The Worker interface of the Web Workers API represents a background task that can be easily created and can send messages back to its creator. Creating a worker is as simple as calling the Worker() constructor and specifying a script to be run in the worker thread.

Workers may in turn spawn new workers as long as those workers are hosted within the same origin as the parent page (Note: nested workers are currently not implemented in WebKit). In addition workers may use XMLHttpRequest for network I/O, with the stipulation that the responseXML and channel attributes on XMLHttpRequest always return null.

Not all interfaces and functions are available to the script associated with a Worker.

In Firefox, if you want to use workers in extensions and would like to have access to js-ctypes, you should use the ChromeWorker object instead.


Creates a dedicated web worker that executes the script at the specified URL. Workers can also be constructed using Blobs.


Inherits properties from its parent, EventTarget, and implements properties from AbstractWorker.

Event handlers

An EventListener called whenever an ErrorEvent of type error bubbles through to the worker. This is inherited from AbstractWorker.
An EventListener called whenever a MessageEvent of type message bubbles through the worker — i.e. when a message is sent to the parent document from the worker via DedicatedWorkerGlobalScope.postMessage. The message is stored in the event's data property.
Is an EventHandler representing the code to be called when the messageerror event is raised.


Inherits methods from its parent, EventTarget, and implements methods from AbstractWorker.

Sends a message — which can consist of any JavaScript object — to the worker's inner scope.
Immediately terminates the worker. This does not offer the worker an opportunity to finish its operations; it is simply stopped at once. ServiceWorker instances do not support this method.


The following code snippet shows creation of a Worker object using the Worker() constructor and usage of the object:

var myWorker = new Worker('worker.js');
var first = document.querySelector('#number1');
var second = document.querySelector('#number2');

first.onchange = function() {
  console.log('Message posted to worker');

For a full example, see ourBasic dedicated worker example (run dedicated worker).


Specification Status Comment
HTML Living Standard
The definition of 'Worker' in that specification.
Living Standard

Browser compatibilityUpdate compatibility data on GitHub

Support varies for different types of workers. See each worker type's page for specifics.

Chrome Edge Firefox Internet Explorer Opera Safari
Basic support 4 Yes 3.5 10 10.6 4
Worker() constructor 4 Yes 3.5 10 10.6 4
onmessage 4 12 3.5 10 10.6 4
onmessageerror 60 ? 57 ? 47 ?
postMessage Yes 12 Yes 10
Internet Explorer does not support Transferable objects.
47 Yes
terminate 4 12 3.5 10 10.6 4
Android webview Chrome for Android Edge Mobile Firefox for Android Opera for Android iOS Safari Samsung Internet
Basic support 4 18 Yes 4 11.5 5.1 Yes
Worker() constructor 4 18 Yes 4 11.5 5.1 Yes
onmessage 4 18 Yes 4 11.5 5.1 Yes
onmessageerror 60 60 ? 57 47 ? No
postMessage Yes Yes Yes Yes 47 Yes Yes
terminate 4 18 Yes 4 11.5 5.1 Yes

Cross-origin worker error behaviour

In earlier browser versions, trying to load a cross-origin worker script threw a SecurityError; in newer browsers an error event is thrown instead due to a spec change. Find out more information on how to deal with this in Loading cross-origin worker now fires error event instead of throwing; worker in sandboxed iframe no longer allowed.

See also

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