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 Using a JavaScript module as a worker
        3. 10.1.2.3 Shared workers introduction
        4. 10.1.2.4 Shared state using a shared worker
        5. 10.1.2.5 Delegation
        6. 10.1.2.6 Providing libraries
      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 Runtime script errors
      4. 10.2.4 Creating workers
        1. 10.2.4.1 Properties present on both Worker and SharedWorker
        2. 10.2.4.2 Dedicated workers and the Worker interface
        3. 10.2.4.3 Shared workers and the SharedWorker interface
      5. 10.2.5 Concurrent hardware capabilities
    3. 10.3 APIs available to workers
      1. 10.3.1 The WorkerNavigator interface
      2. 10.3.2 The WorkerLocation interface

10 Web workers

Web_Workers_API

Support in all current engines.

Firefox3.5+Safari4+Chrome4+
Opera10.6+Edge79+
Edge (Legacy)12+Internet Explorer10+
Firefox Android4+Safari iOS5+Chrome Android18+WebView Android4+Samsung Internet1.0+Opera Android11+

Web_Workers_API/Using_web_workers

10.1 Introduction

10.1.1 Scope

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

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

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 lang="en">
 <head>
  <meta charset="utf-8">
  <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 Using a JavaScript module as a worker

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.

As the import statement is available, the importScripts() method will automatically fail inside module workers.

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

<p>
  <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>
</p>

<p>
  <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>
</p>

<div id="output"></div>

<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 = () => {
      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.replaceChildren(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.

10.1.2.3 Shared workers introduction

SharedWorker

Firefox29+Safari5–7Chrome4+
Opera10.6+Edge79+
Edge (Legacy)NoInternet ExplorerNo
Firefox Android33+Safari iOS5–7Chrome AndroidNoWebView AndroidNoSamsung Internet4.0–5.0Opera Android11–14

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>
<html lang="en">
<meta charset="utf-8">
<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>
<html lang="en">
<meta charset="utf-8">
<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>
<html lang="en">
<meta charset="utf-8">
<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>
<html lang="en">
<meta charset="utf-8">
<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.

10.1.2.4 Shared state using a shared worker

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 lang="en">
 <head>
  <meta charset="utf-8">
  <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 lang="en">
 <head>
  <meta charset="utf-8">
  <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.

10.1.2.5 Delegation

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 lang="en">
 <head>
  <meta charset="utf-8">
  <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.

10.1.2.6 Providing libraries

Suppose that a cryptography library is made available that provides three tasks:

Generate a public/private key pair
Takes a port, on which it will send two messages, first the public key and then the private key.
Given a plaintext and a public key, return the corresponding ciphertext
Takes a port, to which any number of messages can be sent, the first giving the public key, and the remainder giving the plaintext, each of which is encrypted and then sent on that same channel as the ciphertext. The user can close the port when it is done encrypting content.
Given a ciphertext and a private key, return the corresponding plaintext
Takes a port, to which any number of messages can be sent, the first giving the private key, and the remainder giving the ciphertext, each of which is decrypted and then sent on that same channel as the plaintext. The user can close the port when it is done decrypting content.

The library itself is as follows:

function handleMessage(e) {
  if (e.data == "genkeys")
    genkeys(e.ports[0]);
  else if (e.data == "encrypt")
    encrypt(e.ports[0]);
  else if (e.data == "decrypt")
    decrypt(e.ports[0]);
}

function genkeys(p) {
  var keys = _generateKeyPair();
  p.postMessage(keys[0]);
  p.postMessage(keys[1]);
}

function encrypt(p) {
  var key, state = 0;
  p.onmessage = function (e) {
    if (state == 0) {
      key = e.data;
      state = 1;
    } else {
      p.postMessage(_encrypt(key, e.data));
    }
  };
}

function decrypt(p) {
  var key, state = 0;
  p.onmessage = function (e) {
    if (state == 0) {
      key = e.data;
      state = 1;
    } else {
      p.postMessage(_decrypt(key, e.data));
    }
  };
}

// support being used as a shared worker as well as a dedicated worker
if ('onmessage' in this) // dedicated worker
  onmessage = handleMessage;
else // shared worker
  onconnect = function (e) { e.port.onmessage = handleMessage; }


// the "crypto" functions:

function _generateKeyPair() {
  return [Math.random(), Math.random()];
}

function _encrypt(k, s) {
  return 'encrypted-' + k + ' ' + s;
}

function _decrypt(k, s) {
  return s.substr(s.indexOf(' ')+1);
}

Note that the crypto functions here are just stubs and don't do real cryptography.

