Why is module.exports=router is needed? - node.js

Question

In my node.js server conf file, I set app.use('/user', user.js), which maps /user route to a user.js file.
Then I create subrouting in my user.js file to handle my get or post requests.
My question is: what's the responsibility of module.exports=router at the end of this file?
If I remove it, routing stops working, so I don't understand if it is here to tell my server conf file that there are sub paths in user.js?
var express = require('express');
var user = require('../../models/user');
var db = require('../../models/index');
var router = express.Router();
router.get('/addUser',function (req, res, next) {
db.user.create(req.body)
.then(user => res.json({
data: user,
}))
.catch(error => res.json({
error: true,
data: [],
error: error
}));
});
module.exports = router;

Answer 1

When you do
var user = require('../../models/user');
the user object would get whatever is being exported from the module in user.js. So in user.js, the module.exports=router is mapping a router and all logic that's required to map /user (along with the right callbacks etc...)
If you remove it, your require statement can't acquire an exported object from that module, which is why it would fail. Your user object will be nullified effectively.
Check out here for more info: https://www.tutorialsteacher.com/nodejs/nodejs-module-exports

Answer 2

A router cannot listen(PORT) for requests on its own. The router is useful when you have lots of routes. It's useful for separating your app into multiple modules.
const app = express()
app.listen(port)
app is listening for the requests(not the Router) while your user.js is just a separate js file with some codes.
In module.export ,module is a variable that represents the current module and export is an object. Anything you assign to the module.exports will be expose as a module.
copied: Module in Node.js is a simple or complex functionality organized in single or multiple JavaScript files which can be reused throughout the Node.js application.
Once you do module.export = Router Now you have a newly created module. In nodeJs, you need to require a module before use it. const user = require('./user.js') will do that process. Once you require the node module into your app, you need to tell it to execute by app.use('/' , user)
Or you can do something like below too
in your user.js file,
var user = require('../../models/user');
var db = require('../../models/index');
module.export = (app) =>{
app.get('/addUser',function (req, res, next) {
db.user.create(req.body)
.then(user => res.json({
data: user,
}))
.catch(error => res.json({
error: true,
data: [],
error: error
}));
});
}
in your main index.js,
const app = express()
require('./user.js')(app)