The Document Object Model is a cross-platform and language-independent convention for representing
and interacting with objects in HTML, XHTML and XML documents. Objects in the DOM tree
may be addressed and manipulated by using methods on the objects. The public interface
of a DOM is specified in its application programming interface. The history of the Document Object
Model is intertwined with the history of the "browser wars" of the late 1990s between Netscape
Navigator and Microsoft Internet Explorer, as well as with that of JavaScript and JScript,
the first scripting languages to be widely implemented in the layout engines of web browsers.
Legacy DOM JavaScript was released by Netscape Communications
in 1995 within Netscape Navigator 2.0. Netscape's competitor, Microsoft, released Internet Explorer
3.0 the following year with a port of JavaScript called JScript. JavaScript and JScript let
web developers create web pages with client-side interactivity. The limited facilities for
detecting user-generated events and modifying the HTML document in the first generation
of these languages eventually became known as "DOM Level 0" or "Legacy DOM." No independent
standard was developed for DOM Level 0, but it was partly described in the specification
of HTML 4. Legacy DOM was limited in the kinds of elements
that could be accessed. Form, link and image elements could be referenced with a hierarchical
name that began with the root document object. A hierarchical name could make use of either
the names or the sequential index of the traversed elements. For example, a form input element
could be accessed as either "document.formName.inputName" or "document.forms[0].elements[0]."
The Legacy DOM enabled client-side form validation and the popular "rollover" effect.
Intermediate DOM In 1997, Netscape and Microsoft released version
4.0 of Netscape Navigator and Internet Explorer respectively, adding support for Dynamic HTML,
functionality enabling changes to a loaded HTML document. DHTML required extensions to
the rudimentary document object that was available in the Legacy DOM implementations. Although
the Legacy DOM implementations were largely compatible since JScript was based on JavaScript,
the DHTML DOM extensions were developed in parallel by each browser maker and remained
incompatible. These versions of the DOM became known as the "Intermediate DOM."
Standardization The World Wide Web Consortium, founded in
1994 to promote open standards for the World Wide Web, brought Netscape Communications
and Microsoft together with other companies to develop a standard for browser scripting
languages, called "ECMAScript." The first version of the standard was published in 1997.
Subsequent releases of JavaScript and JScript would implement the ECMAScript standard for
greater cross-browser compatibility. After the release of ECMAScript, W3C began
working on a standardized DOM. The initial DOM standard, known as "DOM Level 1," was
recommended by W3C in late 1998. About the same time, Internet Explorer 5.0 shipped with
limited support for DOM Level 1. DOM Level 1 provided a complete model for an entire
HTML or XML document, including means to change any portion of the document. Non-conformant
browsers such as Internet Explorer 4.x and Netscape 4.x were still widely used as late
as 2000. DOM Level 2 was published in late 2000. It
introduced the "getElementById" function as well as an event model and support for XML
namespaces and CSS. DOM Level 3, the current release of the DOM
specification, published in April 2004, added support for XPath and keyboard event handling,
as well as an interface for serializing documents as XML.
DOM Level 4 is currently being developed. Last Call Working Draft was released in February
2014. By 2005, large parts of W3C DOM were well-supported
by common ECMAScript-enabled browsers, including Microsoft Internet Explorer version 6, Opera,
Safari and Gecko-based browsers. Applications
Web browsers To render a document such as an HTML page,
most web browsers use an internal model similar to the DOM. The nodes of every document are
organized in a tree structure, called the DOM tree, with topmost node named "Document
object". When an HTML page is rendered in browsers, the browser downloads the HTML into
local memory and automatically parses it to display the page on screen. The DOM is also
the way JavaScript transmits the state of the browser in HTML pages.
Implementations Because DOM supports navigation in any direction
and allows for arbitrary modifications, an implementation must at least buffer the document
that has been read so far. Layout engines
Web browsers rely on layout engines to parse HTML into a DOM. Some layout engines, such
as Trident/MSHTML, are associated primarily or exclusively with a particular browser,
such as Internet Explorer. Others, such as Blink, WebKit, and Gecko, JavaScript rendered
in html pages, collection of web pages shared by a number of browsers, such as Google Chrome,
Opera, Safari, and Firefox. The different layout engines implement the DOM standards
to varying degrees of compliance.
Libraries DOM implementations:
libxml2 MSXML
Xerces is a collection of DOM implementations written in C++, Java and Perl
XML for is a JavaScript-based DOM implementation
APIs that expose DOM implementations: JAXP is an API for accessing DOM providers
Lazarus contains two variants of the DOM - with UTF-8 and ANSI format
Inspection tools DOM Inspector is a web developer tool
See also Ajax—a methodology employing DOM in combination
with techniques for retrieving data without reloading a page.
Application Object Model DOM scripting
JDOM—a Java-based document object model for XML that integrates with DOM and SAX and
uses parsers to build the document. jQuery
JSON Prototype.js
SAX—serial access parser API for XML, an alternative to DOM.
SXML—a model for representing XML and HTML in the form of S-expressions.
TinyXml—efficient platform-independent XML library for C++.
Sources Flanagan, David. JavaScript: The Definitive
Guide. O'Reilly & Associates. pp. 312–313. ISBN 0-596-10199-6.
Koch, Peter-Paul. "The Document Object Model: an Introduction". Digital Web Magazine. Retrieved
January 10, 2009. Le Hégaret, Philippe. "The W3C Document Object
Model". World Wide Web Consortium. Retrieved January 10, 2009.
Guisset, Fabian. "What does each DOM Level bring?". Mozilla Developer Center. Mozilla
Project. Retrieved January 10, 2009. References
External links Document Object Model by the World Wide Web
Consortium Technology Reports
What does your user agent claim to support? W3C DOM scripts and compatibility tables
Gecko DOM Reference IB DOM Utilities: Mapping JavaScript Objects
to DOM Elements XJR with DOM, SAX2, and XPath interfaces
Firefox plugin that lets you visualize a Web's page DOM in 3D