The Document Object Model (DOM) is an application programming interface (API) for HTML and XML documents. It defines the logical structure of documents and the way a document is accessed and manipulated. In the DOM specification, the term "document" is used in the broad sense - increasingly, XML is being used as a way of representing many different kinds of information that may be stored in diverse systems, and much of this would traditionally be seen as data rather than as documents. Nevertheless, XML presents this data as documents, and the DOM may be used to manage this data.
With the Document Object Model, programmers can build documents, navigate their structure, and add, modify, or delete elements and content. Anything found in an HTML or XML document can be accessed, changed, deleted, or added using the Document Object Model, with a few exceptions - in particular, the DOM interfaces for the XML internal and external subsets have not yet been specified.
As a W3C specification, one important objective for the Document
Object Model is to provide a standard programming interface that
can be used in a wide variety of environments and applications.
The DOM is designed to be used with any programming
language. In order to provide a precise, language-independent specification
of the DOM interfaces, we have chosen to define
the specifications in OMG IDL, as defined in the CORBA 2.2
In addition to the OMG IDL specification,
we provide language bindings for Java and ECMAScript (an
JScript). Note: OMG IDL is used only as a language-independent and
implementation-neutral way to specify interfaces. Various other
IDLs could have been used.. In general, IDLs are designed for
specific computing environments. The Document Object Model
can be implemented in any computing environment, and does not
require the object binding runtimes generally associated with
The DOM is a programming API for documents. It closely resembles the structure of the documents it models. For instance, consider this table, taken from an HTML document:
<TABLE> <TBODY> <TR> <TD>Shady Grove</TD> <TD>Aeolian</TD> </TR> <TR> <TD>Over the River, Charlie</TD> <TD>Dorian</TD> </TR> </TBODY> </TABLE>
The DOM represents this table like this:
In the DOM, documents have a logical structure which is very much like a tree; to be more precise, it is like a "forest" or "grove", which can contain more than one tree. However, the DOM does not specify that documents must be implemented as a tree or a grove , nor does it specify how the relationships among objects be implemented. the DOM is a logical model that may be implemented in any convenient manner. In this specification, we use the term structure model to describe the tree-like representation of a document; we specifically avoid terms like "tree" or "grove" in order to avoid implying a particular implementation. One important property of DOM structure models is structural isomorphism: if any two Document Object Model implementations are used to create a representation of the same document, they will create the same structure model, with precisely the same objects and relationships.
The name "Document Object Model" was chosen because it is an "object model" in the traditional object oriented design sense: documents are modeled using objects, and the model encompasses not only the structure of a document, but also the behavior of a document and the objects of which it is composed. In other words, the nodes in the above diagram do not represent a data structure, they represent objects, which have functions and identity. As an object model, the DOM identifies:
The structure of SGML documents has traditionally been represented by an abstract data model, not by an object model. In an abstract data model, the model is centered around the data. In object oriented programming languages, the data itself is encapsulated in objects that hide the data, protecting it from direct external manipulation. The functions associated with these objects determine how the objects may be manipulated, and they are part of the object model.
The Document Object Model currently consists of two parts,
DOM Core and DOM HTML. The DOM Core represents the
functionality used for XML documents, and also serves as the
basis for DOM HTML.
A compliant implementation of the DOM must implement all of the
fundamental interfaces in the Core chapter with the semantics as
defined. Further, it must implement at least one of the HTML DOM
and the extended (XML) interfaces with the semantics as defined.
This section is designed to give a more precise understanding of the DOM by distinguishing it from other systems that may seem to be like it.
The DOM originated as a specification to
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 DOM
Working Group was formed at W3C, 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 DOM
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 an APIs for SGML/XML
editors or document repositories, and these object models have
also influenced the DOM.
In the fundamental DOM interfaces, there are no objects representing entities. Numeric character references, and references to the pre-defined entities in HTML and XML, are replaced by the single character that makes up the entity's replacement. For example, in:
<p>This is a dog & a cat</p>the "&" will be replaced by the character "&", and the text in the <p> element will form a single continuous sequence of characters. The representation of general entities, both internal and external, are defined within the extended (XML) interfaces of the Level 1 specification. Note: When a DOM representation of a document is serialized as XML or HTML text, applications will need to check each character in text data to see if it needs to be escaped using a numeric or pre-defined entity. Failing to do so could result in invalid HTML or XML.
The DOM specifies interfaces which may be used to manage XML or HTML documents. It is important to realize that these interfaces are an abstraction - much like "abstract base classes" in C++, they are a means of specifying a way to access and manipulate an application's internal representation of a document. Interfaces do not imply a particular concrete implementation. Each DOM application is free to maintain documents in any convenient representation, as long as the interfaces shown in this specification are supported. Some DOM implementations will be existing programs that use the DOM interfaces to access software written long before the DOM specification existed. Therefore, the DOM is designed to avoid implementation dependencies; in particular,
The DOM Level 1 specification is intentionally limited to those methods needed to represent and manipulate document structure and content. The plan is for future Levels of the DOM specification to provide: