1. 10 Web workers
    1. 10.1 Introduction
      1. 10.1.1 Scope
      2. 10.1.2 Examples
        1. 10.1.2.1 A background number-crunching worker
        2. 10.1.2.2 Worker used for background I/O
        3. 10.1.2.3 Using a JavaScript module as a worker
        4. 10.1.2.4 Shared workers introduction
        5. 10.1.2.5 Shared state using a shared worker
        6. 10.1.2.6 Delegation
      3. 10.1.3 Tutorials
        1. 10.1.3.1 Creating a dedicated worker
        2. 10.1.3.2 Communicating with a dedicated worker
        3. 10.1.3.3 Shared workers
    2. 10.2 Infrastructure
      1. 10.2.1 The global scope
        1. 10.2.1.1 The WorkerGlobalScope common interface
        2. 10.2.1.2 Dedicated workers and the DedicatedWorkerGlobalScope interface
        3. 10.2.1.3 Shared workers and the SharedWorkerGlobalScope interface
      2. 10.2.2 The event loop
      3. 10.2.3 The worker's lifetime
      4. 10.2.4 Processing model
      5. 10.2.5 Runtime script errors
      6. 10.2.6 Creating workers
        1. 10.2.6.1 The AbstractWorker abstract interface
        2. 10.2.6.2 Script settings for workers
        3. 10.2.6.3 Dedicated workers and the Worker interface
        4. 10.2.6.4 Shared workers and the SharedWorker interface
      7. 10.2.7 Concurrent hardware capabilities
    3. 10.3 APIs available to workers
      1. 10.3.1 Importing scripts and libraries
      2. 10.3.2 The WorkerNavigator object
      3. 10.3.3 Worker locations

10 Web workers

10.1 Introduction

10.1.1 Scope

This section is non-normative.

This specification defines an API for running scripts in the background independently of any user interface scripts.

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.

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.

Generally, workers are expected to be long-lived, have a high start-up performance cost, and a high per-instance memory cost.

10.1.2 Examples

This section is non-normative.

There are a variety of uses that workers can be put to. The following subsections show various examples of this use.

10.1.2.1 A background number-crunching worker

This section is non-normative.

The simplest use of workers is for performing a computationally expensive task without interrupting the user interface.

In this example, the main document spawns a worker to (naïvely) compute prime numbers, and progressively displays the most recently found prime number.

The main page is as follows:

<!DOCTYPE HTML>
<html>
 <head>
  <title>Worker example: One-core computation</title>
 </head>
 <body>
  <p>The highest prime number discovered so far is: <output id="result"></output></p>
  <script>
   var worker = new Worker('worker.js');
   worker.onmessage = function (event) {
     document.getElementById('result').textContent = event.data;
   };
  </script>
 </body>
</html>

The Worker() constructor call creates a worker and returns a Worker object representing that worker, which is used to communicate with the worker. That object's onmessage event handler allows the code to receive messages from the worker.

The worker itself is as follows:

var n = 1;
search: while (true) {
  n += 1;
  for (var i = 2; i <= Math.sqrt(n); i += 1)
    if (n % i == 0)
     continue search;
  // found a prime!
  postMessage(n);
}

The bulk of this code is simply an unoptimized search for a prime number. The postMessage() method is used to send a message back to the page when a prime is found.

View this example online.

10.1.2.2 Worker used for background I/O

This section is non-normative.

In this example, the main document uses two workers, one for fetching stock updates at regular intervals, and one for performing search queries that the user requests.

The main page is as follows:

<!DOCTYPE HTML>
<html>
 <head>
  <title>Worker example: Stock ticker</title>
  <script>
   // 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);
   }
  </script>
  <meta http-equiv="Refresh" content="120; URL=../">
 </head>
 <body onload="search('')">
  <p><output id="symbol"></output> <output id="value"></output></p>
  <script>
   ticker.onmessage = function (event) {
     var data = event.data.split(' ');
     document.getElementById('symbol').textContent = data[0];
     document.getElementById('value').textContent = data[1];
   };
   ticker.postMessage(symbol);
  </script>
  <p><label>Search: <input type="text" autofocus oninput="search(this.value)"></label></p>
  <ul id="results"></ul>
  <script>
   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 < 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);
     }
   };
  </script>
  <p>(The data in this example is not real. Try searching for "Google" or "Apple".)</p>
 </body>
</html>

The two workers use a common library for performing the actual network calls. This library is as follows:

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
  }
}

The stock updater worker is as follows:

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();
};

The search query worker is as follows:

importScripts('io.js');
onmessage = function (event) {
  postMessage(get('search.cgi?' + event.data));
};

View this example online.

10.1.2.3 Using a JavaScript module as a worker

This section is non-normative.

All of our examples so far show workers that run classic scripts. Workers can instead be instantiated using module scripts, which have the usual benefits: the ability to use the JavaScript import statement to import other modules; strict mode by default; and top-level declarations not polluting the worker's global scope.

Note that such module-based workers follow different restrictions regarding cross-origin content, compared to classic workers. Unlike classic workers, module workers can be instantiated using a cross-origin script, as long as that script is exposed using the CORS protocol. Additionally, the importScripts() method will automatically fail inside module workers; the JavaScript import statement is generally a better choice.

In this example, the main document uses a worker to do off-main-thread image manipulation. It imports the filters used from another module.

