DIV任何时候都居中的JS代码

<html>
<body onresize="doThis()" onscroll="doThis()">
<input type="button" value="sd" onclick="doThis();"/><br/> 
<div id="111" style="position:absolute;width:200px;height:200px;background-color:#666666;">
<div id="a0" name="nContainer">nContainer[0]</div>

<div id="a1" name="nContainer"">nContainer[1]</div>

<div id="a2" name="nContainer">nContainer[2]</div>

<div id="a3" name="nContainer">nContainer[3]</div>

<div id="a4" name="nContainer">nContainer[4]</div>

<div id="a5" name="nContainer">nContainer[5]</div>
</div>
<br/><br/><br/><br/><br/><br/><br/><br/><br/><br/>
<br/><br/><br/><br/><br/><br/><br/><br/><br/><br/>
<br/><br/><br/><br/><br/><br/><br/><br/><br/><br/>
<br/><br/><br/><br/><br/><br/><br/><br/><br/><br/>
<input type="button" value="sd" onclick="doThis();"/>

<SCRIPT LANGUAGE="JavaScript">
function doThis()
{
   var a = document.getElementById("111");
   a.style.left=(document.body.clientWidth/2-a.clientWidth/2)+"px";
   a.style.top=(document.body.scrollTop+document.body.clientHeight/2-a.clientHeight/2)+"px";
}
</SCRIPT> 
</body>
</html>

[提示：你可先修改部分代码，再按运行]