NodeJS = asynchronous event driven JavaScript runtime, Node is designed to build scalable network applications. In the following "hello world" example, many connections can be handled concurrently. Upon each connection the callback is fired, but if there is no work to be done Node is sleeping

*******It has a built in HTTP server library --- you don't need a separate web server like Apache --- the nodeJS application can do the http listening -----***********************

simple NodeJS application that is an http listener and always returns Hello World

const http = require('http');
const hostname = '127.0.0.1';
const port = 3000;
 const server = http.createServer((req, res) => {
    res.statusCode = 200;
    res.setHeader('Content-Type', 'text/plain');
    res.end('Hello World\n');  });

 server.listen(port, hostname, () => {
    console.log(`Server running at http://${hostname}:${port}/`);
  });  

This is in contrast to today's more common concurrency model where OS threads are employed. 

• Created by Ryan Dahl in 2009
  • Development && maintenance sponsored by Joyent
  • Licence MIT
  • Last release : 0.10.31
  • Based on Google V8 Engine
  • +99 000 packages
  

2 Ways to create a module

module.exports = {
    sayHelloInEnglish: function() {
      return "HELLO";    },
    sayHelloInSpanish: function() {
      return "Hola";    }
  };

// greetings.js  
 var exports = module.exports = {};
 exports.sayHelloInEnglish = function() {
    return "HELLO";  };
 exports.sayHelloInSpanish = function() {
    return "Hola";  };

// main.js
  var greetings = require("./greetings.js");

// "Hello"
  greetings.sayHelloInEnglish();

// "Hola"
    greetings.sayHelloInSpanish();