# CHAPTER 4 SUMMARY

## Key Concepts Introduced
1. **CRUD Operations** - Create, Read, Update, Delete - Used in chapters: 5, 8, 10, 13
2. **POST for Creation** - tf.post() with automatic JSON handling - Used in chapters: 5, 8, 10, 11
3. **PUT vs PATCH** - Complete replacement vs partial update - Used in chapters: 10, 13
4. **DELETE Operations** - Removing resources with tf.delete() - Used in chapters: 10
5. **Content Types** - FormData, URLSearchParams, text/plain - Used in chapters: 9
6. **Optimistic Updates** - Update UI before server confirms - Used in chapters: 10, 11
7. **Bulk Operations** - Multiple creates/updates/deletes - Used in chapters: 10, 13
8. **Idempotency** - Safe retries with idempotency keys - Used in chapters: 5, 10
9. **Conditional Updates** - Using ETags and If-Match - Used in chapters: 6, 10
10. **Authentication Headers** - Bearer tokens in requests - Used in chapters: 8

## Code Patterns Established
```javascript
// Pattern 1: Basic CRUD operations
await tf.post('/api/resource', { data: newItem })
await tf.get('/api/resource/123')
await tf.patch('/api/resource/123', { data: updates })
await tf.put('/api/resource/123', { data: fullItem })
await tf.delete('/api/resource/123')

// Pattern 2: Error handling for mutations
try {
  const { data } = await tf.post(url, { data })
} catch (error) {
  if (error.response?.status === 409) {
    // Handle conflict
  }
}

// Pattern 3: Optimistic updates
updateUI(newState)
try {
  await tf.patch(url, { data: newState })
} catch (error) {
  revertUI(oldState)
}

// Pattern 4: Authenticated requests
const api = tf.create({
  headers: () => ({
    'Authorization': `Bearer ${getToken()}`
  })
})
```

## API Endpoints Used
- jsonplaceholder.typicode.com/todos - Todo CRUD examples
- api.myapp.com/auth/register - User registration
- api.weatherbuddy.com/* - Full CRUD Weather Buddy backend
- /oauth/token - OAuth token endpoint example

## Advanced Patterns Introduced
1. **FormData Upload** - File uploads with multipart/form-data
2. **URLSearchParams** - OAuth and form-encoded data
3. **Bulk Operations** - Efficient multi-item processing
4. **Idempotency Keys** - Safe payment/order creation
5. **Conditional Updates** - Prevent lost updates with ETags
6. **Soft Deletes** - Mark as deleted vs hard delete

## Building Blocks for Next Chapter
- Learned: All CRUD operations
- Mastered: Error response handling basics
- Ready for: Deep dive into error handling, retries, circuit breakers

## Weather Buddy App Status
- Version 4.0: Full user system
- Features: Registration, login, save cities, preferences
- Added: Share dashboard, bulk operations
- Database: User preferences persisted
- Next: Error resilience and offline support (Chapter 5)

## Best Practices Established
1. Use correct HTTP methods for operations
2. Show loading states during mutations
3. Validate client-side before sending
4. Handle specific error status codes
5. Use PATCH for partial updates
6. Implement optimistic updates for better UX
7. Make requests idempotent when possible

## Common Mistakes to Avoid
- Using GET for state changes
- PUT with partial data (use PATCH)
- Forgetting loading states
- Not handling specific errors
- Ignoring conflict resolution
- Missing authentication headers