This content originally appeared on DEV Community and was authored by Achal Tiwari

What is Node.js?

Let's dive into Node.js! This article covers many concepts, so grab your favorite coffee or tea, sit back, and let's get started. If you're in a rush, feel free to bookmark it for later. Let's begin!

Introduction to Node.js

Node.js is an open-source, cross-platform JavaScript runtime environment that lets you run JavaScript code on the server side. Here’s why it’s important and how it works:

Node.js is an open-source server environment
An open source server environment is a computer system that processes requests via HTTP
Node.js is a cross-platform, open-source JavaScript runtime environment To understand the magic of Node.js, let's compare it with PHP or ASP in handling file requests:

Traditional PHP or ASP Approach:

Sends the task to the computer's file system.
Waits while the file system opens and reads the file.
Returns the content to the client.
Ready to handle the next request. ##### Node.js Approach:
Sends the task to the computer's file system.
Ready to handle the next request.
When the file system has opened and read the file, the server returns the content to the client. It runs on a single thread using non-blocking, asynchronous programming, which makes it very memory-efficient.

Understanding Node.js Modules

Modules in Node.js are like libraries in JavaScript. Here are the types of modules:

Core Modules: Built into Node.js and available without any additional installation.
Local Modules: Created within your project.
Third-Party Modules: Available through the npm registry and can be installed using npm.

Core Modules

Core modules are part of the Node.js framework. Some examples include:

fs: File System module for interacting with the file system.
http: HTTP module for creating web servers.
path: Path module for handling and transforming file paths.
os: Operating System module for fetching OS-related information.

Example: Using the fs Module

const fs = require('fs');
// Reading a file
fs.readFile('example.txt', 'utf8', (err, data) => {
  if (err) {
    console.error(err);
    return;
  }
  console.log(data);
});

Local Modules

They can be a single file or a collection of files organized in a directory.

Example: Creating and Using a Local Module

Create a Local Module

// maths.js
function add(a, b) {
  return a + b;
}

function subtract(a, b) {
  return a - b;
}

module.exports = { add, subtract };

Use the Local Module

// app.js
const math = require('./math');

console.log(math.add(5, 3)); // Output: 8
console.log(math.subtract(5, 3)); // Output: 2

Third-Party Modules

Example: Installing and Using a Third-Party Module

Install the Module

npm install lodash

Use the Installed Module

const _ = require('lodash');

const array = [1, 2, 3, 4, 5];
const reversedArray = _.reverse(array.slice());
console.log(reversedArray); // Output: [5, 4, 3, 2, 1]

Module Caching

Node.js caches modules after they are loaded for the first time.
This means that subsequent require calls for the same module will return the cached version.

Node.js HTTP Module

The http module in Node.js is a core module that provides functionality to create web servers and handle HTTP requests and responses.

const http = require('http');

// Using the `http.createServer()` method to create an HTTP server
const server = http.createServer((req, res) => {  // a callback function to handle incoming requests and send responses.
  res.statusCode = 200; // HTTP status code
  res.setHeader('Content-Type', 'text/plain'); // Set response headers
  res.end('Hello, World!\n'); // Send response
});

// Make the server listen on port 3000
server.listen(3000, '127.0.0.1', () => {
  console.log('Server is running at http://127.0.0.1:3000/');
});

Handling Different Request Methods

HTTP servers can handle different types of requests, such as GET, POST, PUT, and DELETE.
Important Methods:

request.on(event, callback):
- This method is used to set up event listeners for various events that may occur during an HTTP request.
- The event parameter specifies the event to listen for, such as 'data' for when a chunk of data is received, or 'end' for when the entire response has been received.
- The callback function is called when the specified event occurs.
request.end([data], [encoding], [callback]):
- This method is used to send the request to the server.
- It finalizes the request, so no more data can be written to the request.
- Optionally, you can pass data to be sent in the request body, encoding to specify the encoding of the data, and a callback function that will be called when the request has been sent.

const http = require('http');

const server = http.createServer((req, res) => {
  if (req.method === 'GET') {
    res.statusCode = 200;
    res.setHeader('Content-Type', 'text/plain');
    res.end('Received a GET request\n');
  } 
  else if (req.method === 'POST') {\
    let body = '';
    req.on('data', chunk => {
    // chunk will be a buffer like <Buffer 6d 65 73 ... >, 
    // therefore we convered it to String
      body += chunk.toString(); // Convert Buffer to string
    });

    req.on('end', () => {
      res.statusCode = 200;
      res.setHeader('Content-Type', 'application/json');
      res.end(`Received a POST request with body: ${body}\n`);
    });
  } 
  else {
    res.statusCode = 405; // Method Not Allowed
    res.setHeader('Content-Type', 'text/plain');
    res.end('Method Not Allowed\n');
  }
});

// Listen on port 3000
server.listen(3000, '127.0.0.1', () => {
  console.log('Server is running at http://127.0.0.1:3000/');
});

We can use test this code by the use of curl command, to send a post request to our page.

