Instrumentation for the event loop

testTimeout.js

/*
  Paste this script in the console of a live website to quantify its lag.
  This script calls `setTimeout` every 50ms. If the event loop takes more than 2x that long
  to hit the callback, it's considered lag.
  Consecutive laggy loops are counted together and then, after the lag passes, they are
  logged to the console as a single warning.
  Like this:
    Lagged for 254 ms over 2 loop(s). Should have taken 100 ms (39% of actual).
 */

(function (timeoutDurationMs) {
  var lastQuickDate = null, lastDate = null, loopsSinceLastQuickDate = 0;
  function testTimeout() {
    var d = new Date();
    if (!lastQuickDate) {
      lastQuickDate = lastDate = d;
    }
    if (d - lastDate < timeoutDurationMs * 2) {
      if (lastDate !== lastQuickDate) {
        var slowdown = lastDate - lastQuickDate;
        var expectedTime = loopsSinceLastQuickDate * timeoutDurationMs;
        var percent = Math.round(expectedTime / slowdown * 100);
        console.warn('Lagged for ' + slowdown + ' ms over ' + loopsSinceLastQuickDate
         + ' loop(s). Should have taken ' + expectedTime + ' ms (' + percent + '% of actual).');
      }
      lastQuickDate = d;
      loopsSinceLastQuickDate = 0;
    } else {
      loopsSinceLastQuickDate++;
    }
    lastDate = d;
    setTimeout(testTimeout, timeoutDurationMs);
  }
  testTimeout();
  return 'Okay';
})(50);