https://3fx.ch/typing-is-hard.html#typescriptTypeScript
undecidable, unsound. The TypeScript documentation mentions its unsoundness and the motivations behind them.
Undecidability: TypeScript’s type system was proven Turing complete until they disallowed self-referential types. However Robbie Ostrow wrote a program checking the Collatz conjecture, and as the generalized form of the Collatz conjecture is undecidable3, the TypeScript type system is undecidable as well.