Node.js v23.5.0 documentation

The Module object#

• <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#

• <string[]>

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;copy

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'); copy

module.findPackageJSON(specifier[, base])#

• specifier <string> | <URL> The specifier for the module whose package.json to retrieve. When passing a bare specifier, the package.json at the root of the package is returned. When passing a relative specifier or an absolute specifier, the closest parent package.json is returned.
• base <string> | <URL> The absolute location (file: URL string or FS path) of the containing module. For CJS, use __filename (not __dirname!); for ESM, use import.meta.url. You do not need to pass it if specifier is an absolute specifier.
• Returns: <string> | <undefined> A path if the package.json is found. When startLocation is a package, the package's root package.json; when a relative or unresolved, the closest package.json to the startLocation.

Caveat: Do not use this to try to determine module format. There are many things effecting that determination; the type field of package.json is the least definitive (ex file extension superceeds it, and a loader hook superceeds that).

/path/to/project
  ├ packages/
    ├ bar/
      ├ bar.js
      └ package.json // name = '@foo/bar'
    └ qux/
      ├ node_modules/
        └ some-package/
          └ package.json // name = 'some-package'
      ├ qux.js
      └ package.json // name = '@foo/qux'
  ├ main.js
  └ package.json // name = '@foo' copy

// /path/to/project/packages/bar/bar.js
import { findPackageJSON } from 'node:module';

findPackageJSON('..', import.meta.url);
// '/path/to/project/package.json'
// Same result when passing an absolute specifier instead:
findPackageJSON(new URL('../', import.meta.url));
findPackageJSON(import.meta.resolve('../'));

findPackageJSON('some-package', import.meta.url);
// '/path/to/project/packages/bar/node_modules/some-package/package.json'
// When passing an absolute specifier, you might get a different result if the
// resolved module is inside a subfolder that has nested `package.json`.
findPackageJSON(import.meta.resolve('some-package'));
// '/path/to/project/packages/bar/node_modules/some-package/some-subfolder/package.json'

findPackageJSON('@foo/qux', import.meta.url);
// '/path/to/project/packages/qux/package.json'// /path/to/project/packages/bar/bar.js
const { findPackageJSON } = require('node:module');
const { pathToFileURL } = require('node:url');
const path = require('node:path');

findPackageJSON('..', __filename);
// '/path/to/project/package.json'
// Same result when passing an absolute specifier instead:
findPackageJSON(pathToFileURL(path.join(__dirname, '..')));

findPackageJSON('some-package', __filename);
// '/path/to/project/packages/bar/node_modules/some-package/package.json'
// When passing an absolute specifier, you might get a different result if the
// resolved module is inside a subfolder that has nested `package.json`.
findPackageJSON(pathToFileURL(require.resolve('some-package')));
// '/path/to/project/packages/bar/node_modules/some-package/some-subfolder/package.json'

findPackageJSON('@foo/qux', __filename);
// '/path/to/project/packages/qux/package.json'copy

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 copy

module.register(specifier[, parentURL][, options])#

• specifier <string> | <URL> Customization hooks to be registered; this should be the same string that would be passed to import(), except that if it is relative, it is resolved relative to parentURL.
• parentURL <string> | <URL> If you want to resolve specifier relative to a base URL, such as import.meta.url, you can pass that URL here. Default: 'data:'
• options <Object>
  • parentURL <string> | <URL> If you want to resolve specifier relative to a base URL, such as import.meta.url, you can pass that URL here. This property is ignored if the parentURL is supplied as the second argument. Default: 'data:'
  • data <any> Any arbitrary, cloneable JavaScript value to pass into the initialize hook.
  • transferList <Object[]> transferable objects to be passed into the initialize hook.

Register a module that exports hooks that customize Node.js module resolution and loading behavior. See Customization hooks.

module.registerHooks(options)#

• options <Object>
  • load <Function> | <undefined> See load hook. Default: undefined.
  • resolve <Function> | <undefined> See resolve hook. Default: undefined.

