Introduction

A variety of interfaces are available to customize the compilation process. Some features overlap between interfaces, e.g. a configuration option may be available via a CLI flag, while others exist only through a single interface. The following high-level information should get you started.

CLI

The Command Line Interface (CLI) to configure and interact with your build. It is especially useful in the case of early prototyping and profiling. For the most part, the CLI is simply used to kick off the process using a configuration file and a few flags (e.g. --env).

Learn more about the CLI!

Module

When processing modules with webpack, it is important to understand the different module syntaxes – specifically the methods and variables – that are supported.

Learn more about modules!

Node

While most users can get away with just using the CLI along with a configuration file, more fine-grained control of the compilation can be achieved via the Node interface. This includes passing multiple configurations, programmatically running or watching, and collecting stats.

Learn more about the Node API!

Loaders

Loaders are transformations that are applied to the source code of a module. They are written as functions that accept source code as a parameter and return a new version of that code with transformations applied.

Learn more about loaders!

Plugins

The plugin interface allows users to tap directly into the compilation process. Plugins can register handlers on lifecycle hooks that run at different points throughout a compilation. When each hook is executed, the plugin will have full access to the current state of the compilation.

Learn more about plugins!

Command Line Interface

For proper usage and easy distribution of this configuration, webpack can be configured with webpack.config.js. Any parameters sent to the CLI will map to a corresponding parameter in the configuration file.

Read the installation guide if you don't already have webpack and CLI installed.

Commands

webpack-cli offers a variety of commands to make working with webpack easy. By default webpack ships with

Flags

By default webpack ships with the following flags:

Negated Flags

Core Flags

Starting CLI v4 and webpack v5, CLI imports the entire configuration schema from webpack core to allow tuning almost every configuration option from the command line.

Here's the list of all the core flags supported by webpack v5 with CLI v4 - link

For example if you want to enable performance hints in your project you'd use this option in configuration, with core flags you can do -

npx webpack --performance-hints warning

Usage

With configuration file

npx webpack [--config webpack.config.js]

See configuration for the options in the configuration file.

Without configuration file

npx webpack --entry <entry> -o <output-path>

example

npx webpack --entry ./first.js --entry ./second.js --output-path /build

<entry>

A filename or a set of named filenames which act as the entry point to build your project. You can pass multiple entries (every entry is loaded on startup).

<output>

A path for the bundled file to be saved in. It will be mapped to the configuration options output.path.

Example

If your project structure is as follows -

.
├── dist
├── index.html
└── src
    ├── index.js
    ├── index2.js
    └── others.js

npx webpack ./src/index.js -o dist

This will bundle your source code with entry as index.js, and the output bundle file will have a path of dist.

asset main.js 142 bytes [compared for emit] [minimized] (name: main)
./src/index.js 30 bytes [built] [code generated]
./src/others.js 1 bytes [built] [code generated]
webpack 5.1.0 compiled successfully in 187 ms

npx webpack ./src/index.js ./src/others2.js -o dist/

This will form the bundle with both the files as separate entry points.

asset main.js 142 bytes [compared for emit] [minimized] (name: main)
./src/index.js 30 bytes [built] [code generated]
./src/others2.js 1 bytes [built] [code generated]
./src/others.js 1 bytes [built] [code generated]
webpack 5.1.0 compiled successfully in 198 ms

Default Configurations

CLI will look for some default configurations in the path of your project, here are the config files picked up by CLI.

This is the lookup priority in increasing order

example - config file lookup will be in order of .webpack/webpackfile > .webpack/webpack.config.js > webpack.config.js

'webpack.config',
'.webpack/webpack.config',
'.webpack/webpackfile',

Common Options

List basic commands and flags available on the cli

npx webpack --help

List all supported commands and flags by cli

npx webpack --help=verbose

Show version of installed packages and sub-packages

To inspect the version of webpack and webpack-cli you are using just run command:

npx webpack --version

This will output the following result:

webpack 5.11.1
webpack-cli 4.3.1

It will output the version of webpack-dev-server as well if you have it installed:

webpack 5.11.1
webpack-cli 4.3.1
webpack-dev-server 3.11.1

To inspect the version of any webpack-cli sub-package (like @webpack-cli/init) just run command similar to the following:

npx webpack init --version

This will output the following result:

@webpack-cli/init 1.0.3
webpack-cli 4.2.0
webpack 5.4.0

Build source using a configuration file

Specify a different configuration file other than webpack.config.js, which is one of the defaults.

npx webpack --config example.config.js

In case your configuration file exports multiple configurations, you can use --config-name to specify which configuration to run.

Consider the following webpack.config.js:

module.exports = [
  {
    output: {
      filename: './dist-first.js',
    },
    name: 'first',
    entry: './src/first.js',
    mode: 'development',
  },
  {
    output: {
      filename: './dist-second.js',
    },
    name: 'second',
    entry: './src/second.js',
    mode: 'development',
  },
  {
    output: {
      filename: './dist-third.js',
    },
    name: 'third',
    entry: './src/third.js',
    mode: 'none',
    stats: 'verbose',
  },
];

To run only the second configuration:

npx webpack --config-name second

You can also pass multiple values:

npx webpack --config-name first --config-name second

Merge two or more different webpack configurations

You can merge two or more different webpack configurations with the help of --merge:

npx webpack --config ./first.js --config ./second.js --merge

Print result of webpack as a JSON

npx webpack --json

If you want to store stats as json instead of printing it, you can use -

npx webpack --json stats.json

In every other case, webpack prints out a set of stats showing bundle, chunk and timing details. Using this option, the output can be a JSON object. This response is accepted by webpack's analyse tool, or chrisbateman's webpack-visualizer, or th0r's webpack-bundle-analyzer. The analyse tool will take in the JSON and provide all the details of the build in graphical form.

Environment Options

When the webpack configuration exports a function, an "environment" may be passed to it.

npx webpack --env production    # env.production = true

The --env argument accepts multiple values:

NODE_ENV

you can use --node-env option to set process.env.NODE_ENV:

webpack-cli --node-env production   # process.env.NODE_ENV = 'production'

Configuration Options

Analyzing Bundle

You can also use webpack-bundle-analyzer to analyze your output bundles emitted by webpack. You can use --analyze flag to invoke it via CLI.

npx webpack --analyze

Progress

To check the progress of any webpack compilation you can use the --progress flag.

npx webpack --progress

To collect profile data for progress steps you can pass profile as value to --progress flag.

npx webpack --progress=profile

Pass CLI arguments to Node.js

To pass arguments directly to Node.js process, you can use the NODE_OPTIONS option.

For example, to increase the memory limit of Node.js process to 4 GB

NODE_OPTIONS="--max-old-space-size=4096" webpack

Also, you can pass multiple options to Node.js process

NODE_OPTIONS="--max-old-space-size=4096 -r /path/to/preload/file.js" webpack

Exit codes and their meanings

Node Interface

webpack provides a Node.js API which can be used directly in Node.js runtime.

The Node.js API is useful in scenarios in which you need to customize the build or development process since all the reporting and error handling must be done manually and webpack only does the compiling part. For this reason the stats configuration options will not have any effect in the webpack() call.

