Refactoring for Reusability: Shared Jest Test Utilities

Many projects involve repetitive setup and assertion logic within their test suites. This can lead to bloated test files and increased maintenance overhead. This challenge focuses on identifying and extracting common testing patterns into reusable utility functions using Jest in a TypeScript environment.

Problem Description

Your task is to refactor an existing, albeit simplified, set of Jest tests by extracting common setup and assertion logic into shared utility functions. You will then apply these utilities to simplify the existing tests and demonstrate the benefits of code reusability in testing.

What needs to be achieved:

1. Identify common patterns in the provided test file.
2. Create a new TypeScript file (e.g., src/test-utils.ts) to house these reusable utility functions.
3. Refactor the existing tests to utilize these new utility functions.
4. Ensure the refactored tests pass with the same outcomes as the original tests.

Key requirements:

• Create at least two distinct utility functions.
• One utility should handle common test setup (e.g., initializing a mock service).
• Another utility should handle common assertion logic (e.g., checking for specific error structures).
• All utility functions must be written in TypeScript and exported from a dedicated file.
• The original test file should be significantly cleaner and more concise after refactoring.

Expected behavior: The refactored test suite should produce the exact same results (pass/fail status and console output) as the original test suite.

Important edge cases to consider:

• How to handle varying parameters passed to the utility functions.
• Ensuring the utilities are well-typed for TypeScript integration.

Examples

Let's imagine a scenario where we are testing a UserService which interacts with a UserRepository.

Original Test File (src/user.test.ts - simplified):

import { UserService } from './user-service';
import { UserRepository } from './user-repository';

describe('UserService', () => {
  let mockUserRepository: jest.Mocked<UserRepository>;
  let userService: UserService;

  beforeEach(() => {
    mockUserRepository = {
      getUserById: jest.fn(),
      createUser: jest.fn(),
      deleteUser: jest.fn(),
    };
    userService = new UserService(mockUserRepository);
  });

  describe('getUserById', () => {
    it('should return user data when found', async () => {
      const mockUser = { id: '123', name: 'Alice' };
      mockUserRepository.getUserById.mockResolvedValue(mockUser);

      const user = await userService.getUserById('123');
      expect(user).toEqual(mockUser);
      expect(mockUserRepository.getUserById).toHaveBeenCalledWith('123');
    });

    it('should throw an error if user is not found', async () => {
      mockUserRepository.getUserById.mockResolvedValue(undefined);

      await expect(userService.getUserById('456')).rejects.toThrow('User not found');
      await expect(userService.getUserById('456')).rejects.toMatchObject({
        statusCode: 404,
        message: 'User not found',
      });
      expect(mockUserRepository.getUserById).toHaveBeenCalledWith('456');
    });
  });

  // ... other tests for createUser, deleteUser ...
});

Your Goal: Create a src/test-utils.ts that contains utilities to:

1. Initialize mockUserRepository and userService.
2. Assert that a rejected promise matches a specific error structure (like statusCode and message).

After Refactoring (Conceptual):

Your src/user.test.ts might look something like this:

import { UserService } from './user-service';
import { UserRepository } from './user-repository';
import { setupUserService, expectErrorResponse } from './test-utils'; // Assuming these utilities are created

describe('UserService', () => {
  let mockUserRepository: jest.Mocked<UserRepository>;
  let userService: UserService;

  beforeEach(() => {
    ({ mockUserRepository, userService } = setupUserService()); // Using the setup utility
  });

  describe('getUserById', () => {
    it('should return user data when found', async () => {
      const mockUser = { id: '123', name: 'Alice' };
      mockUserRepository.getUserById.mockResolvedValue(mockUser);

      const user = await userService.getUserById('123');
      expect(user).toEqual(mockUser);
      expect(mockUserRepository.getUserById).toHaveBeenCalledWith('123');
    });

    it('should throw an error if user is not found', async () => {
      mockUserRepository.getUserById.mockResolvedValue(undefined);

      await expect(userService.getUserById('456')).rejects.toThrow('User not found');
      await expectErrorResponse(
        userService.getUserById('456'),
        404,
        'User not found'
      ); // Using the assertion utility
      expect(mockUserRepository.getUserById).toHaveBeenCalledWith('456');
    });
  });

  // ... other tests refactored ...
});

Constraints

• You must use Jest as the testing framework.
• The solution must be written in TypeScript.
• The provided example code (or a similar structure for a hypothetical service) serves as the starting point for refactoring. Assume you have user-service.ts and user-repository.ts files with basic implementations.
• The refactored code must maintain all original test logic and outcomes.

Notes

• Consider using Jest's jest.mock if your services had actual dependencies that needed mocking, though for this challenge, direct mock object creation is sufficient.
• Think about how to make your utility functions flexible enough to be used in different test contexts.
• Well-named utility functions and clear type definitions are crucial for maintainability.