html代码：
<button style="position: fixed;right: 30px;bottom:20px" id="back" onclick="back()">返回顶部</button>
javascript代码：
for(var i = 0; i < 100; i++){
document.write("<li>" + i + "</li>");
}
function back(){
var back = document.getElementById("back");
var iSpeed = 40;
var time = setInterval(function(){
var scrollTop = document.body.scrollTop || document.documentElement.scrollTop;
if(scrollTop > 0){
document.body.scrollTop = document.documentElement.scrollTop =scrollTop/3 - iSpeed;
}else{
clearInterval(time);
}
},100)
}
...)
