Web Workers

As of webpack 5, you can use Web Workers without worker-loader.

Syntax

new Worker(new URL("./worker.js", import.meta.url));

// or customize the chunk name with magic comments
// see https://webpack.js.org/api/module-methods/#magic-comments
new Worker(
  /* webpackChunkName: "foo-worker" */ new URL("./worker.js", import.meta.url),
);

The syntax was chosen to allow running code without bundler, it is also available in native ECMAScript modules in the browser.

Note that while the Worker API suggests that Worker constructor would accept a string representing the URL of the script, in webpack 5 you can only use URL instead.

When using new Worker(), webpack can resolve worker modules by export condition names defined in the package's exports field. This allows packages to provide worker-specific versions of modules automatically.

package.json (in a dependency package):

{
  "name": "my-package",
  "exports": {
    ".": {
      "worker": "./index.worker.js",
      "default": "./index.js"
    }
  }
}

When you import this package inside a worker context:

// Inside a worker file
import { someFunction } from "my-package";

Webpack will automatically resolve to index.worker.js when the module is used in a worker context, without requiring any additional configuration.

Example

src/index.js

const worker = new Worker(new URL("./deep-thought.js", import.meta.url));
worker.postMessage({
  question:
    "The Answer to the Ultimate Question of Life, The Universe, and Everything.",
});
worker.onmessage = ({ data: { answer } }) => {
  console.log(answer);
};

src/deep-thought.js

globalThis.onmessage = ({ data: { question } }) => {
  self.postMessage({
    answer: 42,
  });
};

Set a public path from a variable

When you set __webpack_public_path__ from a variable, and use publicPath equal to auto, worker chunks will get a separate runtime, and Webpack runtime will set publicPath to automatically calculated public path, that is probably is not what you expect.

To work around this issue, you need to set __webpack_public_path__ from within the worker code. Here is an example:

worker.js

globalThis.onmessage = ({ data: { publicPath, ...otherData } }) => {
  if (publicPath) {
    __webpack_public_path__ = publicPath;
  }

  // rest of the worker code
};

app.js

const worker = new Worker(new URL("./worker.js", import.meta.url));

worker.postMessage({ publicPath: globalThis.__MY_GLOBAL_PUBLIC_PATH_VAR__ });

When to use this:

This pattern is only required when a worker needs to load additional chunks and the asset base URL is determined at runtime (for example, when using a CDN or a multi-domain deployment).

Since workers run in an isolated global scope, the automatically detected public path may differ from the one used by the main thread. In such cases, the public path (__webpack_public_path__) must be explicitly passed to the worker and set inside the worker runtime.

Note: This is an advanced use case. If your worker does not load additional chunks or your assets are served from a static, same-origin path, you typically do not need to set __webpack_public_path__ manually.

Node.js

This section describes using Web Workers in a Node.js environment via the worker_threads module.

Similar syntax is supported in Node.js (>= 12.17.0):

import { Worker } from "node:worker_threads";

new Worker(new URL("./worker.js", import.meta.url));

Note that this is only available in ESM. Worker in CommonJS syntax is not supported by either webpack or Node.js.