JavaScript technologies overview

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).

JavaScript, the core language (ECMAScript)

The core language of JavaScript is standardized by the ECMA TC39 committee as a language named ECMAScript. "ECMAScript" is the term for the language standard, but "ECMAScript" and "JavaScript" can be used interchangeably.

This core language is also used in non-browser environments, for example in Node.js.

What falls under the ECMAScript scope?

Among other things, ECMAScript defines:

  • Language syntax (parsing rules, keywords, control flow, object literal initialization, ...)
  • Error handling mechanisms (throw, try...catch, ability to create user-defined Error types)
  • Types (boolean, number, string, function, object, ...)
  • A prototype-based inheritance mechanism
  • Built-in objects and functions, including JSON, Math, Array methods, parseInt, decodeURI, etc.
  • Strict mode
  • A module system
  • Basic memory model

Standardization process

ECMAScript editions are approved and published as a standard by the ECMA General Assembly on a yearly basis. All development is public on the Ecma TC39 GitHub organization, which hosts proposals, the official specification text, and meeting notes.

Before the 6th edition of ECMAScript (known as ES6), specifications were published once several years, and are commonly referred by their major version numbers — ES3, ES5, etc. After ES6, the specification is named by the publishing year — ES2017, ES2018, etc. ES6 is synonymous with ES2015. ESNext is a dynamic name that refers to whatever the next version is at the time of writing. ESNext features are more correctly called proposals, because, by definition, the specification has not been finalized yet.

The current committee-approved snapshot of ECMA-262 can be found in PDF form and HTML form. ECMA-262 and ECMA-402 are continuously maintained and kept up to date by the specification editors; the TC39 website hosts the latest, up-to-date ECMA-262 and ECMA-402 versions.

New language features, including introduction of new syntaxes and APIs and revision of existing behaviors, are discussed in the form of proposals. Each proposal goes through a 4-stage process, and is typically implemented by JavaScript engines at stage 3 or stage 4 and thus available for public consumption.

See Wikipedia ECMAScript entry for more information on ECMAScript history.

Internationalization API

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.

There are a variety of ways you can participate in or just track current work on the ECMAScript Language Specification and the ECMAScript Internationalization API Specification and related resources:



The WebIDL specification provides the glue between the DOM technologies and ECMAScript.

The Core of the DOM

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 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:

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 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.

Other notable APIs

Non-browser environments (like Node.js) often do not have DOM APIs — because they don't interact with a document — but they still usually implement many web APIs, such as fetch() and setTimeout().


A JavaScript shell allows you to quickly test snippets of JavaScript code without having to reload a web page. They are extremely useful for developing and debugging code.

Standalone JavaScript shells

The following JavaScript shells are stand-alone environments, like Perl or Python.

  • Node.js - Node.js is a platform for easily building fast, scalable network applications.
  • GraalJS - A high performance implementation of the JavaScript programming language. Built on the GraalVM by Oracle Labs.
  • ShellJS - Portable Unix shell commands for Node.js.

Browser-based JavaScript shells

The following JavaScript shells run code through the browser's JavaScript engine.