Register hooks that customize Node.js module resolution and loading behavior. See Customization hooks.

module.stripTypeScriptTypes(code[, options])#

• code <string> The code to strip type annotations from.
• options <Object>
  • mode <string> Default: 'strip'. Possible values are:
    • 'strip' Only strip type annotations without performing the transformation of TypeScript features.
    • 'transform' Strip type annotations and transform TypeScript features to JavaScript.
  • sourceMap <boolean> Default: false. Only when mode is 'transform', if true, a source map will be generated for the transformed code.
  • sourceUrl <string> Specifies the source url used in the source map.
• Returns: <string> The code with type annotations stripped. module.stripTypeScriptTypes() removes type annotations from TypeScript code. It can be used to strip type annotations from TypeScript code before running it with vm.runInContext() or vm.compileFunction(). By default, it will throw an error if the code contains TypeScript features that require transformation such as Enums, see type-stripping for more information. When mode is 'transform', it also transforms TypeScript features to JavaScript, see transform TypeScript features for more information. When mode is 'strip', source maps are not generated, because locations are preserved. If sourceMap is provided, when mode is 'strip', an error will be thrown.

WARNING: The output of this function should not be considered stable across Node.js versions, due to changes in the TypeScript parser.

import { stripTypeScriptTypes } from 'node:module';
const code = 'const a: number = 1;';
const strippedCode = stripTypeScriptTypes(code);
console.log(strippedCode);
// Prints: const a         = 1;const { stripTypeScriptTypes } = require('node:module');
const code = 'const a: number = 1;';
const strippedCode = stripTypeScriptTypes(code);
console.log(strippedCode);
// Prints: const a         = 1;copy

If sourceUrl is provided, it will be used appended as a comment at the end of the output:

import { stripTypeScriptTypes } from 'node:module';
const code = 'const a: number = 1;';
const strippedCode = stripTypeScriptTypes(code, { mode: 'strip', sourceUrl: 'source.ts' });
console.log(strippedCode);
// Prints: const a         = 1\n\n//# sourceURL=source.ts;const { stripTypeScriptTypes } = require('node:module');
const code = 'const a: number = 1;';
const strippedCode = stripTypeScriptTypes(code, { mode: 'strip', sourceUrl: 'source.ts' });
console.log(strippedCode);
// Prints: const a         = 1\n\n//# sourceURL=source.ts;copy

When mode is 'transform', the code is transformed to JavaScript:

import { stripTypeScriptTypes } from 'node:module';
const code = `
  namespace MathUtil {
    export const add = (a: number, b: number) => a + b;
  }`;
const strippedCode = stripTypeScriptTypes(code, { mode: 'transform', sourceMap: true });
console.log(strippedCode);
// Prints:
// var MathUtil;
// (function(MathUtil) {
//     MathUtil.add = (a, b)=>a + b;
// })(MathUtil || (MathUtil = {}));
// # sourceMappingURL=data:application/json;base64, ...const { stripTypeScriptTypes } = require('node:module');
const code = `
  namespace MathUtil {
    export const add = (a: number, b: number) => a + b;
  }`;
const strippedCode = stripTypeScriptTypes(code, { mode: 'transform', sourceMap: true });
console.log(strippedCode);
// Prints:
// var MathUtil;
// (function(MathUtil) {
//     MathUtil.add = (a, b)=>a + b;
// })(MathUtil || (MathUtil = {}));
// # sourceMappingURL=data:application/json;base64, ...copy

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);
}); copy

Module compile cache#

The module compile cache can be enabled either using the module.enableCompileCache() method or the NODE_COMPILE_CACHE=dir environment variable. After it is enabled, whenever Node.js compiles a CommonJS or a ECMAScript Module, it will use on-disk V8 code cache persisted in the specified directory to speed up the compilation. This may slow down the first load of a module graph, but subsequent loads of the same module graph may get a significant speedup if the contents of the modules do not change.

To clean up the generated compile cache on disk, simply remove the cache directory. The cache directory will be recreated the next time the same directory is used for for compile cache storage. To avoid filling up the disk with stale cache, it is recommended to use a directory under the os.tmpdir(). If the compile cache is enabled by a call to module.enableCompileCache() without specifying the directory, Node.js will use the NODE_COMPILE_CACHE=dir environment variable if it's set, or defaults to path.join(os.tmpdir(), 'node-compile-cache') otherwise. To locate the compile cache directory used by a running Node.js instance, use module.getCompileCacheDir().

Currently when using the compile cache with V8 JavaScript code coverage, the coverage being collected by V8 may be less precise in functions that are deserialized from the code cache. It's recommended to turn this off when running tests to generate precise coverage.

