jquery 弹出层居中设置

var iWidth = document.documentElement.clientWidth;
var iHeight = document.documentElement.clientHeight;
var scrollHeight = document.documentElement.scrollTop;//文档卷去高度

var oShow = document.getElementById('show');

oShow.style.left = (iWidth-oShow.offsetWidth)/2+"px";
oShow.style.top = (iHeight-oShow.offsetHeight)/2+scrollHeight+"px";