js 判断div距离浏览器顶部或者底部的距离

$(window).scroll(function() {             var scrollTop = $(this).scrollTop();             var scrollHeight = $(document).height();             var windowHeight = $(this).height();
            if(scrollTop == 0){                 alert('顶部');             }             if(scrollTop + windowHeight == scrollHeight  ){                 alert('下拉到底了');             }         })