ExpressHTTPRequestResponse.js

const express = require('express')
// import the Express module
const app = express()
// create an Express application that has Express' built-in functionality, such as routing

app.get('/', (req, res) => res.send('Hello World!'))
// define a route (this one is a GET request to 'domainname.com/', in this case, 'localhost:3000/'. When the request is recieved, it will respond with the string "Hello World!")

app.listen(3000, () => console.log('Example app listening on port 3000!'))
// this defines the port that the server should run (3000) and logs that it's running. By running this file (' node filename.js'), the server will start, and you'll see 'Hello World!' by visiting localhost:3000/.