Mocking TypeORM Repositories with Jest: A Practical Guide
TypeORM is a powerful ORM for Node.js, but testing interactions with your database can be tricky. Mocking your repositories with Jest provides a way to isolate your logic and ensure fast, reliable testing. Let's explore how to effectively mock TypeORM repositories for a more streamlined testing process.
Scenario: Testing User Creation Logic
Imagine we have a User entity with a createUser service that relies on a UserRepository. The createUser service attempts to create a new user in the database via the repository.
// src/services/user.service.ts
import { Injectable } from '@nestjs/common';
import { UserRepository } from '../repositories/user.repository';
@Injectable()
export class UserService {
constructor(private readonly userRepository: UserRepository) {}
async createUser(userData: { name: string; email: string }) {
try {
const user = await this.userRepository.create(userData);
return user;
} catch (error) {
throw error;
}
}
}
// src/repositories/user.repository.ts
import { EntityRepository, Repository } from 'typeorm';
import { User } from '../entities/user.entity';
@EntityRepository(User)
export class UserRepository extends Repository<User> {
async create(userData: { name: string; email: string }): Promise<User> {
const newUser = this.create(userData);
await this.save(newUser);
return newUser;
}
}
The Problem: Real Database Interactions in Tests
If we directly test the UserService.createUser function, our test would depend on a real database connection, making it slow and prone to errors. We need a way to mock the UserRepository to control the behavior and avoid real database interactions.
Mocking with Jest
Jest provides excellent mocking capabilities. We'll use the jest.mock function to replace the original UserRepository implementation with a mock one.
// src/services/user.service.spec.ts
import { UserService } from '../services/user.service';
import { UserRepository } from '../repositories/user.repository';
describe('UserService', () => {
let userService: UserService;
let userRepository: UserRepository;
beforeEach(() => {
jest.mock('../repositories/user.repository');
userRepository = new UserRepository();
userService = new UserService(userRepository);
});
it('should create a new user', async () => {
const mockUser = { name: 'John Doe', email: '[email protected]' };
// Mock the create method
userRepository.create.mockResolvedValue(mockUser);
const createdUser = await userService.createUser(mockUser);
expect(createdUser).toEqual(mockUser);
expect(userRepository.create).toHaveBeenCalledWith(mockUser);
});
});
Explanation:
jest.mock('../repositories/user.repository'): This line tells Jest to replace the originalUserRepositorywith a mock.userRepository = new UserRepository(): Creates an instance of the mockedUserRepository.userRepository.create.mockResolvedValue(mockUser): Sets thecreatemethod on the mocked repository to return a predefinedmockUserobject. This simulates successful creation.expect(userRepository.create).toHaveBeenCalledWith(mockUser): Asserts that thecreatemethod was called with the correct arguments.
Benefits of Mocking:
- Faster Tests: Mocking eliminates the need for actual database interactions, speeding up test execution significantly.
- Controlled Environment: Mocks allow you to define the exact behavior of the repository, ensuring predictable test results.
- Isolation: Mocking isolates your service logic from external dependencies, improving test clarity and maintainability.
Conclusion:
Mocking TypeORM repositories with Jest is crucial for robust and efficient unit testing. By replacing real database interactions with controlled mocks, we can achieve faster test execution, improved isolation, and predictable results. This allows us to confidently validate our service logic without relying on external factors. Remember to carefully design your mocks to accurately represent the behavior of your real repository.
