var i = 0,

    timer;

document.querySelector('.btn-toggle').addEventListener('click', start, false);

​

function start() {

  timer = setInterval(increment, 1);

  document.querySelector('.btn-toggle').innerHTML = 'Pause';

}

​

function increment() {

  i++;

  document.querySelector('.stopwatch').innerHTML = i/1000;

}

​

function stop() {

  clearInterval(timer);

}