Web Workers API

Web Workers makes it possible to run a script operation in a background thread separate from the main execution thread of a web application. The advantage of this is that laborious processing can be performed in a separate thread, allowing the main (usually the UI) thread to run without being blocked/slowed down.

Note: Web Workers can also use the Web Worker API (i.e. workers can spawn workers, provided they are hosted within the same origin as the parent page).

Web Workers concepts and usage

A worker is an object created using a constructor (e.g. Worker()) that runs a named JavaScript file — this file contains the code that will run in the worker thread.

In addition to the standard JavaScript set of functions (such as StringArrayObjectJSON, etc.), you can run almost any code you like inside a worker thread. There are some exceptions: for example, you can't directly manipulate the DOM from inside a worker, or use some default methods and properties of the window object. For information about the code that you can run see worker global context and functions, and supported web APIs below.

Data is sent between workers and the main thread via a system of messages — both sides send their messages using the postMessage() method, and respond to messages via the onmessage event handler (the message is contained within the Message event's data property). The data is copied rather than shared.

Workers may in turn spawn new workers, as long as those workers are hosted within the same origin as the parent page. In addition, workers may use XMLHttpRequest for network I/O, with the exception that the responseXML and channel attributes on XMLHttpRequest always return null.

Worker types

There are a number of different types of workers:

  • Dedicated workers are workers that are utilized by a single script. This context is represented by a DedicatedWorkerGlobalScope object.
  • Shared workers are workers that can be utilized by multiple scripts running in different windows, IFrames, etc., as long as they are in the same domain as the worker. They are a little more complex than dedicated workers — scripts must communicate via an active port.
  • Service Workers essentially act as proxy servers that sit between web applications, the browser, and the network (when available). They are intended, among other things, to enable the creation of effective offline experiences, intercept network requests and take appropriate action based on whether the network is available, and update assets residing on the server. They will also allow access to push notifications and background sync APIs.

Note: As per the Web workers Spec, worker error events should not bubble (see bug 1188141. This has been implemented in Firefox 42.

Worker global contexts and functions

Workers run in a different global context than the current window! While  Window is not directly available to workers, many of the same methods are defined in a shared mixin (WindowOrWorkerGlobalScope), and made available to workers through their own WorkerGlobalScope-derived contexts:

Some of the functions (a subset) that are common to all workers and to the main thread (from WindowOrWorkerGlobalScope) are: atob(), btoa(), clearInterval()clearTimeout(),dump() , setInterval(), setTimeout().

The following functions are only available to workers:

Supported Web APIs

Note: If a listed API is supported by a platform in a particular version, then it can generally be assumed to be available in web workers. You can also test support for a particular object/function using the site: https://worker-playground.glitch.me/

The following Web APIs are available to workers: Barcode Detection API, Broadcast Channel API, Cache API, Channel Messaging API,Console APIWeb Crypto API (Crypto), CustomEvent, Data Store (Firefox only), DOMRequest and DOMCursor, Encoding API (TextEncoder, TextDecoder, etc.), Fetch APIFileReaderFileReaderSync (only works in workers!), FormDataImageDataIndexedDBNetwork Information APINotifications API, Performance API (including: PerformancePerformanceEntry, PerformanceMeasure, PerformanceMark, PerformanceObserver, PerformanceResourceTiming), PromiseServer-sent eventsServiceWorkerRegistrationURL API (e.g. URL), WebGL with OffscreenCanvas (enabled behind a feature preference setting gfx.offscreencanvas.enabled), WebSocketXMLHttpRequest.

Workers can also spawn other workers, so these APIs are also available: WorkerWorkerGlobalScopeWorkerLocationWorkerNavigator.

Web Worker interfaces


Represents a running worker thread, allowing you to pass messages to the running worker code.


Defines the absolute location of the script executed by the Worker.


Represents a specific kind of worker that can be accessed from several browsing contexts, being several windows, iframes or even workers.


Represents the generic scope of any worker (doing the same job as Window does for normal web content). Different types of worker have scope objects that inherit from this interface and add more specific features.


Represents the scope of a dedicated worker, inheriting from WorkerGlobalScope and adding some dedicated features.


Represents the scope of a shared worker, inheriting from WorkerGlobalScope and adding some dedicated features.


Represents the identity and state of the user agent (the client):


We have created a couple of simple demos to show basic usage:

You can find out more information on how these demos work in Using Web Workers.


HTML Living Standard # workers

See also