mediumConceptScenarioMidSenior
A request fails but your Express error-handling middleware never runs. How do you work out why?
An Express error handler is skipped when it is not registered after routes, lacks the four-argument signature, or an async error never reaches next(). Express 4 needs explicit next(err) for rejected promises; Express 5 forwards them automatically.
4 min readUpdated 2026-08-15
Answer
Short answer
If Express error middleware is not catching errors, check three things: it must have four parameters (err, req, res, next), it must be registered after the routes, and async failures must be passed to next(err) in Express 4. Express 5 forwards rejected async handlers, but ordering and signature still matter.
Mention error handling where it changes the risk, the owner, or the next check.
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