var ajaxObj = null;

/* Function that starts a new AJAX request. */
function startAjaxReq()
{
  /* If there is an ongoing AJAX request, we stop it now. */
  if (ajaxObj != null)
  {
    ajaxObj.abort();
    ajaxObj = null;
  }

  ajaxObj = new XMLHttpRequest();
  /* Etc... */
}