JavaScript固定导航栏

<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
    <style>
        * {
            margin: 0;
            padding: 0
        }

        img {
            vertical-align: top;
        }

        .main {
            margin: 0 auto;
            width: 1000px;
            margin-top: 10px;

        }

        .fixed {
            position: fixed;
            top: 0;
            left: 0;
        }
    </style>
</head>
<body>
<div class="top" id="topPart">
    <img src="images/top.png" alt=""/>
</div>
<div class="nav" id="navBar">
    <img src="images/nav.png" alt=""/>
</div>
<div class="main" id="mainPart">
    <img src="images/main.png" alt=""/>
</div>
<script src="common.js"></script>
<script>
    //获取页面向上或者向左卷曲出去的距离的值
    function getScroll() {
        return {
            left: window.pageXOffset || document.documentElement.scrollLeft || document.body.scrollLeft || 0,
            top: window.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop || 0
        };
    }
    //滚动事件
    window.onscroll = function () {
        //向上卷曲出去的距离和最上面的div的高度对比
        if (getScroll().top >= my$("topPart").offsetHeight) {
            //设置第二个div的类样式
            my$("navBar").className = "nav fixed";
            //设置第三个div的marginTop的值
            my$("mainPart").style.marginTop = my$("navBar").offsetHeight + 10 + "px";
        } else {
            my$("navBar").className = "nav";
            my$("mainPart").style.marginTop = "10px";
        }
    };
</script>
</body>
</html>
main.png
nav.png
top.png