let books = [
    { name: 'Name of the Wind', genre: 'Horror', id: '1', authorID: '3' },
    { name: 'The Final Empire', genre: 'Fantasy', id: '2', authorID: '1' },
    { name: 'The Long Earth', genre: 'Sci-Fi', id: '3', authorID: '2' },
];
const RootQuery = new GraphQLObjectType({
    name: 'RootQueryType',
    fields: {
        book: {
            type: BookType,
            args: { id: { type: GraphQLString } },
            //this forEach is not working
            resolve(parent, args){
                books.forEach( function(book) {
                    if(book.id == args.id) {
                        console.log(book);
                        return book;
                    }
                });
            }
        }
    }
});

request:
{
  book(id: "2") {
    name
    genre
  }
}
response:
{
  "data": {
    "book": null
  }
}

return books.find(function(book) {
    return book.id == args.id;
});

let books = [
    { name: 'Name of the Wind', genre: 'Horror', id: '1', authorID: '3' },
    { name: 'The Final Empire', genre: 'Fantasy', id: '2', authorID: '1' },
    { name: 'The Long Earth', genre: 'Sci-Fi', id: '3', authorID: '2' },
];
let bookIds = new Set(books.map(({id}) => id));

return bookIds.has(args.id);