How to unit test typeorm getRepository with Jest?

When unit testing getRepository from typeorm with Jest, we typically employ mocking techniques to simulate the results of database operations, thereby validating our code logic. Here is a step-by-step guide on how to perform unit testing with Jest and typeorm:

Assume we have a service named UserService that uses typeorm's getRepository for data operations.

typescript
// UserService.ts
import { getRepository } from 'typeorm';
import { User } from './entity/User';

export class UserService {
  async findUser(id: number): Promise<User | undefined> {
    const userRepository = getRepository(User);
    return userRepository.findOne(id);
  }
}

To unit test the findUser function in the above code, we need to follow these steps:

Step 1: Set up the Jest testing environment

First, we need to install Jest along with TypeScript support and relevant type definitions:

bash
npm install --save-dev jest ts-jest @types/jest

Then configure jest.config.js to support TypeScript:

javascript
// jest.config.js
module.exports = {
  // ...
  preset: 'ts-jest',
  // ...
};

Step 2: Mock the getRepository function

Next, we need to mock the getRepository function from typeorm:

typescript
// UserService.test.ts
import { getRepository } from 'typeorm';
import { User } from './entity/User';
import { UserService } from './UserService';

// Mock typeorm getRepository
jest.mock('typeorm', () => ({
  getRepository: jest.fn(),
}));

Step 3: Write test cases

Now we can write test cases. We will mock the object returned by getRepository, particularly its findOne method:

typescript
describe('UserService', () => {
  test('findUser should return a user if it exists', async () => {
    // Arrange
    const mockUser = { id: 1, name: 'John Doe' };
    const userRepositoryMock = {
      findOne: jest.fn().mockResolvedValue(mockUser),
    };

    (getRepository as jest.Mock).mockReturnValue(userRepositoryMock);

    const userService = new UserService();

    // Act
    const user = await userService.findUser(1);

    // Assert
    expect(userRepositoryMock.findOne).toHaveBeenCalledWith(1);
    expect(user).toEqual(mockUser);
  });
});

In this test case, we first mock the findOne method to return an expected user object. Then, we use the mock object returned by getRepository (i.e., userRepositoryMock). Next, we call the findUser method in UserService and expect it to return the correct user object, while verifying that the findOne method is called correctly.

By doing this, we can perform unit tests on services involving typeorm without requiring a real database connection. The benefits include faster test execution, immunity to external factors, and the ability to focus on validating business logic correctness.