testing-strategies

Testing Strategies

Guidance for writing effective, maintainable tests. Covers unit testing patterns, integration testing, mocking strategies, and test organization.

Core Principles

1. Test behavior, not implementation: Tests should verify what code does, not how
2. Arrange-Act-Assert: Structure every test with clear setup, action, and verification
3. One assertion per concept: Each test should verify one logical concept
4. Independent tests: Tests should not depend on each other or shared state
5. Fast feedback: Unit tests should run in milliseconds

Test Structure

Arrange-Act-Assert (AAA)

describe('UserService', () => {
  describe('createUser', () => {
    it('should create user with valid data', async () => {
      // Arrange
      const userData = { name: 'John', email: '[email protected]' };
      const mockRepo = { save: jest.fn().mockResolvedValue({ id: '1', ...userData }) };
      const service = new UserService(mockRepo);

      // Act
      const result = await service.createUser(userData);

      // Assert
      expect(result.id).toBe('1');
      expect(result.name).toBe('John');
      expect(mockRepo.save).toHaveBeenCalledWith(userData);
    });
  });
});

Test Naming

// Pattern: should [expected behavior] when [condition]
it('should return null when user not found', () => {});
it('should throw ValidationError when email is invalid', () => {});
it('should retry 3 times when connection fails', () => {});

// Or: [condition] => [expected behavior]
it('invalid email => throws ValidationError', () => {});
it('user not found => returns null', () => {});

Test Organization

src/
  services/
    user.ts
    user.test.ts          # Co-located unit tests
tests/
  integration/
    user.integration.ts   # Integration tests
  e2e/
    user-flow.e2e.ts      # End-to-end tests
  fixtures/
    users.ts              # Shared test data
  helpers/
    test-db.ts            # Test utilities