JS按钮控制内容上下滚动

运行效果如下：
代码如下：
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>JS按钮控制内容上下滚动</title>
    <style>
    *{ padding: 0; margin: 0;}
    li{ list-style: none;}
    a{ text-decoration: none; color: #333;}
    a:hover{ color: #f00;}
    .center{ text-align: center; margin-top: 20px; font-family: "Microsoft Yahei";}
    #box{ width: 168px; margin: 20px auto; border: 1px solid #ccc; background: #fff; font-size: 12px; position: relative;}
    #up ,#down{ position: absolute; z-index: 2; cursor: pointer; background: #eee; width: 100%; text-align: center; height: 28px; line-height: 28px;}
    #up{ top: 0;}
    #down{ bottom: 0;}
    .content{ margin-top: 28px; height: 475px; overflow: hidden; position: relative; background: #ccc;}
    #wrap{ position: absolute; top: 0;}
    #wrap ul{ padding: 0 10px;}
    #wrap li{ width: 148px; height: 100px; background: purple; margin-top: 10px;}
    </style>
</head>
<body>
    <h3 class="center">先向上滚动试试，然后向下滚动试试</h3>
    <div id="box">
        <a id="up" href="javascript:;">向上滚动</a>
        <a id="down" href="javascript:;">向下滚动</a>
        <div class="content">
            <div id="wrap">
                <ul>
                    <li></li>
                    <li></li>
                    <li></li>
                    <li></li>
                    <li></li>
                    <li></li>
                </ul>
            </div>
        </div>
    </div>
    <script>
    // 封装getStyle函数
    function getStyle(obj ,attr){
        return obj.currentStyle ? obj.currentStyle[attr] :getComputedStyle(obj ,false)[attr];
    }

    window.onload = function(){
        var oBox = document.getElementById('box');
        // alert(getStyle(oBox,'width'))        // 168px
        var oUp = document.getElementById('up');
        var oDown = document.getElementById('down');
        var oWrap = document.getElementById('wrap');
        var num = 0;
        var timer = null;

        // 鼠标按下向上链接，开启定时器
        oUp.onmousedown = function(){
            oWrap.timer = setInterval(function(){
                // 先获取内容的top值[获取的是字符串，所以要用parseInt转换成数字]，然后让它每隔300ms减少5px使之向上运动
                var dis = parseInt(getStyle(oWrap,'top')) - 5;
                // 这里的200是根据当前案例设置的，可以根据实际情况调整数值
                if( dis < -220){
                    dis = -220;
                }
                oWrap.style.top = dis + 'px';
            },30);
        };

        // 鼠标移开向上链接，关闭定时器
        oUp.onmouseup = function(){
            clearInterval(oWrap.timer);
        };

        // 鼠标按下向下链接，开启定时器
        oDown.onmousedown = function(){
            oWrap.timer = setInterval(function(){
                var dis = parseInt(getStyle(oWrap,'top')) + 5;
                if(dis > 0){
                    dis = 0;
                }
                oWrap.style.top = dis + 'px';
            },30);
        };

        // 鼠标移开向下链接，关闭定时器
        oDown.onmouseup = function(){
            clearInterval(oWrap.timer);
        };
    };
    </script>
</body>
</html>