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web-worker npm

Native cross-platform Web Workers. Works in published npm modules.

In Node, it's a web-compatible Worker implementation atop Node's worker_threads.

In the browser (and when bundled for the browser), it's simply an alias of Worker.


Here's how this is different from worker_threads:

  • makes Worker code compatible across browser and Node
  • supports Module Workers ({type:'module'}) natively in Node 12.8+
  • uses DOM-style events (Event.data, Event.type, etc)
  • supports event handler properties (worker.onmessage=..)
  • Worker() accepts a module URL, Blob URL or Data URL
  • emulates browser-style WorkerGlobalScope within the worker

Usage Example

In its simplest form:

import Worker from 'web-worker';

const worker = new Worker('data:,postMessage("hello")');
worker.onmessage = e => console.log(e.data);  // "hello"
main.js worker.js
import Worker from 'web-worker';

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

worker.addEventListener('message', e => {
  console.log(e.data)  // "hiya!"

addEventListener('message', e => {
  if (e.data === 'hello') {

👉 Notice how new URL('./worker.js', import.meta.url) is used above to load the worker relative to the current module instead of the application base URL. Without this, Worker URLs are relative to a document's URL, which in Node.js is interpreted to be process.cwd().

Support for this pattern in build tools and test frameworks is still limited. We are working on growing this.

Module Workers

Module Workers are supported in Node 12.8+ using this plugin, leveraging Node's native ES Modules support. In the browser, they can be used natively in Chrome 80+, or in all browsers via worker-plugin or rollup-plugin-off-main-thread. As with classic workers, there is no difference in usage between Node and the browser:

main.mjs worker.mjs
import Worker from 'web-worker';

const worker = new Worker(
  new URL('./worker.mjs', import.meta.url),
  { type: 'module' }
worker.addEventListener('message', e => {
  console.log(e.data)  // "200 OK"
import fetch from 'isomorphic-fetch';

addEventListener('message', async e => {
  const url = e.data;
  const res = await fetch(url)
  const text = await res.text();

Data URLs

Instantiating Worker using a Data URL is supported in both module and classic workers:

import Worker from 'web-worker';

const worker = new Worker(`data:application/javascript,postMessage(42)`);
worker.addEventListener('message', e => {
  console.log(e.data)  // 42

Special Thanks

This module aims to provide a simple and forgettable piece of infrastructure, and as such it needed an obvious and descriptive name. @calvinmetcalf, who you may recognize as the author of Lie and other fine modules, gratiously offered up the name from his web-worker package. Thanks Calvin!

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npm i @eshaz/web-worker

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