The answer
In controllers or services, not in routes. Routes should be thin: they declare the endpoint and bind it to a controller. router.get('/', listUsers) is good. Putting logic in the router makes the file hard to maintain.
Express Router: Grouping and Mounting Explained
Where should business logic live in an Express app?
More FAQs in Express Router: Grouping and Mounting Explained
For grouping routes by feature in separate files. You create a router in routes/users.js, add routes to it, and mount it in app.js with app.use('/users', usersRouter). This keeps app.js clean as features grow.
Use app.use('/path', router). The router's paths are then relative to the mount path. For example, router.get('/:id') mounted at /users responds to /users/:id.
Yes. Use router.use(middleware) at the top of the router file. Every route in the router now runs that middleware. Useful for auth and role checks across a feature.
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