html文件：
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title></title>
<style>
*{padding: 0px; margin: 0px;}
.content{
width: 100%;
height: 500px;
/* background: #F27729; */
}
.content .select{
width: 300px;
height: 40px;
background: #FFFFFF;
margin: 0px auto;
position: relative;
cursor: pointer;
border: #666666 solid 1px;
}
.content .select::after{
content: "";
display: block;
width: 10px;
height: 10px;
border-left: 1px solid #666;
border-bottom: 1px solid #666;
top: 12px;
right: 12px;
position: absolute;
transform: rotate(-46deg);
transition: all .3s ease-in;
}
.content .select p{
width: 300px;
line-height: 40px;
font-size: 16px;
font-family: "microsoft yahei";
color: #666666;
padding: 0px 15px;
}
.content .select ul{
width: 300px;
display: block;
font-size: 16px;
background: #FFFFFF;
position: absolute;
top: 40px;
left: 0px;
max-height: 0px;
overflow: hidden;
transition: max-height .3s ease-in;
border: #666666 solid 1px;
}
.content .select ul li{
width: 100%;
height: 30px;
line-height: 30px;
padding: 0px 15px;
list-style: none;
color: #666666;
}
.content .select ul li.Selected{
background: yellowgreen;
color: #FFFFFF;
}
.content .select ul li:hover{
background: #D0D0D0;
}
@-webkit-keyframes slide-down{
0%{transform: scale(1,0);}
25%{transform: scale(1,1.2);}
50%{transform: scale(1,0.85);}
75%{transform: scale(1,1.05);}
100%{transform: scale(1,1);}
}
.content .select.open ul{
max-height: 250px;
transform-origin: 50% 0;
-webkit-animation: slide-down .5s ease-in;
transition: max-height .2s ease-in;
}
.content .select.open::after{
transform: rotate(134deg);
transition: all .3s ease-in;
top: 18px;
}
</style>
<script src="https://cdn.bootcdn.net/ajax/libs/jquery/3.5.1/jquery.min.js"></script>
<script>
$(function(){
$(".select p").click(function(e){
$(".select").toggleClass('open');
e.stopPropagation();
});
$(".content .select ul li").click(function(e){
var _this=$(this);
$(".select > p").text(_this.attr('data-value'));
_this.addClass("Selected").siblings().removeClass("Selected");
$(".select").removeClass("open");
e.stopPropagation();
});
$(document).on('click',function(){
$(".select").removeClass("open");
})
});
</script>
</head>
<body>
<div class="content">
<div class="select">
<p data-value="所有选项">所有选项</p>
<ul>
<li data-value="所有选项" class="Selected">所有选项</li>
<li data-value="Html">Html</li>
<li data-value="Html5">Html5</li>
<li data-value="JavaScript">JavaScript</li>
<li data-value="Jquery">Jquery</li>
<li data-value="Java">Java</li>
<li data-value="c#">c#</li>
</ul>
</div>
</div>
</body>
</html>