# Modules: `node:module` API

<!--introduced_in=v12.20.0-->

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## 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`

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* {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.

Note: the list doesn't contain [prefix-only modules][] like `node:test`.

`module` in this context isn't the same object that's provided
by the [module wrapper][]. To access it, require the `Module` module:

```mjs
// module.mjs
// In an ECMAScript module
import { builtinModules as builtin } from 'node:module';
```

```cjs
// module.cjs
// In a CommonJS module
const builtin = require('node:module').builtinModules;
```

### `module.createRequire(filename)`

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added: v12.2.0
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* `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

```mjs
import { createRequire } from 'node:module';
const require = createRequire(import.meta.url);

// sibling-module.js is a CommonJS module.
const siblingModule = require('./sibling-module');
```

### `module.isBuiltin(moduleName)`

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* `moduleName` {string} name of the module
* Returns: {boolean} returns true if the module is builtin else returns false

```mjs
import { isBuiltin } from 'node:module';
isBuiltin('node:fs'); // true
isBuiltin('fs'); // true
isBuiltin('wss'); // false
```

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

<!-- YAML
added: v20.6.0
changes:
  - version: v20.8.0
    pr-url: https://github.com/nodejs/node/pull/49655
    description: Add support for WHATWG URL instances.
-->

> Stability: 1.2 - Release candidate

* `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\[]} [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()`

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added: v12.12.0
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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][].

```js
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);
});
```

<i id="module_customization_hooks"></i>

## Customization Hooks

<!-- YAML
added: v8.8.0
changes:
  - version: v20.6.0
    pr-url: https://github.com/nodejs/node/pull/48842
    description: Added `initialize` hook to replace `globalPreload`.
  - version:
    - v18.6.0
    - v16.17.0
    pr-url: https://github.com/nodejs/node/pull/42623
    description: Add support for chaining loaders.
  - version: v16.12.0
    pr-url: https://github.com/nodejs/node/pull/37468
    description: Removed `getFormat`, `getSource`, `transformSource`, and
                 `globalPreload`; added `load` hook and `getGlobalPreload` hook.
-->

> Stability: 1.2 - Release candidate

<!-- type=misc -->

<i id="enabling_module_customization_hooks"></i>

### 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:

```bash
node --import ./register-hooks.js ./my-app.js
```

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

register('./hooks.mjs', import.meta.url);
```

```cjs
// register-hooks.js
const { register } = require('node:module');
const { pathToFileURL } = require('node:url');

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

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

```bash
node --import some-package/register ./my-app.js
```

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:

```mjs
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');
```

```cjs
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');
```

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`:

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

### Chaining

It's possible to call `register` more than once:

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

register('./foo.mjs', import.meta.url);
register('./bar.mjs', import.meta.url);
await import('./my-app.mjs');
```

```cjs
// 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');
```

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.

### 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.

```mjs
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],
});
```

```cjs
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],
});
```

### 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.

```mjs
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.
}
```

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.

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()`

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> Stability: 1.2 - Release candidate

* `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:

```mjs
// path-to-my-hooks.js

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

Caller code:

```mjs
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],
});
```

```cjs
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],
});
```

#### `resolve(specifier, context, nextResolve)`

<!-- YAML
changes:
  - version: v20.10.0
    pr-url: https://github.com/nodejs/node/pull/50140
    description: The property `context.importAssertions` is replaced with
                 `context.importAttributes`. Using the old name is still
                 supported and will emit an experimental warning.
  - version:
    - v18.6.0
    - v16.17.0
    pr-url: https://github.com/nodejs/node/pull/42623
    description: Add support for chaining resolve hooks. Each hook must either
      call `nextResolve()` or include a `shortCircuit` property set to `true`
      in its return.
  - version:
    - v17.1.0
    - v16.14.0
    pr-url: https://github.com/nodejs/node/pull/40250
    description: Add support for import assertions.
-->

> Stability: 1.2 - Release candidate

* `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][Conditional exports] 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][Conditional exports] 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.

```mjs
export async function resolve(specifier, context, nextResolve) {
  const { parentURL = null } = context;

  if (Math.random() > 0.5) { // Some condition.
    // For some or all specifiers, do some custom logic for resolving.
    // Always return an object of the form {url: <string>}.
    return {
      shortCircuit: true,
      url: parentURL ?
        new URL(specifier, parentURL).href :
        new URL(specifier).href,
    };
  }

  if (Math.random() < 0.5) { // Another condition.
    // When calling `defaultResolve`, the arguments can be modified. In this
    // case it's adding another value for matching conditional exports.
    return nextResolve(specifier, {
      ...context,
      conditions: [...context.conditions, 'another-condition'],
    });
  }

  // Defer to the next hook in the chain, which would be the
  // Node.js default resolve if this is the last user-specified loader.
  return nextResolve(specifier);
}
```

#### `load(url, context, nextLoad)`

<!-- YAML
changes:
  - version: v20.6.0
    pr-url: https://github.com/nodejs/node/pull/47999
    description: Add support for `source` with format `commonjs`.
  - version:
    - v18.6.0
    - v16.17.0
    pr-url: https://github.com/nodejs/node/pull/42623
    description: Add support for chaining load hooks. Each hook must either
      call `nextLoad()` or include a `shortCircuit` property set to `true` in
      its return.
-->

> Stability: 1.2 - Release candidate

* `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}
  * `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 assertion.