The main page is as follows:

<!DOCTYPE html>
<meta charset="utf-8">
<title>Worker example: image decoding</title>

<label>
  Type an image URL to decode
  <input type="url" id="image-url" list="image-list">
  <datalist id="image-list">
    <option value="https://html.spec.whatwg.org/images/drawImage.png">
    <option value="https://html.spec.whatwg.org/images/robots.jpeg">
    <option value="https://html.spec.whatwg.org/images/arcTo2.png">
  </datalist>
</label>

<label>
  Choose a filter to apply
  <select id="filter">
    <option value="none">none</option>
    <option value="grayscale">grayscale</option>
    <option value="brighten">brighten by 20%</option>
  </select>
</label>

<script type="module">
  const worker = new Worker("worker.js", { type: "module" });
  worker.onmessage = receiveFromWorker;

  const url = document.querySelector("#image-url");
  const filter = document.querySelector("#filter");
  const output = document.querySelector("#output");

  url.oninput = updateImage;
  filter.oninput = sendToWorker;

  let imageData, context;

  function updateImage() {
    const img = new Image();
    img.src = url.value;

    img.onload = () => {
      output.innerHTML = "";

      const canvas = document.createElement("canvas");
      canvas.width = img.width;
      canvas.height = img.height;

      context = canvas.getContext("2d");
      context.drawImage(img, 0, 0);
      imageData = context.getImageData(0, 0, canvas.width, canvas.height);

      sendToWorker();
      output.appendChild(canvas);
    };
  }

  function sendToWorker() {
    worker.postMessage({ imageData, filter: filter.value });
  }

  function receiveFromWorker(e) {
    context.putImageData(e.data, 0, 0);
  }
</script>

The worker file is then:

import * as filters from "./filters.js";

self.onmessage = e => {
  const { imageData, filter } = e.data;
  filters[filter](imageData);
  self.postMessage(imageData, [imageData.data.buffer]);
};

Which imports the file filters.js:

export function none() {}

export function grayscale({ data: d }) {
  for (let i = 0; i < d.length; i += 4) {
    const [r, g, b] = [d[i], d[i + 1], d[i + 2]];

    // CIE luminance for the RGB
    // The human eye is bad at seeing red and blue, so we de-emphasize them.
    d[i] = d[i + 1] = d[i + 2] = 0.2126 * r + 0.7152 * g + 0.0722 * b;
  }
};

export function brighten({ data: d }) {
  for (let i = 0; i < d.length; ++i) {
    d[i] *= 1.2;
  }
};

View this example online.

10.1.2.4 Shared workers introduction

Support: Chrome 4+Chrome for Android NoneiOS Safari NoneUC Browser for Android 9.9+Firefox 29+IE NoneOpera Mini NoneSamsung Internet 4+Android Browser NoneSafari NoneEdge NoneOpera 10.6+

Source: caniuse.com

This section is non-normative.

This section introduces shared workers using a Hello World example. Shared workers use slightly different APIs, since each worker can have multiple connections.

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.

Here is the HTML page:

<!DOCTYPE HTML>
<title>Shared workers: demo 1</title>
<pre id="log">Log:</pre>
<script>
  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;
  }
</script>

Here is the JavaScript worker:

onconnect = function(e) {
  var port = e.ports[0];
  port.postMessage('Hello World!');
}

View this example online.


This second example extends the first one by changing two things: first, messages are received using addEventListener() instead of an event handler IDL attribute, and second, a message is sent to the worker, causing the worker to send another message in return. Received messages are again displayed in a log.

Here is the HTML page:

<!DOCTYPE HTML>
<title>Shared workers: demo 2</title>
<pre id="log">Log:</pre>
<script>
  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');
</script>

Here is the JavaScript worker:

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
  }
}

View this example online.


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 iframe on the first page, but the same principle would apply to an entirely separate page in a separate top-level browsing context.

Here is the outer HTML page:

<!DOCTYPE HTML>
<title>Shared workers: demo 3</title>
<pre id="log">Log:</pre>
<script>
  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');
</script>
<iframe src="inner.html"></iframe>

Here is the inner HTML page:

<!DOCTYPE HTML>
<title>Shared workers: demo 3 inner frame</title>
<pre id=log>Inner log:</pre>
<script>
  var worker = new SharedWorker('test.js');
  var log = document.getElementById('log');
  worker.port.onmessage = function(e) {
   log.textContent += '\n' + e.data;
  }
</script>

Here is the JavaScript worker:

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');
  }
}

View this example online.

10.1.2.5 Shared state using a shared worker

This section is non-normative.

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.

The main page isn't interesting, it merely provides a way to open the viewers:

<!DOCTYPE HTML>
<html>
 <head>
  <title>Workers example: Multiviewer</title>
  <script>
   function openViewer() {
     window.open('viewer.html');
   }
  </script>
 </head>
 <body>
  <p><button type=button onclick="openViewer()">Open a new
  viewer</button></p>
  <p>Each viewer opens in a new window. You can have as many viewers
  as you like, they all view the same data.</p>
 </body>
</html>

The viewer is more involved:

<!DOCTYPE HTML>
<html>
 <head>
  <title>Workers example: Multiviewer viewer</title>
  <script>
   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)[0];
     // 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 < 3; y += 1) {
       for (var x = 0; x < 3; x += 1) {
         var tile = data[y * 3 + x];
         if (tile == '0')
           context.fillStyle = 'green';
         else 
           context.fillStyle = 'maroon';
         context.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)[0];
     var message = event.data.substr(4 + name.length + 1);
     // display "<name> message" in public chat
     var public = document.getElementById('public');
     var p = document.createElement('p');
     var n = document.createElement('button');
     n.textContent = '<' + name + '> ';
     n.onclick = function () { worker.port.postMessage('msg ' + name); };
     p.appendChild(n);
     var m = document.createElement('span');
     m.textContent = message;
     p.appendChild(m);
     public.appendChild(p);
   }
   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)[0];
     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 div = document.createElement('div');
     var addMessage = function(name, message) {
       var p = document.createElement('p');
       var n = document.createElement('strong');
       n.textContent = '<' + name + '> ';
       p.appendChild(n);
       var t = document.createElement('span');
       t.textContent = message;
       p.appendChild(t);
       div.appendChild(p);
     };
     port.onmessage = function (event) {
       addMessage(name, event.data);
     };
     li.appendChild(div);
     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);
     ul.appendChild(li);
   }
   worker.port.addEventListener('message', startPrivateChat, false);

