GRAPHQL

GraphQL with Node.js Skill

Master building flexible, efficient GraphQL APIs using Node.js with Apollo Server, type-safe schemas, and real-time subscriptions.

Quick Start

GraphQL API in 4 steps:

1. Define Schema - Types, queries, mutations
2. Write Resolvers - Data fetching logic
3. Setup Server - Apollo Server configuration
4. Connect Data - Database integration

Core Concepts

Schema Definition (SDL)

type User {
  id: ID!
  name: String!
  email: String!
  posts: [Post!]!
}

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

type Query {
  user(id: ID!): User
  users(limit: Int, offset: Int): [User!]!
  post(id: ID!): Post
}

type Mutation {
  createUser(input: CreateUserInput!): User!
  updateUser(id: ID!, input: UpdateUserInput!): User
  deleteUser(id: ID!): Boolean!
}

input CreateUserInput {
  name: String!
  email: String!
  password: String!
}

type Subscription {
  postCreated: Post!
}

Apollo Server Setup

const { ApolloServer } = require('@apollo/server');
const { expressMiddleware } = require('@apollo/server/express4');
const { readFileSync } = require('fs');

const typeDefs = readFileSync('./schema.graphql', 'utf-8');

const resolvers = {
  Query: {
    user: async (_, { id }, { dataSources }) => {
      return dataSources.userAPI.getUser(id);
    },
    users: async (_, { limit = 10, offset = 0 }, { dataSources }) => {
      return dataSources.userAPI.getUsers(limit, offset);
    }
  },
  Mutation: {
    createUser: async (_, { input }, { dataSources }) => {
      return dataSources.userAPI.createUser(input);
    }
  },
  User: {
    posts: async (parent, _, { dataSources }) => {
      return dataSources.postAPI.getPostsByAuthor(parent.id);
    }
  }
};

const server = new ApolloServer({ typeDefs, resolvers });
await server.start();

app.use('/graphql', express.json(), expressMiddleware(server, {
  context: async ({ req }) => ({
    user: req.user,
    dataSources: {
      userAPI: new UserAPI(),
      postAPI: new PostAPI()
    }
  })
}));

Learning Path

Beginner (2-3 weeks)

• ✅ Understand GraphQL concepts
• ✅ Define schemas with SDL
• ✅ Write basic resolvers
• ✅ Query and mutation basics

Intermediate (4-6 weeks)

• ✅ Field resolvers and relationships
• ✅ Input validation
• ✅ Error handling
• ✅ DataLoader for N+1

Advanced (8-10 weeks)

• ✅ Subscriptions (real-time)
• ✅ Authentication and authorization
• ✅ Federation for microservices
• ✅ Performance optimization

DataLoader for N+1 Problem

const DataLoader = require('dataloader');

const createUserLoader = () =>
  new DataLoader(async (userIds) => {
    const users = await User.find({ _id: { $in: userIds } });
    const userMap = new Map(users.map(u => [u.id, u]));
    return userIds.map(id => userMap.get(id));
  });

// Use in resolver
User: {
  posts: (parent, _, { loaders }) => {
    return loaders.userLoader.load(parent.authorId);
  }
}

Authentication & Authorization

const { shield, rule, allow } = require('graphql-shield');

const isAuthenticated = rule()((parent, args, { user }) => {
  return user !== null;
});

const isAdmin = rule()((parent, args, { user }) => {
  return user?.role === 'admin';
});

const permissions = shield({
  Query: { '*': allow, users: isAuthenticated },
  Mutation: {
    createPost: isAuthenticated,
    deleteUser: isAdmin
  }
});

Error Handling

const { ApolloError, UserInputError } = require('apollo-server-express');

class NotFoundError extends ApolloError {
  constructor(message) {
    super(message, 'NOT_FOUND');
  }
}

// In resolvers
const resolvers = {
  Query: {
    user: async (_, { id }) => {
      const user = await User.findById(id);
      if (!user) throw new NotFoundError(`User ${id} not found`);
      return user;
    }
  }
};

Unit Test Template

const { createTestClient } = require('apollo-server-testing');

describe('GraphQL API', () => {
  let query, mutate;

  beforeAll(() => {
    const server = new ApolloServer({ typeDefs, resolvers });
    const testClient = createTestClient(server);
    query = testClient.query;
    mutate = testClient.mutate;
  });

  it('should return users list', async () => {
    const GET_USERS = `query { users { id name } }`;
    const res = await query({ query: GET_USERS });

    expect(res.errors).toBeUndefined();
    expect(res.data.users).toBeDefined();
  });
});

Troubleshooting

When to Use

Use GraphQL when:

• Clients need flexible data fetching
• Multiple clients with different data needs
• Avoiding over-fetching/under-fetching
• Real-time updates needed
• API evolution without versioning

Related Skills

• Express REST API (alternative approach)
• Database Integration (data sources)
• JWT Authentication (securing API)

Resources

• Apollo Server Docs
• GraphQL Specification
• DataLoader