How do you implement a GraphQL API with Apollo Server in Node.js?

Question

Accepted Answer

**GraphQL** — query language for APIs with flexible data fetching.

```javascript
const { ApolloServer, gql } = require("apollo-server");

const typeDefs = gql`
    type User {
        id: ID!
        name: String!
        email: String!
        posts: [Post!]
    }

type Post {
        id: ID!
        title: String!
        author: User!
    }

type Query {
        users: [User!]
        user(id: ID!): User
    }

type Mutation {
        createUser(name: String!, email: String!): User!
    }
`;

const resolvers = {
    Query: {
        users: () => User.find(),
        user: (_, { id }) => User.findById(id),
    },
    User: {
        posts: (user) => Post.find({ authorId: user.id }),
    },
    Mutation: {
        createUser: (_, { name, email }) => User.create({ name, email }),
    },
};

const server = new ApolloServer({ typeDefs, resolvers });
server.listen().then(({ url }) => console.log(`Server at ${url}`));
```

**Key points:**
- N+1 problem: use DataLoader.
- Authentication via context.
- Schema-first vs code-first.

Node.js: How do you implement a GraphQL API with Apollo Server in Node.js?