Error handling middleware trong Express hoạt động như thế nào?

class AppError extends Error {
  constructor(
    public statusCode: number,
    message: string,
    public isOperational = true,
  ) {
    super(message)
  }
}

// bọc route async để khỏi lặp try/catch
const asyncHandler = (fn) => (req, res, next) =>
  Promise.resolve(fn(req, res, next)).catch(next)

app.use((err, req, res, next) => {
  if (err.isOperational) return res.status(err.statusCode).json({ message: err.message })
  logger.error(err)
  res.status(500).json({ message: 'Internal server error' })
})