자바스크립트 팝업을 가운데 띄우고 포커스를 팝업으로 하기

<script>
function new_popup() {  
    var url = http://saybox.tistory.com;
    var n = "";
    var w = "800";
    var h = "300";
    var s = "";
    var winl = (screen.width - w) / 2;
    var wint = (screen.height - h) / 2;
    winprops = 'height='+h+',width='+w+',top='+wint+',left='+winl+',menubar=no,scrollbars=yes,status=no,toolbar=no,resizable=yes';
    win = window.open(url, n, winprops);
    if (parseInt(navigator.appVersion) >= 4) {
      win.window.focus();
}
</script>