小知识：
$(window).scrollTop()---获取当前滚轮滑动的值
$(window).scrollTop(0)----括号中为0则为回到顶部
代码示例：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<style>
.hide{
display:none;
}
.returnTop{
height: 60px;
100px;
background-color: darkgray;
position: fixed;
right: 0;
bottom: 0;
color: greenyellow;
line-height: 60px;
text-align: center;
</style>
</head>
<body>
<div class="div1 div">
<p>hello</p>
<p>hello</p>
<p>hello</p>
<p>hello</p>
<p>hello</p>
<p>hello</p>
<p>hello</p>
<p>hello</p>
<p>hello</p>
<p>hello</p>
<p>hello</p>
<p>hello</p>
<p>hello</p>
<p>hello</p>
<p>hello</p>
<p>hello</p>
<p>hello</p>
<p>hello</p>
</div>
<div class="div2 div"></div>
<div class="div3 div"></div>
<div class="returnTop hide" onclick="returnTop();">返回顶部</div>
<script src="jquery-3.2.1.js"></script>
<script>
// 点击事件
function returnTop() {
$(window).scrollTop(0)
}
//滚轮事件
window.onscroll=function () {
var current=$(window).scrollTop();
if(current>10){
$('.returnTop').removeClass('hide')
}else{
$('.returnTop').addClass('hide')
}
}
</script>
</body>
</html>