事件侦听器不在外部文件中工作

Question

当我的JavaScript代码在同一个HTML页面上时，事件监听器正在运行：

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.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;}

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<div class="dropdown">
		<button id="dropBtn" class="dropbtn">Dropdown</button>
			  <div id="myDropdown" class="dropdown-content">
			    <a href="#">Link 1</a>
			    <a href="#">Link 2</a>
			    <a href="#">Link 3</a>
			</div>
	</div>
  
  <script>
    
		
		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');
		      }
		    }
		  }
		}

	
    </script>

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但是当我将相同的javascript代码放入外部JS文件时，事件监听器无效。

我在关闭</body>标记之前包含了JS文件：

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

如何使我的事件监听器能够从外部文件中运行，而不仅仅是在HTML页面内？

Answer 1

你错过了window.onload（）函数。

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');
          }
        }
      }
    }
}

此外，如果这是外部JS，则不应包含<script>标记。