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js監聽滾動條滾動 改變元素

<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width,initial-scale=1.0, minimum-scale=1.0, maximum-scale=1.0, user-scalable=no" />
    <title>js監聽滾動條滾動事件-改變元素</title>
    <style type="text/css">
        body {
            background: #F5F5F5;
            height: 3000px;
        }
        #header-top {
            position: fixed;
            left: 0;
            top: 0;
            right: 0;
            z-index: 50;
            height: 40px;
            text-align: center;
            background: #ff0000;
            color: #fff;
            font-size:20px;
            line-height: 40px;
        }
    </style>
    <script>
        window.onscroll=function(){
            var t =document.documentElement.scrollTop||document.body.scrollTop;
            var headerTop =document.getElementById("header-top");
            if( t >=40) {
                headerTop.style.background="rgba(255,0,255,1)";
            }else{
                headerTop.style.background="#ff0000";
            }
        }
    </script>
</head>
<body>
    <div id="header-top">標題</div>
</body>
</html>