Error Middleware: One Place to Handle Failures

Error middleware is the reliability backbone: normalize failures into a stable response shape while preserving logs, stack traces, and root causes.

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Why Error Middleware Exists

Production systems must not leak internals and must return consistent error responses. Error middleware is where you translate exceptions into a safe, observable HTTP contract.

Error Handler Must Be Last

Express only routes errors to middleware with four arguments: (err, req, res, next). It must be registered after routes.

import type { Request, Response, NextFunction } from 'express';

export function errorHandler(err: unknown, req: Request, res: Response, _next: NextFunction) {
  const requestId = (req as any).requestId as string | undefined;

  const status = 500;
  res.status(status).json({
    error: { code: 'INTERNAL_ERROR', message: 'Unexpected error', requestId }
  });
}

Production Rules

Always log the original error with context (route, requestId)
Return stable client-facing codes and messages
Do not expose stack traces in production responses

← Middleware Concept: The Request Pipeline

Validation Strategy (DTOs + schema validation) →