Test-Driven Development (TDD)

Overview

TDD enforces a strict "tests before code" methodology through the Red-Green-Refactor cycle. Write a failing test first (Red), implement minimal code to pass it (Green), then improve the code while keeping tests green (Refactor). Target: 80%+ test coverage.

Test Types

Edge Cases You Must Test

1. Null/Undefined input
2. Empty arrays/strings
3. Invalid types
4. Boundary values (min/max)
5. Error paths (network failures, DB errors)
6. Race conditions (concurrent operations)
7. Large data (10k+ items)
8. Special characters (Unicode, emojis, SQL chars)

Testing Patterns

Unit Test (Jest/Vitest)

describe('Button Component', () => {
  it('renders with correct text', () => {
    render(<Button>Click me</Button>)
    expect(screen.getByText('Click me')).toBeInTheDocument()
  })

  it('calls onClick when clicked', () => {
    const handleClick = jest.fn()
    render(<Button onClick={handleClick}>Click</Button>)
    fireEvent.click(screen.getByRole('button'))
    expect(handleClick).toHaveBeenCalledTimes(1)
  })
})

API Integration Test

describe('GET /api/markets', () => {
  it('returns markets successfully', async () => {
    const request = new NextRequest('http://localhost/api/markets')
    const response = await GET(request)
    const data = await response.json()
    expect(response.status).toBe(200)
    expect(data.success).toBe(true)
  })

  it('validates query parameters', async () => {
    const request = new NextRequest('http://localhost/api/markets?limit=invalid')
    const response = await GET(request)
    expect(response.status).toBe(400)
  })
})

E2E Test (Playwright)

test('user can search and filter markets', async ({ page }) => {
  await page.goto('/')
  await page.click('a[href="/markets"]')
  await page.fill('input[placeholder="Search markets"]', 'election')
  await page.waitForTimeout(600)
  const results = page.locator('[data-testid="market-card"]')
  await expect(results).toHaveCount(5, { timeout: 5000 })
})

Mocking External Services

// Supabase
jest.mock('@/lib/supabase', () => ({
  supabase: {
    from: jest.fn(() => ({
      select: jest.fn(() => ({
        eq: jest.fn(() => Promise.resolve({ data: [...], error: null }))
      }))
    }))
  }
}))

// Redis
jest.mock('@/lib/redis', () => ({
  searchMarketsByVector: jest.fn(() => Promise.resolve([...])),
  checkRedisHealth: jest.fn(() => Promise.resolve({ connected: true }))
}))

Anti-Patterns to Avoid

Coverage Requirements

• 80% minimum for all code (branches, functions, lines, statements)
• 100% required for: financial calculations, authentication, security-critical code, core business logic

Best Practices

1. Write the test FIRST, before any implementation
2. One assert per test — focus on single behavior
3. Use Arrange-Act-Assert structure
4. Mock external dependencies to isolate units
5. Keep tests fast (unit tests < 50ms each)
6. Clean up after tests — no side effects
7. Use watch mode during development: npm test -- --watch

Integration with Other Commands

Remember: Tests are not optional. They are the safety net that enables confident refactoring, rapid development, and production reliability.