express.Router() 与多个 express() 对象之间的区别？

Question

I know using the new express 4 router we can organize multiple route paths into different files like this:

// In cars.js
 const router = express.Router();
 router.get('/brands', function(req, res) { ... });
 router.get('/models', function(req, res) { ... });
 module.exports = router;

// In animals.js
const routerTwo = express.Router();
routerTwo.get('/domestic', function(req, res) { ... });
routerTwo.get('/wild', function(req, res) { ... });
module.exports = routerTwo;

// In main.js
app.use('/cars', router);  // matches `/cars/brands`, `/cars/models`.
app.use('/animals', routerTwo); // matches `animals/domestic`, `animals/wild`.

Now I want to know if I change the express.Router() with simply express() in both cars.js and animals.js file, it'll work because both acts as new instance of express() which we called mini-app. Then what is the use of new express.Router() object?

Aren't the same methods exposed in express.Router() object, with their functionality, achievable via multiple express() instance like modular router handlers etc.?

Answer 1

In simplest terms, Router is a lightweight version of the express app, or as Express docs put it , a mini express application.

The global express object comes with many more resources to support views and various settings while the router basically provides the routing APIs like .use , .get , .param , and route . A router object represents an isolated instance of middleware and routes and is only capable of performing middleware and routing functions, which makes it perfect for efficiently modularizing your route handling.

Now I want to know if I change the express.Router() with simply express() in both cars.js and animals.js file, it'll work because both act as new instance of express() which we called mini-app. Then what is the use of new express.Router() object?

Well, you should still be able to use express() instead of express.Router() for isolated routing purposes but with that you are also bringing much extra code that you probably won't use so it's an unnecessary overhead.

That is why the express.Router() was created: to provide route modularization without the overhead of creating an additional app.

Related: https://en.wikipedia.org/wiki/Liskov_substitution_principle