---
title: Worker
slug: Web/API/Worker
tags:
  - API
  - DOM
  - Interface
  - NeedsTranslation
  - Reference
  - TopicStub
  - Web Workers
  - Worker
  - Workers
translation_of: Web/API/Worker
---
<p>{{APIRef("Web Workers API")}}</p>

<p>The <strong><code>Worker</code></strong> interface of the <a href="/en-US/docs/Web/API/Web_Workers_API">Web Workers API</a> represents a background task that can be easily created and can send messages back to its creator. Creating a worker is as simple as calling the <code>Worker()</code> constructor and specifying a script to be run in the worker thread.</p>

<p>Workers may in turn spawn new workers as long as those workers are hosted within the same origin as the parent page (Note: nested workers are <a href="https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/5R3B4RN4GHU">currently not implemented in Blink</a>).  In addition workers may use <a class="internal" href="/en/DOM/XMLHttpRequest" title="En/XMLHttpRequest"><code>XMLHttpRequest</code></a> for network I/O, with the stipulation that the <code>responseXML</code> and <code>channel</code> attributes on <code>XMLHttpRequest</code> always return <code>null</code>.</p>

<p>Not <a href="/En/DOM/Worker/Functions_available_to_workers" title="En/DOM/Worker/Functions available to workers">all interfaces and functions are available</a> to the script associated with a <code>Worker</code>.</p>

<p>In Firefox, if you want to use workers in extensions and would like to have access to <a href="/en/js-ctypes" title="en/js-ctypes">js-ctypes</a>, you should use the {{ domxref("ChromeWorker") }} object instead.</p>

<h2 id="Properties">Properties</h2>

<p><em>Inherits properties from its parent, {{domxref("EventTarget")}}, and implements properties from {{domxref("AbstractWorker")}}.</em></p>

<h3 id="Event_handlers">Event handlers</h3>

<dl>
 <dt>{{domxref("AbstractWorker.onerror")}}</dt>
 <dd>An {{ domxref("EventListener") }} called whenever an {{domxref("ErrorEvent")}} of type <code>error</code> bubbles through to the worker. This is inherited from {{domxref("AbstractWorker")}}.</dd>
 <dt>{{domxref("Worker.onmessage")}}</dt>
 <dd>An {{ domxref("EventListener") }} called whenever a {{domxref("MessageEvent")}} of type <code>message</code> bubbles through the worker — i.e. when a message is sent to the parent document from the worker via {{domxref("DedicatedWorkerGlobalScope.postMessage")}}. The message is stored in the event's {{domxref("MessageEvent.data", "data")}} property.</dd>
</dl>

<h2 id="Constructors">Constructors</h2>

<dl>
 <dt>{{domxref("Worker.Worker", "Worker()")}}</dt>
 <dd>Creates a dedicated web worker that executes the script at the specified URL. Workers can also be constructed using <a href="/en-US/docs/Web/API/Blob">Blobs</a>.</dd>
</dl>

<h2 id="Methods">Methods</h2>

<p><em>Inherits methods from its parent, {{domxref("EventTarget")}}, and implements properties from {{domxref("AbstractWorker")}}.</em></p>

<dl>
 <dt>{{domxref("Worker.postMessage()")}}</dt>
 <dd>Sends a message — which can consist of <code>any</code> JavaScript object — to the worker's inner scope.</dd>
 <dt>{{domxref("Worker.terminate()")}}</dt>
 <dd>Immediately terminates the worker. This does not offer the worker an opportunity to finish its operations; it is simply stopped at once. ServiceWorker instances do not support this method.</dd>
</dl>

<h2 id="Example">Example</h2>

<p>The following code snippet shows creation of a {{domxref("Worker")}} object using the {{domxref("Worker.Worker", "Worker()")}} constructor and usage of the object:</p>

<pre class="brush: js">var myWorker = new Worker("worker.js");

first.onchange = function() {
  myWorker.postMessage([first.value,second.value]);
  console.log('Message posted to worker');
}</pre>

<p>For a full example, see our<a class="external external-icon" href="https://github.com/mdn/simple-web-worker">Basic dedicated worker example</a> (<a class="external external-icon" href="http://mdn.github.io/simple-web-worker/">run dedicated worker</a>).</p>

<h2 id="Specifications">Specifications</h2>

<table class="standard-table">
 <tbody>
  <tr>
   <th scope="col">Specification</th>
   <th scope="col">Status</th>
   <th scope="col">Comment</th>
  </tr>
  <tr>
   <td>{{SpecName('HTML WHATWG', "#worker", "Worker")}}</td>
   <td>{{Spec2('HTML WHATWG')}}</td>
   <td>No change from {{SpecName("Web Workers")}}.</td>
  </tr>
  <tr>
   <td>{{SpecName('Web Workers', "#worker", "Worker")}}</td>
   <td>{{Spec2('Web Workers')}}</td>
   <td>Initial definition.</td>
  </tr>
 </tbody>
</table>

<h2 id="Browser_compatibility">Browser compatibility</h2>

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

{{Compat("api.Worker")}}

<h2 id="See_also">See also</h2>

<ul>
 <li><a class="internal" href="/En/Using_web_workers" title="en/Using DOM workers">Using web workers</a></li>
 <li><a href="/En/DOM/Worker/Functions_available_to_workers" title="https://developer.mozilla.org/En/DOM/Worker/Functions_available_to_workers">Functions available to workers</a></li>
 <li>Other kind of workers: {{ domxref("SharedWorker") }} and <a href="/en-US/docs/Web/API/ServiceWorker_API">ServiceWorker</a>.</li>
 <li>Non-standard, Gecko-specific workers: {{ domxref("ChromeWorker") }}, used by extensions.</li>
</ul>