The enabled module compile cache can be disabled by the NODE_DISABLE_COMPILE_CACHE=1 environment variable. This can be useful when the compile cache leads to unexpected or undesired behaviors (e.g. less precise test coverage).

Compilation cache generated by one version of Node.js can not be reused by a different version of Node.js. Cache generated by different versions of Node.js will be stored separately if the same base directory is used to persist the cache, so they can co-exist.

At the moment, when the compile cache is enabled and a module is loaded afresh, the code cache is generated from the compiled code immediately, but will only be written to disk when the Node.js instance is about to exit. This is subject to change. The module.flushCompileCache() method can be used to ensure the accumulated code cache is flushed to disk in case the application wants to spawn other Node.js instances and let them share the cache long before the parent exits.

module.constants.compileCacheStatus#

The following constants are returned as the status field in the object returned by module.enableCompileCache() to indicate the result of the attempt to enable the module compile cache.

module.enableCompileCache([cacheDir])#

• cacheDir <string> | <undefined> Optional path to specify the directory where the compile cache will be stored/retrieved.
• Returns: <Object>
  • status <integer> One of the module.constants.compileCacheStatus
  • message <string> | <undefined> If Node.js cannot enable the compile cache, this contains the error message. Only set if status is module.constants.compileCacheStatus.FAILED.
  • directory <string> | <undefined> If the compile cache is enabled, this contains the directory where the compile cache is stored. Only set if status is module.constants.compileCacheStatus.ENABLED or module.constants.compileCacheStatus.ALREADY_ENABLED.

Enable module compile cache in the current Node.js instance.

If cacheDir is not specified, Node.js will either use the directory specified by the NODE_COMPILE_CACHE=dir environment variable if it's set, or use path.join(os.tmpdir(), 'node-compile-cache') otherwise. For general use cases, it's recommended to call module.enableCompileCache() without specifying the cacheDir, so that the directory can be overridden by the NODE_COMPILE_CACHE environment variable when necessary.

Since compile cache is supposed to be a quiet optimization that is not required for the application to be functional, this method is designed to not throw any exception when the compile cache cannot be enabled. Instead, it will return an object containing an error message in the message field to aid debugging. If compile cache is enabled successfully, the directory field in the returned object contains the path to the directory where the compile cache is stored. The status field in the returned object would be one of the module.constants.compileCacheStatus values to indicate the result of the attempt to enable the module compile cache.

This method only affects the current Node.js instance. To enable it in child worker threads, either call this method in child worker threads too, or set the process.env.NODE_COMPILE_CACHE value to compile cache directory so the behavior can be inherited into the child workers. The directory can be obtained either from the directory field returned by this method, or with module.getCompileCacheDir().

module.flushCompileCache()#

Flush the module compile cache accumulated from modules already loaded in the current Node.js instance to disk. This returns after all the flushing file system operations come to an end, no matter they succeed or not. If there are any errors, this will fail silently, since compile cache misses should not interfere with the actual operation of the application.

module.getCompileCacheDir()#

• Returns: <string> | <undefined> Path to the module compile cache directory if it is enabled, or undefined otherwise.

Customization Hooks#

There are two types of module customization hooks that are currently supported:

1. module.register(specifier[, parentURL][, options]) which takes a module that exports asynchronous hook functions. The functions are run on a separate loader thread.
2. module.registerHooks(options) which takes synchronous hook functions that are run directly on the thread where the module is loaded.

Enabling#

Module resolution and loading can be customized by:

1. Registering a file which exports a set of asynchronous hook functions, using the register method from node:module,
2. Registering a set of synchronous hook functions using the registerHooks method from node:module.

