What is express.Router?
A mini-app you can mount with app.use('/path', router). It lets you group routes by feature in separate files. Routers have their own middleware stack and can be nested.
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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 without repeating per route.
Yes. Mount one router inside another: router.use('/:userId/posts', postsRouter). Now /users/:userId/posts/... hits postsRouter. Limit nesting to one level; for deeper resources, use a separate top-level router with a filter.
Put public routes (signup, login) before router.use(auth). Then call router.use(auth) and put protected routes below. This keeps the structure clear and prevents accidental auth on public endpoints.
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