//判断页面滚动到顶部和底部

    $(window).scroll(function(){

        var doc_height = $(document).height();

        var scroll_top = $(document).scrollTop();

        var window_height = $(window).height();

        if(scroll_top == 0){

            alert("到顶啦");

        }else if(scroll_top + window_height >= doc_height){

            alert("到底啦啦");

        }

    });