api-patterns

API Patterns Skill

Quick Reference

Use when: Building REST APIs, implementing authentication, handling errors, validating inputs

Key Patterns:

• Request validation
• Standardized error responses
• Authentication middleware
• Rate limiting
• Pagination
• API documentation

---

1. Request Validation

Always validate incoming requests at the boundary.

Pattern with schema validation:

// Define schema for expected input
const CreateUserSchema = {
  email: 'string, email format, required',
  password: 'string, min 12 chars, required',
  name: 'string, 2-100 chars, required',
  age: 'number, integer, min 18, optional'
};

// Validate in route handler
function createUserHandler(req, res) {
  const validation = validateSchema(CreateUserSchema, req.body);

  if (!validation.success) {
    return res.status(400).json({
      error: 'Validation failed',
      details: validation.errors
    });
  }

  // Proceed with validated data
  const user = await createUser(validation.data);
  res.status(201).json(user);
}

Key Points:

• Validate at API boundary, not deep in business logic
• Return specific error messages
• Use 400 status code for validation failures

---

2. Standardized Error Responses

Use consistent error format across all endpoints.

// Error response structure
interface ErrorResponse {
  error: string;        // Human-readable message
  code?: string;        // Machine-readable error code
  details?: unknown;    // Additional context (validation errors, etc.)
}

// Custom error class
class AppError extends Error {
  constructor(
    message: string,
    public statusCode: number = 500,
    public code?: string,
    public details?: unknown
  ) {
    super(message);
  }
}

// Usage
throw new AppError('User not found', 404, 'USER_NOT_FOUND');
throw new AppError('Validation failed', 400, 'VALIDATION_ERROR', errors);

Error Handler Pattern:

function errorHandler(err, req, res, next) {
  // Known application errors
  if (err instanceof AppError) {
    return res.status(err.statusCode).json({
      error: err.message,
      code: err.code,
      details: err.details
    });
  }

  // Log unexpected errors (don't expose to user)
  console.error('Unexpected error:', err);

  res.status(500).json({
    error: 'Internal server error'
  });
}

---

3. Authentication Middleware

Protect routes with authentication middleware.

// Auth middleware pattern
function requireAuth(req, res, next) {
  const token = req.headers.authorization?.replace('Bearer ', '');

  if (!token) {
    throw new AppError('Authentication required', 401, 'NO_TOKEN');
  }

  try {
    const decoded = verifyToken(token);
    req.userId = decoded.userId;
    next();
  } catch (err) {
    throw new AppError('Invalid token', 401, 'INVALID_TOKEN');
  }
}

// Optional auth (for routes that work with or without auth)
function optionalAuth(req, res, next) {
  const token = req.headers.authorization?.replace('Bearer ', '');

  if (token) {
    try {
      const decoded = verifyToken(token);
      req.userId = decoded.userId;
    } catch {
      // Ignore invalid tokens in optional auth
    }
  }

  next();
}

// Usage
router.get('/api/profile', requireAuth, getProfileHandler);
router.get('/api/posts', optionalAuth, getPostsHandler);

---

4. Rate Limiting

Protect endpoints from abuse.

// General rate limit
const generalLimiter = {
  windowMs: 15 * 60 * 1000,  // 15 minutes
  max: 100                    // 100 requests per window
};

// Strict limit for auth endpoints
const authLimiter = {
  windowMs: 15 * 60 * 1000,  // 15 minutes
  max: 5,                     // Only 5 attempts
  skipSuccessfulRequests: true
};

// Apply to routes
app.use('/api/', rateLimitMiddleware(generalLimiter));
router.post('/api/auth/login', rateLimitMiddleware(authLimiter), loginHandler);

---

5. Pagination

Standard pagination for list endpoints.

// Pagination parameters
const DEFAULT_PAGE = 1;
const DEFAULT_LIMIT = 20;
const MAX_LIMIT = 100;

function parsePagination(query) {
  const page = Math.max(1, parseInt(query.page) || DEFAULT_PAGE);
  const limit = Math.min(MAX_LIMIT, Math.max(1, parseInt(query.limit) || DEFAULT_LIMIT));
  const offset = (page - 1) * limit;

  return { page, limit, offset };
}

// Response format
function paginatedResponse(data, total, page, limit) {
  return {
    data,
    pagination: {
      page,
      limit,
      total,
      totalPages: Math.ceil(total / limit),
      hasNext: page < Math.ceil(total / limit),
      hasPrev: page > 1
    }
  };
}

// Usage in handler
async function listUsersHandler(req, res) {
  const { page, limit, offset } = parsePagination(req.query);

  const [users, total] = await Promise.all([
    db.users.findMany({ skip: offset, take: limit }),
    db.users.count()
  ]);

  res.json(paginatedResponse(users, total, page, limit));
}

---

6. API Response Format

Use consistent response structure.

// Success response
{
  data: T,                    // The requested resource(s)
  meta?: {
    pagination?: {...},       // For lists
    timestamp: string         // ISO timestamp
  }
}

// Error response
{
  error: string,              // Human-readable message
  code?: string,              // Machine-readable code
  details?: unknown           // Additional context
}

---

7. HTTP Status Codes

Use appropriate status codes:

Code	Meaning	When to Use
200	OK	Successful GET, PUT, PATCH
201	Created	Successful POST (resource created)
204	No Content	Successful DELETE
400	Bad Request	Validation errors, malformed request
401	Unauthorized	Missing or invalid authentication
403	Forbidden	Authenticated but not authorized
404	Not Found	Resource doesn't exist
409	Conflict	Resource conflict (duplicate, etc.)
429	Too Many Requests	Rate limit exceeded
500	Internal Error	Unexpected server error

---

8. RESTful Resource Naming

Follow conventions:

• GET /api/resources - List all (with pagination)
• GET /api/resources/:id - Get one
• POST /api/resources - Create new
• PUT /api/resources/:id - Replace entirely
• PATCH /api/resources/:id - Partial update
• DELETE /api/resources/:id - Delete

Nested resources:

• GET /api/users/:userId/posts - User's posts
• POST /api/users/:userId/posts - Create post for user

---

Checklist for New Endpoints

• Input validation with schema
• Authentication check (if protected)
• Authorization check (if resource-specific)
• Rate limiting configured
• Error handling with appropriate codes
• Consistent response format
• Pagination for lists
• Documentation/comments
• Tests covering success and error cases