   worker.port.start();
  </script>
 </head>
 <body>
  <h1>Viewer</h1>
  <h2>Map</h2>
  <p><canvas id="map" height=150 width=150></canvas></p>
  <p>
   <button type=button onclick="worker.port.postMessage('mov left')">Left</button>
   <button type=button onclick="worker.port.postMessage('mov up')">Up</button>
   <button type=button onclick="worker.port.postMessage('mov down')">Down</button>
   <button type=button onclick="worker.port.postMessage('mov right')">Right</button>
   <button type=button onclick="worker.port.postMessage('set 0')">Set 0</button>
   <button type=button onclick="worker.port.postMessage('set 1')">Set 1</button>
  </p>
  <h2>Public Chat</h2>
  <div id="public"></div>
  <form onsubmit="worker.port.postMessage('txt ' + message.value); message.value = ''; return false;">
   <p>
    <input type="text" name="message" size="50">
    <button>Post</button>
   </p>
  </form>
  <h2>Private Chat</h2>
  <ul id="private"></ul>
 </body>
</html>

There are several key things worth noting about the way the viewer is written.

Multiple listeners. 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.

Registering event listeners in this way also allows you to unregister specific listeners when you are done with them, as is done with the configure() method in this example.

Finally, the worker:

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 < 0) return wrapX(x + map[0].length);
  if (x >= map[0].length) return wrapX(x - map[0].length);
  return x;
}

function wrapY(y) {
  if (y < 0) return wrapY(y + map.length);
  if (y >= map[0].length) return wrapY(y - map.length);
  return y;
}

function wrap(val, min, max) {
  if (val < min)
    return val + (max-min)+1;
  if (val > max)
    return val - (max-min)-1;
  return val;
}

function sendMapData(viewer) {
  var data = '';
  for (var y = viewer.y-1; y <= viewer.y+1; y += 1) {
    for (var x = viewer.x-1; x <= viewer.x+1; x += 1) {
      if (data != '')
        data += ',';
      data += map[wrap(y, 0, map[0].length-1)][wrap(x, 0, map.length-1)];
    }
  }
  viewer.port.postMessage('map ' + data);
}

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

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._data);
      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]);
      break;
    case 'txt ':
      var name = event.target._data.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.target._data;
      var party2 = viewers[event.data.substr(4).split(' ', 1)[0]];
      if (party2) {
        var channel = new MessageChannel();
        party1.port.postMessage('msg ' + party2.name, [channel.port1]);
        party2.port.postMessage('msg ' + party1.name, [channel.port2]);
      }
      break;
  }
}

Connecting to multiple pages. The script uses the onconnect event listener to listen for multiple connections.

Direct channels. 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.

View this example online.

10.1.2.6 Delegation

This section is non-normative.

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.

The main page is as follows, it just reports the result:

<!DOCTYPE HTML>
<html>
 <head>
  <title>Worker example: Multicore computation</title>
 </head>
 <body>
  <p>Result: <output id="result"></output></p>
  <script>
   var worker = new Worker('worker.js');
   worker.onmessage = function (event) {
     document.getElementById('result').textContent = event.data;
   };
  </script>
 </body>
</html>

The worker itself is as follows:

// 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 < 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 <= 0)
    postMessage(result); // finished!
}

It consists of a loop to start the subworkers, and then a handler that waits for all the subworkers to respond.

The subworkers are implemented as follows:

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 < end; i += 1) {
    // perform some complex calculation here
    result += 1;
  }
  postMessage(result);
  close();
}

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.

View this example online.

10.1.3 Tutorials

10.1.3.1 Creating a dedicated worker

This section is non-normative.

Creating a worker requires a URL to a JavaScript file. The Worker() constructor is invoked with the URL to that file as its only argument; a worker is then created and returned:

var worker = new Worker('helper.js');

If you want your worker script to be interpreted as a module script instead of the default classic script, you need to use a slightly different signature:

var worker = new Worker('helper.js', { type: "module" });
10.1.3.2 Communicating with a dedicated worker

This section is non-normative.

Dedicated workers use MessagePort objects behind the scenes, and thus support all the same features, such as sending structured data, transferring binary data, and transferring other ports.

To receive messages from a dedicated worker, use the onmessage event handler IDL attribute on the Worker object:

worker.onmessage = function (event) { ... };

You can also use the addEventListener() method.

The implicit MessagePort used by dedicated workers has its port message queue implicitly enabled when it is created, so there is no equivalent to the MessagePort interface's start() method on the Worker interface.

To send data to a worker, use the postMessage() method. Structured data can be sent over this communication channel. To send ArrayBuffer objects efficiently (by transferring them rather than cloning them), list them in an array in the second argument.

worker.postMessage({
  operation: 'find-edges',
  input: buffer, // an ArrayBuffer object
  threshold: 0.6,
}, [buffer]);

To receive a message inside the worker, the onmessage event handler IDL attribute is used.

onmessage = function (event) { ... };

You can again also use the addEventListener() method.

In either case, the data is provided in the event object's data attribute.

