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Module Methods

This section covers all methods available in code compiled with webpack. When using webpack to bundle your application, you can pick from a variety of module syntax styles including ES6, CommonJS, and AMD.

While webpack supports multiple module syntaxes, we recommend following a single syntax for consistency and to avoid odd behaviors/bugs. Actually webpack would enforce the recommendation for .mjs files, .cjs files or .js files when their nearest parent package.json file contains a "type" field with a value of either "module" or "commonjs". Please pay attention to these enforcements before you read on:

  • .mjs or .js with "type": "module" in package.json
    • No CommonJS allowed, for example, you can't use require, module.exports or exports
    • File extensions are required when importing, e.g, you should use import './src/App.mjs' instead of import './src/App' (you can disable this enforcement with Rule.resolve.fullySpecified)
  • .cjs or .js with "type": "commonjs" in package.json
    • Neither import nor export is available
  • .wasm with "type": "module" in package.json
    • File extensions are required when importing wasm file

Version 2 of webpack supports ES6 module syntax natively, meaning you can use import and export without a tool like babel to handle this for you. Keep in mind that you will still probably need babel for other ES6+ features. The following methods are supported by webpack:

import

Statically import the exports of another module.

import MyModule from './my-module.js';
import { NamedExport } from './other-module.js';

You can also import Data URI:

import 'data:text/javascript;charset=utf-8;base64,Y29uc29sZS5sb2coJ2lubGluZSAxJyk7';
import {
  number,
  fn,
} from 'data:text/javascript;charset=utf-8;base64,ZXhwb3J0IGNvbnN0IG51bWJlciA9IDQyOwpleHBvcnQgY29uc3QgZm4gPSAoKSA9PiAiSGVsbG8gd29ybGQiOw==';

export

Export anything as a default or named export.

// Named exports
export var Count = 5;
export function Multiply(a, b) {
  return a * b;
}

// Default export
export default {
  // Some data...
};

import()

function(string path):Promise

Dynamically load modules. Calls to import() are treated as split points, meaning the requested module and its children are split out into a separate chunk.

if (module.hot) {
  import('lodash').then((_) => {
    // Do something with lodash (a.k.a '_')...
  });
}

Dynamic expressions in import()

It is not possible to use a fully dynamic import statement, such as import(foo). Because foo could potentially be any path to any file in your system or project.

The import() must contain at least some information about where the module is located. Bundling can be limited to a specific directory or set of files so that when you are using a dynamic expression - every module that could potentially be requested on an import() call is included. For example, import(`./locale/${language}.json`) will cause every .json file in the ./locale directory to be bundled into the new chunk. At run time, when the variable language has been computed, any file like english.json or german.json will be available for consumption.

// imagine we had a method to get language from cookies or other storage
const language = detectVisitorLanguage();
import(`./locale/${language}.json`).then((module) => {
  // do something with the translations
});

Magic Comments

Inline comments to make features work. By adding comments to the import, we can do things such as name our chunk or select different modes. For a full list of these magic comments see the code below followed by an explanation of what these comments do.

// Single target
import(
  /* webpackChunkName: "my-chunk-name" */
  /* webpackMode: "lazy" */
  /* webpackExports: ["default", "named"] */
  'module'
);

// Multiple possible targets
import(
  /* webpackInclude: /\.json$/ */
  /* webpackExclude: /\.noimport\.json$/ */
  /* webpackChunkName: "my-chunk-name" */
  /* webpackMode: "lazy" */
  /* webpackPrefetch: true */
  /* webpackPreload: true */
  `./locale/${language}`
);
import(/* webpackIgnore: true */ 'ignored-module.js');

webpackIgnore: Disables dynamic import parsing when set to true.

webpackChunkName: A name for the new chunk. Since webpack 2.6.0, the placeholders [index] and [request] are supported within the given string to an incremented number or the actual resolved filename respectively. Adding this comment will cause our separate chunk to be named [my-chunk-name].js instead of [id].js.