This library could be used as follows:

<!DOCTYPE HTML>
<html lang="en">
 <head>
  <meta charset="utf-8">
  <title>Worker example: Crypto library</title>
  <script>
   const cryptoLib = new Worker('libcrypto-v1.js'); // or could use 'libcrypto-v2.js'
   function startConversation(source, message) {
     const messageChannel = new MessageChannel();
     source.postMessage(message, [messageChannel.port2]);
     return messageChannel.port1;
   }
   function getKeys() {
     let state = 0;
     startConversation(cryptoLib, "genkeys").onmessage = function (e) {
       if (state === 0)
         document.getElementById('public').value = e.data;
       else if (state === 1)
         document.getElementById('private').value = e.data;
       state += 1;
     };
   }
   function enc() {
     const port = startConversation(cryptoLib, "encrypt");
     port.postMessage(document.getElementById('public').value);
     port.postMessage(document.getElementById('input').value);
     port.onmessage = function (e) {
       document.getElementById('input').value = e.data;
       port.close();
     };
   }
   function dec() {
     const port = startConversation(cryptoLib, "decrypt");
     port.postMessage(document.getElementById('private').value);
     port.postMessage(document.getElementById('input').value);
     port.onmessage = function (e) {
       document.getElementById('input').value = e.data;
       port.close();
     };
   }
  </script>
  <style>
   textarea { display: block; }
  </style>
 </head>
 <body onload="getKeys()">
  <fieldset>
   <legend>Keys</legend>
   <p><label>Public Key: <textarea id="public"></textarea></label></p>
   <p><label>Private Key: <textarea id="private"></textarea></label></p>
  </fieldset>
  <p><label>Input: <textarea id="input"></textarea></label></p>
  <p><button onclick="enc()">Encrypt</button> <button onclick="dec()">Decrypt</button></p>
 </body>
</html>

A later version of the API, though, might want to offload all the crypto work onto subworkers. This could be done as follows:

function handleMessage(e) {
  if (e.data == "genkeys")
    genkeys(e.ports[0]);
  else if (e.data == "encrypt")
    encrypt(e.ports[0]);
  else if (e.data == "decrypt")
    decrypt(e.ports[0]);
}

function genkeys(p) {
  var generator = new Worker('libcrypto-v2-generator.js');
  generator.postMessage('', [p]);
}

function encrypt(p) {
  p.onmessage = function (e) {
    var key = e.data;
    var encryptor = new Worker('libcrypto-v2-encryptor.js');
    encryptor.postMessage(key, [p]);
  };
}

function encrypt(p) {
  p.onmessage = function (e) {
    var key = e.data;
    var decryptor = new Worker('libcrypto-v2-decryptor.js');
    decryptor.postMessage(key, [p]);
  };
}

// support being used as a shared worker as well as a dedicated worker
if ('onmessage' in this) // dedicated worker
  onmessage = handleMessage;
else // shared worker
  onconnect = function (e) { e.ports[0].onmessage = handleMessage };

The little subworkers would then be as follows.

For generating key pairs:

onmessage = function (e) {
  var k = _generateKeyPair();
  e.ports[0].postMessage(k[0]);
  e.ports[0].postMessage(k[1]);
  close();
}

function _generateKeyPair() {
  return [Math.random(), Math.random()];
}

For encrypting:

onmessage = function (e) {
  var key = e.data;
  e.ports[0].onmessage = function (e) {
    var s = e.data;
    postMessage(_encrypt(key, s));
  }
}

function _encrypt(k, s) {
  return 'encrypted-' + k + ' ' + s;
}

For decrypting:

onmessage = function (e) {
  var key = e.data;
  e.ports[0].onmessage = function (e) {
    var s = e.data;
    postMessage(_decrypt(key, s));
  }
}

function _decrypt(k, s) {
  return s.substr(s.indexOf(' ')+1);
}

Notice how the users of the API don't have to even know that this is happening — the API hasn't changed; the library can delegate to subworkers without changing its API, even though it is accepting data using message channels.

View this example online.

10.1.3 Tutorials

10.1.3.1 Creating a dedicated worker

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.mjs', { type: "module" });
10.1.3.2 Communicating with a dedicated worker

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

SharedWorker

Firefox29+Safari5–7Chrome4+
Opera10.6+Edge79+
Edge (Legacy)NoInternet ExplorerNo
Firefox Android33+Safari iOS5–7Chrome AndroidNoWebView AndroidNoSamsung Internet4.0–5.0Opera Android11–14

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

This standard defines 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. Service Workers defines a third kind. [SW]

10.2.1 The global scope

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

10.2.1.1 The WorkerGlobalScope common interface

WorkerGlobalScope

Support in all current engines.