To send messages back, you again use postMessage(). It supports the structured data in the same manner.

postMessage(event.data.input, [event.data.input]); // transfer the buffer back
10.1.3.3 Shared workers

This section is non-normative.

Shared workers are identified by the URL of the script used to create it, optionally with an explicit name. The name allows multiple instances of a particular shared worker to be started.

Shared workers are scoped by origin. Two different sites using the same names will not collide. However, if a page tries to use the same shared worker name as another page on the same site, but with a different script URL, it will fail.

Creating shared workers is done using the SharedWorker() 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.

var worker = new SharedWorker('service.js');

Communicating with shared workers is done with explicit MessagePort objects. The object returned by the SharedWorker() constructor holds a reference to the port on its port attribute.

worker.port.onmessage = function (event) { ... };
worker.port.postMessage('some message');
worker.port.postMessage({ foo: 'structured', bar: ['data', 'also', 'possible']});

Inside the shared worker, new clients of the worker are announced using the connect event. The port for the new client is given by the event object's source attribute.

onconnect = function (event) {
  var newPort = event.source;
  // 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
};

10.2 Infrastructure

There are two kinds of workers; dedicated workers, and shared workers. Dedicated workers, once created, 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 origin can obtain a reference to that worker and communicate with it.

10.2.1 The global scope

The global scope is the "inside" of a worker.

10.2.1.1 The WorkerGlobalScope common interface
[Exposed=Worker] 
interface WorkerGlobalScope : EventTarget {
  readonly attribute WorkerGlobalScope self;
  readonly attribute WorkerLocation location;
  readonly attribute WorkerNavigator navigator;
  void importScripts(USVString... urls);

  attribute OnErrorEventHandler onerror;
  attribute EventHandler onlanguagechange;
  attribute EventHandler onoffline;
  attribute EventHandler ononline;
  attribute EventHandler onrejectionhandled;
  attribute EventHandler onunhandledrejection;
};

A WorkerGlobalScope object has an associated type ("classic" or "module"). It is set during creation.

A WorkerGlobalScope object has an associated url (null or a URL). It is initially null.

A WorkerGlobalScope object has an associated HTTPS state (an HTTPS state value). It is initially "none".

A WorkerGlobalScope object has an associated referrer policy (a referrer policy). It is initially the empty string.

A WorkerGlobalScope object has an associated CSP list. It is initially an empty list.

A WorkerGlobalScope object has an associated module map. It is a module map, initially empty.

workerGlobal . self
Returns workerGlobal.
workerGlobal . location
Returns workerGlobal's WorkerLocation object.
workerGlobal . navigator
Returns workerGlobal's WorkerNavigator object.
workerGlobal . importScripts(urls...)
Fetches each URL in urls, executes them one-by-one in the order they are passed, and then returns (or throws if something went amiss).

The self attribute must return the WorkerGlobalScope object itself.

The location attribute must return the WorkerLocation object whose associated WorkerGlobalScope object is the WorkerGlobalScope object.

While the WorkerLocation object is created after the WorkerGlobalScope object, this is not problematic as it cannot be observed from script.


The following are the event handlers (and their corresponding event handler event types) that must be supported, as event handler IDL attributes, by objects implementing the WorkerGlobalScope interface:

Event handler Event handler event type
onerror error
onlanguagechange languagechange
onoffline offline
ononline online
onrejectionhandled rejectionhandled
onunhandledrejection unhandledrejection
10.2.1.2 Dedicated workers and the DedicatedWorkerGlobalScope interface
[Global=(Worker,DedicatedWorker),Exposed=DedicatedWorker]
interface DedicatedWorkerGlobalScope : WorkerGlobalScope {
  void postMessage(any message, optional sequence<object> transfer = []);

  void close();

  attribute EventHandler onmessage;
};

DedicatedWorkerGlobalScope objects act as if they had an implicit MessagePort 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 DedicatedWorkerGlobalScope object.

All messages received by that port must immediately be retargeted at the DedicatedWorkerGlobalScope object.

dedicatedWorkerGlobal . postMessage(message [, transfer ])
Clones message and transmits it to the Worker object associated with dedicatedWorkerGlobal. transfer can be passed as a list of objects that are to be transferred rather than cloned.
dedicatedWorkerGlobal . close()
Aborts dedicatedWorkerGlobal.

The postMessage() method on DedicatedWorkerGlobalScope 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.

To close a worker, given a workerGlobal, run these steps:

  1. Discard any tasks that have been added to workerGlobal's event loop's task queues.

  2. Set workerGlobal's closing flag to true. (This prevents any further tasks from being queued.)

The close() method, when invoked, must close a worker with this DedicatedWorkerGlobalScope object.


The following are the event handlers (and their corresponding event handler event types) that must be supported, as event handler IDL attributes, by objects implementing the DedicatedWorkerGlobalScope interface:

Event handler Event handler event type
onmessage message

For the purposes of the application cache networking model, a dedicated worker is an extension of the cache host from which it was created.

10.2.1.3 Shared workers and the SharedWorkerGlobalScope interface
[Global=(Worker,SharedWorker),Exposed=SharedWorker]
interface SharedWorkerGlobalScope : WorkerGlobalScope {
  readonly attribute DOMString name;
  readonly attribute ApplicationCache applicationCache; // deprecated

  void close();

  attribute EventHandler onconnect;
};

A SharedWorkerGlobalScope object has an associated constructor url and name.

Shared workers receive message ports through connect events on their SharedWorkerGlobalScope object for each connection.

sharedWorkerGlobal . name
Returns sharedWorkerGlobal's name.
sharedWorkerGlobal . close()
Aborts sharedWorkerGlobal.

