Example
html
<html>
<head>
<title>My Document</title>
<script>
function change() {
// document.getElementsByTagName("H1") returns a NodeList of the h1
// elements in the document, and the first is number 0:
const header = document.getElementsByTagName("H1").item(0);
// the firstChild of the header is a Text node:
header.firstChild.data = "A dynamic document";
// now the header is "A dynamic document".
const para = document.getElementsByTagName("P").item(0);
para.firstChild.data = "This is the first paragraph.";
// create a new Text node for the second paragraph
const newText = document.createTextNode(
"This is the second paragraph.",
);
// create a new Element to be the second paragraph
const newElement = document.createElement("P");
// put the text in the paragraph
newElement.appendChild(newText);
// and put the paragraph on the end of the document by appending it to
// the BODY (which is the parent of para)
para.parentNode.appendChild(newElement);
}
</script>
</head>
<body>
<input type="button" value="Change this document." onclick="change()" />
<h1>Header</h1>
<p>Paragraph</p>
</body>
</html>