Comment by bern4444

It's pretty trivial to create derived and augmented types with Pick, Omit, Required, Partial. Combined with a few parsing functions that return an object typed to whatever specification you need and you are set IE:

    type User = { name: string; verified: boolean; email?: string; lastName: string; birthday?: string | { year: string; month: string; date: string; }}

    type Birthday = Required<Pick<User, 'birthday'>>;
    type UserWithBirthday = User & { birthday: Birthday } 
    type VerifiedUser = User & { verified: true; email: string; }
    type VerifiedUserWithBirthday = User & UserWithBirthday & VerifiedUser;


    const userHasBDayAndEmail = (user: User): user is VerifiedUserWithBirthday => {
        if (user.email === undefined || user.birthday === undefined) {
            return false
        }

        return true
    }

Any caller of userHasBDayAndEmail knows for the rest of its nested call stack if the provided user is a User object or a VerifiedUserWithBirthday.

The types are cheap to write (they're all derived) and have no runtime impact (types are erased at build/compile time) and these parsing functions are quite small to write

https://www.typescriptlang.org/play/?#code/FAFwngDgpgBAqgZyg...