The name attribute must return the SharedWorkerGlobalScope object's name. Its value represents the name that can be used to obtain a reference to the worker using the SharedWorker constructor.

The close() method, when invoked, must close a worker with this SharedWorkerGlobalScope object.


The following are the event handlers (and their corresponding event handler event types) that must be supported, as event handler IDL attributes, by objects implementing the SharedWorkerGlobalScope interface:

Event handler Event handler event type
onconnect connect

For the purposes of the application cache networking model, a shared worker is its own cache host. The run a worker algorithm takes care of associating the worker with an application cache.

The applicationCache attribute returns the ApplicationCache object for the worker.

10.2.2 The event loop

Each WorkerGlobalScope object has a distinct event loop, separate from those used by units of related similar-origin browsing contexts. This event loop has no associated browsing context, and its task queues only have events, callbacks, and networking activity as tasks. These event loops are created by the run a worker algorithm.

Each WorkerGlobalScope object also has a closing flag, which must initially be false, but which can get set to true by the algorithms in the processing model section below.

Once the WorkerGlobalScope's closing flag is set to true, the event loop's task queues must discard any further tasks that would be added to them (tasks already on the queue are unaffected except where otherwise specified). Effectively, once the closing flag is true, timers stop firing, notifications for all pending background operations are dropped, etc.

10.2.3 The worker's lifetime

Workers communicate with other workers and with browsing contexts through message channels and their MessagePort objects.

Each WorkerGlobalScope worker global scope has a list of the worker's ports, which consists of all the MessagePort objects that are entangled with another port and that have one (but only one) port owned by worker global scope. This list includes the implicit MessagePort in the case of dedicated workers.

Each WorkerGlobalScope also has a list of the worker's workers. Initially this list is empty; it is populated when the worker creates or obtains further workers.

Finally, each WorkerGlobalScope also has a list of the worker's Documents. Initially this list is empty; it is populated when the worker is created.

Whenever a Document d is added to the worker's Documents, the user agent must, for each worker q in the list of the worker's workers whose list of the worker's Documents does not contain d, add d to q's WorkerGlobalScope owner's list of the worker's Documents.

Whenever a Document object is discarded, it must be removed from the list of the worker's Documents of each worker whose list contains that Document.

Given an environment settings object o when creating or obtaining a worker, the list of relevant Document objects to add depends on the type of global object specified by o. If o specifies a global object that is a WorkerGlobalScope object (i.e. if we are creating a nested worker), then the relevant Documents are the the worker's Documents of the global object specified by o. Otherwise, o specifies a global object that is a Window object, and the relevant Document is just the responsible document specified by o.


A worker is said to be a permissible worker if its list of the worker's Documents is not empty, or if its list has been empty for no more than a short user-agent-defined timeout value, its WorkerGlobalScope is actually a SharedWorkerGlobalScope object (i.e. the worker is a shared worker), and the user agent has a browsing context whose Document is not complete loaded.

The second part of this definition allows a shared worker to survive for a short time while a page is loading, in case that page is going to contact the shared worker again. This can be used by user agents as a way to avoid the cost of restarting a shared worker used by a site when the user is navigating from page to page within that site.

A worker is said to be an active needed worker if any of the Document objects in the worker's Documents are fully active.

A worker is said to be a protected worker if it is an active needed worker and either it has outstanding timers, database transactions, or network connections, or its list of the worker's ports is not empty, or its WorkerGlobalScope is actually a SharedWorkerGlobalScope object (i.e. the worker is a shared worker).

A worker is said to be a suspendable worker if it is not an active needed worker but it is a permissible worker.

10.2.4 Processing model

When a user agent is to run a worker for a script with Worker or SharedWorker object worker, URL url, environment settings object outside settings, MessagePort outside port, and a WorkerOptions dictionary options, it must run the following steps:

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

    For the purposes of timing APIs, this is the official moment of creation of the worker.

  2. Let is shared be true if worker is a SharedWorker object, and false otherwise.

  3. Let docs be the list of relevant Document objects to add given outside settings.

  4. Let parent worker global scope be null.

  5. If outside settings's global object is a WorkerGlobalScope object (i.e. we are creating a nested worker), set parent worker global scope to outside settings's global object.

  6. Call the JavaScript InitializeHostDefinedRealm() abstract operation with the following customizations:

  7. Set up a worker environment settings object with realm execution context and outside settings, and let inside settings be the result.

  8. Let destination be "sharedworker" if is shared is true, and "worker" otherwise.

  9. Obtain script by switching on the value of options's type member:

    "classic"
    Fetch a classic worker script given url, outside settings, and destination.
    "module"
    Fetch a module worker script tree given url, outside settings, destination, the value of the credentials member of options, and inside settings.

    In both cases, to perform the fetch given request, perform the following steps if the is top-level flag is set:

    1. Fetch request, and asynchronously wait to run the remaining steps as part of fetch's process response for the response response.

    2. Set worker global scope's url to response's url.

    3. Set worker global scope's HTTPS state to response's HTTPS state.

    4. Set worker global scope's referrer policy to the result of parsing the `Referrer-Policy` header of response.

    5. Execute the Initialize a global object's CSP list algorithm on worker global scope and response. [CSP]

    6. Asynchronously complete the perform the fetch steps with response.

    If the algorithm asynchronously completes with null, queue a task to fire a simple event named error at worker, and abort these steps. Otherwise, continue the rest of these steps after the algorithm's asynchronous completion, with script being the asynchronous completion value.

  10. Associate worker with worker global scope.

  11. Create a new MessagePort object whose owner is inside settings. Let inside port be this new object.

  12. Associate inside port with worker global scope.

  13. Entangle outside port and inside port.

  14. Add to worker global scope's list of the worker's Documents the Document objects in docs.

  15. If parent worker global scope is not null, add worker global scope to the list of the worker's workers of parent worker global scope.

