What does the following type implement and what is it useful for? type Opaque = T & { readonly __brand: Brand }; type UserId = Opaque; type ProductId = Opaque;

Question

TypeScript Professional — Hard

What does the following type implement and what is it useful for? type Opaque = T & { readonly __brand: Brand }; type UserId = Opaque; type ProductId = Opaque;

Accepted Answer

The type Opaque with a __brand property creates a nominal typing simulation, ensuring UserId and ProductId are distinct despite both being strings. This prevents them from being interchangeable at runtime.