Getting Started with Node JS

What is NodeJS?

• Definition: NodeJS is an open-source, cross-platform JavaScript runtime environment that allows you to execute JavaScript code outside of a web browser.
• Purpose: It’s 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?

• Definition: 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?

• Definition: 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?

• Using the http Module: 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?

• Definition: 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".

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