KEYUP is not working

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Javascript

I am using this function to load data when i press down key of my keyboard but problem is when i press up key its not moving up please help

window.onkeydown = keydown;
      function keydown() {
          window.scrollBy(0, 10);
         if ($(window).scrollTop() == $(document).height() - $(window).height()) {
             CallWebService();
         }
      }