63 NodeJS Interview Questions and Answers PDF 2025

Node.js is huge in web development right now, especially when you need to build backend systems that can handle lots of users without slowing down. What makes it special is how it handles multiple tasks at once without getting stuck, it runs on a single thread, and you get to use JavaScript for everything. This makes it perfect for apps that need to work in real-time think chat apps, APIs that respond instantly, or streaming platforms.

When you’re interviewing for Node.js jobs, they’re going to test two main things, do you really understand how Node.js works under the hood, and can you actually build stuff with it.

They’ll ask about frameworks like Express, how you handle code that doesn’t run right away (async stuff), fixing performance problems, and writing code that won’t break when real users start using it. We put together this list of interview questions to help you get ready.

NodeJS Interview Questions and Answers for Freshers

Que 1. What is Node.js and how is it different from JavaScript?

Answer:
Node.js is a runtime environment that allows JavaScript to be executed outside the browser, typically on the server side. While JavaScript was originally used for client-side scripting, Node.js enables backend development using JavaScript. It uses the V8 engine from Chrome and supports non-blocking I/O operations.

Que 2. Explain the concept of event-driven programming in Node.js.

Answer:
Node.js uses an event-driven architecture where the flow of the program is determined by events such as user actions or messages from other programs. The core idea is that the system responds to events using callback functions.

Que 3. What are the advantages of using Node.js?

Answer:

• Asynchronous and non-blocking I/O
• Fast execution using V8 engine
• Single programming language for full stack
• Active community support
• Scalable for microservices

Que 4. What is the purpose of the package.json file?

Answer:
The package.json file is the metadata file of a Node.js project. It includes information like the project’s name, version, description, dependencies, scripts, and more. It’s used to manage and install project dependencies using npm.

Que 5. How do you export and import modules in Node.js?

Answer:
To export a function or variable:

// math.js
module.exports.add = (a, b) => a + b;

To import it in another file:

const math = require('./math');
console.log(math.add(2, 3));

Que 6. What is the difference between require and import in Node.js?

Answer:

• require is CommonJS syntax, used by default in Node.js.
• import is ES6 module syntax, and requires setting "type": "module" in package.json.

Que 7. What is the Event Loop in Node.js?

Answer:
The Event Loop handles asynchronous operations in Node.js. It continuously checks the call stack and task queues, executing callbacks when the stack is clear.

Que 8. How does Node.js handle asynchronous I/O?

Answer:
Node.js uses non-blocking I/O and the event loop to manage asynchronous operations like file reads and API calls. It allows the server to process multiple requests without waiting for one to finish.

Que 9. What are callbacks in Node.js?

Answer:
Callbacks are functions passed as arguments to other functions, executed after the completion of a task. They’re commonly used in async operations.

Example:

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

Que 10. What are Promises and how are they used?

Answer:
Promises represent the future result of an asynchronous operation. They help in writing cleaner async code and avoid callback hell.

Example:

fetch(url)
  .then(res => res.json())
  .then(data => console.log(data))
  .catch(err => console.error(err));

Que 11. What is npm in Node.js?

Answer:
npm (Node Package Manager) is the default package manager for Node.js. It allows developers to install, share, and manage packages and dependencies.

Que 12. How can you create a simple HTTP server in Node.js?

Answer:

const http = require('http');
http.createServer((req, res) => {
  res.writeHead(200, {'Content-Type': 'text/plain'});
  res.end('Hello World');
}).listen(3000);

Que 13. What is middleware in Node.js?

Answer:
Middleware functions are used in frameworks like Express to handle requests and responses. They have access to req, res, and next().

Que 14. What are streams in Node.js?

Answer:
Streams are objects that allow reading or writing data continuously. Node.js has four types of streams: Readable, Writable, Duplex, and Transform.

Que 15. How do you handle exceptions in Node.js?

Answer:

• Use try...catch for synchronous code
• Use .catch() with promises
• Use error-first callbacks for async code
• Listen to uncaughtException event for process-level errors

Que 16. What is the use of the buffer module in Node.js?

Answer:
Buffers handle binary data directly in memory without converting it to strings. They’re useful for processing files, streams, and network protocols.

Que 17. What is the difference between process.nextTick() and setImmediate()?

Feature	process.nextTick()	setImmediate()
Timing	Executes after current operation	Executes on the next event loop cycle
Use Case	Prioritize task execution	Defer execution

Que 18. How can you handle file operations in Node.js?

Answer:
You can use the fs module to perform file operations.

Example:

const fs = require('fs');
fs.writeFile('test.txt', 'Hello Node', (err) => {
  if (err) throw err;
});

Que 19. How do you install and use external packages?

Answer:
Use npm:

npm install lodash

Then import in code:

const _ = require('lodash');
console.log(_.capitalize('hello'));

Que 20. What is the difference between synchronous and asynchronous methods in Node.js?

