrotliDecompress([options])
#
options
<brotli options>
Creates and returns a new BrotliDecompress
object.
zlib.createDeflate([options])
#
options
<zlib options>
Creates and returns a new Deflate
object.
zlib.createDeflateRaw([options])
#
options
<zlib options>
Creates and returns a new DeflateRaw
object.
An upgrade of zlib from 1.2.8 to 1.2.11 changed behavior when windowBits
is set to 8 for raw deflate streams. zlib would automatically set windowBits
to 9 if was initially set to 8. Newer versions of zlib will throw an exception,
so Node.js restored the original behavior of upgrading a value of 8 to 9,
since passing windowBits = 9
to zlib actually results in a compressed stream
that effectively uses an 8-bit window only.
zlib.createGunzip([options])
#
options
<zlib options>
Creates and returns a new Gunzip
object.
zlib.createGzip([options])
#
options
<zlib options>
zlib.createInflate([options])
#
options
<zlib options>
Creates and returns a new Inflate
object.
zlib.createInflateRaw([options])
#
options
<zlib options>
Creates and returns a new InflateRaw
object.
zlib.createUnzip([options])
#
options
<zlib options>
Creates and returns a new Unzip
object.
Convenience methods#
All of these take a Buffer
, TypedArray
, DataView
,
ArrayBuffer
or string as the first argument, an optional second argument
to supply options to the zlib
classes and will call the supplied callback
with callback(error, result)
.
Every method has a *Sync
counterpart, which accept the same arguments, but
without a callback.
zlib.brotliCompress(buffer[, options], callback)
#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<brotli options>callback
<Function>
zlib.brotliCompressSync(buffer[, options])
#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<brotli options>
Compress a chunk of data with BrotliCompress
.
zlib.brotliDecompress(buffer[, options], callback)
#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<brotli options>callback
<Function>
zlib.brotliDecompressSync(buffer[, options])
#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<brotli options>
Decompress a chunk of data with BrotliDecompress
.
zlib.deflate(buffer[, options], callback)
#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<zlib options>callback
<Function>
zlib.deflateSync(buffer[, options])
#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<zlib options>
Compress a chunk of data with Deflate
.
zlib.deflateRaw(buffer[, options], callback)
#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<zlib options>callback
<Function>
zlib.deflateRawSync(buffer[, options])
#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<zlib options>
Compress a chunk of data with DeflateRaw
.
zlib.gunzip(buffer[, options], callback)
#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<zlib options>callback
<Function>
zlib.gunzipSync(buffer[, options])
#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<zlib options>
Decompress a chunk of data with Gunzip
.
zlib.gzip(buffer[, options], callback)
#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<zlib options>callback
<Function>
zlib.gzipSync(buffer[, options])
#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<zlib options>
Compress a chunk of data with Gzip
.
zlib.inflate(buffer[, options], callback)
#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<zlib options>callback
<Function>
zlib.inflateSync(buffer[, options])
#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<zlib options>
Decompress a chunk of data with Inflate
.
zlib.inflateRaw(buffer[, options], callback)
#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<zlib options>callback
<Function>
zlib.inflateRawSync(buffer[, options])
#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<zlib options>
Decompress a chunk of data with InflateRaw
.
zlib.unzip(buffer[, options], callback)
#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<zlib options>callback
<Function>
zlib.unzipSync(buffer[, options])
#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<zlib options>
Decompress a chunk of data with Unzip
.