JS实现简单的顶部定时关闭层效果

<script type="text/javascript"> 
var time = 300; 
var h = 0; 
function addCount() 
{ 
if(time>0) 
{ 
time--; 
h = h+5; 
} 
else 
{ 
return; 
} 
if(h>300) //高度 
{ 
return; 
} 
document.getElementById("ads").style.display = ""; 
document.getElementById("ads").style.height = h+"px"; 
setTimeout("addCount()",30); 
} 
window.onload = function showAds() 
{ 
addCount(); 
setTimeout("noneAds()",7000); //停留时间自己调了 
} 
var T = 300; 
var N = 300; //高度 
function noneAds() 
{ 
if(T>0) 
{ 
T--; 
N = N-5; 
} 
else 
{ 
return; 
} 
if(N<0) 
{ 
document.getElementById("ads").style.display = "none"; 
return; 
} 
document.getElementById("ads").style.height = N+"px"; 
setTimeout("noneAds()",30); 
} 
</script> 
 
 代码如下:

<div style="MARGIN: 0px auto; WIDTH: 900px; BACKGROUND: #369; HEIGHT: 300px; COLOR: #fff" id="ads">脚本之家_www.gxlcms.com</div>