Whereas HTML defines a webpage's structure and content and CSS sets the formatting and appearance, JavaScript adds interactivity to a webpage and creates rich web applications.
However, the umbrella term "JavaScript" as understood in a web browser context contains several very different elements. One of them is the core language (ECMAScript), another is the collection of the Web APIs, including the DOM (Document Object Model).
The core language of JavaScript is standardized by the ECMA TC39 committee as a language named ECMAScript.
This core language is also used in non-browser environments, for example in node.js.
Among other things, ECMAScript defines:
parseInt, parseFloat, decodeURI, encodeURI...JSON, Math, Array.prototype methods, Object introspection methods...)As of October 2016, the current versions of the major Web browsers implement ECMAScript 5.1 and ECMAScript 2015, but older versions (still in use) implement ECMAScript 5 only.
The major 6th Edition of ECMAScript was officially approved and published as a standard on June 17, 2015 by the ECMA General Assembly. Since then ECMAScript Editions are published on a yearly basis.
The ECMAScript Internationalization API Specification is an addition to the ECMAScript Language Specification, also standardized by Ecma TC39. The internationalization API provides collation (string comparison), number formatting, and date-and-time formatting for JavaScript applications, letting the applications choose the language and tailor the functionality to their needs. The initial standard was approved in December 2012; the status of implementations in browsers is tracked in the documentation of the Intl object. The Internationalization specification is nowadays also ratified on a yearly basis and browsers constantly improve their implementation.
The WebIDL specification provides the glue between the DOM technologies and ECMAScript.
The Document Object Model (DOM) is a cross-platform, 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 {{glossary("W3C")}} standardizes the Core Document Object Model, which defines language-agnostic interfaces that abstract HTML and XML documents as objects, and also defines mechanisms to manipulate this abstraction. Among the things defined by the DOM, we can find:
Node, Element, DocumentFragment, Document, DOMImplementation, Event, EventTarget, …From the ECMAScript point of view, objects defined in the DOM specification are called "host objects".
HTML, the Web's markup language, is specified in terms of the DOM. Layered above the abstract concepts defined in DOM Core, HTML also defines the meaning of elements. The HTML DOM includes such things as the className property on HTML elements, or APIs such as {{ domxref("document.body") }}.
The HTML specification also defines restrictions on documents; for example, it requires all children of a ul element, which represents an unordered list, to be li elements, as those represent list items. In general, it also forbids using elements and attributes that aren't defined in a standard.
Looking for the Document object, Window object, and the other DOM elements? Read the DOM documentation.
setTimeout and setInterval functions were first specified on the Window interface in HTML Standard.As every web developer has experienced, the DOM is a mess. Browser support uniformity varies a lot from feature to feature, mainly because many important DOM features have very unclear, specifications (if any), and different web browsers add incompatible features for overlapping use cases (like the Internet Explorer event model). As of June 2011, the W3C and particularly the WHATWG are defining older features in detail to improve interoperability, and browsers in turn can improve their implementations based on these specifications.
One common, though perhaps not the most reliable, approach to cross-browser compatibility is to use JavaScript libraries, which abstract DOM features and keep their APIs working the same in different browsers. Some of the most widely used frameworks are jQuery, prototype, and YUI.