server.js

const express = require('express')
const expressGraphQL = require('express-graphql')
const {
  GraphQLSchema,
  GraphQLObjectType,
  GraphQLString,
  GraphQLList,
  GraphQLInt,
  GraphQLNonNull
} = require('graphql')
const app = express()

const authors = [
	{ id: 1, name: 'J. K. Rowling' },
	{ id: 2, name: 'J. R. R. Tolkien' },
	{ id: 3, name: 'Brent Weeks' }
]

const books = [
	{ id: 1, name: 'Harry Potter and the Chamber of Secrets', authorId: 1 },
	{ id: 2, name: 'Harry Potter and the Prisoner of Azkaban', authorId: 1 },
	{ id: 3, name: 'Harry Potter and the Goblet of Fire', authorId: 1 },
	{ id: 4, name: 'The Fellowship of the Ring', authorId: 2 },
	{ id: 5, name: 'The Two Towers', authorId: 2 },
	{ id: 6, name: 'The Return of the King', authorId: 2 },
	{ id: 7, name: 'The Way of Shadows', authorId: 3 },
	{ id: 8, name: 'Beyond the Shadows', authorId: 3 }
]

const BookType = new GraphQLObjectType({
  name: 'Book',
  description: 'This represents a book written by an author',
  fields: () => ({
    id: { type: GraphQLNonNull(GraphQLInt) },
    name: { type: GraphQLNonNull(GraphQLString) },
    authorId: { type: GraphQLNonNull(GraphQLInt) },
    author: {
      type: AuthorType,
      resolve: (book) => {
        return authors.find(author => author.id === book.authorId)
      }
    }
  })
})

const AuthorType = new GraphQLObjectType({
  name: 'Author',
  description: 'This represents a author of a book',
  fields: () => ({
    id: { type: GraphQLNonNull(GraphQLInt) },
    name: { type: GraphQLNonNull(GraphQLString) },
    books: {
      type: new GraphQLList(BookType),
      resolve: (author) => {
        return books.filter(book => book.authorId === author.id)
      }
    }
  })
})

const RootQueryType = new GraphQLObjectType({
  name: 'Query',
  description: 'Root Query',
  fields: () => ({
    book: {
      type: BookType,
      description: 'A Single Book',
      args: {
        id: { type: GraphQLInt }
      },
      resolve: (parent, args) => books.find(book => book.id === args.id)
    },
    books: {
      type: new GraphQLList(BookType),
      description: 'List of All Books',
      resolve: () => books
    },
    authors: {
      type: new GraphQLList(AuthorType),
      description: 'List of All Authors',
      resolve: () => authors
    },
    author: {
      type: AuthorType,
      description: 'A Single Author',
      args: {
        id: { type: GraphQLInt }
      },
      resolve: (parent, args) => authors.find(author => author.id === args.id)
    }
  })
})

const RootMutationType = new GraphQLObjectType({
  name: 'Mutation',
  description: 'Root Mutation',
  fields: () => ({
    addBook: {
      type: BookType,
      description: 'Add a book',
      args: {
        name: { type: GraphQLNonNull(GraphQLString) },
        authorId: { type: GraphQLNonNull(GraphQLInt) }
      },
      resolve: (parent, args) => {
        const book = { id: books.length + 1, name: args.name, authorId: args.authorId }
        books.push(book)
        return book
      }
    },
    addAuthor: {
      type: AuthorType,
      description: 'Add an author',
      args: {
        name: { type: GraphQLNonNull(GraphQLString) }
      },
      resolve: (parent, args) => {
        const author = { id: authors.length + 1, name: args.name }
        authors.push(author)
        return author
      }
    }
  })
})

const schema = new GraphQLSchema({
  query: RootQueryType,
  mutation: RootMutationType
})

app.use('/graphql', expressGraphQL({
  schema: schema,
  graphiql: true //work without postman server in web browser
}))
app.listen(5000, () => console.log('Server Running'))