包含导航栏子菜单代码的外部Java脚本文件不起作用

我想设计一个固定高度的侧导航条，我面临的问题是当我把java脚本代码放在html文件中时，它工作正常，但是当我把相同的代码放在外部JS文件中时，它不工作

<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css">
<style>
body {
  font-family: "Lato", sans-serif;
}
/* Fixed sidenav, full height */
.sidenav {
  height: 100%;
  width: 200px;
  position: fixed;
  z-index: 1;
  top: 0;
  left: 0;
  background-color: #111;
  overflow-x: hidden;
  padding-top: 20px;
}
/* Style the sidenav links and the dropdown button */
.sidenav a, .dropdown-btn {
  padding: 6px 8px 6px 16px;
  text-decoration: none;
  font-size: 20px;
  color: #818181;
  display: block;
  border: none;
  background: none;
  width: 100%;
  text-align: left;
  cursor: pointer;
  outline: none;
}
/* On mouse-over */
.sidenav a:hover, .dropdown-btn:hover {
  color: #f1f1f1;
}
/* Main content */
.main {
  margin-left: 200px; /* Same as the width of the sidenav */
  font-size: 20px; /* Increased text to enable scrolling */
  padding: 0px 10px;
}
/* Add an active class to the active dropdown button */
.active {
  background-color: green;
  color: white;
}
/* Dropdown container (hidden by default). Optional: add a lighter background color and some left padding to change the design of the dropdown content */
.dropdown-container {
  display: none;
  background-color: #262626;
  padding-left: 8px;
}
/* Optional: Style the caret down icon */
.fa-caret-down {
  float: right;
  padding-right: 8px;
}
/* Some media queries for responsiveness */
@media screen and (max-height: 450px) {
  .sidenav {padding-top: 15px;}
  .sidenav a {font-size: 18px;}
}
</style>
</head>
<body>
<div class="sidenav">
  <a href="#about">About</a>
  <a href="#services">Services</a>
  <a href="#clients">Clients</a>
  <a href="#contact">Contact</a>
  <button class="dropdown-btn">Dropdown 
    <i class="fa fa-caret-down"></i>
  </button>
  <div class="dropdown-container">
    <a href="#">Link 1</a>
    <a href="#">Link 2</a>
    <a href="#">Link 3</a>
  </div>
  <a href="#contact">Search</a>
</div>
<div class="main">
  <h2>Sidebar Dropdown</h2>
  <p>Click on the dropdown button to open the dropdown menu inside the side navigation.</p>
  <p>This sidebar is of full height (100%) and always shown.</p>
  <p>Some random text..</p>
</div>
</body>
<script src="abc.js"></script>
</html>

这是我的档案(abc.js)

/在所有下拉按钮之间循环切换隐藏和显示其下拉内容-这允许用户有多个下拉而没有任何冲突/

var dropdown = document.getElementsByClassName("dropdown-btn");
var i;
for (i = 0; i < dropdown.length; i++) 
{
  dropdown[i].addEventListener("click", function() 
  {
    this.classList.toggle("active");
    var dropdownContent = this.nextElementSibling;
    if (dropdownContent.style.display === "block")
    {
      dropdownContent.style.display = "none";
    } 
    else 
    {
      dropdownContent.style.display = "block";
    }
  });
}

包含导航栏子菜单代码的外部Java脚本文件不起作用

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