What's the difference between application- and router level- middleware when routing in Express?

Question

In the Express docs, it says:

Application level middleware are bound to an instance of express, using app.use() and app.VERB().

Router level middleware work just like application level middleware except they are bound to an instance of express.Router(). The middleware system created at the application level in the example above, can be replicated at the router level using the following code.

In the app provided by the Express generator, I see in the main app.js, there is:

var routes = require('./routes/index'); app.use('/', routes); 

And in ./routes/index.js, I see this:

var express = require('express'); var router = express.Router();  /* GET home page. */ router.get('/', function(req, res, next) {   res.render('index', { title: 'Express' }); });  module.exports = router; 

What is the purpose of app.use passing it to router.get instead of simply using app.get? And generally, what's the difference between app.VERB and router.VERB in terms of routing?

Answer 1

What is the purpose of app.use passing it to router.get instead of simply using app.get?

This is all just designing for modularity. It allows an application to be divided into several smaller loosely-coupled pieces and because they all do not have any direct knowledge of the shared app instance that combines them together, the modularity is achieved.

For example, you could build a whole user account subsystem that handles sign up, login, forgot password, etc, and share it between several different applications by "mounting" it within them via app.use(require("my-user-system")).

That's the only purpose behind this. There no functional, technical, or performance difference otherwise.

And generally, what's the difference between app.VERB and router.VERB in terms of routing?

No difference. The app has it's own main/primary router and app.VERB is just convenience sugar for the equivalent of app.router.VERB.

Answer 2

An example will help here:

In the birds.js:

// birds.js  var express = require('express'); var router = express.Router();  // middleware that is specific to this router router.use(function timeLog(req, res, next) {     console.log('Time: ', Date.now());     next(); }); // define the home page route router.get('/', function (req, res) {     res.send('Birds home page'); }); 

In the app.js:

// app.js  const express = require('express') const app = express()  var birds = require('./birds'); app.use('/birds', birds);  app.listen(3000, () => console.log('app listening on port 3000!\naccess http://localhost:3000/birds/')) 

Now requests to http://localhost:3000/birds/ will go to birds.js file.

This modular approach will make easy to maintain a large code base, as the application grows big.

Source: https://expressjs.com/en/guide/routing.html