js网页滚动条滚动事件实例分析

<style type="text/css"> 
#_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 _div = document.getElementById( "_div" ); 
 if( t >= 300 ) { 
  _div.style.display = "inline"; 
 } else { 
  _div.style.display = "none"; 
 } 
} 
</script> 
<a name="">顶部<a> 
<div id="_div"><a href="#">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>