Edit in JSFiddle

Web Worker Demo by Troy Whorten 3/20/2014
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<br/>These web workers start computing primes and continuously return them, once a second.
<br/>Each new worker cycles through the colors of the rainbow!
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<br/>
<button onclick="getPrimes()">Add Worker!</button><label for="color">| Change color to:</label><input type="color" id="color" />
<div id="killZone"></div>
<br/>
<br/>
<span id="prime"></span>

<script type="javascript/worker" id="worker">
    // this is the prime numbers code from the spec example
    var x = 1;
    var color;

    self.onmessage = function(e) {
        color = e.data;
    };

    function isPrime(n) {
        if (n < 2) return false;
        var q = Math.sqrt(n);
        for (var i = 2; i <= q; i++) {
            if (n % i == 0) {
                return false;
            }
        }
        return true;
    }

    self.setInterval(function() {
        while (!isPrime(x))
        x++;
        postMessage("<span style='color:" + color + "'>" + x + " </span>");
        x++;
    }, 1000);
</script>

var workBlob = new Blob([document.getElementById("worker").textContent]);
var blobURL = window.webkitURL.createObjectURL(workBlob) || window.URL.createObjectURL(workBlob);
var worker = new Array();
var colors = ["purple", "blue", "green", "yellow", "orange", "red", "fuchsia"];


function getPrimes() {
    //worker.terminate();
    initWorker();
    var primes = document.getElementById('prime');
    var wrkIndex = worker.length - 1;
    worker[wrkIndex].postMessage(colors[worker.length % colors.length]);

    worker[wrkIndex].onmessage = function (event) {
        primes.innerHTML += event.data;
    }
    //primes.textContent += " End of getPrimes()";
}


function killWorker(num, el) {
    worker[num].terminate();
    el.parentNode.removeChild(el.nextSibling);
    el.parentNode.removeChild(el);   
    //alert(worker.length);
}

function colorWorker(num, color) {
    worker[num].postMessage(color);
}

function initWorker() {
    worker.push(new Worker(blobURL));

    var killZone = document.getElementById("killZone");
    var btn = document.createElement("button");
    btn.setAttribute("value", (Number(worker.length) - 1));
    btn.onclick = function () {
        killWorker(this.getAttribute("value"), this);
    }
    btn.innerHTML = "Kill " + (Number(worker.length) - 1) + "!";
    killZone.appendChild(btn);
    
    var clrbtn = document.createElement("button");
    clrbtn.setAttribute("value", (Number(worker.length) - 1));
    clrbtn.onclick = function () {
        colorWorker(this.getAttribute("value"), document.getElementById("color").value);
    }
    clrbtn.innerHTML = "Color " + (Number(worker.length) - 1) + "!";
    killZone.appendChild(clrbtn);
    
}