How does error-handling middleware work in Express?

Answer: Express distinguishes error middleware from normal middleware by its arity: it takes four arguments.

// Registered AFTER all routes
app.use((err, req, res, next) => {
  console.error(err.stack);
  const status = err.statusCode || 500;
  res.status(status).json({ error: err.message || 'Internal Server Error' });
});

How Express reaches it:

• You call next(err) from any middleware/route.
• A synchronous throw in a handler is caught by Express and routed here automatically.
• Async rejections: in Express 5, a rejected promise from an async handler is forwarded automatically. In Express 4, it is not — you must catch and next(err) yourself, often via a wrapper:

const asyncHandler = fn => (req, res, next) =>
  Promise.resolve(fn(req, res, next)).catch(next);

app.get('/users/:id', asyncHandler(async (req, res) => {
  const user = await db.getUser(req.params.id); // rejection → next(err)
  res.json(user);
}));

Best practices:

• Keep a single central error handler; don't scatter response formatting.
• Log full details server-side, but return a safe, generic message to clients (don't leak stack traces or internals in production).
• Combine with custom error classes carrying statusCode so the handler maps errors to the right HTTP code.
• Add a catch-all 404 handler before the error handler for unmatched routes.