ECMAScript modules

Ajouté en: v8.5.0

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:

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

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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, or the --input-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.

M 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 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.

M file: URLs

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

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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.

M data: imports

Ajouté en: v12.10.0

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

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

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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.

M 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.

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Import assertions

Ajouté en: v17.1.0, v16.14.0

The Import Assertions proposal adds an inline syntax for module import statements to pass on more information alongside the module specifier.

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Node.js supports the following type values, for which the assertion is mandatory:

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().

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M import() expressions

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

M import.meta

• Object

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

M 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:

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M import.meta.resolve(specifier[, parent])

This feature is only available with the --experimental-import-meta-resolve command flag enabled.

• specifier string The module specifier to resolve relative to parent.
• parent string | URL The absolute parent module URL to resolve from. If none is specified, the value of import.meta.url is used as the default.
• Returns: Promise

Provides a module-relative resolution function scoped to each module, returning the URL string.

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import.meta.resolve also accepts a second argument which is the parent module from which to resolve from:

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This function is asynchronous because the ES module resolver in Node.js is allowed to be asynchronous.

Interoperability with CommonJS

M 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.

M 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:

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The ECMAScript Module Namespace representation of a CommonJS module is always a namespace with a default export key pointing to the CommonJS module.exports value.

This Module Namespace Exotic Object can be directly observed either when using import * as m from 'cjs' or a dynamic import:

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For better compatibility with existing usage in the JS ecosystem, Node.js in addition attempts to determine the CommonJS named exports of every imported CommonJS module to provide them as separate ES module exports using a static analysis process.

For example, consider a CommonJS module written:

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The preceding module supports named imports in ES modules:

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As can be seen from the last example of the Module Namespace Exotic Object being logged, the name export is copied off of the module.exports object and set directly on the ES module namespace when the module is imported.

Live binding updates or new exports added to module.exports are not detected for these named exports.

The detection of named exports is based on common syntax patterns but does not always correctly detect named exports. In these cases, using the default import form described above can be a better option.

Named exports detection covers many common export patterns, reexport patterns and build tool and transpiler outputs. See cjs-module-lexer for the exact semantics implemented.

Differences between ES modules and CommonJS

No require, exports, or module.exports

In most cases, the ES module import can be used to load CommonJS modules.

If needed, a require function can be constructed within an ES module using module.createRequire().

No __filename or __dirname

These CommonJS variables are not available in ES modules.

__filename and __dirname use cases can be replicated via import.meta.url.

No Native Module Loading

Native modules are not currently supported with ES module imports.

They can instead be loaded with module.createRequire() or process.dlopen.

No require.resolve

Relative resolution can be handled via new URL('./local', import.meta.url).

For a complete require.resolve replacement, there is a flagged experimental import.meta.resolve API.

Alternatively module.createRequire() can be used.

No NODE_PATH

NODE_PATH is not part of resolving import specifiers. Please use symlinks if this behavior is desired.

No require.extensions

require.extensions is not used by import. The expectation is that loader hooks can provide this workflow in the future.

No require.cache

require.cache is not used by import as the ES module loader has its own separate cache.

JSON modules

JSON files can be referenced by import:

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The assert { type: 'json' } syntax is mandatory; see Import Assertions.

The imported JSON only exposes a default export. There is no support for named exports. A cache entry is created in the CommonJS cache to avoid duplication. The same object is returned in CommonJS if the JSON module has already been imported from the same path.

Wasm modules

Importing WebAssembly modules is supported under the --experimental-wasm-modules flag, allowing any .wasm files to be imported as normal modules while also supporting their module imports.

This integration is in line with the ES Module Integration Proposal for WebAssembly.

For example, an index.mjs containing:

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executed under:

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would provide the exports interface for the instantiation of module.wasm.

Top-level await

Ajouté en: v14.8.0

The await keyword may be used in the top level body of an ECMAScript module.

Assuming an a.mjs with

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And a b.mjs with

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If a top level await expression never resolves, the node process will exit with a 13 status code.

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HTTPS and HTTP imports

Importing network based modules using https: and http: is supported under the --experimental-network-imports flag. This allows web browser-like imports to work in Node.js with a few differences due to application stability and security concerns that are different when running in a privileged environment instead of a browser sandbox.

Imports are limited to HTTP/1

Automatic protocol negotiation for HTTP/2 and HTTP/3 is not yet supported.

HTTP is limited to loopback addresses

http: is vulnerable to man-in-the-middle attacks and is not allowed to be used for addresses outside of the IPv4 address 127.0.0.0/8 (127.0.0.1 to 127.255.255.255) and the IPv6 address ::1. Support for http: is intended to be used for local development.

Authentication is never sent to the destination server.

Authorization, Cookie, and Proxy-Authorization headers are not sent to the server. Avoid including user info in parts of imported URLs. A security model for safely using these on the server is being worked on.

CORS is never checked on the destination server

CORS is designed to allow a server to limit the consumers of an API to a specific set of hosts. This is not supported as it does not make sense for a server-based implementation.

