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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 (run dedicated worker).

Specifications

Specification Status Comment
HTML Living Standard
The 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
DesktopMobile
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung Internet
WorkerChrome Full support 4Edge Full support 12Firefox Full support 3.5IE Full support 10Opera Full support 10.6Safari Full support 4WebView Android Full support 4Chrome Android Full support 18Firefox Android Full support 4Opera Android Full support 11Safari iOS Full support 5.1Samsung Internet Android Full support 1.0
Worker() constructorChrome Full support 4Edge Full support 12Firefox Full support 3.5IE Full support 10Opera Full support 10.6Safari Full support 4WebView Android Full support 4Chrome Android Full support 18Firefox Android Full support 4Opera Android Full support 11Safari iOS Full support 5.1Samsung Internet Android Full support 1.0
message eventChrome Full support 4Edge Full support 12Firefox Full support 3.5IE Full support 10Opera Full support 10.6Safari Full support 4WebView Android Full support 4Chrome Android Full support 18Firefox Android Full support 4Opera Android Full support 11.5Safari iOS Full support 5.1Samsung Internet Android Full support 1.0
messageerror eventChrome Full support 60Edge Full support 18Firefox Full support 57IE ? Opera Full support 47Safari ? WebView Android Full support 60Chrome Android Full support 60Firefox Android Full support 57Opera Android Full support 47Safari iOS ? Samsung Internet Android Full support 8.0
onmessageChrome Full support 4Edge Full support 12Firefox Full support 3.5IE Full support 10Opera Full support 10.6Safari Full support 4WebView Android Full support 4Chrome Android Full support 18Firefox Android Full support 4Opera Android Full support 11Safari iOS Full support 5.1Samsung Internet Android Full support 1.0
onmessageerrorChrome Full support 60Edge Full support 18Firefox Full support 57IE No support NoOpera Full support 47Safari ? WebView Android Full support 60Chrome Android Full support 60Firefox Android Full support 57Opera Android Full support 44Safari iOS ? Samsung Internet Android Full support 8.0
postMessageChrome Full support YesEdge Full support 12Firefox Full support YesIE Full support 10
Notes
Full support 10
Notes
Notes Internet Explorer does not support Transferable objects.
Opera Full support 47Safari Full support YesWebView Android Full support YesChrome Android Full support YesFirefox Android Full support YesOpera Android Full support 44Safari iOS Full support YesSamsung Internet Android Full support Yes
terminateChrome Full support 4Edge Full support 12Firefox Full support 3.5IE Full support 10Opera Full support 10.6Safari Full support 4WebView Android Full support 4Chrome Android Full support 18Firefox Android Full support 4Opera Android Full support 11Safari iOS Full support 5.1Samsung 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.

See also