test-driven-development

SKILL: Test-Driven Development

Purpose: Red-Green-Refactor cycle for reliable code with tests Target: Coder agent implementing features or bugfixes

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Quick Start

When to Use This Skill

• Implementing new features
• Fixing bugs
• Refactoring existing code
• Adding new functionality

Quick Reference

Phase	Action	Verify
RED	Write failing test	npm test → FAIL
GREEN	Minimal implementation	npm test → PASS
REFACTOR	Clean up code	npm test && npm run lint → PASS
REPEAT	Until criteria met	All quality gates pass

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Core Concepts

The TDD Cycle

Step 1: RED - Write a failing test

// src/auth.service.test.ts
describe('AuthService', () => {
  it('should authenticate valid user', async () => {
    const service = new AuthService();
    const result = await service.authenticate('[email protected]', 'password');
    expect(result).toBe(true);
  });
});

Verify test fails: npm test → FAIL: AuthService is not defined

Step 2: GREEN - Write minimal code to pass

// src/auth.service.ts
export class AuthService {
  async authenticate(email: string, password: string): Promise<boolean> {
    return true;  // Minimal implementation
  }
}

Verify test passes: npm test → PASS

Step 3: REFACTOR - Clean up

// src/auth.service.ts
export class AuthService {
  constructor(private users: UserRepository) {}

  async authenticate(email: string, password: string): Promise<boolean> {
    const user = await this.users.findByEmail(email);
    if (!user) return false;
    return await user.verifyPassword(password);
  }
}

Verify tests still pass: npm test && npm run lint → PASS

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Test Patterns

Arrange-Act-Assert

it('should calculate total with discount', () => {
  const cart = new Cart();  // Arrange
  cart.applyDiscount('SUMMER20');  // Act
  expect(cart.total).toBe(80);  // Assert
});