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   <p><a href="http://www.w3.org/"><img alt="W3C" height="48" src="http://www.w3.org/Icons/w3c_home" width="72"></a></p>

   <h1>Web Workers</h1>
   
   <h2 class="no-num no-toc" id="lcwd-01-september-2011">W3C Working Draft 01 September 2011</h2>
   <dl>
    <dt>This Version:</dt>
    <dd><a href="http://www.w3.org/TR/2011/WD-workers-20110901/">http://www.w3.org/TR/2011/WD-workers-20110901/</a></dd>
    <dt>Latest Published Version:</dt>
    <dd><a href="http://www.w3.org/TR/workers/">http://www.w3.org/TR/workers/</a></dd>
    <dt>Latest Editor's Draft:</dt>
    <dd><a class="latest-link" href="http://dev.w3.org/html5/workers/">http://dev.w3.org/html5/workers/</a></dd>

    <dt>Previous Versions:</dt>
    <dd><a href="http://www.w3.org/TR/2011/WD-workers-20110310/">http://www.w3.org/TR/2011/WD-workers-20110310/</a></dd>
    <dd><a href="http://www.w3.org/TR/2011/WD-workers-20110208/">http://www.w3.org/TR/2011/WD-workers-20110208/</a></dd>
    <dd><a href="http://www.w3.org/TR/2009/WD-workers-20091029/">http://www.w3.org/TR/2009/WD-workers-20091029/</a></dd>
    <dd><a href="http://www.w3.org/TR/2009/WD-workers-20090423/">http://www.w3.org/TR/2009/WD-workers-20090423/</a></dd>
<!-- :ZZZ -->
    <dt>Editor:</dt>
    <dd><a href="mailto:ian@hixie.ch">Ian Hickson</a>, Google, Inc.</dd>
   </dl><p class="copyright"><a href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a>
   &copy; 2011 <a href="http://www.w3.org/"><abbr title="World Wide
   Web Consortium">W3C</abbr></a><sup>&reg;</sup> (<a href="http://www.csail.mit.edu/"><abbr title="Massachusetts
   Institute of Technology">MIT</abbr></a>, <a href="http://www.ercim.eu/"><abbr title="European Research
   Consortium for Informatics and Mathematics">ERCIM</abbr></a>, <a href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. W3C
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   use</a> rules apply.</p>


   <!-- UNDER NO CIRCUMSTANCES IS THE FOLLOWING PARAGRAPH TO BE REMOVED OR EDITED WITHOUT TALKING TO IAN FIRST -->
   <p class="alt">The bulk of the text of this specification is also
   available in the WHATWG <a href="http://www.whatwg.org/specs/web-apps/current-work/complete.html#workers">Web Applications 1.0</a> specification, under a license that permits
   reuse of the specification text.</p>
   <!-- UNDER NO CIRCUMSTANCES IS THE PRECEDING PARAGRAPH TO BE REMOVED OR EDITED WITHOUT TALKING TO IAN FIRST -->


  </div><hr class="top"><h2 class="no-num no-toc" id="abstract">Abstract</h2><p>This specification defines an API that allows Web application
  authors to spawn background workers running scripts in parallel to
  their main page. This allows for thread-like operation with
  message-passing as the coordination mechanism.<h2 class="no-num no-toc" id="status-of-this-document">Status of This document</h2><p><em>This section describes the status of this document at the
  time of its publication. Other documents may supersede this
  document. A list of current W3C publications and the

  most recently formally published <!-- DO NOT CHANGE THIS BACK TO THE STANDARD BOILERPLATE, AS IT IS INACCURATE -->

  revision of this technical report can be found in the <a href="http://www.w3.org/TR/">W3C technical reports index</a> at
  http://www.w3.org/TR/.</em></p><p>If you wish to make comments regarding this document in a manner
  that is tracked by the W3C, please submit them via using <a href="http://www.w3.org/Bugs/Public/enter_bug.cgi?product=HTML%20WG">our
  public bug database</a>. If you do not have an account then you can
  enter feedback using this form:<form action="http://www.whatwg.org/specs/web-apps/current-work/file-spam.cgi" method="post">
   <fieldset><legend>Feedback Comments</legend>
    <input name="id" type="hidden" value="top"><input name="component" type="hidden" value="Web Workers (editor: Ian Hickson)"><input name="response" type="hidden" value="html"><p><label for="feedbackBox">Please enter your feedback, carefully
    indicating the title of the section for which you are submitting
    feedback, quoting the text that's wrong today if appropriate. If
    you're suggesting a new feature, it's really important to say
    <em>what</em> the problem you're trying to solve is. That's more
    important than the solution, in fact.</label></p>
    <p><textarea cols="79" id="feedbackBox" name="text" rows="10"></textarea></p>
    <p class="note">Please don't use section numbers as these tend to
    change rapidly and make your feedback harder to understand.</p>
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        if (form.elements.text.value.match(/^Please enter your feedback, carefully/)) {
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        } else if (form.elements.text.value.match(/ [^ ]+ [^ ]+ [^ ]+ [^ ]+ /)) {
          form.action = "http://www.whatwg.org/specs/web-apps/current-work/file-bug.cgi";
          return true;
        } else {
          alert('Please include significantly more detail about exactly what problem you are trying to solve.');
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     <input onclick="return checkFeedbackForm(form)" type="submit" value="Submit feedback"><small>(Note: Your IP address and user agent will be publicly recorded for spam prevention purposes.)</small>
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   </fieldset></form><p>You can also e-mail feedback to <a href="mailto:public-webapps@w3.org">public-webapps@w3.org</a> (<a href="mailto:public-webapps-request@w3.org?subject=subscribe">subscribe</a>,
  <a href="http://lists.w3.org/Archives/Public/public-webapps/">archives</a>),
  or <a href="mailto:whatwg@whatwg.org">whatwg@whatwg.org</a> (<a href="http://lists.whatwg.org/listinfo.cgi/whatwg-whatwg.org">subscribe</a>,
  <a href="http://lists.whatwg.org/pipermail/whatwg-whatwg.org/">archives</a>).
  All feedback is welcome.</p><p><strong>Implementors who are not taking part in the
  discussions are likely to find the specification changing out from
  under them in incompatible ways.</strong> Vendors interested in
  implementing this specification before it eventually reaches the
  Candidate Recommendation stage should join the aforementioned
  mailing lists and take part in the discussions.<div id="multipage-common">
  </div><p>The latest
  stable version of the editor's draft of this specification is always
  available on <a href="http://dev.w3.org/html5/workers/">the W3C CVS server</a>
  and in the <a href="http://svn.whatwg.org/webapps/">WHATWG
  Subversion repository</a>. The <a href="http://www.whatwg.org/specs/web-apps/current-work/complete.html">latest
  editor's working copy</a> (which may contain unfinished text in the
  process of being prepared) contains the latest draft text of this
  specification (amongst others). For more details, please see the <a href="http://wiki.whatwg.org/wiki/FAQ#What_are_the_various_versions_of_the_spec.3F">WHATWG
  FAQ</a>.<p>Notifications of changes to this specification are sent along
  with notifications of changes to related specifications using the
  following mechanisms:<dl><dt>E-mail notifications of changes</dt>
   <dd>Commit-Watchers mailing list (complete source diffs): <a href="http://lists.whatwg.org/listinfo.cgi/commit-watchers-whatwg.org">http://lists.whatwg.org/listinfo.cgi/commit-watchers-whatwg.org</a></dd>
   <dt>Browsable version-control record of all changes:</dt>
   <dd>CVSWeb interface with side-by-side diffs: <a href="http://dev.w3.org/cvsweb/html5/">http://dev.w3.org/cvsweb/html5/</a></dd>
   <dd>Annotated summary with unified diffs: <a href="http://html5.org/tools/web-apps-tracker">http://html5.org/tools/web-apps-tracker</a></dd>
   <dd>Raw Subversion interface: <code>svn checkout http://svn.whatwg.org/webapps/</code></dd>
  </dl><p>The W3C <a href="http://www.w3.org/2008/webapps/">Web Applications
  Working Group</a> is the W3C working group responsible for this
  specification's progress along the W3C Recommendation track.
  This specification is the 01 September 2011 Last Call Working Draft. The Last Call review period will end on 27 September 2011. 
  </p><p>This document was produced by a group operating under the <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/">5
  February 2004 W3C Patent Policy</a>. W3C maintains a <a href="http://www.w3.org/2004/01/pp-impl/42538/status" rel="disclosure">public list of
  any patent disclosures</a> made in connection with the deliverables
  of the group; that page also includes instructions for disclosing a
  patent. An individual who has actual knowledge of a patent which the
  individual believes contains <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/#def-essential">Essential
  Claim(s)</a> must disclose the information in accordance with <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/#sec-Disclosure">section
  6 of the W3C Patent Policy</a>.<h2 class="no-num no-toc" id="contents">Table of Contents</h2>
<ol class="toc">
 <li><a href="#introduction"><span class="secno">1 </span>Introduction</a>
  <ol>
   <li><a href="#scope"><span class="secno">1.1 </span>Scope</a></li>
   <li><a href="#examples"><span class="secno">1.2 </span>Examples</a>
    <ol>
     <li><a href="#a-background-number-crunching-worker"><span class="secno">1.2.1 </span>A background number-crunching worker</a></li>
     <li><a href="#a-worker-for-updating-a-client-side-database"><span class="secno">1.2.2 </span>A worker for updating a client-side database</a></li>
     <li><a href="#worker-used-for-background-i-o"><span class="secno">1.2.3 </span>Worker used for background I/O</a></li>
     <li><a href="#shared-workers-introduction"><span class="secno">1.2.4 </span>Shared workers introduction</a></li>
     <li><a href="#shared-state-using-a-shared-worker"><span class="secno">1.2.5 </span>Shared state using a shared worker</a></li>
     <li><a href="#delegation"><span class="secno">1.2.6 </span>Delegation</a></ol></li>
   <li><a href="#tutorials"><span class="secno">1.3 </span>Tutorials</a>
    <ol>
     <li><a href="#creating-a-dedicated-worker"><span class="secno">1.3.1 </span>Creating a dedicated worker</a></li>
     <li><a href="#communicating-with-a-dedicated-worker"><span class="secno">1.3.2 </span>Communicating with a dedicated worker</a></li>
     <li><a href="#shared-workers"><span class="secno">1.3.3 </span>Shared workers</a></ol></ol></li>
 <li><a href="#conformance-requirements"><span class="secno">2 </span>Conformance requirements</a>
  <ol>
   <li><a href="#dependencies"><span class="secno">2.1 </span>Dependencies</a></ol></li>
 <li><a href="#terminology"><span class="secno">3 </span>Terminology</a></li>
 <li><a href="#infrastructure"><span class="secno">4 </span>Infrastructure</a>
  <ol>
   <li><a href="#dependencies-0"><span class="secno">4.1 </span>Dependencies</a></li>
   <li><a href="#the-global-scope"><span class="secno">4.2 </span>The global scope</a>
    <ol>
     <li><a href="#the-workerglobalscope-abstract-interface"><span class="secno">4.2.1 </span>The <code>WorkerGlobalScope</code> abstract interface</a></li>
     <li><a href="#dedicated-workers-and-the-dedicatedworkerglobalscope-interface"><span class="secno">4.2.2 </span>Dedicated workers and the <code>DedicatedWorkerGlobalScope</code> interface</a></li>
     <li><a href="#shared-workers-and-the-sharedworkerglobalscope-interface"><span class="secno">4.2.3 </span>Shared workers and the <code>SharedWorkerGlobalScope</code> interface</a></ol></li>
   <li><a href="#origins-of-workers"><span class="secno">4.3 </span>Origins of workers</a></li>
   <li><a href="#the-event-loop"><span class="secno">4.4 </span>The event loop</a></li>
   <li><a href="#the-worker-s-lifetime"><span class="secno">4.5 </span>The worker's lifetime</a></li>
   <li><a href="#processing-model"><span class="secno">4.6 </span>Processing model</a></li>
   <li><a href="#runtime-script-errors"><span class="secno">4.7 </span>Runtime script errors</a></li>
   <li><a href="#creating-workers"><span class="secno">4.8 </span>Creating workers</a>
    <ol>
     <li><a href="#the-abstractworker-abstract-interface"><span class="secno">4.8.1 </span>The <code>AbstractWorker</code> abstract interface</a></li>
     <li><a href="#dedicated-workers-and-the-worker-interface"><span class="secno">4.8.2 </span>Dedicated workers and the <code>Worker</code> interface</a></li>
     <li><a href="#shared-workers-and-the-sharedworker-interface"><span class="secno">4.8.3 </span>Shared workers and the <code>SharedWorker</code> interface</a></ol></ol></li>
 <li><a href="#apis-available-to-workers"><span class="secno">5 </span>APIs available to workers</a>
  <ol>
   <li><a href="#importing-scripts-and-libraries"><span class="secno">5.1 </span>Importing scripts and libraries</a></li>
   <li><a href="#the-workernavigator-object"><span class="secno">5.2 </span>The <code>WorkerNavigator</code> object</a></li>
   <li><a href="#interface-objects-and-constructors"><span class="secno">5.3 </span>Interface objects and constructors</a></li>
   <li><a href="#worker-locations"><span class="secno">5.4 </span>Worker locations</a></ol></li>
 <li><a class="no-num" href="#references">References</a></li>
 <li><a class="no-num" href="#acknowledgements">Acknowledgements</a></ol>
<hr><h2 id="introduction"><span class="secno">1 </span>Introduction</h2><h3 id="scope"><span class="secno">1.1 </span>Scope</h3><p><i>This section is non-normative.</i><p>This specification defines an API for running scripts in the
  background independently of any user interface scripts.<p>This allows for long-running scripts that are not interrupted by
  scripts that respond to clicks or other user interactions, and
  allows long tasks to be executed without yielding to keep the page
  responsive.<p>Workers (as these background scripts are called herein) are
  relatively heavy-weight, and are not intended to be used in large
  numbers. For example, it would be inappropriate to launch one worker
  for each pixel of a four megapixel image. The examples below show
  some appropriate uses of workers.<p>Generally, workers are expected to be long-lived, have a high
  start-up performance cost, and a high per-instance memory cost.<h3 id="examples"><span class="secno">1.2 </span>Examples</h3><p><i>This section is non-normative.</i><p>There are a variety of uses that workers can be put to. The
  following subsections show various examples of this use.<h4 id="a-background-number-crunching-worker"><span class="secno">1.2.1 </span>A background number-crunching worker</h4><p><i>This section is non-normative.</i><p>The simplest use of workers is for performing a computationally
  expensive task without interrupting the user interface.<p>In this example, the main document spawns a worker to
  (na&iuml;vely) compute prime numbers, and progressively displays the
  most recently found prime number.<p>The main page is as follows:<pre>&lt;!DOCTYPE HTML&gt;
&lt;html&gt;
 &lt;head&gt;
  &lt;title&gt;Worker example: One-core computation&lt;/title&gt;
 &lt;/head&gt;
 &lt;body&gt;
  &lt;p&gt;The highest prime number discovered so far is: &lt;output id="result"&gt;&lt;/output&gt;&lt;/p&gt;
  &lt;script&gt;
   var worker = new Worker('worker.js');
   worker.onmessage = function (event) {
     document.getElementById('result').textContent = event.data;
   };
  &lt;/script&gt;
 &lt;/body&gt;
&lt;/html&gt;</pre><p>The <code title="dom-Worker"><a href="#dom-worker">Worker()</a></code> constructor call
  creates a worker and returns a <code><a href="#worker">Worker</a></code> object
  representing that worker, which is used to communicate with the
  worker. That object's <code title="handler-Worker-onmessage"><a href="#handler-worker-onmessage">onmessage</a></code> event handler allows the code to receive messages from the worker.<p>The worker itself is as follows:<pre>var n = 1;
search: while (true) {
  n += 1;
  for (var i = 2; i &lt;= Math.sqrt(n); i += 1)
    if (n % i == 0)
     continue search;
  // found a prime!
  postMessage(n);
}</pre><p>The bulk of this code is simply an unoptimized search for a prime
  number. The <code title="dom-DedicatedWorkerGlobalScope-postMessage"><a href="#dom-dedicatedworkerglobalscope-postmessage">postMessage()</a></code>
  method is used to send a message back to the page when a prime is
  found.<p><a href="http://www.whatwg.org/demos/workers/primes/page.html">View this example online</a>.<h4 id="a-worker-for-updating-a-client-side-database"><span class="secno">1.2.2 </span>A worker for updating a client-side database</h4><p><i>This section is non-normative.</i><p>In this example, the main document spawns a worker whose only
  task is to listen for notifications from the server, and, when
  appropriate, either add or remove data from the client-side
  database.<p>Since no communication occurs between the worker and the main
  page, the main page can start the worker by just doing:<pre>&lt;script&gt;
 new Worker('worker.js');
&lt;/script&gt;</pre><p>The worker itself is as follows:<pre>var server = new WebSocket('ws://whatwg.org/database');
var database = openDatabase('demobase', '1.0', 'Demo Database', 10240);
server.onmessage = function (event) {
  // data is in the format "command key value"
  var data = event.data.split(' ');
  switch (data[0]) {
    case '+':
     database.transaction(function(tx) {
       tx.executeSql('INSERT INTO pairs (key, value) VALUES (?, ?)', data[1], data[2]);
     });
    case '-':
     database.transaction(function(tx) {
       tx.executeSql('DELETE FROM pairs WHERE key=? AND value=?', data[1], data[2]);
     });
  }
};</pre><p>This connects to the server using the <code>WebSocket</code>
  mechanism and opens the local database (which, we presume, has been
  created earlier). The worker then just listens for messages from the
  server and acts on them as appropriate, forever (or until the main
  page is closed).<p><a href="http://www.whatwg.org/demos/workers/database-updater/page.html">View
  this example online</a>. (This example will not actually function,
  since the server does not actually exist and the database is not
  created by this sample code.)<h4 id="worker-used-for-background-i-o"><span class="secno">1.2.3 </span>Worker used for background I/O</h4><p><i>This section is non-normative.</i><p>In this example, the main document uses two workers, one for
  fetching stock updates for at regular intervals, and one for
  fetching performing search queries that the user requests.<p>The main page is as follows:<pre>&lt;!DOCTYPE HTML&gt;
&lt;html&gt;
 &lt;head&gt;
  &lt;title&gt;Worker example: Stock ticker&lt;/title&gt;
  &lt;script&gt;
   // TICKER
   var symbol = 'GOOG'; // default symbol to watch
   var ticker = new Worker('ticker.js');

