forEach循环返回未定义的值

Question

I've dummy data (books) which I want see from graphiql gui . But when I use a forEach loop to iterate through the books, looking for a specific id , it's returning undefined values, but if I use a normal for loop it works fine.

This is my code:

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;
                    }
                }); 
            }
        }
    }
});

When I print out the book data, it's showing that particular book in the console but not in the GUI response:

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

Answer 1

A return <value> in a forEach callback is meaningless. The returned value goes nowhere, and the loop is not interrupted either.

Instead use .find :

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

When performance is important, and you have lots of books, then it is better to first preprocess the books and create a Set:

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));

... and then no loop is needed to know whether a book ID is valid or not:

return bookIds.has(args.id);