Difference Between app.use() and router.use() in Express

Question

I was just reading the documentation on express and found these two terms, app.use(); and router.use();

I know app.use(); is u

Answer 1

router.use(); mounts middleware for the routes served by the specific router, app.use(); mounts middleware for all routes of the app (or those matching the routes specified if you use app.use('/ANYROUTESHERE', yourMiddleware());).

Example use case could be an app with one router with standard routes and one router that handles api routes, which need a valid user.

You would then mount the authentication middleware for the api router only with router.use(yourAuthMiddleware());.

If you would have an app though that requires a valid user for all routes, mount the middleware for the app with app.use(yourAuthMiddleware());