<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<style type="text/css">
*{margin: 0;padding: 0;}
html,body{height: 3000px;}
#t{
width: 60px;
height: 60px;
background: red;
position: fixed;
right: 0;
text-align: center;
line-height: 60px;
top: 400px;
display: none;
}
</style>
</head>
<body>
<div id="t">回到顶部</div>
</body>
</html>
<script type="text/javascript">
var box=document.getElementById("t");
window.onscroll=function(){
var sTop=document.documentElement.scrollTop||document.body.scrollTop;
if(sTop>400){
box.style.display="block";
}else{
box.style.display="none";
}
}
box.onclick=function(){
document.documentElement.scrollTop||document.body.scrollTop==0;
}
</script>