转载自：http://www.cnblogs.com/foodoir/p/5885174.html
完整代码：
<!DOCTYPE
html>
<html>
<head>
<meta charset="UTF-8">
<title>Javascript 回到顶部效果</title>
<style type="text/css">
#btn {
width: 40px;
height: 40px;
position: fixed;
display: none;
right: 0px;
bottom: 30px;
background: url(2.jpg) no-repeat left top;
}
#btn:hover {
background: url(2.jpg) no-repeat 0 -40px;
}
.box {
width: 1190px;
margin: 0 auto;
}
</style>
</head>
<body>
<div class="box">
<img src="1.jpg" />
</div>
<a href="javascript:;" id="btn" title="回到顶部"></a>
<script type="text/javascript">
window.onload = function() {
var obtn = document.getElementById('btn');
var timer = null;
var isTop = true;
//获取页面的可视窗口高度
var clientHeight = document.documentElement.clientHeight || document.body.clientHeight;
//滚动条滚动时触发
window.onscroll = function(){
//在滚动的时候增加判断
var osTop = document.documentElement.scrollTop || document.body.scrollTop;//特别注意这句，忘了的话很容易出错
if (osTop >= clientHeight) {
obtn.style.display = 'block';
}else{
obtn.style.display = 'none';
}
if (!isTop) {
clearInterval(timer);
}
isTop = false;
};
btn.onclick = function(){
//设置定时器
timer = setInterval(function(){
//获取滚动条距离顶部的高度
var osTop = document.documentElement.scrollTop || document.body.scrollTop; //同时兼容了ie和Chrome浏览器
//减小的速度
var isSpeed = Math.floor(-osTop / 6);
document.documentElement.scrollTop = document.body.scrollTop = osTop + isSpeed;
//console.log( osTop + isSpeed);
isTop = true;
//判断，然后清除定时器
if (osTop == 0) {
clearInterval(timer);
}
},30);
};
}
</script>
</body>
</html>