Code before modification
<!DOCTYPE HTML>
<html lang="en-US">
<head>
<meta charset="UTF-8">
<title></title>
</head>
<body>
<button id='btn'>创建元素</button>
<script type="text/javascript">
var btn = document.getElementById('btn');
//注册点击事件
btn.onclick = function(){
//创建一个元素
var h1 = document.createElement('h1');
h1.innerHTML = "这是新增h1标签";
console.log( h1 );
//使用appendChild的方式
//document.body.appendChild( h1 );
//使用insertBefore的方式
//是在父元素中，先找一个节点，然后插入到它之前
document.body.insertBefore(h1,btn); //新插入的节点是h1，作为第一个参数
}
</script>
</body>
</html>Modified code
<!DOCTYPE HTML>
<html lang="en-US">
<head>
<meta charset="UTF-8">
<title></title>
</head>
<body>
<p id="box">
<button id='btn'>创建元素</button>
</p>
<script type="text/javascript">
var btn = document.getElementById('btn');
//注册点击事件
btn.onclick = function(){
//创建一个元素
var h1 = document.createElement('h1');
var box = document.createElement('box');
h1.innerHTML = "这是新增h1标签";
console.log( h1 );
//使用appendChild的方式
//document.body.appendChild( h1 );
//使用insertBefore的方式
//是在父元素中，先找一个节点，然后插入到它之前
document.box.insertBefore(h1,btn); //新插入的节点是h1，作为第一个参数
}
</script>
</body>
</html>After modification, elements cannot be added. What is the problem. The previous addition had body as the parent element and was used directly without passing it. I have nested a p and obtained the elements in advance. But still cannot insert new elements
![[Web front-end] Node.js quick start](https://img.php.cn/upload/course/000/000/067/662b5d34ba7c0227.png)
The boxes you created are not in the document. Where should you insert them?
1. createElement can only create labels. Box is not considered a label, right? Replace var box = document.createElement('box'); with var box = document.createElement('p');
2. This tag only exists in the Body, you cannot get it directly through document.box,
You have done it before What is written is var box=document.createElement('box');
document.box Isn't that equivalent to document.document.createElement('box')?
First add a class name to get it