center

var wid = "800";
var hei = "900";
var popwid = "400";
var pophei = "450";
var leftPos = (wid-popwid)/2;
var topPos = (hei-pophei)/2;
window.open('popup','width=' + popwid + ',height='+pophei+',top='+topPos+','left='+leftPos);