Interfaces

const Dog = t.object({
  name: 'Dog',
  fields: {
    species: t.string,
    isLoyal: t.boolean,
  },
});

const Cat = t.object({
  name: 'Cat',
  fields: {
    species: t.string,
    isDomesticated: t.boolean,
  },
});

const Pet = t.interface({
  name: 'Pet',
  fields: {
    species: t.string,
  },

  /**
   * the library makes sure that object types
   * passed here correctly implement the interface.
  */
  implementors: [Dog, Cat],
  resolveType: () => {
    /**
     * 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 'Dog' as const;
    } else {
      return 'Cat' as const;
    }
  }
})

/** TypeScript */
type Pet = {
  species: string;
};

# GraphQL

interface Pet {
  species: String!
}

type Dog implements Pet {
  species: String!
  isLoyal: Boolean!
}

type Cat implements Pet {
  species: String!
  isDomesticated: Boolean!
}