JS 回到顶部

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title></title>
    <style>
        #goTop{
            display:none;
            position:fixed;
            bottom:20px;
            background:url(http://cdn.w3cfuns.com/resource/images/scrolltop.png) no-repeat 0 0;
            width:34px;
            height:34px;
            cursor:pointer;
            right:20px;
        }
    </style>
    <script>
        window.onload=function(){
            var timer=null;
            var goTop=document.getElementById("goTop");
            goTop.onclick=function(){
                timer=setInterval(function(){
                    var top=document.body.scrollTop+document.documentElement.scrollTop;
                    var speed=top/5;
                    if(document.body.scrollTop){
                        document.body.scrollTop-=speed;
                    }
                    else{
                        document.documentElement.scrollTop-=speed;
                    }
                    if(top==0){
                        clearInterval(timer);
                    }
                },30);
            };
            window.onscroll=function(){
                if(document.body.scrollTop+document.documentElement.scrollTop>=500){
                    goTop.style.display="block";
                }
                else{
                    goTop.style.display="none";
                }
            };
        };
    </script>
</head>
<body>
    <p>123</p><br/>
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    <p>123</p><br/>
    <span id="goTop"></span>
</body>
</html>