次要依靠于滚轮监听实现。scroll值须要正确的了解
代码实现如下：
<!DOCTYPE html>
<html lang='en'>
<head>
<meta charset='UTF-8'>
<title>回到顶部</title>
<style>
*{
margin:0;
padding:0;
}
body{
height:3000px;
}
#box{
position:fixed;
top:0;
display:none;
width:100%;
background-color:black;
height:60px;
}
#top{
position:fixed;
right:30px;
bottom:30px;
baclground-color:blue;
width:30px;
height:30px;
}
</style>
</head>
<body>
<div id='box'></div>
<div id='top'></div>
<script>
window.onscroll = function(){
if(window.scrollY >= 300){
box.style.display = 'block';
}else{
box.style.display = 'none';
}
}
var top = document.getElementById('top');
top.onclick = function(){
window.scrollTo(0,200);
}
</script>
</body>
</html>
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