Installation

To start using the webpack Node.js API, first install webpack if you haven’t yet:

npm install --save-dev webpack

Then require the webpack module in your Node.js script:

const webpack = require('webpack');

Or if you prefer ES2015:

import webpack from 'webpack';

webpack()

The imported webpack function is fed a webpack Configuration Object and runs the webpack compiler if a callback function is provided:

const webpack = require('webpack');

webpack({
  // [Configuration Object](/configuration/)
}, (err, stats) => { // [Stats Object](#stats-object)
  if (err || stats.hasErrors()) {
    // [Handle errors here](#error-handling)
  }
  // Done processing
});

Compiler Instance

If you don’t pass the webpack runner function a callback, it will return a webpack Compiler instance. This instance can be used to manually trigger the webpack runner or have it build and watch for changes, much like the CLI. The Compiler instance provides the following methods:

• .run(callback)
• .watch(watchOptions, handler)

Typically, only one master Compiler instance is created, although child compilers can be created in order to delegate specific tasks. The Compiler is ultimately just a function which performs bare minimum functionality to keep a lifecycle running. It delegates all the loading, bundling, and writing work to registered plugins.

The hooks property on a Compiler instance is used to register a plugin to any hook event in the Compiler's lifecycle. The WebpackOptionsDefaulter and WebpackOptionsApply utilities are used by webpack to configure its Compiler instance with all the built-in plugins.

The run method is then used to kickstart all compilation work. Upon completion, the given callback function is executed. The final logging of stats and errors should be done in this callback function.

Run

Calling the run method on the Compiler instance is much like the quick run method mentioned above:

const webpack = require('webpack');

const compiler = webpack({
  // [Configuration Object](/configuration/)
});

compiler.run((err, stats) => { // [Stats Object](#stats-object)
  // ...
});

Watching

Calling the watch method triggers the webpack runner, but then watches for changes (much like CLI: webpack --watch), as soon as webpack detects a change, runs again. Returns an instance of Watching.

watch(watchOptions, callback);

const webpack = require('webpack');

const compiler = webpack({
  // [Configuration Object](/configuration/)
});

const watching = compiler.watch({
  // Example [watchOptions](/configuration/watch/#watchoptions)
  aggregateTimeout: 300,
  poll: undefined
}, (err, stats) => { // [Stats Object](#stats-object)
  // Print watch/build result here...
  console.log(stats);
});

Watching options are covered in detail here.

Close Watching

The watch method returns a Watching instance that exposes .close(callback) method. Calling this method will end watching:

watching.close(() => {
  console.log('Watching Ended.');
});

Invalidate Watching

Using watching.invalidate, you can manually invalidate the current compiling round, without stopping the watch process:

watching.invalidate();

Stats Object

The stats object that is passed as a second argument of the webpack() callback, is a good source of information about the code compilation process. It includes:

• Errors and Warnings (if any)
• Timings
• Module and Chunk information

The webpack CLI uses this information to display nicely formatted output in your console.

This stats object exposes the following methods:

stats.hasErrors()

Can be used to check if there were errors while compiling. Returns true or false.

stats.hasWarnings()

Can be used to check if there were warnings while compiling. Returns true or false.

stats.toJson(options)

Returns compilation information as a JSON object. options can be either a string (a preset) or an object for more granular control:

stats.toJson('minimal'); // [more options: 'verbose', etc](/configuration/stats).

stats.toJson({
  assets: false,
  hash: true
});

All available options and presets are described in the stats documentation.

Here’s an [example]

(https://raw.githubusercontent.com/webpack/analyse/master/app/pages/upload/example2.json) of this function’s output.

stats.toString(options)

Returns a formatted string of the compilation information (similar to CLI output).

Options are the same as stats.toJson(options) with one addition:

stats.toString({
  // Add console colors
  colors: true
});

Here’s an example of stats.toString() usage:

const webpack = require('webpack');

webpack({
  // [Configuration Object](/configuration/)
}, (err, stats) => {
  if (err) {
    console.error(err);
    return;
  }

  console.log(stats.toString({
    chunks: false,  // Makes the build much quieter
    colors: true    // Shows colors in the console
  }));
});

MultiCompiler

The MultiCompiler module allows webpack to run multiple configurations in separate compilers. If the options parameter in the webpack's NodeJS api is an array of options, webpack applies separate compilers and calls the callback after all compilers have been executed.

var webpack = require('webpack');

webpack([
  { entry: './index1.js', output: { filename: 'bundle1.js' } },
  { entry: './index2.js', output: { filename: 'bundle2.js' } }
], (err, stats) => { // [Stats Object](#stats-object)
  process.stdout.write(stats.toString() + '\n');
})

Error Handling

For good error handling, you need to account for these three types of errors:

• Fatal webpack errors (wrong configuration, etc)
• Compilation errors (missing modules, syntax errors, etc)
• Compilation warnings

Here’s an example that does all that:

const webpack = require('webpack');

webpack({
  // [Configuration Object](/configuration/)
}, (err, stats) => {
  if (err) {
    console.error(err.stack || err);
    if (err.details) {
      console.error(err.details);
    }
    return;
  }

  const info = stats.toJson();

  if (stats.hasErrors()) {
    console.error(info.errors);
  }

  if (stats.hasWarnings()) {
    console.warn(info.warnings);
  }

  // Log result...
});

Custom File Systems

By default, webpack reads files and writes files to disk using a normal file system. However, it is possible to change the input or output behavior using a different kind of file system (memory, webDAV, etc). To accomplish this, one can change the inputFileSystem or outputFileSystem. For example, you can replace the default outputFileSystem with memfs to write files to memory instead of to disk:

const { createFsFromVolume, Volume } = require('memfs');
const webpack = require('webpack');

const fs = createFsFromVolume(new Volume());
const compiler = webpack({ /* options */ });

compiler.outputFileSystem = fs;
compiler.run((err, stats) => {
  // Read the output later:
  const content = fs.readFileSync('...');
});

Note that this is what webpack-dev-middleware, used by webpack-dev-server and many other packages, uses to mysteriously hide your files but continue serving them up to the browser!

Stats Data

When compiling source code with webpack, users can generate a JSON file containing statistics about modules. These statistics can be used to analyze an application's dependency graph as well as to optimize compilation speed. The file is typically generated with the following CLI command:

npx webpack --profile --json=compilation-stats.json

The --json=compilation-stats.json flag indicates to webpack that it should emit the compilation-stats.json containing the dependency graph and various other build information. Typically, the --profile flag is also added so that a profile section is added to each modules object containing module-specific compilation stats.

