팝업창 화면 중앙에 띄우기

<script language="javascript" type="text/javascript">
function MoveCenterWindow(){
  var cx = document.body.clientWidth;
  var cy = document.body.clientHeight;
  var sw = screen.availWidth;
  var sh = screen.availHeight;
  var px = (sw-cx)/2;
  var py = (sh-cy)/2

  window.moveTo(px, py);
}

MoveCenterWindow();
</script>