  16. Set worker global scope's type to the value of the type member of options.

  17. If is shared is true, and there are any relevant application caches that are identified by a manifest URL with the same origin as url and that have url as one of their entries, not excluding entries marked as foreign, then associate worker global scope with the most appropriate application cache of those that match.

  18. Create a new WorkerLocation object and associate it with worker global scope.

  19. Closing orphan workers: Start monitoring the worker such that no sooner than it stops being a protected worker, and no later than it stops being a permissible worker, worker global scope's closing flag is set to true.

  20. Suspending workers: Start monitoring the worker, such that whenever worker global scope's closing flag is false and the worker is a suspendable worker, the user agent suspends execution of script in that worker until such time as either the closing flag switches to true or the worker stops being a suspendable worker.

  21. If script is a classic script, then run the classic script script. Otherwise, it is a module script; run the module script script.

    In addition to the usual possibilities of returning a value or failing due to an exception, this could be prematurely aborted by the "kill a worker" or "terminate a worker" algorithms defined below.

  22. Enable outside port's port message queue.

  23. If is shared is false, enable the port message queue of the worker's implicit port.

  24. If is shared is true, create a trusted event that uses the MessageEvent interface, with the name connect, which does not bubble, is not cancelable, has no default action, has a data attribute whose value is initialized to the empty string, has a ports attribute whose value is initialized to a new frozen array containing inside port, has a source attribute whose value is initialized to inside port. Queue a task, using the DOM manipulation task source, to dispatch the event at worker global scope.

  25. Enable the client message queue of the ServiceWorkerContainer object whose associated service worker client is worker global scope's relevant settings object.

  26. Event loop: Run the responsible event loop specified by inside settings until it is destroyed.

    The handling of events or the execution of callbacks by tasks run by the event loop might get prematurely aborted by the "kill a worker" or "terminate a worker" algorithms defined below.

    The worker processing model remains on this step until the event loop is destroyed, which happens after the closing flag is set to true, as described in the event loop processing model.

  27. Empty the worker global scope's list of active timers.

  28. Disentangle all the ports in the list of the worker's ports.

  29. Empty the worker's list of the worker's Documents.


When a user agent is to kill a worker it must run the following steps in parallel with the worker's main loop (the "run a worker" processing model defined above):

  1. Set the worker's WorkerGlobalScope object's closing flag to true.

  2. If there are any tasks queued in the WorkerGlobalScope object's event loop's task queues, discard them without processing them.

  3. Wait a user-agent-defined amount of time.

  4. Abort the script currently running in the worker.

User agents may invoke the "kill a worker" 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 active needed worker if the worker continues executing even after its closing flag was set to true.


When a user agent is to terminate a worker it must run the following steps in parallel with the worker's main loop (the "run a worker" processing model defined above):

  1. Set the worker's WorkerGlobalScope object's closing flag to true.

  2. If there are any tasks queued in the WorkerGlobalScope object's event loop's task queues, discard them without processing them.

  3. Abort the script currently running in the worker.

  4. If the worker's WorkerGlobalScope object is actually a DedicatedWorkerGlobalScope object (i.e. the worker is a dedicated worker), then empty the port message queue of the port that the worker's implicit port is entangled with.


The task source for the tasks mentioned above is the DOM manipulation task source.

10.2.5 Runtime script errors

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 report the error for that script, with the position (line number and column number) where the error occurred, using the WorkerGlobalScope object as the target.

For shared workers, if the error is still not handled afterwards, the error may be reported to a developer console.

For dedicated workers, if the error is still not handled afterwards, the user agent must queue a task to fire a trusted event that uses the ErrorEvent interface, with the name error, that doesn't bubble and is cancelable, with its message, filename, lineno, colno, attributes initialized appropriately, and with the error attribute initialized to null, at the Worker 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 Worker object is in, thus repeating the entire runtime script error reporting process one level up.

If the implicit port connecting the worker to its Worker object has been disentangled (i.e. if the parent worker has been terminated), then the user agent must act as if the Worker object had no error event handler and as if that worker's onerror attribute was null, but must otherwise act as described above.

Thus, error reports propagate up to the chain of dedicated workers up to the original Document, even if some of the workers along this chain have been terminated and garbage collected.

The task source for the task mentioned above is the DOM manipulation task source.

10.2.6 Creating workers

10.2.6.1 The AbstractWorker abstract interface
[NoInterfaceObject, Exposed=(Window,Worker)]
interface AbstractWorker {
  attribute EventHandler onerror;
};

The following are the event handlers (and their corresponding event handler event types) that must be supported, as event handler IDL attributes, by objects implementing the AbstractWorker interface:

Event handler Event handler event type
onerror error
10.2.6.2 Script settings for workers

When the user agent is required to set up a worker environment settings object, given a JavaScript execution context execution context and environment settings object outside settings, it must run the following steps:

  1. Let inherited responsible browsing context be outside settings's responsible browsing context.

  2. Let inherited origin be outside settings's origin.

  3. Let worker event loop be a newly created event loop.

  4. Let realm be the value of execution context's Realm component.

  5. Let worker global scope be realm's global object.

  6. Let settings object be a new environment settings object whose algorithms are defined as follows:

    The realm execution context

    Return execution context.

    The module map

    Return worker global scope's module map.

    The responsible browsing context

    Return inherited responsible browsing context.

    The responsible event loop

    Return worker event loop.

    The responsible document

    Not applicable (the responsible event loop is not a browsing context event loop).

