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Web Workers API
A web worker is JavaScript that runs in the background, without affecting the performance of the page.
What is a Web Worker?
When executing scripts on an HTML page, the page becomes unresponsive until the script is finished.
A web worker is JavaScript running in the background, independently of other scripts, without affecting the performance of the page. You can to do activities like: clicking, selecting things, etc., while the web worker runs in the background.
Web Workers Example
The example below creates a simple web worker that counts numbers in the background.
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Web Workers API</h2>
<p>Count numbers: <output id=”result”></output></p>
<button onclick=”startWorker()”>Start Worker</button>
<button onclick=”stopWorker()”>Stop Worker</button>
<script>
let w;
function startWorker() {
if(typeof(w) == “undefined”) {
w = new Worker(“demo_workers.js”);
}
w.onmessage = function(event) {
document.getElementById(“result”).innerHTML = event.data;
};
}
function stopWorker() {
w.terminate();
w = undefined;
}
</script>
</body>
</html>
Check Web Worker Support
To create a web worker, always make sure to check whether the user’s browser supports it:
Example
if (typeof(Worker) !== “undefined”) {
// Yes! Web worker support!
// Some code…..
} else {
// Sorry! No Web Worker support..
}
Create a Web Worker File
Create a web worker in external JavaScript.
Here, we create a script that counts. The script is stored in the “demo_workers.js” file:
Example
let i = 0;
function timedCount() {
i ++;
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.
Create a Web Worker Object
After the web worker file, call it from an HTML page.
The following lines check if the worker already exists, if not – it creates a new web worker object and runs the code in “demo_workers.js”:
Example
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 inside 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 continues 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>
<h2>JavaScript Web Workers API</h2>
<p>Count numbers: <output id=”result”></output></p>
<button onclick=”startWorker()”>Start Worker</button>
<button onclick=”stopWorker()”>Stop Worker</button>
<script>
let w;
function startWorker() {
if(typeof(w) == “undefined”) {
w = new Worker(“demo_workers.js”);
}
w.onmessage = function(event) {
document.getElementById(“result”).innerHTML = event.data;
};
}
function stopWorker() {
w.terminate();
w = undefined;
}
</script>
</body>
</html>