京东，大v店很多商城都有类似定时抢购选项卡，可以滚动至顶部然后固定；
具体实现如下，贴上代码：
<!DOCTYPE HTML>
<html>
<head>
<meta charset="UTF-8">
<title>滚动至顶部后固定</title>
<style type="text/css">
.wrapper{width:100%;height:2000px;margin-left:auto;margin-right:auto;}
.header{height:30px;width:100%;background-color: greenyellow;position: fixed;top:0;}
#testbody{padding-top: 30px;height: 120px;background-color: #2AC845;}
#nav_test_1{height:30px;position:relative;top:0;background:#125430;width:100%;}
#nav_test_2{height:30px;width:100%;position:relative;top:0;background-color: red;}
#nav_test_3{height:30px;width:100%;position:relative;top:0;background-color: blueviolet;}
</style>
</head>
<body>
<div class="wrapper">
<div id="top_1" class="header">标题</div>
<div id="testbody">
<p>212112122</p>
<p>212112122</p>
<p>212112122</p>
</div>
<div id="nav_test_1">
顶端导航测试1
</div>
<div id="testbody">
<p>212112122</p>
<p>212112122</p>
<p>212112122</p>
</div>
<div id="nav_test_2">
顶端导航测试2
</div>
<div id="testbody">
<p>212112122</p>
<p>212112122</p>
<p>212112122</p>
</div>
<div id="nav_test_3">
顶端导航测试3
</div>
<div id="testbody">
<p>212112122</p>
<p>212112122</p>
<p>212112122</p>
</div>
</div>
<script type="text/javascript" >
var ha = new Array();
var fixHight = 0; //由于postion变动高度变成不计算
function menuFixed(preid,id){
var obj = document.getElementById(id); //当前div ID对象
var preobj = document.getElementById(preid); //前一个DIV ID对象
var _getTop = obj.offsetTop-fixHight; //当前div距离顶端距离
var _preTop = preobj.offsetTop;//前一个DIV距离顶端距离
var _preHeight = preobj.offsetHeight;//前一个DIV高度
fixHight = fixHight+obj.offsetHeight;
console.log('preid:'+preid+" _preHeight:"+_preHeight);
ha.push({id:id,_getTop:_getTop,_preTop:_preTop,_preHeight:_preHeight,preid:preid});
console.log("arry:"+JSON.stringify(ha));
}
window.onscroll = function(){
var scrollTop = document.documentElement.scrollTop || document.body.scrollTop;
var i = getIndex(scrollTop);
console.log('index:'+i);
changePos(ha[i].preid,ha[i]._preHeight,ha[i].id,ha[i]._getTop,scrollTop);
}
function getIndex(scrollTop){
var length = ha.length;
for(var i=0;i<length;i++){
if(ha[i]._getTop >= scrollTop && scrollTop >= ha[i]._preTop)
return i;
}
return length-1;
}
function changePos(preid,preheight,id,gettop,scrollTop){
var obj = document.getElementById(id);
var preobj = document.getElementById(preid);
console.log('scrollTop:'+scrollTop+' gettop:'+gettop);
if(scrollTop < gettop){
preobj.style.position = 'fixed';
obj.style.position = 'relative';
if(scrollTop-gettop <= 0 && scrollTop-gettop >= -preheight){
preobj.style.top = gettop - preheight - scrollTop + 'px';
}else if(scrollTop-gettop <= 0){
preobj.style.top = 0;
}else{
preobj.style.top = -preheight+'px';
}
}else{
obj.style.position = 'fixed';
console.log('id:'+id);
}
}
</script>
<script type="text/javascript">
window.onload = function(){
menuFixed('top_1','nav_test_1');
menuFixed('nav_test_1','nav_test_2');
menuFixed('nav_test_2','nav_test_3');
}
</script>
</body>
</html>实现效果图:
