Node JS
Interview Questions with Answers

21. How do you create a simple HTTP server in Node.js?

You can create an HTTP server using the built-in http module. The createServer() method handles incoming requests and responses.

Example:

const http = require('http');
const server = http.createServer((req, res) => {
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  res.end('Hello from Node.js Server!');
});
server.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});

22. How do you send an HTTP response in Node.js?

You use the response object (res) methods like writeHead() to set headers and end() to send the response body.

Example:

const http = require('http');
http.createServer((req, res) => {
  res.writeHead(200, { 'Content-Type': 'text/html' });
  res.end('<h1>Welcome to My Server</h1>');
}).listen(4000);

23. How do you handle query parameters in a Node.js HTTP server?

You use the built-in url module to parse the request URL and extract query parameters.

Example:

const http = require('http');
const url = require('url');
http.createServer((req, res) => {
  const query = url.parse(req.url, true).query;
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  res.end(`Hello ${query.name || 'Guest'}`);
}).listen(5000);

**If you visit http://localhost:5000/?name=Neha, it will respond with:

Hello Neha

24. How do you handle POST requests in Node.js?

POST data comes in chunks, so you must collect it using the request’s data and end events.

Example:

const http = require('http');
http.createServer((req, res) => {
  if (req.method === 'POST') {
    let body = '';
    req.on('data', chunk => {
      body += chunk.toString();
    });
    req.on('end', () => {
      res.writeHead(200, { 'Content-Type': 'text/plain' });
      res.end(`Data received: ${body}`);
    });
  }
}).listen(6000);

25. What’s the difference between http and https modules in Node.js?

• http module: Used for creating servers that run on the HTTP protocol. It’s unencrypted and not secure.
• https module: Used for creating servers that run on the HTTPS protocol. It uses SSL/TLS certificates for encrypted communication.

Example use case:

• Use http for local testing or internal apps.
• Use https for production apps where security matters, like online banking or e-commerce.

26. What is the EventEmitter class in Node.js?

The EventEmitter class (from the built-in events module) allows objects to emit events and register listeners for those events. It’s the foundation of Node.js’s event-driven architecture.

Example:

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

27. How do you register an event listener in Node.js?

You use the ‘on()’ method of EventEmitter to register a listener for a specific event.

Example:

const EventEmitter = require('events');
const emitter = new EventEmitter();
emitter.on('greet', (name) => {
  console.log(`Hello, ${name}`);
});
emitter.emit('greet', 'Neha');

Output:

Hello, Neha

28. How do you trigger an event in Node.js?

You trigger (emit) an event using the emit() method of EventEmitter.

Example:

const EventEmitter = require('events');
const emitter = new EventEmitter();
emitter.on('message', () => console.log('Message event triggered'));
emitter.emit('message');

Output:

Message event triggered

29. What is the difference between on and once in EventEmitter?

• on: Registers a listener that runs every time the event is emitted.
• once: Registers a listener that runs only the first time, then removes itself automatically.

Example:

const EventEmitter = require('events');
const emitter = new EventEmitter();
emitter.once('start', () => console.log('This runs only once'));
emitter.on('start', () => console.log('This runs every time'));
emitter.emit('start');
emitter.emit('start');

Output:

This runs only once
This runs every time
This runs every time

30. Can you remove an event listener in Node.js? If yes, how?

Yes, you can remove an event listener using removeListener() or off() (modern syntax).

Example:

const EventEmitter = require('events');
const emitter = new EventEmitter();
function greet(name) {
  console.log(`Hello, ${name}`);
}
emitter.on('greet', greet);
emitter.emit('greet', 'Neha'); // Works
emitter.off('greet', greet); // Remove listener
emitter.emit('greet', 'Tushar'); // Nothing happens

31. How do you initialize a Node.js project with npm?

To initialize a Node.js project, we use the command:

npm init

It asks questions like project name, version, description, etc., and then creates a package.json file.

If you want to skip all questions and create a default setup:

npm init -y

Example:
Ramesh is starting a project for his “Online Library” system. He runs npm init -y and gets a package.json file instantly.

32. What is the difference between dependencies and devDependencies?

• dependencies: These are packages required for the project to run in production.
• devDependencies: These are packages needed only during development, not in production.

Example:

• Seema is building a shopping app. She installs Express.js for the server, so it goes into dependencies.
• She also installs Jest for testing, so it goes into devDependencies.

33. How do you install a specific version of a package with npm?

You can install a specific version using @version.

Example:

npm install express@4.18.2

This installs Express.js version 4.18.2.

Imagine Ankit is working in a team where everyone uses Express v4.18.2 for consistency. He installs the same version using the above command.

34. What is the purpose of the package.json file?

The package.json file is the heart of a Node.js project. It:

• Stores project metadata (name, version, author).
• Defines dependencies and devDependencies.
• Allows scripts (like npm start).
• Helps others set up the project easily.

Example:
Priya shares her project with a friend. Instead of sending all installed libraries, she just shares package.json. Her friend runs npm install and gets everything automatically.

35. What is the difference between npm install and npm ci?

• npm install: Installs dependencies from package.json. If package-lock.json exists, it updates it if needed.
• npm ci: Installs dependencies strictly from package-lock.json. It’s faster and ensures exact versions.

Example:

• When Rahul is developing and may add new packages, he uses npm install.
• When his team deploys the app on a server, they use npm ci to get the exact same versions without changes.

36. What is Express.js, and why is it used with Node.js?

Express.js is a lightweight web framework for Node.js. It simplifies handling routes, requests, and responses.

It is used because:

• It reduces the boilerplate code of raw http module.
• It provides middleware support.
• It makes building APIs and web apps faster.

Example:
If Neha wants to build an API for her food delivery app, Express.js helps her manage routes like /menu, /orders, /cart very easily.

37. How do you install Express.js?

You install Express.js using npm:

npm install express

Example:
Amit is starting a “Student Management System” project. He runs npm install express to add Express.js to his project.

38. How do you create a simple route in Express.js?

You first import Express, create an app, and then define a route with app.get().

Example:

const express = require('express');
const app = express();
app.get('/hello', (req, res) => {
  res.send('Hello from Learn2Earn Labs!');
});
app.listen(3000, () => console.log('Server running on port 3000'));

**When Sunita visits http://localhost:3000/hello, she sees:

Hello from Learn2Earn Labs!

39. How do you send a JSON response in Express.js?

You use the res.json() method.

Example:

const express = require('express');
const app = express();
app.get('/student', (req, res) => {
  res.json({ name: 'Arjun', course: 'Full Stack Development' });
});
app.listen(4000);
Visiting /student gives:
{ "name": "Arjun", "course": "Full Stack Development" }

40. What is middleware in Express.js?

Middleware is a function that runs between the request and the response. It can modify requests, add logic, or stop requests before reaching the final route handler.

Types:

• Application-level middleware (applies to all routes).
• Router-level middleware (applies to specific routes).
• Built-in middleware like express.json().

Example:

const express = require('express');
const app = express();
// Middleware for logging
app.use((req, res, next) => {
  console.log(`Request from: ${req.url}`);
  next();
});
app.get('/', (req, res) => res.send('Welcome Rajesh!'));
app.listen(5000);

Here, whenever Rajesh visits any route, the middleware logs the URL before sending the response.