HTML DOM: How to use Document Object Model

A Document object represents the HTML document that is displayed in that window. The Document object has various properties that refer to other objects which allow access to and modification of document content. In this article, we will understand HTML DOM.

• HTML DOM Concept
• What the HTML DOM is
• What the HTML DOM is NOT
• Where the DOM Came From
• Properties of  HTML DOM
• Finding HTML Elements

HTML DOM Concept

The way a documented content is accessed and modified is called the Document Object Model, or DOM. The Objects are organized in a hierarchy. This hierarchical structure applies to the organization of objects in a Web document.

• Window object − Top of the hierarchy. It is the utmost element of the object hierarchy.
• Document object − Each HTML document that gets loaded into a window becomes a document object. The document contains the contents of the page.
• Form object − Everything enclosed in the <form>…</form> tags sets the form object.
• Form control elements − The form object contains all the elements defined for that object such as text fields, buttons, radio buttons, and checkboxes.

What the HTML DOM is

The Document Object Model is a programming API for documents. The object model itself closely resembles the structure of the documents it models. For instance, consider this table, taken from an HTML document:

      <TABLE>
      <ROWS> 
      <TR> 
      <TD>Shady Grove</TD>
      <TD>Aeolian</TD> 
      </TR> 
      <TR>
      <TD>Over the River, Charlie</TD>
      <TD>Dorian</TD> 
      </TR> 
      </ROWS>
      </TABLE>

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What the HTML DOM is NOT

This section is designed to give a more precise understanding of the Document Object Model by distinguishing it from other systems that may seem to be like it.

Although the Document Object Model was strongly influenced by Dynamic HTML, in Level 1, it does not implement all of Dynamic HTML. In particular, events have not yet been defined. Level 1 is designed to lay a firm foundation for this kind of functionality by providing a robust, flexible model of the document itself.

The Document Object Model is not a binary specification. Document Object Model programs written in the same language will be source code compatible across platforms, but the Document Object Model does not define any form of binary interoperability.

The Document Object Model is not a way of persisting objects to XML or HTML. Instead of specifying how objects may be represented in XML, the Document Object Model specifies how XML and HTML documents are represented as objects, so that they may be used in object-oriented programs.

The Document Object Model is not a set of data structures, it is an object model that specifies interfaces. Although this document contains diagrams showing parent/child relationships, these are logical relationships defined by the programming interfaces, not representations of any particular internal data structures.

The Document Object Model does not define “the true inner semantics” of XML or HTML. The semantics of those languages are defined by the languages themselves.

The Document Object Model is a programming model designed to respect these semantics. The Document Object Model does not have any ramifications for the way you write XML and HTML documents; any document that can be written in these languages can be represented in the Document Object Model.

The Document Object Model, despite its name, is not a competitor to the Component Object Model (COM). COM, like CORBA, is a language independent way to specify interfaces and objects; the Document Object Model is a set of interfaces and objects designed for managing HTML and XML documents. The DOM may be implemented using language-independent systems like COM or CORBA; it may also be implemented using language-specific bindings like the Java or ECMAScript bindings specified in this document.

Where the Document Object Model Came From

The Document Object Model originated as a specification to allow JavaScript scripts and Java programs to be portable among web browsers. Dynamic HTML was the immediate ancestor of the Document Object Model, and it was originally thought of largely in terms of browsers.

However, when the Document Object Model Working Group was formed, it was also joined by vendors in other domains, including HTML or XML editors and document repositories. Several of these vendors had worked with SGML before XML was developed; as a result, the Document Object Model has been influenced by SGML Groves and the HyTime standard. Some of these vendors had also developed their own object models for documents in order to provide programming APIs for SGML/XML editors or document repositories, and these object models have also influenced the Document Object Model.

Properties of HTML DOM

Let’s see the properties of the document objects that can be accessed and modified by the document object.

1. Window Object: Window Object is always at the top of the hierarchy.
2. Document object: When an HTML document is loaded into a window, it becomes a document object.
3. Form Object: It is represented by form tags.
4. Link Objects: It is represented by link tags.
5. Anchor Objects: It is represented by a href tags.
6. Form Control Elements: Form can have many control elements such as text fields, buttons, radio buttons, and checkboxes, etc.

Methods of Document Object:

1. write(“string”): writes the given string on the document.
2. getElementById(): returns the element having the given id value.
3. getElementsByName(): returns all the elements having the given name value.
4. getElementsByTagName(): returns all the elements having the given tag name.
5. getElementsByClassName(): returns all the elements having the given class name.

Finding HTML Elements

When you want to access HTML elements with JavaScript, you have to find the elements first.

There are a couple of ways to do this:

• Finding HTML elements by id
• Finding HTML elements by tag name
• Finding HTML elements by class name

Finding HTML Element by Id

The easiest way to find an HTML element in the DOM, is by using the element id.

Example

Finding HTML Elements by Tag Name

This example finds the element with id=”main”, and then finds all <p> elements inside “main”:

With this, we come to an end of this HTML DOM article. I hope you got an understanding of the various aspects of the HTML DOM, the Document Object Model. 

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