js平滑滚动回到顶部

优先使用 requestAnimationFrame实现。
实验中发现，IE中的requestAnimationFrame 对象，在页面载入的时候，似乎是不可用的。 所以在 domContentLoaded事件以后再读取，才是安全的。

<script>
 //没有输出!
console.log(window.requestAnimationFrame || window.msRequestAnimationFrame );

window.onDomContentLoaded = function(){
        //这里就有了。
	console.log(window.requestAnimationFrame || window.msRequestAnimationFrame);	
}
</script>

function scrollUp(){
		
		//IE10+/Android Browser4.4+/
		var requestAnimationFrame = window.requestAnimationFrame || window.mozRequestAnimationFrame 
			|| window.webkitRequestAnimationFrame || window.msRequestAnimationFrame 
			|| function(clb){
				return setTimeout(clb,1000/60);
		    };

		var cancelAnimationFrame = window.cancelAnimationFrame || window.mozCancelAnimationFrame 
			|| window.webkitCancelAnimationFrame || window.msCancelAnimationFrame 
			|| function(id){clearTimeout(id);};

		//document.getElementById("imagePreview").scrollIntoView();
		//document.body.scrollTop = "0px";
		
		var top = document.body.scrollTop || document.documentElement.scrollTop;
		
		//滚动时长
		var duration = 300; //300ms
		
		//计算步长
		var step =  top / (duration / ( 1000 / 60)) >> 0; //取整
		
		//window.console && console.log(requestAnimationFrame);
		
		function fn(){

			if(top >= 0){
				top -= step;
				document.documentElement.scrollTop = document.body.scrollTop = top;
				fn.rafTimer = requestAnimationFrame(fn);
			}else{
				document.body.scrollTop = 0;
				cancelAnimationFrame(fn.rafTimer);
			}
		}
		fn.rafTimer = requestAnimationFrame(fn);
	}