requestAnimationFrame优化动画效果的API

(function() {    var lastTime = 0;    var vendors = ['webkit', 'moz'];    for(var x = 0; x < vendors.length && !window.requestAnimationFrame; ++x) {        window.requestAnimationFrame = window[vendors[x]+'RequestAnimationFrame'];        window.cancelAnimationFrame =          window[vendors[x]+'CancelAnimationFrame'] || window[vendors[x]+'CancelRequestAnimationFrame'];    }    if (!window.requestAnimationFrame)        window.requestAnimationFrame = function(callback, element) {            var currTime = new Date().getTime();            var timeToCall = Math.max(0, 16 - (currTime - lastTime));            var id = window.setTimeout(function() { callback(currTime + timeToCall); },              timeToCall);            lastTime = currTime + timeToCall;            return id;        };    if (!window.cancelAnimationFrame)        window.cancelAnimationFrame = function(id) {            clearTimeout(id);        };}());