index.js

const {gql} = require('apollo-server');
 const typeDefs = gql`
    extend type Query {
        book(id: ID!): Book
        books: [Book]
    }
     type Book {
        id: ID!
        title: String
        author: Author
    }
     extend type Author {
        books: [Book]
    }
`;
 const resolvers = {
    Query: {
        book(_, {id}) {
            return {
                id,
                title: 'test',
                author: {
                        id: Math.round(Math.random() * 100000),
                        name: 'Daniele',
                        surname: 'Zurico'
                    }
            }
        },
        books() {
            return [
                {
                    id: Math.round(Math.random() * 100000),
                    title: 'test',
                    author:{
                        id: Math.round(Math.random() * 100000),
                        name: 'Daniele',
                        surname: 'Zurico'
                    }
                },
                {
                    id: Math.round(Math.random() * 100000),
                    title: 'test2',
                    author: {
                        id: Math.round(Math.random() * 100000),
                        name: 'Alex',
                        surname: 'Michaels'
                    },
                }
            ]
        }
    },
    Author: {
        books() {
            return [{
                id: Math.round(Math.random() * 100000),
                title: 'test',
            }]
        }
    }
 };
 module.exports = {
    typeDefs,
    resolvers
}