Demonstrating Express.js `Router()` and RESTful API

Live Code

Create a routes.js file
Demonstrate creating a new Express app instance and prompt students:
- What would be wrong with initializing a new application?
```
const express = require('express')
const app = express()
```
Introduce 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.
```
const router = require('express').Router()
```
Copy and paste all of the routes from the app.js file
- Update app to router
Export the router
```
module.exports = router
```

Near the top of the app.js file, import in the routes using require
```
const routes = require('./routes.js')
```
Use the routes
```
app.use(routes)
```
For further granularity, we can break the routes file into two separate files. In this case, we would have to require them both in then.
- countriesRouter.js
- citiesRouter.js
```
app.use('/countries', require('./countriesRouter.js'))
app.use('/cities', require('./citiesRouter.js'))
```

POST requests/routes are designated for creating something. For example, let's say we want to create a new city. We can send some data to do so in our HTTP request message in a couple of ways:
- Form data (URL encoded) is sent as strings or arrays, which we must use Express' app.use(express.urlencoded({ extended: false })) to parse easily.
- JSON object, which we must use Express' app.use(express.json()) to parse easily.
Demonstrate using Postman