分享程序员开发的那些事...
更新时间：2023-08-23 08:20:46
将脚本放入外部文件中，然后从<head>加载该外部脚本时，它将在DOM准备就绪之前加载.这意味着，如果您的脚本像使用document.querySelectorAll()一样尝试引用页面中的元素，那么这些元素将不存在.
When you put the script in an external file and then load that external script from the <head>, it loads BEFORE the DOM is ready. That means that if your script tries to reference elements in the page like you are with document.querySelectorAll(), those elements will not exist yet.
最简单的解决方法是简单地移动脚本标签:
The simpleset way to fix that is to simply move the script tag:
<script type="text/javascript" src="menu.js"></script>
在</body>之前.然后，在脚本运行如下之前，将解析页面中的所有元素并在DOM中进行解析:
to right before </body>. Then, all the elements of the page will be parsed and in the DOM before your script runs like this:
<!DOCTYPE html>
<html>
<head>
<title>Test</title>
</head>
<body>
<div class="container">
<button name="one">Button 1</button>
<p>
Lorem ipsum dolor sit amet
</p>
</div>
<div class="container">
<button name="two">Button 2</button>
<p>
Lorem ipsum dolor sit amet
</p>
</div>
<div class="container">
<button name="three">Button 3</button>
<p>
Lorem ipsum dolor sit amet
</p>
</div>
<script type="text/javascript" src="menu.js"></script>
</body>
</html>
或者，您可以使用一个脚本，该脚本将在DOM准备就绪时通知您，然后可以调用您的代码.
Alternately, you can use a script that will notify you when the DOM is ready and you can then call your code.
See this reference and highly voted answer for a plain Javascript and cross browser way of waiting until the DOM is ready if you'd rather keep your code in the <head> section or make it so you code can be located anywhere.