Web Workers

A web worker is a mechanism for running background tasks outside of the browser context. Heavy and resource consuming tasks can instead be moved to a background task so as to not block any UI threads in the main window. Communication is done through postMessage and onmessage handlers in both the worker instance and the browser.

Fusion.js supports loading web workers using the workerUrl virtual module exposed by fusion-core. The virtual module allows Fusion.js to bundle your web worker with Babel.


The method signature for workerUrl is:

workerUrl(url: string): string

It takes as input the path to your web worker file and will return the internal Babel-bundled URL. This URL can then be passed to the Worker constructor. For example, the following code creates a web worker within a Fusion.js client-side plugin.

// web-worker-plugin.js
import {createPlugin, workerUrl} from 'fusion-core';

export default __BROWSER__ && createPlugin({
  middleware: () => {
    return (ctx, next) => {
      const url = workerUrl('./path/to/worker.js');
      const worker = new Worker(url);

      worker.onmessage = e => {
        console.log('Worker sent back', e.data);
      worker.postMessage('Some message');

      return next();

Because the web worker has been transpiled and bundled with Babel, you can also use ES2015+ syntax as well as import other modules within the worker file.

// worker.js

import add from '../utils/add.js';

class SomeWorkerClass() {
  // ...


Since web workers are meant only run on the client, make sure to properly code fence your plugins and worker registration code with __BROWSER__ guards so the code is not executed on the server.

// app.js

if (__BROWSER__) {