class AppError extends Error {
  constructor(
    public message: string,
    public statusCode: number = 500,
    public code?: string
  ) {
    super(message);
    this.name = 'AppError';
    Error.captureStackTrace(this, this.constructor);
  }
}

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

class ValidationError extends AppError {
  constructor(message: string) {
    super(message, 400, 'VALIDATION_ERROR');
  }
}

2. Async Error Handling

// ✅ Async/await with try-catch
async function fetchUser(id: string) {
  try {
    const response = await api.get(`/users/${id}`);
    return response.data;
  } catch (error) {
    if (error instanceof AxiosError) {
      throw new AppError(error.message, error.response?.status);
    }
    throw error;
  }
}

// ✅ Promise error handling
fetchUser(id)
  .then(handleSuccess)
  .catch(handleError);

3. Express Error Middleware

// Error handler middleware
app.use((err: Error, req: Request, res: Response, next: NextFunction) => {
  console.error(err.stack);
  
  if (err instanceof AppError) {
    return res.status(err.statusCode).json({
      success: false,
      error: {
        message: err.message,
        code: err.code
      }
    });
  }
  
  res.status(500).json({
    success: false,
    error: {
      message: 'Internal server error',
      code: 'INTERNAL_ERROR'
    }
  });
});

4. React Error Boundaries

class ErrorBoundary extends React.Component<Props, State> {
  state = { hasError: false, error: null };

  static getDerivedStateFromError(error: Error) {
    return { hasError: true, error };
  }

  componentDidCatch(error: Error, errorInfo: ErrorInfo) {
    console.error('Error caught:', error, errorInfo);
    // Log to error reporting service
  }

  render() {
    if (this.state.hasError) {
      return <ErrorFallback error={this.state.error} />;
    }
    return this.props.children;
  }
}

5. API Error Response Format

// Standard error response
interface ApiError {
  success: false;
  error: {
    message: string;
    code: string;
    details?: unknown;
  };
}

// Example response
{
  "success": false,
  "error": {
    "message": "User not found",
    "code": "USER_NOT_FOUND"
  }
}

6. Logging

import winston from 'winston';

const logger = winston.createLogger({
  level: 'info',
  format: winston.format.json(),
  transports: [
    new winston.transports.File({ filename: 'error.log', level: 'error' }),
    new winston.transports.File({ filename: 'combined.log' })
  ]
});

// Usage
logger.error('Database connection failed', { error, context });

2. Async Error Handling

3. Express Error Middleware

4. React Error Boundaries

5. API Error Response Format

6. Logging

References

errors

Error Handling Patterns

Instructions

1. Custom Error Classes