> Yes, but what about mutating objects/primitives in a way that causes all validations to pass but still be passing the wrong data? > What is the advantage of strong types if your data structures are immutable? Is there any? I'm genuinely trying to understand if TS is just for cases where uncontrolled mutation is the norm and test coverage is poor.

Can you give an example? I'm not following what you mean. Why would strong types be less useful if your data structures are immutable? OCaml mostly uses immutable data structures for example but its strong type system is fundamental to why it can be used to write such robust code.

Typescript has a stronger type system than JavaScript so Typescript can therefore capture more program properties statically compared to JavaScript whether you're using mutable data structures or not.