Bootstrap下拉列表在单击时不会显示其内容列表。我在bootstrap.min.js之前添加了jquery库(这是我在堆栈溢出中找到的解决方案，但对我来说不起作用(。
<html>
<head>
<title>Demo</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="css/main.css">
<link rel="stylesheet"href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/css/bootstrap.min.css" integrity="sha384-MCw98/SFnGE8fJT3GXwEOngsV7Zt27NXFoaoApmYm81iuXoPkFOJwJ8ERdknLPMO" crossorigin="anonymous">
<script src="https://code.jquery.com/jquery-3.3.1.slim.min.js" integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo" crossorigin="anonymous"></script>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.min.js"></script>
<script src="http://code.jquery.com/jquery-1.11.1.min.js"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/js/bootstrap.min.js" integrity="sha384-ChfqqxuZUCnJSK3+MXmPNIyE6ZbWh2IMqE241rYiqJxyMiZ6OW/JmZQ5stwEULTy" crossorigin="anonymous"></script>
</head>
<body>
<div class="container features">
<div class="row">
<div class="col-lg-4 col-md-4 col-sm-12">
</div>
<div class="col-lg-4 col-md-4 col-sm-12">
<div class="signin-div">
<h3 class="feature-title">Login Here..</h3>
<div class="form-group">
<!-- Basic dropdown -->
<div class="dropdown">
<button class="btn btn-primary dropdown-toggle" type="button" data-toggle="dropdown">Dropdown Example
<span class="caret"></span></button>
<ul class="dropdown-menu">
<li><a href="#">HTML</a></li>
<li><a href="#">CSS</a></li>
<li><a href="#">JavaScript</a></li>
</ul>
</div>
<!-- Basic dropdown -->
</div>
</div>
</div>
<div class="col-lg-4 col-md-4 col-sm-12">
</div>
</div>
</div>
</body>
</html>
试试这个代码。
我在boostrap.js 之前添加了popper.js
<html>
<head>
<title>Demo</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="css/main.css">
<link rel="stylesheet"href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" i crossorigin="anonymous">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.0.4/popper.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.min.js" crossorigin="anonymous"></script>
</head>
<body>
<div class="container features">
<div class="row">
<div class="col-lg-4 col-md-4 col-sm-12">
</div>
<div class="col-lg-4 col-md-4 col-sm-12">
<div class="signin-div">
<h3 class="feature-title">Login Here..</h3>
<div class="form-group">
<!-- Basic dropdown -->
<div class="dropdown">
<button class="btn btn-primary dropdown-toggle" type="button" data-toggle="dropdown">Dropdown Example
<span class="caret"></span></button>
<ul class="dropdown-menu">
<li><a href="#">HTML</a></li>
<li><a href="#">CSS</a></li>
<li><a href="#">JavaScript</a></li>
</ul>
</div>
<!-- Basic dropdown -->
</div>
</div>
</div>
<div class="col-lg-4 col-md-4 col-sm-12">
</div>
</div>
</div>
</body>
</html>
将此代码粘贴到head中
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous">
将此代码粘贴到正文中
<script src="https://code.jquery.com/jquery-3.2.1.slim.min.js" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.9/umd/popper.min.js" crossorigin="anonymous"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.min.js" integrity="sha384-JZR6Spejh4U02d8jOt6vLEHfe/JQGiRRSQQxSfFWpi1MquVdAyjUar5+76PVCmYl" crossorigin="anonymous"></script>