<!doctype html>
<html>
<head>
<meta charset="utf-8">
<meta name="keywords" content="">
<meta name="description" content="">
<title>study html</title>
<script src="js/jquery-3.2.1.min.js" type="text/javascript"></script>
<style type="text/css">
div{
width:500px;
height:500px;
background:green;
border:1px solid #f00;
}
.xa{width:40px;height:40px;background:#000;color:#fff;display:none;position:fixed;}
</style>
</head>
<body>
<span class="xa">返回</span>
<div></div>
<div></div>
<div></div>
<div></div>
<div></div>
<div></div>
<script type="text/javascript">
$(window).scroll(function(){
var top=$(this).scrollTop();
if(top>=500){
$(".xa").show();
}else{
$(".xa").hide();
}
});
$("span").click(function(){
$("body").animate({scrollTop:0},3000);
});
</script>
</body>
</html>
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