我正在跟随一个视频课程，并完全按照讲师的要求做。他的导航栏汉堡包在点击时折叠，但我的没有。我试过：不同的浏览器重新安排链接和JQuery和bootstrap脚本标签的顺序和位置通过bootstrap css文件查看是否有一个类属性被禁用。
谁能帮我解决一下为什么汉堡包按钮不能折叠？
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>Bootstrap Navbars</title>
<link
rel="stylesheet"
type="text/css"
href="https://stackpath.bootstrapcdn.com/bootstrap/3.4.1/css/bootstrap.min.css"
integrity="sha384-HSMxcRTRxnN+Bdg0JdbxYKrThecOKuH5zCYotlSAcp1+c8xmyTe9GYg1l9a69psu"
crossorigin="anonymous"
/>
</head>
<body>
<nav class="navbar navbar-default">
<div class="container">
<div class="navbar-header">
<button
type="button"
class="navbar-toggle collapsed"
data-toggle="collapse"
data-target="#demo"
aria-expanded="false"
>
<span class="sr-only">Toggle navigation</span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
</button>
<a href="#" class="navbar-brand">Koffee</a>
</div>
<div class="collapse navbar-collapse" id="demo">
<ul class="nav navbar-nav">
<li><a href="#">About</a></li>
<li><a href="#">Contact</a></li>
</ul>
<ul class="nav navbar-nav navbar-right">
<li><a href="#">Sign Up</a></li>
<li><a href="#">Login</a></li>
</ul>
</div>
</div>
</nav>
<!-- DEFAULT NAVBAR -->
<nav class="navbar navbar-default">
<div class="container-fluid">
<!-- Brand and toggle get grouped for better mobile display -->
<div class="navbar-header">
<button
type="button"
class="navbar-toggle collapsed"
data-toggle="collapse"
data-target="#bs-example-navbar-collapse-1"
aria-expanded="false"
>
<span class="sr-only">Toggle navigation</span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
</button>
<a class="navbar-brand" href="#">Brand</a>
</div>
<!-- Collect the nav links, forms, and other content for toggling -->
<div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">
<ul class="nav navbar-nav">
<li class="active">
<a href="#">Link <span class="sr-only">(current)</span></a>
</li>
<li><a href="#">Link</a></li>
<li class="dropdown">
<a
href="#"
class="dropdown-toggle"
data-toggle="dropdown"
role="button"
aria-haspopup="true"
aria-expanded="false"
>Dropdown <span class="caret"></span
></a>
<ul class="dropdown-menu">
<li><a href="#">Action</a></li>
<li><a href="#">Another action</a></li>
<li><a href="#">Something else here</a></li>
<li role="separator" class="divider"></li>
<li><a href="#">Separated link</a></li>
<li role="separator" class="divider"></li>
<li><a href="#">One more separated link</a></li>
</ul>
</li>
</ul>
<form class="navbar-form navbar-left">
<div class="form-group">
<input type="text" class="form-control" placeholder="Search" />
</div>
<button type="submit" class="btn btn-default">Submit</button>
</form>
<ul class="nav navbar-nav navbar-right">
<li><a href="#">Link</a></li>
<li class="dropdown">
<a
href="#"
class="dropdown-toggle"
data-toggle="dropdown"
role="button"
aria-haspopup="true"
aria-expanded="false"
>Dropdown <span class="caret"></span
></a>
<ul class="dropdown-menu">
<li><a href="#">Action</a></li>
<li><a href="#">Another action</a></li>
<li><a href="#">Something else here</a></li>
<li role="separator" class="divider"></li>
<li><a href="#">Separated link</a></li>
</ul>
</li>
</ul>
</div>
<!-- /.navbar-collapse -->
</div>
<!-- /.container-fluid -->
</nav>
<script
src="https://code.jquery.com/jquery-3.5.0.js"
integrity="sha256-r/AaFHrszJtwpe+tHyNi/XCfMxYpbsRg2Uqn0x3s2zc="
crossorigin="anonymous"
></script>
<script
src="https://stackpath.bootstrapcdn.com/bootstrap/3.4.1/js/bootstrap.min.js"
integrity="sha384-aJ21OjlMXNL5UyIl/XNwTMqvzeRMZH2w8c5cRVpzpU8Y5bApTppSuUkhZXN0VxHd"
crossorigin="anonymous"
></script>
</body>
</html>
问题是关于你的库。把它们改成