    The API URL character encoding

    Return UTF-8.

    The API base URL

    Return worker global scope's url.

    The origin

    Return inherited origin.

    The creation URL

    Return worker global scope's url.

    The HTTPS state

    Return worker global scope's HTTPS state.

    The referrer policy

    Return worker global scope's referrer policy.

  7. Set realm's [[HostDefined]] field to settings object.

  8. Return settings object.

10.2.6.3 Dedicated workers and the Worker interface
[Constructor(USVString scriptURL, optional WorkerOptions options), Exposed=(Window,Worker)]
interface Worker : EventTarget {
  void terminate();

  void postMessage(any message, optional sequence<object> transfer = []);
  attribute EventHandler onmessage;
};

dictionary WorkerOptions {
  WorkerType type = "classic";
  RequestCredentials credentials = "omit"; // credentials is only used if type is "module"
};

enum WorkerType { "classic", "module" };

Worker implements AbstractWorker;
worker = new Worker(scriptURL [, options ])
Returns a new Worker object. scriptURL will be fetched and executed in the background, creating a new global environment for which worker represents the communication channel. options can be used to ensure this new global environment supports JavaScript modules (specify type: "module") and if that is specified, can also be used to specify how scriptURL is fetched through the credentials option.
worker . terminate()
Aborts worker's associated global environment.
worker . postMessage(message [, transfer ])
Clones message and transmits it to worker's global environment. transfer can be passed as a list of objects that are to be transferred rather than cloned.

The terminate() method, when invoked, must cause the "terminate a worker" algorithm to be run on the worker with which the object is associated.

Worker objects act as if they had an implicit MessagePort 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 Worker object.

All messages received by that port must immediately be retargeted at the Worker object.

The postMessage() method on Worker 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.

The postMessage() method's first argument can be structured data:

worker.postMessage({opcode: 'activate', device: 1938, parameters: [23, 102]});

The following are the event handlers (and their corresponding event handler event types) that must be supported, as event handler IDL attributes, by objects implementing the Worker interface:

Event handler Event handler event type
onmessage message

When the Worker(scriptURL, options) constructor is invoked, the user agent must run the following steps:

  1. The user agent may throw a "SecurityError" DOMException and abort these steps if the request violates a policy decision (e.g. if the user agent is configured to not allow the page to start dedicated workers).

  2. Let outside settings be the current settings object.

  3. Parse the scriptURL argument relative to outside settings.

  4. If this fails, throw a "SyntaxError" DOMException and abort these steps.

  5. Let worker URL be the resulting URL record.

    Any same-origin URL will do, including blob: URLs.

  6. Let worker be a new Worker object.

  7. Create a new MessagePort object whose owner is outside settings. Let this be the outside port.

  8. Associate the outside port with worker.

  9. Let docs be the list of relevant Document objects to add given outside settings.

  10. Return worker, and run the following step in parallel.

  11. Run a worker given worker, worker URL, outside settings, outside port, and options.

10.2.6.4 Shared workers and the SharedWorker interface
[Constructor(USVString scriptURL, optional DOMString name = "", optional WorkerOptions options), Exposed=(Window,Worker)]
interface SharedWorker : EventTarget {
  readonly attribute MessagePort port;
};
SharedWorker implements AbstractWorker;
sharedWorker = new SharedWorker(scriptURL [, name [, options ]])
Returns a new SharedWorker object. scriptURL will be fetched and executed in the background, creating a new global environment for which sharedWorker represents the communication channel. name can be used to define the name of that global environment. options can be used to ensure this new global environment supports JavaScript modules (specify type: "module") and if that is specified, can also be used to specify how scriptURL is fetched through the credentials option.
sharedWorker . port
Returns sharedWorker's MessagePort object which can be used to communicate with the global environment.

The port attribute must return the value it was assigned by the object's constructor. It represents the MessagePort for communicating with the shared worker.

When the SharedWorker(scriptURL, name, options) constructor is invoked, the user agent must run the following steps:

  1. The user agent may throw a "SecurityError" DOMException and abort these steps if the request violates a policy decision (e.g. if the user agent is configured to not allow the page to start shared workers).

  2. Let outside settings be the current settings object.

  3. Parse scriptURL relative to outside settings.

  4. If this fails, throw a "SyntaxError" DOMException and abort these steps.

  5. Let urlString be the resulting URL string and urlRecord be the resulting URL record.

    Any same-origin URL will do, including blob: URLs.

  6. Execute the following substeps atomically:

    1. Let worker be a new SharedWorker object.

    2. Create a new MessagePort object whose owner is outside settings. Let this be the outside port.

    3. Assign outside port to the port attribute of worker.

    4. Let worker global scope be null.

    5. If there exists a SharedWorkerGlobalScope object whose closing flag is false, whose name is exactly equal to name, whose constructor url is equal to urlString, and whose relevant settings object's origin is same origin with outside settings's origin, then let worker global scope be that SharedWorkerGlobalScope object.

    6. If worker global scope is not null, but the user agent has been configured to disallow communication between the worker represented by the worker global scope and the scripts whose settings object is outside settings, then set worker global scope to null.

      For example, a user agent could have a development mode that isolates a particular top-level browsing context from all other pages, and scripts in that development mode could be blocked from connecting to shared workers running in the normal browser mode.

