TDD Skill

Guide for Test-Driven Development workflow in Guidr.

TDD Cycle: RED-GREEN-REFACTOR

1. RED Phase - Write Failing Test

Write a test that describes the desired behavior. It should fail because the implementation doesn't exist yet.

Example (adding a new entity):

typescript

// src/domain/entities/Timer.test.ts
import { Timer } from './Timer'

describe('Timer', () => {
  it('should create timer with duration', () => {
    const timer = new Timer('timer-1', 300)

    expect(timer.id).toBe('timer-1')
    expect(timer.duration).toBe(300)
    expect(timer.remaining).toBe(300)
  })
})

Run test: npm test -- Timer.test.ts Expected: ❌ FAIL (Timer class doesn't exist)

2. GREEN Phase - Make It Pass

Write the minimal code needed to pass the test.

Example:

typescript

// src/domain/entities/Timer.ts
export class Timer {
  readonly id: string
  readonly duration: number
  private _remaining: number

  constructor(id: string, duration: number) {
    this.id = id
    this.duration = duration
    this._remaining = duration
  }

  get remaining(): number {
    return this._remaining
  }
}

Run test: npm test -- Timer.test.ts Expected: ✅ PASS

3. REFACTOR Phase - Improve Code

Clean up implementation while keeping tests green. Add more tests for edge cases.

Example (adding validation):

typescript

// Timer.test.ts - Add validation tests
it('should throw error if duration is negative', () => {
  expect(() => new Timer('timer-1', -10)).toThrow('Duration must be positive')
})

// Timer.ts - Add validation
constructor(id: string, duration: number) {
  if (duration < 0) {
    throw new Error('Duration must be positive')
  }
  // ... rest of constructor
}

Run tests: npm test Expected: ✅ All tests pass

4. VERIFY Phase - Quality Checks

Before committing:

bash

npm test           # All tests pass
npm run lint       # No lint errors
npm run typecheck  # No type errors

TDD for Services

Services use mocked repositories. Follow this pattern:

typescript

describe('TimerService', () => {
  let service: TimerService
  let mockRepository: jest.Mocked<ITimerRepository>

  beforeEach(() => {
    mockRepository = {
      findById: jest.fn(),
      save: jest.fn(),
    }
    service = new TimerService(mockRepository)
  })

  it('should create timer with generated ID', async () => {
    const timer = await service.createTimer(300)

    expect(timer.id).toBeDefined()
    expect(timer.duration).toBe(300)
    expect(mockRepository.save).toHaveBeenCalledWith(timer)
  })
})

TDD for Screens

Screens use mocked storage/services:

typescript

import { render, fireEvent } from '@testing-library/react-native'

describe('TimerScreen', () => {
  let mockService: jest.Mocked<TimerService>

  beforeEach(() => {
    mockService = {
      createTimer: jest.fn(),
    }
  })

  it('should create timer when button pressed', async () => {
    mockService.createTimer.mockResolvedValue(new Timer('timer-1', 300))

    const { getByText } = render(<TimerScreen service={mockService} />)

    fireEvent.press(getByText('Start Timer'))

    await waitFor(() => {
      expect(mockService.createTimer).toHaveBeenCalledWith(300)
    })
  })
})

Common Mistakes to Avoid

•Writing implementation first - Always write test first (RED phase)
•Over-engineering in GREEN phase - Write minimal code to pass
•Skipping REFACTOR phase - Tests green doesn't mean code is clean
•Skipping VERIFY phase - Always run full suite + lint + typecheck
•Testing implementation details - Test behavior, not internal structure

Benefits of TDD

•Confidence: Comprehensive test coverage from day one
•Design: Writing tests first leads to better API design
•Documentation: Tests document expected behavior
•Refactoring: Safe to improve code with test safety net
•Debugging: Failing test pinpoints exact problem