scroll down up のクラス名を使用しておきたい

[Jung Issei]

  GlobalMenu.prototype.gm_position_fixed_view_out = function() {
    let _ = this,
        is_fixed = 'is_fixed',
        is_scroll_up = 'is_scroll_up',
        is_scroll_down = 'is_scroll_down',
        last_position = 0,
        curr_position = 0;

    $(window).on('scroll', function(){
      if(parseInt($(area_header).offset().top + $(area_header).height()) < $(window).scrollTop()){

        curr_position = $(this).scrollTop();
        let remove_class = curr_position <= last_position?
              is_scroll_down : is_scroll_up,
            add_class = curr_position <= last_position?
              is_scroll_up : is_scroll_down;

        $area_header
          .removeClass(remove_class)
          .addClass(
            add_class + ' ' +
            is_fixed
          );

        last_position = curr_position;

      }else{
        $area_header.removeClass(
          is_scroll_up + ' ' +
          is_scroll_down + ' ' +
          is_fixed
        );

      }

    });
  }