webpackMode: Since webpack 2.6.0, different modes for resolving dynamic imports can be specified. The following options are supported:

  • 'lazy' (default): Generates a lazy-loadable chunk for each import()ed module.
  • 'lazy-once': Generates a single lazy-loadable chunk that can satisfy all calls to import(). The chunk will be fetched on the first call to import(), and subsequent calls to import() will use the same network response. Note that this only makes sense in the case of a partially dynamic statement, e.g. import(`./locales/${language}.json`), where multiple module paths that can potentially be requested.
  • 'eager': Generates no extra chunk. All modules are included in the current chunk and no additional network requests are made. A Promise is still returned but is already resolved. In contrast to a static import, the module isn't executed until the call to import() is made.
  • 'weak': Tries to load the module if the module function has already been loaded in some other way (e.g. another chunk imported it or a script containing the module was loaded). A Promise is still returned, but only successfully resolves if the chunks are already on the client. If the module is not available, the Promise is rejected. A network request will never be performed. This is useful for universal rendering when required chunks are always manually served in initial requests (embedded within the page), but not in cases where app navigation will trigger an import not initially served.

webpackPrefetch: Tells the browser that the resource is probably needed for some navigation in the future. Check out the guide for more information on how webpackPrefetch works.

webpackPreload: Tells the browser that the resource might be needed during the current navigation. Check out the guide for more information on how webpackPreload works.

webpackInclude: A regular expression that will be matched against during import resolution. Only modules that match will be bundled.

webpackExclude: A regular expression that will be matched against during import resolution. Any module that matches will not be bundled.

webpackExports: tells webpack to only bundle the specified exports of a dynamically import()ed module. It can decrease the output size of a chunk. Available since webpack 5.0.0-beta.18.

CommonJS

The goal of CommonJS is to specify an ecosystem for JavaScript outside the browser. The following CommonJS methods are supported by webpack:

require

require(dependency: String);

Synchronously retrieve the exports from another module. The compiler will ensure that the dependency is available in the output bundle.

var $ = require('jquery');
var myModule = require('my-module');

It's possible to enable magic comments for require as well, see module.parser.javascript.commonjsMagicComments for more.

require.resolve

require.resolve(dependency: String);

Synchronously retrieve a module's ID. The compiler will ensure that the dependency is available in the output bundle. It is recommended to treat it as an opaque value which can only be used with require.cache[id] or __webpack_require__(id) (best to avoid such usage).

See module.id for more information.

require.cache

Multiple requires of the same module result in only one module execution and only one export. Therefore a cache in the runtime exists. Removing values from this cache causes new module execution and a new export.

var d1 = require('dependency');
require('dependency') === d1;
delete require.cache[require.resolve('dependency')];
require('dependency') !== d1;
// in file.js
require.cache[module.id] === module;
require('./file.js') === module.exports;
delete require.cache[module.id];
require.cache[module.id] === undefined;
require('./file.js') !== module.exports; // in theory; in praxis this causes a stack overflow
require.cache[module.id] !== module;

require.ensure

require.ensure(
  dependencies: String[],
  callback: function(require),
  errorCallback: function(error),
  chunkName: String
)

Split out the given dependencies to a separate bundle that will be loaded asynchronously. When using CommonJS module syntax, this is the only way to dynamically load dependencies. Meaning, this code can be run within execution, only loading the dependencies if certain conditions are met.

var a = require('normal-dep');

if (module.hot) {
  require.ensure(['b'], function (require) {
    var c = require('c');

    // Do something special...
  });
}

The following parameters are supported in the order specified above:

  • dependencies: An array of strings declaring all modules required for the code in the callback to execute.
  • callback: A function that webpack will execute once the dependencies are loaded. An implementation of the require function is sent as a parameter to this function. The function body can use this to further require() modules it needs for execution.
  • errorCallback: A function that is executed when webpack fails to load the dependencies.
  • chunkName: A name given to the chunk created by this particular require.ensure(). By passing the same chunkName to various require.ensure() calls, we can combine their code into a single chunk, resulting in only one bundle that the browser must load.

