So basically it just means that I have to write more interfaces and implementations for them, because I don't have base classes to fall onto. Instead, in derived type/class instances I have an optional (maybe null) "pointer" to a base type/class instance. If the derived instance never uses its base class features, then the pointer stays null and no base instance is created.

Often derived objects in JS are only used for those derived features, so I save live memory. But: the derived object type needs its own version of at least some of the base class methods, so I pay more in instruction memory (executable size).