Structure

The top-level structure of the output JSON file is fairly straightforward but there are a few nested data structures as well. Each nested structure has a dedicated section below to make this document more consumable. Note that you can click links within the top-level structure below to jump to relevant sections and documentation:

{
  'version': '5.9.0', // Version of webpack used for the compilation
  'hash': '11593e3b3ac85436984a', // Compilation specific hash
  'time': 2469, // Compilation time in milliseconds
  'publicPath': 'auto',
  'outputPath': '/', // path to webpack output directory
  'assetsByChunkName': {
    // Chunk name to emitted asset(s) mapping
    'main': [
      'web.js?h=11593e3b3ac85436984a'
    ],
    'named-chunk': [
      'named-chunk.web.js'
    ],
    'other-chunk': [
      'other-chunk.js',
      'other-chunk.css'
    ]
  },
  'assets': [
    // A list of [asset objects](#asset-objects)
  ],
  'chunks': [
    // A list of [chunk objects](#chunk-objects)
  ],
  'modules': [
    // A list of [module objects](#module-objects)
  ],
  'entryPoints': {
    // A list of [entry objects](#entry-objects)
  },
  'errors': [
    // A list of [error objects](#errors-and-warnings)
  ],
  'errorsCount': 0, // number of errors
  'warnings': [
    // A list of [warning objects](#errors-and-warnings)
  ],
  'warningsCount': 0, // nummber of warnings
}

Asset Objects

Each assets object represents an output file emitted from the compilation. They all follow a similar structure:

{
  'chunkNames': [], // The chunks this asset contains
  'chunks': [ 10, 6 ], // The chunk IDs this asset contains
  'comparedForEmit': false, // Indicates whether or not the asset was compared with the same file on the output file system
  'emitted': true, // Indicates whether or not the asset made it to the `output` directory
  'name': '10.web.js', // The `output` filename
  'size': 1058, // The size of the file in bytes
  'info': {
    'immutable': true, // A flag telling whether the asset can be long term cached (contains a hash)
    'size': 1058, // The size in bytes, only becomes available after asset has been emitted
    'development': true, // A flag telling whether the asset is only used for development and doesn't count towards user-facing assets
    'hotModuleReplacement': true, // A flag telling whether the asset ships data for updating an existing application (HMR)
    'sourceFilename': 'originalfile.js', // sourceFilename when asset was created from a source file (potentially transformed)
    'javascriptModule': true // true, when asset is javascript and an ESM
  }
}

Chunk Objects

Each chunks object represents a group of modules known as a chunk. Each object follows the following structure:

{
  "entry": true, // Indicates whether or not the chunk contains the webpack runtime
  "files": [
    // An array of filename strings that contain this chunk
  ],
  "filteredModules": 0, // See the description in the [top-level structure](#structure) above
  "id": 0, // The ID of this chunk
  "initial": true, // Indicates whether this chunk is loaded on initial page load or [on demand](/guides/lazy-loading)
  "modules": [
    // A list of [module objects](#module-objects)
    "web.js?h=11593e3b3ac85436984a"
  ],
  "names": [
    // An list of chunk names contained within this chunk
  ],
  "origins": [
    // See the description below...
  ],
  "parents": [], // Parent chunk IDs
  "rendered": true, // Indicates whether or not the chunk went through Code Generation
  "size": 188057 // Chunk size in bytes
}

The chunks object will also contain a list of origins describing how the given chunk originated. Each origins object follows the following schema:

{
  "loc": "", // Lines of code that generated this chunk
  "module": "(webpack)\\test\\browsertest\\lib\\index.web.js", // Path to the module
  "moduleId": 0, // The ID of the module
  "moduleIdentifier": "(webpack)\\test\\browsertest\\lib\\index.web.js", // Path to the module
  "moduleName": "./lib/index.web.js", // Relative path to the module
  "name": "main", // The name of the chunk
  "reasons": [
    // A list of the same `reasons` found in [module objects](#module-objects)
  ]
}

Module Objects

What good would these statistics be without some description of the compiled application's actual modules? Each module in the dependency graph is represented by the following structure:

{
  "assets": [
    // A list of [asset objects](#asset-objects)
  ],
  "built": true, // Indicates that the module went through [Loaders](/concepts/loaders), Parsing, and Code Generation
  "cacheable": true, // Whether or not this module is cacheable
  "chunks": [
    // IDs of chunks that contain this module
  ],
  "errors": 0, // Number of errors when resolving or processing the module
  "failed": false, // Whether or not compilation failed on this module
  "id": 0, // The ID of the module (analogous to [`module.id`](/api/module-variables/#moduleid-commonjs))
  "identifier": "(webpack)\\test\\browsertest\\lib\\index.web.js", // A unique ID used internally
  "name": "./lib/index.web.js", // Path to the actual file
  "optional": false, // All requests to this module are with `try... catch` blocks (irrelevant with ESM)
  "prefetched": false, // Indicates whether or not the module was [prefetched](/plugins/prefetch-plugin)
  "profile": {
    // Module specific compilation stats corresponding to the [`--profile` flag](/api/cli/#profiling) (in milliseconds)
    "building": 73, // Loading and parsing
    "dependencies": 242, // Building dependencies
    "factory": 11 // Resolving dependencies
  },
  "reasons": [
    // See the description below...
  ],
  "size": 3593, // Estimated size of the module in bytes
  "source": "// Should not break it...\r\nif(typeof...", // The stringified raw source
  "warnings": 0 // Number of warnings when resolving or processing the module
}

Every module also contains a list of reasons objects describing why that module was included in the dependency graph. Each "reason" is similar to the origins seen above in the chunk objects section:

{
  "loc": "33:24-93", // Lines of code that caused the module to be included
  "module": "./lib/index.web.js", // Relative path to the module based on [context](/configuration/entry-context/#context)
  "moduleId": 0, // The ID of the module
  "moduleIdentifier": "(webpack)\\test\\browsertest\\lib\\index.web.js", // Path to the module
  "moduleName": "./lib/index.web.js", // A more readable name for the module (used for "pretty-printing")
  "type": "require.context", // The [type of request](/api/module-methods) used
  "userRequest": "../../cases" // Raw string used for the `import` or `require` request
}

Entry Objects

"main": {
  "name": "main",
  "chunks": [
    179
  ],
  "assets": [
    {
      "name": "main.js",
      "size": 22
    }
  ],
  "filteredAssets": 0,
  "assetsSize": 22,
  "auxiliaryAssets": [],
  "filteredAuxiliaryAssets": 0,
  "auxiliaryAssetsSize": 0,
  "children": {},
  "childAssets": {},
  "isOverSizeLimit": false
}

Errors and Warnings

The errors and warnings properties each contain a list of objects. Each object contains a message, a stack trace and various other properties:

{
  "moduleIdentifier": "C:\\Repos\\webpack\\test\\cases\\context\\issue-5750\\index.js",
  "moduleName": "(webpack)/test/cases/context/issue-5750/index.js",
  "loc": "3:8-47",
  "message": "Critical dependency: Contexts can't use RegExps with the 'g' or 'y' flags.",
  "moduleId": 29595,
  "moduleTrace": [
    {
      "originIdentifier": "C:\\Repos\\webpack\\test\\cases|sync|/^\\.\\/[^/]+\\/[^/]+\\/index\\.js$/",
      "originName": "(webpack)/test/cases sync ^\\.\\/[^/]+\\/[^/]+\\/index\\.js$",
      "moduleIdentifier": "C:\\Repos\\webpack\\test\\cases\\context\\issue-5750\\index.js",
      "moduleName": "(webpack)/test/cases/context/issue-5750/index.js",
      "dependencies": [
        {
          "loc": "./context/issue-5750/index.js"
        }
      ],
      "originId": 32582,
      "moduleId": 29595
    },
    {
      "originIdentifier": "C:\\Repos\\webpack\\testCases.js",
      "originName": "(webpack)/testCases.js",
      "moduleIdentifier": "C:\\Repos\\webpack\\test\\cases|sync|/^\\.\\/[^/]+\\/[^/]+\\/index\\.js$/",
      "moduleName": "(webpack)/test/cases sync ^\\.\\/[^/]+\\/[^/]+\\/index\\.js$",
      "dependencies": [
        {
          "loc": "1:0-70"
        }
      ],
      "originId": 8198,
      "moduleId": 32582
    }
  ],
  "details": "at RequireContextDependency.getWarnings (C:\\Repos\\webpack\\lib\\dependencies\\ContextDependency.js:79:5)\n    at Compilation.reportDependencyErrorsAndWarnings (C:\\Repos\\webpack\\lib\\Compilation.js:1727:24)\n    at C:\\Repos\\webpack\\lib\\Compilation.js:1467:10\n    at _next2 (<anonymous>:16:1)\n    at eval (<anonymous>:42:1)\n    at C:\\Repos\\webpack\\node_modules\\neo-async\\async.js:2830:7\n    at Object.each (C:\\Repos\\webpack\\node_modules\\neo-async\\async.js:2850:39)\n    at C:\\Repos\\webpack\\lib\\FlagDependencyExportsPlugin.js:219:18\n    at C:\\Repos\\webpack\\node_modules\\neo-async\\async.js:2830:7\n    at Object.each (C:\\Repos\\webpack\\node_modules\\neo-async\\async.js:2850:39)\n    at C:\\Repos\\webpack\\lib\\FlagDependencyExportsPlugin.js:40:16\n    at Hook.eval [as callAsync] (<anonymous>:38:1)\n    at Hook.CALL_ASYNC_DELEGATE [as _callAsync] (C:\\Repos\\tapable\\lib\\Hook.js:18:14)\n    at Compilation.finish (C:\\Repos\\webpack\\lib\\Compilation.js:1462:28)\n    at C:\\Repos\\webpack\\lib\\Compiler.js:909:18\n    at processTicksAndRejections (internal/process/task_queues.js:75:11)\n",
  "stack": "ModuleDependencyWarning: Critical dependency: Contexts can't use RegExps with the 'g' or 'y' flags.\n    at Compilation.reportDependencyErrorsAndWarnings (C:\\Repos\\webpack\\lib\\Compilation.js:1732:23)\n    at C:\\Repos\\webpack\\lib\\Compilation.js:1467:10\n    at _next2 (<anonymous>:16:1)\n    at eval (<anonymous>:42:1)\n    at C:\\Repos\\webpack\\node_modules\\neo-async\\async.js:2830:7\n    at Object.each (C:\\Repos\\webpack\\node_modules\\neo-async\\async.js:2850:39)\n    at C:\\Repos\\webpack\\lib\\FlagDependencyExportsPlugin.js:219:18\n    at C:\\Repos\\webpack\\node_modules\\neo-async\\async.js:2830:7\n    at Object.each (C:\\Repos\\webpack\\node_modules\\neo-async\\async.js:2850:39)\n    at C:\\Repos\\webpack\\lib\\FlagDependencyExportsPlugin.js:40:16\n    at Hook.eval [as callAsync] (<anonymous>:38:1)\n    at Hook.CALL_ASYNC_DELEGATE [as _callAsync] (C:\\Repos\\tapable\\lib\\Hook.js:18:14)\n    at Compilation.finish (C:\\Repos\\webpack\\lib\\Compilation.js:1462:28)\n    at C:\\Repos\\webpack\\lib\\Compiler.js:909:18\n    at processTicksAndRejections (internal/process/task_queues.js:75:11)\n"
}

Hot Module Replacement

If Hot Module Replacement has been enabled via the HotModuleReplacementPlugin, its interface will be exposed under the module.hot property. Typically, users will check to see if the interface is accessible, then begin working with it. As an example, here's how you might accept an updated module:

if (module.hot) {
  module.hot.accept('./library.js', function() {
    // Do something with the updated library module...
  });
}

The following methods are supported...

Module API

accept

Accept updates for the given dependencies and fire a callback to react to those updates.

module.hot.accept(
  dependencies, // Either a string or an array of strings
  callback // Function to fire when the dependencies are updated
);

When using ESM import all imported symbols from dependencies are automatically updated. Note: The dependency string must match exactly with the from string in the import. In some cases callback can even be omitted. Using require() in the callback doesn't make sense here.

When using CommonJS you need to update dependencies manually by using require() in the callback. Omitting the callback doesn't make sense here.

accept (self)

Accept updates for itself.

module.hot.accept(
  errorHandler // Function to handle errors when evaluating the new version
);

When this module or dependencies are updated, this module can be disposed and re-evaluated without informing parents. This makes sense if this module has no exports (or exports are updated in another way).

The errorHandler is fired when the evaluation of this module (or dependencies) has thrown an exception.

decline

Reject updates for the given dependencies forcing the update to fail with a 'decline' code.

module.hot.decline(
  dependencies // Either a string or an array of strings
);

Flag a dependency as not-update-able. This makes sense when changing exports of this dependency can be handled or handling is not implemented yet. Depending on your HMR management code, an update to these dependencies (or unaccepted dependencies of it) usually causes a full-reload of the page.

decline (self)

Reject updates for itself.

module.hot.decline();

Flag this module as not-update-able. This makes sense when this module has irreversible side-effects, or HMR handling is not implemented for this module yet. Depending on your HMR management code, an update to this module (or unaccepted dependencies) usually causes a full-reload of the page.

dispose (or addDisposeHandler)

Add a handler which is executed when the current module code is replaced. This should be used to remove any persistent resource you have claimed or created. If you want to transfer state to the updated module, add it to the given data parameter. This object will be available at module.hot.data after the update.

module.hot.dispose(data => {
  // Clean up and pass data to the updated module...
});

invalidate

Calling this method will invalidate the current module, which disposes and recreates it when the HMR update is applied. This bubbles like a normal update of this module. invalidate can't be self-accepted by this module.

When called during the idle state, a new HMR update will be created containing this module. HMR will enter the ready state.

When called during the ready or prepare state, this module will be added to the current HMR update.

When called during the check state, this module will be added to the update when an update is available. If no update is available it will create a new update. HMR will enter the ready state.

When called during the dispose or apply state, HMR will pick it up after getting out of those states.