Firefox3.5+Safari4+Chrome4+
Opera10.6+Edge79+
Edge (Legacy)12+Internet Explorer10+
Firefox Android4+Safari iOS5+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android11+

WorkerGlobalScope serves as the base class for specific types of worker global scope objects, including DedicatedWorkerGlobalScope, SharedWorkerGlobalScope, and ServiceWorkerGlobalScope.

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

Event handler Event handler event type
onerror

WorkerGlobalScope/onerror

Support in all current engines.

Firefox3.5+Safari4+Chrome4+
Opera11.5+Edge79+
Edge (Legacy)12+Internet Explorer10+
Firefox Android4+Safari iOS5+Chrome Android40+WebView Android37+Samsung Internet4.0+Opera AndroidYes
error
onlanguagechange

WorkerGlobalScope/onlanguagechange

Support in all current engines.

Firefox74+Safari4+Chrome4+
Opera11.5+Edge79+
Edge (Legacy)12+Internet ExplorerYes
Firefox AndroidNoSafari iOS5+Chrome Android40+WebView Android37+Samsung Internet4.0+Opera AndroidYes
languagechange
onoffline

WorkerGlobalScope/onoffline

Support in all current engines.

Firefox29+Safari8+Chrome4+
Opera?Edge79+
Edge (Legacy)NoInternet ExplorerNo
Firefox Android29+Safari iOS8+Chrome Android40+WebView Android40+Samsung Internet4.0+Opera Android?
offline
ononline

WorkerGlobalScope/ononline

Support in all current engines.

Firefox29+Safari8+Chrome4+
Opera?Edge79+
Edge (Legacy)NoInternet ExplorerNo
Firefox Android29+Safari iOS8+Chrome Android40+WebView Android40+Samsung Internet4.0+Opera Android?
online
onrejectionhandled rejectionhandled
onunhandledrejection unhandledrejection
10.2.1.2 Dedicated workers and the DedicatedWorkerGlobalScope interface

DedicatedWorkerGlobalScope

Support in all current engines.

Firefox3.5+Safari4+Chrome4+
Opera10.6+Edge79+
Edge (Legacy)12+Internet Explorer10+
Firefox Android4+Safari iOS5+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android11+

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.

dedicatedWorkerGlobal.name

Returns dedicatedWorkerGlobal's name, i.e. the value given to the Worker constructor. Primarily useful for debugging.

dedicatedWorkerGlobal.postMessage(message [, transfer ])
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 following are the event handlers (and their corresponding event handler event types) supported, as event handler IDL attributes, by objects implementing the DedicatedWorkerGlobalScope interface:

Event handler Event handler event type
onmessage

DedicatedWorkerGlobalScope/onmessage

Support in all current engines.

Firefox3.5+Safari4+Chrome4+
Opera10.6+Edge79+
Edge (Legacy)12+Internet Explorer10+
Firefox Android4+Safari iOS5+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android11+
message
onmessageerror

DedicatedWorkerGlobalScope/onmessageerror

Firefox57+SafariNoChrome60+
Opera47+Edge79+
Edge (Legacy)18Internet ExplorerNo
Firefox Android57+Safari iOSNoChrome Android60+WebView Android60+Samsung Internet8.0+Opera Android44+
messageerror
10.2.1.3 Shared workers and the SharedWorkerGlobalScope interface

SharedWorkerGlobalScope

Support in all current engines.

Firefox29+Safari5+Chrome4+
Opera10.6+Edge79+
Edge (Legacy)NoInternet ExplorerNo
Firefox Android29+Safari iOS7+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android11+

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

sharedWorkerGlobal.name

Returns sharedWorkerGlobal's name, i.e. the value given to the SharedWorker constructor. Multiple SharedWorker objects can correspond to the same shared worker (and SharedWorkerGlobalScope), by reusing the same name.

sharedWorkerGlobal.close()

Aborts sharedWorkerGlobal.

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

Event handler Event handler event type
onconnect

SharedWorkerGlobalScope/onconnect

Firefox29+SafariNoChrome4+
Opera10.6+Edge79+
Edge (Legacy)NoInternet ExplorerNo
Firefox Android29+Safari iOSNoChrome Android18+WebView Android37+Samsung Internet1.0+Opera Android11+
connect

10.2.2 The event loop

A worker event loop's task queues only have events, callbacks, and networking activity as tasks.

Each WorkerGlobalScope object has a closing flag, initially false, but which can get set to true when the worker is requested to close.

