Implement a Timer in javascript. With tests. Took 1 hour to implement from scratch. Could be optimized.

timer.js

var expect = function(value1) {
  return {
    toBe: function(value2) {
      document.getElementById('test').innerHTML += 
        "<pre>"
      + ((JSON.stringify(value1) === JSON.stringify(value2)) ? "Pass " : "Fail ")
      + "(val1: " + JSON.stringify(value1)
      + " val2: " + JSON.stringify(value2)
      + ")</pre><br\>";
    }
  };
};
//expect(1).toBe(1);
//expect([0,1]).toBe([0,1]);

var Timer = (function(){
  var Timer = function(id) {
    this._startDate = new Date();
    this._id = id;
    this._elem = document.getElementById(id);
  };
  Timer.prototype.getTime = function() {
    var d = new Date (new Date() - this._startDate);
    return d.toISOString().match(/\d\d:\d\d:\d\d/)[0];
  };
  Timer.prototype.start = function() {
    var self = this;
    window.setInterval(function() {
      self._elem.innerHTML = self.getTime();
    },1000);
  };
  return Timer;
})();

var c = new Timer("timer");

expect(c.getTime()).toBe("00:00:00");
window.setTimeout(function() {
  expect(c.getTime()).toBe("00:00:01");
},1000);

c.start();

zindex.html

<h2>Timer</h2>
<div id="timer"></div>
<div id="test"></div>
<hr/>

I did this on codepen. Saving it here.