​

var timer = document.querySelector('#o');

var calls = 0;

var start = +new Date;

​

setInterval(function () {

  if (window.stopTimer) { return; }

    var time = (new Date - start) / 1000;

    var avg  = ++calls / time;

    timer.textContent = 

       calls + " calls in " + time.toFixed(3) + "s is " + avg.toFixed(6) + " calls/second";

}, 10);

​

​

document.querySelector('#stop').onclick = function() {

  window.stopTimer = true;

}