// index.html
<p id="timer">
  00:00:00
</p>

<button onClick="hangTheBrowser()">
  Hang the browser
</button>

<p id="message">Your message here</p>

/* ---- */

// script.js
var worker = new Worker('worker.js');

worker.addEventListener('message', function(e) {
  $('#message').text(e.data);
});

let count = 0;
let rAF_ID;

var rAFCallback = function(callback) {
  let count = callback;

  let ms = Math.floor(count % 1000);
  let s = Math.floor((count /  1000)) % 60;
  let m = Math.floor((count / 60000)) % 60;

  $('#timer').text(m + ":" + s + ":" + ms);
  rAF_ID = requestAnimationFrame( rAFCallback );
}

// request animation frame on render
rAF_ID = requestAnimationFrame( rAFCallback );

function hangTheBrowser() {
  worker.postMessage('hang the browser');
}

/* ---- */

// worker.js
self.addEventListener('message', function(e) {
  if(e.data === 'hang the browser') {
    let val = "";

    for (let i = 0; i < 10000; i++) {
      for (let j = 0; j < 10000; j++) {
        val = "Worker returned: " + i + j;
      }
    }

    self.postMessage(val);
  }
});