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HTML Web Workers API

A web worker is a JavaScript running in the background, without affecting the performance of the page.

What is a Web Worker?

When executing scripts in an HTML page, the page becomes unresponsive until the script is finished.

A web worker is a JavaScript that runs in the background, independently of other scripts, without affecting the performance of the page. You can continue to do whatever you want: clicking, selecting things, etc., while the web worker runs in the background.

Browser Support

The numbers in the table specify the first browser version that fully support Web Workers.

Web Workers 4.0 10.0 3.5 4.0 11.5

HTML Web Workers Example

The example below creates a simple web worker that count numbers in the background:


Count numbers:

Try it Yourself »

Check Web Worker Support

Before creating a web worker, check whether the user's browser supports it:

if (typeof(Worker) !== "undefined") {
  // Yes! Web worker support!
  // Some code.....
} else {
  // Sorry! No Web Worker support..

Create a Web Worker File

Now, let's create our web worker in an external JavaScript.

Here, we create a script that counts. The script is stored in the "demo_workers.js" file:

var i = 0;

function timedCount() {
  i = i + 1;


The important part of the code above is the postMessage() method - which is used to post a message back to the HTML page.

Note: Normally web workers are not used for such simple scripts, but for more CPU intensive tasks.

Create a Web Worker Object

Now that we have the web worker file, we need to call it from an HTML page.

The following lines checks if the worker already exists, if not - it creates a new web worker object and runs the code in "demo_workers.js":

if (typeof(w) == "undefined") {
  w = new Worker("demo_workers.js");

Then we can send and receive messages from the web worker.

Add an "onmessage" event listener to the web worker.

w.onmessage = function(event){
  document.getElementById("result").innerHTML = event.data;

When the web worker posts a message, the code within the event listener is executed. The data from the web worker is stored in event.data.

Terminate a Web Worker

When a web worker object is created, it will continue to listen for messages (even after the external script is finished) until it is terminated.

To terminate a web worker, and free browser/computer resources, use the terminate() method:


Reuse the Web Worker

If you set the worker variable to undefined, after it has been terminated, you can reuse the code:

w = undefined;

Full Web Worker Example Code

We have already seen the Worker code in the .js file. Below is the code for the HTML page:


<!DOCTYPE html>

<p>Count numbers: <output id="result"></output></p>
<button onclick="startWorker()">Start Worker</button>
<button onclick="stopWorker()">Stop Worker</button>

var w;

function startWorker() {
  if (typeof(Worker) !== "undefined") {
    if (typeof(w) == "undefined") {
      w = new Worker("demo_workers.js");
    w.onmessage = function(event) {
      document.getElementById("result").innerHTML = event.data;
  } else {
    document.getElementById("result").innerHTML = "Sorry! No Web Worker support.";

function stopWorker() {
  w = undefined;

Try it Yourself »

Web Workers and the DOM

Since web workers are in external files, they do not have access to the following JavaScript objects:

  • The window object
  • The document object
  • The parent object