js实现html页面滚动条向下拉到一定的程度时，某个div就固定在顶部，向上拉时返回原位置。
实现代码
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>test</title>
<style>
*{
padding:0;
margin:0;
}
#header{
position: fixed;
top: 0;
left: 0;
height: 30px;
width: 100%;
background: #555;
opacity: 0.8;
}
#nav,.nav{
height: 30px;
width: 100%;
background: #000;
opacity: 0.8;
}
.fixed{
position: fixed;
top:30px;
left:0;
}
</style>
</head>
<body>
<div id="header"></div>
<div style="margin-top:30px;">
<script type="text/javascript">
//document.write('<div id="nav"></div>');
for(var i=0;i<100;i++)
document.write('<div>'+i+' hello world'+'</div>');
</script>
</div>
<div id="nav"></div>
<script type="text/javascript">
for(var i=100;i<200;i++)
document.write('<div>'+i+' hello world'+'</div>');
</script>
<script type="text/javascript">
function getElementViewTop(element){
var actualTop = element.offsetTop;
var current = element.offsetParent;
while (current !== null){
actualTop += current.offsetTop;
current = current.offsetParent;
}
var elementScrollTop=0;
if (documentpatMode == "BackCompat"){
elementScrollTop=document.body.scrollTop;
} else {
elementScrollTop=document.documentElement.scrollTop;
}
return actualTop;
// return actualTop-elementScrollTop;
}
function getScroll()
{
var top, left, width, height;
if (document.documentElement && document.documentElement.scrollTop) {
top = document.documentElement.scrollTop;
left = document.documentElement.scrollLeft;
width = document.documentElement.scrollWidth;
height = document.documentElement.scrollHeight;
} else if (document.body) {
top = document.body.scrollTop;
left = document.body.scrollLeft;
width = document.body.scrollWidth;
height = document.body.scrollHeight;
}
return { 'top': top, 'left': left, 'width': width, 'height': height };
}
var nav = ElementById('nav');
var posTop = getElementViewTop(nav);
window.addEventListener('scroll',function(event){
var scrollTop = getScroll().top;
var output = ElementById('output');
output.innerHTML = posTop +' '+scrollTop+' '+(posTop-scrollTop);
if(posTop>=30 && posTop-scrollTop <= 30)
nav.className = 'nav fixed';
else nav.className = 'nav';
},false);
</script>
<div id="output" style="position:fixed;width:60px;height:60px;right:100px;bottom:100px;background:#DDD">0</div>
</body>
</html>
