Introduction to Document Object Model

The Document Object Model (DOM) is a cross-platform and language-independent interface that treats an XML or HTML document as a tree structure wherein each node is an object representing a part of the document. The DOM represents a document with a logical tree. Each branch of the tree ends in a node, and each node contains objects. DOM methods allow programmatic access to the tree; with them one can change the structure, style or content of a document. Nodes can have event handlers attached to them. Once an event is triggered, the event handlers get executed.

The principal standardization of the DOM was handled by the World Wide Web Consortium, which last developed a recommendation in 2004. WHATWG took over the development of the standard, publishing it as a living document. The W3C now publishes stable snapshots of the WHATWG standard.

The W3C DOM Working Group published its final recommendation and subsequently disbanded in 2004. Development efforts migrated to the WHATWG, which continues to maintain a living standard. In 2009, the Web Applications group reorganized DOM activities at the W3C. In 2013, due to a lack of progress and the impending release of HTML5, the DOM Level 4 specification was reassigned to the HTML Working Group to expedite its completion. Meanwhile, in 2015, the Web Applications group was disbanded and DOM stewardship passed to the Web Platform group. Beginning with the publication of DOM Level 4 in 2015, the W3C creates new recommendations based on snapshots of the WHATWG standard.

  • DOM Level 1 provided a complete model for an entire HTML or XML document, including the 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, 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 was published in 2015. It is a snapshot of the WHATWG living standard.

Web browsers:

To render a document such as a 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 the 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.

JavaScript:

When a web page is loaded, the browser creates a Document Object Model of the page, which is an object oriented representation of an HTML document that acts as an interface between JavaScript and the document itself. This allows the creation of dynamic web pages, because within a page JavaScript can:

  • add, change, and remove any of the HTML elements and attributes
  • change any of the CSS styles
  • react to all the existing events
  • create new events

Because the 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).

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, including 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.

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
  • PHP.Gt DOM is a server-side DOM implementation based on libxml2 and brings DOM level 4 compatibility to the PHP programming language
  • Domino is a Server-side (Node.js) DOM implementation based on Mozilla’s dom.js. Domino is used in the MediaWiki stack with Visual Editor.

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:

  • DOM Inspector is a web developer tool

The above is a brief about Document Object Model. Watch this space for more updates on the latest trends in Technology.

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