Introduction to the DOM

The Document Object Model (DOM) is the data representation of the objects that comprise the structure and content of a document on the web. This guide will introduce the DOM, look at how the DOM represents an HTML document in memory and how to use APIs to create web content and applications.

The Document Object Model (DOM) is a programming interface for web documents. It represents the page so that programs can change the document structure, style, and content. The DOM represents the document as nodes and objects; that way, programming languages can interact with the page.

A web page is a document that can be either displayed in the browser window or as the HTML source. In both cases, it is the same document but the Document Object Model (DOM) representation allows it to be manipulated. As an object-oriented representation of the web page, it can be modified with a scripting language such as JavaScript.

For example, the DOM specifies that the querySelectorAll method in this code snippet must return a list of all the <p> elements in the document:

const paragraphs = document.querySelectorAll("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. For example, the document object that represents the document itself, any table objects that implement the HTMLTableElement DOM interface for accessing HTML tables, and so forth, are all objects.

The DOM is built using multiple APIs that work together. The core DOM defines the entities describing any document and the objects within it. This is expanded upon as needed by other APIs that add new features and capabilities to the DOM. For example, the HTML DOM API adds support for representing HTML documents to the core DOM, and the SVG API adds support for representing SVG documents.

The previous short example, like nearly all examples, is JavaScript. That is to say, it is written in JavaScript, but 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 web pages, HTML documents, SVG documents, and their component parts. The document as a whole, the head, tables within the document, table headers, text within the table cells, and all other elements in a document are parts of the document object model for that document. They can all be accessed and manipulated using the DOM and a scripting language like JavaScript.

The DOM is not part of the JavaScript language, but is instead a Web API used to build websites. JavaScript can also be used in other contexts. For example, Node.js runs JavaScript programs on a computer, but provides a different set of APIs, and the DOM API is not a core part of the Node.js runtime.

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. Even if most web developers will only use the DOM through JavaScript, 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(r"C:\Projects\Py\chap1.xml")
doc.nodeName # DOM property of document object
p_list = doc.getElementsByTagName("para")

For more information on what technologies are involved in writing JavaScript on the web, see JavaScript technologies overview.

You don't have to do anything special to begin using the DOM. You use the API directly in JavaScript from within what is called a script, a program run by a browser.

When you create a script, whether inline in a <script> element or included in the web page, you can immediately begin using the API for the document or window objects to manipulate the document itself, or any of the various elements in the web page (the descendant elements of the document). Your DOM programming may be something as simple as the following example, which displays a message on the console by using the console.log() function:

<body onload="console.log('Welcome to my home page!');">

As it is generally not recommended to mix the structure of the page (written in HTML) and manipulation of the DOM (written in JavaScript), the JavaScript parts will be grouped together here, and separated from the HTML.

For example, the following function creates a new <h1> element, adds text to that element, and then adds it to the tree for the document:

<html>
  <head>
    <script>
       // run this function when the document is loaded
       window.onload = function() {

         // create a couple of elements in an otherwise empty HTML page
         const heading = document.createElement("h1");
         const heading_text = document.createTextNode("Big Head!");
         heading.appendChild(heading_text);
         document.body.appendChild(heading);
      }
    </script>
  </head>
  <body>
  </body>
</html>

This page tries to describe the various objects and types in simple terms. But there are a number of different data types being passed around the API that you should be aware of.

The following table briefly describes these data types.

There are also some common terminology considerations to keep in mind. It's common to refer to any Attr node as an attribute, for example, and to refer to an array of DOM nodes as a nodeList. You'll find these terms and others to be introduced and used throughout the documentation.

This guide is about the objects and the actual things you can use to manipulate the DOM hierarchy. There are many points where understanding how these work can be confusing. For example, the object representing the HTML form element gets its name property from the HTMLFormElement interface but its className property from the HTMLElement interface. In both cases, the property you want is in that form object.

But the relationship between objects and the interfaces that they implement in the DOM can be confusing, and so this section attempts to say a little something about the actual interfaces in the DOM specification and how they are made available.

Many objects borrow from several different interfaces. The table object, for example, implements a specialized HTMLTableElement interface, which includes such methods as createCaption and insertRow. But since it's also an HTML element, table implements the Element interface described in the DOM Element Reference chapter. And finally, since an HTML element is also, as far as the DOM is concerned, a node in the tree of nodes that make up the object model for an HTML or XML page, the table object also implements the more basic Node interface, from which Element derives.

When you get a reference to a table object, as in the following example, you routinely use all three of these interfaces interchangeably on the object, perhaps without knowing it.

const table = document.getElementById("table");
const tableAttrs = table.attributes; // Node/Element interface
for (let i = 0; i < tableAttrs.length; i++) {
  // HTMLTableElement interface: border attribute
  if(tableAttrs[i].nodeName.toLowerCase() == "border")
    table.border = "1";
}
// HTMLTableElement interface: summary attribute
table.summary = "note: increased border";

This section lists some of the most commonly-used interfaces in the DOM. The idea is not to describe what these APIs do here but to give you an idea of the sorts of methods and properties you will see very often as you use the DOM. These common APIs are used in the longer examples in the DOM Examples chapter at the end of this book.

The document and window objects are the objects whose interfaces you generally use most often in DOM programming. In simple terms, the window object represents something like the browser, and the document object is the root of the document itself. Element inherits from the generic Node interface, and together these two interfaces provide many of the methods and properties you use on individual elements. These elements may also have specific interfaces for dealing with the kind of data those elements hold, as in the table object example in the previous section.

The following is a brief list of common APIs in web and XML page scripting using the DOM.

• document.querySelector(selector)
• document.querySelectorAll(name)
• document.createElement(name)
• parentNode.appendChild(node)
• element.innerHTML
• element.style.left
• element.setAttribute()
• element.getAttribute()
• element.addEventListener()
• window.content
• GlobalEventHandlers/onload
• window.scrollTo()

The following simple example illustrates using the DOM Document API — specifically, it illustrates using the body property of the Document API to change:

• the document's text color
• the document's background color
• the documents's link color (that is, the color of any hypertext links anywhere in the document)

<html>
<head>
  <title>Simple Document API example</title>
  <script>
    function setBodyAttr(attr, value) {
      if (document.body) document.body[attr] = value;
      else throw new Error("no support");
    }
  </script>
</head>
<body>
  <div>
    <form>
      <p><b><code>text</code></b></p>
      <select onChange="setBodyAttr('text',
        this.options[this.selectedIndex].value);">
        <option value="black">black</option>
        <option value="red">red</option>
      </select>
      <p><b><code>bgColor</code></b></p>
      <select onChange="setBodyAttr('bgColor',
        this.options[this.selectedIndex].value);">
        <option value="white">white</option>
        <option value="lightgrey">gray</option>
      </select>
      <p><b><code>link</code></b></p>
      <select onChange="setBodyAttr('link',
        this.options[this.selectedIndex].value);">
        <option value="blue">blue</option>
        <option value="green">green</option>
      </select>
      <small>
        <a href="http://some.website.tld/page.html" id="sample">
          (sample link)
        </a>
      </small>
    </form>
  </div>
</body>
</html>