src/middleware/errorHandler.js

const errorHandler = (err, req, res, next) => {
  console.error('Error:', err.message);

  // Mongoose validation error
  if (err.name === 'ValidationError') {
    const messages = Object.values(err.errors).map((e) => e.message);
    return res.status(400).json({ message: 'Validation Error', errors: messages });
  }

  // Mongoose duplicate key error
  if (err.code === 11000) {
    const field = Object.keys(err.keyValue)[0];
    return res.status(400).json({ message: `Duplicate value for '${field}'. This already exists.` });
  }

  // Mongoose cast error (invalid ObjectId)
  if (err.name === 'CastError') {
    return res.status(400).json({ message: `Invalid ${err.path}: ${err.value}` });
  }

  // Default server error
  const statusCode = err.statusCode || 500;
  res.status(statusCode).json({
    message: err.message || 'Internal Server Error',
    ...(process.env.NODE_ENV === 'development' && { stack: err.stack }),
  });
};

module.exports = errorHandler;