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:

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