console 命令行工具 X clear

console

 var obj = document.getElementById("wrap");
    var ot = obj.offsetTop;
    document.onscroll = function () {
        var st = document.body.scrollTop || document.documentElement.scrollTop;
        obj.setAttribute("data-fixed",st >= ot?"fixed":"")}

<body>
<div id="wrap"></div>
</body>

*{
          margin: 0;
          padding: 0;
      }
    body{
        height: 2000px;
        background-image: linear-gradient(-180deg, #15f09d 0%, #25A0FF 50%, #fca72b 100%);
    }
    #wrap{
        background-color: rgba(0,0,0,0.2);
        width: 100%;
        height: 100px;
        margin-top: 100px;
    }
    #wrap[data-fixed="fixed"]{
        position: fixed;
        top:0;
        left: 0;
        margin: 0;
    }