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.

Event handlers

Worker.onerror
An EventListener called whenever an ErrorEvent of type error event occurs.
Worker.onmessage
An EventListener called whenever a MessageEvent of type message occurs — 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 event handler representing the code to be called when the messageerror event is raised.

Methods

Inherits methods from its parent, EventTarget.

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 our Basic dedicated worker example (run dedicated worker).

Specifications

Specification
HTML Standard (HTML)
# dedicated-workers-and-the-worker-interface

Browser compatibility

BCD tables only load in the browser

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

Cross-origin worker error behavior

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