Use Cases

Conditional Accepting

A module can accept a dependency, but can call invalidate when the change of the dependency is not handleable:

import { x, y } from './dep';
import { processX, processY } from 'anotherDep';

const oldY = y;

processX(x);
export default processY(y);

module.hot.accept('./dep', () => {
  if(y !== oldY) {
    // This can't be handled, bubble to parent
    module.hot.invalidate();
    return;
  }
  // This can be handled
  processX(x);
});

Conditional self accept

A module can self-accept itself, but can invalidate itself when the change is not handleable:

const VALUE = 'constant';

export default VALUE;

if(module.hot.data && module.hot.data.value && module.hot.data.value !== VALUE) {
  module.hot.invalidate();
} else {
  module.hot.dispose(data => {
    data.value = VALUE;
  });
  module.hot.accept();
}

Triggering custom HMR updates

const moduleId = chooseAModule();
const code = __webpack_modules__[moduleId].toString();
__webpack_modules__[moduleId] = eval(`(${makeChanges(code)})`);
if(require.cache[moduleId]) {
  require.cache[moduleId].hot.invalidate();
  module.hot.apply();
}

removeDisposeHandler

Remove the handler added via dispose or addDisposeHandler.

module.hot.removeDisposeHandler(callback);

Management API

status

Retrieve the current status of the hot module replacement process.

module.hot.status(); // Will return one of the following strings...

check

Test all loaded modules for updates and, if updates exist, apply them.

module.hot.check(autoApply).then(outdatedModules => {
  // outdated modules...
}).catch(error => {
  // catch errors
});

The autoApply parameter can either be a boolean or options to pass to the apply method when called.

apply

Continue the update process (as long as module.hot.status() === 'ready').

module.hot.apply(options).then(outdatedModules => {
  // outdated modules...
}).catch(error => {
  // catch errors
});

The optional options object can include the following properties:

• ignoreUnaccepted (boolean): Ignore changes made to unaccepted modules.
• ignoreDeclined (boolean): Ignore changes made to declined modules.
• ignoreErrored (boolean): Ignore errors thrown in accept handlers, error handlers and while reevaluating module.
• onDeclined (function(info)): Notifier for declined modules
• onUnaccepted (function(info)): Notifier for unaccepted modules
• onAccepted (function(info)): Notifier for accepted modules
• onDisposed (function(info)): Notifier for disposed modules
• onErrored (function(info)): Notifier for errors

The info parameter will be an object containing some of the following values:

{
  type: 'self-declined' | 'declined' |
        'unaccepted' | 'accepted' |
        'disposed' | 'accept-errored' |
        'self-accept-errored' | 'self-accept-error-handler-errored',
  moduleId: 4, // The module in question.
  dependencyId: 3, // For errors: the module id owning the accept handler.
  chain: [1, 2, 3, 4], // For declined/accepted/unaccepted: the chain from where the update was propagated.
  parentId: 5, // For declined: the module id of the declining parent
  outdatedModules: [1, 2, 3, 4], // For accepted: the modules that are outdated and will be disposed
  outdatedDependencies: { // For accepted: The location of accept handlers that will handle the update
    5: [4]
  },
  error: new Error(...), // For errors: the thrown error
  originalError: new Error(...) // For self-accept-error-handler-errored:
                                // the error thrown by the module before the error handler tried to handle it.
}

addStatusHandler

Register a function to listen for changes in status.

module.hot.addStatusHandler(status => {
  // React to the current status...
});

removeStatusHandler

Remove a registered status handler.

module.hot.removeStatusHandler(callback);

Loader Interface

A loader is just a JavaScript module that exports a function. The loader runner calls this function and passes the result of the previous loader or the resource file into it. The this context of the function is filled-in by webpack and the loader runner with some useful methods that allow the loader (among other things) to change its invocation style to async, or get query parameters.

The first loader is passed one argument: the content of the resource file. The compiler expects a result from the last loader. The result should be a String or a Buffer (which is converted to a string), representing the JavaScript source code of the module. An optional SourceMap result (as a JSON object) may also be passed.

A single result can be returned in sync mode. For multiple results the this.callback() must be called. In async mode this.async() must be called to indicate that the loader runner should wait for an asynchronous result. It returns this.callback(). Then the loader must return undefined and call that callback.

Examples

The following sections provide some basic examples of the different types of loaders. Note that the map and meta parameters are optional, see this.callback below.

Synchronous Loaders

Either return or this.callback can be used to return the transformed content synchronously:

sync-loader.js

module.exports = function(content, map, meta) {
  return someSyncOperation(content);
};

The this.callback method is more flexible as it allows multiple arguments to be passed as opposed to just the content.

sync-loader-with-multiple-results.js

module.exports = function(content, map, meta) {
  this.callback(null, someSyncOperation(content), map, meta);
  return; // always return undefined when calling callback()
};

Asynchronous Loaders

For asynchronous loaders, this.async is used to retrieve the callback function:

async-loader.js

module.exports = function(content, map, meta) {
  var callback = this.async();
  someAsyncOperation(content, function(err, result) {
    if (err) return callback(err);
    callback(null, result, map, meta);
  });
};

async-loader-with-multiple-results.js

module.exports = function(content, map, meta) {
  var callback = this.async();
  someAsyncOperation(content, function(err, result, sourceMaps, meta) {
    if (err) return callback(err);
    callback(null, result, sourceMaps, meta);
  });
};

"Raw" Loader

By default, the resource file is converted to a UTF-8 string and passed to the loader. By setting the raw flag to true, the loader will receive the raw Buffer. Every loader is allowed to deliver its result as a String or as a Buffer. The compiler converts them between loaders.

raw-loader.js

module.exports = function(content) {
  assert(content instanceof Buffer);
  return someSyncOperation(content);
  // return value can be a `Buffer` too
  // This is also allowed if loader is not "raw"
};
module.exports.raw = true;

Pitching Loader

Loaders are always called from right to left. There are some instances where the loader only cares about the metadata behind a request and can ignore the results of the previous loader. The pitch method on loaders is called from left to right before the loaders are actually executed (from right to left).

For the following configuration of use:

module.exports = {
  //...
  module: {
    rules: [
      {
        //...
        use: [
          'a-loader',
          'b-loader',
          'c-loader'
        ]
      }
    ]
  }
};

These steps would occur:

|- a-loader `pitch`
  |- b-loader `pitch`
    |- c-loader `pitch`
      |- requested module is picked up as a dependency
    |- c-loader normal execution
  |- b-loader normal execution
|- a-loader normal execution

So why might a loader take advantage of the "pitching" phase?

First, the data passed to the pitch method is exposed in the execution phase as well under this.data and could be useful for capturing and sharing information from earlier in the cycle.

module.exports = function(content) {
  return someSyncOperation(content, this.data.value);
};

module.exports.pitch = function(remainingRequest, precedingRequest, data) {
  data.value = 42;
};

Second, if a loader delivers a result in the pitch method, the process turns around and skips the remaining loaders. In our example above, if the b-loaders pitch method returned something:

module.exports = function(content) {
  return someSyncOperation(content);
};

module.exports.pitch = function(remainingRequest, precedingRequest, data) {
  if (someCondition()) {
    return 'module.exports = require(' + JSON.stringify('-!' + remainingRequest) + ');';
  }
};

The steps above would be shortened to:

|- a-loader `pitch`
  |- b-loader `pitch` returns a module
|- a-loader normal execution

The Loader Context

The loader context represents the properties that are available inside of a loader assigned to the this property.

Given the following example, this require call is used:

In /abc/file.js:

require('./loader1?xyz!loader2!./resource?rrr');

this.version

Loader API version. Currently 2. This is useful for providing backwards compatibility. Using the version you can specify custom logic or fallbacks for breaking changes.

this.context

The directory of the module. Can be used as a context for resolving other stuff.

