useReducer with TypeScript

const [{ firstName, lastName }, setName] = useReducer(
    (prev, next) => ({ ...prev, ...next }),
    {
      firstName: "",
      lastName: ""
    }
  );

type TName = { firstName: string; lastName: string };
const [{ firstName, lastName }, setName] = useReducer(
    (prev: TName, next: Partial<TName>) => ({ ...prev, ...next }),
    {
      firstName: "",
      lastName: ""
    }
  );