What are common Express middleware mistakes?
Forgetting next(), calling next() after responding, wrong order, sync heavy work, repeating middleware per route, sync throws in try/catch, async middleware without wrapper, mutating req.body without validating, error handler in the middle, and no error handler at all.
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More FAQs in Common Express Middleware Mistakes (and How to Fix Them)
A middleware forgot to call next() or send a response. Every middleware must do one or the other. Without it, the request never ends and the client eventually times out.
No. Once you send a response, the cycle is over. Calling next() can lead to downstream code trying to respond again, causing 'headers already sent' errors. Use return res.json(...) to be safe.
Express 4 does not catch promise rejections from async middlewares. Wrap with Promise.resolve(fn(req, res, next)).catch(next) so rejections go to the error handler. Express 5 catches them automatically.
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