developing-with-tdd

TDD Cycle

Red → Green → Refactor

1. Red: Write a failing test that describes desired behavior
2. Green: Write the minimum code to make it pass (no gold-plating)
3. Refactor: Improve the code without changing behavior (tests stay green)

Never skip to Green without a failing Red test first.

Practical Rules

Rule 1: One test at a time Write one failing test. Make it pass. Only then write the next test.

Rule 2: Minimum viable production code In Green phase, write only what's needed to pass the test. No future-proofing.

Rule 3: Test behavior, not implementation Bad: expect(user._hash).toBeDefined() Good: expect(await user.verifyPassword('secret')).toBe(true)

Rule 4: Fast tests Unit tests must run in <100ms. Use mocks for I/O (database, network, filesystem).

Structure

src/
  auth/
    auth.service.ts         ← Implementation
    auth.service.test.ts    ← Tests (co-located)

Test Template (TypeScript/Vitest)

import { describe, it, expect, beforeEach, vi } from 'vitest';
import { AuthService } from './auth.service';

describe('AuthService', () => {
  let service: AuthService;

  beforeEach(() => {
    service = new AuthService({ /* mock deps */ });
  });

  describe('login()', () => {
    it('returns a JWT token for valid credentials', async () => {
      // Arrange
      const credentials = { email: 'user@test.com', password: 'correct' };
      // Act
      const result = await service.login(credentials);
      // Assert
      expect(result.token).toMatch(/^eyJ/);
    });

    it('throws for invalid password', async () => {
      await expect(service.login({ email: 'user@test.com', password: 'wrong' }))
        .rejects.toThrow('Invalid credentials');
    });
  });
});

Mocking External Dependencies

// Mock the database
const mockUserRepo = {
  findByEmail: vi.fn().mockResolvedValue({ id: '1', email: 'user@test.com', passwordHash: '$2b...' }),
};

When to Stop TDD

• Configuration files
• Simple data transformations (use property-based testing instead)
• Exploratory code (spike) — delete tests after learning, rewrite properly

Definition of Done

• All tests pass
• No test mocks real behavior (only I/O)
• Coverage ≥ 80% on the changed module
• Tests run in < 1 second total