Express 4 router - node.js

Question

At this point, I am pretty clear on how the Express 4 router works, but I am confused about how the primary router instance declared in the app.js file, like it is immediately below, interacts with other routers in the Express server.
so you start off with this in your app.js file:
var express = require('express');
var router = express.Router();
var app = express();
in your Express 4 app.js file you will then do:
app.use(router); //what is this doing? maybe nothing
then you might do (in the same file app.js):
var loginRoute = require('../routes/login');
var logoutRoute = require('../routes/logout');
app.use('/login',loginRoute);
app.use('/logout',logoutRoute);
and inside loginRoute we have
var express = require('express');
var router = express.Router();
router.get('/',function(req,res,next){
}
router.post('/',function(req,res,next){
}
and in logoutRoute we might have pretty much the same skeleton as loginRoute:
var express = require('express');
var router = express.Router();
router.get('/',function(req,res,next){
}
router.post('/',function(req,res,next){
}
I am warming up to this new router for Express 4, but I still am in the dark about how it works. So my question is, how does the Express router in app.js interact with the other routers?
and what exactly does the line app.use(require('express').Router()) supposed to be doing? My guess is that calling app.use(router); tells Express 4 about your intentions - your intention to use that router instead of the single basic router for app.
But given the code, I don't see how app.use(router) interacts or has anything to do with the routers used by app.use('/login'..) and app.use('/logout'..).
Can Express 4 routers delegate to each other? Can you give an example of this?
I hope your understand my confusion.

Answer 1

One way to think about routers in Express 4+ is as micro-services that can be mounted on either the app itself or on other routers.
For example when mounting on the app itself:
var routerA = express.Router();
var routerB = express.Router();
routerA
.post('/bar', function (req, res, next) {...})
.get('/bar', function (req, res, next) {...});
routerB
.post('/bar', function (req, res, next) {...})
.get('/bar', function (req, res, next) {...});
// Mount routerA at `/foo`
// handles both get and post requests to /foo/bar but not /bar/bar
app.use('/foo', routerA);
// Mount routerB at `/bar`
// handles both get and post requests to /bar/bar but not /foo/bar
app.use('/bar', routerB);
And if you wanted to mount one router to another:
var routerA = express.Router();
var routerB = express.Router();
routerA
.post('/bar', function (req, res, next) {...})
.get('/bar', function (req, res, next) {...});
routerB
.post('/bar', function (req, res, next) {...})
.get('/bar', function (req, res, next) {...});
// Mount routerB to routerA at `/foo`
// handles both get and post requests to /foo/bar at routerA's mount point
routerA.use('/foo', routerB);
// Mount routerA at `/foobar`
// handles both get and post requests to /foobar/bar with routerA
// and /foobar/foo/bar with routerB
app.use('/foobar', routerA);
Using routers in this way can make your code a bit more modular while also allowing for things like this.
Regarding the use of app.use(router) without anything assigned to that router: it's useless. You could remove it without an impact to the application. The routers that are mounted after are the ones actually doing the routing.