<script src="https://code.jquery.com/jquery-2.2.4.min.js" integrity="sha256-BbhdlvQf/xTY9gja0Dq3HiwQF8LaCRTXxZKRutelT44=" crossorigin="anonymous"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js" integrity="sha384-Tc5IQib027qvyjSMfHjOMaLkfuWVxZxUPnCJA7l2mCWNIpG9mGCD8wGNIcPD7Txa" crossorigin="anonymous"></script>
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>Bootstrap Navbars</title>
<link
rel="stylesheet"
type="text/css"
href="https://stackpath.bootstrapcdn.com/bootstrap/3.4.1/css/bootstrap.min.css"
integrity="sha384-HSMxcRTRxnN+Bdg0JdbxYKrThecOKuH5zCYotlSAcp1+c8xmyTe9GYg1l9a69psu"
crossorigin="anonymous"
/>
</head>
<body>
<nav class="navbar navbar-default">
<div class="container">
<div class="navbar-header">
<button
type="button"
class="navbar-toggle collapsed"
data-toggle="collapse"
data-target="#demo"
aria-expanded="false"
>
<span class="sr-only">Toggle navigation</span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
</button>
<a href="#" class="navbar-brand">Koffee</a>
</div>
<div class="collapse navbar-collapse" id="demo">
<ul class="nav navbar-nav">
<li><a href="#">About</a></li>
<li><a href="#">Contact</a></li>
</ul>
<ul class="nav navbar-nav navbar-right">
<li><a href="#">Sign Up</a></li>
<li><a href="#">Login</a></li>
</ul>
</div>
</div>
</nav>
<!-- DEFAULT NAVBAR -->
<nav class="navbar navbar-default">
<div class="container-fluid">
<!-- Brand and toggle get grouped for better mobile display -->
<div class="navbar-header">
<button
type="button"
class="navbar-toggle collapsed"
data-toggle="collapse"
data-target="#bs-example-navbar-collapse-1"
aria-expanded="false"
>
<span class="sr-only">Toggle navigation</span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
</button>
<a class="navbar-brand" href="#">Brand</a>
</div>
<!-- Collect the nav links, forms, and other content for toggling -->
<div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">
<ul class="nav navbar-nav">
<li class="active">
<a href="#">Link <span class="sr-only">(current)</span></a>
</li>
<li><a href="#">Link</a></li>
<li class="dropdown">
<a
href="#"
class="dropdown-toggle"
data-toggle="dropdown"
role="button"
aria-haspopup="true"
aria-expanded="false"
>Dropdown <span class="caret"></span
></a>
<ul class="dropdown-menu">
<li><a href="#">Action</a></li>
<li><a href="#">Another action</a></li>
<li><a href="#">Something else here</a></li>
<li role="separator" class="divider"></li>
<li><a href="#">Separated link</a></li>
<li role="separator" class="divider"></li>
<li><a href="#">One more separated link</a></li>
</ul>
</li>
</ul>
<form class="navbar-form navbar-left">
<div class="form-group">
<input type="text" class="form-control" placeholder="Search" />
</div>
<button type="submit" class="btn btn-default">Submit</button>
</form>
<ul class="nav navbar-nav navbar-right">
<li><a href="#">Link</a></li>
<li class="dropdown">
<a
href="#"
class="dropdown-toggle"
data-toggle="dropdown"
role="button"
aria-haspopup="true"
aria-expanded="false"
>Dropdown <span class="caret"></span
></a>
<ul class="dropdown-menu">
<li><a href="#">Action</a></li>
<li><a href="#">Another action</a></li>
<li><a href="#">Something else here</a></li>
<li role="separator" class="divider"></li>
<li><a href="#">Separated link</a></li>
</ul>
</li>
</ul>
</div>
<!-- /.navbar-collapse -->
</div>
<!-- /.container-fluid -->
</nav>
<script src="https://code.jquery.com/jquery-2.2.4.min.js" integrity="sha256-BbhdlvQf/xTY9gja0Dq3HiwQF8LaCRTXxZKRutelT44=" crossorigin="anonymous"></script>
<!--Boostrap jQuery-->
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js" integrity="sha384-Tc5IQib027qvyjSMfHjOMaLkfuWVxZxUPnCJA7l2mCWNIpG9mGCD8wGNIcPD7Txa" crossorigin="anonymous"></script>
.
</body>
</html>