JavaScript Engines

JavaScript is an object-oriented scripting language used to enable programmatic access to objects within both the client application and other applications. It is primarily used in the form of client-side JavaScript, implemented as an integrated component of the web browser, allowing the development of enhanced user interfaces and dynamic websites.

A JavaScript engine (also known as JavaScript interpreter or JavaScript implementation) is an interpreter that interprets JavaScript source code and executes the script accordingly. The first JavaScript engine was created by Brendan Eich at Netscape Communications Corporation, for the Netscape Navigator web browser.

There are now a ton of JavaScript engines on the market (even when you only look at the ones being actively used in browsers):

1. JavaScriptCore: The engine that powers Safari/WebKit (up until Safari 3.1).
2. SquirrelFish: The engine used by Safari 4.0. Note: The latest WebKit nightly for Windows crashes on Dromaeo, so it's passed for now.
3. V8: The engine used by Google Chrome.
4. SpiderMonkey: The engine that powers Firefox (up to, and including, Firefox 3.0).
5. TraceMonkey: The engine that will power Firefox 3.1 and newer (currently in nightlies, but disabled by default).
6. Futhark: The engine used in Opera 9.5 and newer.
7. IE JScript: The engine that powers Internet Explorer.

ref:

http://ejohn.org/blog/javascript-performance-rundown/