Global Error Handling

A safe error page is useless if one forgotten route sends an exception straight to the client. In Express, error-handling middleware gives you one final boundary between private failures and public responses.

You create error-handling middleware by giving it four parameters: (err, req, res, next). Register it after your routes so Express can call it when a route throws an exception or passes an error to next(err):

import type { ErrorRequestHandler } from "express";

const errorHandler: ErrorRequestHandler = (err, req, res, next) => {
  if (res.headersSent) {
    next(err);
    return;
  }

  const details = err instanceof Error ? err : new Error(String(err));

  logger.error("Unhandled error", {
    message: details.message,
    stack: details.stack,
    path: req.path,
  });

  res.status(500).json({ error: "Something went wrong" });
};

app.use(errorHandler);

Some errors are expected or route-specific and should be handled in the route. A missing concert ticket should get a deliberate 404, and invalid checkout input should get a client-safe 400. An unexpected database or filesystem failure belongs at the global boundary, where it can be logged and returned as a safe 500.