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HTML / CSS / JS

// middleware/errors.js  (expanded status-code handling)
const HTTP_ERRORS = {
    400: 'Bad Request',
    401: 'Unauthorized',
    403: 'Forbidden',
    404: 'Not Found',
    409: 'Conflict',
    422: 'Unprocessable Entity',
    429: 'Too Many Requests',
    500: 'Internal Server Error',
};

function errorHandler(err, req, res, next) {
    // Handle express-validator errors
    if (err.type === 'validation') {
        return res.status(422).json({ errors: err.details });
    }

// Handle known Mongoose/Sequelize duplicate key
    if (err.code === 11000 || err.code === 'ER_DUP_ENTRY') {
        return res.status(409).json({ error: { message: 'Resource already exists', code: 'CONFLICT' } });
    }

const status  = err.status  || 500;
    const message = err.message || HTTP_ERRORS[status] || 'Unknown error';
    const level   = status >= 500 ? 'error' : 'warn';

console[level](JSON.stringify({
        level, status, message,
        requestId: req.requestId,
        method:    req.method,
        path:      req.path,
        stack:     process.env.NODE_ENV !== 'production' ? err.stack : undefined,
    }));

res.status(status).json({
        error: { message: status >= 500 && process.env.NODE_ENV === 'production'
            ? HTTP_ERRORS[500] : message }
    });
}

module.exports = errorHandler;