ActionScript 3 has been chosen as the source language for JavaScript "programming in the large", because is adds language features that

• address real-world problems,
• ease development,
• add to the strengths of JavaScript, and
• can be translated to efficient, debuggable JavaScript 1.x.

The currently supported language features are:

• classes and interfaces with inheritance,
• packages,
• static members,
• private members, and
• type annotations.

All details are described in the Jangaroo language reference.
Some advanced features are described in the Jangaroo Runtime documentation.