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+---
+title: About JavaScript
+slug: Web/JavaScript/About_JavaScript
+tags:
+  - Beginner
+  - Guide
+  - Introduction
+  - JavaScript
+---
+{{JsSidebar}}
+
+## What is JavaScript?
+
+**JavaScript** (often shortened to **JS**) is a lightweight, interpreted,
+object-oriented language with
+[first-class functions](https://en.wikipedia.org/wiki/First-class_function), and
+is best known as the scripting language for Web pages, but it's
+[used in many non-browser environments](https://en.wikipedia.org/wiki/JavaScript#Uses_outside_Web_pages)
+as well. It is a
+[prototype-based](https://en.wikipedia.org/wiki/Prototype-based_programming "Prototype-based"),
+multi-paradigm scripting language that is dynamic, and supports object-oriented,
+imperative, and functional programming styles.
+
+JavaScript runs on the client side of the web, which can be used to design /
+program how the web pages behave on the occurrence of an event. JavaScript is an
+easy to learn and also powerful scripting language, widely used for controlling
+web page behavior.
+
+Contrary to popular misconception, **JavaScript is _not_ "Interpreted Java"**.
+In a nutshell, JavaScript is a dynamic scripting language supporting
+[prototype based](/en-US/docs/Web/JavaScript/Guide/Details_of_the_Object_Model#class-based_vs._prototype-based_languages)
+object construction. The basic syntax is intentionally similar to both Java and
+C++ to reduce the number of new concepts required to learn the language.
+Language constructs, such as `if` statements, `for` and `while` loops, and
+`switch` and `try ... catch` blocks function the same as in these languages (or
+nearly so).
+
+JavaScript can function as both a
+[procedural](https://en.wikipedia.org/wiki/Procedural_programming) and an
+[object oriented language](/en-US/docs/Learn/JavaScript/Objects). Objects are
+created programmatically in JavaScript, by attaching methods and properties to
+otherwise empty objects **at run time**, as opposed to the syntactic class
+definitions common in compiled languages like C++ and Java. Once an object has
+been constructed it can be used as a blueprint (or prototype) for creating
+similar objects.
+
+JavaScript's dynamic capabilities include runtime object construction, variable
+parameter lists, function variables, dynamic script creation (via
+[`eval`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval)), object
+introspection (via `for ... in`), and source code recovery (JavaScript programs
+can decompile function bodies back into their source text).
+
+For a more in depth discussion of JavaScript programming follow the
+[JavaScript resources](#javascript_resources) links below.
+
+## What JavaScript implementations are available?
+
+The Mozilla project provides two JavaScript implementations. The first **ever**
+JavaScript was created by Brendan Eich at Netscape, and has since been updated
+to conform to ECMA-262 Edition 5 and later versions. This engine, code named
+[SpiderMonkey](/en-US/docs/Mozilla/Projects/SpiderMonkey), is implemented in
+C/C++. The [Rhino](/en-US/docs/Rhino) engine, created primarily by Norris Boyd
+(also at Netscape) is a JavaScript implementation written in Java. Like
+SpiderMonkey, Rhino is ECMA-262 Edition 5 compliant.
+
+Several major runtime optimizations such as TraceMonkey (Firefox 3.5),
+JägerMonkey (Firefox 4) and IonMonkey were added to the SpiderMonkey JavaScript
+engine over time. Work is always ongoing to improve JavaScript execution
+performance.
+
+Besides the above implementations, there are other popular JavaScript engines
+such as:-
+
+- Google's [V8](https://code.google.com/p/v8/), which is used in the Google
+  Chrome browser and recent versions of Opera browser. This is also the engine
+  used by [Node.js](http://nodejs.org).
+- The [JavaScriptCore](https://www.webkit.org/projects/javascript/index.html)
+  (SquirrelFish/Nitro) used in some WebKit browsers such as Apple Safari.
+- [Carakan](http://my.opera.com/ODIN/blog/carakan-faq) in old versions of Opera.
+- The [Chakra](https://en.wikipedia.org/wiki/Chakra_%28JScript_engine%29) engine
+  used in Internet Explorer (although the language it implements is formally
+  called "JScript" in order to avoid trademark issues).
+
+Each of Mozilla's 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
+[DOM](https://www.w3.org/DOM/) into JavaScript.
+
+Another common application for JavaScript is as a (Web) server side scripting
+language. A JavaScript web server would expose host objects representing a HTTP
+request and response objects, which could then be manipulated by a JavaScript
+program to dynamically generate web pages. [Node.js](http://nodejs.org) is a
+popular example of this.
+
+## JavaScript resources
+
+- [SpiderMonkey](/en-US/docs/Mozilla/Projects/SpiderMonkey)
+  - : Information specific to Mozilla's implementation of JavaScript in C/C++
+    engine (aka SpiderMonkey), including how to embed it in applications.
+- [Rhino](/en-US/docs/Rhino)
+  - : Information specific to the JavaScript implementation written in Java (aka
+    Rhino).
+- [Language resources](/en-US/docs/Web/JavaScript/Language_Resources)
+  - : Pointers to published JavaScript standards.
+- [A re-introduction to JavaScript](/en-US/docs/Web/JavaScript/A_re-introduction_to_JavaScript)
+  - : [JavaScript guide](/en-US/docs/Web/JavaScript/Guide) and
+    [JavaScript reference](/en-US/docs/Web/JavaScript/Reference).
+
+JavaScript® is a trademark or registered trademark of Oracle in the U.S. and
+other countries.