Then, paradoxically, with no error checking at runtime, it becomes fully possible for JS code to call into TS code in a way that breaks the shit out of the TS compiler's assumptions. In philosophy then TS and JS are as incompatible as GPL and EULA
Sure, you can check if they gave you a string instead of a number. But if you receive an array of nested objects, are you going to traverse the whole graph and check every property? If the caller gives you a callback, do you check if it returns the correct value? If that callback itself returns a function, do you check that function's return type too? And will you check these things at every single function boundary?
This kind of paranoid runtime type-checking would completely dominate the code, and nobody does it. Many invariants which exist at compile-time cannot be meaningfully checked at runtime, even if you wanted to. All you can do is offer a type-safe interface, trust your callers to respect it, and check for a few common mistakes at the boundary. You cannot protect your code against other code calling it incorrectly, and in practice nobody does. This is equally true for JS and TS.
However, if your point is that Typescript can lull people into a false sense of safety, then sure, I take your point. You have to understand where type assertions are coming into play, and if that's obscured then the type safety can be illusory. The benefits of Typescript require you to make sure that the runtime inputs to your program are sufficiently validated.
Usually there's little if any protection against a JS caller providing wrong-type args to TS functions.
I'm also not sure we're actually disagreeing on anything, so perhaps my reply was pointless. I agree that if you mix JS and TS in the way you describe, you'll have problems. My reply was just to say "yes, and that's not really Typescript's fault", but perhaps you weren't implying that to begin with.
* I'm aware that you can't run Typescript directly, but I hope my point here is clear... you're running a program that wasn't type-checked by TS.
If you write a C library, nothing stops someone from writing an assembly-language program that calls functions in your library with the wrong types.
If it's a service, yes, and that's true no matter what technology the service is using. If it's a library, no, because...
> and if you write type assertions without ensuring that the assertions are accurate, then that's on you, not Typescript.
That's on whoever is using the library, not the library author. If the library author provides type definitions, and you as the consumer choose to ignore them, then it's on you.
To bridge runtime and compile time (as your application will likely get some external data) you've got to use a proper parser such as zod[1] or if you want to stretch it even further effect-schema[2].
[1] https://zod.dev/