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
.
What falls under the ECMAScript scope?
Among other things, ECMAScript defines:
Language syntax (parsing rules, keywords, control flow, object literal initialization, ...)
Error handling mechanisms (
,
, ability to create user-defined
types)
Types (boolean, number, string, function, object, ...)
A prototype-based inheritance mechanism
Built-in objects and functions, including
,
,
methods,
,
, etc.
A
Basic memory model
ECMAScript editions are approved and published as a standard by the ECMA General Assembly on a yearly basis. All development is public on the
, which hosts proposals, the official specification text, and meeting notes.
Before the 6th edition of ECMAScript (known as ES6), specifications were published once every 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 is available in PDF and HTML format on Ecma International's
ECMA-262 language specification page
. 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
and
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
, and is typically implemented by JavaScript engines at stage 3 or stage 4 and thus available for public consumption.
See
for more information on ECMAScript history.
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
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:
ECMAScript Language Specification repo
ECMAScript Internationalization API Specification repo
ECMAScript conformance test suite repo
ECMAScript spec discussion; current mailing list
ECMAScript spec discussion; historical mailing-list archives (until March 2021)
The
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. Nowadays, the
spec is maintained by
(superseding the
version). It defines language-agnostic interfaces that abstract HTML and XML documents as objects, and also defines mechanisms to manipulate this abstraction. This includes:
,
,
,
,
,
,
, and more.
From the ECMAScript point of view, objects defined in the DOM specification are called "host objects".
, 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
.
The HTML specification also defines restrictions on documents; for example, it requires all children of a
element, which represents an unordered list, to be
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
object,
object, and the other DOM elements? Read the
.
The
and
functions were first specified on the
interface in HTML Standard.
makes it possible to send asynchronous HTTP requests.
The
provides a more ergonomic abstraction for network requests.
The
abstract CSS rules as objects.
allows parallel computation.
allows low-level bidirectional communication.
is a drawing API for
.
The
provides utilities for communication between JavaScript code and
modules.
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
and
.
JavaScript engines used in current web browsers include:
Mozilla's
, used in Firefox, Servo, and Flow. Other non-browser usage includes MongoDB, CouchDB, and more. This was the first ever JavaScript engine, created by Brendan Eich at Netscape.
Google's
, used in Chrome and Chromium-based browsers such as Opera, Edge, and Brave. Other non-browser usage includes
,
,
, and more.
Apple's
(also known as SquirrelFish/Nitro), used in Safari and other WebKit-based browsers. Other non-browser usage includes
.
, used in
.
Some JavaScript engines used in earlier browsers include:
, used in Opera before it became a Chromium-based browser.
Microsoft's
, used in Internet Explorer (although the language it implements is formally called "JScript" to avoid trademark issues). Earlier versions of Edge used another JavaScript engine, confusingly also called
, before it became a Chromium-based browser.
Some JavaScript engines specifically tailored for non-browser purposes include:
, written in JavaScript and intended essentially as a reference implementation of the language.
Meta's
, optimized for
.
Mozilla's
, written in Java.
Oracle's
, written in Java and built on top of GraalVM.
, intended for IoT/embedded systems.
, intended to be small and lightweight.
JavaScript engines expose a public API which application developers can use to integrate JavaScript into their software. By far, the most common host environment for JavaScript is web browsers. Web browsers typically use the public API to create host objects responsible for reflecting the
into JavaScript.
Another common application for JavaScript is as a (Web) server-side scripting language. A JavaScript web server exposes host objects representing an HTTP request and response objects, which can then be manipulated by a JavaScript program to dynamically generate web pages.
is a popular example of this.
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.
The following JavaScript shells are stand-alone environments, like Perl or Python.
- Node.js is a platform for easily building fast, scalable network applications.
- Portable Unix shell commands for Node.js.
Browser-based JavaScript shells
The following JavaScript shells run code through the browser's JavaScript engine.
Firefox has a
, which support multi-line editing.
- A browser-based
for experimenting with future JavaScript.
— A browser-based playground for experimenting both new JavaScript features (via the tsc compiler) and TypeScript syntax.
Helpful tools for writing and debugging your JavaScript code.
,
,
, and more.
An excellent resource for aspiring web developers — Learn JavaScript in an interactive environment, with short lessons and interactive tests, guided by automated assessment. The first 40 lessons are free, and the complete course is available for a small one-time payment.
Collaboration made easy. By adding TogetherJS to your site, your users can help each other out on a website in real-time!
Stack Overflow questions tagged with "JavaScript".
Edit JavaScript, CSS, and HTML and get live results. Use external resources and collaborate with your team online.
Plunker is an online community for creating, collaborating on, and sharing your web development ideas. Edit your JavaScript, CSS, and HTML files and get live results and file structure.
JS Bin is an open-source collaborative web development debugging tool.
CodePen is another collaborative web development tool used as a live result playground.
StackBlitz is another online playground/debugging tool, which can host and deploy full-stack applications using React, Angular, etc.
RunJS is a desktop playground/scratchpad tool, which provides live results and access to both Node and Browser APIs.