计算window窗口大小

var width = window.innerWidth;

var height = window.innerHeight;

if(typeof width != 'number'){//IE 5/6/7/8

if(document.compatMode == 'CSS1Compat'){

width = document.documentElement.clientWidth;

height = document.docuementElement.clientHeight;

}else{

width = document.body.clientWidth;

height = document.body.clientHeight;

}