Cuihua Error Handler

// ❌ BEFORE - Fragile code
async function getUserById(id) {
  const res = await fetch(`/api/users/${id}`);
  return res.json();
}

// 🔍 DETECTED ISSUES:
// - No error handling for network failures
// - No handling for non-200 responses
// - No handling for invalid JSON
// - No logging for debugging

// ✅ AFTER - Bulletproof code
class UserServiceError extends Error {
  constructor(message, options = {}) {
    super(message);
    this.name = 'UserServiceError';
    this.statusCode = options.statusCode;
    this.originalError = options.cause;
  }
}

async function getUserById(id) {
  try {
    // Validation
    if (!id || typeof id !== 'string') {
      throw new UserServiceError('Invalid user ID', { statusCode: 400 });
    }

    // Network request with timeout
    const controller = new AbortController();
    const timeout = setTimeout(() => controller.abort(), 5000);
    
    const res = await fetch(`/api/users/${id}`, {
      signal: controller.signal
    });
    
    clearTimeout(timeout);

    // HTTP error handling
    if (!res.ok) {
      if (res.status === 404) {
        throw new UserServiceError(`User ${id} not found`, { statusCode: 404 });
      }
      if (res.status >= 500) {
        throw new UserServiceError('Server error, please retry', { statusCode: 502 });
      }
      throw new UserServiceError(`HTTP ${res.status}`, { statusCode: res.status });
    }

    // JSON parsing with error handling
    let data;
    try {
      data = await res.json();
    } catch (parseError) {
      throw new UserServiceError('Invalid response format', {
        statusCode: 502,
        cause: parseError
      });
    }

    return data;

  } catch (error) {
    // Network errors (timeout, connection refused)
    if (error.name === 'AbortError') {
      logger.error('getUserById timeout', { id, timeout: 5000 });
      throw new UserServiceError('Request timeout', {
        statusCode: 504,
        cause: error
      });
    }

    if (error.message.includes('fetch failed')) {
      logger.error('getUserById network error', { id, error: error.message });
      throw new UserServiceError('Network error', {
        statusCode: 503,
        cause: error
      });
    }

    // Re-throw UserServiceError
    if (error instanceof UserServiceError) {
      logger.error('getUserById failed', { id, error: error.message });
      throw error;
    }

    // Unexpected errors
    logger.error('getUserById unexpected error', { id, error });
    throw new UserServiceError('Internal error', {
      statusCode: 500,
      cause: error
    });
  }
}

async function retryWithBackoff(fn, options = {}) {
  const {
    maxRetries = 3,
    initialDelay = 1000,
    maxDelay = 10000,
    backoffFactor = 2,
    shouldRetry = (error) => true
  } = options;

  let lastError;
  let delay = initialDelay;

  for (let attempt = 0; attempt <= maxRetries; attempt++) {
    try {
      return await fn();
    } catch (error) {
      lastError = error;

      // Check if we should retry
      if (attempt === maxRetries || !shouldRetry(error)) {
        throw error;
      }

      // Wait before retry
      logger.warn(`Retry attempt ${attempt + 1}/${maxRetries} after ${delay}ms`, {
        error: error.message
      });

      await new Promise(resolve => setTimeout(resolve, delay));

      // Exponential backoff
      delay = Math.min(delay * backoffFactor, maxDelay);
    }
  }

  throw lastError;
}

// Usage
async function fetchUserWithRetry(id) {
  return retryWithBackoff(
    () => getUserById(id),
    {
      maxRetries: 3,
      shouldRetry: (error) => {
        // Retry on network errors and 5xx
        return error.statusCode >= 500 || error.name === 'NetworkError';
      }
    }
  );
}

class CircuitBreaker {
  constructor(fn, options = {}) {
    this.fn = fn;
    this.failureThreshold = options.failureThreshold || 5;
    this.resetTimeout = options.resetTimeout || 60000;
    this.state = 'CLOSED'; // CLOSED, OPEN, HALF_OPEN
    this.failureCount = 0;
    this.nextAttempt = Date.now();
  }

  async execute(...args) {
    if (this.state === 'OPEN') {
      if (Date.now() < this.nextAttempt) {
        throw new Error('Circuit breaker is OPEN');
      }
      // Try to recover
      this.state = 'HALF_OPEN';
    }

    try {
      const result = await this.fn(...args);
      this.onSuccess();
      return result;
    } catch (error) {
      this.onFailure();
      throw error;
    }
  }

  onSuccess() {
    this.failureCount = 0;
    if (this.state === 'HALF_OPEN') {
      this.state = 'CLOSED';
      logger.info('Circuit breaker recovered');
    }
  }

  onFailure() {
    this.failureCount++;
    if (this.failureCount >= this.failureThreshold) {
      this.state = 'OPEN';
      this.nextAttempt = Date.now() + this.resetTimeout;
      logger.error('Circuit breaker opened', {
        failureCount: this.failureCount,
        resetTimeout: this.resetTimeout
      });
    }
  }
}

// Usage
const getUserBreaker = new CircuitBreaker(getUserById, {
  failureThreshold: 5,
  resetTimeout: 60000
});

async function fetchUserSafely(id) {
  try {
    return await getUserBreaker.execute(id);
  } catch (error) {
    if (error.message === 'Circuit breaker is OPEN') {
      // Return cached data or default
      return getCachedUser(id) || { id, name: 'Unknown', error: true };
    }
    throw error;
  }
}

async function getUserWithFallback(id) {
  try {
    // Try primary source
    return await getUserById(id);
  } catch (error) {
    logger.warn('Primary source failed, trying fallback', { id, error: error.message });

    try {
      // Try cache
      const cached = await cache.get(`user:${id}`);
      if (cached) {
        logger.info('Returned cached user', { id });
        return { ...cached, _cached: true };
      }
    } catch (cacheError) {
      logger.error('Cache also failed', { id, error: cacheError.message });
    }

    // Return default user
    logger.warn('Returning default user', { id });
    return {
      id,
      name: 'Guest User',
      _default: true,
      _error: error.message
    };
  }
}

🛡️ Error Handling Coverage Report
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📁 Files analyzed: 42
🔍 Functions analyzed: 156
  - Async functions: 89
  - Sync functions: 67

📊 Overall coverage: 68%

❌ Missing error handling (28 functions):

Critical (needs immediate attention):
  1. api/payment.js:45 - processPayment()
     Risk: High - Financial transaction
     Recommendation: Add try/catch + transaction rollback

  2. api/auth.js:23 - verifyToken()
     Risk: High - Security critical
     Recommendation: Add error handling + logging

  3. db/users.js:78 - updateUserProfile()
     Risk: Medium - Data mutation
     Recommendation: Add validation + rollback

⚠️  Weak error handling (12 functions):

  1. api/users.js:12 - getUserById()
     Issue: Generic catch block without specific handling
     Recommendation: Add error type differentiation

  2. services/email.js:34 - sendWelcomeEmail()
     Issue: Swallowed errors (empty catch block)
     Recommendation: Add logging + alerting

💡 Recommendations:
  - Add error handling to 28 unprotected functions
  - Improve 12 weak error handlers
  - Target coverage: 90%+ for production readiness