Getting Started with NodeJS

What is NodeJS?

NodeJS is an open-source, cross-platform JavaScript runtime environment that allows you to execute JavaScript code outside of a web browser.

NodeJS is primarily used for server-side scripting, where JavaScript is used to produce dynamic web content before the page is sent to the user's web browser.

• Key Features:
  • Event-Driven Architecture: NodeJS uses an event-driven, non-blocking I/O model, making it efficient and lightweight.
  • Single-Threaded: Although single-threaded, NodeJS handles concurrent operations using its asynchronous nature and the event loop.
  • Built on V8: NodeJS is built on Google Chrome's V8 JavaScript engine, making it extremely fast in executing JavaScript code.

How Does NodeJS Work in the Background?

Event Loop

• NodeJS operates on a single-threaded event loop, which allows it to handle multiple concurrent requests without blocking the thread.
  • Phases of Event Loop:
  • Timers: Executes callbacks scheduled by setTimeout() and setInterval().
  • Pending Callbacks: Executes I/O callbacks deferred to the next loop iteration.
  • Idle, Prepare: Used internally by NodeJS.
  • Poll: Retrieves new I/O events and executes I/O-related callbacks.
  • Check: Executes callbacks scheduled by setImmediate().
  • Close Callbacks: Executes close event callbacks.

Non-blocking I/O

NodeJS handles I/O operations asynchronously, meaning it doesn’t wait for operations to complete before moving on to the next task.

Example:

 const fs = require('fs'); console.log("Start"); // Reading a file asynchronously fs.readFile('example.txt', 'utf8', (err, data) => { if (err) throw err; console.log(data); }); console.log("End"); 

Output:

 Start End (contents of example.txt) 

Explanation: NodeJS continues to execute the code after the fs.readFile() function is called, without waiting for the file to be read. This demonstrates its non-blocking I/O model.

What are Modules in NodeJS?

Modules are blocks of encapsulated code that communicate with an external application based on their related functionality.

• Types of Modules:
  • Core Modules: Built into NodeJS (e.g., fs, http, path, etc.).
  • Local Modules: Created by users to organize and structure code.
  • Third-Party Modules: Installed via npm (e.g., express, lodash).

Ways of Importing and Exporting Modules in JavaScript and NodeJS

In JavaScript (ES6 Modules):

• Exporting:

 // Named export export const add = (a, b) => a + b; // Default export export default function subtract(a, b) { return a - b; } 

• Importing:

 // Named import import { add } from './math.js'; // Default import import subtract from './math.js'; 

In NodeJS (CommonJS Modules):

• Exporting:

 // Using module.exports module.exports.add = (a, b) => a + b; // Using exports shorthand exports.subtract = (a, b) => a - b; 

• Importing:

 // Importing modules const math = require('./math.js'); const add = math.add; const subtract = math.subtract; 

What is File Handling in NodeJS?

File handling in NodeJS allows you to work with the file system on your machine, including reading, writing, updating, and deleting files.

Important Functions:

• Some of the most important fs Module Functions:
  • fs.readFile(): Asynchronously reads the contents of a file.
  • fs.writeFile(): Asynchronously writes data to a file, replacing the file if it already exists.
  • fs.appendFile(): Appends data to a file. If the file does not exist, it creates a new file.
  • fs.unlink(): Deletes a file.
  • fs.rename(): Renames a file.

Example:

 const fs = require('fs'); // Writing to a file fs.writeFile('example.txt', 'Hello, NodeJS!', (err) => { if (err) throw err; console.log('File written successfully.'); // Reading the file fs.readFile('example.txt', 'utf8', (err, data) => { if (err) throw err; console.log('File contents:', data); // Appending to the file fs.appendFile('example.txt', ' This is an appended text.', (err) => { if (err) throw err; console.log('File appended successfully.'); // Renaming the file fs.rename('example.txt', 'newExample.txt', (err) => { if (err) throw err; console.log('File renamed successfully.'); // Deleting the file fs.unlink('newExample.txt', (err) => { if (err) throw err; console.log('File deleted successfully.'); }); }); }); }); }); 

Output:

 File written successfully. File contents: Hello, NodeJS! File appended successfully. File renamed successfully. File deleted successfully. 

How to Build a Server in NodeJS?

The http module is a core module in NodeJS that allows you to create a server that listens for requests on a specific port and sends responses.

Example:

 const http = require('http'); // Creating a server const server = http.createServer((req, res) => { res.statusCode = 200; res.setHeader('Content-Type', 'text/plain'); res.end('Hello, World!\n'); }); // Listening on port 3000 server.listen(3000, '127.0.0.1', () => { console.log('Server running at http://127.0.0.1:3000/'); }); 

Output:

 Server running at http://127.0.0.1:3000/ 

Explanation: The server responds with "Hello, World!" every time it receives a request. The server listens on localhost (127.0.0.1) at port 3000.

What is an HTTP Module?

The http module in NodeJS provides functionalities to create HTTP servers and clients.

Important Functions?

• Some of the most important functions of HTTP module are:
  • http.createServer(): Creates an HTTP server that listens to requests and sends responses.
  • req.method: Retrieves the request method (GET, POST, etc.).
  • req.url: Retrieves the URL of the request.
  • res.writeHead(): Sets the status code and headers for the response.
  • res.end(): Signals to the server that all of the response headers and body have been sent.

Example:

 const http = require('http'); const server = http.createServer((req, res) => { if (req.url === '/') { res.writeHead(200, { 'Content-Type': 'text/plain' }); res.end('Welcome to the homepage!\n'); } else if (req.url === '/about') { res.writeHead(200, { 'Content-Type': 'text/plain' }); res.end('Welcome to the about page!\n'); } else { res.writeHead(404, { 'Content-Type': 'text/plain' }); res.end('404 Not Found\n'); } }); server.listen(3000, '127.0.0.1', () => { console.log('Server running at http://127.0.0.1:3000/'); }); 

Output:

• If you navigate to http://127.0.0.1:3000/, the server will display "Welcome to the homepage!".
• If you navigate to http://127.0.0.1:3000/about, the server will display "Welcome to the about page!".
• If you navigate to any other URL, the server will display "404 Not Found".

To learn about URLs, HTTP Methods, Express Framework, and Versioning, please click here.