Error handling in one place

📘What it really is

• Define error classes for expected problems:

export class AppError extends Error {
  status: number;
  constructor(status: number, message: string) {
    super(message);
    this.status = status;
  }
}
export class NotFoundError extends AppError { constructor(m: string) { super(404, m); } }
export class ConflictError extends AppError { constructor(m: string) { super(409, m); } }

• One error-handling middleware, added last, with four parameters:

app.use((err, req, res, next) => {
  if (err instanceof z.ZodError) {
    return res.status(422).json({ error: "Validation failed", fields: z.flattenError(err).fieldErrors });
  }
  if (err instanceof AppError) {
    return res.status(err.status).json({ error: err.message });
  }
  console.error(err);                      // Module 45: proper logging
  res.status(500).json({ error: "Something went wrong. Please try again." });
});

• Expected errors (not found, invalid input, conflicts) get specific statuses and helpful messages.

• Unexpected errors (bugs, a crashed disk) are logged in full on the server, and the client gets a generic 500 message, never details.

• Express 5 sends errors thrown in async handlers to this middleware automatically.

• Never let the process crash silently: also log unhandledRejection and uncaughtException, then restart cleanly (your host or process manager restarts it, Level 6).