js回到顶部

 1 <!DOCTYPE html>
 2 <html lang="en">
 3 <head>
 4 <meta charset="UTF-8">
 5 <meta name="viewport" content="width=device-width, initial-scale=1.0">
 6 <meta http-equiv="X-UA-Compatible" content="ie=edge">
 7 <title>回到顶部</title>
 8 <style>
 9    * { padding: 0; margin: 0; }
10    #box {width: 100%;height: 2400px;margin: auto;border: 1px solid red;box-sizing: border-box;}
11    #top {width: 60px;height: 50px;line-height: 50px;background-color: orange;color: #fff;font-size: 12px;text-align: center;position: fixed;
12          right: 20px;bottom: 20px;text-decoration: none;display: none;
13    }
14 </style>
15 </head>
16 <body>
17   <div id="box"></div>
18   <a href="javascript:;" id="top">返回顶部</a>
19 </body>
20 <script>
21  window.onload = function () {
22   var oTop = document.getElementById('top')
23   // 获取页面可视区的高度
24   var clientHeight = document.documentElement.clientHeight
25   var timer = null
26   var isTop = true
27  
28   // 滚动条滚动时触发
29   window.onscroll = function () {
30   var osTop = document.documentElement.scrollTop || document.body.scrollTop
31   if (osTop >= clientHeight) {
32     oTop.style.display = 'block'
33   } else {
34     oTop.style.display = 'none'
35   }
36   if (!isTop) {
37     clearInterval(timer)
38   }
39   isTop = false
40 }
41 
42 oTop.onclick = function () {
43   timer = setInterval(function () {
44     // 获取滚动条距离顶部的高度
45     var osTop = document.documentElement.scrollTop || document.body.scrollTop
46     var ispeed = Math.floor(-osTop / 5)
47     document.documentElement.scrollTop = document.body.scrollTop = osTop + ispeed
48     isTop = true
49     if (osTop === 0) {
50        clearInterval(timer)
51     }
52   }, 30)
53  }
54 }
55 </script>
56 </html>
57