---

name: error-handling description: Implement robust error handling in Node.js with custom error classes, async patterns, Express middleware, and production monitoring version: "2.1.0" sasmp_version: "1.3.0" bonded_agent: 06-testing-debugging bond_type: PRIMARY_BOND

Node.js Error Handling Skill

Master error handling patterns for building resilient Node.js applications that fail gracefully and provide meaningful feedback.

Quick Start

Error handling in 4 layers:

1. Custom Error Classes - Typed, informative errors
2. Try/Catch Patterns - Sync and async handling
3. Express Middleware - Centralized API errors
4. Process Handlers - Uncaught exceptions

Core Concepts

Custom Error Classes

// Base application error
class AppError extends Error {
  constructor(message, statusCode, code) {
    super(message);
    this.statusCode = statusCode;
    this.code = code;
    this.isOperational = true;
    Error.captureStackTrace(this, this.constructor);
  }
}

// Specific error types
class ValidationError extends AppError {
  constructor(message, details = []) {
    super(message, 400, 'VALIDATION_ERROR');
    this.details = details;
  }
}

class NotFoundError extends AppError {
  constructor(resource = 'Resource') {
    super(`${resource} not found`, 404, 'NOT_FOUND');
  }
}

class UnauthorizedError extends AppError {
  constructor(message = 'Authentication required') {
    super(message, 401, 'UNAUTHORIZED');
  }
}

class ForbiddenError extends AppError {
  constructor(message = 'Access denied') {
    super(message, 403, 'FORBIDDEN');
  }
}

Async Error Handling

// Async wrapper for Express routes
const asyncHandler = (fn) => (req, res, next) => {
  Promise.resolve(fn(req, res, next)).catch(next);
};

// Usage
router.get('/users/:id', asyncHandler(async (req, res) => {
  const user = await User.findById(req.params.id);
  if (!user) throw new NotFoundError('User');
  res.json(user);
}));

Express Error Middleware

const errorHandler = (err, req, res, next) => {
  // Log error
  logger.error({
    message: err.message,
    stack: err.stack,
    code: err.code,
    url: req.originalUrl
  });

  // Handle operational errors
  if (err.isOperational) {
    return res.status(err.statusCode).json({
      success: false,
      error: {
        message: err.message,
        code: err.code,
        ...(err.details && { details: err.details })
      }
    });
  }

  // Production: hide internal errors
  if (process.env.NODE_ENV === 'production') {
    return res.status(500).json({
      success: false,
      error: { message: 'Internal server error', code: 'INTERNAL_ERROR' }
    });
  }

  // Development: show stack
  res.status(500).json({
    success: false,
    error: { message: err.message, stack: err.stack }
  });
};

Learning Path

Beginner (1-2 weeks)

• ✅ Understand Error types
• ✅ Try/catch for sync code
• ✅ Promise .catch() handling
• ✅ Basic Express error middleware

Intermediate (3-4 weeks)

• ✅ Custom error classes
• ✅ Async error wrapper
• ✅ Centralized error handling
• ✅ Error logging with Winston

Advanced (5-6 weeks)

• ✅ Production error monitoring
• ✅ Graceful shutdown
• ✅ Circuit breaker patterns
• ✅ Error recovery strategies

Process-Level Error Handling

// Uncaught exceptions
process.on('uncaughtException', (error) => {
  console.error('Uncaught Exception:', error);
  logger.fatal({ error }, 'Uncaught exception');
  process.exit(1);
});

// Unhandled promise rejections
process.on('unhandledRejection', (reason, promise) => {
  console.error('Unhandled Rejection:', reason);
  logger.error({ reason }, 'Unhandled rejection');
});

// Graceful shutdown
process.on('SIGTERM', async () => {
  console.log('SIGTERM received. Graceful shutdown...');
  server.close(() => console.log('HTTP server closed'));
  await mongoose.connection.close();
  process.exit(0);
});

Retry with Exponential Backoff

async function retryOperation(fn, options = {}) {
  const { maxRetries = 3, delay = 1000, backoff = 2 } = options;
  let lastError;

  for (let attempt = 1; attempt <= maxRetries; attempt++) {
    try {
      return await fn();
    } catch (error) {
      lastError = error;
      if (attempt === maxRetries) throw error;

      const waitTime = delay * Math.pow(backoff, attempt - 1);
      await new Promise(resolve => setTimeout(resolve, waitTime));
    }
  }
  throw lastError;
}

Error Response Format

// Success
{ "success": true, "data": { ... } }

// Error
{
  "success": false,
  "error": {
    "message": "User not found",
    "code": "NOT_FOUND",
    "details": [],
    "timestamp": "2024-01-15T10:30:00.000Z"
  }
}

Unit Test Template

describe('Error Handling', () => {
  describe('ValidationError', () => {
    it('should create error with correct properties', () => {
      const error = new ValidationError('Invalid input', [
        { field: 'email', message: 'Required' }
      ]);

      expect(error.statusCode).toBe(400);
      expect(error.code).toBe('VALIDATION_ERROR');
      expect(error.isOperational).toBe(true);
    });
  });

  describe('asyncHandler', () => {
    it('should catch async errors', async () => {
      const mockNext = jest.fn();
      const handler = asyncHandler(async () => {
        throw new Error('Test error');
      });

      await handler({}, {}, mockNext);
      expect(mockNext).toHaveBeenCalledWith(expect.any(Error));
    });
  });
});

Troubleshooting

Problem	Cause	Solution
Unhandled rejection warnings	Missing .catch()	Add process handler + local catches
Error swallowed silently	Empty catch block	Log or rethrow errors
Stack trace missing	Error.captureStackTrace not called	Use AppError base class
Memory leak from errors	Storing error references	Use WeakMap or clean up

When to Use

Use proper error handling when:

• Building production Node.js applications
• Creating REST APIs with Express
• Handling database operations
• Processing external API calls
• Implementing retry logic

Related Skills

• Express REST API (API error responses)
• Async Programming (promise rejections)
• Testing & Debugging (error testing)

Resources

• Node.js Error Handling
• Express Error Handling
• Sentry for Node.js