/Web APIs


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.

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



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

Instance properties

Inherits properties from its parent, EventTarget.

Instance methods

Inherits methods from its parent, EventTarget.


Sends a message — consisting of any JavaScript object — to the worker's inner scope.


Immediately terminates the worker. This does not let worker finish its operations; it is halted at once. ServiceWorker instances do not support this method.



Fires when an error occurs in the worker.


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


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


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


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:

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).


Browser compatibility

Desktop Mobile
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet
Worker 4 12 3.5 10 10.6 4 4.4 18 4 11 5 1.0
Worker 2 12 3.5 10 10.6 4 4.4 18 4 11 5 1.0
error_event 4 12 3.5 10 10.6 4 4.4 18 4 11 5 1.0
message_event 4 12 3.5 10 10.6 4 4.4 18 4 11.5 5 1.0
messageerror_event 60 18 57 No 47
16.4See bug 171216.
60 60 57 47
16.4See bug 171216.
postMessage 2 12 3.5
10Internet Explorer does not support Transferable objects.
10.6 4 4.4 18 4 11 5 1.0
terminate 2 12 3.5 10 10.6 4 4.4 18 4 11 5 1.0

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

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