Unions

const Animal = t.object({
  name: 'Animal',
  fields: {
    id: t.id,
    name: t.string,
    species: t.string,
  },
});

const Person = t.object({
  name: 'Person',
  fields: {
    id: t.id,
    fullName: t.string,
  },
});

const BestFriend = t.union({
  name: 'BestFriend',
  types: [Animal, Person],
  resolveType: (x) => {
    /**
     * All abstract types need a resolveType function that will
     * figure out the type name of a resolved object so that
     * GraphQL can understand which Object type this returned
     * piece of data falls under.
    */

    if (/** some condition */) {
      return 'Person' as const;
    } else {
      return 'Animal' as const;
    }
  },
});

const User = t.object({
  name: 'User',
  fields: {
    id: t.id,
    email: t.string,
    bestFriend: BestFriend,
  }
});

/** TypeScript */
type User = {
  id: string | number;
  email: string;
  bestFriend:
    | {
        id: string | number;
        name: string;
        species: string;
      }
    | {
        id: string | number;
        fullName: string;
      };
};

# GraphQL

type Animal {
  id: ID!
  name: String!
  species: String!
}

type Person {
  id: ID!
  fullName: String!
}

union BestFriend = Animal | Person

type User {
  id: ID!
  email: String!
  bestFriend: BestFriend!
}