   // SEARCHER
   var searcher = new Worker('searcher.js');
   function search(query) {
     searcher.postMessage(query);
   }

   // SYMBOL SELECTION UI
   function select(newSymbol) {
     symbol = newSymbol;
     ticker.postMessage(symbol);
   }
  &lt;/script&gt;
 &lt;/head&gt;
 &lt;body onload="search('')"&gt;
  &lt;p&gt;&lt;output id="symbol"&gt;&lt;/output&gt; &lt;output id="value"&gt;&lt;/output&gt;&lt;/p&gt;
  &lt;script&gt;
   ticker.onmessage = function (event) {
     var data = event.data.split(' ');
     document.getElementById('symbol').textContent = data[0];
     document.getElementById('value').textContent = data[1];
   };
   ticker.postMessage(symbol);
  &lt;/script&gt;
  &lt;p&gt;&lt;label&gt;Search: &lt;input type="text" autofocus oninput="search(this.value)"&gt;&lt;/label&gt;&lt;/p&gt;
  &lt;ul id="results"&gt;&lt;/ul&gt;
  &lt;script&gt;
   searcher.onmessage = function (event) {
     var data = event.data.split(' ');
     var results = document.getElementById('results');
     while (results.hasChildNodes()) // clear previous results
       results.removeChild(results.firstChild);
     for (var i = 0; i &lt; data.length; i += 1) {
       // add a list item with a button for each result
       var li = document.createElement('li');
       var button = document.createElement('button');
       button.value = data[i];
       button.type = 'button';
       button.onclick = function () { select(this.value); };
       button.textContent = data[i];
       li.appendChild(button);
       results.appendChild(li);
     }
   };
  &lt;/script&gt;
  &lt;p&gt;(The data in this example is not real. Try searching for "Google" or "Apple".)&lt;/p&gt;
 &lt;/body&gt;
&lt;/html&gt;</pre><p>The two workers use a common library for performing the actual
  network calls. This library is as follows:<pre>function get(url) {
  try {
    var xhr = new XMLHttpRequest();
    xhr.open('GET', url, false);
    xhr.send();
    return xhr.responseText;
  } catch (e) {
    return ''; // turn all errors into empty results
  }
}</pre><p>The stock updater worker is as follows:<pre>importScripts('io.js');
var timer;
var symbol;
function update() {
  postMessage(symbol + ' ' + get('stock.cgi?' + symbol));
  timer = setTimeout(update, 10000);
}
onmessage = function (event) {
  if (timer)
    clearTimeout(timer);
  symbol = event.data;
  update();
};</pre><p>The search query worker is as follows:<pre>importScripts('io.js');
onmessage = function (event) {
  postMessage(get('search.cgi?' + event.data));
};</pre><p><a href="http://www.whatwg.org/demos/workers/stocks/page.html">View this example online</a>.<h4 id="shared-workers-introduction"><span class="secno">1.2.4 </span>Shared workers introduction</h4><p><i>This section is non-normative.</i><p>This section introduces shared workers using a Hello World
  example. Shared workers use slightly different APIs, since each
  worker can have multiple connections.<p>This first example shows how you connect to a worker and how a
  worker can send a message back to the page when it connects to
  it. Received messages are displayed in a log.<p>Here is the HTML page:<pre>&lt;!DOCTYPE HTML&gt;
&lt;title&gt;Shared workers: demo 1&lt;/title&gt;
&lt;pre id="log"&gt;Log:&lt;/pre&gt;
&lt;script&gt;
  var worker = new SharedWorker('test.js');
  var log = document.getElementById('log');
  worker.port.onmessage = function(e) { // note: not worker.onmessage!
    log.textContent += '\n' + e.data;
  }
&lt;/script&gt;
</pre><p>Here is the JavaScript worker:<pre>onconnect = function(e) {
  var port = e.ports[0];
  port.postMessage('Hello World!');
}
</pre><p><a href="http://www.whatwg.org/demos/workers/shared/001/test.html">View this example online</a>.<hr><p>This second example extends the first one by changing two things:
  first, messages are received using <code title="">addEventListener()</code> instead of an <span title="event
  handler IDL attributes">event handler IDL attribute</span>, and
  second, a message is sent <em>to</em> the worker, causing the worker
  to send another message in return. Received messages are again
  displayed in a log.<p>Here is the HTML page:<pre>&lt;!DOCTYPE HTML&gt;
&lt;title&gt;Shared workers: demo 2&lt;/title&gt;
&lt;pre id="log"&gt;Log:&lt;/pre&gt;
&lt;script&gt;
  var worker = new SharedWorker('test.js');
  var log = document.getElementById('log');
  worker.port.addEventListener('message', function(e) {
    log.textContent += '\n' + e.data;
  }, false);
  worker.port.start(); // note: need this when using addEventListener
  worker.port.postMessage('ping');
&lt;/script&gt;
</pre><p>Here is the JavaScript worker:<pre>onconnect = function(e) {
  var port = e.ports[0];
  port.postMessage('Hello World!');
  port.onmessage = function(e) {
    port.postMessage('pong'); // not e.ports[0].postMessage!
    // e.target.postMessage('pong'); would work also
  }
}
</pre><p><a href="http://www.whatwg.org/demos/workers/shared/002/test.html">View this example online</a>.<hr><p>Finally, the example is extended to show how two pages can
  connect to the same worker; in this case, the second page is merely
  in an <code>iframe</code> on the first page, but the same principle
  would apply to an entirely separate page in a separate
  <span>top-level browsing context</span>.<p>Here is the outer HTML page:<pre>&lt;!DOCTYPE HTML&gt;
&lt;title&gt;Shared workers: demo 3&lt;/title&gt;
&lt;pre id="log"&gt;Log:&lt;/pre&gt;
&lt;script&gt;
  var worker = new SharedWorker('test.js');
  var log = document.getElementById('log');
  worker.port.addEventListener('message', function(e) {
    log.textContent += '\n' + e.data;
  }, false);
  worker.port.start();
  worker.port.postMessage('ping');
&lt;/script&gt;
&lt;iframe src="inner.html"&gt;&lt;/iframe&gt;
</pre><p>Here is the inner HTML page:<pre>&lt;!DOCTYPE HTML&gt;
&lt;title&gt;Shared workers: demo 3 inner frame&lt;/title&gt;
&lt;pre id=log&gt;Inner log:&lt;/pre&gt;
&lt;script&gt;
  var worker = new SharedWorker('test.js');
  var log = document.getElementById('log');
  worker.port.onmessage = function(e) {
   log.textContent += '\n' + e.data;
  }
&lt;/script&gt;
</pre><p>Here is the JavaScript worker:<pre>var count = 0;
onconnect = function(e) {
  count += 1;
  var port = e.ports[0];
  port.postMessage('Hello World! You are connection #' + count);
  port.onmessage = function(e) {
    port.postMessage('pong');
  }
}
</pre><p><a href="http://www.whatwg.org/demos/workers/shared/003/test.html">View this example online</a>.<h4 id="shared-state-using-a-shared-worker"><span class="secno">1.2.5 </span>Shared state using a shared worker</h4><p><i>This section is non-normative.</i><p>In this example, multiple windows (viewers) can be opened that
  are all viewing the same map. All the windows share the same map
  information, with a single worker coordinating all the viewers. Each
  viewer can move around independently, but if they set any data on
  the map, all the viewers are updated.<p>The main page isn't interesting, it merely provides a way to open
  the viewers:<pre>&lt;!DOCTYPE HTML&gt;
&lt;html&gt;
 &lt;head&gt;
  &lt;title&gt;Workers example: Multiviewer&lt;/title&gt;
  &lt;script&gt;
   function openViewer() {
     window.open('viewer.html');
   }
  &lt;/script&gt;
 &lt;/head&gt;
 &lt;body&gt;
  &lt;p&gt;&lt;button type=button onclick="openViewer()"&gt;Open a new
  viewer&lt;/button&gt;&lt;/p&gt;
  &lt;p&gt;Each viewer opens in a new window. You can have as many viewers
  as you like, they all view the same data.&lt;/p&gt;
 &lt;/body&gt;
&lt;/html&gt;</pre><p>The viewer is more involved:<pre>&lt;!DOCTYPE HTML&gt;
&lt;html&gt;
 &lt;head&gt;
  &lt;title&gt;Workers example: Multiviewer viewer&lt;/title&gt;
  &lt;script&gt;
   var worker = new SharedWorker('worker.js', 'core');

