simple countdown timer for some stuff

timer.js

function Countdown(seconds, update, done) {
  this.length = seconds;
  this.update = update;
  this.done = done;
}

Countdown.prototype.start = function () {
  var self = this;

  self.update(self.length);

  this.timer = setInterval(function () {
    if (--self.length < 1) {
      clearInterval(self.timer);
      self.update(self.length);
      self.done();
    } else {
      self.update(self.length);
    }
  }, 1e3);
}

Countdown.toClock = function (time) {
  return ('0' + Math.floor(time / 60)).slice(-2) + ':' + ('0' + time % 60).slice(-2)
}



document.getElementsByTagName('button')[0].onclick = function () {
  (new Countdown(64, function(time) {
      timer.innerHTML = Countdown.toClock(time);
  }, function() {
      timer.innerHTML += 'go';
  })).start()
}