<!DOCTYPE html>
<html>
<head>
<title>Bootstrap 示例</title>
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://www.w3schools.cn/lib/bootstrap/4.5.2/css/bootstrap.min.css">
<script src="https://ajax.aspnetcdn.com/ajax/jQuery/jquery-3.5.1.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.16.0/umd/popper.min.js"></script>
<script src="https://www.w3schools.cn/lib/bootstrap/4.5.2/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container mt-3">
<h2>下拉示例</h2>
<p><strong>toggle</strong> 方法切换下拉列表。</p>
<div class="dropdown">
<button type="button" class="btn btn-primary dropdown-toggle" data-toggle="dropdown">
下拉按钮
</button>
<div class="dropdown-menu">
<a class="dropdown-item" href="#">链接 1</a>
<a class="dropdown-item" href="#">链接 2</a>
<a class="dropdown-item" href="#">链接 3</a>
</div>
</div>
<p><strong>注意：</strong>无论是否调用 dropdown() 方法，data-toggle="dropdown" 属性都是必需的。</p>
</div>
<script>
$(document).ready(function(){ $(".dropdown-toggle").dropdown("toggle");});
</script>
</body>
</html>