Node.js v27.0.0 documentation

Virtual File System#

The node:vfs module provides a virtual file system with a node:fs-like API. It is useful for tests, fixtures, embedded assets, and other scenarios where you need a self-contained file system without touching the actual file-system.

To access it:

import vfs from 'node:vfs';
const vfs = require('node:vfs');
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This module is only available under the node: scheme, and only when Node.js is started with the --experimental-vfs flag.

Security#

The VFS API is not a sandbox, permission system, or access-control mechanism. It does not isolate untrusted code from the host file system or from other Node.js capabilities. Code that can access a VirtualFileSystem instance, mount it, select its provider, or pass paths to it is trusted application code.

Mounting a VFS only redirects supported node:fs calls whose resolved paths are under the mount point. It does not prevent code from using other paths or other Node.js APIs to access resources available to the process. RealFSProvider maps VFS paths under its configured root and rejects paths that resolve outside that root, but that check is not a security boundary. Do not rely on VFS to run untrusted code; use operating-system-level isolation, such as separate users, containers, or platform sandboxes, when a security boundary is required.

Basic usage#

const vfs = require('node:vfs');

const myVfs = vfs.create();
myVfs.mkdirSync('/dir', { recursive: true });
myVfs.writeFileSync('/dir/hello.txt', 'Hello, VFS!');

console.log(myVfs.readFileSync('/dir/hello.txt', 'utf8')); // 'Hello, VFS!'
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vfs.create() returns a VirtualFileSystem instance backed by a MemoryProvider by default. The instance exposes synchronous, callback-based, and promise-based file system methods that mirror the shape of the node:fs API. All paths are POSIX-style and absolute (starting with /).

By default, the file tree is private to the VFS instance. To expose it through the global node:fs module, require(), and import, call vfs.mount(); call vfs.unmount() (or rely on a using declaration) to detach again.

vfs.create([provider][, options])#

• provider <VirtualProvider> The provider to use. Default: new MemoryProvider().
• options <Object>
  • emitExperimentalWarning <boolean> Whether to emit the experimental warning when the instance is created. Default: true.
• Returns: <VirtualFileSystem>

Convenience factory equivalent to new VirtualFileSystem(provider, options).

const vfs = require('node:vfs');

// Default in-memory provider
const memoryVfs = vfs.create();

// Explicit provider
const realVfs = vfs.create(new vfs.RealFSProvider('/tmp/vfs-root'));
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Class: VirtualFileSystem#

A VirtualFileSystem wraps a VirtualProvider and exposes a node:fs-like API. Each instance maintains its own file tree.

new VirtualFileSystem([provider][, options])#

• provider <VirtualProvider> The provider to use. Default: new MemoryProvider().
• options <Object>
  • emitExperimentalWarning <boolean> Whether to emit the experimental warning. Default: true.

vfs.mount()#

• Returns: <string> The absolute mount point.

Mounts the virtual file system and returns the resulting mount point. After mounting, files in the VFS can be accessed through the node:fs module and resolved through require() and import using paths under the returned mount point.

Mount points always live inside a reserved namespace that cannot have child file system entries, so virtual paths never conflate with (or shadow) real paths. The virtual path scheme is subject to change and users should not manually construct them based on assumptions. Instead, obtain them from what vfs.mount() returns or vfs.mountPoint.

const vfs = require('node:vfs');
const fs = require('node:fs');

const myVfs = vfs.create();
myVfs.writeFileSync('/data.txt', 'Hello');
const mountPoint = myVfs.mount();
// e.g. '/dev/null/vfs/0'

fs.readFileSync(`${mountPoint}/data.txt`, 'utf8'); // 'Hello'
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Each VirtualFileSystem instance may be mounted at most once at a time. Attempting to mount an already-mounted instance throws ERR_INVALID_STATE. Because each instance mounts inside its own per-layer namespace, mounts from different instances can never overlap.

The VFS supports the Explicit Resource Management proposal. Use a using declaration to unmount automatically when leaving scope:

const vfs = require('node:vfs');
const fs = require('node:fs');

let mountPoint;
{
  using myVfs = vfs.create();
  myVfs.writeFileSync('/data.txt', 'Hello');
  mountPoint = myVfs.mount();

  fs.readFileSync(`${mountPoint}/data.txt`, 'utf8'); // 'Hello'
} // VFS is automatically unmounted here

fs.existsSync(`${mountPoint}/data.txt`); // false
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vfs.unmount()#

Unmounts the virtual file system. After unmounting, virtual files are no longer reachable through node:fs, require(), or import. The same instance may be mounted again by calling mount().

