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 versions that fully support Web Workers:
| Chrome | Internet explorer | Mozila firefox | Safari | Opera |
|---|---|---|---|---|
| Chrome 4 | IE 10 | Firefox 3.5 | Safari 4 | Opera 11.5 |
| Jan 2010 | Sep 2012 | Jun 2009 | Jun 2009 | Jun 2011 |
Count numbers:
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;
postMessage(i);
setTimeout("timedCount()",500);
}
timedCount(); 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:
w.terminate();
Reuse the Web Worker #
If you set the worker variable to undefined, after it has been terminated, you can reuse the code:
w = undefined;
<!DOCTYPE html>
<html>
<body>
<p>Count numbers: <output id="result"></output></p>
<button onclick="startWorker()">Start Worker</button>
<button onclick="stopWorker()">Stop Worker</button>
<script>
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.terminate();
w = undefined;
}
</script>
</body>
</html> 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
