1. Introduction

In this tutorial, we will learn about best practices for testing GraphQL APIs. Testing is a critical part of any software development process and it ensures that your code works as expected and helps to prevent regressions when you add new features.

What you will learn:
- The fundamentals of testing GraphQL APIs
- How to write test cases for your APIs
- Best practices for effective testing

Prerequisites:
- Basic knowledge of GraphQL and how it works
- Familiarity with JavaScript and Node.js
- Understanding of API testing concepts

2. Step-by-Step Guide

Understanding GraphQL Testing

Testing GraphQL involves checking the server (queries, mutations, and subscriptions) and the client (components and hooks). GraphQL makes it easy to predict the shape of your data, hence making testing easier.

Using the Right Tools

Use tools that are specifically designed for testing GraphQL APIs like 'apollo-server-testing' for server-side testing and 'react-apollo' for client-side testing.

Write Tests for Each Resolver

Resolvers are at the heart of GraphQL APIs. They provide the instructions for turning a GraphQL operation into data. Test for each scenario your resolver should handle.

Test Your Schema

Testing your schema helps catch breaking changes before they go to production. You can use tools like 'graphql-schema-linter' to validate your schema against certain rules.

Error Handling

Make sure you write tests to ensure your API handles errors gracefully. You want to ensure that an error in one field doesn't affect the rest of your results.

3. Code Examples

GraphQL Resolver Test

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

const { query } = createTestClient(server);
const res = await query({ query: GET_DATA });

// Check if response returns data as expected
expect(res).toMatchSnapshot();

In the above code snippet, we create a test client using 'apollo-server-testing'. We then perform a query and check if the response matches the expected data.

Schema Test

const { expect } = require('chai');
const { validate } = require('graphql-schema-linter');

const schema = `
  type Query {
    data: String
  }
`;

const errors = validate(schema);

// Check if schema validates without errors
expect(errors).to.be.empty;

Here, we defined a simple GraphQL schema and validate it using 'graphql-schema-linter'. We expect no errors from the validation.

4. Summary

In this tutorial, we have covered the fundamental concepts of testing GraphQL APIs, including writing tests for resolvers and schemas, and error handling. The next step would be to explore more complex scenarios and learn about more advanced testing techniques.

5. Practice Exercises

1. Write a test for a mutation that creates a new item in your database.
2. Write a test for a query that fetches data with specific criteria from your database.
3. Write a test that checks if the error handling works properly when a wrong query is made.

Remember to run your tests after writing them to ensure they pass. Also, make sure you understand why they passed or failed. This will help you improve your testing skills. Keep practicing and happy testing!