Bootstrap v3.3.6 Navbar切换按钮显示但不显示菜单

Question

当我尝试折叠它时，我的Navbar工作正常并且有关于表单的所有内容，它显示了所有内容，包含按钮但不显示菜单项。我不知道为什么。我正在使用Bootstrap 3.3.6。

<nav class="navbar navbar-inverse navbar-fixed-top">
 <div class="container-fluid">
  <div class="navbar-header">
    <button type="button" class="navbar-toggle" data-toggle="collapse" data-target="#mainNavBar">
      <span class="icon-bar"></span>
      <span class="icon-bar"></span>
      <span class="icon-bar"></span>
    </button>
    <a class="navbar-brand" href="#">Welcome</a>
  </div>
    <div class="collapse navbar-collapse" id="mainNavBar">
        <ul class="nav navbar-nav">
            <li><a href="#">Home</a> </li>
            <li><a href="#">About</a> </li>
            <li><a href="#">Contact</a> </li>
        </ul>
    </div><!--/.nav-collapse -->
</div>

Answer 1

It is working fine in Codepen So my guess would be you forgot to include JQuery or bootstrap.js in your html page.

You can download JQuery from here.