The hooks can be registered before the application code is run by using the --import or --require flag:

node --import ./register-hooks.js ./my-app.js
node --require ./register-hooks.js ./my-app.js copy

// register-hooks.js
// This file can only be require()-ed if it doesn't contain top-level await.
// Use module.register() to register asynchronous hooks in a dedicated thread.
import { register } from 'node:module';
register('./hooks.mjs', import.meta.url);// register-hooks.js
const { register } = require('node:module');
const { pathToFileURL } = require('node:url');
// Use module.register() to register asynchronous hooks in a dedicated thread.
register('./hooks.mjs', pathToFileURL(__filename));copy

// Use module.registerHooks() to register synchronous hooks in the main thread.
import { registerHooks } from 'node:module';
registerHooks({
  resolve(specifier, context, nextResolve) { /* implementation */ },
  load(url, context, nextLoad) { /* implementation */ },
});// Use module.registerHooks() to register synchronous hooks in the main thread.
const { registerHooks } = require('node:module');
registerHooks({
  resolve(specifier, context, nextResolve) { /* implementation */ },
  load(url, context, nextLoad) { /* implementation */ },
});copy

The file passed to --import or --require can also be an export from a dependency:

node --import some-package/register ./my-app.js
node --require some-package/register ./my-app.js copy

Where some-package has an "exports" field defining the /register export to map to a file that calls register(), like the following register-hooks.js example.

Using --import or --require ensures that the hooks are registered before any application files are imported, including the entry point of the application and for any worker threads by default as well.

Alternatively, register() and registerHooks() can be called from the entry point, though dynamic import() must be used for any ESM code that should be run after the hooks are registered.

import { register } from 'node:module';

register('http-to-https', import.meta.url);

// Because this is a dynamic `import()`, the `http-to-https` hooks will run
// to handle `./my-app.js` and any other files it imports or requires.
await import('./my-app.js');const { register } = require('node:module');
const { pathToFileURL } = require('node:url');

register('http-to-https', pathToFileURL(__filename));

// Because this is a dynamic `import()`, the `http-to-https` hooks will run
// to handle `./my-app.js` and any other files it imports or requires.
import('./my-app.js');copy

Customization hooks will run for any modules loaded later than the registration and the modules they reference via import and the built-in require. require function created by users using module.createRequire() can only be customized by the synchronous hooks.

In this example, we are registering the http-to-https hooks, but they will only be available for subsequently imported modules — in this case, my-app.js and anything it references via import or built-in require in CommonJS dependencies.

If the import('./my-app.js') had instead been a static import './my-app.js', the app would have already been loaded before the http-to-https hooks were registered. This due to the ES modules specification, where static imports are evaluated from the leaves of the tree first, then back to the trunk. There can be static imports within my-app.js, which will not be evaluated until my-app.js is dynamically imported.

If synchronous hooks are used, both import, require and user require created using createRequire() are supported.

import { registerHooks, createRequire } from 'node:module';

registerHooks({ /* implementation of synchronous hooks */ });

const require = createRequire(import.meta.url);

// The synchronous hooks affect import, require() and user require() function
// created through createRequire().
await import('./my-app.js');
require('./my-app-2.js');const { register, registerHooks } = require('node:module');
const { pathToFileURL } = require('node:url');

registerHooks({ /* implementation of synchronous hooks */ });

const userRequire = createRequire(__filename);

// The synchronous hooks affect import, require() and user require() function
// created through createRequire().
import('./my-app.js');
require('./my-app-2.js');
userRequire('./my-app-3.js');copy

Finally, if all you want to do is register hooks before your app runs and you don't want to create a separate file for that purpose, you can pass a data: URL to --import:

node --import 'data:text/javascript,import { register } from "node:module"; import { pathToFileURL } from "node:url"; register("http-to-https", pathToFileURL("./"));' ./my-app.js copy

Chaining#

It's possible to call register more than once:

// entrypoint.mjs
import { register } from 'node:module';

register('./foo.mjs', import.meta.url);
register('./bar.mjs', import.meta.url);
await import('./my-app.mjs');// entrypoint.cjs
const { register } = require('node:module');
const { pathToFileURL } = require('node:url');

const parentURL = pathToFileURL(__filename);
register('./foo.mjs', parentURL);
register('./bar.mjs', parentURL);
import('./my-app.mjs');copy

In this example, the registered hooks will form chains. These chains run last-in, first out (LIFO). If both foo.mjs and bar.mjs define a resolve hook, they will be called like so (note the right-to-left): node's default ← ./foo.mjs ← ./bar.mjs (starting with ./bar.mjs, then ./foo.mjs, then the Node.js default). The same applies to all the other hooks.

The registered hooks also affect register itself. In this example, bar.mjs will be resolved and loaded via the hooks registered by foo.mjs (because foo's hooks will have already been added to the chain). This allows for things like writing hooks in non-JavaScript languages, so long as earlier registered hooks transpile into JavaScript.

