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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
<head>
    <title>Javascript 回到顶部效果</title>
    <link rel="stylesheet" type="text/css" href="style.css" />
    <script type="text/javascript" src="script.js"></script>
    <style type="text/css">

#btn {
    width:40px;
    height:40px;
    position:fixed;
    left:50%; 
    display: none;
    margin-left:610px;
    bottom:30px; 
    background:url(http://img1.sycdn.imooc.com//535e0dc100010e9c00400080.jpg) no-repeat left top;
    }
#btn:hover {
    background:url(http://img1.sycdn.imooc.com//535e0dc100010e9c00400080.jpg) no-repeat 0 -39px;
    }
.box {
    width:1190px; 
    margin:0 auto; 
    }</style>
</head>
<body>  
    <div>
        <img src="http://img1.sycdn.imooc.com//535e0ce800015b7511902787.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');
    //获取页面可视区的高度
    var clientHeight=document.documentElement.clientHeight || document.body.clientHeight;
    var timer = null;
    //var isTop = true;
    
    window.onscroll=function(){
        var osTop=document.documentElement.scrollTop || document.body.scrollTop;
        
        if (osTop >= clientHeight){
            //显示按钮
            obtn.style.display="block";
        }else {
            //隐藏按钮
            obtn.style.display="none";
        }
        //if (!isTop){
         //   clearInterval(timer);
        //}
        //isTop = false;
    }
    
        
    obtn.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;    
            var ispeed = Math.floor(-osTop / 6);
            document.documentElement.scrollTop = document.body.scrollTop = osTop +ispeed;
            
            //isTop = true;
            console.log(osTop +ispeed);
            if (osTop == 0){
                clearInterval(timer);
            }
        },30);
    }
}
    


</script>
</body>
</html>

老师注释的部分有什么作用，不要也没什么问题吧