MDN Specification
Notes
- Creating a worker is done by calling the
Worker("path/to/worker/script") constructor.
- Workers have no access to the page-specific objects/APIs (like
document, etc)
- We can send data back and forth between the main application and our worker script. Calling the
postMessage() function can send JSON, Strings, numbers, and/or arrays. //main.js
worker.postMessage("hello world"); // Send this to the worker script.
//worker.js
postMessage("hi from worker"); // Send this back to the main script.
-
We need to add event listeners to both sides to receive the messages above. We can either use onmessage or addEventListener.
//worker.js
self.addEventListener('message', function(e) {
// Send the message back.
self.postMessage('You said: ' + e.data);
}, false);
Note that we use self for the worker to give a global reference to web workers because it is not a window object.
Event listener in the main script:
//main.js
let worker = new Worker('worker.js');
worker.addEventListener('message', function(e) {
// Log the workers message.
console.log(e.data);
}, false);
MDN Reference: Functions and classes available to Web Workers
The following functions are available to workers:
- WorkerGlobalScope.atob()
- WorkerGlobalScope.btoa()
- WorkerGlobalScope.clearInterval()
- WorkerGlobalScope.clearTimeout()
- WorkerGlobalScope.createImageBitmap()
- WorkerGlobalScope.dump() Non-standard
- WorkerGlobalScope.fetch()
- WorkerGlobalScope.queueMicrotask()
- WorkerGlobalScope.reportError()
- WorkerGlobalScope.setInterval()
- WorkerGlobalScope.setTimeout()
- WorkerGlobalScope.structuredClone()
- DedicatedWorkerGlobalScope.postMessage() (dedicated workers only)
- DedicatedWorkerGlobalScope.requestAnimationFrame() (dedicated workers only)
- DedicatedWorkerGlobalScope.cancelAnimationFrame() (dedicated workers only)
The following functions are only available to workers:
- WorkerGlobalScope.importScripts()
The following Web APIs are available to workers:
- Background Fetch API
- Background Synchronization API
- Barcode Detection API
- Broadcast Channel API
- Canvas API
- Channel Messaging API
- Compression Streams API
- Compute Pressure API
- Console API
- Content Index API
- Cookie Store API (service workers only)
- CSS Font Loading API
- Encoding API
- Fetch API
- File API
- File System API
- Idle Detection API
- IndexedDB API
- Media Capabilities API
- Media Source Extensions API (dedicated workers only)
- Network Information API
- Notifications API
- Payment Handler API
- Performance API
- Permissions API
- Prioritized Task Scheduling API
- Push API
- Reporting API
- Server-Sent Events
- Service Worker API
- Streams API
- Trusted Types API
- URL API
- URL Pattern API
- User-Agent Client Hints API
- Web Crypto API
- Web Locks API
- Web Serial API
- Web Periodic Background Synchronization API
- WebCodecs API
- WebGL API
- WebGPU API
- WebHID API (dedicated and service workers only)
- WebUSB API
- WebSockets API
- XMLHttpRequest API
Tags:
platform
browser
Last modified 26 July 2025