The register method cannot be called from within the module that defines the hooks.

Chaining of registerHooks work similarly. If synchronous and asynchronous hooks are mixed, the synchronous hooks are always run first before the asynchronous hooks start running, that is, in the last synchronous hook being run, its next hook includes invocation of the asynchronous hooks.

// entrypoint.mjs
import { registerHooks } from 'node:module';

const hook1 = { /* implementation of hooks */ };
const hook2 = { /* implementation of hooks */ };
// hook2 run before hook1.
registerHooks(hook1);
registerHooks(hook2);// entrypoint.cjs
const { registerHooks } = require('node:module');

const hook1 = { /* implementation of hooks */ };
const hook2 = { /* implementation of hooks */ };
// hook2 run before hook1.
registerHooks(hook1);
registerHooks(hook2);copy

Communication with module customization hooks#

Asynchronous hooks run on a dedicated thread, separate from the main thread that runs application code. This means mutating global variables won't affect the other thread(s), and message channels must be used to communicate between the threads.

The register method can be used to pass data to an initialize hook. The data passed to the hook may include transferable objects like ports.

import { register } from 'node:module';
import { MessageChannel } from 'node:worker_threads';

// This example demonstrates how a message channel can be used to
// communicate with the hooks, by sending `port2` to the hooks.
const { port1, port2 } = new MessageChannel();

port1.on('message', (msg) => {
  console.log(msg);
});
port1.unref();

register('./my-hooks.mjs', {
  parentURL: import.meta.url,
  data: { number: 1, port: port2 },
  transferList: [port2],
});const { register } = require('node:module');
const { pathToFileURL } = require('node:url');
const { MessageChannel } = require('node:worker_threads');

// This example showcases how a message channel can be used to
// communicate with the hooks, by sending `port2` to the hooks.
const { port1, port2 } = new MessageChannel();

port1.on('message', (msg) => {
  console.log(msg);
});
port1.unref();

register('./my-hooks.mjs', {
  parentURL: pathToFileURL(__filename),
  data: { number: 1, port: port2 },
  transferList: [port2],
});copy

Synchronous module hooks are run on the same thread where the application code is run. They can directly mutate the globals of the context accessed by the main thread.

Hooks#

Asynchronous hooks accepted by module.register()#

The register method can be used to register a module that exports a set of hooks. The hooks are functions that are called by Node.js to customize the module resolution and loading process. The exported functions must have specific names and signatures, and they must be exported as named exports.

export async function initialize({ number, port }) {
  // Receives data from `register`.
}

export async function resolve(specifier, context, nextResolve) {
  // Take an `import` or `require` specifier and resolve it to a URL.
}

export async function load(url, context, nextLoad) {
  // Take a resolved URL and return the source code to be evaluated.
} copy

Asynchronous hooks are run in a separate thread, isolated from the main thread where application code runs. That means it is a different realm. The hooks thread may be terminated by the main thread at any time, so do not depend on asynchronous operations (like console.log) to complete. They are inherited into child workers by default.

Synchronous hooks accepted by module.registerHooks()#

The module.registerHooks() method accepts synchronous hook functions. initialize() is not supported nor necessary, as the hook implementer can simply run the initialization code directly before the call to module.registerHooks().

function resolve(specifier, context, nextResolve) {
  // Take an `import` or `require` specifier and resolve it to a URL.
}

function load(url, context, nextLoad) {
  // Take a resolved URL and return the source code to be evaluated.
} copy

Synchronous hooks are run in the same thread and the same realm where the modules are loaded. Unlike the asynchronous hooks they are not inherited into child worker threads by default, though if the hooks are registered using a file preloaded by --import or --require, child worker threads can inherit the preloaded scripts via process.execArgv inheritance. See the documentation of Worker for detail.

In synchronous hooks, users can expect console.log() to complete in the same way that they expect console.log() in module code to complete.

Conventions of 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 (in LIFO order).

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. Return shortCircuit: true from a hook to signal that the chain is intentionally ending at your hook.

initialize()#

• data <any> The data from register(loader, import.meta.url, { data }).

The initialize hook is only accepted by register. registerHooks() does not support nor need it since initialization done for synchronous hooks can be run directly before the call to registerHooks().

