express.Router() vs express() in express

Question

As mentioned in express routing guide and this answer, we can create "mini-app" and use it from the main app. However I saw a code where it uses app instead of router in the module

app.js

var express = require('express');
var userRoutes = require('./routes/user');

var app = express();

app.use('/user', userRoutes);

module.exports = app;

routes/user.js

var express = require('express');
var app = express(); // not express.Router() !!

app.get('/:name', function(req, res) {
  var userName = req.params.name;
  res.render('user.jade', {
    userName: userName
  });
});

module.exports = app;

I assumed the correct usage in routes/user.js should be

router = express.Router()

instead of

app = express()

but app = express() also works! what are the differences and why router = express.Router() is better?

Answer 1

When you are working with a server where there are many routes, it can be confusing to leave them in a Main file together. The let router = express.Router() option works differently than let app = express().

While the app returns an app object, router will return a small app fragment, similar to the app, where you will use logic to call them later on the Main.

The most important, about your question, is that a router, which is isolated, will not interfere with others in the application, being a single environment.

https://expressjs.com/en/api.html#router

A router object is an isolated instance of middleware and routes. You can think of it as a “mini-application,” capable only of performing middleware and routing functions. Every Express application has a built-in app router.

A router behaves like middleware itself, so you can use it as an argument to app.use() or as the argument to another router’s use() method.