JavaScript实现点击返回顶部按钮的完整代码与原理

核心代码实现与关键步骤解析

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>无标题文档</title>
<style>
body{margin:0; padding:0}
#to_top{width:30px; height:40px; padding:20px; font:14px/20px arial; text-align:center; background:#06c; position:absolute; cursor:pointer; color:#fff}
</style>
<script>
window.onload = function(){
 var oTop = document.getElementById("to_top");
 var screenw = document.documentElement.clientWidth || document.body.clientWidth;
 var screenh = document.documentElement.clientHeight || document.body.clientHeight;
 oTop.style.left = screenw - oTop.offsetWidth +"px";
 oTop.style.top = screenh - oTop.offsetHeight + "px";
 window.onscroll = function(){
  var scrolltop = document.documentElement.scrollTop || document.body.scrollTop;
  oTop.style.top = screenh - oTop.offsetHeight + scrolltop +"px";
 }
 oTop.onclick = function(){
  document.documentElement.scrollTop = document.body.scrollTop =0;
 }
} 
</script>
</head>
<body style="height:1000px;">
<h1>返回顶部</h1>
<div id="to_top">返回顶部</div>
</body>
</html>