Error Handling

Error Handling

Recoverable vs Fatal

Not all errors are equal. Classify before you handle.

Rule: Never catch a TypeError, RangeError, or ReferenceError unless you specifically expect it. These indicate bugs, not runtime conditions.

Typed Error Classes

Give errors machine-readable codes. Avoid stringly-typed message checks.

// ✅ CORRECT — typed, discriminable, chainable
export class AppError extends Error {
  constructor(
    message: string,
    public readonly code: string,
    public readonly statusCode: number = 500,
    options?: ErrorOptions,
  ) {
    super(message, options); // passes { cause } through
    this.name = this.constructor.name;
    // Fix prototype chain for instanceof checks in transpiled code
    Object.setPrototypeOf(this, new.target.prototype);
  }
}

export class NotFoundError extends AppError {
  constructor(resource: string, id: string, options?: ErrorOptions) {
    super(`${resource} with id "${id}" not found`, 'NOT_FOUND', 404, options);
  }
}

export class ValidationError extends AppError {
  constructor(
    message: string,
    public readonly fields: Record<string, string[]>,
    options?: ErrorOptions,
  ) {
    super(message, 'VALIDATION_FAILED', 422, options);
  }
}

export class ConflictError extends AppError {
  constructor(message: string, options?: ErrorOptions) {
    super(message, 'CONFLICT', 409, options);
  }
}

// ❌ WRONG — string-matching error messages is fragile
try {
  await getUser(id);
} catch (err) {
  if ((err as Error).message.includes('not found')) {
    return null;
  }
  throw err;
}

// ✅ CORRECT — discriminate by type or code
try {
  await getUser(id);
} catch (err) {
  if (err instanceof NotFoundError) return null;
  throw err;
}

Result Pattern for Expected Failures

When a function has a known failure mode that the caller must handle, prefer returning a Result<T, E> over throwing.

// Result type (no library needed)
type Result<T, E = AppError> =
  | { ok: true; value: T }
  | { ok: false; error: E };

const ok = <T>(value: T): Result<T, never> => ({ ok: true, value });
const err = <E>(error: E): Result<never, E> => ({ ok: false, error });

// ✅ CORRECT — caller is forced to handle both cases
async function findUser(id: string): Promise<Result<User, NotFoundError>> {
  const user = await db.users.findUnique({ where: { id } });
  if (!user) return err(new NotFoundError('User', id));
  return ok(user);
}

const result = await findUser(userId);
if (!result.ok) {
  // TypeScript knows result.error is NotFoundError here
  return res.status(404).json({ error: result.error.code });
}
// TypeScript knows result.value is User here
doSomethingWith(result.value);

// ❌ WRONG — caller can forget to handle the failure
async function findUser(id: string): Promise<User | null> {
  const user = await db.users.findUnique({ where: { id } });
  return user ?? null;
  // Caller can forget the null check entirely
}

Never Swallow Errors Silently

An empty catch block is one of the most dangerous patterns in any codebase.

// ❌ WRONG — error disappears completely
try {
  await sendWelcomeEmail(user.email);
} catch (_) {
  // silence
}

// ❌ WRONG — logged but the original error chain is lost
try {
  await sendWelcomeEmail(user.email);
} catch (err) {
  console.log('email failed');
}

// ✅ CORRECT — log with context, decide if it's fatal
try {
  await sendWelcomeEmail(user.email);
} catch (err) {
  logger.warn({ err, userId: user.id }, 'Welcome email delivery failed');
  // Intentionally non-fatal: user still gets registered, email can be retried
  // If this MUST succeed, re-throw instead
}

Rule: Every catch block must do at least one of: log, re-throw, or return an error value. A comment explaining why you chose silence is mandatory when silence is truly correct.

Propagate with Context (Error Cause Chain)

When re-throwing, wrap with context. Never discard the original error.

// ❌ WRONG — original stack trace and cause are gone
try {
  await db.query(sql);
} catch {
  throw new Error('Database query failed');
}

// ✅ CORRECT — chain with { cause }, preserving the full error trail
try {
  await db.query(sql);
} catch (err) {
  throw new AppError(
    `Failed to load user profile for id="${userId}"`,
    'DB_QUERY_FAILED',
    500,
    { cause: err }, // native Error cause chain (Node 16.9+)
  );
}

Printing the full cause chain:

function formatErrorChain(err: unknown): string {
  if (!(err instanceof Error)) return String(err);
  const parts = [err.message];
  let cause = err.cause;
  while (cause instanceof Error) {
    parts.push(`caused by: ${cause.message}`);
    cause = cause.cause;
  }
  return parts.join('\n  ');
}

User-Facing vs Log-Only Messages

Never expose internal details (SQL, stack traces, internal IDs) in API responses.

// ❌ WRONG — leaks DB schema, query, and stack trace
app.use((err: unknown, req: Request, res: Response, _next: NextFunction) => {
  res.status(500).json({
    error: (err as Error).message, // may contain SQL or file paths
    stack: (err as Error).stack,
  });
});

// ✅ CORRECT — safe external message + full detail in logs
app.use((err: unknown, req: Request, res: Response, _next: NextFunction) => {
  const appErr = err instanceof AppError ? err : null;
  const statusCode = appErr?.statusCode ?? 500;

  // Full details go to structured log (internal only)
  logger.error({
    err,
    req: { method: req.method, url: req.url, traceId: req.traceId },
  }, 'Request failed');

  // Sanitized response goes to client
  res.status(statusCode).json({
    error: {
      code: appErr?.code ?? 'INTERNAL_ERROR',
      message: appErr?.statusCode && appErr.statusCode < 500
        ? appErr.message            // 4xx: safe to expose, it's the caller's fault
        : 'An unexpected error occurred', // 5xx: hide internals
    },
  });
});

Rule:

• 4xx errors (client mistakes): message is safe to show — it describes what the caller did wrong.
• 5xx errors (server failures): use a generic message. Log the real error internally.

Unhandled Rejection / Uncaught Exception

// Register at process entry point — do this exactly once
process.on('unhandledRejection', (reason, promise) => {
  logger.fatal({ reason, promise }, 'Unhandled Promise rejection');
  // Exit cleanly — let the process manager restart
  process.exit(1);
});

process.on('uncaughtException', (err) => {
  logger.fatal({ err }, 'Uncaught exception');
  process.exit(1);
});

Rule: Crash fast and let the process manager (systemd, PM2, container orchestrator) restart. A process running in an unknown state is more dangerous than a restarted one.

Checklist

• Every error has a machine-readable code property
• No empty catch blocks — each one logs, re-throws, or returns an error value
• Re-throws use { cause: originalErr } to preserve the chain
• 5xx responses never include stack traces, SQL, or internal paths
• process.on('unhandledRejection') registered at startup
• Typed error subclasses for all expected failure modes
• instanceof checks used for error discrimination, not message.includes()