tasks-backend/middleware/errorHandler.js

const logger = require('../config/logger');
const { AppError } = require('../utils/AppError');

/**
 * Handle Sequelize validation errors
 */
const handleSequelizeValidationError = (error) => {
  const errors = error.errors.map(err => ({
    field: err.path,
    message: err.message,
    value: err.value
  }));
  
  return {
    statusCode: 400,
    message: 'Validation error',
    errors
  };
};

/**
 * Handle Sequelize unique constraint errors
 */
const handleSequelizeUniqueConstraintError = (error) => {
  const field = error.errors[0]?.path;
  const value = error.errors[0]?.value;
  
  return {
    statusCode: 409,
    message: `${field} '${value}' already exists`
  };
};

/**
 * Handle Sequelize foreign key constraint errors
 */
const handleSequelizeForeignKeyConstraintError = (error) => {
  return {
    statusCode: 400,
    message: 'Invalid reference to related resource'
  };
};

/**
 * Handle Sequelize database connection errors
 */
const handleSequelizeConnectionError = (error) => {
  return {
    statusCode: 503,
    message: 'Database connection error. Please try again later.'
  };
};

/**
 * Handle JWT errors
 */
const handleJWTError = () => {
  return {
    statusCode: 401,
    message: 'Invalid token. Please log in again.'
  };
};

/**
 * Handle JWT expired errors
 */
const handleJWTExpiredError = () => {
  return {
    statusCode: 401,
    message: 'Your token has expired. Please log in again.'
  };
};

/**
 * Handle Sequelize errors
 */
const handleSequelizeError = (error) => {
  // Validation error
  if (error.name === 'SequelizeValidationError') {
    return handleSequelizeValidationError(error);
  }
  
  // Unique constraint violation
  if (error.name === 'SequelizeUniqueConstraintError') {
    return handleSequelizeUniqueConstraintError(error);
  }
  
  // Foreign key constraint violation
  if (error.name === 'SequelizeForeignKeyConstraintError') {
    return handleSequelizeForeignKeyConstraintError(error);
  }
  
  // Database connection error
  if (error.name === 'SequelizeConnectionError' || 
      error.name === 'SequelizeConnectionRefusedError' ||
      error.name === 'SequelizeHostNotFoundError' ||
      error.name === 'SequelizeAccessDeniedError') {
    return handleSequelizeConnectionError(error);
  }
  
  // Generic database error
  return {
    statusCode: 500,
    message: 'Database error occurred'
  };
};

/**
 * Send error response in development
 */
const sendErrorDev = (err, res) => {
  res.status(err.statusCode).json({
    status: err.status,
    message: err.message,
    error: err,
    stack: err.stack,
    ...(err.errors && { errors: err.errors })
  });
};

/**
 * Send error response in production
 */
const sendErrorProd = (err, res) => {
  // Operational, trusted error: send message to client
  if (err.isOperational) {
    res.status(err.statusCode).json({
      status: err.status,
      message: err.message,
      ...(err.errors && { errors: err.errors })
    });
  } 
  // Programming or unknown error: don't leak error details
  else {
    // Log error for debugging
    logger.error('ERROR 💥', err);
    
    // Send generic message
    res.status(500).json({
      status: 'error',
      message: 'Something went wrong. Please try again later.'
    });
  }
};

/**
 * Centralized error handling middleware
 */
const errorHandler = (err, req, res, next) => {
  err.statusCode = err.statusCode || 500;
  err.status = err.status || 'error';

  // Log the error
  logger.logError(err, req);

  // Handle specific error types
  let error = { ...err };
  error.message = err.message;
  error.stack = err.stack;

  // Sequelize errors
  if (err.name && err.name.startsWith('Sequelize')) {
    const handled = handleSequelizeError(err);
    error.statusCode = handled.statusCode;
    error.message = handled.message;
    error.isOperational = true;
    if (handled.errors) error.errors = handled.errors;
  }

  // JWT errors
  if (err.name === 'JsonWebTokenError') {
    const handled = handleJWTError();
    error.statusCode = handled.statusCode;
    error.message = handled.message;
    error.isOperational = true;
  }

  if (err.name === 'TokenExpiredError') {
    const handled = handleJWTExpiredError();
    error.statusCode = handled.statusCode;
    error.message = handled.message;
    error.isOperational = true;
  }

  // Multer errors (file upload)
  if (err.name === 'MulterError') {
    error.statusCode = 400;
    error.message = `File upload error: ${err.message}`;
    error.isOperational = true;
  }

  // Send error response
  if (process.env.NODE_ENV === 'development') {
    sendErrorDev(error, res);
  } else {
    sendErrorProd(error, res);
  }
};

/**
 * Handle async errors (wrap async route handlers)
 */
const catchAsync = (fn) => {
  return (req, res, next) => {
    fn(req, res, next).catch(next);
  };
};

/**
 * Handle 404 Not Found errors
 */
const notFoundHandler = (req, res, next) => {
  const error = new AppError(
    `Cannot find ${req.originalUrl} on this server`,
    404
  );
  next(error);
};

/**
 * Log unhandled rejections
 */
process.on('unhandledRejection', (reason, promise) => {
  logger.error('Unhandled Rejection at:', {
    promise,
    reason: reason.stack || reason
  });
  // Optional: Exit process in production
  // process.exit(1);
});

/**
 * Log uncaught exceptions
 */
process.on('uncaughtException', (error) => {
  logger.error('Uncaught Exception:', {
    message: error.message,
    stack: error.stack
  });
  // Exit process on uncaught exception
  process.exit(1);
});

module.exports = {
  errorHandler,
  catchAsync,
  notFoundHandler
};