Web Worker Study Note

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Reference List


Web worker create a extra thread. The JS file running by this work will run through this thread.

There are some special things to be noticed when using web worker:

  1. Instead of window, the JS file will run in another global context. So make sure using self to get current gloabl context in a worker.

  2. You cannot directly manipulate DOM inside a web worker. However some items are still available, like WebSockets or IndexedDB.

The way to communicate between main thread and web worker thread is using messages. To send message, use postMessage() method. onmessage event handler or message event listener is used to receive message. Workers may use XMLHttpRequest for network I/O.

During this process, the data passed between main thread and web worker is copied instead of shared.

There are two workers: dedicated worker and shared worker. Dedicated worker is only accessible from the script that first spawned it. Shared worker can be accessed from multiple scripts.

Dedicated Worker

The best practice is to wrap web worker code in following if loop:

    //Add Web Worker Code

Message being sent/received can be a string/array or JSON object. JSON is supported by most modern browers.


Basic operations are: create, post and receive message, handler errors and terminate works. see code below:

//main thread
var myWorker = new Worker('worker.js'); //Declare new web worker

myWorker.addEventListener('message', 'onError', function(e){
    console.log(e.data); //main thread response to message from worker.js
}, false);

worker.postMessage(['Hello World','hello']); //main thread pass data to worker

worker.terminate(); //Terminate worker from main thread

//Error Handle Function. Notice three attributes for error event
function onError(e){
    console.log(e.message + e.filename + e.lineno);

self.addEventListener('message', function(e){
    //worker receive data from main thread and return it back. Notice self is used here
}, false);

self.close(); //Stop this work from worker thread

Workers can import scripts and use them:

importScripts('one.js', 'two.js');

If execution fails, NETWORK_ERROR will be thrown out. However, function declarations after importScripts() will still work because they are hoisted.

Data Passing

When web worker passing data, the data is always serialized on one hand and de-serialized on the other hand.

Main thread and worker thread do not share the same instance, so the end result is duplicated.

Shared Worker


Shared worker is accessible by multiple scripts.

Similar to dedicated worker, shared work can be declared by statement like var myWorker = new SharedWorker("worker.js");.


Shared worker communicate via a port object. Scripts can communicate with workers in a opened port.

Port can be started by:

  1. Use a onmessage event handler

  2. Instead above if you have a message event listener, then use start() function to start explicitly.


myWorker.port.start(); //main thread
port.start();//If a worker thread has port variable, call this line in worker


For shared worker, postMessage() should be used from port object:

testpart.onchange = function(){
    myWorker.port.postMessage('hello world');
myWorker.port.onmessage = function(e){

self.addEventListener('connect', function(e){
    var port = e.ports[0]; //Get first port
    port.onmessage = function(e){
        port.postMessage('hello world back');
    //If onmessage event listener is not used, needs to start port here


Compute in Background

This example is from MDN, counting fibonacii in worker thread:

var worker = new Worker("fibonacci.js");

worker.onmessage = function(event) {
  document.getElementById("result").textContent = event.data;
  dump("Got: " + event.data + "\n");

worker.onerror = function(error) {
  dump("Worker error: " + error.message + "\n");
  throw error;



var results = [];

function resultReceiver(event) {
  //After result returned, push to results array. This array has two elements: previous result and newly returned item
  if (results.length == 2) {
    postMessage(results[0] + results[1]);

function errorReceiver(event) {
  throw event.data;

onmessage = function(event) {
  var n = parseInt(event.data);

  if (n == 0 || n == 1) {

  /*Recursively create new worker thread and counting 4, 3, 2, 1, then return back to previous worker */
  for (var i = 1; i <= 2; i++) {
    var worker = new Worker("fibonacci.js");
    worker.onmessage = resultReceiver;
    worker.onerror = errorReceiver;

A very clean/simple example from html5rocks.com

This example cerate clean function to handle difference cases. Functions can be added to attribute of an object to be more organized.

function sayHI() {
    worker.postMessage({'cmd': 'start', 'msg': 'Hi'});

  function stop() {
    // Calling worker.terminate() from this script would also stop the worker.
    worker.postMessage({'cmd': 'stop', 'msg': 'Bye'});

  function unknownCmd() {
    worker.postMessage({'cmd': 'foobard', 'msg': '???'});

  var worker = new Worker('doWork2.js');

  worker.addEventListener('message', function(e) {
    document.getElementById('result').textContent = e.data;
  }, false);

  self.addEventListener('message', function(e) {
      var data = e.data;
      switch (data.cmd) {
        case 'start':
          self.postMessage('WORKER STARTED: ' + data.msg);
        case 'stop':
          self.postMessage('WORKER STOPPED: ' + data.msg + '. (buttons will no longer work)');
          self.close(); // Terminates the worker.
          self.postMessage('Unknown command: ' + data.msg);
    }, false);
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