This method is idempotent: calling unmount() on a VFS that is not currently mounted has no effect.

vfs.mounted#

• <boolean>

true while the VFS is mounted; false otherwise.

vfs.mountPoint#

• <string> | <null>

The current mount point as an absolute string (the value returned by the last vfs.mount() call), or null when the VFS is not mounted.

vfs.mountPointURL#

• <string> | <null>

The current mount point as a file: URL string (the vfs.mountPoint path converted with url.pathToFileURL()), or null when the VFS is not mounted.

This is a convenience for addressing mounted files with URL-based APIs such as dynamic import():

import vfs from 'node:vfs';

const myVfs = vfs.create();
myVfs.writeFileSync('/mod.mjs', 'export const value = 42;');
myVfs.mount();

const { value } = await import(`${myVfs.mountPointURL}/mod.mjs`);
console.log(value); // 42

myVfs.unmount();
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vfs.provider#

• <VirtualProvider>

The provider backing this VFS instance.

vfs.readonly#

• <boolean>

true when the underlying provider is read-only.

APIs#

VirtualFileSystem implements the following methods, with the same signatures as their node:fs counterparts:

Synchronous API#

• existsSync(path)
• statSync(path[, options])
• lstatSync(path[, options])
• readFileSync(path[, options])
• writeFileSync(path, data[, options])
• appendFileSync(path, data[, options])
• readdirSync(path[, options])
• mkdirSync(path[, options])
• rmdirSync(path)
• unlinkSync(path)
• renameSync(oldPath, newPath)
• copyFileSync(src, dest[, mode])
• realpathSync(path[, options])
• readlinkSync(path[, options])
• symlinkSync(target, path[, type])
• accessSync(path[, mode])
• rmSync(path[, options])
• truncateSync(path[, len])
• ftruncateSync(fd[, len])
• linkSync(existingPath, newPath)
• chmodSync(path, mode)
• chownSync(path, uid, gid)
• lchownSync(path, uid, gid)
• utimesSync(path, atime, mtime)
• lutimesSync(path, atime, mtime)
• mkdtempSync(prefix)
• opendirSync(path[, options])
• openAsBlob(path[, options])
• File-descriptor ops: openSync, closeSync, readSync, writeSync, fstatSync
• Streams: createReadStream, createWriteStream
• Watchers: watch, watchFile, unwatchFile

Callback API#

readFile, writeFile, stat, lstat, readdir, realpath, readlink, access, open, close, read, write, rm, fstat, truncate, ftruncate, link, mkdtemp, opendir. Each takes a Node.js-style callback (err, ...result) => {}.

Promise API#

vfs.promises exposes the promise-based variants:

const vfs = require('node:vfs');

async function example() {
  const myVfs = vfs.create();
  await myVfs.promises.writeFile('/file.txt', 'hello');
  const data = await myVfs.promises.readFile('/file.txt', 'utf8');
  return data;
}
example();
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The promise namespace mirrors fs.promises and includes readFile, writeFile, appendFile, stat, lstat, readdir, mkdir, rmdir, unlink, rename, copyFile, realpath, readlink, symlink, access, rm, truncate, link, mkdtemp, chmod, chown, lchown, utimes, lutimes, open, lchmod, and watch.

Module loader integration#

Once a VirtualFileSystem is mounted, paths under the mount point participate in module resolution and loading. The CommonJS resolution algorithm used by require() and require.resolve() and the ES modules resolution algorithm used by import and import.meta.resolve() are unchanged; instead, every file system operation those algorithms perform is dispatched on the path being probed: paths under a mount point are served by the owning VFS, and all other paths are served by the real file system. Files served from the VFS therefore behave as first-class modules.

Because mounted paths live in a reserved namespace that cannot exist on disk, any given path is served either by exactly one VFS or by the real file system, never both. There is no search order or fallback between the two: if a path under a mount point does not exist in the VFS, resolution fails with ENOENT without consulting the disk, and a mounted layer never shadows a real directory.

For resolution purposes the mount point behaves as a file system root: package.json scope lookups and loading from node_modules folders stop at the mount point. For example, when ${mountPoint}/foo/bar/main.cjs calls require('baz'), the lookup goes through:

• ${mountPoint}/foo/bar/node_modules/baz
• ${mountPoint}/foo/node_modules/baz
• ${mountPoint}/node_modules/baz
• If $NODE_PATH is set, the folders listed in $NODE_PATH
• $HOME/.node_modules/baz
• $HOME/.node_libraries/baz
• $PREFIX/lib/node/baz

The last four entries are the global folders, which are legacy CommonJS behavior and do not apply to import. Absolute specifiers may cross the boundary in either direction: a module on the real file system can require() a mounted path, and a virtual module can require() a real one.

