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');

            }

        })