   // CONFIGURATION
   function configure(event) {
     if (event.data.substr(0, 4) != 'cfg ') return;
     var name = event.data.substr(4).split(' ', 1);
     // update display to mention our name is name
     document.getElementsByTagName('h1')[0].textContent += ' ' + name;
     // no longer need this listener
     worker.port.removeEventListener('message', configure, false);
   }
   worker.port.addEventListener('message', configure, false);

   // MAP
   function paintMap(event) {
     if (event.data.substr(0, 4) != 'map ') return;
     var data = event.data.substr(4).split(',');
     // display tiles data[0] .. data[8]
     var canvas = document.getElementById('map');
     var context = canvas.getContext('2d');
     for (var y = 0; y &lt; 3; y += 1) {
       for (var x = 0; x &lt; 3; x += 1) {
         var tile = data[y * 3 + x];
         if (tile == '0')
           context.fillStyle = 'green';
         else 
           context.fillStyle = 'maroon';
         fillRect(x * 50, y * 50, 50, 50);
       }
     }
   }
   worker.port.addEventListener('message', paintMap, false);

   // PUBLIC CHAT
   function updatePublicChat(event) {
     if (event.data.substr(0, 4) != 'txt ') return;
     var name = event.data.substr(4).split(' ', 1);
     var message = event.data.substr(4 + length(name) + 1);
     // display "&lt;name&gt; message" in public chat
     var dialog = document.getElementById('public');
     var dt = document.createElement('dt');
     dt.textContent = name;
     dialog.appendChild(dt);
     var dd = document.createElement('dd');
     dd.textContent = message;
     dialog.appendChild(dd);
   }
   worker.port.addEventListener('message', updatePublicChat, false);

   // PRIVATE CHAT
   function startPrivateChat(event) {
     if (event.data.substr(0, 4) != 'msg ') return;
     var name = event.data.substr(4).split(' ', 1);
     var port = event.ports[0];
     // display a private chat UI
     var ul = document.getElementById('private');
     var li = document.createElement('li');
     var h3 = document.createElement('h3');
     h3.textContent = 'Private chat with ' + name;
     li.appendChild(h3);
     var dialog = document.createElement('dialog');
     var addMessage = function(name, message) {
       var dt = document.createElement('dt');
       dt.textContent = name;
       dialog.appendChild(dt);
       var dd = document.createElement('dd');
       dd.textContent = message;
       dialog.appendChild(dd);
     };
     port.onmessage = function (event) {
       addMessage(name, event.data);
     };
     li.appendChild(dialog);
     var form = document.createElement('form');
     var p = document.createElement('p');
     var input = document.createElement('input');
     input.size = 50;
     p.appendChild(input);
     p.appendChild(document.createTextNode(' '));
     var button = document.createElement('button');
     button.textContent = 'Post';
     p.appendChild(button);
     form.onsubmit = function () {
       port.postMessage(input.value);
       addMessage('me', input.value);
       input.value = '';
       return false;
     };
     form.appendChild(p);
     li.appendChild(form);
   }
   worker.port.addEventListener('message', startPrivateChat, false);

   worker.port.start();
  &lt;/script&gt;
 &lt;/head&gt;
 &lt;body&gt;
  &lt;h1&gt;Viewer&lt;/h1&gt;
  &lt;h2&gt;Map&lt;/h2&gt;
  &lt;p&gt;&lt;canvas id="map" height=150 width=150&gt;&lt;/canvas&gt;&lt;/p&gt;
  &lt;p&gt;
   &lt;button type=button onclick="worker.port.postMessage('mov left')"&gt;Left&lt;/button&gt;
   &lt;button type=button onclick="worker.port.postMessage('mov up')"&gt;Up&lt;/button&gt;
   &lt;button type=button onclick="worker.port.postMessage('mov down')"&gt;Down&lt;/button&gt;
   &lt;button type=button onclick="worker.port.postMessage('mov right')"&gt;Right&lt;/button&gt;
   &lt;button type=button onclick="worker.port.postMessage('set 0')"&gt;Set 0&lt;/button&gt;
   &lt;button type=button onclick="worker.port.postMessage('set 1')"&gt;Set 1&lt;/button&gt;
  &lt;/p&gt;
  &lt;h2&gt;Public Chat&lt;/h2&gt;
  &lt;dialog id="public"&gt;&lt;/dialog&gt;
  &lt;form onsubmit="worker.port.postMessage('txt ' + message.value); message.value = ''; return false;"&gt;
   &lt;p&gt;
    &lt;input type="text" name="message" size="50"&gt;
    &lt;button&gt;Post&lt;/button&gt;
   &lt;/p&gt;
  &lt;/form&gt;
  &lt;h2&gt;Private Chat&lt;/h2&gt;
  &lt;ul id="private"&gt;&lt;/ul&gt;
 &lt;/body&gt;
&lt;/html&gt;
</pre><p>There are several key things worth noting about the way the
  viewer is written.<p><strong>Multiple listeners</strong>. Instead of a single message
  processing function, the code here attaches multiple event
  listeners, each one performing a quick check to see if it is
  relevant for the message. In this example it doesn't make much
  difference, but if multiple authors wanted to collaborate using a
  single port to communicate with a worker, it would allow for
  independent code instead of changes having to all be made to a
  single event handling function.<p>Registering event listeners in this way also allows you to
  unregister specific listeners when you are done with them, as is
  done with the <code title="">configure()</code> method in this
  example.<p>Finally, the worker:<pre>
var nextName = 0;
function getNextName() {
  // this could use more friendly names
  // but for now just return a number
  return nextName++;
}

var map = [
 [0, 0, 0, 0, 0, 0, 0],
 [1, 1, 0, 1, 0, 1, 1],
 [0, 1, 0, 1, 0, 0, 0],
 [0, 1, 0, 1, 0, 1, 1],
 [0, 0, 0, 1, 0, 0, 0],
 [1, 0, 0, 1, 1, 1, 1],
 [1, 1, 0, 1, 1, 0, 1],
];

function wrapX(x) {
  if (x &lt; 0) return wrapX(x + map[0].length);
  if (x &gt;= map[0].length) return wrapX(x - map[0].length);
  return x;
}

function wrapY(y) {
  if (y &lt; 0) return wrapY(y + map.length);
  if (y &gt;= map[0].length) return wrapY(y - map.length);
  return y;
}

function sendMapData(callback) {
  var data = '';
  for (var y = viewer.y-1; y &lt;= viewer.y+1; y += 1) {
    for (var x = viewer.x-1; x &lt;= viewer.x+1; x += 1) {
      if (data != '')
        data += ',';
      data += map[y][x];
    }
  }
  callback('map ' + data);
}

var viewers = {};
onconnect = function (event) {
  event.ports[0]._name = getNextName();
  event.ports[0]._data = { port: event.port, x: 0, y: 0, };
  viewers[event.ports[0]._name] = event.port._data;
  event.ports[0].postMessage('cfg ' + name);
  event.ports[0].onmessage = getMessage;
  sendMapData(event.ports[0].postMessage);
};

function getMessage(event) {
  switch (event.data.substr(0, 4)) {
    case 'mov ':
      var direction = event.data.substr(4);
      var dx = 0;
      var dy = 0;
      switch (direction) {
        case 'up': dy = -1; break;
        case 'down': dy = 1; break;
        case 'left': dx = -1; break;
        case 'right': dx = 1; break;
      }
      event.target._data.x = wrapX(event.target._data.x + dx);
      event.target._data.y = wrapY(event.target._data.y + dy);
      sendMapData(event.target.postMessage);
      break;
    case 'set ':
      var value = event.data.substr(4);
      map[event.target._data.y][event.target._data.x] = value;
      for (var viewer in viewers)
        sendMapData(viewers[viewer].port.postMessage);
      break;
    case 'txt ':
      var name = event.target._name;
      var message = event.data.substr(4);
      for (var viewer in viewers)
        viewers[viewer].port.postMessage('txt ' + name + ' ' + message);
      break;
    case 'msg ':
      var party1 = event._data;
      var party2 = viewers[event.data.substr(4).split(' ', 1)];
      if (party2) {
        var channel = new MessageChannel();
        party1.port.postMessage('msg ' + party2.name, [channel.port1]);
        party2.port.postMessage('msg ' + party1.name, [channel.port2]);
      }
      break;
  }
}</pre><p><strong>Connecting to multiple pages</strong>. The script uses
  the <code title="handler-SharedWorkerGlobalScope-onconnect"><a href="#handler-sharedworkerglobalscope-onconnect">onconnect</a></code>
  event listener to listen for multiple connections.<p><strong>Direct channels</strong>. When the worker receives a
  "msg" message from one viewer naming another viewer, it sets up a
  direct connection between the two, so that the two viewers can
  communicate directly without the worker having to proxy all the
  messages.<p><a href="http://www.whatwg.org/demos/workers/multiviewer/page.html">View this example online</a>.<h4 id="delegation"><span class="secno">1.2.6 </span>Delegation</h4><p><i>This section is non-normative.</i><p>With multicore CPUs becoming prevalent, one way to obtain better
  performance is to split computationally expensive tasks amongst
  multiple workers. In this example, a computationally expensive task
  that is to be performed for every number from 1 to 10,000,000 is
  farmed out to ten subworkers.<p>The main page is as follows, it just reports the result:<pre>&lt;!DOCTYPE HTML&gt;
&lt;html&gt;
 &lt;head&gt;
  &lt;title&gt;Worker example: Multicore computation&lt;/title&gt;
 &lt;/head&gt;
 &lt;body&gt;
  &lt;p&gt;Result: &lt;output id="result"&gt;&lt;/output&gt;&lt;/p&gt;
  &lt;script&gt;
   var worker = new Worker('worker.js');
   worker.onmessage = function (event) {
     document.getElementById('result').textContent = event.data;
   };
  &lt;/script&gt;
 &lt;/body&gt;
&lt;/html&gt;</pre><p>The worker itself is as follows:<pre>// settings
var num_workers = 10;
var items_per_worker = 1000000;

// start the workers
var result = 0;
var pending_workers = num_workers;
for (var i = 0; i &lt; num_workers; i += 1) {
  var worker = new Worker('core.js');
  worker.postMessage(i * items_per_worker);
  worker.postMessage((i+1) * items_per_worker);
  worker.onmessage = storeResult;
}

// handle the results
function storeResult(event) {
  result += 1*event.data;
  pending_workers -= 1;
  if (pending_workers &lt;= 0)
    postMessage(result); // finished!
}</pre><p>It consists of a loop to start the subworkers, and then a handler
  that waits for all the subworkers to respond.<p>The subworkers are implemented as follows:<pre>var start;
onmessage = getStart;
function getStart(event) {
  start = 1*event.data;
  onmessage = getEnd;
}

var end;
function getEnd(event) {
  end = 1*event.data;
  onmessage = null;
  work();
}

