JSX is an open source programming language, or an AltJS with classes and static types. The source code is compiled into JavaScript which is highly optimized.

JSX is an -like syntax extension to ECMAScript without any defined semantics. It is used by , a client-side JavaScript view framework developed at Facebook. JSX is intended to be used by various preprocessors (transpilers) to transform these tokens into standard ECMAScript.

You can find out more about JSX on the project home page.