jquery判斷頁面是否滑動到最底部

// 滾動到底部，向下的箭頭消失

       var $down = $('.down');

       var $window = $(window);

       var $document = $(document);

       $window.scroll(function(){

           if ($document.scrollTop() + $window.height() >= $document.height()) {

               $down.hide();

           } else {

               $down.show();

           }

       });