1. Introduction

Tutorial's Goal

In this tutorial, we'll explore the best practices for writing and managing GraphQL resolvers. By following these practices, you will be able to write cleaner, highly efficient, and maintainable code.

Learning Outcome

After going through this tutorial, you will be proficient in:
- Understanding and using GraphQL resolvers effectively.
- Applying best practices while writing resolvers.
- Writing more maintainable and efficient GraphQL resolvers.

Prerequisites

This tutorial assumes that you have a basic understanding of GraphQL and JavaScript.

2. Step-by-Step Guide

What is a Resolver?

In GraphQL, a resolver is a function that's responsible for populating the data for a single field in your schema. It's where you define how data is fetched and what data to fetch.

Best Practices for GraphQL Resolvers

1. Keep Resolvers Thin

Resolvers should be thin with regards to business logic. They should delegate the heavy lifting to the service layer. This way, our resolvers remain clean, and the business logic becomes easier to test and reuse.

// Bad
const resolvers = {
  Query: {
    user: (_, { id }) => {
      // Business logic inside resolver
      const user = fetchUserFromDb(id);
      return formatUser(user);
    },
  },
};

// Good
const resolvers = {
  Query: {
    user: (_, { id }, { dataSources }) => {
      // Business logic moved to data source
      return dataSources.userAPI.getUser(id);
    },
  },
};

2. Use DataLoader for Batch Loading

DataLoader is a utility provided by Facebook that reduces the number of requests to your backend by batching and caching requests.

const userLoader = new DataLoader(userIds =>
  myBatchGetUsers(userIds),
);

const resolvers = {
  Query: {
    user: (_, { id }) => {
      return userLoader.load(id);
    },
  },
};

3. Error Handling

Never suppress errors in your resolvers. Instead, let GraphQL catch it and send it to the client with the rest of the data.

const resolvers = {
  Query: {
    user: (_, { id }, { dataSources }) => {
      return dataSources.userAPI.getUser(id);
      // If an error happens in getUser, it will be caught by GraphQL
    },
  },
};

3. Code Examples

Example 1: Simple Resolver

// Define your resolver
const resolvers = {
  Query: {
    hello: () => 'Hello world!',
  },
};

// The resolver will return 'Hello world!' when the hello query is executed.

Example 2: Resolver with Argument

// Define your resolver
const resolvers = {
  Query: {
    greet: (_, { name }) => `Hello ${name}!`,
  },
};

// The resolver will return 'Hello [name]!' when the greet query is executed with an argument.

4. Summary

In this tutorial, we've covered the best practices for writing and managing GraphQL resolvers. We've discussed how to keep resolvers thin, the use of DataLoader for batch loading, and effective error handling.

For further learning, you can delve into advanced topics like resolver optimization and complex schema stitching.

5. Practice Exercises

Exercise 1: Write a resolver for a query getUser(id: ID!) which fetches user data from a data source.

Exercise 2: Enhance the resolver written in Exercise 1 to use DataLoader for batch loading.

Exercise 3: Write a resolver for a mutation addUser(name: String!, email: String!) which adds a new user to the data source.

Remember to follow the best practices discussed in the tutorial while writing these resolvers. Happy coding!