W3School TIY Editor

<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
body {
  font-family: Arial, Helvetica, sans-serif;
  font-size: 20px;
}

#myBtn {
  display: none;
  position: fixed;
  bottom: 20px;
  right: 30px;
  z-index: 99;
  font-size: 18px;
  border: none;
  outline: none;
  background-color: red;
  color: white;
  cursor: pointer;
  padding: 15px;
  border-radius: 4px;
}

#myBtn:hover {
  background-color: #555;
}
</style>
</head>
<body>

<div style="background-color:black;color:white;padding:30px">向下滚动</div>
<div style="background-color:lightgrey;padding:30px 30px 2500px">这个例子展示了如何创建一个“返回顶部”按钮，该按钮在用户开始滚动页面时变得可见。</div>

let mybutton = document.getElementById("myBtn");

// 当用户从文档顶部向下滚动 20px 时，显示该按钮
window.onscroll = function() {scrollFunction()};

function scrollFunction() {
  if (document.body.scrollTop > 20 || document.documentElement.scrollTop > 20) {
    mybutton.style.display = "block";
  } else {
    mybutton.style.display = "none";
  }
}

// 当用户单击按钮时，滚动回文档顶部
function topFunction() {
  document.body.scrollTop = 0; // For Safari
  document.documentElement.scrollTop = 0; // 适用于 Chrome、Firefox、IE 和 Opera 浏览器
}
</script>

</body>
</html>