The answer
Body parser missing or after routes, no error handler, hardcoded port, CORS misconfigured, .env not loaded, routers not mounted, handlers that forget to respond, misuse of next, sync heavy work in handlers, and console.log in production.
Common Express Server Setup Mistakes (and How to Fix Them)
What are the most common Express setup mistakes?
More FAQs in Common Express Server Setup Mistakes (and How to Fix Them)
Because express.json() was not added or was added after the routes that read req.body. Add app.use(express.json()) before any route that needs to parse JSON request bodies.
Routers need to be mounted in app.js with app.use('/path', router). Creating a router in routes/auth.js is not enough; you must attach it to the Express app.
Your handler did the work but forgot to send a response. Every handler must call res.send, res.json, or res.end. Better: use a wrapper that catches errors and sends a response automatically.
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