HTML5 回到顶部

图片：

html

<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8" />
		<title>回到顶部</title>
		<link rel="stylesheet" type="text/css" href="css/try.css" />
		<script src="js/app.js"></script>
	</head>
	<body>
		<div  class="box">
                  <img   src="img/jkxy.png"/>
        </div>
        <a style="display:none" href="javascript:;"  id="btn"  title="回到顶部"></a>
	</body>
</html>

　　css

*{
    margin: 0;
    padding: 0;
}
.box {
    1000px;
    margin:  0  auto;
}
#btn {
    40px;
    height: 40px;
   position:fixed;
    left:50%;
    bottom:30px;
    background:url(../img/顶部.png)  no-repeat  left top ;
    margin-left: 610px;
}
#btn:hover {
    background:url(../img/顶部.png)  no-repeat  left -40px;
}

indow.onload = function () {
    var topbtn = document.getElementById("btn");
    var timer = null;
    //获取屏幕的高度
    var pagelookheight = document.documentElement.clientHeight;

    window.onscroll = function(){
        //滚动超出高度，显示按钮，否则隐藏
        var backtop = document.body.scrollTop;
           //  滚动超过一频    应该显示
        if(backtop >= pagelookheight){
            topbtn.style.display = "block";
        }
      //   不显示
       else{
            topbtn.style.display = "none";
        }
    }

    topbtn.onclick = function () {
         timer = setInterval(function () {
            var backtop = document.body.scrollTop;  
              //速度操作  减速
            var speedtop = backtop/5;   
            document.body.scrollTop = backtop -speedtop;  //高度不断减少
            if(backtop ==0){  //滑动到顶端
                clearInterval(timer);  //清除计时器
            }
        }, 30);
    }
}

　　效果：