Edit in JSFiddle

(function (d, w, undefined) {
    const ww = new Worker(getInlineJS());
    const tresholdselector = d.querySelector('#treshold');
    const print = Helpers.log2Screen;
    
    tresholdselector.addEventListener('change', start);
    start();
    
    ww.onmessage = e => {
        // status message during calculation
        if (e.data.status) {
            return print( 'Found <code>' + e.data.found +
                          '</code> prime numbers', 
                          {clear: true, direct: true} );
        }
        
        d.querySelector('#waiting').classList.add("hidden");
        $('#warning').addClass('hidden');
        
        // print result
        print( e.data.msg, ', calculation took ', e.data.duration, 
               ' millisecond', e.data.duration != 1 ? 's': '',
               e.data.primes && e.data.primes.length > 10000 ?
                '<br><b class="warn">Busy</b> ' +
                '(reflow/repaint takes time and unfortunaly that\'s blocking)' : '',
               '<h3>Prime numbers listing</h3>', {clear: true, direct: true} );
        warnWait();
        setTimeout(() => {       
            let xresult = d.createElement('div');
            xresult.id = "PRIMES";
            xresult.style.display = "none";
            xresult.innerHTML = e.data.primes.join(' ');
            d.body.appendChild(xresult);
            xresult.style.display = "";
            warnWait(true, "Done");
        }, 10);
    };
    
    function warnWait(end, replaceText) {
        const warner = d.querySelector('b.warn');
        if (!warner) {return true;}
        var action = end ? "remove" : "add";
        warner.classList[action]("spin");
        warner.textContent = replaceText || warner.textContent;
    }
    
    function start() {
        const previous = d.querySelector("#PRIMES");
        if (previous) {d.body.removeChild(previous);}
        print('start working...', {clear:true, direct: true});
        $('#warning')[+tresholdselector.value > 100000 ? 'removeClass' : 'addClass']('hidden');
        d.querySelector('#waiting').classList.remove("hidden");
        ww.postMessage({value: tresholdselector.value});
    }
    
    function getInlineJS() {
        const ecmaCode = d.querySelector('[data-isworker]').textContent;
        const ecmaBlob = new Blob([ecmaCode], {"type": "application\/javascript"});
        return URL.createObjectURL(ecmaBlob);
    }
}(document, window))

<div class="solink" data-linkid="26312971"></div>

<h3>Using a web worker to calculate prime numbers from 0 to [Threshold]</h3>

<p id="chunking"></p>

<div id="waiting" class="hidden"><b class="working spin">Working</b> (Browser is <i>not</i> blocked)</div>

Threshold: <select id="treshold">
    <option value="10">10</option>
    <option value="100" selected="selected">100</option>
    <option value="1000">1000</option>
    <option value="10000">10,000</option>
    <option value="100000">100,000</option>
    <option value="1000000">1,000,000</option>
    <option value="10000000">10,000,000</option>
    <option value="100000000">100,000,000</option>
</select>

<script type="text/ecmaworker" data-isworker="true">
		String.prototype.padLeft =  String.prototype.padLeft ||
          function padLeft(base, chr) {
            var len = (String(base || 10).length - String(this).length) + 1;
            return len > 0 ? new Array(len).join(chr || '0') + this : this;
          };
    Number.prototype.pretty = function (usa, noprecision){
        const somenum = this;
        const dec = (''+somenum).split(/[.,]/);
        const lendec = dec[1] ? dec[1].length : 0
        const precision = lendec && !noprecision ? decPrecise(somenum,lendec) : dec[1]
        const sep = usa ? ',' : '.';
        const decsep = usa ? '.' : ',';

        // from http://stackoverflow.com/questions/10473994/javascript-adding-decimal-numbers-issue/10474209
        function decPrecise(d,l){
          return String(d.toFixed(12)).split(/[.,]/)[1].substr(0,l);
        }

        function xsep(num,sep) {
          let n = String(num).split('');
          let i = -3;
          while (n.length + i > 0) {
              n.splice(i, 0, sep);
              i -= 4;
          }
          return n.join('');
        }

        return xsep(dec[0],sep) + (dec[1] ? decsep+precision :'');
      }
    
      onmessage = function (e) {
          let n = 0;
          let total = 0;
          const THRESHOLD = +e.data.value;
          const startTime = new Date;
          const padFactor = Math.floor(THRESHOLD/10);
          let primes = [];

          while (++n < THRESHOLD) {
            let isprime = isPrime(n);
            total += isprime ? 1 : 0;
            void( isprime && (primes.push(String(n).padLeft(padFactor, '&nbsp;')),
                  primes.length % 100 < 1 &&  postMessage({status: true, found: primes.length})) );
          	}

          postMessage({ msg: 'Found <code>' + total.pretty(1) + 
                         '</code> prime numbers using threshold <code>' + 
                         THRESHOLD.pretty(1) + '</code>',
                    primes: primes,
                    duration: (new Date - startTime).pretty(1)});

          function isPrime(number) {
              if (number % 2 === 0 && number > 2) { return false; }
              let start = 2;
              const limit = Math.sqrt(number);
              while (start <= limit) {
                  if (number % start++ < 1) return false;
              }
              return number > 1;
          }
      }
</script>

#theChunks {
    margin-top: 1.5em;
}

#warning {
    color: red;
    font-weight: bold;
}

.hidden {
    display: none;
}

.numbers {
    font-family: courier new;
}

#PRIMES {
    font-family: courier new;
    white-space: pre-line;
}

#waiting {
    width: 300px;
    height: 1.2em;
    position: relative;
    margin-bottom: 1em;
}

.wait {
    width: 20px;
    position: absolute;
    top: 0;
    bottom: 0;
    background-color: red;
    transition: all ease-in-out 0.1s 0s;
}

@keyframes spinner  {
    from {
      -moz-transform: rotateY(0deg);
      -ms-transform: rotateY(0deg);
      transform: rotateY(0deg);
    }
    to {
      -moz-transform: rotateY(-360deg);
      -ms-transform: rotateY(-360deg);
      transform: rotateY(-360deg);
    }
  }

.warn, .working {
    color: red;
}

.spin {
    display: inline-block;
    animation: spinner 1s linear infinite;    
}