Events

Key Concepts

EventEmitter:
- Core of Node asynchronous event-driven architecture
- It is a module that facilitates communication/interaction between objects in Node.
- Emitter objects emit named events that cause previously registered listeners to be called.
- So, an emitter object basically has two main features:
  - Emitting name events.
  - Registering and unregistering listener functions.

Creating an event

const EventEmitter = require('events');
// Creating an event
const myEmitter = new EventEmitter();

// Define an event listener
const greetListener = (name) => {
  console.log(`Hello, ${name}!`);
};

myEmitter.on('greet', greetListener);

// Emit the event, this will emit the event named 'greet'
// passing 'World' as an argument
myEmitter.emit('greet', 'World');

console.log(myEmitter.listenerCount('greet')); // 1

// Removing the listener
myEmitter.off('greet', greetListener);

// Now, even if you emit the event, nothing will happen
myEmitter.emit('greet', 'World');

console.log(myEmitter.listenerCount('greet')); // 0
console.log(myEmitter.rawListeners('greet')); // []

//-----------Output----------
Hello, World!
1
[ [Function: greetListener] ]
0
[]

Built-in Events

const http = require('http');
const server = http.createServer((req, res) => {
  res.end('Hello, World!');
});

server.on('request', (req, res) => {
  console.log(`Request received: ${req.url}`);
});

server.listen(3000, () => {
  console.log('Server listening on port 3000');
});

Serving Static Files

This code will display your image1.png when at localhost:3000/ or 127.0.0.1:3000

const http = require('http');
const fs = require('fs');
const path = require('path');

// Create the server
const server = http.createServer((req, res) => {
  const filePath = path.join(__dirname, req.url === '/' ? 'image1.png' : req.url);
  const extname = path.extname(filePath);
  let contentType = 'text/html';

  // Set content type based on file extension
  switch (extname) {
    case '.js':
      contentType = 'text/javascript';
      break;
    case '.css':
      contentType = 'text/css';
      break;
    case '.json':
      contentType = 'application/json';
      break;
    case '.png':
      contentType = 'image/png';
      break;
    case '.jpg':
      contentType = 'image/jpg';
      break;
  }

  // Read and serve the file
  fs.readFile(filePath, (err, content) => {
    if (err) {
      if (err.code === 'ENOENT') {
        // File not found
        res.statusCode = 404;
        res.setHeader('Content-Type', 'text/html');
        res.end('<h1>404 Not Found</h1>');
      } else {
        // Server error
        res.statusCode = 500;
        res.setHeader('Content-Type', 'text/html');
        res.end('<h1>500 Server Error</h1>');
      }
    } else {
      // Serve the file
      res.statusCode = 200;
      res.setHeader('Content-Type', contentType);
      res.end(content);
    }
  });
});

// Listen on port 3000
server.listen(3000, '127.0.0.1', () => {
  console.log('Server is running at http://127.0.0.1:3000/');
});

URL Module

var url = require('url');  
var adr = 'http://localhost:8080/default.htm?year=2017&month=february';  
var q = url.parse(adr, true);  

console.log(q.host); //returns 'localhost:8080'  
console.log(q.pathname); //returns '/default.htm'  
console.log(q.search); //returns '?year=2017&month=february'  

var qdata = q.query; //returns an object: { year: 2017, month: 'february' }  
console.log(qdata.month); //returns 'february'

Query using URL Parameters

const http = require('http');
const url = require('url');

const server = http.createServer((req, res) => {
  // Parse the URL
  const parsedUrl = url.parse(req.url, true);
  // Extract the pathname (e.g., '/filename.ext')
  const pathname = parsedUrl.pathname;
  // Extract the filename from the pathname
  const filename = pathname.substring(pathname.lastIndexOf('/') + 1);
  // Correctly display the filename with spaces and other special characters
  const decodedFilename = decodeURIComponent(filename); 

  // Set the response header and status code
  res.statusCode = 200;
  res.setHeader('Content-Type', 'text/html');

  // Send the response
  res.end(`<h1>Filename: ${decodedFilename}</h1>`);
});

// Listen on port 3000
server.listen(3000, '127.0.0.1', () => {
  console.log('Server is running at http://127.0.0.1:3000/');
});

Open your web browser and navigate to http://127.0.0.1:3000/filename.ext (replace filename.ext with any filename you want to test). The server will display the filename extracted from the URL on the webpage.

Thank you for sticking through this long read. If you have any questions or need further clarification, feel free to reach out in the comments. Happy coding!

This content originally appeared on DEV Community and was authored by Achal Tiwari

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