Web/API/Worker

The Worker interface of the Web Workers API represents a background task that can be created via script, which can send messages back to its creator. Creating a worker is done by calling the Worker("path/to/worker/script") constructor.

Workers may themselves spawn new workers, as long as those workers are hosted at the same origin as the parent page. (Note: nested workers are not yet implemented in WebKit).

Not all interfaces and functions are available to scripts inside a Worker. Workers may use XMLHttpRequest for network communication, but its responseXML and channel attributes are always null. (fetch is also available, with no such restrictions.)

Constructors

Worker(): Creates a dedicated web worker that executes the script at the specified URL. This also works for Blob URLs.

Properties

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

Event handlers

AbstractWorker.onerror: An EventListener called whenever an ErrorEvent of type error bubbles through to the worker. This is inherited from AbstractWorker.
Worker.onmessage: 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.
Worker.onmessageerror: Is an EventHandler representing the code to be called when the messageerror event is raised.

Methods

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

Worker.postMessage(): Sends a message — consisting of any JavaScript object — to the worker's inner scope.
Worker.terminate(): Immediately terminates the worker. This does not let worker finish its operations; it is halted at once. ServiceWorker instances do not support this method.

Events

message: Fires when the worker's parent receives a message from that worker. Also available via the onmessage property.
messageerror: Fires when a Worker object receives a message that can't be deserialized. Also available via the onmessageerror property.

rejectionhandled: Fires every time a Promise rejects, regardless of whether or not there is a handler to catch the rejection. Also available through the onrejectionhandled event handler property.
unhandledrejection: Fires when a Promise rejects with no handler to catch the rejection. Also available using the onunhandledrejection event handler property.

Example

The following code snippet creates a Worker object using the Worker() constructor, then uses the worker object:

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

first.onchange = function() {
  myWorker.postMessage([first.value, second.value]);
  console.log('Message posted to worker');
}

For a full example, see ourBasic dedicated worker example ([[../../../../../../mdn.github.io/simple-web-worker/index|run dedicated worker]]).

Specifications

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

Browser compatibility

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

Update compatibility data on GitHub

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
`Worker`	Chrome Full support 4	Edge Full support 12	Firefox Full support 3.5	IE Full support 10	Opera Full support 10.6	Safari Full support 4	WebView Android Full support 4	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 11	Safari iOS Full support 5.1	Samsung Internet Android Full support 1.0
`Worker()` constructor	Chrome Full support 4	Edge Full support 12	Firefox Full support 3.5	IE Full support 10	Opera Full support 10.6	Safari Full support 4	WebView Android Full support 4	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 11	Safari iOS Full support 5.1	Samsung Internet Android Full support 1.0
`message` event	Chrome Full support 4	Edge Full support 12	Firefox Full support 3.5	IE Full support 10	Opera Full support 10.6	Safari Full support 4	WebView Android Full support 4	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 11.5	Safari iOS Full support 5.1	Samsung Internet Android Full support 1.0
`messageerror` event	Chrome Full support 60	Edge Full support 18	Firefox Full support 57	IE ?	Opera Full support 47	Safari ?	WebView Android Full support 60	Chrome Android Full support 60	Firefox Android Full support 57	Opera Android Full support 47	Safari iOS ?	Samsung Internet Android Full support 8.0
`onmessage`	Chrome Full support 4	Edge Full support 12	Firefox Full support 3.5	IE Full support 10	Opera Full support 10.6	Safari Full support 4	WebView Android Full support 4	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 11	Safari iOS Full support 5.1	Samsung Internet Android Full support 1.0
`onmessageerror`	Chrome Full support 60	Edge Full support 18	Firefox Full support 57	IE No support No	Opera Full support 47	Safari ?	WebView Android Full support 60	Chrome Android Full support 60	Firefox Android Full support 57	Opera Android Full support 44	Safari iOS ?	Samsung Internet Android Full support 8.0
`postMessage`	Chrome Full support Yes	Edge Full support 12	Firefox Full support Yes	IE Full support 10 Notes' Full support 10 Notes' Notes' Internet Explorer does not support `Transferable` objects.	Opera Full support 47	Safari Full support Yes	WebView Android Full support Yes	Chrome Android Full support Yes	Firefox Android Full support Yes	Opera Android Full support 44	Safari iOS Full support Yes	Samsung Internet Android Full support Yes
`terminate`	Chrome Full support 4	Edge Full support 12	Firefox Full support 3.5	IE Full support 10	Opera Full support 10.6	Safari Full support 4	WebView Android Full support 4	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 11	Safari iOS Full support 5.1	Samsung Internet Android Full support 1.0

Legend

Full support: Full support
No support: No support
Compatibility unknown: Compatibility unknown
See implementation notes.': See implementation notes.

Cross-origin worker error behaviour

In early versions of the spec, loading a cross-origin worker script threw a SecurityError. Nowadays, an error event is thrown instead. Find out how to deal with this in Loading cross-origin worker now fires error event instead of throwing; worker in sandboxed iframe no longer allowed.