index.html

<!DOCTYPE html>
<html>
<body>

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

<script>
    var w = null;

    function startWorker(){
        if(typeof(Worker)!=="undefined"){
            if(w==null){
                w=new Worker("worker.js");
            }
            w.onmessage = function (event) {
                document.getElementById("result").innerHTML=event.data;
            };
        }
        else{
            document.getElementById("result").innerHTML="Sorry, your browser does not support Web Workers...";
        }
    }

    function stopWorker(){
        if(w==null){
            return;
        }
        w.terminate();
        w = null;
    }
</script>

</body>
</html>

worker.js

var i=0;

function timedCount(){
	i=i+1;
	postMessage(i);
	setTimeout("timedCount()",500);
}

timedCount();