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

| `format`     | Description                    | Acceptable types for `source` returned by `load`                           |
| ------------ | ------------------------------ | -------------------------------------------------------------------------- |
| `'builtin'`  | Load a Node.js builtin module  | Not applicable                                                             |
| `'commonjs'` | Load a Node.js CommonJS module | { [`string`][], [`ArrayBuffer`][], [`TypedArray`][], `null`, `undefined` } |
| `'json'`     | Load a JSON file               | { [`string`][], [`ArrayBuffer`][], [`TypedArray`][] }                      |
| `'module'`   | Load an ES module              | { [`string`][], [`ArrayBuffer`][], [`TypedArray`][] }                      |
| `'wasm'`     | Load a WebAssembly module      | { [`ArrayBuffer`][], [`TypedArray`][] }                                    |

The value of `source` is ignored for type `'builtin'` because currently it is
not possible to replace the value of a Node.js builtin (core) module.

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.

When `node` is run with `--experimental-default-type=commonjs`, 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:

```mjs
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;
}
```

> **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`.

```mjs
export async function load(url, context, nextLoad) {
  const { format } = context;

  if (Math.random() > 0.5) { // Some condition
    /*
      For some or all URLs, do some custom logic for retrieving the source.
      Always return an object of the form {
        format: <string>,
        source: <string|buffer>,
      }.
    */
    return {
      format,
      shortCircuit: true,
      source: '...',
    };
  }

  // Defer to the next hook in the chain.
  return nextLoad(url);
}
```

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

#### `globalPreload()`

<!-- YAML
changes:
  - version:
    - v18.6.0
    - v16.17.0
    pr-url: https://github.com/nodejs/node/pull/42623
    description: Add support for chaining globalPreload hooks.
-->

> Stability: 1.0 - Early development

> **Warning:** This hook will be removed in a future version. Use
> [`initialize`][] instead. When a hooks module has an `initialize` export,
> `globalPreload` will be ignored.

* `context` {Object} Information to assist the preload code
  * `port` {MessagePort}
* Returns: {string} Code to run before application startup

Sometimes it might be necessary to run some code inside of the same global
scope that the application runs in. This hook allows the return of a string
that is run as a sloppy-mode script on startup.

Similar to how CommonJS wrappers work, the code runs in an implicit function
scope. The only argument is a `require`-like function that can be used to load
builtins like "fs": `getBuiltin(request: string)`.

If the code needs more advanced `require` features, it has to construct
its own `require` using  `module.createRequire()`.

```mjs
export function globalPreload(context) {
  return `\
globalThis.someInjectedProperty = 42;
console.log('I just set some globals!');

const { createRequire } = getBuiltin('module');
const { cwd } = getBuiltin('process');

const require = createRequire(cwd() + '/<preload>');
// [...]
`;
}
```

Another argument is provided to the preload code: `port`. This is available as a
parameter to the hook and inside of the source text returned by the hook. This
functionality has been moved to the `initialize` hook.

Care must be taken in order to properly call [`port.ref()`][] and
[`port.unref()`][] to prevent a process from being in a state where it won't
close normally.

```mjs
/**
 * This example has the application context send a message to the hook
 * and sends the message back to the application context
 */
export function globalPreload({ port }) {
  port.onmessage = (evt) => {
    port.postMessage(evt.data);
  };
  return `\
    port.postMessage('console.log("I went to the hook and back");');
    port.onmessage = (evt) => {
      eval(evt.data);
    };
  `;
}
```

### 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.

```mjs
// 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);
}
```

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

console.log(VERSION);
```

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][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.

```mjs
// 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, '..'));
}
```

```coffee
# 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}"
```

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

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).

```mjs
// 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);
}
```

With these files:

```mjs
// main.js
import 'a-module';
```

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

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

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

<!-- YAML
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> Stability: 1 - Experimental

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`][].

```mjs
// module.mjs
// In an ECMAScript module
import { findSourceMap, SourceMap } from 'node:module';
```

```cjs
// module.cjs
// In a CommonJS module
const { findSourceMap, SourceMap } = require('node:module');
```

<!-- Anchors to make sure old links find a target -->

<a id="module_module_findsourcemap_path_error"></a>

### `module.findSourceMap(path)`

<!-- YAML
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-->

* `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`

<!-- YAML
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-->

#### `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

[CommonJS]: modules.md
[Conditional exports]: packages.md#conditional-exports
[Customization hooks]: #customization-hooks
[ES Modules]: esm.md
[HTTPS and HTTP imports]: esm.md#https-and-http-imports
[Source map v3 format]: https://sourcemaps.info/spec.html#h.mofvlxcwqzej
[`"exports"`]: packages.md#exports
[`--enable-source-maps`]: cli.md#--enable-source-maps
[`ArrayBuffer`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer
[`NODE_V8_COVERAGE=dir`]: cli.md#node_v8_coveragedir
[`SharedArrayBuffer`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer
[`SourceMap`]: #class-modulesourcemap
[`TypedArray`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray
[`Uint8Array`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array
[`initialize`]: #initialize
[`module`]: modules.md#the-module-object
[`port.ref()`]: worker_threads.md#portref
[`port.unref()`]: worker_threads.md#portunref
[`register`]: #moduleregisterspecifier-parenturl-options
[`string`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String
[`util.TextDecoder`]: util.md#class-utiltextdecoder
[chain]: #chaining
[hooks]: #customization-hooks
[load hook]: #loadurl-context-nextload
[module wrapper]: modules.md#the-module-wrapper
[prefix-only modules]: modules.md#built-in-modules-with-mandatory-node-prefix
[realm]: https://tc39.es/ecma262/#realm
[source map include directives]: https://sourcemaps.info/spec.html#h.lmz475t4mvbx
[transferrable objects]: worker_threads.md#portpostmessagevalue-transferlist