Worker

Note: This feature is available in Web Workers, except for Service Workers.

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 that not all interfaces and functions are available to web workers. See Functions and classes available to Web Workers for details.

Constructors

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.

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.

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

HTML

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.