自动关闭的层js

<SCRIPT LANGUAGE="JavaScript1.2">
adTime=5;
chanceAd=1;

var ns=(document.layers);
var ie=(document.all);
var w3=(document.getElementById && !ie);
adCount=0;
function initAd(){
if(!ns && !ie && !w3) return;
if(ie) adDiv=eval('document.all.sponsorAdDiv.style');
else if(ns) adDiv=eval('document.layers["sponsorAdDiv"]');
else if(w3) adDiv=eval('document.getElementById("sponsorAdDiv").style');
randAd=Math.ceil(Math.random()*chanceAd);
if (ie||w3)
adDiv.visibility="visible";
else
adDiv.visibility ="show";
if(randAd==1) showAd();
}
function showAd(){
if(adCount<adTime*10){adCount+=1;
if (ie){documentWidth =document.body.offsetWidth/2+document.body.scrollLeft-20;
documentHeight =document.body.offsetHeight/2+document.body.scrollTop-20;}
else if (ns){documentWidth=window.innerWidth/2+window.pageXOffset-20;
documentHeight=window.innerHeight/2+window.pageYOffset-20;}
else if (w3){documentWidth=self.innerWidth/2+window.pageXOffset-20;
documentHeight=self.innerHeight/2+window.pageYOffset-20;}
adDiv.left=documentWidth-200;adDiv.top =documentHeight-200;
setTimeout("showAd()",100);}else closeAd();
}
function closeAd(){
if (ie||w3)
adDiv.display="none";
else
adDiv.visibility ="hide";
}
onload=initAd;
//End-->
</script>
<div id="sponsorAdDiv" style="visibility:hidden">
<table bgcolor="#008000"><tr><td>
<table width="300" bgcolor="#F0FFF0"><tr><td align="center" valign="middle">
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(该层将在5秒钟后自动关闭)

</td></tr></table></td></tr></table>
</div>

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