Node.js

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Table of contents

- Modules: `node:module` API
  - The `Module` object
    - `module.builtinModules`
    - `module.createRequire(filename)`
    - `module.isBuiltin(moduleName)`
    - `module.register()`
    - `module.syncBuiltinESMExports()`
  - Source map v3 support
    - `module.findSourceMap(path)`
    - Class: `module.SourceMap`
      - `new SourceMap(payload)`
      - `sourceMap.payload`
      - `sourceMap.findEntry(lineOffset, columnOffset)`
      - `sourceMap.findOrigin(lineNumber, columnNumber)`

## Modules: `node:module` API#

Added in: v0.3.7

### 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`#

Added in: v9.3.0, v8.10.0, v6.13.0

- <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;

#### `module.createRequire(filename)`#

Added in: v12.2.0

- `filename` <string> | <URL> Filename to be used to construct the require function. Must be a file URL object, file URL string, or absolute path string.
- Returns: <require> Require function

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

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

#### `module.isBuiltin(moduleName)`#

Added in: v18.6.0, v16.17.0

- `moduleName` <string> name of the module
- Returns: <boolean> returns true if the module is builtin else returns false

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

#### `module.register()`#

Added in: REPLACEME

In addition to using the `--experimental-loader` option in the CLI, loaders can be registered programmatically using the `module.register()` method.

    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
    // before importing `./my-app.mjs`.
    await import('./my-app.mjs');

In the example above, we are registering the `http-to-https` loader, but it will only be available for subsequently imported modules—in this case, `my-app.mjs`. If the `await import('./my-app.mjs')` had instead been a static `import './my-app.mjs'`, the app would already have been loaded before the `http-to-https` hooks were registered. This is part of the design of ES modules, where static imports are evaluated from the leaves of the tree first back to the trunk. There can be static imports within `my-app.mjs`, which will not be evaluated until `my-app.mjs` is when it's dynamically imported.

The `--experimental-loader` flag of the CLI can be used together with the `register` function; the loaders registered with the function will follow the same evaluation chain of loaders registered within the CLI:

    node \
      --experimental-loader unpkg \
      --experimental-loader http-to-https \
      --experimental-loader cache-buster \
      entrypoint.mjs

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

    const loaderURL = new URL('./my-programmatically-loader.mjs', import.meta.url);

    register(loaderURL);
    await import('./my-app.mjs');

The `my-programmatic-loader.mjs` can leverage `unpkg`, `http-to-https`, and `cache-buster` loaders.

It's also possible to use `register` more than once:

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

    register(new URL('./first-loader.mjs', import.meta.url));
    register('./second-loader.mjs', import.meta.url);
    await import('./my-app.mjs');

Both loaders (`first-loader.mjs` and `second-loader.mjs`) can use all the resources provided by the loaders registered in the CLI. But remember that they will only be available in the next imported module (`my-app.mjs`). The evaluation order of the hooks when importing `my-app.mjs` and consecutive modules in the example above will be:

    resolve: second-loader.mjs
    resolve: first-loader.mjs
    resolve: cache-buster
    resolve: http-to-https
    resolve: unpkg
    load: second-loader.mjs
    load: first-loader.mjs
    load: cache-buster
    load: http-to-https
    load: unpkg
    globalPreload: second-loader.mjs
    globalPreload: first-loader.mjs
    globalPreload: cache-buster
    globalPreload: http-to-https
    globalPreload: unpkg

#### `module.syncBuiltinESMExports()`#

Added in: v12.12.0

The `module.syncBuiltinESMExports()` method updates all the live bindings for builtin ES Modules to match the properties of the CommonJS exports. It does not add or remove exported names from the ES Modules.

    const fs = require('node:fs');
    const assert = require('node:assert');
    const { syncBuiltinESMExports } = require('node:module');

    fs.readFile = newAPI;

    delete fs.readFileSync;

    function newAPI() {
      // ...
    }

    fs.newAPI = newAPI;

    syncBuiltinESMExports();

    import('node:fs').then((esmFS) => {
      // It syncs the existing readFile property with the new value
      assert.strictEqual(esmFS.readFile, newAPI);
      // readFileSync has been deleted from the required fs
      assert.strictEqual('readFileSync' in fs, false);
      // syncBuiltinESMExports() does not remove readFileSync from esmFS
      assert.strictEqual('readFileSync' in esmFS, true);
      // syncBuiltinESMExports() does not add names
      assert.strictEqual(esmFS.newAPI, undefined);
    });

### Source map v3 support#

Added in: v13.7.0, v12.17.0

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`.

    // module.mjs
    // In an ECMAScript module
    import { findSourceMap, SourceMap } from 'node:module';// module.cjs
    // In a CommonJS module
    const { findSourceMap, SourceMap } = require('node:module');

#### `module.findSourceMap(path)`#

Added in: v13.7.0, v12.17.0

- `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`#

Added in: v13.7.0, v12.17.0

##### `new SourceMap(payload)`#

- `payload` <Object>

Creates a new `sourceMap` instance.

`payload` is an object with keys matching the Source map v3 format:

- `file`: <string>
- `version`: <number>
- `sources`: <string[]>
- `sourcesContent`: <string[]>
- `names`: <string[]>
- `mappings`: <string>
- `sourceRoot`: <string>

##### `sourceMap.payload`#

- Returns: <Object>

Getter for the payload used to construct the `SourceMap` instance.

##### `sourceMap.findEntry(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