The initialize hook provides a way to define a custom function that runs in the hooks thread when the hooks module is initialized. Initialization happens when the hooks module is registered via register.

This hook can receive data from a register invocation, including ports and other transferable objects. The return value of initialize can be a <Promise>, in which case it will be awaited before the main application thread execution resumes.

Module customization code:

// path-to-my-hooks.js

export async function initialize({ number, port }) {
  port.postMessage(`increment: ${number + 1}`);
} copy

Caller code:

import assert from 'node:assert';
import { register } from 'node:module';
import { MessageChannel } from 'node:worker_threads';

// This example showcases how a message channel can be used to communicate
// between the main (application) thread and the hooks running on the hooks
// thread, by sending `port2` to the `initialize` hook.
const { port1, port2 } = new MessageChannel();

port1.on('message', (msg) => {
  assert.strictEqual(msg, 'increment: 2');
});
port1.unref();

register('./path-to-my-hooks.js', {
  parentURL: import.meta.url,
  data: { number: 1, port: port2 },
  transferList: [port2],
});const assert = require('node:assert');
const { register } = require('node:module');
const { pathToFileURL } = require('node:url');
const { MessageChannel } = require('node:worker_threads');

// This example showcases how a message channel can be used to communicate
// between the main (application) thread and the hooks running on the hooks
// thread, by sending `port2` to the `initialize` hook.
const { port1, port2 } = new MessageChannel();

port1.on('message', (msg) => {
  assert.strictEqual(msg, 'increment: 2');
});
port1.unref();

register('./path-to-my-hooks.js', {
  parentURL: pathToFileURL(__filename),
  data: { number: 1, port: port2 },
  transferList: [port2],
});copy

resolve(specifier, context, nextResolve)#

• specifier <string>
• context <Object>
  • conditions <string[]> Export conditions of the relevant package.json
  • importAttributes <Object> An object whose key-value pairs represent the attributes for the module to import
  • parentURL <string> | <undefined> The module importing this one, or undefined if this is the Node.js entry point
• nextResolve <Function> The subsequent resolve hook in the chain, or the Node.js default resolve hook after the last user-supplied resolve hook
  • specifier <string>
  • context <Object>
• Returns: <Object> | <Promise> The asynchronous version takes either an object containing the following properties, or a Promise that will resolve to such an object. The synchronous version only accepts an object returned synchronously.
  • format <string> | <null> | <undefined> A hint to the load hook (it might be ignored) 'builtin' | 'commonjs' | 'json' | 'module' | 'wasm'
  • importAttributes <Object> | <undefined> The import attributes 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 of resolve hooks. Default: false
  • url <string> The absolute URL to which this input resolves

Warning In the case of the asynchronous version, despite support for returning promises and async functions, calls to resolve may still block the main thread which can impact performance.

The resolve hook chain is responsible for telling Node.js where to find and how to cache a given import statement or expression, or require call. It can optionally return a 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 attributes are part of the cache key for saving loaded modules into the internal module cache. The resolve hook is responsible for returning an importAttributes object if the module should be cached with different attributes than were present in the source code.

The conditions property in context is an array of conditions that will be used to match package exports conditions for 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.

// Asynchronous version accepted by module.register().
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);
} copy

// Synchronous version accepted by module.registerHooks().
function resolve(specifier, context, nextResolve) {
  // Similar to the asynchronous resolve() above, since that one does not have
  // any asynchronous logic.
} copy

load(url, context, nextLoad)#

• url <string> The URL returned by the resolve chain
• context <Object>
  • conditions <string[]> Export conditions of the relevant package.json
  • format <string> | <null> | <undefined> The format optionally supplied by the resolve hook chain
  • importAttributes <Object>
• nextLoad <Function> The subsequent load hook in the chain, or the Node.js default load hook after the last user-supplied load hook
  • url <string>
  • context <Object>
• Returns: <Object> | <Promise> The asynchronous version takes either an object containing the following properties, or a Promise that will resolve to such an object. The synchronous version only accepts an object returned synchronously.
  • format <string>
  • shortCircuit <undefined> | <boolean> A signal that this hook intends to terminate the chain of load 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 attributes.

The final value of format must be one of the following:

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.

