javascript的计算div高度的问题

我想实现一个功能，有一个div，开始时相对于顶端300px，鼠标滚轮向下，当div层滚到相对于顶端为0px是，fixed定位，一直留在那

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head></head>
<body>
<style>
.mclass{
position:absolute;
right:0px;
top:300px;
width:200px;
height:200px;
border:1px solid #000;
}</style>
 
 
 
 
 <br <br <br 
 <br 
 
<div class='mclass' id='div'></div>
<script>var div = document.getElementById('div');window.setInterval(function(){
 var scrolltop = document.documentElement.scrollTop;
 if(scrolltop >= 300) div.style.top = scrolltop+'px'
 else div.style.top = '300px'
},1)
</script>
</body>
</html>
或者:window.onscroll=function(){
var scrolltop = document.documentElement.scrollTop;
   if(scrolltop >= 300) div.style.top = scrolltop+'px'
   else div.style.top = '300px'
}