TypeScript: type-level programming — пример кода

// A recursive DeepReadonly — mapped + conditional + recursion together
type DeepReadonly<T> = T extends (infer E)[]
  ? ReadonlyArray<DeepReadonly<E>>
  : T extends object
  ? { readonly [K in keyof T]: DeepReadonly<T[K]> }
  : T; // primitives bottom out the recursion

type Config = { db: { host: string; ports: number[] } };
type Frozen = DeepReadonly<Config>;
// { readonly db: { readonly host: string; readonly ports: readonly number[] } }

// Template-literal + infer as a typed string parser (real use: typed i18n keys)
type Params<S extends string> =
  S extends `${string}:${infer P}/${infer Rest}` ? P | Params<Rest>
  : S extends `${string}:${infer P}` ? P
  : never;
type P = Params<'/users/:userId/posts/:postId'>; // 'userId' | 'postId'