效果先看
知识点
bootstrap下拉框由div>button ul>li>a 构成
div的类为：dropdown
a标签必须写
button的类为： dropdown-toggle 必带属性：data-toggle="dropdown"
ul的类为：dropdown-menu
获取事件触发的对象为：event.target
完整代码
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>下拉框</title>
<link rel="stylesheet" type="text/css" href="css/bootstrap.min.css"/>
</head>
<body>
<div id="" class="dropdown">
<button type="button" class="btn btn-default dropdown-toggle" data-toggle="dropdown">切换</button>
<ul class="dropdown-menu">
<li><a href="javascrip:void(0)">嘻嘻</a></li>
<li><a href="javascrip:void(0)">哈哈</a></li>
<li><a href="javascrip:void(0)">吼吼</a></li>
</ul>
</div>
<script src="js/jQuery-3.4.1.js" type="text/javascript" charset="utf-8"></script>
<script src="js/bootstrap.min.js" type="text/javascript" charset="utf-8"></script>
<script type="text/javascript">
var btn = $(".dropdown-toggle");
var ul = $(".dropdown-menu");
ul.click(function(){
btn.html(event.target.innerHTML);
})
</script>
</body>
</html>