js网页滚动条滚动事件实例分析

<style type="text/css"> 
#top_div{ 
 position:fixed; 
 bottom:80px; 
 right:0; 
 display:none; 
} 
</style> 
<script type="text/javascript"> 
window.onscroll = function(){ 
 var t = document.documentElement.scrollTop || document.body.scrollTop; 
 var top_div = document.getElementById( "top_div" ); 
 if( t >= 300 ) { 
  top_div.style.display = "inline"; 
 } else { 
  top_div.style.display = "none"; 
 } 
} 
</script> 
<a name="top">顶部<a> 
<div id="top_div"><a href="#top">返回顶部</a></div> 
<br /> 
<br /> 
<div> 
这里尽量多些<br />以便页面出现滚动条，限于篇幅本文此处略去 
</div>