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.| API | |||||
|---|---|---|---|---|---|
| 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:Example
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");
}Add an "onmessage" event listener to the web worker.
w.onmessage = function(event){
document.getElementById("result").innerHTML = 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;Full Web Worker Example Code
We have already seen the Worker code in the .js file. Below is the code for the HTML page:Example
<!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
