看下这段代码，想优化下
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=gb2312" /><title>无标题文档</title></head><body><SCRIPT LANGUAGE="JavaScript">function initEcAd() {document.all.AdLayer1.style.posTop = -200;//设置onLoad事件激发以后，广告层相对于固定后的y方向位置document.all.AdLayer1.style.visibility = 'visible'//设置层为可见MoveLayer1('AdLayer1');//调用函数MoveLayer()}function MoveLayer1(layerName) {var x = 5;//浮动广告层固定于浏览器的x方向位置var y =document.documentElement .clientHeight/2-25;//浮动广告层固定于浏览器的y方向位置var diff = (document.documentElement .scrollTop + y - document.all.AdLayer1.style.posTop)*.40;var y = document.documentElement .scrollTop + y - diff;eval("document.all." + layerName + ".style.posTop = y");eval("document.all." + layerName + ".style.posLeft = x");//移动广告层setTimeout("MoveLayer1('AdLayer1');", 20);//设置20毫秒后再调用函数MoveLayer1()}document.write("<div id=AdLayer1 style='position: absolute;visibility:hidden;z-index:1'><img src=http://img.baidu.com/img/logo-zhidao.gif></div>");initEcAd()</SCRIPT></body></html>