Gathering actual performance from the browser in real time

gistfile1.js

(function (window) {

  var iterations = 100;
  var sampling = 1000;
  var counters = [];
  
  var i = 0, j, sum = 0, mean;
  var lastNow = window.performance.now();

  var interval = setInterval(function () {
    var now = window.performance.now();
    counters.push(now - lastNow - sampling);
    lastNow = now;
    i++;
    if (i >= iterations) {
      clearInterval(interval);
      
      for(j=0;j<counters.length;j++) {
        sum += counters[j];
      }
      
      mean = sum/counters.length;
  
      console.log('performance result. Sum: ' + sum + ' Mean: ' + mean);
    }
  }, sampling);

})(window);