向上滚动时导航栏不显示

Question

参考this snippet here，每当用户向上滚动时，我都试图将导航显示在顶部。 向下滚动时，导航栏可以正常消失，但是向上滚动时，导航栏应该会显示。

以下是代码段：

<script> src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.4.1/core.js"</script>
<nav>
  <div class="container">
    <a href="#" id="brand">Brand</a>
    <button>
      <span></span>
      <span></span>
      <span></span>
    </button>
    
    <ul class="navbar-menu">
      <li><a href="#">Home</a></li>
      <li><a href="#">page a</a></li>
      <li><a href="#">page b</a></li>
      <li><a href="#">page c</a></li>
      <li><a href="#">page d</a></li>
    </ul>
    
  </div>
</nav>

body {
    background: #eee;
    min-height: 3000px;
    padding: 0;
    margin: 0;
    font-family: 'Open Sans', sans-serif;
}

.add_menu{
  background: red;
}

.container {
    width: 80%;
    margin: 0 auto;
    clear: both;
}

a {
    display: inline-block;
    color: #333;
    text-decoration: none;
}

nav {
    background: #fff;
    height: 80px;
    line-height: 80px;
    box-shadow: 1px 1px 1px rgba(0, 0, 0, 0.2);
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    z-index: 9998;
    transition: all 0.5s;
}

nav.scrollUp {
    transform: translateY(-80px);
}

nav ul.navbar-menu {
    margin: 0;
    padding: 0;
    display: inline-block;
    float: right;
}

nav ul.navbar-menu li {
    display: inline-block;
    margin: 0 10px;
}

nav ul.navbar-menu li a {
    color: #666;
    font-size: 14px;
}

nav a#brand {
    text-transform: uppercase;
    foat: left;
    font-weight: 800;
    font-size: 20px;
}

nav button {
    background: none;
    width: 30px;
    height: 40px;
    margin-top: 20px;
    border: none;
    float: right;
    display: inline-block;
    cursor: pointer;
    display: none;
}

nav button span {
    width: 30px;
    height: 40px;
    height: 2px;
    background: #333;
    display: block;
    margin: 5px 0;
}

@media (max-width: 768px) {
    nav ul.navbar-menu {
        display: none;
    }
    nav button {
        display: block;
    }
}

$(document).ready(function () {
  
   var c, currentScrollTop = 0,
       navbar = $('nav');

   $(window).scroll(function () {
      var a = $(window).scrollTop();
      var b = navbar.height();
     
      currentScrollTop = a;
     
      if (c < currentScrollTop && a > b + b) {
        navbar.addClass("scrollUp");
      } else if (c > currentScrollTop && !(a <= b)) {
        navbar.removeClass("scrollUp");
      }
      c = currentScrollTop;
  });
  
});

通过检查器查看页面时，出现未捕获的参考错误：

Uncaught ReferenceError: $ is not defined

但是，如片段所示，我导入了jQuery。

我该如何解决？

Answer 1

我正在编辑您已修复的部分代码，请检查。

$(document).ready(function () {
  
   var c, currentScrollTop = 0,
       navbar = $('nav');

   $(window).scroll(function () {
      var a = $(window).scrollTop();
      var b = navbar.height();
     
      currentScrollTop = a;
     
      if (c < currentScrollTop && a > b + b) {
        navbar.addClass("scrollUp");
      } else if (c > currentScrollTop && !(a <= b)) {
        navbar.removeClass("scrollUp");
      }
      c = currentScrollTop;
  });
  
});

body {
    background: #eee;
    min-height: 3000px;
    padding: 0;
    margin: 0;
    font-family: 'Open Sans', sans-serif;
}

.add_menu{
  background: red;
}

.container {
    width: 80%;
    margin: 0 auto;
    clear: both;
}

a {
    display: inline-block;
    color: #333;
    text-decoration: none;
}

nav {
    background: #fff;
    height: 80px;
    line-height: 80px;
    box-shadow: 1px 1px 1px rgba(0, 0, 0, 0.2);
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    z-index: 9998;
    transition: all 0.5s;
}

nav.scrollUp {
    transform: translateY(-80px);
}

nav ul.navbar-menu {
    margin: 0;
    padding: 0;
    display: inline-block;
    float: right;
}

nav ul.navbar-menu li {
    display: inline-block;
    margin: 0 10px;
}

nav ul.navbar-menu li a {
    color: #666;
    font-size: 14px;
}

nav a#brand {
    text-transform: uppercase;
    foat: left;
    font-weight: 800;
    font-size: 20px;
}

nav button {
    background: none;
    width: 30px;
    height: 40px;
    margin-top: 20px;
    border: none;
    float: right;
    display: inline-block;
    cursor: pointer;
    display: none;
}

nav button span {
    width: 30px;
    height: 40px;
    height: 2px;
    background: #333;
    display: block;
    margin: 5px 0;
}

@media (max-width: 768px) {
    nav ul.navbar-menu {
        display: none;
    }
    nav button {
        display: block;
    }
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

<nav>
  <div class="container">
    <a href="#" id="brand">Brand</a>
    <button>
      <span></span>
      <span></span>
      <span></span>
    </button>
    
    <ul class="navbar-menu">
      <li><a href="#">Home</a></li>
      <li><a href="#">page a</a></li>
      <li><a href="#">page b</a></li>
      <li><a href="#">page c</a></li>
      <li><a href="#">page d</a></li>
    </ul>
    
  </div>
</nav>