What is the DOM?

The Document Object Model (DOM) is a programming interface for HTML and XML documents. It provides a structured representation of the document and it defines a way that the structure can be accessed from programs so that they can change the document structure, style and content. The DOM provides a representation of the document as a structured group of nodes and objects that have properties and methods. Essentially, it connects web pages to scripts or programming languages.

A Web page is a document. This document can be either displayed in the browser window, or as the HTML source. But it is the same document in both cases. The Document Object Model (DOM) provides another way to represent, store and manipulate that same document. The DOM is a fully object-oriented representation of the web page, and it can be modified with a scripting language such as JavaScript.

For example, the W3C DOM specifies that the getElementsByTagName method in the code below must return a list of all the <P> elements in the document:

paragraphs = document.getElementsByTagName("P");
// paragraphs[0] is the first <p> element
// paragraphs[1] is the second <p> element, etc.
alert(paragraphs[0].nodeName);     

All of the properties, methods, and events available for manipulating and creating web pages are organized into objects (e.g., the document object that represents the document itself, the table object that implements the special HTMLTableElement DOM interface for accessing HTML tables, and so forth). This documentation provides an object-by-object reference to the DOM implemented in Gecko-based browsers.

DOM and JavaScript

The short example above, like nearly all of the examples in this reference, is JavaScript. That is to say, it's written in JavaScript, but it uses the DOM to access the document and its elements. The DOM is not a programming language, but without it, the JavaScript language wouldn't have any model or notion of the web pages, XML pages and elements with which it is usually concerned. Every element in a document—the document as a whole, the head, tables within the document, table headers, text within the table cells—is part of the document object model for that document, so they can all be accessed and manipulated using the DOM and a scripting language like JavaScript.

In the beginning, JavaScript and the DOM were tightly intertwined, but eventually they evolved into separate entities. The page content is stored in DOM and may be accessed and manipulated via JavaScript, so that we may write this approximative equation:

API(web or XML page) = DOM + JS(scripting language)

The DOM was designed to be independent of any particular programming language, making the structural representation of the document available from a single, consistent API. Though we focus exclusively on JavaScript in this reference documentation, implementations of the DOM can be built for any language, as this Python example demonstrates:

# Python DOM example
import xml.dom.minidom as m
doc = m.parse("C:\\Projects\\Py\\chap1.xml");
doc.nodeName # DOM property of document object;
p_list = doc.getElementsByTagName("para");

How Do I Access the DOM?

You don't have to do anything special to begin using the DOM. Different browsers have different implementations of the DOM, and these implementations exhibit varying degrees of conformance to the actual DOM standard (a subject we try to avoid in this documentation), but every web browser uses some document object model to make web pages accessible to script.

Please refere to the Kaboodle source of this document to understand how the DOM is referenced in this example.  For a comprehensive understanding of the DOM and Javascript please refer to the Gecko DOM Reference

A Kaboodle example

NOTE: This is an example demonstrating the use of javascript to capture information from the document object model (DOM) and will not work from the preview mode in the editor.  To view this script in action you will need to create a test menu item linked to this document.