Node.js Basics Cheat Sheet

1. Installation

• Download and Install:
  https://nodejs.org/
• Check Installation:

  node -v      # Node.js version
  npm -v       # npm (Node Package Manager) version
  

2. Running Node.js

1. Interactive Mode (REPL):

   node
   

   • Type JavaScript directly into the terminal.

   console.log("Hello, Node!");
   

2. Run a Script:

   node app.js
   

3. Creating a Basic Node.js App

1. Create Project Directory:

   mkdir myapp
   cd myapp
   

2. Initialize Node Project:

   npm init -y
   

3. Create app.js:

   console.log("Node.js App Running!");
   

4. Run App:

   node app.js
   

4. Core Modules

const fs = require('fs');        // File System  
const http = require('http');    // HTTP Server  
const path = require('path');    // Path Utilities  
const os = require('os');        // OS Information  
const events = require('events'); // Event Emitter  

5. Creating an HTTP Server

const http = require('http');

const server = http.createServer((req, res) => {
    res.writeHead(200, { 'Content-Type': 'text/plain' });
    res.end('Hello, World!');
});

server.listen(3000, () => {
    console.log('Server running at http://localhost:3000/');
});

• Visit http://localhost:3000 in your browser.

6. File System (fs)

Reading and Writing Files

const fs = require('fs');

// Read File
fs.readFile('text.txt', 'utf8', (err, data) => {
    if (err) throw err;
    console.log(data);
});

// Write to File
fs.writeFile('output.txt', 'Hello Node.js!', (err) => {
    if (err) throw err;
    console.log('File written!');
});

7. Path Module

const path = require('path');

console.log(__dirname);                 // Directory name
console.log(path.basename(__filename)); // File name
console.log(path.join(__dirname, 'src', 'index.html'));

8. Events and Event Emitters

const EventEmitter = require('events');
const eventEmitter = new EventEmitter();

eventEmitter.on('greet', () => {
    console.log('Hello, Event!');
});

eventEmitter.emit('greet');

9. NPM (Node Package Manager)

Install Packages:

npm install express

Install as Dev Dependency:

npm install nodemon --save-dev

Uninstall Package:

npm uninstall express

Global Installation:

npm install -g nodemon

10. Package.json Overview

• Dependencies: Packages required for production.
• devDependencies: Packages required during development.

{
  "name": "myapp",
  "version": "1.0.0",
  "main": "app.js",
  "dependencies": {
    "express": "^4.18.1"
  },
  "scripts": {
    "start": "node app.js",
    "dev": "nodemon app.js"
  }
}

11. Express.js (Basic Setup)

const express = require('express');
const app = express();

app.get('/', (req, res) => {
    res.send('Welcome to Express!');
});

app.listen(3000, () => {
    console.log('Server running at http://localhost:3000/');
});

12. Middleware in Express

app.use(express.json());

app.use((req, res, next) => {
    console.log(`${req.method} ${req.url}`);
    next();
});

13. API Routes (CRUD Example)

let users = [{ id: 1, name: 'John' }];

app.get('/users', (req, res) => {
    res.json(users);
});

app.post('/users', (req, res) => {
    const newUser = req.body;
    users.push(newUser);
    res.status(201).send('User added');
});

14. Working with JSON

const data = {
    name: "Node.js",
    version: "18.0"
};

const jsonData = JSON.stringify(data);  // Convert to JSON
console.log(JSON.parse(jsonData));      // Parse JSON back to object

15. Handling Promises

const fetchData = () => {
    return new Promise((resolve, reject) => {
        setTimeout(() => resolve('Data Loaded'), 2000);
    });
};

fetchData()
    .then((data) => console.log(data))
    .catch((err) => console.error(err));

16. Asynchronous/Await

async function loadData() {
    const data = await fetchData();
    console.log(data);
}
loadData();

17. Environment Variables

PORT=3000

require('dotenv').config();
console.log(process.env.PORT);

18. Useful Node.js Commands

node app.js             # Run app
nodemon app.js          # Auto-reload app (install nodemon globally)
npm start               # Run start script
npm run dev             # Run dev script
npm list                # List installed packages

19. Error Handling

app.use((err, req, res, next) => {
    console.error(err.stack);
    res.status(500).send('Something broke!');
});

20. Deployment (PM2 Process Manager)

npm install pm2 -g
pm2 start app.js
pm2 list
pm2 stop app
pm2 logs