worker-loader

Web Worker loader for webpack 4.

Note that this is specific to webpack 4. To use Web Workers in webpack 5, see https://webpack.js.org/guides/web-workers/.

Getting Started

To begin, you'll need to install worker-loader:

$ npm install worker-loader --save-dev

Inlined

App.js

import Worker from "worker-loader!./Worker.js";

Config

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.worker\.js$/,
        use: { loader: "worker-loader" },
      },
    ],
  },
};

App.js

import Worker from "./file.worker.js";

const worker = new Worker();

worker.postMessage({ a: 1 });
worker.onmessage = function (event) {};

worker.addEventListener("message", function (event) {});

And run webpack via your preferred method.

Options

worker

Type: String|Object Default: Worker

Set the worker type.

String

Allows to set web worker constructor name.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.worker\.(c|m)?js$/i,
        loader: "worker-loader",
        options: {
          worker: "SharedWorker",
        },
      },
    ],
  },
};

Object

Allow to set web worker constructor name and options.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.worker\.(c|m)?js$/i,
        loader: "worker-loader",
        options: {
          worker: {
            type: "SharedWorker",
            options: {
              type: "classic",
              credentials: "omit",
              name: "my-custom-worker-name",
            },
          },
        },
      },
    ],
  },
};

publicPath

Type: String|Function Default: based on output.publicPath

The publicPath specifies the public URL address of the output files when referenced in a browser. If not specified, the same public path used for other webpack assets is used.

String

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.worker\.(c|m)?js$/i,
        loader: "worker-loader",
        options: {
          publicPath: "/scripts/workers/",
        },
      },
    ],
  },
};

Function

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.worker\.(c|m)?js$/i,
        loader: "worker-loader",
        options: {
          publicPath: (pathData, assetInfo) => {
            return `/scripts/${pathData.hash}/workers/`;
          },
        },
      },
    ],
  },
};

filename

Type: String|Function Default: based on output.filename, adding worker suffix, for example - output.filename: '[name].js' value of this option will be [name].worker.js

The filename of entry chunks for web workers.

String

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.worker\.(c|m)?js$/i,
        loader: "worker-loader",
        options: {
          filename: "[name].[contenthash].worker.js",
        },
      },
    ],
  },
};

Function

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.worker\.(c|m)?js$/i,
        loader: "worker-loader",
        options: {
          filename: (pathData) => {
            if (
              /\.worker\.(c|m)?js$/i.test(pathData.chunk.entryModule.resource)
            ) {
              return "[name].custom.worker.js";
            }

            return "[name].js";
          },
        },
      },
    ],
  },
};

chunkFilename

Type: String Default: based on output.chunkFilename, adding worker suffix, for example - output.chunkFilename: '[id].js' value of this option will be [id].worker.js

The filename of non-entry chunks for web workers.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.worker\.(c|m)?js$/i,
        loader: "worker-loader",
        options: {
          chunkFilename: "[id].[contenthash].worker.js",
        },
      },
    ],
  },
};

inline

Type: 'fallback' | 'no-fallback' Default: undefined

Allow to inline the worker as a BLOB.

Inline mode with the fallback value will create file for browsers without support web workers, to disable this behavior just use no-fallback value.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.worker\.(c|m)?js$/i,
        loader: "worker-loader",
        options: {
          inline: "fallback",
        },
      },
    ],
  },
};

esModule

Type: Boolean Default: true

By default, worker-loader generates JS modules that use the ES modules syntax.

You can enable a CommonJS modules syntax using:

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.worker\.(c|m)?js$/i,
        loader: "worker-loader",
        options: {
          esModule: false,
        },
      },
    ],
  },
};

Examples

Basic

The worker file can import dependencies just like any other file:

index.js

import Worker from "./my.worker.js";

var worker = new Worker();

var result;

worker.onmessage = function (event) {
  if (!result) {
    result = document.createElement("div");
    result.setAttribute("id", "result");

    document.body.append(result);
  }

  result.innerText = JSON.stringify(event.data);
};

const button = document.getElementById("button");

button.addEventListener("click", function () {
  worker.postMessage({ postMessage: true });
});

my.worker.js

onmessage = function (event) {
  var workerResult = event.data;

  workerResult.onmessage = true;

  postMessage(workerResult);
};