function work() {
  var result = 0;
  for (var i = start; i &lt; end; i += 1) {
    // perform some complex calculation here
    result += 1;
  }
  postMessage(result);
  close();
}</pre><p>They receive two numbers in two events, perform the computation
  for the range of numbers thus specified, and then report the result
  back to the parent.<p><a href="http://www.whatwg.org/demos/workers/multicore/page.html">View this example online</a>.</p><h3 id="tutorials"><span class="secno">1.3 </span>Tutorials</h3><h4 id="creating-a-dedicated-worker"><span class="secno">1.3.1 </span>Creating a dedicated worker</h4><p><i>This section is non-normative.</i><p>Creating a worker requires a URL to a JavaScript file. The <code title="dom-Worker"><a href="#dom-worker">Worker()</a></code> constructor is invoked with the
  URL to that file as its only argument; a worker is then created and
  returned:<pre>var worker = new Worker('helper.js');</pre><h4 id="communicating-with-a-dedicated-worker"><span class="secno">1.3.2 </span>Communicating with a dedicated worker</h4><p><i>This section is non-normative.</i><p>Dedicated workers use <code><a href="#messageport">MessagePort</a></code> objects behind the
  scenes, and thus support all the same features, such as sending
  structured data, transferring binary data, and transferring other
  ports.<p>To receive messages from a dedicated worker, use the <code title="handler-worker-onmessage"><a href="#handler-worker-onmessage">onmessage</a></code> <span title="event
  handler IDL attributes">event handler IDL attribute</span> on the
  <code><a href="#worker">Worker</a></code> object:<pre>worker.onmessage = function (event) { ... };</pre><p>You can also use the <code title="dom-EventTarget-addEventListener">addEventListener()</code> method.<p class="note">The implicit <code><a href="#messageport">MessagePort</a></code> used by
  dedicated workers has its <span>port message queue</span> implicitly
  enabled when it is created, so there is no equivanet to the
  <code><a href="#messageport">MessagePort</a></code> interface's <code title="dom-MessagePort-start">start()</code> method on the
  <code><a href="#worker">Worker</a></code> interface.<p>To <em>send</em> data to a worker, use the <code title="dom-Worker-postMessage"><a href="#dom-worker-postmessage">postMessage()</a></code> method.
  Structured data can be sent over this communication channel. To send
  <code>ArrayBuffer</code> objects efficiently (by transferring them
  rather than cloning them), list them in an array in the second
  argument.<pre>worker.postMessage({
  operation: 'find-edges',
  input: buffer, // an ArrayBuffer object
  threshold: 0.6,
}, [buffer]);</pre><p>To receive a message inside the worker, the <code title="handler-DedicatedWorkerGlobalScope-onmessage"><a href="#handler-dedicatedworkerglobalscope-onmessage">onmessage</a></code>
  <span title="event handler IDL attributes">event handler IDL
  attribute</span> is used.<pre>onmessage = function (event) { ... };</pre><p>You can again also use the <code title="dom-EventTarget-addEventListener">addEventListener()</code>
  method.<p>In either case, the data is provided in the event object's <code title="dom-MessageEvent-data">data</code> attribute.<p>To send messages back, you again use <code title="dom-DedicatedWorkerGlobalScope-postMessage"><a href="#dom-dedicatedworkerglobalscope-postmessage">postMessage()</a></code>.
  It supports the structured data in the same manner.<pre>postMessage(event.data.input, [event.data.input]); // transfer the buffer back</pre><h4 id="shared-workers"><span class="secno">1.3.3 </span>Shared workers</h4><p><i>This section is non-normative.</i><p>Shared workers are identified in one of two ways: either by the
  URL of the script used to create it, or by explicit name. When
  created by name, the URL used by the first page to create the worker
  with that name is the URL of the script that will be used for that
  worker. This allows multiple applications on a domain to all use a
  single shared worker to provide a common service, without the
  applications having to keep track of a common URL for the script
  used to provide the service.<p class="note">In either case, shared workers are scoped by
  <span>origin</span>. Two different sites using the same names will
  not collide.<p>Creating shared workers is done using the <code title="dom-SharedWorker"><a href="#dom-sharedworker">SharedWorker()</a></code> constructor. This
  constructor takes the URL to the script to use for its first
  argument, and the name of the worker, if any, as the second
  argument.<pre>var worker = new SharedWorker('service.js');</pre><p>Communicating with shared workers is done with explicit
  <code><a href="#messageport">MessagePort</a></code> objects. The object returned by the <code title="dom-SharedWorker"><a href="#dom-sharedworker">SharedWorker()</a></code> constructor holds a
  reference to the port on its <code title="dom-SharedWorker-port"><a href="#dom-sharedworker-port">port</a></code> attribute.<pre>worker.port.onmessage = function (event) { ... };
worker.port.postMessage('some message');
worker.port.postMessage({ foo: 'structured'; bar: ['data', 'also', 'possible']});</pre><p>Inside the shared worker, new clients of the worker are announced
  using the <code title="event-connect">connect</code> event. The port
  for the new client is given by the event object's <code title="dom-messageevent-ports">ports</code> array as its first (and
  only) value.<pre>onconnect = function (event) {
  var newPort = event.ports[0];
  // set up a listener
  newPort.onmessage = function (event) { ... };
  // send a message back to the port
  newPort.postMessage('ready!'); // can also send structured data, of course
};</pre><h2 id="conformance-requirements"><span class="secno">2 </span>Conformance requirements</h2><p>All diagrams, examples, and notes in this specification are
  non-normative, as are all sections explicitly marked non-normative.
  Everything else in this specification is normative.<p>The key words "MUST", "MUST NOT", "REQUIRED",  "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
  "OPTIONAL" in the normative parts of this document are to be
  interpreted as described in RFC2119. For readability, these words do
  not appear in all uppercase letters in this specification. <a href="#refsRFC2119">[RFC2119]</a><p>Requirements phrased in the imperative as part of algorithms
  (such as "strip any leading space characters" or "return false and
  abort these steps") are to be interpreted with the meaning of the
  key word ("must", "should", "may", etc) used in introducing the
  algorithm.<p>Some conformance requirements are phrased as requirements on
  attributes, methods or objects. Such requirements are to be
  interpreted as requirements on user agents.<p>Conformance requirements phrased as algorithms or specific steps
  may be implemented in any manner, so long as the end result is
  equivalent. (In particular, the algorithms defined in this
  specification are intended to be easy to follow, and not intended to
  be performant.)<p>The only conformance class defined by this specification is user
  agents.<p>User agents may impose implementation-specific limits on
  otherwise unconstrained inputs, e.g. to prevent denial of service
  attacks, to guard against running out of memory, or to work around
  platform-specific limitations.<p>When support for a feature is disabled (e.g. as an emergency
  measure to mitigate a security problem, or to aid in development, or
  for performance reasons), user agents must act as if they had no
  support for the feature whatsoever, and as if the feature was not
  mentioned in this specification. For example, if a particular
  feature is accessed via an attribute in a Web IDL interface, the
  attribute itself would be omitted from the objects that implement
  that interface &mdash; leaving the attribute on the object but
  making it return null or throw an exception is insufficient.<h3 id="dependencies"><span class="secno">2.1 </span>Dependencies</h3><p>This specification relies on several other underlying
  specifications.<dl><dt>HTML</dt>

   <dd>

    <p>Many fundamental concepts from HTML are used by this
    specification. <a href="#refsHTML">[HTML]</a></p>

   </dd>

   <dt>WebIDL</dt>

   <dd>

    <p>The IDL blocks in this specification use the semantics of the
    WebIDL specification. <a href="#refsWEBIDL">[WEBIDL]</a></p>

   </dd>

  </dl><h2 id="terminology"><span class="secno">3 </span>Terminology</h2><p>The construction "a <code title="">Foo</code> object", where
  <code title="">Foo</code> is actually an interface, is sometimes
  used instead of the more accurate "an object implementing the
  interface <code title="">Foo</code>".<p>The term DOM is used to refer to the API set made available to
  scripts in Web applications, and does not necessarily imply the
  existence of an actual <code>Document</code> object or of any other
  <code>Node</code> objects as defined in the DOM Core
  specifications. <a href="#refsDOMCORE">[DOMCORE]</a><p>An IDL attribute is said to be <em>getting</em> when its value is
  being retrieved (e.g. by author script), and is said to be
  <em>setting</em> when a new value is assigned to it.<p>The term "JavaScript" is used to refer to ECMA262, rather than
  the official term ECMAScript, since the term JavaScript is more
  widely known. <a href="#refsECMA262">[ECMA262]</a><h2 id="infrastructure"><span class="secno">4 </span>Infrastructure</h2><p>There are two kinds of workers; dedicated workers, and shared
  workers. Dedicated workers, once created, and are linked to their
  creator; but message ports can be used to communicate from a
  dedicated worker to multiple other browsing contexts or
  workers. Shared workers, on the other hand, are named, and once
  created any script running in the same <span>origin</span> can
  obtain a reference to that worker and communicate with it.<h3 id="dependencies-0"><span class="secno">4.1 </span>Dependencies</h3><p>The <dfn id="messageport"><code>MessagePort</code></dfn> interface is defined in
  the HTML specification. <a href="#refsHTML">[HTML]</a><h3 id="the-global-scope"><span class="secno">4.2 </span>The global scope</h3><p>The global scope is the "inside" of a worker.<h4 id="the-workerglobalscope-abstract-interface"><span class="secno">4.2.1 </span>The <code><a href="#workerglobalscope">WorkerGlobalScope</a></code> abstract interface</h4><pre class="idl">interface <dfn id="workerglobalscope">WorkerGlobalScope</dfn> : <span>EventTarget</span> {
  readonly attribute <a href="#workerglobalscope">WorkerGlobalScope</a> <a href="#dom-workerglobalscope-self" title="dom-WorkerGlobalScope-self">self</a>;
  readonly attribute <a href="#workerlocation">WorkerLocation</a> <a href="#dom-workerglobalscope-location" title="dom-WorkerGlobalScope-location">location</a>;

  void <a href="#dom-workerglobalscope-close" title="dom-WorkerGlobalScope-close">close</a>();
           attribute <span>Function</span>? <a href="#handler-workerglobalscope-onerror" title="handler-WorkerGlobalScope-onerror">onerror</a>;
           attribute <span>Function</span>? <a href="#handler-workerglobalscope-onoffline" title="handler-WorkerGlobalScope-onoffline">onoffline</a>;
           attribute <span>Function</span>? <a href="#handler-workerglobalscope-ononline" title="handler-WorkerGlobalScope-ononline">ononline</a>;
};
<a href="#workerglobalscope">WorkerGlobalScope</a> implements <a href="#workerutils">WorkerUtils</a>;</pre><p>The <dfn id="dom-workerglobalscope-self" title="dom-WorkerGlobalScope-self"><code>self</code></dfn> attribute
  must return the <code><a href="#workerglobalscope">WorkerGlobalScope</a></code> object itself.<p>The <dfn id="dom-workerglobalscope-location" title="dom-WorkerGlobalScope-location"><code>location</code></dfn>
  attribute must return the <code><a href="#workerlocation">WorkerLocation</a></code> object created
  for the <code><a href="#workerglobalscope">WorkerGlobalScope</a></code> object when the worker was
  created. It represents the <span>absolute URL</span> of the script
  that was used to initialize the worker, after any redirects.<hr><p>When a script invokes the <dfn id="dom-workerglobalscope-close" title="dom-WorkerGlobalScope-close"><code>close()</code></dfn>
  method on a <code><a href="#workerglobalscope">WorkerGlobalScope</a></code> object, the user agent
  must run the following steps (atomically):<ol><li><p>Discard any <span title="concept-task">tasks</span> that
   have been added to the <span>event loop</span>'s <span title="task
   queue">task queues</span>.</p>


   <li><p>Set the worker's <code><a href="#workerglobalscope">WorkerGlobalScope</a></code> object's
   <a href="#dom-workerglobalscope-closing" title="dom-WorkerGlobalScope-closing">closing</a> flag to
   true. (This prevents any further tasks from being queued.)</li>