AMD

Asynchronous Module Definition (AMD) is a JavaScript specification that defines an interface for writing and loading modules. The following AMD methods are supported by webpack:

define (with factory)

define([name: String], [dependencies: String[]], factoryMethod: function(...))

If dependencies are provided, factoryMethod will be called with the exports of each dependency (in the same order). If dependencies are not provided, factoryMethod is called with require, exports and module (for compatibility!). If this function returns a value, this value is exported by the module. The compiler ensures that each dependency is available.

define(['jquery', 'my-module'], function ($, myModule) {
  // Do something with $ and myModule...

  // Export a function
  return function doSomething() {
    // ...
  };
});

define (with value)

define(value: !Function)

This will export the provided value. The value here can be anything except a function.

define({
  answer: 42,
});

require (amd-version)

require(dependencies: String[], [callback: function(...)])

Similar to require.ensure, this will split the given dependencies into a separate bundle that will be loaded asynchronously. The callback will be called with the exports of each dependency in the dependencies array.

require(['b'], function (b) {
  var c = require('c');
});

Labeled Modules

The internal LabeledModulesPlugin enables you to use the following methods for exporting and requiring within your modules:

export label

Export the given value. The label can occur before a function declaration or a variable declaration. The function name or variable name is the identifier under which the value is exported.

export: var answer = 42;
export: function method(value) {
  // Do something...
};

require label

Make all exports from the dependency available in the current scope. The require label can occur before a string. The dependency must export values with the export label. CommonJS or AMD modules cannot be consumed.

some-dependency.js

export: var answer = 42;
export: function method(value) {
  // Do something...
};
require: 'some-dependency';
console.log(answer);
method(...);

Webpack

Aside from the module syntaxes described above, webpack also allows a few custom, webpack-specific methods:

require.context

require.context(
  (directory: String),
  (includeSubdirs: Boolean) /* optional, default true */,
  (filter: RegExp) /* optional, default /^\.\/.*$/, any file */,
  (mode: String) /* optional, 'sync' | 'eager' | 'weak' | 'lazy' | 'lazy-once', default 'sync' */
);

Specify a whole group of dependencies using a path to the directory, an option to includeSubdirs, a filter for more fine grained control of the modules included, and a mode to define the way how loading will work. Underlying modules can then be easily resolved later on:

var context = require.context('components', true, /\.html$/);
var componentA = context.resolve('componentA');

If mode is set to 'lazy', the underlying modules will be loaded asynchronously:

var context = require.context('locales', true, /\.json$/, 'lazy');
context('localeA').then((locale) => {
  // do something with locale
});

The full list of available modes and their behavior is described in import() documentation.

require.include

require.include((dependency: String));

Include a dependency without executing it. This can be used for optimizing the position of a module in the output chunks.

require.include('a');
require.ensure(['a', 'b'], function (require) {
  /* ... */
});
require.ensure(['a', 'c'], function (require) {
  /* ... */
});

This will result in the following output:

  • entry chunk: file.js and a
  • anonymous chunk: b
  • anonymous chunk: c

Without require.include('a') it would be duplicated in both anonymous chunks.

require.resolveWeak

Similar to require.resolve, but this won't pull the module into the bundle. It's what is considered a "weak" dependency.

if (__webpack_modules__[require.resolveWeak('module')]) {
  // Do something when module is available...
}
if (require.cache[require.resolveWeak('module')]) {
  // Do something when module was loaded before...
}

// You can perform dynamic resolves ("context")
// similarly to other require/import methods.
const page = 'Foo';
__webpack_modules__[require.resolveWeak(`./page/${page}`)];

Further Reading

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