router.get is only for defining subpaths. Consider this example:

var router = express.Router();
app.use('/first', router);  // Mount the router as middleware at path /first
router.get('/sud', smaller);
router.get('/user', bigger);

• If you open /first/sud, then the smaller function will get called.
• If you open /first/user, then the bigger function will get called.
  In short, app.use('/first', router) mounts the middleware at path /first, then router.get sets the subpath accordingly(相应的).

But if we instead use the following:

app.use('/first', fun);
app.get('/sud', bigger);
app.get('/user', smaller);

• If you open /first in your browser, fun will get called,
• For /sud, bigger will get called
• For /user, smaller will get called
  But remember for /first/sud, no function will get called.