  </ol><p>The following are the <span>event handlers</span> (and their
  corresponding <span title="event handler event type">event handler
  event types</span>) that must be supported, as IDL attributes, by
  objects implementing the <code><a href="#workerglobalscope">WorkerGlobalScope</a></code>
  interface:<table><thead><tr><th><span title="event handlers">Event handler</span> <th><span>Event handler event type</span>
   <tbody><tr><td><dfn id="handler-workerglobalscope-onerror" title="handler-WorkerGlobalScope-onerror"><code>onerror</code></dfn> <td> <code title="event-error">error</code>
    <tr><td><dfn id="handler-workerglobalscope-onoffline" title="handler-WorkerGlobalScope-onoffline"><code>onoffline</code></dfn> <td> <code title="event-offline">offline</code> 
    <tr><td><dfn id="handler-workerglobalscope-ononline" title="handler-WorkerGlobalScope-ononline"><code>ononline</code></dfn> <td> <code title="event-online">online</code> 
  </table><hr><p>The <code><a href="#workerglobalscope">WorkerGlobalScope</a></code> interface must not exist if
  the interface's <span>relevant namespace object</span> is a
  <code>Window</code> object. <a href="#refsWEBIDL">[WEBIDL]</a><h4 id="dedicated-workers-and-the-dedicatedworkerglobalscope-interface"><span class="secno">4.2.2 </span>Dedicated workers and the <code><a href="#dedicatedworkerglobalscope">DedicatedWorkerGlobalScope</a></code> interface</h4><pre class="idl">[Supplemental, NoInterfaceObject]
interface <dfn id="dedicatedworkerglobalscope">DedicatedWorkerGlobalScope</dfn> : <a href="#workerglobalscope">WorkerGlobalScope</a> {
  void <a href="#dom-dedicatedworkerglobalscope-postmessage" title="dom-DedicatedWorkerGlobalScope-postMessage">postMessage</a>(any message, optional sequence&lt;<span>Transferable</span>&gt; transfer);
           attribute <span>Function</span>? <a href="#handler-dedicatedworkerglobalscope-onmessage" title="handler-DedicatedWorkerGlobalScope-onmessage">onmessage</a>;
};</pre><p><code><a href="#dedicatedworkerglobalscope">DedicatedWorkerGlobalScope</a></code> objects act as if they
  had an implicit <code><a href="#messageport">MessagePort</a></code> associated with them. This
  port is part of a channel that is set up when the worker is created,
  but it is not exposed. This object must never be garbage collected
  before the <code><a href="#dedicatedworkerglobalscope">DedicatedWorkerGlobalScope</a></code> object.<p>All messages received by that port must immediately be retargeted
  at the <code><a href="#dedicatedworkerglobalscope">DedicatedWorkerGlobalScope</a></code> object.<p>The <dfn id="dom-dedicatedworkerglobalscope-postmessage" title="dom-DedicatedWorkerGlobalScope-postMessage"><code>postMessage()</code></dfn>  method on
  <code><a href="#dedicatedworkerglobalscope">DedicatedWorkerGlobalScope</a></code> objects must act as if, when
  invoked, it immediately invoked the
  method of the same name on the port, with the same arguments, and
  returned the same return value.<p>The following are the <span>event handlers</span> (and their
  corresponding <span title="event handler event type">event handler
  event types</span>) that must be supported, as IDL attributes, by
  objects implementing the <code><a href="#dedicatedworkerglobalscope">DedicatedWorkerGlobalScope</a></code>
  interface:<table><thead><tr><th><span title="event handlers">Event handler</span> <th><span>Event handler event type</span>
   <tbody><tr><td><dfn id="handler-dedicatedworkerglobalscope-onmessage" title="handler-DedicatedWorkerGlobalScope-onmessage"><code>onmessage</code></dfn> <td> <code title="event-message">message</code>
  </table><p>For the purposes of the <span>application cache</span> networking
  model, a dedicated worker is an extension of the <span>cache
  host</span> from which it was created.<h4 id="shared-workers-and-the-sharedworkerglobalscope-interface"><span class="secno">4.2.3 </span>Shared workers and the <code><a href="#sharedworkerglobalscope">SharedWorkerGlobalScope</a></code> interface</h4><pre class="idl">[Supplemental, NoInterfaceObject]
interface <dfn id="sharedworkerglobalscope">SharedWorkerGlobalScope</dfn> : <a href="#workerglobalscope">WorkerGlobalScope</a> {
  readonly attribute DOMString <a href="#dom-sharedworkerglobalscope-name" title="dom-SharedWorkerGlobalScope-name">name</a>;
  readonly attribute <span>ApplicationCache</span> <span title="dom-SharedWorkerGlobalScope-applicationCache">applicationCache</span>;
           attribute <span>Function</span>? <a href="#handler-sharedworkerglobalscope-onconnect" title="handler-SharedWorkerGlobalScope-onconnect">onconnect</a>;
};</pre><p>Shared workers receive message ports through <code title="event-WorkerGlobalScope-connect">connect</code> events on
  their global object for each connection.<p>The <dfn id="dom-sharedworkerglobalscope-name" title="dom-SharedWorkerGlobalScope-name"><code>name</code></dfn>
  attribute must return the value it was assigned when the
  <code><a href="#sharedworkerglobalscope">SharedWorkerGlobalScope</a></code> object was created by the
  "<a href="#run-a-worker">run a worker</a>" algorithm. Its value represents the name
  that can be used to obtain a reference to the worker using the
  <code><a href="#sharedworker">SharedWorker</a></code> constructor.<p>The following are the <span>event handlers</span> (and their
  corresponding <span title="event handler event type">event handler
  event types</span>) that must be supported, as IDL attributes, by
  objects implementing the <code><a href="#sharedworkerglobalscope">SharedWorkerGlobalScope</a></code>
  interface:<table><thead><tr><th><span title="event handlers">Event handler</span> <th><span>Event handler event type</span>
   <tbody><tr><td><dfn id="handler-sharedworkerglobalscope-onconnect" title="handler-SharedWorkerGlobalScope-onconnect"><code>onconnect</code></dfn> <td> <code title="event-connect">connect</code>
  </table><p>For the purposes of the <span>application cache</span> networking
  model, a shared worker is its own <span>cache host</span>. The
  <a href="#run-a-worker">run a worker</a> algorithm takes care of associating the
  worker with an <span>application cache</span>.<p class="note">The <code title="dom-SharedWorkerGlobalScope-applicationCache">applicationCache</code>
  attribute returns the <code>ApplicationCache</code> object for the
  worker.</p><h3 id="origins-of-workers"><span class="secno">4.3 </span>Origins of workers</h3><p>Both the <span>origin</span> and <span>effective script
  origin</span> of scripts running in workers are the
  <span>origin</span> of the <span>absolute URL</span> given in that
  the worker's <code title="dom-WorkerGlobalScope-location"><a href="#dom-workerglobalscope-location">location</a></code> attribute
  represents.<h3 id="the-event-loop"><span class="secno">4.4 </span>The event loop</h3><p>Each <code><a href="#workerglobalscope">WorkerGlobalScope</a></code> object has an <span>event
  loop</span> distinct from those defined for <span title="unit of
  related similar-origin browsing contexts">units of related
  similar-origin browsing contexts</span>. This <span>event
  loop</span> has no associated <span>browsing context</span>, and its
  <span title="task queue">task queues</span> only have events,
  callbacks, and networking activity as <span title="concept-task">tasks</span>. The processing model of these
  <span title="event loop">event loops</span> is defined below in the
  <a href="#run-a-worker">run a worker</a> algorithm.<p>Each <code><a href="#workerglobalscope">WorkerGlobalScope</a></code> object also has a <dfn id="dom-workerglobalscope-closing" title="dom-WorkerGlobalScope-closing">closing</dfn> flag, which must
  initially be false, but which can get set to true by the algorithms
  in the processing model section below.<p>Once the <code><a href="#workerglobalscope">WorkerGlobalScope</a></code>'s <a href="#dom-workerglobalscope-closing" title="dom-WorkerGlobalScope-closing">closing</a> flag is set to
  true, the <span>event loop</span>'s <span title="task queue">task
  queues</span> must discard any further <span title="concept-task">tasks</span> that would be added to them (tasks
  already on the queue are unaffected except where otherwise
  specified). Effectively, once the <a href="#dom-workerglobalscope-closing" title="dom-WorkerGlobalScope-closing">closing</a> flag is true,
  timers stop firing, notifications for all pending asynchronous
  operations are dropped, etc.<h3 id="the-worker-s-lifetime"><span class="secno">4.5 </span>The worker's lifetime</h3><p>Workers communicate with other workers and with <span title="browsing context">browsing contexts</span> through <span title="channel messaging">message channels</span> and their
  <code><a href="#messageport">MessagePort</a></code> objects.<p>Each <code><a href="#workerglobalscope">WorkerGlobalScope</a></code> <var title="">worker global
  scope</var> has a list of <dfn id="the-worker-s-ports">the worker's ports</dfn>, which
  consists of all the <code><a href="#messageport">MessagePort</a></code> objects that are
  entangled with another port and that have one (but only one) port
  owned by <var title="">worker global scope</var>. This list includes
   the implicit
  <code><a href="#messageport">MessagePort</a></code> in the case of <a href="#dedicatedworkerglobalscope" title="DedicatedWorkerGlobalScope">dedicated workers</a>.<p>Each <code><a href="#workerglobalscope">WorkerGlobalScope</a></code> also has a list of <dfn id="the-worker-s-workers">the
  worker's workers</dfn>. Initially this list is empty; it is
  populated when the worker creates or obtains further workers.<p>Finally, each <code><a href="#workerglobalscope">WorkerGlobalScope</a></code> also has a list of
  <dfn id="the-worker-s-documents">the worker's <code>Document</code>s</dfn>. Initially this list
  is empty; it is populated when the worker is created.<p>Whenever a <code>Document</code> <var title="">d</var> is <dfn id="add-a-document-to-the-worker-s-documents" title="add a document to the worker's documents">added to the
  worker's <code>Document</code>s</dfn>, the user agent must, for each
  worker in the list of <a href="#the-worker-s-workers">the worker's workers</a> whose list
  of <a href="#the-worker-s-documents">the worker's <code>Document</code>s</a> does not contain
  <var title="">d</var>, <a href="#add-a-document-to-the-worker-s-documents" title="add a document to the worker's
  documents">add <var title="">d</var> to <var title="">q</var>'s
  <code>WorkerGlobalScope</code> owner's list of <span>the worker's
  <code>Document</code>s</span></a>.</p><p>Whenever a <code>Document</code> object is <span title="discard a
  Document">discarded</span>, it must be removed from the list of
  <a href="#the-worker-s-documents">the worker's <code>Document</code>s</a> of each worker
  whose list contains that <code>Document</code>.<p>Given a <span>script's global object</span> <var title="">o</var>
  when creating or obtaining a worker, the <dfn id="list-of-relevant-document-objects-to-add">list of relevant
  <code>Document</code> objects to add</dfn> depends on the type of
  <var title="">o</var>. If <var title="">o</var> is a
  <code><a href="#workerglobalscope">WorkerGlobalScope</a></code> object (i.e. if we are creating a
  nested worker), then the relevant <code>Document</code>s are the
  <code>Document</code>s that are in <var title="">o</var>'s own list
  of <a href="#the-worker-s-documents">the worker's <code>Document</code>s</a>. Otherwise, <var title="">o</var> is a <code>Window</code> object, and the relevant
  <code>Document</code> is just the <code>Document</code> that is the
  <span>active document</span> of the <code>Window</code> object <var title="">o</var>.<hr><p>A worker is said to be a <dfn id="permissible-worker">permissible worker</dfn> if its
  list of <a href="#the-worker-s-documents">the worker's <code>Document</code>s</a> is not
  empty.<p>A worker is said to be a <dfn id="protected-worker">protected worker</dfn> if it is a
  <a href="#permissible-worker">permissible worker</a> and either it has outstanding
  timers, database transactions, or network connections, or its list
  of <a href="#the-worker-s-ports">the worker's ports</a> is not empty, or its
  <code><a href="#workerglobalscope">WorkerGlobalScope</a></code> is actually a
  <code><a href="#sharedworkerglobalscope">SharedWorkerGlobalScope</a></code> object (i.e. the worker is a
  shared worker).<p>A worker is said to be an <dfn id="active-needed-worker">active needed worker</dfn> if any
  of the <code>Document</code> objects in <a href="#the-worker-s-documents">the worker's
  <code>Document</code>s</a> are <span>fully active</span>.<p>A worker is said to be a <dfn id="suspendable-worker">suspendable worker</dfn> if it is
  not an <a href="#active-needed-worker">active needed worker</a> but it is a
  <a href="#permissible-worker">permissible worker</a>.<h3 id="processing-model"><span class="secno">4.6 </span>Processing model</h3><p>When a user agent is to <dfn id="run-a-worker">run a worker</dfn> for a script with
  <span>URL</span> <var title="">url</var>, a <span>browsing
  context</span> <var title="">owner browsing context</var>, a
  <code>Document</code> <var title="">owner document</var>, an
  <span>origin</span> <var title="">owner origin</var>, and with
  global scope <var title="">worker global scope</var>, it must run
  the following steps:<ol><li>

    <p>Create a separate parallel execution environment (i.e. a
    separate thread or process or equivalent construct), and run the
    rest of these steps asynchronously in that context.</p>

   </li>

   <li><p>If <var title="">worker global scope</var> is actually a
   <code><a href="#sharedworkerglobalscope">SharedWorkerGlobalScope</a></code> object (i.e. the worker is a
   shared worker), and there are any <span title="relevant application
   cache">relevant application caches</span> that are identified by a
   manifest URL with the <span>same origin</span> as <var title="">url</var> and that have <var title="">url</var> as one of
   their entries, <em>not</em> excluding entries marked as <span title="concept-appcache-foreign">foreign</span>, then associate the
   <var title="">worker global scope</var> with the <span title="concept-appcache-selection">most appropriate application
   cache</span> of those that match.</li>

   <li>

    <p>Attempt to <span>fetch</span> the resource identified by <var title="">url</var>, from the <var title="">owner origin</var>,
    with the <i>synchronous flag</i> set and the <i>force same-origin
    flag</i> set.</p> 
    <p>If the attempt fails, then for each <code><a href="#worker">Worker</a></code> or
    <code><a href="#sharedworker">SharedWorker</a></code> object associated with <var title="">worker global scope</var>, <span>queue a task</span> to
    <span>fire a simple event</span> named <code title="event-error">error</code> at that object. Abort these
    steps.</p>

    <p>If the attempt succeeds, then let <var title="">source</var> be
    the script resource <span>decoded as UTF-8, with error
    handling</span>.
    
    <a href="#refsHTML">[HTML]</a>
    
    </p>

    <p>Let <var title="">language</var> be JavaScript.</p>

    <p class="note">As with <code>script</code> elements, the MIME
    type of the script is ignored. Unlike with <code>script</code>
    elements, there is no way to override the type. It's always
    assumed to be JavaScript.</p>

   </li>

   <li>

    <p>A new <span title="concept-script">script</span> is now
    created, as follows.</p>

    <p>Create a new <span>script execution environment</span>
    set up as appropriate for the scripting language <var title="">language</var>.</p>

    <p>Parse/compile/initialize <var title="">source</var> using that
    <span>script execution environment</span>, as appropriate for <var title="">language</var>, and thus obtain a <span>list of code
    entry-points</span>; set the <i>initial code entry-point</i> to
    the entry-point for any executable code to be immediately run.</p>

    <p>Set the <span>script's global object</span> to <var title="">worker global scope</var>.</p>

    <p>Set the <span>script's browsing context</span> to <var title="">owner browsing context</var>.</p>

    <p>Set the <span>script's document</span> to <var title="">owner
    document</var>.</p>

    <p>Set the <span>script's URL character encoding</span> to
    UTF-8. (This is just used for encoding non-ASCII characters in the
    query component of URLs.)</p>

    <p>Set the <span>script's base URL</span> to <var title="">url</var>.</p>

   </li>

   <li>

    <p><strong>Closing orphan workers</strong>: Start monitoring the
    worker such that no sooner than it stops being either a
    <a href="#protected-worker">protected worker</a> or a <a href="#suspendable-worker">suspendable
    worker</a>, and no later than it stops being a
    <a href="#permissible-worker">permissible worker</a>, <var title="">worker global
    scope</var>'s <a href="#dom-workerglobalscope-closing" title="dom-WorkerGlobalScope-closing">closing</a> flag is set
    to true.</p>

   </li>

   <li>

    <p><strong>Suspending workers</strong>: Start monitoring the
    worker, such that whenever <var title="">worker global
    scope</var>'s <a href="#dom-workerglobalscope-closing" title="dom-WorkerGlobalScope-closing">closing</a> flag is false
    and the worker is a <a href="#suspendable-worker">suspendable worker</a>, the user
    agent suspends execution of script in that worker until such time
    as either the <a href="#dom-workerglobalscope-closing" title="dom-WorkerGlobalScope-closing">closing</a> flag switches
    to true or the worker stops being a <a href="#suspendable-worker">suspendable
    worker</a>.</p>

