如何解决我是初学者，面临着 Javascript 和 html 链接的问题在 HTML 中看不到 Javascript 的效果
我创建了一个 admin.html 文件，其中 h1 标签中有 div 类。我已经使用脚本连接了 admin.js 文件：src 但是当我在 Javascript 中更改颜色时无法看到效果，但是当我在 Chrome 中检查时相同的代码工作。感谢帮助。
admin.html
<!DOCTYPE html>
<html lang="en">
<head>
<Meta charset="UTF-8">
<Meta http-equiv="X-UA-Compatible" content="IE=edge">
<Meta name="viewport" content="width=device-width,initial-scale=1.0">
<title>Admin</title>
<link rel="stylesheet" href="style2.css">
<script src="admin.js"></script>
</head>
<body>
<ul class="list">
<li id="dashbord"style="font-size: 40px; padding-top: 8px; height: 50px;"><a href="#dashbord">Dashboard</a></li>
<br>
<li><a class="active" href="#home">Home</a></li>
<li><a href="#contact">Contact</a></li>
<li><a href="#about">About</a></li>
</ul>
<div class="div" style="margin-left:25%;padding:1px 16px;height:1000px;">
<h2>Fixed Full-height Side Nav</h2>
<h3>Try to scroll this area,and see how the sidenav sticks to the page</h3>
<p>Notice that this div element has a left margin of 25%. This is because the side navigation is set to 25% width. If you remove the margin,the sidenav will overlay/sit on top of this div.</p>
<p>Also notice that we have set overflow:auto to sidenav. This will add a scrollbar when the sidenav is too long (for example if it has over 50 links inside of it).</p>
<p>Some text..</p>
<p>Some text..</p>
<p>Some text..</p>
<p>Some text..</p>
<p>Some text..</p>
<p>Some text..</p>
<p>Some text..</p>
</div>
</body>
</html>
admin.js
const q = document.querySelector('.div h2');
q.style.color = 'red';
解决方法
原因是在读取 HTML 文件和构建 DOM 之前对脚本进行了解析和评估。 DOM 元素此时不存在。
将 script 标签移到 body 的末尾。
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width,initial-scale=1.0">
<title>Admin</title>
<link rel="stylesheet" href="style2.css">
</head>
<body>
<ul class="list">
<li id="dashbord"style="font-size: 40px; padding-top: 8px; height: 50px;"><a href="#dashbord">Dashboard</a></li>
<br>
<li><a class="active" href="#home">Home</a></li>
<li><a href="#contact">Contact</a></li>
<li><a href="#about">About</a></li>
</ul>
<div class="div" style="margin-left:25%;padding:1px 16px;height:1000px;">
<h2>Fixed Full-height Side Nav</h2>
<h3>Try to scroll this area,and see how the sidenav sticks to the page</h3>
<p>Notice that this div element has a left margin of 25%. This is because the side navigation is set to 25% width. If you remove the margin,the sidenav will overlay/sit on top of this div.</p>
<p>Also notice that we have set overflow:auto to sidenav. This will add a scrollbar when the sidenav is too long (for example if it has over 50 links inside of it).</p>
<p>Some text..</p>
<p>Some text..</p>
<p>Some text..</p>
<p>Some text..</p>
<p>Some text..</p>
<p>Some text..</p>
<p>Some text..</p>
</div>
<script src="admin.js"></script>
</body>
</html>
某些框架（如 Webpack）将 script 标记添加到头部并添加 defer，但我不确定这是否总是有效。根据 https://developer.mozilla.org/en-US/docs/Web/HTML/Element/script，它应该始终有效。
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width,initial-scale=1.0">
<title>Admin</title>
<link rel="stylesheet" href="style2.css">
<script src="admin.js" defer></script>
</head>
<body>
<ul class="list">
<li id="dashbord"style="font-size: 40px; padding-top: 8px; height: 50px;"><a href="#dashbord">Dashboard</a></li>
<br>
<li><a class="active" href="#home">Home</a></li>
<li><a href="#contact">Contact</a></li>
<li><a href="#about">About</a></li>
</ul>
<div class="div" style="margin-left:25%;padding:1px 16px;height:1000px;">
<h2>Fixed Full-height Side Nav</h2>
<h3>Try to scroll this area,the sidenav will overlay/sit on top of this div.</p>
<p>Also notice that we have set overflow:auto to sidenav. This will add a scrollbar when the sidenav is too long (for example if it has over 50 links inside of it).</p>
<p>Some text..</p>
<p>Some text..</p>
<p>Some text..</p>
<p>Some text..</p>
<p>Some text..</p>
<p>Some text..</p>
<p>Some text..</p>
</div>
</body>
</html>
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