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HTML

              
                <p>Do a prime number search from <input id="from" value="1"> to <input id="to" value="2000000" size="100">.</p>
<button onclick="doSearch()" id="searchButton">Start Searching</button>
<button onclick="cancelSearch()">Cancel</button>

<div id="primeContainer">
</div>

<div id="status"></div>


<script id="FindPrimes" type="javascript/worker">
   
onmessage = function(event) {  
  var primes = findPrimes(event.data.from, event.data.to);
  postMessage(
   {messageType: "PrimeList", data: primes}
  );
};

function findPrimes(fromNumber, toNumber) {
  // Create an array containing all integers between the two specified numbers.
  var list = [];
  for (var i=fromNumber; i<=toNumber; i++) {
    if (i>1) list.push(i);
  }

  // Test for primes.
  var maxDiv = Math.round(Math.sqrt(toNumber));
  var primes = [];

  var previousProgress;

  for (var i=0; i<list.length; i++) {
    var failed = false;
    for (var j=2; j<=maxDiv; j++) {
      if ((list[i] != j) && (list[i] % j == 0)) {
        failed = true;
      } else if ((j==maxDiv) && (failed == false)) {
        primes.push(list[i]);
      }
    }

    // Give a progress update.
    var progress = Math.round(i/list.length*100);
    if (progress != previousProgress) {
      postMessage(
       {messageType: "Progress", data: progress}
      );
      previousProgress = progress;   
    }
  }

  return primes;
}

  </script>
              
            
!

CSS

              
                body {
  font-family: "Lucida Sans Unicode", "Lucida Grande", sans-serif;
  margin: 20px;
}

#primeContainer {
  border: solid 1px black;
  padding: 3px;
  height: 300px;
  max-width: 500px;
  overflow: scroll;
  overflow-x: hidden;
  font-size: x-small;
  margin-top: 20px;
  margin-bottom: 10px;
}

input {
  width: 80px;
}

button {
  padding: 3px;
}

p {
  margin-bottom: 3px;
}

#status {
  color: darkred;
}

              
            
!

JS

              
                var statusDisplay;
var worker;
var searchButton;

window.onload = function() {
  statusDisplay = document.getElementById("status");
  searchButton = document.getElementById("searchButton");
};

function doSearch() {
  // Get the two numbers in the text boxes. This is the search range.
  searchButton.disabled = true;

  var fromNumber = document.getElementById("from").value;
  var toNumber = document.getElementById("to").value;

  // Ordinarily you would give the web worker JavaScript in a separate
  // file, using a line of code like this.  
  //worker = new Worker("PrimeWorker.js");
  
  // Because this example is hosted on CodePen,
  // which doesn't let you use separate JS files,
  // we need to use a workaround to grab the
  // PrimeWorker.js code from the findPrimes() function
  // on this page.  
  var blob = new Blob([document.querySelector('#FindPrimes').textContent]); 
  blobURL = window.URL.createObjectURL(blob);

  worker = new Worker(blobURL);
  worker.onmessage = receivedWorkerMessage;
  worker.onerror = workerError;
  
  worker.postMessage(
   { from: fromNumber,
     to: toNumber
   }
  );

  statusDisplay.innerHTML = "A web worker is on the job ("+
   fromNumber + " to " + toNumber + ") ...";  
}

function receivedWorkerMessage(event) {
  var message = event.data;

  if (message.messageType == "PrimeList") {
    var primes = message.data;

    // Show the prime number list on the page.
    var primeList = "";
    for (var i=0; i<primes.length; i++) {
      primeList += primes[i];
      if (i != primes.length-1) primeList += ", ";
    }
    var displayList = document.getElementById("primeContainer");
    displayList.innerHTML = primeList;

    if (primeList.length == 0) {
      statusDisplay.innerHTML = "Search failed to find any results.";
    }
    else {
      statusDisplay.innerHTML = "The results are here!";
    }
    searchButton.disabled = false;
  }
  else if (message.messageType == "Progress") {
    statusDisplay.innerHTML = message.data + "% done ...";
  }
}

function workerError(error) {
  statusDisplay.innerHTML = error.message;
}

function cancelSearch() {
  worker.terminate();
  worker = null;
  statusDisplay.innerHTML = "Search cancelled.";
  searchButton.disabled = false;
}


              
            
!
999px

Console