   </li>

   <li>

    <p><span title="jump to a code entry-point">Jump</span> to the
    <span title="concept-script">script</span>'s <i>initial code
    entry-point</i>, and let that run until it either returns, fails
    to catch an exception, or gets prematurely aborted by the
    "<a href="#kill-a-worker">kill a worker</a>" or "<a href="#terminate-a-worker">terminate a worker</a>"
    algorithms defined below.</p>


   </li>

   <li><p>If <var title="">worker global scope</var> is actually a
   <code><a href="#dedicatedworkerglobalscope">DedicatedWorkerGlobalScope</a></code> object (i.e. the worker is
   a dedicated worker), then enable the <span>port message
   queue</span> of the worker's implicit port.</li>

   <li>

    <p><i title="">Event loop</i>: Wait until either there is a <span title="concept-task">task</span> in one of the <span>event
    loop</span>'s <span title="task queue">task queues</span> or <var title="">worker global scope</var>'s <a href="#dom-workerglobalscope-closing" title="dom-WorkerGlobalScope-closing">closing</a> flag is set
    to true.</p>

   </li>

   <li>

    <p>Run the oldest task on one of the <span>event loop</span>'s
    <span title="task queue">task queues</span>, if any. The user
    agent may pick any <span>task queue</span>.</p>

    <p class="note">The handling of events or the execution of
    callbacks might get prematurely aborted by the "<a href="#kill-a-worker">kill a
    worker</a>" or "<a href="#terminate-a-worker">terminate a worker</a>" algorithms
    defined below.</p>

   </li>

   <li>

    <p>If the <span>storage mutex</span> is now owned by the worker's
    <span>event loop</span>, release it so that it is once again
    free.</p>

   </li>

   <li>

    <p>Remove the task just run in the earlier step, if any, from its
    <span>task queue</span>.</p>

   </li>

   <li>

    <p>If there are any more events in the <span>event loop</span>'s
    <span title="task queue">task queues</span> or if <var title="">worker global scope</var>'s <a href="#dom-workerglobalscope-closing" title="dom-WorkerGlobalScope-closing">closing</a> flag is set
    to false, then jump back to the step above labeled <i>event
    loop</i>.</p>

   </li>

   <li>

    <p>Empty the <var title="">worker global scope</var>'s <span>list
    of active timeouts</span> and its <span>list of active
    intervals</span>.</p>

   </li>

   <li>

    <p>Disentangle all the ports in the list of <a href="#the-worker-s-ports">the worker's
    ports</a>.</p>

   </li>

  </ol><hr><p>When a user agent is to <dfn id="kill-a-worker">kill a worker</dfn> it must
  run the following steps in parallel with the worker's main loop (the
  "<a href="#run-a-worker">run a worker</a>" processing model defined above):<ol><li><p>Set the worker's <code><a href="#workerglobalscope">WorkerGlobalScope</a></code> object's <a href="#dom-workerglobalscope-closing" title="dom-WorkerGlobalScope-closing">closing</a> flag to
   true.</li>


   <li><p>If there are any <span title="concept-task">tasks</span>
   queued in the <span>event loop</span>'s <span title="task
   queue">task queues</span>, discard them without processing them.</li>


   <li><p>Wait a user-agent-defined amount of time.</li>

   <li><p>Abort the script currently running in the worker.</li>

  </ol><p>User agents may invoke the "<a href="#kill-a-worker">kill a worker</a>"
  processing model on a worker at any time, e.g. in response to user
  requests, in response to CPU quota management, or when a worker
  stops being an <a href="#active-needed-worker">active needed worker</a> if the worker
  continues executing even after its <a href="#dom-workerglobalscope-closing" title="dom-WorkerGlobalScope-closing">closing</a> flag was
  set to true.<hr><p>When a user agent is to <dfn id="terminate-a-worker">terminate a worker</dfn> it must run
  the following steps in parallel with the worker's main loop (the
  "<a href="#run-a-worker">run a worker</a>" processing model defined above):<ol><li><p>Set the worker's <code><a href="#workerglobalscope">WorkerGlobalScope</a></code> object's
   <a href="#dom-workerglobalscope-closing" title="dom-WorkerGlobalScope-closing">closing</a> flag to
   true.</li>

   <li><p>If there are any <span title="concept-task">tasks</span>
   queued in the <span>event loop</span>'s <span title="task
   queue">task queues</span>, discard them without processing
   them.</li>

   <li><p>Abort the script currently running in the worker.</li>

   <li><p>If the worker's <code><a href="#workerglobalscope">WorkerGlobalScope</a></code> object is
   actually a <code><a href="#dedicatedworkerglobalscope">DedicatedWorkerGlobalScope</a></code> object (i.e. the
   worker is a dedicated worker), then empty the <span>port message
   queue</span> of the port that the worker's implicit port is
   entangled with.</li>

  </ol><hr><p>The <span>task source</span> for the tasks mentioned above is the
  <span>DOM manipulation task source</span>.<h3 id="runtime-script-errors"><span class="secno">4.7 </span>Runtime script errors</h3><p>Whenever an uncaught runtime script error occurs in one of the
  worker's scripts, if the error did not occur while handling a
  previous script error, the user agent must <span>report the
  error</span> using the <code><a href="#workerglobalscope">WorkerGlobalScope</a></code> object's <code title="handler-WorkerGlobalScope-onerror"><a href="#handler-workerglobalscope-onerror">onerror</a></code>
  attribute.
  
  <a href="#refsHTML">[HTML]</a>
  
  <p>For shared workers, if the error is still <i title="concept-error-nothandled">not handled</i> afterwards, or if
  the error occurred while handling a previous script error, the error
  may be reported to the user.
  
  <a href="#refsHTML">[HTML]</a>
  
  <p>For dedicated workers, if the error is still <i title="concept-error-nothandled">not handled</i> afterwards, or if
  the error occurred while handling a previous script error, the user
  agent must <span>queue a task</span> to fire an event that uses the
  <code><a href="#errorevent">ErrorEvent</a></code> interface, with the name <code title="event-error">error</code>, that doesn't bubble and is
  cancelable, with its <code title="dom-ErrorEvent-message"><a href="#dom-errorevent-message">message</a></code>, <code title="dom-ErrorEvent-filename"><a href="#dom-errorevent-filename">filename</a></code>, and <code title="dom-ErrorEvent-lineno"><a href="#dom-errorevent-lineno">lineno</a></code> attributes set
  appropriately, at the <code><a href="#worker">Worker</a></code> object associated with the
  worker. If the event is not canceled, the user agent must act as if the
  uncaught runtime script error had occurred in the global scope that
  the <code><a href="#worker">Worker</a></code> object is in, thus repeating the entire
  runtime script error reporting process one level up.<p>If the implicit port connecting the worker to its
  <code><a href="#worker">Worker</a></code> object has been disentangled (i.e. if the parent
  worker has been terminated), then the user agent must act as if the
  <code><a href="#worker">Worker</a></code> object had no <code title="event-error">error</code> event handler and as if that
  worker's <code title="handler-WorkerGlobalScope-onerror"><a href="#handler-workerglobalscope-onerror">onerror</a></code> attribute
  was null, but must otherwise act as described above.<p class="note">Thus, error reports proagate up to the chain of
  dedicated workers up to the original <code>Document</code>, even if
  some of the workers along this chain have been terminated and
  garbage collected.<p>The <span>task source</span> for the task mentioned above is the
  <span>DOM manipulation task source</span>.<hr><pre class="idl">interface <dfn id="errorevent">ErrorEvent</dfn> : <span>Event</span> {
  readonly attribute DOMString <a href="#dom-errorevent-message" title="dom-ErrorEvent-message">message</a>;
  readonly attribute DOMString <a href="#dom-errorevent-filename" title="dom-ErrorEvent-filename">filename</a>;
  readonly attribute unsigned long <a href="#dom-errorevent-lineno" title="dom-ErrorEvent-lineno">lineno</a>;
  void <a href="#dom-errorevent-initerrorevent" title="dom-ErrorEvent-initErrorEvent">initErrorEvent</a>(DOMString typeArg, boolean canBubbleArg, boolean cancelableArg, DOMString messageArg, DOMString filenameArg, unsigned long linenoArg);
};</pre><p>The <dfn id="dom-errorevent-initerrorevent" title="dom-ErrorEvent-initErrorEvent"><code>initErrorEvent()</code></dfn>
  method must initialize the event in a manner analogous to the
  similarly-named method in the DOM Events interfaces. <a href="#refsDOMEVENTS">[DOMEVENTS]</a><p>The <dfn id="dom-errorevent-message" title="dom-ErrorEvent-message"><code>message</code></dfn>
  attribute represents the error message.<p>The <dfn id="dom-errorevent-filename" title="dom-ErrorEvent-filename"><code>filename</code></dfn>
  attribute represents the <span>absolute URL</span> of the script in
  which the error originally occurred.<p>The <dfn id="dom-errorevent-lineno" title="dom-ErrorEvent-lineno"><code>lineno</code></dfn>
  attribute represents the line number where the error occurred in the
  script.<h3 id="creating-workers"><span class="secno">4.8 </span>Creating workers</h3><h4 id="the-abstractworker-abstract-interface"><span class="secno">4.8.1 </span>The <code><a href="#abstractworker">AbstractWorker</a></code> abstract interface</h4><pre class="idl">[Supplemental, NoInterfaceObject]
interface <dfn id="abstractworker">AbstractWorker</dfn> : <span>EventTarget</span> {
           attribute <span>Function</span>? <a href="#handler-abstractworker-onerror" title="handler-AbstractWorker-onerror">onerror</a>;
};</pre><p>The following are the <span>event handlers</span> (and their
  corresponding <span title="event handler event type">event handler
  event types</span>) that must be supported, as IDL attributes, by
  objects implementing the <code><a href="#abstractworker">AbstractWorker</a></code> interface:<table><thead><tr><th><span title="event handlers">Event handler</span> <th><span>Event handler event type</span>
   <tbody><tr><td><dfn id="handler-abstractworker-onerror" title="handler-AbstractWorker-onerror"><code>onerror</code></dfn> <td> <code title="event-error">error</code>
  </table><h4 id="dedicated-workers-and-the-worker-interface"><span class="secno">4.8.2 </span>Dedicated workers and the <code><a href="#worker">Worker</a></code> interface</h4><pre class="idl">[<a href="#dom-worker" title="dom-Worker">Constructor</a>(DOMString scriptURL)]
interface <dfn id="worker">Worker</dfn> : <a href="#abstractworker">AbstractWorker</a> {
  void <a href="#dom-worker-terminate" title="dom-Worker-terminate">terminate</a>();

  void <a href="#dom-worker-postmessage" title="dom-Worker-postMessage">postMessage</a>(any message, optional sequence&lt;<span>Transferable</span>&gt; transfer);           attribute <span>Function</span>? <a href="#handler-worker-onmessage" title="handler-Worker-onmessage">onmessage</a>;
};</pre><p>The <dfn id="dom-worker-terminate" title="dom-Worker-terminate"><code>terminate()</code></dfn> method,
  when invoked, must cause the "<a href="#terminate-a-worker">terminate a worker</a>"
  algorithm to be run on the worker with with the object is
  associated.<p><code><a href="#worker">Worker</a></code> objects act as if they had an implicit
  <code><a href="#messageport">MessagePort</a></code> associated with them. This port is part of
  a channel that is set up when the worker is created, but it is not
  exposed. This object must never be garbage collected before the
  <code><a href="#worker">Worker</a></code> object.<p>All messages received by that port must immediately be retargeted
  at the <code><a href="#worker">Worker</a></code> object.<p>The <dfn id="dom-worker-postmessage" title="dom-Worker-postMessage"><code>postMessage()</code></dfn>  method on <code><a href="#worker">Worker</a></code> objects
  must act as if, when invoked, it
  immediately invoked the method of the same name on the port, with
  the same arguments, and returned the same return value.<p>The following are the <span>event handlers</span> (and their
  corresponding <span title="event handler event type">event handler
  event types</span>) that must be supported, as IDL attributes, by
  objects implementing the <code><a href="#worker">Worker</a></code> interface:<table><thead><tr><th><span title="event handlers">Event handler</span> <th><span>Event handler event type</span>
   <tbody><tr><td><dfn id="handler-worker-onmessage" title="handler-Worker-onmessage"><code>onmessage</code></dfn> <td> <code title="event-message">message</code>
  </table><hr><p>When the <dfn id="dom-worker" title="dom-Worker"><code>Worker(<var title="">scriptURL</var>)</code></dfn> constructor is invoked, the
  user agent must run the following steps:<ol><li><p><span title="resolve a url">Resolve</span> the <var title="">scriptURL</var> argument relative to the <span>entry
   script</span>'s <span title="script's base URL">base URL</span>,
   when the method is invoked.</li>

   <li><p>If this fails, throw a <code>SYNTAX_ERR</code>
   exception.</li>

   <li>

    <p>If the <span>origin</span> of the resulting <span>absolute
    URL</span> is not the <span title="same origin">same</span> as the
    origin of the <span>entry script</span>, then throw a
    <code>SECURITY_ERR</code> exception.</p>

