Your first Javascript code in terminal 😃

Prerequisites

• Node.js installed in your system Nodejs website
• Here is a mini tutorial on how to install Node.js in your system
• type node -v in terminal to check if it is installed

Lets write your first line of code

• open up your terminal make a directory hello-modules

  mkdir hello-modules
  

  cd hello-modules
  

  npm init -y
  

• it will generate a file named package.json

  {
  "name": "hello-modules",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC"
  }
  

• now create two new files
  first one -> hello-modules/index.js
  is it necessary to create a file with the same name mentioned in package.json
  "main": "index.js" ? NO.
  If you are not going to publish it to npm using npm publish
  This field "main" does not serve any purpose.
  second one -> hello-modules/universe.js
  

• and your directory will look like
  

  /**
  * hello-modules
  * -- index.js
  * -- package.json
  * -- universe.js
  */
  

• write some code in hello-modules/index.js
  

  console.log('hello modules from index.js')
  

• then in terminal type

  node index.js
  

  *make sure your terminal is in current directory ...\hello-modules>

• your terminal would give output

  hello modules from index.js
  

  Woohoo, your javascript code now runs in the terminal.

  That's why Node.js® is called JavaScript runtime.

Now its time for fancy stuffs like

module.exports & require()

Let me clear you a few things before we go ahead

When you start doing cool stuffs with javascript in terminal instead of browser.

you write javascript code in file like index.js, that file is known as a module.

Now there are two types of modules

• CommonJs module
• ECMAScript module

by default node supports Commonjs module system

What it feels like writing code in CommonJS module system

you can communicate between two files ( modules ) and use one another codes, functions, classes

• go to hello-modules/universe.js
  

  // declare a const 
  const milkyWay = " it has billions of stars "
  // and export it with a magic spell
  module.exports = milkyWay;
  

• in hello-modules/index.js
  

  // magic spell to get that const from another file (module)
  // is to make that module (file) available in this file first
  const milkyWaySomeWhereElse = require('./universe.js');
  // now console it to terminal
  console.log(milkyWaySomeWhereElse);
  

• now in terminal type

  node index.js
  

• will give output like

  it has billions of stars
  

  Congrats 🎉, You just used two magical spells exports and require

Lets do some experiments 🧪

Experiment #1

• clear everything from hello-modules/universe.js & make it empty
  and in hello-modules/index.js write

  const milkyWaySomeWhereElse = require('./universe.js');
  console.log(milkyWaySomeWhereElse);
  

• in terminal type

  node index.js
  

• it will give output like

  {}
  

Experiment #2

• in hello-modules/universe.js write

  const solarSystem = "It has a planet called Earth";
  const pandora = "this planet exists in movie Avatar";
  module.exports.solarSystem = solarSystem;
  module.exports.pandora = pandora;
  // you can remove module. from above lines
  // exports.solarSystem = solarSystem;
  // module.exports.pandora = pandora;
  // they will work the same way
  

• then in hello-modules/index.js keep it same

  const milkyWaySomeWhereElse = require('./universe.js');
  console.log(milkyWaySomeWhereElse);
  

• in terminal type

  node index.js
  

• you will get an output

  {
  solarSystem: 'It has a planet called Earth',
  pandora: 'this planet exists in movie Avatar'
  }
  

  from this experiment, what I conclude is
  

  • if we export from a module (file) like
    module.exports.solarSytem = solarSystem
module.exports.pandora = pandora
    

  • and require from another file (module)
    const milkyWaySomeWhereElse = require('./universe.js')
    we get an object with keys pandora and solarSystem
    

  • and we access them like we do with objects (Associative Array)
    

    // either by dot Notation
       const pandora = milkyWaySomeWhereElse.pandora;
       const solarSystem = milkyWaySomeWhereElse.solarSystem;
    // or from bracket Notation
       const pandora =  milkyWaySomeWhereElse["pandora"];
       const solarSystem = milkyWaySomeWhereElse["solarSystem"];
    // or by destructuring assignment 
    // most popular one used in modules while importing
       const { pandora, solarSystem } = milkyWaySomeWhereElse;
    // most of the times you will see
       const { pandora, solarSystem } = require('./universe.js');
    

Experiment #3

• in hello-modules/universe.js write

  const solarSystem = "It has a planet called Earth";
  const milkyWay = " it has billions of stars "
  module.exports.solarSystem = solarSystem;
  module.exports =  milkyWay;
  

• keep hello-modules/index.js same

  const milkyWaySomeWhereElse = require('./universe.js');
  console.log(milkyWaySomeWhereElse);
  

• in terminal type

  node index.js
  

• the output you will get is

  it has billions of stars
  

from this experiment, what I conclude is

• in hello-modules/universe.js
module.exports = milkyWay;
  only this line worked and milkyWay got exported

• whatever you put in place of milkyWay get exported as it is from that whole file (module), leaving other exports like module.exports.solarSystem = solarSystem useless
  

• In above two experiments, we have seen a file (module) exports a object.
  It is not always the case.
  Experiment #1 and #2 were special cases.
  
• You can export anything from a module ( lets say from hello-modules/universe.js ) using module.exports = anything.
  

• And require ( import that module) from hello-modules/index.js like
  

  const milkyWaySomeWhereElse = require('./universe.js');
  console.log(milkyWaySomeWhereElse);
  

  ---

• in hello-modules/universe.js

  module.exports = "you can export a string";
  

  milkyWaySomeWhereElse will be equal to "you can export a string"

  ---

  module.exports = ["an", "array", "with", 1, "boolean", "value", true];
  

  now milkyWaySomeWhereElse will be equal to ["an", "array", "with", 1, "boolean", "value", true]

  ---

  module.exports = {
  sky:"it is blue",
  hello:"you can export a object"
  };
  

  In this case milkyWaySomeWhereElse will be equal to
  { sky:"it is blue", hello:"you can export a object" }

  ---

  module.exports = function(){
  return "you can a export a function which returns some value";
  }
  // or
  // module.exports = function(){
  //  console.log("you can a export a function which do not returns any value");
  // }
  

  milkyWaySomeWhereElse is a function now
  the function is declared only and get assigned to another variable,
  but not called yet!
  or not executed yet
  This function is same as normal functions in javascript,
  to call or execute write
  milkyWaySomeWhereElse()
console.log(milkyWaySomeWhereElse()) will output the string
  "you can a export a function which returns some value"

---

• In hello-modules/universe.js you can export a class.
  

  class Planet {
      constructor(name) {
        this.name = name;
    }
  
    getName() {
        return this.name;
    }
  
    sayHiTo(name) {
        console.log(`Hi ${name}, I'm ${this.name}`);
    }
  }
  
  module.exports = Planet;
  

• from hello-modules/index.js
  

  const Planet = require('./universe.js');
  
  const mars = new Planet('Mars');
  
  console.log(mars.getName());
  mars.sayHiTo('Elon Musk');
  

• in terminal type

  node index.js
  

• output would be like

  Mars
  Hi Elon Musk, I'm Mars
  

  ---

  Its your turn 🎯

  I have written a coding-challenge based on this topic.

• Go ahead make modules ( javascript files ) and practice with this coding challenge.
  
• Do cool stuffs.
  
• Happy Learning !
  

NOTE: I will write about ECMAScript (ES6) modules in next post.