README.md

Node.js Module System (Notes App)

1. Introduction

In this section the focus is going to be on one of the most fundamental features of Nodejs, the Nodejs Module System. It allows us to load functionality into our app and use it to do interesting things (read or write from the file system, connect to a database or start up a web server...).

There is three different ways to use it:

1. Load a core Nodejs module (included in the node installation).
2. Load in third party modules written by other developers.
3. Load in modules created by ourselves.

2. Importing Nodejs Core Modules

We're going to explore the File system module – this module is going to allow us to access the operating systems file system. We'll be able to read and write, append files and figure out if a given file or directory exists.

fs.writeFile(file, data[, options], callback) (async) fs.writeFileSync(file, data[, options]) (sync)

This method is used to write some data to a file on our file system from the Nodejs application.

// node-course/notes-app/app.js
const fs = require('fs');

fs.writeFileSync('notes.txt', 'This file was created by Nodejs!');
fs.appendFileSync('notes.txt', '\nThis line should be append');

3. Importing Your Own Files

// node-course/notes-app/utils.js
console.log('utils.js');

const name = 'Max';

module.exports = name;

// node-course/notes-app/app.js
const name = require('./utils.js');

console.log(name);

---

// node-course/notes-app/utils.js
console.log('utils.js');

const add = function (a, b) {
  return a + b;
};

module.exports = add;

// node-course/notes-app/app.js
const add = require('./utils.js');

const sum = add(4, -2);
console.log(sum);

---

// node-course/notes-app/notes.js
const getNotes = () => {
  return 'Your notes...';
};

module.exports = getNotes;

// node-course/notes-app/app.js
const getNotes = require('./notes.js');

const msg = getNotes();

console.log(msg);

4. Importing npm Modules

node -v
# v12.18.0
npm -v
# 6.14.7

npm init to initialize npm. It creates a package.json file. This file is going to be used to manage all of the dependencies that our application needs to run.

Then, let's install a npm package – validator.

npm i validator
# same as -> npm install validator

When we ran that command two things happened: package-lock.json is added and node_modules is created.

node_modules is a folder which contains all of the code for the dependencies we installed. We have a single directory validator. That's the package we installed. It should never be manually editing. NPM is maintaining this directory.

package-lock.json is a file which contains extra information, making NPM a bit faster and a bit more secure. It lists out the exact versions of all our dependencies as well as where they were fetched from. And we also have a char hash making sure that we're getting the exact code that we got previously if we were to install a dependency again. Once again this is not a file we should ever be editing. Once again this will be maintained by NPM.

// node-course/notes-app/app.js
const validator = require('validator');

console.log(validator.isEmail('[email protected]'));

5. Printing in Color

# install a specific version here (just for the example)
npm i [email protected]

Loading chalk into app.js:

const chalk = require('chalk');
const log = console.log;

console.log(chalk.green('Success!'));
// We can be a bit more creative – and composing multiple styles using the chainable API
log(chalk.green.inverse.bold('Success!'));

6. Global npm Modules and nodemon

Let's install nodemon globally.

# install globally
npm i nodemon -g

# test if nodemon exists
nodemon -v

# launch app with nodemon
nodemon app.js