`Error-handling middleware`

Special middleware with four arguments that handles errors passed via next(err).

Since Express 4; async auto-forward Express 5 Spec ↗

Syntax

app.use((err, req, res, next) => { ... })

Parameters

Name	Type	Required	Description
`err`	`any`	Yes	The error forwarded from `next(err)` or a thrown/rejected handler.
`req`	`Request`	Yes	The request object.
`res`	`Response`	Yes	The response object.
`next`	`function`	Yes	Pass to the next error handler, or default handler.

Returns

void — Sends the error response.

Examples

app.get('/x', (req, res, next) => {
  next(new Error('boom'));
});

app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(err.status || 500).json({ error: err.message });
});

Output

(500) {"error":"boom"}

// Express 5: rejected async handlers reach here automatically
app.get('/data', async (req, res) => {
  const d = await mightThrow();
  res.json(d);
});
app.use((err, req, res, next) => res.sendStatus(500));

Output

(500 Internal Server Error)

Notes

Identified solely by its four-argument signature, so keep the unused `next`. Register it LAST, after all routes. In Express 5 rejected promises and thrown errors in async handlers are forwarded automatically; in Express 4 you must call `next(err)` yourself. Never leak stack traces to clients in production.