​x
 
<!DOCTYPE html>
<html>
<head>
<style>
.test {
  background-color: yellow;
}
</style>
</head>
<body style="height:1500px">
​
<p>向下滚动此页面</p>
​
<p id="myP" style="position:fixed">当您从该页面顶部滚动 50 像素后，将类 "test"（黄色背景色）添加到该段落。再次向上滚动以删除类。</p>
​
<script>
window.onscroll = function() {myFunction()};
​
function myFunction() {
  if (document.body.scrollTop > 50 || document.documentElement.scrollTop > 50) {
    document.getElementById("myP").className = "test";
  } else {
    document.getElementById("myP").className = "";
  }
}
</script>
​
</body>
</html>
​