What is the difference between “express.Router” and routing using “app.get”?

Here’s a simple example:

// myroutes.js
var router = require('express').Router();

router.get("https://stackoverflow.com/", function(req, res) {
    res.send('Hello from the custom router!');
});

module.exports = router;

// main.js
var app = require('express')();

app.use('/routepath', require('./myroutes'));

app.get("https://stackoverflow.com/", function(req, res) {
    res.send('Hello from the root path!');
});

Here, app.use() is mounting the Router instance at /routepath, so that any routes added to the Router instance will be relative to /routepath.