<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title></title>
</head>
<style>
.up {
display: none; width: 100px; height: 100px; background: red;
position: fixed; right: 10px; bottom: 20px;
}
</style>
<script>
var num = 0;
window.onscroll = function(){
var up = document.getElementById("top");
//当前页面滚动的距离？？
var _top = document.body.scrollTop || document.documentElement.scrollTop;
if(_top >= 1000){
up.style.display = "block";
} else {
up.style.display = "none";
}
}
function goTop(){
document.body.scrollTop = document.documentElement.scrollTop = 0;
}
</script>
<body>
<p>aaaaaaaaaaaaaaaaaaaaaaa</p>
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<p>aaaaaaaaaaaaaaaaaaaaaaa</p>
<p>aaaaaaaaaaaaaaaaaaaaaaa</p>
<p>aaaaaaaaaaaaaaaaaaaaaaa</p>
<p>aaaaaaaaaaaaaaaaaaaaaaa</p>
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<p>aaaaaaaaaaaaaaaaaaaaaaa</p>
<p>aaaaaaaaaaaaaaaaaaaaaaa</p>
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<p>aaaaaaaaaaaaaaaaaaaaaaa</p>
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<p>aaaaaaaaaaaaaaaaaaaaaaa</p>
<p>aaaaaaaaaaaaaaaaaaaaaaa</p>
<p>aaaaaaaaaaaaaaaaaaaaaaa</p>
<p>aaaaaaaaaaaaaaaaaaaaaaa</p>
<p>aaaaaaaaaaaaaaaaaaaaaaa</p>
<p>aaaaaaaaaaaaaaaaaaaaaaa</p>
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<div >回到顶部</div>
</body>
</html>