js 返回顶部小火箭

<script type="text/javascript">
	window.onload = function() {
		function $(id) {return document.getElementById(id);}
		function show(obj){obj.style.display = 'block';}
		function hide(obj){obj.style.display = 'none';}
		function scroll() {
		      if(window.pageYOffset != null ){//ie9+浏览器
				return {
					left:window.pageXOffset,
					top:window.pageYOffset
				}
			} else if(document.compatMode == "CSS1Compat"){//标准浏览器
					return {
						left:document.documentElement.scrollLeft,
						top:document.documentElement.scrollTop
					}
				}
				return {//其它浏览器
					left:document.body.scrollLeft,
					top:document.body.scrollTop
				}
			}
			var goTop = $("gotop");
			window.onscroll = function() {
				scroll().top > 0 ? show(goTop) : hide(goTop);
				leader = scroll().top;
			}
			var leader = 0,target = 0,timer = null;
			goTop.onclick = function() {
				target = 0; //可以省略
				timer = setInterval(function(){
					leader = leader + (target - leader) / 10;
					window.scroll(0,leader);
					if (leader == target) {
						clearInterval(timer);
					}
				},30)
			}
		}
	</script>