Experiments

experiments

boolean: false

experiments option was introduced in webpack 5 to empower users with the ability to activate and try out experimental features.

Available options:

• asyncWebAssembly: Support the new WebAssembly according to the updated specification, it makes a WebAssembly module an async module. And it is enabled by default when experiments.futureDefaults is set to true.
• backCompat
• buildHttp
• cacheUnaffected
• css
• futureDefaults
• layers: Enable module and chunk layers.
• lazyCompilation
• outputModule
• syncWebAssembly: Support the old WebAssembly like in webpack 4.
• topLevelAwait

webpack.config.js

module.exports = {
  //...
  experiments: {
    asyncWebAssembly: true,
    buildHttp: true,
    layers: true,
    lazyCompilation: true,
    outputModule: true,
    syncWebAssembly: true,
    topLevelAwait: true,
  },
};

experiments.backCompat

Enable backward-compat layer with deprecation warnings for many webpack 4 APIs.

• Type: boolean

module.exports = {
  //...
  experiments: {
    backCompat: true,
  },
};

experiments.buildHttp

When enabled, webpack can build remote resources that begin with the http(s): protocol.

• Type:

  • (string | RegExp | ((uri: string) => boolean))[]

    A shortcut for experiments.buildHttp.allowedUris.

  • HttpUriOptions

    {
      allowedUris: (string|RegExp|(uri: string) => boolean)[],
      cacheLocation?: false | string,
      frozen?: boolean,
      lockfileLocation?: string,
      upgrade?: boolean
    }

• Available: 5.49.0+

• Example

  // webpack.config.js
  module.exports = {
    //...
    experiments: {
      buildHttp: true,
    },
  };

  // src/index.js
  import pMap1 from 'https://cdn.skypack.dev/p-map';
  // with `buildHttp` enabled, webpack will build pMap1 like a regular local module
  console.log(pMap1);

experiments.buildHttp.allowedUris

A list of allowed URIs.

• Type: (string|RegExp|(uri: string) => boolean)[]

• Example

  // webpack.config.js
  module.exports = {
    //...
    experiments: {
      buildHttp: {
        allowedUris: [
          'http://localhost:9990/',
          'https://raw.githubusercontent.com/',
        ],
      },
    },
  };

experiments.buildHttp.cacheLocation

Define the location for caching remote resources.

• Type

  • string
  • false

• Example

  // webpack.config.js
  module.exports = {
    //...
    experiments: {
      buildHttp: {
        cacheLocation: false,
      },
    },
  };

By default webpack would use <compiler-name.>webpack.lock.data/ for caching, but you can disable it by setting its value to false.

Note that you should commit files under experiments.buildHttp.cacheLocation into a version control system as no network requests will be made during the production build.

experiments.buildHttp.frozen

Freeze the remote resources and lockfile. Any modification to the lockfile or resource contents will result in an error.

• Type: boolean

experiments.buildHttp.lockfileLocation

Define the location to store the lockfile.

• Type: string

By default webpack would generate a <compiler-name.>webpack.lock file>. Make sure to commit it into a version control system. During the production build, webpack will build those modules beginning with http(s): protocol from the lockfile and caches under experiments.buildHttp.cacheLocation.

experiments.buildHttp.proxy

Specify the proxy server to use for fetching remote resources.

• Type: string

By default, Webpack would imply the proxy server to use for fetching remote resources from the http_proxy (case insensitive) environment variable. However, you can also specify one through the proxy option.

experiments.buildHttp.upgrade

Detect changes to remote resources and upgrade them automatically.

• Type: boolean

experiments.css

Enable native CSS support. Note that it's an experimental feature still under development and will be enabled by default in webpack v6, however you can track the progress on GitHub.

• Type: boolean

Experimental features:

• CSS Modules support: webpack will generate a unique name for each CSS class. Use the .module.css extension for CSS Modules.

• 5.87.0+ Style-specific fields resolution in package.json files: webpack will look for style field in package.json files and use that if it exists for imports inside CSS files.

  For example, if you add @import 'bootstrap'; to your CSS file, webpack will look for bootstrap in node_modules and use the style field in package.json from there. If style field is not found, webpack will use the main field instead to preserve backward compatibility.

• Content hash for CSS files: webpack will generate a content hash for CSS files and use it in the filename. This is useful for long-term caching.

• CSS extraction: webpack will extract CSS into a separate file. This functionality replaces the need for mini-css-extract-plugin and css-loader, as it provides native support.

• CSS imports: webpack will inline CSS imports into the generated CSS file.

• Hot Module Reload (HMR): webpack supports HMR for CSS files. This means that changes made to CSS files will be reflected in the browser without a full page reload.

experiments.cacheUnaffected

Enable additional in-memory caching of modules which are unchanged and reference only unchanged modules.

• Type: boolean

Defaults to the value of futureDefaults.

experiments.futureDefaults

Use defaults of the next major webpack and show warnings in any problematic places.

webpack.config.js

module.exports = {
  //...
  experiments: {
    futureDefaults: true,
  },
};

experiments.lazyCompilation

Compile entrypoints and dynamic imports only when they are in use. It can be used for either Web or Node.js.

• Type

  • boolean

  • object

    {
    // define a custom backend
    backend?: ((
      compiler: Compiler,
      callback: (err?: Error, api?: BackendApi) => void
      ) => void)
      | ((compiler: Compiler) => Promise<BackendApi>)
      | {
        /**
         * A custom client.
        */
        client?: string;
    
        /**
         * Specify where to listen to from the server.
         */
        listen?: number | ListenOptions | ((server: Server) => void);
    
        /**
         * Specify the protocol the client should use to connect to the server.
         */
        protocol?: "http" | "https";
    
        /**
         * Specify how to create the server handling the EventSource requests.
         */
        server?: ServerOptionsImport | ServerOptionsHttps | (() => Server);
    
    },
    entries?: boolean,
    imports?: boolean,
    test?: string | RegExp | ((module: Module) => boolean)
    }

    • backend: Customize the backend.
    • entries: Enable lazy compilation for entries.
    • imports 5.20.0+: Enable lazy compilation for dynamic imports.
    • test 5.20.0+: Specify which imported modules should be lazily compiled.

• Available: 5.17.0+

• Example 1:

  module.exports = {
    // …
    experiments: {
      lazyCompilation: true,
    },
  };

• Example 2:

  module.exports = {
    // …
    experiments: {
      lazyCompilation: {
        // disable lazy compilation for dynamic imports
        imports: false,
  
        // disable lazy compilation for entries
        entries: false,
  
        // do not lazily compile moduleB
        test: (module) => !/moduleB/.test(module.nameForCondition()),
      },
    },
  };

experiments.outputModule

boolean

Once enabled, webpack will output ECMAScript module syntax whenever possible. For instance, import() to load chunks, ESM exports to expose chunk data, among others.

module.exports = {
  experiments: {
    outputModule: true,
  },
};

experiments.topLevelAwait

boolean = true

The topLevelAwait option transforms a module into an async module when an await is used at the top level. Starting from webpack version 5.83.0, this feature is enabled by default. However, in versions prior to that, you can enable it by setting experiments.topLevelAwait to true.

module.exports = {
  experiments: {
    topLevelAwait: true,
  },
};