    <p class="note">Thus, scripts must be external files with the same
    scheme as the original page: you can't load a script from a <span title="data protocol"><code title="">data:</code> URL</span> or
    <span title="javascript protocol"><code title="">javascript:</code> URL</span>, and an <code>https:</code>
    page couldn't start workers using scripts with <code>http:</code>
    URLs.</p>

   </li>

   <li><p>Create a new <code><a href="#dedicatedworkerglobalscope">DedicatedWorkerGlobalScope</a></code>
   object. Let <var title="">worker global scope</var> be this new
   object.</li>

   <li><p>Create a new <code><a href="#worker">Worker</a></code> object, associated with
   <var title="">worker global scope</var>. Let <var title="">worker</var> be this new object.</li>

   <li><p><span>Create a new <code><a href="#messageport">MessagePort</a></code> object</span>
   owned by the <span title="script's global object">global
   object</span> of the <span title="concept-script">script</span> that
   invoked the constructor. Let this be the <var title="">outside
   port</var>.</li>

   <li><p>Associate the <var title="">outside port</var> with <var title="">worker</var>.</li>

   <li><p><span>Create a new <code><a href="#messageport">MessagePort</a></code> object</span>
   owned by <var title="">worker global scope</var>. Let <var title="">inside port</var> be this new object.</li>

   <li><p>Associate <var title="">inside port</var> with <var title="">worker global scope</var>.</li>

   <li><p><span>Entangle</span> <var title="">outside port</var> and
   <var title="">inside port</var>.</li>

   <li><p>Return <var title="">worker</var>, and run the following
   steps asynchronously.</li>


   <li><p>Enable <var title="">outside port</var>'s <span>port message
   queue</span>.</li>

   <li>

    <p>Let <var title="">docs</var> be the <a href="#list-of-relevant-document-objects-to-add">list of relevant
    <code>Document</code> objects to add</a> given the <span title="script's global object">global object</span> of the <span title="concept-script">script</span> that invoked the
    constructor.</p>

   </li>

   <li>

    <p><a href="#add-a-document-to-the-worker-s-documents" title="add a document to the worker's documents">Add to
    <var title="">worker global scope</var>'s list of <span>the
    worker's <code>Document</code>s</span></a> the
    <code>Document</code> objects in <var title="">docs</var>.</p>

   </li>

   <li>

    <p>If the <span title="script's global object">global object</span>
    of the <span title="concept-script">script</span> that invoked the
    constructor is a <code><a href="#workerglobalscope">WorkerGlobalScope</a></code> object (i.e. we
    are creating a nested worker), add <var title="">worker global
    scope</var> to the list of <a href="#the-worker-s-workers">the worker's workers</a> of the
    <code><a href="#workerglobalscope">WorkerGlobalScope</a></code> object that is the <span title="script's global object">global object</span> of the <span title="concept-script">script</span> that invoked the
    constructor.</p>

   </li>

   <li>

    <p><a href="#run-a-worker">Run a worker</a> for the resulting <span>absolute
    URL</span>, with the <span>script's browsing context</span> of the
    script that invoked the method as the <var title="">owner browsing
    context</var>, with the <span>script's document</span> of the
    script that invoked the method as the <var title="">owner
    document</var>, with the <span>origin</span> of the <span>entry
    script</span> as the <var title="">owner origin</var>, and with
    <var title="">worker global scope</var> as the global scope.</p>

   </li>

  </ol><p>This constructor must be visible when the <span>script's global
  object</span> is either a <code>Window</code> object or an object
  implementing the <code><a href="#workerutils">WorkerUtils</a></code> interface.<h4 id="shared-workers-and-the-sharedworker-interface"><span class="secno">4.8.3 </span>Shared workers and the <code><a href="#sharedworker">SharedWorker</a></code> interface</h4><pre class="idl">[<a href="#dom-sharedworker" title="dom-SharedWorker">Constructor</a>(DOMString scriptURL, optional DOMString name)]
interface <dfn id="sharedworker">SharedWorker</dfn> : <a href="#abstractworker">AbstractWorker</a> {
  readonly attribute <a href="#messageport">MessagePort</a> <a href="#dom-sharedworker-port" title="dom-SharedWorker-port">port</a>;
};</pre><p>The <dfn id="dom-sharedworker-port" title="dom-SharedWorker-port"><code>port</code></dfn>
  attribute must return the value it was assigned by the object's
  constructor. It represents the <code><a href="#messageport">MessagePort</a></code> for
  communicating with the shared worker.<p>When the <dfn id="dom-sharedworker" title="dom-SharedWorker"><code>SharedWorker(<var title="">scriptURL</var>, <var title="">name</var>)</code></dfn>
  constructor is invoked, the user agent must run the following
  steps:<ol><li><p><span title="resolve a url">Resolve</span> the <var title="">scriptURL</var> argument.</li>

   <li><p>If this fails, throw a <code>SYNTAX_ERR</code>
   exception.</li>

   <li><p>Otherwise, let <var title="">scriptURL</var> be the
   resulting <span>absolute URL</span>.</li>

   <li><p>Let <var title="">name</var> be the value of the second
   argument, or the empty string if the second argument was
   omitted.</li>

   <li>

    <p>If the <span>origin</span> of <var title="">scriptURL</var> is
    not the <span title="same origin">same</span> as the origin of the
    <span>entry script</span>, then throw a <code>SECURITY_ERR</code>
    exception.</p>

    <p class="note">Thus, scripts must be external files with the same
    scheme as the original page: you can't load a script from a <span title="data protocol"><code title="">data:</code> URL</span> or
    <span title="javascript protocol"><code title="">javascript:</code> URL</span>, and a <code>https:</code>
    page couldn't start workers using scripts with <code>http:</code>
    URLs.</p>

   </li>

   <li>

    <p>Let <var title="">docs</var> be the <a href="#list-of-relevant-document-objects-to-add">list of relevant
    <code>Document</code> objects to add</a> given the <span title="script's global object">global object</span> of the <span title="concept-script">script</span> that invoked the
    constructor.</p>

   </li>

   <li>

    <p>Execute the following substeps atomically:</p>

    <ol><li><p>Create a new <code><a href="#sharedworker">SharedWorker</a></code> object, which will
     shortly be associated with a <code><a href="#sharedworkerglobalscope">SharedWorkerGlobalScope</a></code>
     object. Let this <code><a href="#sharedworker">SharedWorker</a></code> object be <var title="">worker</var>.</li>

     <li><p><span>Create a new <code><a href="#messageport">MessagePort</a></code> object</span>
     owned by the <span title="script's global object">global
     object</span> of the script that invoked the method. Let this be
     the <var title="">outside port</var>.</li>

     <li><p>Assign <var title="">outside port</var> to the <code title="dom-SharedWorker-port"><a href="#dom-sharedworker-port">port</a></code> attribute of <var title="">worker</var>.</li>

     <li><p>Let <var title="">worker global scope</var> be
     null.</li>

     <li><p>If <var title="">name</var> is not the empty string and
     there exists a <code><a href="#sharedworkerglobalscope">SharedWorkerGlobalScope</a></code> object whose
     <a href="#dom-workerglobalscope-closing" title="dom-WorkerGlobalScope-closing">closing</a> flag
     is false, whose <code title="dom-WorkerGlobalScope-name">name</code> attribute is
     exactly equal to <var title="">name</var>, and whose <code title="dom-WorkerGlobalScope-location"><a href="#dom-workerglobalscope-location">location</a></code> attribute
     represents an <span>absolute URL</span> with the <span>same
     origin</span> as <var title="">scriptURL</var>, then let <var title="">worker global scope</var> be that
     <code><a href="#sharedworkerglobalscope">SharedWorkerGlobalScope</a></code> object.</li>

     <li><p>Otherwise, if <var title="">name</var> is the empty string
     and there exists a <code><a href="#sharedworkerglobalscope">SharedWorkerGlobalScope</a></code> object
     whose <a href="#dom-workerglobalscope-closing" title="dom-WorkerGlobalScope-closing">closing</a>
     flag is false, whose <code title="dom-WorkerGlobalScope-name">name</code> attribute is the
     empty string, and whose <code title="dom-WorkerGlobalScope-location"><a href="#dom-workerglobalscope-location">location</a></code> attribute
     represents an <span>absolute URL</span> that is exactly equal to
     <var title="">scriptURL</var>, then let <var title="">worker
     global scope</var> be that <code><a href="#sharedworkerglobalscope">SharedWorkerGlobalScope</a></code>
     object.</li>

     <li>

      <p>If <var title="">worker global scope</var> is not null, then
      run these steps:</p>

      <ol><li><p>If <var title="">worker global scope</var>'s <code title="dom-WorkerGlobalScope-location"><a href="#dom-workerglobalscope-location">location</a></code>
       attribute represents an <span>absolute URL</span> that is not
       exactly equal to <var title="">scriptURL</var>, then throw a
       <code>URL_MISMATCH_ERR</code> exception and abort all these
       steps.</li>

       <li><p>Associate <var title="">worker</var> with <var title="">worker global scope</var>.</li>

       <li><p><span>Create a new <code><a href="#messageport">MessagePort</a></code>
       object</span> owned by <var title="">worker global
       scope</var>. Let this be the <var title="">inside
       port</var>.</li>

       <li><p><span>Entangle</span> <var title="">outside port</var>
       and <var title="">inside port</var>.</li>

       <li><p>Return <var title="">worker</var> and perform the next
       step asynchronously.</li>

       <li><p>Create an event that uses the <code>MessageEvent</code>
       interface, with the name <code title="event-connect">connect</code>, which does not bubble, is
       not cancelable, has no default action, has a <code title="dom-MessageEvent-data">data</code> attribute whose value
       is the empty string and has a <code title="dom-MessageEvent-ports">ports</code> attribute whose
       value is a <span title="dfn-read-only-array">read only</span>
       array containing only the newly created port, and <span>queue a
       task</span> to dispatch the event at <var title="">worker
       global scope</var>.</li>

       <li>

        <p><a href="#add-a-document-to-the-worker-s-documents" title="add a document to the worker's documents">Add to
        <var title="">worker global scope</var>'s list of <span>the
        worker's <code>Document</code>s</span></a> the
        <code>Document</code> objects in <var title="">docs</var>.</p>

       </li>

       <li>

        <p>If the <span title="script's global object">global
        object</span> of the <span title="concept-script">script</span>
        that invoked the constructor is a
        <code><a href="#workerglobalscope">WorkerGlobalScope</a></code> object, add <var title="">worker global scope</var> to the list of <a href="#the-worker-s-workers">the
        worker's workers</a> of the <code><a href="#workerglobalscope">WorkerGlobalScope</a></code>
        object that is the <span title="script's global object">global
        object</span> of the <span title="concept-script">script</span>
        that invoked the constructor.</p>

       </li>

       <li><p>Abort all these steps.</li>

      </ol></li>

     <li><p>Create a new <code><a href="#sharedworkerglobalscope">SharedWorkerGlobalScope</a></code>
     object. Let <var title="">worker global scope</var> be this new
     object.</li>

     <li><p>Associate <var title="">worker</var> with <var title="">worker global scope</var>.</li>

     <li><p>Set the <code title="dom-SharedWorkerGlobalScope-name"><a href="#dom-sharedworkerglobalscope-name">name</a></code> attribute of
     <var title="">worker global scope</var> to <var title="">name</var>.</li>

     <li><p><span>Create a new <code><a href="#messageport">MessagePort</a></code> object</span>
     owned by <var title="">worker global scope</var>. Let <var title="">inside port</var> be this new object.</li>

     <li><p><span>Entangle</span> <var title="">outside port</var> and
     <var title="">inside port</var>.</li>

    </ol></li>

   <li><p>Return <var title="">worker</var> and perform the remaining
   steps asynchronously.</li>

   <li><p>Create an event that uses the <code>MessageEvent</code>
   interface, with the name <code title="event-connect">connect</code>, which does not bubble, is not
   cancelable, has no default action, has a <code title="dom-MessageEvent-data">data</code> attribute whose value is
   the empty string and has a <code title="dom-MessageEvent-ports">ports</code> attribute whose value
   is a <span title="dfn-read-only-array">read only</span> array
   containing only the newly created port, and <span>queue a
   task</span> to dispatch the event at <var title="">worker global
   scope</var>.</li>

   <li>

    <p><a href="#add-a-document-to-the-worker-s-documents" title="add a document to the worker's documents">Add to
    <var title="">worker global scope</var>'s list of <span>the
    worker's <code>Document</code>s</span></a> the
    <code>Document</code> objects in <var title="">docs</var>.</p>

   </li>

   <li>

    <p>If the <span title="script's global object">global object</span>
    of the <span title="concept-script">script</span> that invoked the
    constructor is a <code><a href="#workerglobalscope">WorkerGlobalScope</a></code> object, add <var title="">worker global scope</var> to the list of <a href="#the-worker-s-workers">the
    worker's workers</a> of the <code><a href="#workerglobalscope">WorkerGlobalScope</a></code>
    object that is the <span title="script's global object">global
    object</span> of the <span title="concept-script">script</span>
    that invoked the constructor.</p>

   </li>

   <li>

    <p><a href="#run-a-worker">Run a worker</a> for <var title="">scriptURL</var>,
    with the <span>script's browsing context</span> of the script that
    invoked the method as the <var title="">owner browsing
    context</var>, with the <span>script's document</span> of the
    script that invoked the method as the <var title="">owner
    document</var>, with the <span>origin</span> of the <span>entry
    script</span> as the <var title="">owner origin</var>, and with
    <var title="">worker global scope</var> as the global scope.</p>

