average frame time

calcFrameTime.js

function flashFrameTime (counts, cb) {
  if (!typeof counts === 'number' || !counts > 0) { throw "no!"; }

  var accum    = []
  ,   reqFrame = window.requestAnimationFrame
  ;

  function calcFramerate () {
    return accum
      .slice(0, accum.length-1)
      .map(function (v, i) {
         return accum[i+1] - v;
      })
      .reduce(function (prev, curr) {
        return prev + curr;
      }) / (accum.length - 1);
  }

  (function accumulate () { 
    if (counts-- > 0) { 
      accum.push(Date.now()); 
      reqFrame(accumulate); 
    } else {
      cb && cb(calcFramerate());
    }
  })();
}