返回顶部/底部3

<!DOCTYPE html>
<html lang="en">

<head>
    <title>返回顶部/底部</title>
    <meta charset="UTF-8">
    <script src="http://apps.bdimg.com/libs/jquery/1.9.1/jquery.min.js" type="text/javascript"></script>
    <script type='text/javascript'>
    //显隐按钮
    function showReposBtn() {
        var clientHeight = $(window).height();
        var scrollTop = $(document).scrollTop();
        var maxScroll = $(document).height() - clientHeight;
        //滚动距离超过可视一屏的距离时显示返回顶部按钮
        if (scrollTop > clientHeight) {
            $('#retopbtn').show();
        } else {
            $('#retopbtn').hide();
        }
        //滚动距离到达最底部时隐藏返回底部按钮
        if (scrollTop >= maxScroll) {
            $('#rebtmbtn').hide();
        } else {
            $('#rebtmbtn').show();
        }
    }

    window.onload = function() {
        //获取文档对象
        $body = (window.opera) ? (document.compatMode == "CSS1Compat" ? $("html") : $("body")) : $("html,body");
        //显示按钮
        showReposBtn();
    }

    window.onscroll = function() {
        //滚动时调整按钮显隐
        showReposBtn();
    }

    //返回顶部
    function returnTop() {
        $body.animate({
            scrollTop: 0
        }, 400);
    }

    //返回底部
    function returnBottom() {
        $body.animate({
            scrollTop: $(document).height()
        }, 400);
    }
    </script>
    <style type='text/css'>
    #retopbtn {
        position: fixed;
        bottom: 10px;
        right: 10px;
    }
    
    #rebtmbtn {
        position: fixed;
        top: 10px;
        right: 10px;
    }
    </style>
</head>

<body>
    <button id='rebtmbtn' onclick='returnBottom()'>底部</button>
    <div style=" 600px; height: 500px; background: #ddd;"></div>
    <div style=" 600px; height: 500px; background: #F8B88E;"></div>
    <div style=" 600px; height: 500px; background: #BF6969;"></div>
    <button id='retopbtn' onclick='returnTop()'>顶部</button>
</body>

</html>

效果图：

另外一种写法：

$(function(){
    //返回顶部
    $('#back_top').on('click',move);
    $(window).on('scroll',function(){
        checkPostion($(window).height());
    });
    function move() {
        $('html,body').animate({
            scrollTop:0
        },800);
    }
    function checkPostion(pos) {
        if($(window).scrollTop() > pos) {
            $('#back_top').fadeIn();
        } else {
            $('#back_top').fadeOut();
        }
    }
});