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.

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; workers run in another global context that is different from the current window. This context is represented by either a DedicatedWorkerGlobalScope object (in the case of dedicated workers - workers that are utilized by a single script), or a SharedWorkerGlobalScope (in the case of shared workers - workers that are shared between multiple scripts).

You can run whatever code you like inside the worker thread, with 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. But you can use a large number of items available under window, including WebSockets, and data storage mechanisms like IndexedDB.  See Functions and classes available to workers for more details.

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.

In addition to dedicated workers, there are other types of worker:

  • 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. See SharedWorker for more details.
  • ServiceWorkers 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.
  • Chrome Workers are a Firefox-only type of worker that you can use if you are developing add-ons and want to use workers in extensions and have access to js-ctypes in your worker. See ChromeWorker for more details. 
  • Audio Workers provide the ability for direct scripted audio processing to be done inside a web worker context.

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

Web Worker interfaces

AbstractWorker
Abstracts properties and methods common to all kind of workers (i.e. Worker or SharedWorker).
Worker
Represents a running worker thread, allowing you to pass messages to the running worker code.
WorkerLocation
Defines the absolute location of the script executed by the Worker.
SharedWorker
Represents a specific kind of worker that can be accessed from several browsing contexts, being several windows, iframes or even workers.
WorkerGlobalScope
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.
DedicatedWorkerGlobalScope
Represents the scope of a dedicated worker, inheriting from WorkerGlobalScope and adding some dedicated features.
SharedWorkerGlobalScope
Represents the scope of a shared worker, inheriting from WorkerGlobalScope and adding some dedicated features.
WorkerNavigator
Represents the identity and state of the user agent (the client):

Examples

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.

Specifications

Specification Status Comment
HTML Living Standard
The definition of 'Web Workers' in that specification.
Living Standard Initial definition.

Browser compatibility

We're converting our compatibility data into a machine-readable JSON format. This compatibility table still uses the old format, because we haven't yet converted the data it contains. Find out how you can help!
Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support 4 (Yes) 3.5 (1.9.1) 10 10.6 4
Shared workers 4 No support 29 (29) No support 10.6 4
Passing data using structured cloning 13 (Yes) 8 (8) 10 11.5 6
Passing data using  transferable objects 17webkit
21
No support 18 (18) No support 15 6
Global URL 10webkit
32
(Yes) 21 (21) 11 15 6webkit
7
Feature Android Chrome for Android Edge Firefox Mobile (Gecko) Opera Mobile Safari Mobile
Basic support 4.4 4 (Yes) 1.0 (1.9.1) 11.5 5.1
Shared workers No support 4 No support 29.0 (29) No support No support
Passing data using structured cloning No support 4 (Yes) 8.0 (8) No support No support
Passing data using  transferable objects No support No support No support 18.0 (18) No support No support
Global URL (Yes) (Yes) (Yes) 21.0 (21) (Yes) 6webkit
7

See also