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skills/testing-practices/SKILL.md

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+# Testing Practices
+
+**Comprehensive testing strategies across the stack**
+
+## Testing Pyramid
+
+### Unit Tests (70%)
+- Test individual functions/methods
+- Fast execution
+- No external dependencies
+- High coverage of business logic
+
+### Integration Tests (20%)
+- Test component interactions
+- Database queries
+- API endpoints
+- Service integrations
+
+### E2E Tests (10%)
+- Test user workflows
+- Full stack testing
+- Browser automation
+- Critical paths only
+
+## Frontend Testing
+
+### Component Testing (React)
+```tsx
+import { render, screen, fireEvent } from '@testing-library/react'
+import { Button } from './Button'
+
+describe('Button', () => {
+  it('calls onClick when clicked', () => {
+    const onClick = jest.fn()
+    render(<Button onClick={onClick}>Click me</Button>)
+
+    fireEvent.click(screen.getByText('Click me'))
+
+    expect(onClick).toHaveBeenCalledTimes(1)
+  })
+})
+```
+
+### State Management Testing
+```tsx
+import { renderHook, act } from '@testing-library/react-hooks'
+import { useCounter } from './useCounter'
+
+it('increments counter', () => {
+  const { result } = renderHook(() => useCounter())
+
+  act(() => {
+    result.current.increment()
+  })
+
+  expect(result.current.count).toBe(1)
+})
+```
+
+### E2E Testing (Playwright)
+```typescript
+import { test, expect } from '@playwright/test'
+
+test('user can log in', async ({ page }) => {
+  await page.goto('/login')
+  await page.fill('[name=email]', 'user@example.com')
+  await page.fill('[name=password]', 'password123')
+  await page.click('button[type=submit]')
+
+  await expect(page).toHaveURL('/dashboard')
+  await expect(page.locator('h1')).toContainText('Welcome')
+})
+```
+
+## Backend Testing
+
+### API Testing
+```typescript
+import request from 'supertest'
+import { app } from './app'
+
+describe('POST /api/users', () => {
+  it('creates a new user', async () => {
+    const res = await request(app)
+      .post('/api/users')
+      .send({
+        name: 'John Doe',
+        email: 'john@example.com'
+      })
+      .expect(201)
+
+    expect(res.body).toHaveProperty('id')
+    expect(res.body.email).toBe('john@example.com')
+  })
+})
+```
+
+### Database Testing
+```typescript
+beforeEach(async () => {
+  await db.migrate.rollback()
+  await db.migrate.latest()
+  await db.seed.run()
+})
+
+afterEach(async () => {
+  await db.destroy()
+})
+```
+
+## Test Best Practices
+
+### AAA Pattern
+```typescript
+it('example test', () => {
+  // Arrange
+  const input = { ... }
+
+  // Act
+  const result = functionUnderTest(input)
+
+  // Assert
+  expect(result).toBe(expected)
+})
+```
+
+### Test Isolation
+- Each test should be independent
+- No shared state between tests
+- Clean up after each test
+- Use beforeEach/afterEach appropriately
+
+### Meaningful Test Names
+```typescript
+// ❌ BAD
+it('works', () => { ... })
+
+// ✅ GOOD
+it('returns 404 when user not found', () => { ... })
+```
+
+### Mocking
+```typescript
+jest.mock('./api')
+
+it('handles API errors', async () => {
+  api.fetchUser.mockRejectedValue(new Error('Network error'))
+
+  // Test error handling
+})
+```
+
+## Performance Testing
+
+### Load Testing
+- Apache JMeter
+- k6
+- Artillery
+
+### Stress Testing
+- Identify breaking points
+- Test recovery
+- Monitor resource usage
+
+## Test Coverage
+
+### Coverage Metrics
+- Line coverage: 80%+ target
+- Branch coverage: 75%+ target
+- Function coverage: 90%+ target
+
+### Coverage Tools
+- Jest (--coverage)
+- Istanbul/nyc
+- Codecov/Coveralls
+
+## CI/CD Integration
+
+### GitHub Actions
+```yaml
+name: Tests
+on: [push, pull_request]
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - run: npm install
+      - run: npm test
+      - run: npm run test:e2e
+```
+
+### Test Stages
+1. Lint and type check
+2. Unit tests
+3. Integration tests
+4. Build verification
+5. E2E tests (on staging)
+
+## Debugging Tests
+
+- Use `test.only` to isolate
+- Add console.logs strategically
+- Use debugger statements
+- Check test environment setup
+- Verify mock implementations
+