Unit 05

Node.js is a powerful platform used for building scalable network applications. It allows developers to use JavaScript to write server-side code, which was traditionally only used for client-side scripting in web browsers.

Concept of Node.js

Working of Node.js

Node.js operates differently than traditional web-serving techniques:

Key Features of Node.js

Node.js is composed of several key components that work together to provide a high-performance, asynchronous environment for executing JavaScript on the server side.

In Node.js, the http module is a core built-in module that allows Node.js to transfer data over the HTTP protocol. It is primarily used to create a web server that listens to server ports and gives a response back to the client.

Creating a Server — http.createServer()

The http.createServer() method is used to create an instance of an HTTP server.

var http = require('http');

http.createServer(function (req, res) {
  // Logic goes here
}).listen(8080);

The Request Object (req)

The req (Request) argument represents the request from the client — an object containing information about the incoming data from the web browser.

• Purpose: Used to get information such as the URL, headers, and data sent by the client.
• Example: req.url holds the part of the URL after the domain. Visiting localhost:8080/summer makes req.url return /summer.

The Response Object (res)

The res (Response) argument represents the response the HTTP server sends back when it receives a request.

Summary Example

var http = require('http');

http.createServer(function (req, res) {
  res.writeHead(200, {'Content-Type': 'text/plain'}); // Header
  res.write('Hello World!');                           // Body
  res.end();                                           // Finalize
}).listen(8080);

The require() Function

In Node.js, the require() function is a built-in tool used to include (import) code from other files or modules into your current file. It reads a JavaScript file, executes it, and allows you to use its functions and variables — preventing you from writing all code in one single file.

var myModule = require('module_name_or_path');

Types of Modules it Can Load

User-Defined Modules

User-defined modules are custom JavaScript files created by the developer. They help keep the project organized, clean, and reusable. Creating and using a module involves two steps:

• Step 1 — Exporting: To make a function available to other files, publish it using the module.exports keyword.
• Step 2 — Importing: To use that custom file, use require() with the exact file path (using ./ for files in the same folder).

Example

// Create a simple addition function
var addNumbers = function(a, b) {
    return a + b;
};

// Export it so other files can use it
module.exports = addNumbers;

// Require the local module
var mathModule = require('./math.js');

// Use the function
var result = mathModule(5, 10);

// Output: 15
console.log(result);

Node.js can be used to create web servers that handle HTTP requests and responses. It uses a non-blocking, event-driven architecture, making it efficient for handling multiple client requests simultaneously.

createServer() Method

The createServer() method is used to create an HTTP server in Node.js. It is provided by the http module and takes a callback function with req and res objects. The server listens to client requests and sends responses accordingly.

http.createServer(function (req, res) {
    // handle request and response
});

writeHead() Method

The writeHead() method sends HTTP response headers — specifying the status code and content type. It must be called before sending the response body.

res.writeHead(statusCode, { 'Content-Type': 'text/html' });

res.writeHead(200, { 'Content-Type': 'text/html' });

Reading Query String

A query string contains data sent in the URL after ?. Node.js uses the url module to read query strings.

http://localhost:8080/?name=John&age=21

var url = require('url');
var q = url.parse(req.url, true).query;
// q.name → "John"
// q.age  → "21"

Splitting Query String

When using url.parse(req.url, true), Node.js automatically converts the query string into a key-value object.

var qdata = url.parse(req.url, true).query;

// Input:  name=Faizan&age=20
// Result:
{
  name: "Faizan",
  age:  "20"
}

The fs module stands for File System. It is a built-in tool in Node.js that allows you to work with files on your computer. Since it is built-in, you don't need to install anything — just require it.

Common Uses of the fs Module

Reading a File — fs.readFile()

Used to get the content out of a file to display it or use it. 'utf8' tells Node.js to read the file as human-readable text.

var fs = require('fs');

fs.readFile('message.txt', 'utf8', function(err, data) {
  if (err) throw err;
  console.log(data); // Prints the file content
});

Creating / Writing a File — fs.writeFile()

Creates a new file. If the file already exists, it replaces the old content with the new content.

var fs = require('fs');

fs.writeFile('mynewfile.txt', 'Hello students!', function (err) {
  if (err) throw err;
  console.log('File Created!');
});

Appending to a File — fs.appendFile()

Adds new content at the end of an existing file without deleting the old content.

fs.appendFile('mynewfile.txt', ' This is extra text.', function (err) {
  if (err) throw err;
  console.log('Updated!');
});

Deleting and Renaming Files