JavaScript: Show massage before user leave the page

gistfile1.js

window.onbeforeunload = confirmExit;
  function confirmExit()
  {
    if (needToConfirm)
    {
      // check to see if any changes to the data entry fields have been made
      for (var i = 0; i < values.length; i++)
      {
        var elem = document.getElementById(ids[i]);
        if (elem)
          if ((elem.type == 'checkbox' || elem.type == 'radio')
                  && values[i] != elem.checked)
            return "You have attempted to leave this page.  If you have made any changes to the fields without clicking the Save button, your changes will be lost.  Are you sure you want to exit this page?";
          else if (!(elem.type == 'checkbox' || elem.type == 'radio') &&
                  elem.value != values[i])
            return "You have attempted to leave this page.  If you have made any changes to the fields without clicking the Save button, your changes will be lost.  Are you sure you want to exit this page?";
      }

      // no changes - return nothing      
    }
  }