Node.js v21.0.0 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.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>
  • data <any> Any arbitrary, cloneable JavaScript value to pass into the initialize hook.
  • transferList <Object[]> transferrable 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.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

Customization Hooks#

Enabling#

Module resolution and loading can be customized by registering a file which exports a set of hooks. This can be done using the register method from node:module, which you can run before your application code by using the --import flag:

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

// register-hooks.js
import { register } from 'node:module';

register('./hooks.mjs', import.meta.url);// register-hooks.js
const { register } = require('node:module');
const { pathToFileURL } = require('node:url');

register('./hooks.mjs', pathToFileURL(__filename));copy

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

node --import 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 ensures that the hooks are registered before any application files are imported, including the entry point of the application. Alternatively, register can be called from the entry point, but dynamic import() must be used for any 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

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 (and optionally require). 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.

my-app.js can also be CommonJS. Customization hooks will run for any modules that it references via import (and optionally require).

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('./first.mjs', import.meta.url);
register('./second.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('./first.mjs', parentURL);
register('./second.mjs', parentURL);
import('./my-app.mjs');copy

In this example, the registered hooks will form chains. If both first.mjs and second.mjs define a resolve hook, both will be called, in the order they were registered. The same applies to all the other hooks.

The registered hooks also affect register itself. In this example, second.mjs will be resolved and loaded per the hooks registered by first.mjs. This allows for things like writing hooks in non-JavaScript languages, so long as an earlier registered loader is one that transpiles into JavaScript.

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

Communication with module customization hooks#

Module customization 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 transferrable 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);
});

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);
});

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

Hooks#

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

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

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.

initialize()#

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

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 transferrable 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');
});

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');
});

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>
  • 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 Despite support for returning promises and async functions, calls to resolve may 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 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);
} 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
  • specifier <string>
  • context <Object>
• Returns: <Object>
  • format <string>
  • shortCircuit <undefined> | <boolean> A signal that this hook intends to terminate the chain of resolve 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:

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.

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.

The Node.js internal 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';

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

Warning: 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 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.

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

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#

In current Node.js, specifiers starting with https:// are experimental (see HTTPS and HTTP imports).

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.

// 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);
    });
  // 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, '..'));
} copy

# 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 module, running node --import 'data:text/javascript,import { register } from "node:module"; import { pathToFileURL } from "node:url"; register(pathToFileURL("./coffeescript-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).

// 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

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 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
• columnOffset <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