Lesson Overview

In this lesson you'll learn what the DOM (Document Object Model) is and why it's important. You might not use the DOM too much while learning HTML but it's still important to understand what it is and how it works so you can create valid, efficient web documents.

Prerequisites

It's important that you understand basic HTML Syntax and what the Minimal HTML Elements are for before doing this lesson.

Overview of the HTML DOM

We code our pages using HTML tags and attributes. When that code is loaded into a browser, the browser interprets all of our code and tries to render a DOM. DOM is short for Document Object Model. In other words, a DOM is a model of all the objects in your document.

The objects represent the different elements, attributes, and content of your page. For example, elements like <html> and <img> are represented by Element Objects. Attributes like src and href are represented by Attribute Objects.

These objects contain special properties and methods you would eventually use to manipulate the various items on the page using a language like JavaScript. The W3C defined the HTML DOM standard that determines what objects exist and what properties and methods they have, which helps to make things more consistent: you can actually program an HTML DOM using many different languages and technologies! Many of the components on this page are actually created using JavaScript code that adds objects to the DOM. For example, the headers and footers on all of these tutorial pages are created by using JavaScript to add objects to the DOM for the coures name heading, page title heading, copyright information container, etc.

You can also use the DOM objects when you want to style your HTML pages with CSS. For example, in my tutorial pages, I have CSS code that styles all the list item objects with different bullet styles and all the navigation objects with specific background colours.

When coding HTML, it's important to understand the DOM and The DOM Tree so that you can create efficient HTML code that is valid and easy to maintain.

The DOM Tree

The DOM Tree is a hierarchical structure that displays the various nodes in the DOM. Nodes are the different types of objects in the DOM, and they're depicted with boxes (or other box-like shapes).

For example, if we had the following HTML code:

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Lorum Ipsum</title>
</head>

<body>

    <h1>A Web Page</h1>

    <div>
      <h2>Lorum Ipsum</h2>

      <img src="images/image.png">

      <div class="decoration">Lorem ipsum dolor sit amet, consectetur 
        adipiscing elit. Donec bibendum metus at mi tincidunt, a 
        pellentesque magna laoreet. Duis quis imperdiet enim. Sed 
        nec felis ex.</div>
    </div>
  </body>
</html>

We would then represent that code by the following DOM tree:

Sample DOM Tree

There are three kinds of nodes in a DOM tree for an HTML document:

the dom tree that corresponds to the code above: each node is a box and there are arrows connecting all the nodes
DOM Tree: Node types

HTML elements inside the document, such as the <hmtl> or <div> elements, are element nodes. For example, in the code up above, you can see there are element nodes in the DOM tree for the <html>, <head>, <meta>, <title>, <body>, <h1>, <h2>, <img>, and the <div> elements. All elements inside an HTML document should have corresponding nodes in the DOM tree. Note that an element node appears in the DOM tree whether it has both opening and closing tags, or only the opening tag.

Attributes of HTML elements, such as href or src, are represented as attribute nodes. Note that an attribute node appears in the DOM tree even if it's not assigned an attribute value. In our coded example and the corresponding DOM tree image, you can see there are attribute nodes for the HTML element (lang), META element (charset), IMG element (src), and the innermost DIV element (class).

Some elements contain text (such as the text inside the <title></title> element tags), and these are represented as text nodes. Note that not all element nodes will contain text nodes. For example, an <img> element node will have no text node. Note that even though attributes are assigned values in your HTML code, we don't represent those values as text nodes. Text nodes only show the actual text that is part of the document content. In our example, you can see the text nodes coming out of the TITLE element, H1 and H2 elements, and the innermost DIV element.

The Document Node or Root Node

The entire HTML document is modeled in the DOM by the document node, which is always at the very top of the DOM tree. The document node is the root node of the document tree: it's the parent of all of the other nodes. Note that this node does not represent the <!doctype> element, it just happens to have a similar name.

Parent and Child Element Nodes

DOM nodes (specifically, element nodes) are often (but not always) parent nodes, meaning that they have child nodes coming out of them. Child nodes can be other element nodes, or text nodes. For example, in our DOM tree above, the DIV node that is immediately under the H! node is the parent of the IMG node. In fact, that outermost DIV node has three children: the H2 node, IMG node, and the innermost DIV node.

same dom tree with parents and children labeled
DOM Tree: Parent/Child Nodes

Child nodes are always joined to their parent node with a solid line that connects the top of the child node's box to the bottom of the parent node's box. Do not connect element nodes on the sides: that is reserved for attribute nodes, so if a DOM tree diagram is not in colour, it will be difficult to tell which nodes are elements and which nodes are attributes.

All element nodes, except for the root node, have exactly one parent node. The root node is the only node that has no parent. Any node in the tree can have one or more child nodes: there is no limit on the number of child nodes a node can have. In the actual HTML code, parent/child node relationships occur when one element node is nested inside another, or when an element contains text content: In our example above, there is text inside the <title> element ("Lorum Ipsum"). In the DOM tree, you can see there is a text node that is a child of the TITLE node: that text node represents the text content "Lorum Ipsum" of the <title> element.

Child nodes that share the same parent node are called siblings. For example, the IMG node, inner DIV node, and H2 node are all siblings of each other. In fact, we can get more specific and refer to the H1 node as the first child of the outer DIV node. Furthermore, we can refer to the innermost DIV as the last child of the outermost DIV node. Additionally, when looking at the IMG node, the H1 node is the previous sibling and the lower DIV is the next sibling. These terms are important because when you want to style nodes with CSS or manipulate them with JavaScript, you will often need to refer to a node's parent, children, or siblings. For example, on my notes pages with many sections, there's always a small navigation bar at the end of each section that allows you to easily jump to another section of the same document. I created these in JavScript by adding a <nav> element with the appropriate links, and then adding this <nav> element as the "last child" of each <article> element that defines each section.

same dom tree with siblings labeled
DOM Tree: Sibling Nodes

Summary of Nodes

Exercise

For practice, see if you can use the diagram of this DOM Tree to answer the questions below:

another dom tree, description available from wendi
DOM Tree Exercise
  1. How many children does the BODY node have? What are they?
  2. How many children does the HEADER have?
  3. What is the ARTICLE node's first child? What is the ARTICLE node's last child?
  4. What is the ARTICLE node's previous sibling? What is the ARTICLE node's next sibling?
  5. What is the META tags' parent?

What does the DOM do?

The DOM allows you to edit, add, delete, and access individual document elements/tags, attributes, and their contents. The most common means of doing this is by using JavaScript. The DOM also allows you to create complex styling rules on page elements using CSS. For example, you could specify that the first list item in an unordered list have different styling than all the other list items.

The DOM defines the various properties and objects that represent the standard HTML elements you use to build the structure and content of a web page; it also defines a set of methods you can use to access and manipulate those properties and objects.

The DOM doesn't have to be accessed via JavaScript. Several other languages can be used to access DOM objects, properties, and methods, such as Python, Perl, PHP, and C++.

Exercise

Choose a simple web page (you could use this page if you have trouble finding one). Sketch out the DOM tree for that page. You can view the text version of the DOM tree by loading it in Chrome or Firefox and pressing Ctrl-Shift-I ("I" is short for "Inspect").