Node.js v18.20.5 documentation

Introduction#

ECMAScript modules are the official standard format to package JavaScript code for reuse. Modules are defined using a variety of import and export statements.

The following example of an ES module exports a function:

// addTwo.mjs
function addTwo(num) {
  return num + 2;
}

export { addTwo }; copy

The following example of an ES module imports the function from addTwo.mjs:

// app.mjs
import { addTwo } from './addTwo.mjs';

// Prints: 6
console.log(addTwo(4)); copy

Node.js fully supports ECMAScript modules as they are currently specified and provides interoperability between them and its original module format, CommonJS.

Enabling#

Node.js has two module systems: CommonJS modules and ECMAScript modules.

Authors can tell Node.js to use the ECMAScript modules loader via the .mjs file extension, the package.json "type" field, the --input-type flag, or the --experimental-default-type flag. Outside of those cases, Node.js will use the CommonJS module loader. See Determining module system for more details.

Packages#

This section was moved to Modules: Packages.

import Specifiers#

Terminology#

The specifier of an import statement is the string after the from keyword, e.g. 'node:path' in import { sep } from 'node:path'. Specifiers are also used in export from statements, and as the argument to an import() expression.

There are three types of specifiers:

• Relative specifiers like './startup.js' or '../config.mjs'. They refer to a path relative to the location of the importing file. The file extension is always necessary for these.

• Bare specifiers like 'some-package' or 'some-package/shuffle'. They can refer to the main entry point of a package by the package name, or a specific feature module within a package prefixed by the package name as per the examples respectively. Including the file extension is only necessary for packages without an "exports" field.

• Absolute specifiers like 'file:///opt/nodejs/config.js'. They refer directly and explicitly to a full path.

Bare specifier resolutions are handled by the Node.js module resolution and loading algorithm. All other specifier resolutions are always only resolved with the standard relative URL resolution semantics.

Like in CommonJS, module files within packages can be accessed by appending a path to the package name unless the package's package.json contains an "exports" field, in which case files within packages can only be accessed via the paths defined in "exports".

For details on these package resolution rules that apply to bare specifiers in the Node.js module resolution, see the packages documentation.

Mandatory file extensions#

A file extension must be provided when using the import keyword to resolve relative or absolute specifiers. Directory indexes (e.g. './startup/index.js') must also be fully specified.

This behavior matches how import behaves in browser environments, assuming a typically configured server.

URLs#

ES modules are resolved and cached as URLs. This means that special characters must be percent-encoded, such as # with %23 and ? with %3F.

file:, node:, and data: URL schemes are supported. A specifier like 'https://example.com/app.js' is not supported natively in Node.js unless using a custom HTTPS loader.

file: URLs#

Modules are loaded multiple times if the import specifier used to resolve them has a different query or fragment.

import './foo.mjs?query=1'; // loads ./foo.mjs with query of "?query=1"
import './foo.mjs?query=2'; // loads ./foo.mjs with query of "?query=2" copy

The volume root may be referenced via /, //, or file:///. Given the differences between URL and path resolution (such as percent encoding details), it is recommended to use url.pathToFileURL when importing a path.

data: imports#

data: URLs are supported for importing with the following MIME types:

• text/javascript for ES modules
• application/json for JSON
• application/wasm for Wasm

import 'data:text/javascript,console.log("hello!");';
import _ from 'data:application/json,"world!"' with { type: 'json' }; copy

data: URLs only resolve bare specifiers for builtin modules and absolute specifiers. Resolving relative specifiers does not work because data: is not a special scheme. For example, attempting to load ./foo from data:text/javascript,import "./foo"; fails to resolve because there is no concept of relative resolution for data: URLs.

node: imports#

node: URLs are supported as an alternative means to load Node.js builtin modules. This URL scheme allows for builtin modules to be referenced by valid absolute URL strings.

import fs from 'node:fs/promises'; copy

Import attributes#

Import attributes are an inline syntax for module import statements to pass on more information alongside the module specifier.

import fooData from './foo.json' with { type: 'json' };

const { default: barData } =
  await import('./bar.json', { with: { type: 'json' } }); copy

Node.js only supports the type attribute, for which it supports the following values:

The type: 'json' attribute is mandatory when importing JSON modules.

Builtin modules#

Core modules provide named exports of their public API. A default export is also provided which is the value of the CommonJS exports. The default export can be used for, among other things, modifying the named exports. Named exports of builtin modules are updated only by calling module.syncBuiltinESMExports().

import EventEmitter from 'node:events';
const e = new EventEmitter(); copy

import { readFile } from 'node:fs';
readFile('./foo.txt', (err, source) => {
  if (err) {
    console.error(err);
  } else {
    console.log(source);
  }
}); copy

import fs, { readFileSync } from 'node:fs';
import { syncBuiltinESMExports } from 'node:module';
import { Buffer } from 'node:buffer';

fs.readFileSync = () => Buffer.from('Hello, ESM');
syncBuiltinESMExports();

fs.readFileSync === readFileSync; copy

import() expressions#

Dynamic import() is supported in both CommonJS and ES modules. In CommonJS modules it can be used to load ES modules.

import.meta#

• <Object>

The import.meta meta property is an Object that contains the following properties.

import.meta.url#

• <string> The absolute file: URL of the module.

This is defined exactly the same as it is in browsers providing the URL of the current module file.

This enables useful patterns such as relative file loading:

import { readFileSync } from 'node:fs';
const buffer = readFileSync(new URL('./data.proto', import.meta.url)); copy

import.meta.resolve(specifier)#

• specifier <string> The module specifier to resolve relative to the current module.
• Returns: <string> The absolute URL string that the specifier would resolve to.

import.meta.resolve is a module-relative resolution function scoped to each module, returning the URL string.

const dependencyAsset = import.meta.resolve('component-lib/asset.css');
// file:///app/node_modules/component-lib/asset.css
import.meta.resolve('./dep.js');
// file:///app/dep.js copy

All features of the Node.js module resolution are supported. Dependency resolutions are subject to the permitted exports resolutions within the package.

Caveats:

• This can result in synchronous file-system operations, which can impact performance similarly to require.resolve.
• This feature is not available within custom loaders (it would create a deadlock).

Non-standard API:

When using the --experimental-import-meta-resolve flag, that function accepts a second argument:

• parent <string> | <URL> An optional absolute parent module URL to resolve from. Default: import.meta.url

Interoperability with CommonJS#

import statements#

An import statement can reference an ES module or a CommonJS module. import statements are permitted only in ES modules, but dynamic import() expressions are supported in CommonJS for loading ES modules.

When importing CommonJS modules, the module.exports object is provided as the default export. Named exports may be available, provided by static analysis as a convenience for better ecosystem compatibility.

require#

The CommonJS module require always treats the files it references as CommonJS.

Using require to load an ES module is not supported because ES modules have asynchronous execution. Instead, use import() to load an ES module from a CommonJS module.

CommonJS Namespaces#

CommonJS modules consist of a module.exports object which can be of any type.

When importing a CommonJS module, it can be reliably imported using the ES module default import or its corresponding sugar syntax:

import { default as cjs } from 'cjs';

// The following import statement is "syntax sugar" (equivalent but sweeter)
// for `{ default as cjsSugar }` in the above import statement: