jquery弹出层完美居中

showDiv($("#pop"));

function showDiv(obj){
 $(obj).show();
 center(obj);
 $(window).scroll(function(){
  center(obj);
 });
 $(window).resize(function(){
  center(obj);
 });
}

function center(obj){
 var windowWidth = document.documentElement.clientWidth;
 var windowHeight = document.documentElement.clientHeight;
 var popupHeight = $(obj).height();
 var popupWidth = $(obj).width();
 $(obj).css({
  "position": "absolute",
  "top": (windowHeight-popupHeight)/2+$(document).scrollTop(),
  "left": (windowWidth-popupWidth)/2
 });
}