使元素固定在网页中，不需要用到JS的，只需要用到CSS样式就可以了。
使用css 中的 position:fixed fixed是相对于浏览器的定位，设置了这个样式的元素将不会随页面滚动而改变位置，固定在屏幕中。
如 ：
.classname {
position:fixed
left:0
top:0
z-index:9999
}
给你一个朴素版的导航(包含css和html2部分)：css.css文件
body{
font-size:12px
font-family:Arial,
Helvetica,
sans-serif
margin:0px
padding:0px
color:white
}
ul,li{
margin:0px
padding:0px
}
li{
display:inline
list-style:none
text-align:center
font-weight:bold
float:left
}
a:link{
color:#336601
text-align:center
text-decoration:none
float:left
width:100px
padding:3px
5px
0px
5px
}
a:visited{
color:#336601
text-align:center
text-decoration:none
float:left
padding:3px
5px
0px
5px
width:100px
}
a:hover{
color:white
float:left
padding-left:6px
text-align:center
width:100px
text-decoration:none
background-color:#539D26
}
a:active{
color:white
float:left
padding:3px
3px
0px
20px
width:100px
text-align:center
text-decoration:none
background-color:#539D26
}
#nav{
width:600px
height:30px
border-bottom:0px
padding:0px
5px
position:absolute
z-index:1
left:
198px
top:
25px
}
.list{
line-height:20px
text-align:left
padding:4px
font-weight:normal
}
.menu1{
width:120px
height:auto
margin:6px
4px
0px
0px
border:1px
solid
#9CDD75
background-color:#F1FBEC
color:#336601
padding:6px
0px
0px
cursor:pointer
overflow-y:hidden
filter:Alpha(opacity=70)
-moz-opacity:0.7
}
.menu2{
width:120px
height:18px
margin:6px
4px
0px
0px
background-color:#F5F5F5
color:#999999
border:1px
solid
#EEE8DD
padding:6px
0px
0px
0px
overflow-y:hidden
cursor:pointer
}
<!DOCTYPE html><html>
<head lang="en">
<meta charset="UTF-8">
<title></title>
<style type="text/css">
li {
display: inline-block
width: 100%
height: 30px
line-height: 30px
}
li:active, li:focus, .li-active {
background: #dddddd
color: red
}
</style>
<script type="text/javascript">
onload = function () {
function removeActiveClass(node) {
node.className = ''
}
document.querySelector('ul[id=test]').onclick = function (e) {
Array.prototype.forEach.call(document.querySelectorAll('ul[id=test] >li'), removeActiveClass)
var target = e.target
target.className = 'li-active'
}
}
</script>
</head>
<body>
<ul id="test">
<li>1111</li>
<li>2222</li>
<li>3333</li>
<li>4444</li>
</ul>
</body>
</html>