Caveat in the asynchronous load hook#

When using the asynchronous load hook, omitting vs providing a source for 'commonjs' has very different effects:

• When a source is provided, all require calls from this module will be processed by the ESM loader with registered resolve and load hooks; all require.resolve calls from this module will be processed by the ESM loader with registered resolve hooks; only a subset of the CommonJS API will be available (e.g. no require.extensions, no require.cache, no require.resolve.paths) and monkey-patching on the CommonJS module loader will not apply.
• If source is undefined or null, it will be handled by the CommonJS module loader and require/require.resolve calls will not go through the registered hooks. This behavior for nullish source is temporary — in the future, nullish source will not be supported.

These caveats do not apply to the synchronous load hook, in which case the complete set of CommonJS APIs available to the customized CommonJS modules, and require/require.resolve always go through the registered hooks.

The Node.js internal asynchronous load implementation, which is the value of next for the last hook in the load chain, returns null for source when format is 'commonjs' for backward compatibility. Here is an example hook that would opt-in to using the non-default behavior:

import { readFile } from 'node:fs/promises';

// Asynchronous version accepted by module.register(). This fix is not needed
// for the synchronous version accepted by module.registerSync().
export async function load(url, context, nextLoad) {
  const result = await nextLoad(url, context);
  if (result.format === 'commonjs') {
    result.source ??= await readFile(new URL(result.responseURL ?? url));
  }
  return result;
} copy

This doesn't apply to the synchronous load hook either, in which case the source returned contains source code loaded by the next hook, regardless of module format.

Warning: The asynchronous 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. This does not apply to the synchronous load hook, in which case exports can be used as usual.

These types all correspond to classes defined in ECMAScript.

• The specific ArrayBuffer object is a SharedArrayBuffer.
• The specific TypedArray object is a Uint8Array.

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 a resolved URL. 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.

// Asynchronous version accepted by module.register().
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);
} copy

// Synchronous version accepted by module.registerHooks().
function load(url, context, nextLoad) {
  // Similar to the asynchronous load() above, since that one does not have
  // any asynchronous logic.
} copy

In a more advanced scenario, this can also be used to transform an unsupported source to a supported one (see Examples below).

Examples#

The various module customization hooks can be used together to accomplish wide-ranging customizations of the Node.js code loading and evaluation behaviors.

Import from HTTPS#

The hook 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 these hooks: performance is much slower than loading files from disk, there is no caching, and there is no security.

// https-hooks.mjs
import { get } from 'node:https';

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.setEncoding('utf8');
        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);
} copy

// main.mjs
import { VERSION } from 'https://coffeescript.org/browser-compiler-modern/coffeescript.js';

console.log(VERSION); copy

With the preceding hooks module, running node --import 'data:text/javascript,import { register } from "node:module"; import { pathToFileURL } from "node:url"; register(pathToFileURL("./https-hooks.mjs"));' ./main.mjs prints the current version of CoffeeScript per the module at the URL in main.mjs.

Transpilation#

Sources that are in formats Node.js doesn't understand can be converted into JavaScript using the load hook.

This is less performant than transpiling source files before running Node.js; transpiler hooks should only be used for development and testing purposes.

Asynchronous version#

// coffeescript-hooks.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 extensionsRegex = /\.(coffee|litcoffee|coffee\.md)$/;

export async function load(url, context, nextLoad) {
  if (extensionsRegex.test(url)) {
    // 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);

    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 = coffeescript.compile(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);
    });
  // If package.json existed and contained a `type` field with a value, voilà
  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, '..'));
} copy

Synchronous version#

// coffeescript-sync-hooks.mjs
import { readFileSync } from 'node:fs/promises';
import { registerHooks } from 'node:module';
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 extensionsRegex = /\.(coffee|litcoffee|coffee\.md)$/;

function load(url, context, nextLoad) {
  if (extensionsRegex.test(url)) {
    const format = getPackageType(url);

    const { source: rawSource } = nextLoad(url, { ...context, format });
    const transformedSource = coffeescript.compile(rawSource.toString(), url);

    return {
      format,
      shortCircuit: true,
      source: transformedSource,
    };
  }

  return nextLoad(url);
}

function getPackageType(url) {
  const isFilePath = !!extname(url);
  const dir = isFilePath ? dirname(fileURLToPath(url)) : url;
  const packagePath = resolvePath(dir, 'package.json');