In the example: /abc because resource.js is in this directory

this.rootContext

Since webpack 4, the formerly this.options.context is provided as this.rootContext.

this.request

The resolved request string.

In the example: '/abc/loader1.js?xyz!/abc/node_modules/loader2/index.js!/abc/resource.js?rrr'

this.query

1. If the loader was configured with an options object, this will point to that object.
2. If the loader has no options, but was invoked with a query string, this will be a string starting with ?.

this.getOptions(schema)

Extracts given loader options. Optionally, accepts JSON schema as an argument.

this.callback

A function that can be called synchronously or asynchronously in order to return multiple results. The expected arguments are:

this.callback(
  err: Error | null,
  content: string | Buffer,
  sourceMap?: SourceMap,
  meta?: any
);

1. The first argument must be an Error or null
2. The second argument is a string or a Buffer.
3. Optional: The third argument must be a source map that is parsable by this module.
4. Optional: The fourth option, ignored by webpack, can be anything (e.g. some metadata).

In case this function is called, you should return undefined to avoid ambiguous loader results.

this.async

Tells the loader-runner that the loader intends to call back asynchronously. Returns this.callback.

this.data

A data object shared between the pitch and the normal phase.

this.cacheable

A function that sets the cacheable flag:

cacheable(flag = true: boolean)

By default, loader results are flagged as cacheable. Call this method passing false to make the loader's result not cacheable.

A cacheable loader must have a deterministic result when inputs and dependencies haven't changed. This means the loader shouldn't have dependencies other than those specified with this.addDependency.

this.loaders

An array of all the loaders. It is writable in the pitch phase.

loaders = [{request: string, path: string, query: string, module: function}]

In the example:

[
  {
    request: '/abc/loader1.js?xyz',
    path: '/abc/loader1.js',
    query: '?xyz',
    module: [Function]
  },
  {
    request: '/abc/node_modules/loader2/index.js',
    path: '/abc/node_modules/loader2/index.js',
    query: '',
    module: [Function]
  }
];

this.loaderIndex

The index in the loaders array of the current loader.

In the example: in loader1: 0, in loader2: 1

this.resource

The resource part of the request, including query.

In the example: '/abc/resource.js?rrr'

this.resourcePath

The resource file.

In the example: '/abc/resource.js'

this.resourceQuery

The query of the resource.

In the example: '?rrr'

this.target

Target of compilation. Passed from configuration options.

Example values: 'web', 'node'

this.webpack

This boolean is set to true when this is compiled by webpack.

this.sourceMap

Tells if source map should be generated. Since generating source maps can be an expensive task, you should check if source maps are actually requested.

this.emitWarning

emitWarning(warning: Error)

Emit a warning that will be displayed in the output like the following:

WARNING in ./src/lib.js (./src/loader.js!./src/lib.js)
Module Warning (from ./src/loader.js):
Here is a Warning!
 @ ./src/index.js 1:0-25

this.emitError

emitError(error: Error)

Emit an error that also can be displayed in the output.

ERROR in ./src/lib.js (./src/loader.js!./src/lib.js)
Module Error (from ./src/loader.js):
Here is an Error!
 @ ./src/index.js 1:0-25

this.loadModule

loadModule(request: string, callback: function(err, source, sourceMap, module))

Resolves the given request to a module, applies all configured loaders and calls back with the generated source, the sourceMap and the module instance (usually an instance of NormalModule). Use this function if you need to know the source code of another module to generate the result.

this.loadModule in a loader context uses CommonJS resolve rules by default. Use this.getResolve with an appropriate dependencyType, e.g. 'esm', 'commonjs' or a custom one before using a different semantic.

this.resolve

resolve(context: string, request: string, callback: function(err, result: string))

Resolve a request like a require expression.

• context must be an absolute path to a directory. This directory is used as the starting location for the resolving.
• request is the request to be resolved. Usually either relative requests like ./relative or module requests like module/path are used, but absolute paths like /some/path are also possible as requests.
• callback is a normal Node.js-style callback function giving the resolved path.

All dependencies of the resolving operation are automatically added as dependencies to the current module.

this.getResolve

getResolve(options: ResolveOptions): resolve

resolve(context: string, request: string, callback: function(err, result: string))
resolve(context: string, request: string): Promise<string>

Creates a resolve function similar to this.resolve.

Any options under webpack resolve options are possible. They are merged with the configured resolve options. Note that "..." can be used in arrays to extend the value from resolve options, e.g. { extensions: [".sass", "..."] }.

options.dependencyType is an additional option. It allows us to specify the type of dependency, which is used to resolve byDependency from the resolve options.

All dependencies of the resolving operation are automatically added as dependencies to the current module.

this.addDependency

addDependency(file: string)
dependency(file: string) // shortcut

Add a file as dependency of the loader result in order to make them watchable. For example, sass-loader, less-loader uses this to recompile whenever any imported css file changes.

this.addContextDependency

addContextDependency(directory: string)

Add a directory as dependency of the loader result.

this.clearDependencies

clearDependencies()

Remove all dependencies of the loader result, even initial dependencies and those of other loaders. Consider using pitch.

this.emitFile

emitFile(name: string, content: Buffer|string, sourceMap: {...})

Emit a file. This is webpack-specific.

this.hot

Information about HMR for loaders.

module.exports = function(source) {
  console.log(this.hot); // true if HMR is enabled via --hot flag or webpack configuration
  return source;
};

this.fs

Access to the compilation's inputFileSystem property.

this.mode

Read in which mode webpack is running.

Possible values: 'production', 'development', 'none'

Deprecated context properties

this.value

Pass values to the next loader. If you know what your result exports if executed as a module, set this value here (as an only element array).

this.inputValue

Passed from the last loader. If you would execute the input argument as a module, consider reading this variable for a shortcut (for performance).

this.debug

A boolean flag. It is set when in debug mode.

this.minimize

Tells if result should be minimized.

this._compilation

Hacky access to the Compilation object of webpack.

this._compiler

Hacky access to the Compiler object of webpack.

this._module

Hacky access to the Module object being loaded.

Error Reporting

You can report errors from inside a loader by:

• Using this.emitError. Will report the errors without interrupting module's compilation.
• Using throw (or other uncaught exception). Throwing an error while a loader is running will cause current module compilation failure.
• Using callback (in async mode). Pass an error to the callback will also cause module compilation failure.

For example:

./src/index.js

require('./loader!./lib');

Throwing an error from loader:

./src/loader.js

module.exports = function(source) {
  throw new Error('This is a Fatal Error!');
};

Or pass an error to the callback in async mode:

./src/loader.js

module.exports = function(source) {
  const callback = this.async();
  //...
  callback(new Error('This is a Fatal Error!'), source);
};

The module will get bundled like this:

/***/ "./src/loader.js!./src/lib.js":
/*!************************************!*\
  !*** ./src/loader.js!./src/lib.js ***!
  \************************************/
/*! no static exports found */
/***/ (function(module, exports) {

throw new Error("Module build failed (from ./src/loader.js):\nError: This is a Fatal Error!\n    at Object.module.exports (/workspace/src/loader.js:3:9)");

/***/ })

Then the build output will also display the error (Similar to this.emitError):

ERROR in ./src/lib.js (./src/loader.js!./src/lib.js)
Module build failed (from ./src/loader.js):
Error: This is a Fatal Error!
    at Object.module.exports (/workspace/src/loader.js:2:9)
 @ ./src/index.js 1:0-25

As you can see below, not only error message, but also details about which loader and module are involved:

• the module path: ERROR in ./src/lib.js
• the request string: (./src/loader.js!./src/lib.js)
• the loader path: (from ./src/loader.js)
• the caller path: @ ./src/index.js 1:0-25

Inline matchResource

A new inline request syntax was introduced in webpack v4. Prefixing <match-resource>!=! to a request will set the matchResource for this request.

