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function PopupCenter(pageURL, title,w,h) {
  var left = (screen.width/2)-(w/2);
  var top = (screen.height/2)-(h);
  var targetWin = window.open(pageURL, title, 'toolbar=no, location=no, directories=no, status=no, menubar=no, scrollbars=no, resizable=no, copyhistory=no, width='+w+', height='+h+', top='+top+', left='+left);
  return targetWin;
} 

PopupCenter("https://stackoverflow.com/questions/4068373/center-a-popup-window-on-screen", "google", screen.width,screen.height-(screen.height * 0.13));

function PopupCenter(pageURL, title,w,h) {
  var left = (screen.width/2)-(w/2);
  var top = (screen.height/2)-(h/2);
  var targetWin = window.open(pageURL, title, 'toolbar=no, location=no, directories=no, status=no, menubar=no, scrollbars=no, resizable=no, copyhistory=no, width='+w+', height='+h+', top='+top+', left='+left);
  return targetWin;
} 

PopupCenter("https://stackoverflow.com/questions/4068373/center-a-popup-window-on-screen", "google", 750,500);