判断滚动条到底部、

    $(document).ready(function() {
        $(window).scroll(function() {
            reachBottom();
        });
    });
    function reachBottom() {
        var scrollTop = 0;
        var clientHeight = 0;
        var scrollHeight = 0;
        if (document.documentElement && document.documentElement.scrollTop) {
            scrollTop = document.documentElement.scrollTop;
        } else if (document.body) {
            scrollTop = document.body.scrollTop;
        }
        if (document.body.clientHeight && document.documentElement.clientHeight) {
            clientHeight = (document.body.clientHeight < document.documentElement.clientHeight) ? document.body.clientHeight : document.documentElement.clientHeight;
        } else {
            clientHeight = (document.body.clientHeight > document.documentElement.clientHeight) ? document.body.clientHeight : document.documentElement.clientHeight;
        }
        scrollHeight = Math.max(document.body.scrollHeight, document.documentElement.scrollHeight);
        if (scrollTop + clientHeight == scrollHeight) {
            // return true;
            $("#test").fadeIn();
        } else {
            //return false;
            $("#test").fadeOut();
        }
    }