HTML DOM concepts and usage

In this article, we'll focus on the parts of the HTML DOM that involve engaging with HTML elements. Discussion of other areas, such as Drag and Drop, WebSockets, Web Storage, etc. can be found in the documentation for those APIs.


Structure of an HTML document

The Document Object Model (DOM) is an architecture that describes the structure of a document; each document is represented by an instance of the interface Document. A document, in turn, consists of a hierarchical tree of nodes, in which a node is a fundamental record representing a single object within the document (such as an element or text node).

Nodes may be strictly organizational, providing a means for grouping other nodes together or for providing a point at which a hierarchy can be constructed; other nodes may represent visible components of a document. Each node is based on the Node interface, which provides properties for getting information about the node as well as methods for creating, deleting, and organizing nodes within the DOM.

Nodes don't have any concept of including the content that is actually displayed in the document. They're empty vessels. The fundamental notion of a node that can represent visual content is introduced by the Element interface. An Element object instance represents a single element in a document created using either HTML or an XML vocabulary such as SVG.

For example, consider a document with two elements, one of which has two more elements nested inside it:

Window, Document and Nested Elements

While the Document interface is defined as part of the DOM specification, the HTML specification significantly enhances it to add information specific to using the DOM in the context of a web browser, as well as to using it to represent HTML documents specifically.

Among the things added to Document by the HTML standard are:

  • Support for accessing various information provided by the HTTP headers when loading the page, such as the location from which the document was loaded, cookies, modification date, referring site, and so forth.
  • Access to lists of elements in the document's <head> block and body, as well as lists of the images, links, scripts, etc. contained in the document.
  • Support for interacting with the user by examining focus and by executing commands on editable content.
  • Event handlers for document events defined by the HTML standard to allow access to mouse and keyboard events, drag and drop, media control, and more.
  • Event handlers for events that can be delivered to both elements and documents; these presently include only copy, cut, and paste actions.

HTML element interfaces

The Element interface has been further adapted to represent HTML elements specifically by introducing the HTMLElement interface, which all more specific HTML element classes inherit from. This expands the Element class to add HTML-specific general features to the element nodes. Properties added by HTMLElement include for example hidden and innerText.

An HTML document is a DOM tree in which each of the nodes is an HTML element, represented by the HTMLElement interface. The HTMLElement class, in turn, implements Node, so every element is also a node (but not the other way around). This way, the structural features implemented by the Node interface are also available to HTML elements, allowing them to be nested within each other, created and deleted, moved around, and so forth.

The HTMLElement interface is generic, however, providing only the functionality common to all HTML elements such as the element's ID, its coordinates, the HTML making up the element, information about scroll position, and so forth.

In order to expand upon the functionality of the core HTMLElement interface to provide the features needed by a specific element, the HTMLElement class is subclassed to add the needed properties and methods. For example, the <canvas> element is represented by an object of type HTMLCanvasElement. HTMLCanvasElement augments the HTMLElement type by adding properties such as height and methods like getContext() to provide canvas-specific features.

The overall inheritance for HTML element classes looks like this:


As such, an element inherits the properties and methods of all of its ancestors. For example, consider a <a> element, which is represented in the DOM by an object of type HTMLAnchorElement. The element, then, includes the anchor-specific properties and methods described in that class's documentation, but also those defined by HTMLElement and Element, as well as from Node and, finally, EventTarget.

Each level defines a key aspect of the utility of the element. From Node, the element inherits concepts surrounding the ability for the element to be contained by another element, and to contain other elements itself. Of special importance is what is gained by inheriting from EventTarget: the ability to receive and handle events such as mouse clicks, play and pause events, and so forth.

There are elements that share commonalities and thus have an additional intermediary type. For example, the <audio> and <video> elements both present audiovisual media. The corresponding types, HTMLAudioElement and HTMLVideoElement, are both based upon the common type HTMLMediaElement, which in turn is based upon HTMLElement and so forth. HTMLMediaElement defines the methods and properties held in common between audio and video elements.