Calculate the frame rate at which the current browser is rendering the frame at

gistfile1.js

(function(){
    var TO_RUN = 20;
    var sum = count = time = last = 0;
    var first = false; // ignore the first time loop

    var fn = function(){
        time = new Date().getTime();

        if(first === true){
            sum += time - last;
            count++;
        } else {
            first = true;
        }

        last = time;

        if(count < TO_RUN) {
            requestAnimationFrame(fn);
        } else {
            console.log("Average Fps: " + 1000/(sum/TO_RUN));
        }

    };

    requestAnimationFrame(fn);
})();