  let type;
  try {
    const filestring = readFileSync(packagePath, { encoding: 'utf8' });
    type = JSON.parse(filestring).type;
  } catch (err) {
    if (err?.code !== 'ENOENT') console.error(err);
  }
  if (type) return type;
  return dir.length > 1 && getPackageType(resolvePath(dir, '..'));
}

registerHooks({ load }); copy

Running hooks#

# 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}" copy

# scream.coffee
export scream = (str) -> str.toUpperCase() copy

With the preceding hooks modules, running node --import 'data:text/javascript,import { register } from "node:module"; import { pathToFileURL } from "node:url"; register(pathToFileURL("./coffeescript-hooks.mjs"));' ./main.coffee or node --import ./coffeescript-sync-hooks.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.

Import maps#

The previous two examples defined load hooks. This is an example of a resolve hook. This hooks module reads an import-map.json file that defines which specifiers to override to other URLs (this is a very simplistic implementation of a small subset of the "import maps" specification).

Asynchronous version#

// import-map-hooks.js
import fs from 'node:fs/promises';

const { imports } = JSON.parse(await fs.readFile('import-map.json'));

export async function resolve(specifier, context, nextResolve) {
  if (Object.hasOwn(imports, specifier)) {
    return nextResolve(imports[specifier], context);
  }

  return nextResolve(specifier, context);
} copy

Synchronous version#

// import-map-sync-hooks.js
import fs from 'node:fs/promises';
import module from 'node:module';

const { imports } = JSON.parse(fs.readFileSync('import-map.json', 'utf-8'));

function resolve(specifier, context, nextResolve) {
  if (Object.hasOwn(imports, specifier)) {
    return nextResolve(imports[specifier], context);
  }

  return nextResolve(specifier, context);
}

module.registerHooks({ resolve }); copy

Using the hooks#

With these files:

// main.js
import 'a-module'; copy

// import-map.json
{
  "imports": {
    "a-module": "./some-module.js"
  }
} copy

// some-module.js
console.log('some module!'); copy

Running node --import 'data:text/javascript,import { register } from "node:module"; import { pathToFileURL } from "node:url"; register(pathToFileURL("./import-map-hooks.js"));' main.js or node --import ./import-map-sync-hooks.js main.js should print some module!.

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');copy

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[, { lineLengths }])#

• payload <Object>
• lineLengths <number[]>

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>

lineLengths is an optional array of the length of each line in the generated code.

sourceMap.payload#

• Returns: <Object>

Getter for the payload used to construct the SourceMap instance.

sourceMap.findEntry(lineOffset, columnOffset)#

• lineOffset <number> The zero-indexed line number offset in the generated source
• columnOffset <number> The zero-indexed column number offset in the generated source
• Returns: <Object>

Given a line offset and column offset in the generated source file, returns an object representing the SourceMap range in the original file if found, or an empty object if not.

The object returned contains the following keys:

• generatedLine: <number> The line offset of the start of the range in the generated source
• generatedColumn: <number> The column offset of start of the range in the generated source
• originalSource: <string> The file name of the original source, as reported in the SourceMap
• originalLine: <number> The line offset of the start of the range in the original source
• originalColumn: <number> The column offset of start of the range in the original source
• name: <string>

The returned value represents the raw range as it appears in the SourceMap, based on zero-indexed offsets, not 1-indexed line and column numbers as they appear in Error messages and CallSite objects.

To get the corresponding 1-indexed line and column numbers from a lineNumber and columnNumber as they are reported by Error stacks and CallSite objects, use sourceMap.findOrigin(lineNumber, columnNumber)

sourceMap.findOrigin(lineNumber, columnNumber)#

• lineNumber <number> The 1-indexed line number of the call site in the generated source
• columnNumber <number> The 1-indexed column number of the call site in the generated source
• Returns: <Object>

Given a 1-indexed lineNumber and columnNumber from a call site in the generated source, find the corresponding call site location in the original source.

If the lineNumber and columnNumber provided are not found in any source map, then an empty object is returned. Otherwise, the returned object contains the following keys:

• name: <string> | <undefined> The name of the range in the source map, if one was provided
• fileName: <string> The file name of the original source, as reported in the SourceMap
• lineNumber: <number> The 1-indexed lineNumber of the corresponding call site in the original source
• columnNumber: <number> The 1-indexed columnNumber of the corresponding call site in the original source