    7. If worker global scope is not null, then run these steps:

      1. Let settings object be the relevant settings object for worker global scope.

      2. If the result of executing Is environment settings object a secure context? on outside settings does not match the result of executing the same algorithm on settings object, then throw a "SecurityError" DOMException and abort these steps. [SECURE-CONTEXTS]

      3. Associate worker with worker global scope.

      4. Create a new MessagePort object whose owner is settings object. Let this be the inside port.

      5. Entangle outside port and inside port.

      6. Create a trusted event that uses the MessageEvent interface, with the name connect, which does not bubble, is not cancelable, has no default action, has a data attribute whose value is initialized to the empty string, has a ports attribute whose value is initialized to a new frozen array containing only the newly created port, and has a source attribute whose value is initialized to the newly created port. Queue a task, using the DOM manipulation task source, to dispatch the event at worker global scope.

      7. Add to worker global scope's list of the worker's Documents the list of relevant Document objects to add given outside settings.

      8. If outside settings's global object is a WorkerGlobalScope object, add worker global scope to the list of the worker's workers of outside settings's global object.

      9. Return worker and abort all these steps.

  7. Return worker and perform the remaining steps in parallel.

  8. Run a worker given worker, urlRecord, outside settings, outside port, and options.

[NoInterfaceObject, Exposed=(Window,Worker)]
interface NavigatorConcurrentHardware {
  readonly attribute unsigned long long hardwareConcurrency;
};
window . navigator . hardwareConcurrency
Returns the number of logical processors potentially available to the user agent.

The navigator.hardwareConcurrency attribute's getter must return a number between 1 and the number of logical processors potentially available to the user agent. If this cannot be determined, the getter must return 1. (This is a fingerprinting vector.)

User agents should err toward exposing the number of logical processors available, using lower values only in cases where there are user-agent specific limits in place (such as a limitation on the number of workers that can be created) or when the user agent desires to limit fingerprinting possibilities.

10.3 APIs available to workers

10.3.1 Importing scripts and libraries

When a script invokes the importScripts(urls) method on a WorkerGlobalScope object, the user agent must import scripts into worker global scope given this WorkerGlobalScope object and urls.

To import scripts into worker global scope, given a WorkerGlobalScope object worker global scope and a sequence<DOMString> urls, run these steps. The algorithm may optionally be customized by supplying custom perform the fetch hooks, which if provided will be used when invoking fetch a classic worker-imported script.

  1. If worker global scope's type is "module", throw a TypeError exception and abort these steps.

  2. Let settings object be the incumbent settings object.

  3. If urls is empty, abort these steps.

  4. Parse each value in urls relative to settings object. If any fail, throw a "SyntaxError" DOMException and abort these steps.

  5. For each url in the resulting URL records, run these substeps:

    1. Fetch a classic worker-imported script given url and settings object, passing along any custom perform the fetch steps provided. If this succeeds, let script be the result. Otherwise, rethrow the exception.

    2. Run the classic script script, passing the rethrow errors flag.

      script will run until it either returns, fails to parse, fails to catch an exception, or gets prematurely aborted by the "kill a worker" or "terminate a worker" algorithms defined above.

      If an exception was thrown or if the script was prematurely aborted, then abort all these steps, letting the exception or aborting continue to be processed by the calling script.

Service Workers is an example of a specification that runs this algorithm with its own options for the perform the fetch hook. [SW]

10.3.2 The WorkerNavigator object

The navigator attribute of the WorkerGlobalScope interface must return an instance of the WorkerNavigator interface, which represents the identity and state of the user agent (the client):

[Exposed=Worker]
interface WorkerNavigator {};
WorkerNavigator implements NavigatorID;
WorkerNavigator implements NavigatorLanguage;
WorkerNavigator implements NavigatorOnLine;
WorkerNavigator implements NavigatorConcurrentHardware;

10.3.3 Worker locations

[Exposed=Worker]
interface WorkerLocation {
  stringifier readonly attribute USVString href;
  readonly attribute USVString origin;
  readonly attribute USVString protocol;
  readonly attribute USVString host;
  readonly attribute USVString hostname;
  readonly attribute USVString port;
  readonly attribute USVString pathname;
  readonly attribute USVString search;
  readonly attribute USVString hash;
};

A WorkerLocation object has an associated WorkerGlobalScope object (a WorkerGlobalScope object).

The href attribute's getter must return the associated WorkerGlobalScope object's url, serialized.

The origin attribute's getter must return the Unicode serialization of the associated WorkerGlobalScope object's url's origin.

It returns the Unicode rather than the ASCII serialization for compatibility with MessageEvent.

The protocol attribute's getter must run return the associated WorkerGlobalScope object's url's scheme, followed by ":".

The host attribute's getter must run these steps:

  1. Let url be the associated WorkerGlobalScope object's url.

  2. If url's host is null, return the empty string.

  3. If url's port is null, return url's host, serialized.

  4. Return url's host, serialized, followed by ":" and url's port, serialized.

The hostname attribute's getter must run these steps:

  1. Let host be the associated WorkerGlobalScope object's url's host.

  2. If host is null, return the empty string.

  3. Return host, serialized.

The port attribute's getter must run these steps:

  1. Let port be the associated WorkerGlobalScope object's url's port.

  2. If port is null, return the empty string.

  3. Return port, serialized.

The pathname attribute's getter must run these steps:

  1. Let url be the associated WorkerGlobalScope object's url.

  2. If url's cannot-be-a-base-URL flag is set, return the first string in url's path.

  3. Return "/", followed by the strings in url's path (including empty strings), separated from each other by "/".

The search attribute's getter must run these steps:

  1. Let query be the associated WorkerGlobalScope object's url's query.

  2. If query is either null or the empty string, return the empty string.

  3. Return "?", followed by query.

The hash attribute's getter must run these steps:

  1. Let fragment be the associated WorkerGlobalScope object's url's fragment.

  2. If fragment is either null or the empty string, return the empty string.

  3. Return "#", followed by fragment.