The documentation is really incomplete. It's not at all clear how similar to TypeScript this is...does this support structural subtyping? Type manipulation? Iterators/generators? Async?

ReasonML is just OCaml with a different syntax. It sort of very loosely resembles JS syntactically in that it's got curly braces, but that's about as far as the similarities go - about as much as Rust resembles C. The type system and language are very different from TypeScript or JS, and much more rigid.

C# is a lovely language but it's not that similar to TypeScript. The big distinction (for me) is that it's nominally-typed and much more rigid. Unless things have changed a lot since I last touched it (admittedly a while ago):

You can't synthesize ad hoc union types (https://www.typescriptlang.org/docs/handbook/2/everyday-type...)

There's no notion of literal types (https://www.typescriptlang.org/docs/handbook/2/everyday-type...)

There's no type narrowing (which gets you a kind of sum type) (https://www.typescriptlang.org/docs/handbook/2/narrowing.htm...)

Most of the type maniuplation features (keyof, indexed access types, conditional types, template literal types...) are missing (https://www.typescriptlang.org/docs/handbook/2/types-from-ty...)

Close but not quite.