Once the WorkerGlobalScope's closing flag is set to true, the event loop's task queues 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 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 will 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 error report propagates to the Worker object, then to any containing workers in the same fashion, eventually reaching the Window if it was never handled along the way.

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.

10.2.4 Creating workers

10.2.4.1 Properties present on both Worker and SharedWorker

The following are the event handlers (and their corresponding event handler event types) supported, as event handler IDL attributes, by Worker and SharedWorker objects:

Event handler Event handler event type
onerror

ServiceWorker/onerror

Support in all current engines.

Firefox44+Safari11.1+Chrome40+
Opera27+Edge79+
Edge (Legacy)17+Internet ExplorerNo
Firefox Android44+Safari iOS11.3+Chrome Android40+WebView Android40+Samsung Internet4.0+Opera Android27+

SharedWorker/onerror

Firefox29+Safari5–7Chrome4+
Opera10.6+Edge79+
Edge (Legacy)NoInternet ExplorerNo
Firefox Android33+Safari iOS5–7Chrome AndroidNoWebView AndroidNoSamsung Internet4.0–5.0Opera Android11–14

Worker/onerror

Support in all current engines.

Firefox3.5+Safari4+Chrome4+
Opera10.6+Edge79+
Edge (Legacy)12+Internet Explorer10+
Firefox Android4+Safari iOS5+Chrome Android18+WebView Android4+Samsung Internet1.0+Opera Android11+
error
10.2.4.2 Dedicated workers and the Worker interface

Worker

Support in all current engines.

Firefox3.5+Safari4+Chrome4+
Opera10.6+Edge79+
Edge (Legacy)12+Internet Explorer10+
Firefox Android4+Safari iOS5+Chrome Android18+WebView Android4+Samsung Internet1.0+Opera Android11+
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 define the name of that global environment via the name option, primarily for debugging purposes. It can also 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 ])
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 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) supported, as event handler IDL attributes, by objects implementing the Worker interface:

Event handler Event handler event type
onmessage

Worker/onmessage

Support in all current engines.

Firefox3.5+Safari4+Chrome4+
Opera10.6+Edge79+
Edge (Legacy)12+Internet Explorer10+
Firefox Android4+Safari iOS5+Chrome Android18+WebView Android4+Samsung Internet1.0+Opera Android11+
message
onmessageerror

Worker/onmessageerror

Firefox57+SafariNoChrome60+
Opera47+Edge79+
Edge (Legacy)18Internet ExplorerNo
Firefox Android57+Safari iOSNoChrome Android60+WebView Android60+Samsung Internet8.0+Opera Android44+
messageerror
10.2.4.3 Shared workers and the SharedWorker interface

SharedWorker

Firefox29+Safari5–7Chrome4+
Opera10.6+Edge79+
Edge (Legacy)NoInternet ExplorerNo
Firefox Android33+Safari iOS5–7Chrome AndroidNoWebView AndroidNoSamsung Internet4.0–5.0Opera Android11–14
sharedWorker = new SharedWorker(scriptURL [, name ])

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.

sharedWorker = new SharedWorker(scriptURL [, 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. options can be used to define the name of that global environment via the name option. It can also 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. Note that attempting to construct a shared worker with options whose type or credentials values mismatch an existing shared worker will cause the returned sharedWorker to fire an error event and not connect to the existing shared worker.

sharedWorker.port

Returns sharedWorker's MessagePort object which can be used to communicate with the global environment.

self.navigator.hardwareConcurrency

Returns the number of logical processors potentially available to the user agent.

10.3 APIs available to workers

10.3.1 The WorkerNavigator interface

WorkerNavigator

Support in all current engines.

Firefox3.5+Safari4+Chrome4+
OperaYesEdge79+
Edge (Legacy)12+Internet Explorer10+
Firefox Android4+Safari iOS5+Chrome Android18+WebView Android1+Samsung Internet1.0+Opera AndroidYes

The WorkerNavigator interface implements a subset of the Navigator interface, consisting of the following APIs:

10.3.2 The WorkerLocation interface

WorkerLocation/toString

Support in all current engines.

Firefox3.5+Safari4+Chrome3+
Opera15+Edge79+
Edge (Legacy)12+Internet Explorer10+
Firefox Android4+Safari iOS5+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android14+

WorkerLocation

Support in all current engines.

Firefox3.5+Safari4+Chrome3+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer10+
Firefox Android4+Safari iOS5+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android12.1+

The WorkerLocation interface is like the Location interface, but lacks the assign(), replace(), reload(), and ancestorOrigins members.