const vfs = require('node:vfs');

const myVfs = vfs.create();
myVfs.mkdirSync('/lib');
myVfs.writeFileSync('/lib/greet.js', 'module.exports = () => "hi";');
myVfs.writeFileSync(
  '/lib/package.json', '{"main": "./greet.js"}');
const mountPoint = myVfs.mount();

const greet = require(`${mountPoint}/lib`);
console.log(greet()); // 'hi'

myVfs.unmount();
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For ECMAScript modules, use file: URLs when passing mounted paths to dynamic import(). vfs.mountPointURL provides the mount point in that form; this keeps VFS imports portable on Windows, where mounted paths use Windows path syntax.

import vfs from 'node:vfs';

const myVfs = vfs.create();
myVfs.writeFileSync('/mod.mjs', 'export const value = 42;');
myVfs.mount();

const { value } = await import(`${myVfs.mountPointURL}/mod.mjs`);
console.log(value); // 42

myVfs.unmount();
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CommonJS modules loaded from a mounted VFS are identified by their VFS paths that start with the mount point. This is reflected in, for example, __filename and __dirname in the module, or the errors stack traces involving functions from the VFS modules. ES modules in the VFS are similarly identified by the file: URL of their VFS paths and this is reflected in e.g. import.meta.url.

Like modules loaded from the real file system, modules loaded from the VFS are cached on the first load. When require() or import() is used to load an absolute path or URL that falls under the mounted VFS multiple times, the module is only loaded once and subsequent calls return the same instance.

Calling vfs.unmount() invalidates the modules that were loaded from the mount point: a subsequent require() or import of a path under a re-created mount re-reads the file from the newly mounted VFS rather than returning a stale module. Modules loaded from other VFS instances or from the real file system are unaffected.

Mounting and unmounting do not stop any module execution that is already started, or invalidate any objects materialized from VFS modules that are already executed. As with modules in the real file system, the callers are responsible for avoiding removal or invalidation of modules in the virtual file system while they are being loaded.

Class: VirtualProvider#

The base class for all VFS providers. Subclasses implement the essential primitives (such as open, stat, readdir, mkdir, rmdir, unlink, rename, etc.) and inherit default implementations of the derived methods (such as readFile, writeFile, exists, copyFile, access, etc.).

Capability flags#

• provider.readonly <boolean> Default: false.
• provider.supportsSymlinks <boolean> Default: false.
• provider.supportsWatch <boolean> Default: false.

Creating custom providers#

const { VirtualProvider } = require('node:vfs');

class StaticProvider extends VirtualProvider {
  get readonly() { return true; }

  statSync(path) { /* ... */ }
  openSync(path, flags) { /* ... */ }
  readdirSync(path, options) { /* ... */ }
  // ...
}
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The base class throws ERR_METHOD_NOT_IMPLEMENTED for any primitive that has not been overridden, and rejects writes from a readonly provider with EROFS.

Class: MemoryProvider#

The default in-memory provider. Stores files, directories, and symbolic links in a Map-backed tree, supports symlinks (supportsSymlinks === true), and supports watching (supportsWatch === true).

memoryProvider.setReadOnly()#

Locks the provider into read-only mode. Subsequent writes through any VirtualFileSystem using this provider throw EROFS. There is no way to revert the provider to writable.

const vfs = require('node:vfs');

const provider = new vfs.MemoryProvider();
const myVfs = vfs.create(provider);
myVfs.writeFileSync('/seed.txt', 'initial');

provider.setReadOnly();

myVfs.writeFileSync('/x.txt', 'fail'); // throws EROFS
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Class: RealFSProvider#

A provider that wraps a directory (i.e. one on the actual file system) and exposes its contents through the VFS API. All VFS paths are resolved relative to the root and verified to stay inside it; symbolic links resolving outside the root are rejected. This path mapping is not a sandbox or access-control mechanism.

new RealFSProvider(rootPath)#

• rootPath <string> The absolute file-system path to use as the root. Must be a non-empty string.

const vfs = require('node:vfs');

const realVfs = vfs.create(new vfs.RealFSProvider('/tmp/vfs-root'));
realVfs.writeFileSync('/file.txt', 'hello'); // writes /tmp/vfs-root/file.txt
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realFSProvider.rootPath#

• <string>

The resolved absolute path used as the root.

Implementation details#

Stats objects#

VFS Stats objects are real instances of fs.Stats (or fs.BigIntStats when { bigint: true } is requested). Their fields use synthetic but stable values:

• dev is 4085 (the VFS device id).
• ino is monotonically increasing per process.
• blksize is 4096.
• blocks is Math.ceil(size / 512).
• Times default to the moment the entry was created/last modified.