When a matchResource is set, it will be used to match with the module.rules instead of the original resource. This can be useful if further loaders should be applied to the resource, or if the module type needs to be changed. It's also displayed in the stats and used for matching Rule.issuer and test in splitChunks.

Example:

file.js

/* STYLE: body { background: red; } */
console.log('yep');

A loader could transform the file into the following file and use the matchResource to apply the user-specified CSS processing rules:

file.js (transformed by loader)

import './file.js.css!=!extract-style-loader/getStyles!./file.js';
console.log('yep');

This will add a dependency to extract-style-loader/getStyles!./file.js and treat the result as file.js.css. Because module.rules has a rule matching /\.css$/ and it will apply to this dependency.

The loader could look like this:

extract-style-loader/index.js

const stringifyRequest = require('loader-utils').stringifyRequest;
const getRemainingRequest = require('loader-utils').getRemainingRequest;
const getStylesLoader = require.resolve('./getStyle');

module.exports = function (source) {
  if (STYLES_REGEXP.test(source)) {
    source = source.replace(STYLES_REGEXP, '');
    const remReq = getRemainingRequest(this);
    return `import ${stringifyRequest(`${this.resource}.css!=!${getStylesLoader}!${remReq}`)};${source}`;
  }
  return source;
};

extract-style-loader/getStyles.js

module.exports = function(source) {
  const match = STYLES_REGEXP.match(source);
  return match[0];
};

Logging

Logging API is available since the release of webpack 4.37. When logging is enabled in stats configuration and/or when infrastructure logging is enabled, loaders may log messages which will be printed out in the respective logger format (stats, infrastructure).

• Loaders should prefer to use this.getLogger() for logging which is a shortcut to compilation.getLogger() with loader path and processed file. This kind of logging is stored to the Stats and formatted accordingly. It can be filtered and exported by the webpack user.
• Loaders may use this.getLogger('name') to get an independent logger with a child name. Loader path and processed file is still added.
• Loaders may use special fallback logic for detecting logging support this.getLogger() ? this.getLogger() : console to provide a fallback when an older webpack version is used which does not support getLogger method.

Logger Interface

Logging output is an additional way to display messages to the end users.

webpack logger is available to loaders and plugins. Emitting as part of the Stats and configured by the user in webpack configuration.

Benefits of custom logging API in webpack:

• Common place to configure the logging display level
• Logging output exportable as part of the stats.json
• Stats presets affect logging output
• Plugins can affect logging capturing and display level
• When using multiple plugins and loaders they use a common logging solution
• CLI, UI tools for webpack may choose different ways to display logging
• webpack core can emit logging output, e.g. timing data

By introducing webpack logging API we hope to unify the way webpack plugins and loaders emit logs and allow better ways to inspect build problems. Integrated logging solution supports plugins and loaders developers by improving their development experience. Paves the way for non-CLI webpack solutions like dashboards or other UIs.

Examples of how to get and use webpack logger in loaders and plugins

my-webpack-plugin.js

const PLUGIN_NAME = 'my-webpack-plugin';
export class MyWebpackPlugin {
  apply(compiler) {
    // you can access Logger from compiler
    const logger = compiler.getInfrastructureLogger(PLUGIN_NAME);
    logger.log('log from compiler');

    compiler.hooks.compilation.tap(PLUGIN_NAME, compilation => {
      // you can also access Logger from compilation
      const logger = compilation.getLogger(PLUGIN_NAME);
      logger.info('log from compilation');
    });
  }
}

my-webpack-loader.js

module.exports = function (source) {
  // you can get Logger with `this.getLogger` in your webpack loaders
  const logger = this.getLogger('my-webpack-loader');
  logger.info('hello Logger');
  return source;
};

Logger methods

• logger.error(...): for error messages
• logger.warn(...): for warnings
• logger.info(...): for important information messages. These messages are displayed by default. Only use this for messages that the user really needs to see
• logger.log(...): for unimportant information messages. These messages are displayed only when user had opted-in to see them
• logger.debug(...): for debugging information. These messages are displayed only when user had opted-in to see debug logging for specific modules
• logger.trace(): to display a stack trace. Displayed like logger.debug
• logger.group(...): to group messages. Displayed collapsed like logger.log
• logger.groupEnd(): to end a logging group
• logger.groupCollapsed(...): to group messages together. Displayed collapsed like logger.log. Displayed expanded when logging level is set to 'verbose' or 'debug'.
• logger.status: writes a temporary message, setting a new status, overrides the previous one
• logger.clear(): to print a horizontal line. Displayed like logger.log
• logger.profile(...), logger.profileEnd(...): to capture a profile. Delegated to console.profile when supported

Runtime Logger API

Runtime logger API is only intended to be used as a development tool, it is not intended to be included in production mode.

• const logging = require('webpack/lib/logging/runtime'): to use the logger in runtime, require it directly from webpack
• logging.getLogger('name'): to get individual logger by name
• logging.configureDefaultLogger(...): to override the default logger.

const logging = require('webpack/lib/logging/runtime');
logging.configureDefaultLogger({
  level: 'log',
  debug: /something/
});

• logging.hooks.log: to apply Plugins to the runtime logger

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

ES6 (Recommended)

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 simply 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")
// just as with other require/import methods.
const page = 'Foo';
__webpack_modules__[require.resolveWeak(`./page/${page}`)];

Module Variables

This section covers all variables available in code compiled with webpack. Modules will have access to certain data from the compilation process through module and other variables.

module.loaded (NodeJS)

This is false if the module is currently executing, and true if the sync execution has finished.

module.hot (webpack-specific)

Indicates whether or not Hot Module Replacement is enabled and provides an interface to the process. See the HMR API page for details.

module.id (CommonJS)

The ID of the current module.

module.id === require.resolve('./file.js');

module.exports (CommonJS)

Defines the value that will be returned when a consumer makes a require call to the module (defaults to a new object).

module.exports = function doSomething() {
  // Do something...
};

exports (CommonJS)

This variable is equal to the default value of module.exports (i.e. an object). If module.exports gets overwritten, exports will no longer be exported.

exports.someValue = 42;
exports.anObject = {
  x: 123
};
exports.aFunction = function doSomething() {
  // Do something
};

global (NodeJS)

See node.js global.

For compatibility reasons webpack polyfills the global variable by default.

