I would like the fixed header div to be hidden when it reaches the level of the test div, my code below does not hide the test div

in pure javascript please

<script>
          function showDiv()
          {
            if ($(window).scrollTop() < $(".test").offset().top)
            {
              $('.header').fadeIn('slow');
            }
            else
            {
              $('.header').fadeOut('slow');
            }
          }
          $(window).scroll(showDiv);
          showDiv();
        </script>