事件侦听外部文件不工作

Question

当我的JavaScript代码是相同的HTML页面，事件侦听器工作：

.dropbtn { 
 
    background-color: #4CAF50; 
 
    color: white; 
 
    padding: 16px; 
 
    font-size: 16px; 
 
    border: none; 
 
    cursor: pointer; 
 
} 
 

 
/* Dropdown button on hover & focus */ 
 
.dropbtn:hover, .dropbtn:focus { 
 
    background-color: #3e8e41; 
 
} 
 

 
/* The container <div> - needed to position the dropdown content */ 
 
.dropdown { 
 
    position: relative; 
 
    display: inline-block; 
 
} 
 

 
/* Dropdown Content (Hidden by Default) */ 
 
.dropdown-content { 
 
    display: none; 
 
    position: absolute; 
 
    background-color: #f9f9f9; 
 
    min-width: 160px; 
 
    box-shadow: 0px 8px 16px 0px rgba(0,0,0,0.2); 
 
    z-index: 1; 
 
} 
 

 
/* Links inside the dropdown */ 
 
.dropdown-content a { 
 
    color: black; 
 
    padding: 12px 16px; 
 
    text-decoration: none; 
 
    display: block; 
 
} 
 

 
/* Change color of dropdown links on hover */ 
 
.dropdown-content a:hover {background-color: #f1f1f1} 
 

 
/* Show the dropdown menu (use JS to add this class to the .dropdown-content container when the user clicks on the dropdown button) */ 
 
.show {display:block;}

<div class="dropdown"> 
 
\t \t <button id="dropBtn" class="dropbtn">Dropdown</button> 
 
\t \t \t <div id="myDropdown" class="dropdown-content"> 
 
\t \t \t  <a href="#">Link 1</a> 
 
\t \t \t  <a href="#">Link 2</a> 
 
\t \t \t  <a href="#">Link 3</a> 
 
\t \t \t </div> 
 
\t </div> 
 
    
 
    <script> 
 
    
 
\t \t 
 
\t \t var dropdown = document.getElementById("dropBtn"); 
 
\t \t 
 
\t \t function myFunction() { 
 
\t \t  document.getElementById("myDropdown").classList.toggle("show"); 
 
\t \t } 
 
\t \t 
 
\t \t dropdown.addEventListener("click", myFunction, false); 
 
\t \t 
 
\t \t // Close the dropdown menu if the user clicks outside of it 
 
\t \t window.onclick = function(event) { 
 
\t \t if (!event.target.matches('.dropbtn')) { 
 
\t \t 
 
\t \t  var dropdowns = document.getElementsByClassName("dropdown-content"); 
 
\t \t  var i; 
 
\t \t  for (i = 0; i < dropdowns.length; i++) { 
 
\t \t  var openDropdown = dropdowns[i]; 
 
\t \t  if (openDropdown.classList.contains('show')) { 
 
\t \t   openDropdown.classList.remove('show'); 
 
\t \t  } 
 
\t \t  } 
 
\t \t } 
 
\t \t } 
 

 
\t 
 
    </script>

但是当我把相同的JavaScript代码放到外部JS文件，事件监听器不工作。

我包括JS文件刚刚闭幕</body>标记之前：

<script type="text/javascript" src="js/main.js"></script> 
</body> 

如何让我的事件监听器从外部文件中的作品，不只是里面的HTML页面？

Answer 1

你错过了window.onload（）函数。

它应该是：

window.onload = function() { 

var dropdown = document.getElementById("dropBtn"); 

    function myFunction() { 
     document.getElementById("myDropdown").classList.toggle("show"); 
    } 

    dropdown.addEventListener("click", myFunction, false); 

    // Close the dropdown menu if the user clicks outside of it 
    window.onclick = function(event) { 
     if (!event.target.matches('.dropbtn')) { 

     var dropdowns = document.getElementsByClassName("dropdown-content"); 
     var i; 
     for (i = 0; i < dropdowns.length; i++) { 
      var openDropdown = dropdowns[i]; 
      if (openDropdown.classList.contains('show')) { 
      openDropdown.classList.remove('show'); 
      } 
     } 
     } 
    } 
} 

而且，如果这是一个外部JS，你不应该包括<script>标签。