Payload limit behavior of express.json middleware in Express 4 vs 5

Answer

When express.json receives a body larger than the configured limit, it immediately calls next(err) inside the middleware itself. That call is synchronous in both Express 4 and Express 5. The error object has type entity.too.large and status 413.

Express 4

The body‑parser implementation that powers express.json performs the size check during the request‑body stream parsing. As soon as the size threshold is crossed, the parser emits an entity.too.large error and synchronously hands it to Express via next(err). Express then skips the rest of the normal middleware stack and looks for the next error‑handling middleware.

Express 5

Express 5’s major change is automatic handling of rejected promises in route handlers. Middleware errors, including the size‑limit error described above, are still propagated synchronously. The new promise plumbing does not touch the synchronous next(err) path, so the 413 error follows the same route as in Express 4.

Practical steps to catch the error

1. const express = require('express');
2. const app = express();
3. app.use(express.json({ limit: '10kb' }));
4. app.use((err, req, res, next) => {
5. if (err.type === 'entity.too.large') {
6. return res.status(413).json({ error: 'Payload too large' });
7. }
8. next(err);
9. });
10. app.post('/upload', (req, res) => res.send('OK'));
11. app.listen(3000);

Sending a body larger than 10 KB will trigger the error handler and return a 413 response in both Express 4 and Express 5.

When you need more info

If the error handler never runs, check that it is defined *after* the express.json middleware and that no earlier middleware consumes the request body or calls next() without forwarding the error. If those conditions are met and the handler still misses the error, the issue is likely elsewhere in the stack.