W3cubDocs

/Web APIs

Using the Document Object Model

The Document Object Model (DOM) is an API for manipulating DOM trees of HTML and XML documents (among other tree-like documents). This API is at the root of the description of a page and serves as a base for scripting on the web.

What is a DOM tree?

A DOM tree is a tree structure whose nodes represent an HTML or XML document's contents. Each HTML or XML document has a DOM tree representation. For example, consider the following document:

html

<html lang="en">
  <head>
    <title>My Document</title>
  </head>
  <body>
    <h1>Header</h1>
    <p>Paragraph</p>
  </body>
</html>

It has a DOM tree that looks like this:

The DOM as a tree-like representation of a document that has a root and node elements containing content

Although the above tree is similar to the above document's DOM tree, it's not identical, as the actual DOM tree preserves whitespace.

When a web browser parses an HTML document, it builds a DOM tree and then uses it to display the document.

What does the Document API do?

The Document API, also sometimes called the DOM API, allows you to modify a DOM tree in any way you want. It enables you to create any HTML or XML document from scratch or to change any contents of a given HTML or XML document. Web page authors can edit the DOM of a document using JavaScript to access the document property of the global object. This document object implements the Document interface.

A simple example

Suppose the author wants to change the header of the above document and write two paragraphs instead of one. The following script would do the job:

HTML

html

<html lang="en">
  <head>
    <title>My Document</title>
  </head>
  <body>
    <input type="button" value="Change this document." onclick="change()" />
    <h2>Header</h2>
    <p>Paragraph</p>
  </body>
</html>

JavaScript

function change() {
  // document.getElementsByTagName("h2") returns a NodeList of the <h2>
  // elements in the document, and the first is number 0:
  const header = document.getElementsByTagName("h2").item(0);

  // The firstChild of the header is a Text node:
  header.firstChild.data = "A dynamic document";

  // Now header is "A dynamic document".

  // Access the first paragraph
  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);

  // 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);
}

How can I learn more?