Unit testing is a crucial part of the development process, ensuring that individual components of your application work as expected. In the context of GraphQL, resolvers are the functions responsible for fetching the data for your queries and mutations. This module will guide you through the process of unit testing these resolvers.

• Understand the importance of unit testing resolvers.
• Learn how to set up a testing environment.
• Write unit tests for GraphQL resolvers using popular testing libraries.
• Explore common pitfalls and best practices.

Unit testing resolvers helps to:

• Ensure that each resolver returns the correct data.
• Validate the logic within the resolver.
• Catch bugs early in the development cycle.
• Facilitate refactoring by providing a safety net.

Before writing tests, you need to set up a testing environment. We'll use Jest, a popular testing framework, and Apollo Server for our GraphQL server.

1. Install Dependencies

   npm install --save-dev jest @babel/preset-env @babel/preset-typescript @babel/core @babel/node
   npm install --save-dev @types/jest ts-jest
   npm install apollo-server graphql
   

2. Configure Babel Create a .babelrc file in the root of your project:

   {
     "presets": ["@babel/preset-env", "@babel/preset-typescript"]
   }
   

3. Configure Jest Add the following configuration to your package.json:

   "jest": {
     "preset": "ts-jest",
     "testEnvironment": "node"
   }
   

Let's write unit tests for a simple resolver. Assume we have the following GraphQL schema and resolver:

1. Import Dependencies

   import { ApolloServer, gql } from 'apollo-server';
   import resolvers from './resolvers';
   
   const typeDefs = gql`
     type Query {
       hello: String
     }
   `;
   
   const server = new ApolloServer({ typeDefs, resolvers });
   

2. Write Tests

   describe('Resolvers', () => {
     it('should return "Hello, world!" for hello query', async () => {
       const result = await server.executeOperation({
         query: gql`
           query {
             hello
           }
         `,
       });
   
       expect(result.data.hello).toBe('Hello, world!');
     });
   });
   

Run the tests using the following command:

• Mocking External Dependencies: If your resolver interacts with external services or databases, mock these dependencies to isolate the resolver logic.
• Testing Edge Cases: Ensure you test edge cases and error scenarios to make your resolvers robust.
• Keeping Tests Fast: Unit tests should be fast. Avoid complex setups that slow down the test execution.

1. Schema

   type Query {
     user(id: ID!): User
   }
   
   type User {
     id: ID!
     name: String
     email: String
   }
   

2. Resolver

   const users = [
     { id: '1', name: 'John Doe', email: '[email protected]' },
     { id: '2', name: 'Jane Doe', email: '[email protected]' },
   ];
   
   const resolvers = {
     Query: {
       user: (_, { id }) => users.find(user => user.id === id),
     },
   };
   
   export default resolvers;
   

3. Test File: resolvers.test.js

   import { ApolloServer, gql } from 'apollo-server';
   import resolvers from './resolvers';
   
   const typeDefs = gql`
     type Query {
       user(id: ID!): User
     }
   
     type User {
       id: ID!
       name: String
       email: String
     }
   `;
   
   const server = new ApolloServer({ typeDefs, resolvers });
   
   describe('Resolvers', () => {
     it('should return user data for a given id', async () => {
       const result = await server.executeOperation({
         query: gql`
           query {
             user(id: "1") {
               id
               name
               email
             }
           }
         `,
       });
   
       expect(result.data.user).toEqual({
         id: '1',
         name: 'John Doe',
         email: '[email protected]',
       });
     });
   
     it('should return null for a non-existent user', async () => {
       const result = await server.executeOperation({
         query: gql`
           query {
             user(id: "3") {
               id
               name
               email
             }
           }
         `,
       });
   
       expect(result.data.user).toBeNull();
     });
   });
   

The provided test file should pass, confirming that the resolver correctly handles both existing and non-existent users.

In this module, you learned the importance of unit testing resolvers, how to set up a testing environment, and how to write and run unit tests for GraphQL resolvers. By following best practices and avoiding common pitfalls, you can ensure that your resolvers are reliable and maintainable. This knowledge prepares you for more advanced testing techniques and integration testing, which will be covered in the next topic.