console 命令行工具 X clear

console

//获取元素
var span = document.querySelector('span');
console.log(span);
var div = document.querySelector('div');
console.log(div);

//监听window的滚动事件
window.addEventListener('scroll', function () {
    if (window.pageYOffset >= div.offsetTop) {
        span.style.display = 'block';
    } else {
        span.style.display = 'none';
    }
});
//监听span的点击事件
span.addEventListener('click', function () {
    window.scroll(0, 0);
})

<!doctype html>
<html lang="zh-CN">

<head>
	<meta charset="utf-8">
	<meta http-equiv="X-UA-Compatible" content="IE=edge">
	<meta name="viewport" content="width=device-width, initial-scale=1">
	<!-- 上述3个meta标签*必须*放在最前面，任何其他内容都*必须*跟随其后！ -->
	<title>Bootstrap 101 Template</title>

	<!-- Bootstrap -->
	<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/3.4.1/css/bootstrap.min.css" integrity="sha384-HSMxcRTRxnN+Bdg0JdbxYKrThecOKuH5zCYotlSAcp1+c8xmyTe9GYg1l9a69psu"
	 crossorigin="anonymous">

</head>

<body>
	<!-- jQuery (Bootstrap 的所有 JavaScript 插件都依赖 jQuery，所以必须放在前边) -->
	<script src="https://fastly.jsdelivr.net/npm/jquery@1.12.4/dist/jquery.min.js" integrity="sha384-nvAa0+6Qg9clwYCGGPpDQLVpLNn0fRaROjHqs13t4Ggj3Ez50XnGQqc/r8MhnRDZ"
	 crossorigin="anonymous">

	</script>
	<!-- 加载 Bootstrap 的所有 JavaScript 插件。你也可以根据需要只加载单个插件。 -->
	<script src="https://stackpath.bootstrapcdn.com/bootstrap/3.4.1/js/bootstrap.min.js" integrity="sha384-aJ21OjlMXNL5UyIl/XNwTMqvzeRMZH2w8c5cRVpzpU8Y5bApTppSuUkhZXN0VxHd"
	 crossorigin="anonymous">

	</script>
    <div></div>
    <span>返回顶部</span>
</body>

</html>

div{
    margin: 80px;
    float: left;
    width: 300px;
    height: 1000px;
    background-color: #456;
}
span{
    float: left;
    display: block;
    position: fixed;
    width: 50px;
    height: 50px;
    background-color: red;
    cursor: pointer;
}