HTML

<div class="header">我是顶部导航栏</div>
    <div class="content">
        <div class="sk">秒杀模块</div>
    </div>
    <div class="backtop">
        <img src="./images/close2.png" alt="">
        <a href="javascript:;"></a>
    </div>

CSS

 * {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
        }

        .content {
            overflow: hidden;
            width: 1000px;
            height: 3000px;
            background-color: pink;
            margin: 0 auto;
        }

        .backtop {
            display: none;
            width: 50px;
            left: 50%;
            margin: 0 0 0 505px;
            position: fixed;
            bottom: 60px;
            z-index: 100;
        }

        .backtop a {
            height: 50px;
            width: 50px;
            background: url(./images/bg2.png) 0 -600px no-repeat;
            opacity: 0.35;
            overflow: hidden;
            display: block;
            text-indent: -999em;
            cursor: pointer;
        }

        .header {
            position: fixed;
            top: 0;
            left: 0;
            width: 100%;
            height: 80px;
            background-color: purple;
            text-align: center;
            color: #fff;
            line-height: 80px;
            font-size: 30px;
            transition: all .3s;
        }

        .sk {
            width: 300px;
            height: 300px;
            background-color: skyblue;
            margin-top: 500px;
        }

JS

//获取头部
        const header = document.querySelector('.header')
        //获取指定位置
        const sk = document.querySelector('.sk')
        //给窗口绑定滚动事件
        window.addEventListener('scroll',function(){
            //获取当前窗口滚动的高度
            let n = document.documentElement.scrollTop
            //判断页面滚动的高度是否大于等于指定位置的高度，如果是就显示，不是就隐藏
            header.style.top = n >= sk.offsetTop ? 0 : '-80px'
        })