Cannot load non-network dependencies

These modules cannot access other modules that are not over http: or https:. To still access local modules while avoiding the security concern, pass in references to the local dependencies:

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Network-based loading is not enabled by default

For now, the --experimental-network-imports flag is required to enable loading resources over http: or https:. In the future, a different mechanism will be used to enforce this. Opt-in is required to prevent transitive dependencies inadvertently using potentially mutable state that could affect reliability of Node.js applications.

Loaders

To customize the default module resolution, loader hooks can optionally be provided via a --experimental-loader ./loader-name.mjs argument to Node.js.

When hooks are used they apply to the entry point and all import calls. They won't apply to require calls; those still follow CommonJS rules.

Loaders follow the pattern of --require:

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These are called in the following sequence: cache-buster calls http-to-https which calls unpkg.

Hooks

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.

M resolve(specifier, context, nextResolve)

• specifier string
• context Object
  • conditions string[] Export conditions of the relevant package.json
  • importAssertions Object
  • 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
  • format string | null | undefined A hint to the load hook (it might be ignored) 'builtin' | 'commonjs' | 'json' | 'module' | 'wasm'
  • 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

The resolve hook chain is responsible for resolving file URL for a given module specifier and parent URL, and optionally its 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.

The module specifier is the string in an import statement or import() expression.

The parent URL is the URL of the module that imported this one, or undefined if this is the main entry point for the application.

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.

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M 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
  • importAssertions 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. The value of source is ignored for type 'commonjs' because the CommonJS module loader does not provide a mechanism for the ES module loader to override the CommonJS module return value. This limitation might be overcome in the future.

• 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 an ES module specifier. 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.

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

M globalPreload()

• 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().

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In order to allow communication between the application and the loader, another argument is provided to the preload code: port. This is available as a parameter to the loader hook and inside of the source text returned by the hook. Some 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.

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Examples

The various loader hooks can be used together to accomplish wide-ranging customizations of the Node.js code loading and evaluation behaviors.

HTTPS loader

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

The loader 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 this loader: performance is much slower than loading files from disk, there is no caching, and there is no security.

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With the preceding loader, running node --experimental-loader ./https-loader.mjs ./main.mjs prints the current version of CoffeeScript per the module at the URL in main.mjs.

Transpiler loader

Sources that are in formats Node.js doesn't understand can be converted into JavaScript using the load hook. Before that hook gets called, however, a resolve hook needs to tell Node.js not to throw an error on unknown file types.

This is less performant than transpiling source files before running Node.js; a transpiler loader should only be used for development and testing purposes.

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With the preceding loader, running node --experimental-loader ./coffeescript-loader.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.

Resolution algorithm

Features

The resolver has the following properties:

• FileURL-based resolution as is used by ES modules
• Support for builtin module loading
• Relative and absolute URL resolution
• No default extensions
• No folder mains
• Bare specifier package resolution lookup through node_modules

Resolver algorithm

The algorithm to load an ES module specifier is given through the ESM_RESOLVE method below. It returns the resolved URL for a module specifier relative to a parentURL.

The algorithm to determine the module format of a resolved URL is provided by ESM_FORMAT, which returns the unique module format for any file. The "module" format is returned for an ECMAScript Module, while the "commonjs" format is used to indicate loading through the legacy CommonJS loader. Additional formats such as "addon" can be extended in future updates.

In the following algorithms, all subroutine errors are propagated as errors of these top-level routines unless stated otherwise.

defaultConditions is the conditional environment name array, ["node", "import"].

The resolver can throw the following errors:

• Invalid Module Specifier: Module specifier is an invalid URL, package name or package subpath specifier.
• Invalid Package Configuration: package.json configuration is invalid or contains an invalid configuration.
• Invalid Package Target: Package exports or imports define a target module for the package that is an invalid type or string target.
• Package Path Not Exported: Package exports do not define or permit a target subpath in the package for the given module.
• Package Import Not Defined: Package imports do not define the specifier.
• Module Not Found: The package or module requested does not exist.
• Unsupported Directory Import: The resolved path corresponds to a directory, which is not a supported target for module imports.

Resolver Algorithm Specification

ESM_RESOLVE(specifier, parentURL)

PACKAGE_RESOLVE(packageSpecifier, parentURL)

PACKAGE_SELF_RESOLVE(packageName, packageSubpath, parentURL)

PACKAGE_EXPORTS_RESOLVE(packageURL, subpath, exports, conditions)

PACKAGE_IMPORTS_RESOLVE(specifier, parentURL, conditions)

PACKAGE_IMPORTS_EXPORTS_RESOLVE(matchKey, matchObj, packageURL, isImports, conditions)

PATTERN_KEY_COMPARE(keyA, keyB)

PACKAGE_TARGET_RESOLVE(packageURL, target, patternMatch, isImports, conditions)

ESM_FILE_FORMAT(url)

LOOKUP_PACKAGE_SCOPE(url)

READ_PACKAGE_JSON(packageURL)

Customizing ESM specifier resolution algorithm

The Loaders API provides a mechanism for customizing the ESM specifier resolution algorithm. An example loader that provides CommonJS-style resolution for ESM specifiers is commonjs-extension-resolution-loader.