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.)

EventTarget Worker

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.

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

error

Fires when an error occurs in the worker.

message

Fires when the worker's parent receives a message from that worker.

messageerror

Fires when a Worker object receives a message that can't be deserialized.

rejectionhandled

Fires every time a Promise rejects, regardless of whether or not there is a handler to catch the rejection.

unhandledrejection

Fires when a Promise rejects with no handler to catch the rejection.

Example

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

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

first.onchange = () => {
  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
# 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.

See also