> you can always just... extend a class (or interface) with whatever function you want Not quite. Extension functions are statically resolved and abide by usua

> you can always just... extend a class (or interface) with whatever function you want Not quite. Extension functions are statically resolved and abide by usual access modifiers. So within a class, I can have a private extension method for another class (say C). Now if my class receives any instance of C, I can call these extension methods on this instance. But when I forward this instance to someone else, they wont have access to any of these extensions. Libraries like exposed use this quite extensively. > Most everything else seems to come down to "Typescript is meant to be runtime-only" Yes, this one too because it would involve type directed emit.