   </li>

  </ol><p>This constructor must be visible when the <span>script's global
  object</span> is either a <code>Window</code> object or an object
  implementing the <code><a href="#workerutils">WorkerUtils</a></code> interface.<p>The <span>task source</span> for the tasks mentioned above is the
  <span>DOM manipulation task source</span>.<h2 id="apis-available-to-workers"><span class="secno">5 </span>APIs available to workers</h2><pre class="idl">[NoInterfaceObject]
interface <dfn id="workerutils">WorkerUtils</dfn> {
  void <a href="#dom-workerglobalscope-importscripts" title="dom-WorkerGlobalScope-importScripts">importScripts</a>(DOMString... urls);
  readonly attribute <a href="#workernavigator">WorkerNavigator</a> <a href="#dom-worker-navigator" title="dom-worker-navigator">navigator</a>;
};
<a href="#workerutils">WorkerUtils</a> implements <span>WindowTimers</span>;
<a href="#workerutils">WorkerUtils</a> implements <span>WindowBase64</span>;</pre><p>The <code>WindowTimers</code> and <code>WindowBase64</code>
  interfaces are defined in the HTML specification. <a href="#refsHTML">[HTML]</a><p>The DOM APIs (<code>Node</code> objects, <code>Document</code>
  objects, etc) are not available to workers in this version of this
  specification.<h3 id="importing-scripts-and-libraries"><span class="secno">5.1 </span>Importing scripts and libraries</h3><p>When a script invokes the <dfn id="dom-workerglobalscope-importscripts" title="dom-WorkerGlobalScope-importScripts"><code>importScripts(<var title="">urls</var>)</code></dfn> method on a
  <code><a href="#workerglobalscope">WorkerGlobalScope</a></code> object, the user agent must run the
  following steps:<ol><li><p>If there are no arguments, return without doing
   anything. Abort these steps.</li>

   <li><p><span title="resolve a url">Resolve</span> each
   argument.</li>

   <li><p>If any fail, throw a <code>SYNTAX_ERR</code>
   exception.</li>


   <li>

    <p>Attempt to <span>fetch</span> each resource identified by the
    resulting <span title="absolute URL">absolute URLs</span>, from
    the <span>entry script</span>'s <span>origin</span>, with the
    <i>synchronous flag</i> set.</p> 
   </li>

   <li>

    <p>For each argument in turn, in the order given, starting with
    the first one, run these substeps:</p>

    <ol><li>

      <p>Wait for the fetching attempt for the corresponding resource
      to complete.</p>

      <p>If the fetching attempt failed, throw a
      <code>NETWORK_ERR</code> exception and abort all these
      steps.</p>

      <p>If the attempt succeeds, then let <var title="">source</var> be
      the script resource <span>decoded as UTF-8, with error
      handling</span>.
      
      <a href="#refsHTML">[HTML]</a>
      
      </p>

      <p>Let <var title="">language</var> be JavaScript.</p>

      <p class="note">As with the worker's script, the script here is
      always assumed to be JavaScript, regardless of the MIME
      type.</p>

     </li>

     <li>

      <p><span>Create a script</span>, using <var title="">source</var> as the script source and <var title="">language</var> as the scripting language, using the
      same global object, browsing context, URL character encoding,
      base URL, and script group as the <span title="concept-script">script</span> that was created by the
      worker's <a href="#run-a-worker">run a worker</a> algorithm.</p>

      <p>Let the newly created <span title="concept-script">script</span> run until it either
      returns, fails to parse, fails to catch an exception, or gets
      prematurely aborted by the "<a href="#kill-a-worker">kill a worker</a>" or
      "<a href="#terminate-a-worker">terminate a worker</a>" algorithms defined above.</p>

      <p>If it failed to parse, then throw an ECMAScript
      <code>SyntaxError</code> exception and abort all these
      steps. <a href="#refsECMA262">[ECMA262]</a></p>

      <p>If an exception was raised or if the script was prematurely
      aborted, then abort all these steps, letting the exception or
      aborting continue to be processed by the script that called the
      <code title="dom-WorkerGlobalScope-importScripts"><a href="#dom-workerglobalscope-importscripts">importScripts()</a></code>
      method.</p>

      <p>If the "<a href="#kill-a-worker">kill a worker</a>" or "<a href="#terminate-a-worker">terminate a
      worker</a>" algorithms abort the script then abort all these
      steps.</p>

     </li>

    </ol></li>

  </ol><h3 id="the-workernavigator-object"><span class="secno">5.2 </span>The <code><a href="#workernavigator">WorkerNavigator</a></code> object</h3><p>The <dfn id="dom-worker-navigator" title="dom-worker-navigator"><code>navigator</code></dfn> attribute
  of the <code><a href="#workerutils">WorkerUtils</a></code> interface must return an instance of
  the <code><a href="#workernavigator">WorkerNavigator</a></code> interface, which represents the
  identity and state of the user agent (the client):<pre class="idl">interface <dfn id="workernavigator">WorkerNavigator</dfn> {};
<a href="#workernavigator">WorkerNavigator</a> implements <span>NavigatorID</span>;
<a href="#workernavigator">WorkerNavigator</a> implements <span>NavigatorOnLine</span>;</pre><p>Objects implementing the <code><a href="#workernavigator">WorkerNavigator</a></code> interface
  also implement the <code>NavigatorID</code> and
  <code>NavigatorOnLine</code> interfaces.
  
  <a href="#refsHTML">[HTML]</a>
  
  <p>This <code><a href="#workernavigator">WorkerNavigator</a></code> interface must not exist if the
  interface's <span>relevant namespace object</span> is a
  <code>Window</code> object. <a href="#refsWEBIDL">[WEBIDL]</a><h3 id="interface-objects-and-constructors"><span class="secno">5.3 </span>Interface objects and constructors</h3><p>There must be no interface objects and constructors available in
  the global scope of scripts whose <span>script's global
  object</span> is a <code><a href="#workerglobalscope">WorkerGlobalScope</a></code> object except for
  the following:<ul><li><p><code>XMLHttpRequest</code> and all interface objects and
   constructors defined by the XMLHttpRequest specifications, except
   that the <span>document response entity body</span> must always be
   null. The <span><code>XMLHttpRequest</code> base URL</span> is the
   <span>script's base URL</span>; the
   <span><code>XMLHttpRequest</code> origin</span> is the script's
   <span>origin</span>. <a href="#refsXHR">[XHR]</a></li>

   <li><p>The interface objects and constructors defined by this
   specification.</li>

   <li><p>Constructors defined by specifications that explicitly say
   that they should be visible when the <span>script's global
   object</span> is a <code><a href="#dedicatedworkerglobalscope">DedicatedWorkerGlobalScope</a></code>, a
   <code><a href="#sharedworkerglobalscope">SharedWorkerGlobalScope</a></code>, or an object implementing the
   <code><a href="#workerutils">WorkerUtils</a></code> interface; the interfaces of any objects
   with such constructors; and the interfaces of any objects made
   accessible through APIs exposed by those constructors or made
   accessible through interfaces to be implemented by any objects that
   are themselves accessible to scripts whose <span>script's global
   object</span> implements the <code><a href="#workerutils">WorkerUtils</a></code>
   interface.</li>

  </ul><p class="note">These requirements do not override the requirements
  defined by the Web IDL specification, in particular concerning the
  visibility of interfaces annotated with the <code title="">[NoInterfaceObject]</code> extended attribute.<h3 id="worker-locations"><span class="secno">5.4 </span>Worker locations</h3><pre class="idl">interface <dfn id="workerlocation">WorkerLocation</dfn> {
  // <span>URL decomposition IDL attributes</span>
  stringifier readonly attribute DOMString <a href="#dom-workerlocation-href" title="dom-WorkerLocation-href">href</a>;
  readonly attribute DOMString <a href="#dom-workerlocation-protocol" title="dom-WorkerLocation-protocol">protocol</a>;
  readonly attribute DOMString <a href="#dom-workerlocation-host" title="dom-WorkerLocation-host">host</a>;
  readonly attribute DOMString <a href="#dom-workerlocation-hostname" title="dom-WorkerLocation-hostname">hostname</a>;
  readonly attribute DOMString <a href="#dom-workerlocation-port" title="dom-WorkerLocation-port">port</a>;
  readonly attribute DOMString <a href="#dom-workerlocation-pathname" title="dom-WorkerLocation-pathname">pathname</a>;
  readonly attribute DOMString <a href="#dom-workerlocation-search" title="dom-WorkerLocation-search">search</a>;
  readonly attribute DOMString <a href="#dom-workerlocation-hash" title="dom-WorkerLocation-hash">hash</a>;
};</pre><p>A <code><a href="#workerlocation">WorkerLocation</a></code> object represents an <span>absolute
  URL</span> set at its creation.<p>The <dfn id="dom-workerlocation-href" title="dom-WorkerLocation-href"><code>href</code></dfn>
  attribute must return the <span>absolute URL</span> that the object
  represents.<p>The <code><a href="#workerlocation">WorkerLocation</a></code> interface also has the complement
  of <span>URL decomposition IDL attributes</span>, <dfn id="dom-workerlocation-protocol" title="dom-WorkerLocation-protocol"><code>protocol</code></dfn>,
  <dfn id="dom-workerlocation-host" title="dom-WorkerLocation-host"><code>host</code></dfn>, <dfn id="dom-workerlocation-port" title="dom-WorkerLocation-port"><code>port</code></dfn>, <dfn id="dom-workerlocation-hostname" title="dom-WorkerLocation-hostname"><code>hostname</code></dfn>,
  <dfn id="dom-workerlocation-pathname" title="dom-WorkerLocation-pathname"><code>pathname</code></dfn>,
  <dfn id="dom-workerlocation-search" title="dom-WorkerLocation-search"><code>search</code></dfn>,
  and <dfn id="dom-workerlocation-hash" title="dom-WorkerLocation-hash"><code>hash</code></dfn>.
  These must follow the rules given for <span>URL decomposition IDL
  attributes</span>, with the <span title="concept-uda-input">input</span> being the <span>absolute
  URL</span> that the object represents (same as the <code title="dom-WorkerLocation-href"><a href="#dom-workerlocation-href">href</a></code> attribute), and the
  <span title="concept-uda-setter">common setter action</span> being a
  no-op, since the attributes are defined to be readonly.
  
  <a href="#refsHTML">[HTML]</a>
  
  <p>The <code><a href="#workerlocation">WorkerLocation</a></code> interface must not exist if the
  interface's <span>relevant namespace object</span> is a
  <code>Window</code> object. <a href="#refsWEBIDL">[WEBIDL]</a><h2 class="no-num" id="references">References</h2><p>All references are normative unless marked "Non-normative".</p><dl><dt id="refsDOMCORE">[DOMCORE]</dt>
   <dd><cite><a href="http://dvcs.w3.org/hg/domcore/raw-file/tip/Overview.html">Web DOM Core</a></cite>, A. van Kesteren. W3C.</dd>

   <dt id="refsDOMEVENTS">[DOMEVENTS]</dt>
      <dd><cite><a href="http://dev.w3.org/2006/webapi/DOM-Level-3-Events/html/DOM3-Events.html">Document
   Object Model (DOM) Level 3 Events Specification</a></cite>,
   D. Schepers. W3C.</dd>

   <dt id="refsECMA262">[ECMA262]</dt>
   <dd><cite><a href="http://www.ecma-international.org/publications/standards/Ecma-262.htm">ECMAScript
   Language Specification</a></cite>. ECMA.</dd>

   <dt id="refsHTML">[HTML]</dt>
   <dd><cite><a href="http://dev.w3.org/html5/spec/">HTML5</a></cite>, I. Hickson. W3C.

   <dt id="refsRFC2119">[RFC2119]</dt>
   <dd><cite><a href="http://tools.ietf.org/html/rfc2119">Key words for use in
   RFCs to Indicate Requirement Levels</a></cite>, S. Bradner. IETF.</dd>

   <dt id="refsWEBIDL">[WEBIDL]</dt>
      <dd><cite><a href="http://dev.w3.org/2006/webapi/WebIDL/">Web
   IDL</a></cite>, C. McCormack. W3C.</dd>

   <dt id="refsXHR">[XHR]</dt>
      <dd><cite><a href="http://dev.w3.org/2006/webapi/XMLHttpRequest-2/"><code>XMLHttpRequest</code></a></cite>,
   A. van Kesteren. W3C.</dd>

  </dl><h2 class="no-num" id="acknowledgements">Acknowledgements</h2><!-- ACKS --><p>Thanks to

  Aaron Boodman,
  &#1040;&#1083;&#1077;&#1082;&#1089;&#1077;&#1081; &#1055;&#1088;&#1086;&#1089;&#1082;&#1091;&#1088;&#1103;&#1082;&#1086;&#1074; (Alexey Proskuryakov),
  Anne van Kesteren,
  Ben Turner,
  Dmitry Titov,
  Drew Wilson,
  Jeremy Orlow,
  Jonas Sicking,
  Justin James,
  Kevin Hakanson,
  Maciej Stachowiak,
  Michael Nordman,
  Mike Smith,

  and

  Philip Taylor

  for their useful and substantial comments.<p>Huge thanks to the whole Gears team, who pioneered this
  technology and whose experience has been a huge influence on this
  specification.