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.)

Worker()

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

Inherits properties from its parent, EventTarget.

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.

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.

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).

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

In early versions of the spec, loading a cross-origin worker script threw a SecurityError. Nowadays, an error event is thrown instead.

• Using Web Workers
• Functions and classes available to Web Workers
• Other kind of workers: SharedWorker and Service Worker.