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Document Object Model
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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 (W3C), 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.
In HTML DOM (Document Object Model), every element is a node:
- A document is a document node.
- All HTML elements are element nodes.
- All HTML attributes are attribute nodes.
- Text inserted into HTML elements are text nodes.
- Comments are comment nodes.
Table of contents
DOM tree structure Manipulating the DOM tree Implementations See also
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.elements.
The Legacy DOM enabled client-side form validation and simple interface interactivity like creating tooltips.
After the standardization of ECMAScript, the W3C DOM Working Group began drafting a standard DOM specification. The completed specification, known as "DOM Level 1", became a W3C Recommendation in late 1998. By 2005, large parts of W3C DOM were well-supported by common ECMAScript-enabled browsers, including Internet Explorer 6 (from 2001), Opera, Safari and Gecko-based browsers (like Mozilla, Firefox, SeaMonkey and Camino).
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.
- HTML5 was published in October 2014. Part of HTML5 had replaced DOM Level 2 HTML module.
- DOM Level 4 was published in 2015. It is a snapshot of the WHATWG living standard.
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. However, the DOM does not necessarily need to be represented as a tree, and some browsers have used other internal models.
- 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
DOM tree structure
A Document Object Model (DOM) tree is a hierarchical representation of an HTML or XML document. It consists of a root node, which is the document itself, and a series of child nodes that represent the elements, attributes, and text content of the document. Each node in the tree has a parent node, except for the root node, and can have multiple child nodes.
Elements as Nodes
Elements in an HTML or XML document are represented as nodes in the DOM tree. Each element node has a tag name, attributes, and can contain other element nodes or text nodes as children. For example, an HTML document with the following structure:<html> <head> <title>My Website</title> </head> <body> <h1>Welcome</h1> <p>This is my website.</p> </body> </html>will be represented in the DOM tree as:- Document (root) - html - head - title - "My Website" - body - h1 - "Welcome" - p - "This is my website."Text Nodes
Text content within an element is represented as a text node in the DOM tree. Text nodes do not have attributes or child nodes, and are always leaf nodes in the tree. For example, the text content "My Website" in the title element and "Welcome" in the h1 element in the above example are both represented as text nodes.
Attributes as Properties
Attributes of an element are represented as properties of the element node in the DOM tree. For example, an element with the following HTML:<a href="https://example.com">Link</a>will be represented in the DOM tree as:- a - href: "https://example.com" - "Link"
Manipulating the DOM tree
It's important to note that creating a DOM structure does not necessarily mean that it will be displayed in the web page, it only exists in memory and should be appended to the document body or a specific container to be rendered.
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).
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.
APIs that expose DOM implementations:
- Xerces is a collection of DOM implementations written in C++, Java and Perl
- xml.dom[python.org] for Python
- PHP.Gt DOM[github.com] is a server-side DOM implementation based on libxml2 and brings DOM level 4 compatibility to the PHP programming language
- Domino[github.com] is a Server-side (Node.js) DOM implementation based on Mozilla's dom.js. Domino is used in the MediaWiki stack with Visual Editor.
- SimpleHtmlDom[github.com] is a simple HTML document object model in C#, which can generate HTML string programmatically.
- 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
- DOM Inspector is a web developer tool
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