__dirname (NodeJS)

Depending on the configuration option node.__dirname:

• false: Not defined
• mock: equal to '/'
• true: node.js __dirname

If used inside an expression that is parsed by the Parser, the configuration option is treated as true.

import.meta.url

Returns the absolute file: URL of the module.

src/index.js

console.log(import.meta.url); // output something like `file:///path/to/your/project/src/index.js`

import.meta.webpack

Returns the webpack version.

src/index.js

console.log(import.meta.webpack); // output `5` for webpack 5

__filename (NodeJS)

Depending on the configuration option node.__filename:

• false: Not defined
• mock: equal to '/index.js'
• true: node.js __filename

If used inside an expression that is parsed by the Parser, the configuration option is treated as true.

__resourceQuery (webpack-specific)

The resource query of the current module. If the following require call was made, then the query string would be available in file.js.

require('file.js?test');

file.js

__resourceQuery === '?test';

__webpack_public_path__ (webpack-specific)

Equals the configuration option's output.publicPath.

__webpack_require__ (webpack-specific)

The raw require function. This expression isn't parsed by the Parser for dependencies.

__webpack_chunk_load__ (webpack-specific)

The internal chunk loading function. Takes two arguments:

• chunkId The id for the chunk to load.
• callback(require) A callback function called once the chunk is loaded.

__webpack_modules__ (webpack-specific)

Access to the internal object of all modules.

__webpack_hash__ (webpack-specific)

This variable is only available with the HotModuleReplacementPlugin or the ExtendedAPIPlugin. It provides access to the hash of the compilation.

__non_webpack_require__ (webpack-specific)

Generates a require function that is not parsed by webpack. Can be used to do cool stuff with a global require function if available.

__webpack_exports_info__ (webpack-specific)

In modules, __webpack_exports_info__ is available to allow exports introspection:

• __webpack_exports_info__ is always true

• __webpack_exports_info__.<exportName>.used is false when the export is known to be unused, true otherwise

• __webpack_exports_info__.<exportName>.useInfo is

  • false when the export is known to be unused
  • true when the export is known to be used
  • null when the export usage could depend on runtime conditions
  • undefined when no info is available

• __webpack_exports_info__.<exportName>.provideInfo is

  • false when the export is known to be not provided
  • true when the export is known to be provided
  • null when the export provision could depend on runtime conditions
  • undefined when no info is available

• Accessing the info from nested exports is possible: i. e. __webpack_exports_info__.<exportName>.<exportName>.<exportName>.used

__webpack_is_included__ (webpack-specific)

if (__webpack_is_included__('./module-a.js')) {
  // do something
}

compiler.hooks.someHook.tap('MyPlugin', (params) => {
  /* ... */
});

compiler.hooks.entryOption.tap('MyPlugin', (context, entry) => {
  /* ... */
});

compilationParams = {
  normalModuleFactory,
  contextModuleFactory,
};

compiler.hooks.beforeCompile.tapAsync('MyPlugin', (params, callback) => {
  params['MyPlugin - data'] = 'important stuff my plugin will use later';
  callback();
});

compiler.hooks.shouldEmit.tap('MyPlugin', (compilation) => {
  // return true to emit the output, otherwise false
  return true;
});

compiler.hooks.assetEmitted.tap(
  'MyPlugin',
  (file, { content, source, outputPath, compilation, targetPath }) => {
    console.log(content); // <Buffer 66 6f 6f 62 61 72>
  }
);

compilation.hooks.someHook.tap(/* ... */);

compilation.hooks.buildModule.tap('SourceMapDevToolModuleOptionsPlugin',
  module => {
    module.useSourceMap = true;
  }
);

compilation.hooks.additionalAssets.tapAsync('MyPlugin', callback => {
  download('https://img.shields.io/npm/v/webpack.svg', function(resp) {
    if(resp.status === 200) {
      compilation.assets['webpack-version.svg'] = toAsset(resp);
      callback();
    } else {
      callback(new Error('[webpack-example-plugin] Unable to download the image'));
    }
  });
});

compilation.hooks
  .optimizeChunkAssets
  .tapAsync('MyPlugin', (chunks, callback) => {
    chunks.forEach(chunk => {
      chunk.files.forEach(file => {
        compilation.assets[file] = new ConcatSource(
          '\/**Sweet Banner**\/',
          '\n',
          compilation.assets[file]
        );
      });
    });

    callback();
  });

compilation.hooks.afterOptimizeChunkAssets.tap('MyPlugin', chunks => {
  chunks.forEach(chunk => {
    console.log({
      id: chunk.id,
      name: chunk.name,
      includes: chunk.getModules().map(module => module.request)
    });
  });
});

compilation.hooks.processAssets.tap(
  {
    name: 'MyPlugin',
    stage: Compilation.PROCESS_ASSETS_STAGE_ADDITIONS, // see below for more stages
  },
  (assets) => {
    // code here
  }
);

ContextModuleFactory.hooks.someHook.tap(/* ... */);

compiler.hooks.normalModuleFactory.tap('MyPlugin', factory => {
  factory.hooks.parser.for('javascript/auto').tap('MyPlugin', (parser, options) => {
    parser.hooks.someHook.tap(/* ... */);
  });
});

parser.hooks.evaluateTypeof.for('myIdentifier').tap('MyPlugin', expression => {
  /* ... */
  return expressionResult;
});

const a = typeof myIdentifier;

const myIdentifier = 0;
const b = typeof myIdentifier;

const a = new String();

parser.hooks.evaluate.for('NewExpression').tap('MyPlugin', expression => {
  /* ... */
  return expressionResult;
});

const a = expression.myFunc();

parser.hooks.evaluateCallExpressionMember.for('myFunc').tap('MyPlugin', (expression, param) => {
  /* ... */
  return expressionResult;
});

parser.hooks.statement.tap('MyPlugin', statement => { /* ... */ });

import _ from 'lodash';

parser.hooks.import.tap('MyPlugin', (statement, source) => {
  // source == 'lodash'
});

import _, { has } from 'lodash';

parser.hooks.importSpecifier.tap('MyPlugin', (statement, source, exportName, identifierName) => {
  /* First call
    source == 'lodash'
    exportName == 'default'
    identifierName == '_'
  */
  /* Second call
    source == 'lodash'
    exportName == 'has'
    identifierName == 'has'
  */
});

export const myVar = 'hello'; // also var, let
export function FunctionName(){}
export class ClassName {}

var a = b;

parser.hooks.canRename.for('b').tap('MyPlugin', expression => {
  // returning true allows renaming
  return true;
});

var a = b;

parser.hooks.rename.for('b').tap('MyPlugin', expression => {});

a += b;

parser.hooks.assigned.for('a').tap('MyPlugin', expression => {
  // this is called before parsing b
});

a += b;

parser.hooks.assigned.for('a').tap('MyPlugin', expression => {
  // this is called before parsing a
});

eval(/* something */);

parser.hooks.call.for('eval').tap('MyPlugin', expression => {});

myObj.anyFunc();

parser.hooks.callMemberChain.for('myObj').tap('MyPlugin', (expression, properties) => {});

new MyClass();

parser.hooks.new.for('MyClass').tap('MyPlugin', expression => {});

const a = this;

parser.hooks.expression.for('this').tap('MyPlugin', expression => {});

NormalModuleFactory.hooks.someHook.tap(/* ... */);

NormalModuleFactory.hooks.someHook.for('identifier').tap(/* ... */);