var WindowObjectReference = null; // global variable

function openFFPromotionPopup()
{
  if(WindowObjectReference == null || WindowObjectReference.closed)
  /* if the pointer to the window object in memory does not exist or if such pointer exists but the window was closed */

  {
    WindowObjectReference = window.open("http://resanet.webcamp.fr/resa.php?Camping=tuc40&flag=resa&slg=fr",
   "Resa", "width=820,height=900,resizable=yes,scrollbars=yes,status=yes");
    /* then create it. The new window will be created and  will be brought on top of any other window. */
  }
  else
  {
    WindowObjectReference.focus();
    /* else the window reference must exist and the window is not closed; therefore, we can bring it back on top of any other
       window with the focus() method. There would be no need to re-create the window or to reload the referenced resource. */
  };
}