移动版中导航栏无法折叠

使用Bootstrap中的collapse组件实现导航栏在移动设备上的折叠和展开。

代码示例： 1.添加按钮和导航列表

<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarNav" aria-controls="navbarNav" aria-expanded="false" aria-label="Toggle navigation">
  <span class="navbar-toggler-icon"></span>
</button>

<div class="collapse navbar-collapse" id="navbarNav">
  <ul class="navbar-nav">
    <li class="nav-item">
      <a class="nav-link" href="#">首页</a>
    </li>
    <li class="nav-item">
      <a class="nav-link" href="#">产品</a>
    </li>
    <li class="nav-item">
      <a class="nav-link" href="#">服务</a>
    </li>
  </ul>
</div>

2.在页面中引入Bootstrap的CSS和JS文件

<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.5.2/css/bootstrap.min.css" integrity="sha384-OgVRvuATP1z7JjHLkuOU7Xw704+h722OWLV7Xp8YjZZTtmIwOdjYl9+DBMlshzqZ" crossorigin="anonymous">

<script src="https://code.jquery.com/jquery-3.5.1.slim.min.js" integrity="sha384-DfXdz2htPH0lsSSs5nCTpuj/zy4C+OGpamoFVy38MVBnE+IbbVYUew+OrCXaRkfj" crossorigin="anonymous"></script>

<script src="https://cdn.jsdelivr.net/npm/popper.js@1.16.0/dist/umd/popper.min.js" integrity="sha384-Q6E9RHvbIyZFJoft+2mJbHaEWldlvI9IOYy5n3zV9zzTtmIpyV06z6pT6hhoP+a+" crossorigin="anonymous"></script>

<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.5.2/js/bootstrap.min.js