Something pretty similar to the AST example in another thread can be implemented in Typescript using sub typing (as it happens, trying to do something like that is how I first found out about GADTs). Looking into existential types, I see that one of the things they can be used for is OOP style subtype polymorphism [1]. Are there things you can do with existential types that can't be done with subtyping?

[1] https://wiki.haskell.org/Existential_type#Dynamic_dispatch_m...