Document Object Model

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HTML
Comparisons
Hierarchy of objects in an example HTML DOM—Document Object Model

The Document Object Model (DOM) is a cross-platform and language-independent application programming interface that treats an HTML, XHTML, or XML document as a tree structure wherein each node is an object representing a part of the document. The objects can be manipulated programmatically and any visible changes occurring as a result may then be reflected in the display of the document.[1]

Principal standardization of DOM was handled by the W3C DOM Working Group, which closed in 2008. Current stewardship of DOM-related standards resides with the Web Platform group.

History

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.

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.

In 1997, Netscape and Microsoft released version 4.0 of Netscape Navigator and Internet Explorer respectively, adding support for Dynamic HTML (DHTML), 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."

After the standardization of ECMAScript, the W3C DOM Working Group began drafting a standard DOM specification. The completed specification, known as "DOM Level 1", was recommended by W3C in late 1998. By 2005, large parts of W3C DOM were well-supported by common ECMAScript-enabled browsers, including Microsoft Internet Explorer version 6 (from 2001), Opera, Safari and Gecko-based browsers (like Mozilla, Firefox, SeaMonkey and Camino).

The DOM Working Group was closed in 2008, temporarily decentralizing DOM activity within the W3C. In 2009, the Web Applications group took over stewardship of DOM.[2] This group also closed in 2015, and stewardship passed to the Web Platform group.[3]

Standards

DOM Level 1 provided a complete model for an entire HTML or XML document, including means to change any portion of the document.

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.[4]

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 as "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.

JavaScript

When a web page is loaded, the browser creates a Document Object Model of the page.

With the object model, JavaScript is fully enabled to create dynamic HTML:[5]

  • JavaScript can add, change, and remove all the HTML elements and attributes in the page
  • JavaScript can change all the CSS styles in the page
  • JavaScript can react to all existing events in the page
  • JavaScript can create new events in the page

Implementations

Because DOM supports navigation in any direction (e.g., parent and previous sibling) and allows for arbitrary modifications, an implementation must at least buffer the document that has been read so far (or some parsed form of it).[citation needed]

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, are 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.

See also: Comparison of layout engines (Document Object Model)

Libraries

DOM implementations:

  • libxml2
  • MSXML
  • Xerces is a collection of DOM implementations written in C++, Java and Perl
  • XML for <SCRIPT> is a JavaScript-based DOM implementation[6]

APIs that expose DOM implementations:

  • JAXP (Java API for XML Processing) is an API for accessing DOM providers
  • Lazarus (Free Pascal IDE) contains two variants of the DOM - with UTF-8 and ANSI format

Inspection tools

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 (2006). JavaScript: The Definitive Guide. O'Reilly & Associates. pp. 312–313. ISBN 0-596-10199-6.
  • Koch, Peter-Paul (May 14, 2001). "The Document Object Model: an Introduction". Digital Web Magazine. Retrieved January 10, 2009.
  • Le Hégaret, Philippe (2002). "The W3C Document Object Model (DOM)". 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

  1. ^ "Document Object Model (DOM)". http://www.w3.org/: W3C. Retrieved 2012-01-12. The Document Object Model is a platform- and language-neutral interface that will allow programs and scripts to dynamically access and update the content, structure and style of documents. {{cite web}}: External link in |location= (help)CS1 maint: location (link)
  2. ^ https://www.w3.org/DOM/
  3. ^ https://www.w3.org/2008/webapps/wiki/PubStatus
  4. ^ http://www.w3.org/TR/dom/
  5. ^ http://www.w3schools.com/js/js_htmldom.asp
  6. ^ "Cross Platform XML Parsing in JavaScript"

External links

Wikimedia Commons has media related to
Document object models.
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