Major Differences Between C# and Unity JavaScript - Unity 4 Game Development - page 636

Limited interface support
While Unity's JavaScript does support inheritance and interfaces, it has a very limiting
caveat: you can either inherit your class from an existing class or declare one interface:
// JavaScript user: (Only on allowed)
class MyClass extends MyObject implements IMyObject {…}
// C# user:
class MyClass : MonoBehaviour, IMyObject, IMyItem {…}