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.worker\.(c|m)?js$/i,
        loader: "worker-loader",
        options: {
          esModule: false,
        },
      },
    ],
  },
};

Integrating with ES6+ features

You can even use ES6+ features if you have the babel-loader configured.

index.js

import Worker from "./my.worker.js";

const worker = new Worker();

let result;

worker.onmessage = (event) => {
  if (!result) {
    result = document.createElement("div");
    result.setAttribute("id", "result");

    document.body.append(result);
  }

  result.innerText = JSON.stringify(event.data);
};

const button = document.getElementById("button");

button.addEventListener("click", () => {
  worker.postMessage({ postMessage: true });
});

my.worker.js

onmessage = function (event) {
  const workerResult = event.data;

  workerResult.onmessage = true;

  postMessage(workerResult);
};

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.worker\.(c|m)?js$/i,
        use: [
          {
            loader: "worker-loader",
          },
          {
            loader: "babel-loader",
            options: {
              presets: ["@babel/preset-env"],
            },
          },
        ],
      },
    ],
  },
};

Integrating with TypeScript

To integrate with TypeScript, you will need to define a custom module for the exports of your worker.

Loading with worker-loader!

typings/worker-loader.d.ts

declare module "worker-loader!*" {
  // You need to change `Worker`, if you specified a different value for the `workerType` option
  class WebpackWorker extends Worker {
    constructor();
  }

  // Uncomment this if you set the `esModule` option to `false`
  // export = WebpackWorker;
  export default WebpackWorker;
}

my.worker.ts

const ctx: Worker = self as any;

// Post data to parent thread
ctx.postMessage({ foo: "foo" });

// Respond to message from parent thread
ctx.addEventListener("message", (event) => console.log(event));

index.ts

import Worker from "worker-loader!./Worker";

const worker = new Worker();

worker.postMessage({ a: 1 });
worker.onmessage = (event) => {};

worker.addEventListener("message", (event) => {});

Loading without worker-loader!

Alternatively, you can omit the worker-loader! prefix passed to import statement by using the following notation. This is useful for executing the code using a non-WebPack runtime environment (such as Jest with workerloader-jest-transformer).

typings/worker-loader.d.ts

declare module "*.worker.ts" {
  // You need to change `Worker`, if you specified a different value for the `workerType` option
  class WebpackWorker extends Worker {
    constructor();
  }

  // Uncomment this if you set the `esModule` option to `false`
  // export = WebpackWorker;
  export default WebpackWorker;
}

my.worker.ts

const ctx: Worker = self as any;

// Post data to parent thread
ctx.postMessage({ foo: "foo" });

// Respond to message from parent thread
ctx.addEventListener("message", (event) => console.log(event));

index.ts

import MyWorker from "./my.worker.ts";

const worker = new MyWorker();

worker.postMessage({ a: 1 });
worker.onmessage = (event) => {};

worker.addEventListener("message", (event) => {});

webpack.config.js

module.exports = {
  module: {
    rules: [
      // Place this *before* the `ts-loader`.
      {
        test: /\.worker\.ts$/,
        loader: "worker-loader",
      },
      {
        test: /\.ts$/,
        loader: "ts-loader",
      },
    ],
  },
  resolve: {
    extensions: [".ts", ".js"],
  },
};

Cross-Origin Policy

WebWorkers are restricted by a same-origin policy, so if your webpack assets are not being served from the same origin as your application, their download may be blocked by your browser. This scenario can commonly occur if you are hosting your assets under a CDN domain. Even downloads from the webpack-dev-server could be blocked.

There are two workarounds:

Firstly, you can inline the worker as a blob instead of downloading it as an external script via the inline parameter

App.js

import Worker from "./file.worker.js";

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        loader: "worker-loader",
        options: { inline: "fallback" },
      },
    ],
  },
};

Secondly, you may override the base download URL for your worker script via the publicPath option

App.js

// This will cause the worker to be downloaded from `/workers/file.worker.js`
import Worker from "./file.worker.js";

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        loader: "worker-loader",
        options: { publicPath: "/workers/" },
      },
    ],
  },
};

Contributing

Please take a moment to read our contributing guidelines if you haven't yet done so.

CONTRIBUTING

License

MIT