滚动条监听事件

$(function(){
     $(window).scroll(function(){
         var h=$("body").scrollTop();
         if(h>200){$(".aaa").css("top","0px");}else if(h<200){$(".aaa").css("top","100px");}
     })
})

<!DOCTYPE html>
<html>
<head>
    <title></title>
    <script src="http://cdn.static.runoob.com/libs/jquery/2.1.1/jquery.min.js"></script>
    <script>
    window.onscroll = function () {
    var t = document.documentElement.scrollTop || document.body.scrollTop;
    if (t > 0&&t<500) {
    $(".aaa").css("height","50");
    } else if(t>500&t<1000) {
        $(".aaa").css("height","100");


    }
    </script>
    <style type="text/css">
    .aaa{background-color:red;height:80px;100px;position: fixed;top:100px}
    </style>
</head>

<body style="height:7500px">
<div class="aaa"></div>
</body>
<ml>