Answer:
Synchronous methods block the event loop until the operation is complete, while asynchronous methods allow the event loop to continue running, improving performance and scalability.

Also Check: Java interview Questions and Answers

Node JS Interview Questions and Answers for Experienced Professionals

Que 21. How does Node.js handle concurrency if it runs on a single thread?

Answer:
Node.js handles concurrency using the event loop and non-blocking I/O. Tasks such as file I/O or network calls are offloaded to the system’s kernel or thread pool (via libuv). Once these tasks are completed, their callbacks are added to the event queue to be executed by the main thread.

Que 22. What are worker threads in Node.js and when should you use them?

Answer:
Worker Threads enable running JavaScript in parallel on multiple threads. They are ideal for CPU-intensive operations like data processing or encryption, which would otherwise block the event loop.

Example:

const { Worker } = require('worker_threads');
new Worker('./worker.js');

Que 23. How do you handle large-scale application performance in Node.js?

Answer:

• Use clustering to utilize multiple CPU cores
• Implement caching using Redis or memory
• Apply lazy loading for non-critical modules
• Optimize database queries and use indexes
• Use tools like PM2, New Relic, or Node Clinic to monitor and optimize

Que 24. Explain process.nextTick(), setImmediate(), and setTimeout() differences.

Method	Execution Order
process.nextTick()	Executes after the current operation, before I/O events
setImmediate()	Executes on the next iteration of the event loop
setTimeout()	Executes after specified delay in ms

Que 25. How do you handle memory leaks in Node.js?

Answer:
Memory leaks can be caused by:

• Global variables
• Unclosed closures
• Improper cache usage

To detect:

• Use heapdump, clinic.js, or Chrome DevTools
• Monitor memory usage via process.memoryUsage()

Que 26. What is the role of libuv in Node.js?

Answer:
Libuv is a C library that provides Node.js with:

• Event loop
• Thread pool
• Asynchronous I/O
  It abstracts system-specific APIs and powers much of Node’s asynchronous behavior.

Que 27. How do you scale Node.js applications?

Answer:

• Clustering to distribute load across CPU cores
• Load balancers like Nginx or HAProxy
• Horizontal scaling using containers or microservices
• Caching responses and database queries
• Use PM2 for process management and monitoring

Que 28. What is the significance of middleware in Express.js?

Answer:
Middleware functions in Express.js are functions with access to the req, res, and next objects. They are used for:

• Logging
• Authentication
• Error handling
• Modifying request/response objects

Example:

app.use((req, res, next) => {
  console.log('Request Time:', Date.now());
  next();
});

Que 29. How does the EventEmitter work in Node.js?

Answer:
EventEmitter allows creating and handling custom events. You can use .on() to listen and .emit() to trigger events.

Example:

const EventEmitter = require('events');
const emitter = new EventEmitter();
emitter.on('log', msg => console.log(msg));
emitter.emit('log', 'Hello Node!');

Que 30. How do you implement caching in Node.js?

Answer:

• In-memory caching using node-cache or JavaScript Map
• Distributed caching using Redis or Memcached
• HTTP caching via headers like ETag, Cache-Control
• Database-level caching using query result caches

Que 31. How can you handle uncaught exceptions and unhandled promise rejections?

Answer:

process.on('uncaughtException', (err) => {
  console.error('Uncaught Exception:', err);
});

process.on('unhandledRejection', (reason) => {
  console.error('Unhandled Rejection:', reason);
});

These should be used to log and gracefully shut down the application, not as a replacement for proper error handling.

Que 32. What is backpressure in Node.js streams?

Answer:
Backpressure occurs when the readable stream is producing data faster than the writable stream can consume. It is managed using the .pipe() method and drain events.

Que 33. Explain the lifecycle of an HTTP request in a Node.js Express app.

Answer:

1. Client sends a request
2. Express routes the request to matching middleware
3. Middleware performs tasks like validation, logging
4. Final handler processes the request and sends a response

Que 34. How do you perform authentication and authorization in a Node.js app?

Answer:

• Use Passport.js for strategy-based authentication
• JWTs (JSON Web Tokens) for stateless auth
• Role-based access control using middleware
• OAuth integration for third-party providers

Que 35. How do you manage and isolate configurations for different environments?

Answer:

• Use .env files with dotenv
• Set environment-specific variables: process.env.NODE_ENV
• Create separate config files (config.dev.js, config.prod.js)
• Avoid committing secrets; use vaults or secure storage

Also Check: ExpressJS Interview Questions and Answers

NodeJS Interview Questions and Answers for Senior Developers

Que 36. How do you design a scalable Node.js architecture for high-traffic applications?

Answer:
A scalable Node.js architecture includes:

• Load balancer (e.g., Nginx, HAProxy) in front
• Clustering using cluster or PM2 to utilize multi-core CPUs
• Microservices architecture with service discovery
• Message queues like RabbitMQ or Kafka for async processing
• Caching layers using Redis or Memcached
• Database replication and sharding

Que 37. What are the best practices for handling asynchronous operations in Node.js?

Answer:

• Prefer async/await for better readability
• Use Promise.all for concurrent executions
• Always handle rejections with try-catch or .catch
• Avoid callback hell by using named functions
• Use util.promisify for legacy callback APIs

Que 38. How do you implement rate limiting in a Node.js API?

Answer:

• Use middleware like express-rate-limit
• Implement Redis-based rate limiting for distributed environments
• Define rate-limiting strategy (e.g., IP-based, user-based)
• Set headers like X-RateLimit-Remaining to inform clients

Example:

const rateLimit = require('express-rate-limit');
app.use(rateLimit({
  windowMs: 15 * 60 * 1000,
  max: 100
}));

Que 39. How would you debug performance bottlenecks in Node.js?

Answer:

• Use Node.js built-in profiler (--inspect, --inspect-brk)
• Use Chrome DevTools for flame charts
• Analyze memory leaks with heapdump, clinic.js, or memwatch-next
• Use console.time/timeEnd for manual profiling
• Monitor event loop lag using event-loop-delay

Que 40. What security concerns should be addressed in a Node.js application?

Answer:

• Input validation & sanitization to avoid XSS/SQLi
• Avoid eval and unsafe JSON parsing
• Secure cookies with httpOnly and secure flags
• Rate limiting and brute-force protection
• Use libraries like helmet, csurf, and perform dependency audits

Que 41. Explain how streams can be optimized in a data-heavy Node.js app.

Answer:

• Use stream.pipeline() for cleaner stream composition
• Prefer pause() and resume() to control flow manually
• Use highWaterMark to adjust buffer thresholds
• Process data in chunks to reduce memory footprint

Que 42. How do you manage sessions in a distributed Node.js environment?

Answer:

• Use express-session with Redis or database as a store
• Avoid in-memory sessions in multi-instance setups
• Use sticky sessions if session data is stored in memory (not recommended)
• Implement stateless auth (JWT) when possible for scalability

Que 43. What is the difference between spawn, exec, and fork in Node.js?

Method	Description	Use Case
spawn	Launches a new process with streams	Large data streams
exec	Executes a command in shell, buffers output	Small output
fork	Spawns a new Node.js process with IPC channel	Child process communication

Que 44. How would you handle background jobs and queues in Node.js?

Answer:

• Use libraries like Bull, Agenda, or Kue
• Queue tasks for email, image processing, reports
• Use Redis for job storage and pub/sub
• Monitor job status, retries, and failures using dashboard tools

Que 45. How can you ensure code quality and maintainability in large Node.js codebases?

Answer:

• Follow modular structure and SOLID principles
• Use TypeScript or JSDoc for type safety
• Enforce linting rules with ESLint
• Write unit tests with Jest or Mocha
• Maintain documentation and follow consistent naming conventions

Que 46. What are some common pitfalls with Promises and async/await in Node.js?

Answer:

• Not using try/catch with async/await
• Missing await on a Promise
• Using await inside loops (use Promise.all instead)
• Forgetting to return a Promise in middleware

Que 47. How do you secure communication between microservices in Node.js?

Answer:

• Use mutual TLS (mTLS) or signed JWTs for auth
• Encrypt data using HTTPS and secure tokens
• Use API Gateway for access control and rate limiting
• Validate all incoming requests (headers, tokens, IP)

Que 48. How do you enforce versioning in Node.js APIs?

Answer:

• Use URI versioning: /api/v1/resource
• Use header-based versioning: Accept: application/vnd.myapp.v1+json
• Maintain separate route handlers or modules for each version
• Clearly deprecate old versions with response headers

Que 49. What is Domain-Driven Design (DDD) and how can it be applied in Node.js?

Answer:

• DDD helps model complex domains by separating business logic
• Structure code into layers: Domain, Application, Infrastructure
• Use services and repositories to abstract database and logic
• Helps maintain clean, testable, and scalable architecture

Que 50. How do you test asynchronous code in Node.js?

Answer:

• Use async functions with await inside test cases
• Use done() callback in Mocha or Jest for callback-based tests
• Mock external calls using libraries like nock, sinon
• Use supertest for HTTP route testing in Express

Also Check: React JS Interview Questions and Answers

Common NodeJS Interview Questions and Answers

Sure! Here are 13 more Common Node.js Interview Questions and Answers, starting from Que 51, and following your requested format—no separators, standard markdown, and including tables/lists/code blocks where needed.

Que 51. How does the Node.js event loop work?

Answer:
The event loop is the heart of Node.js’ asynchronous behavior. It handles non-blocking operations by offloading them to the system or a thread pool and listens for their completion to push the callback onto the event queue.

Phases of the event loop:

1. Timers
2. Pending callbacks
3. Idle/prepare
4. Poll
5. Check
6. Close callbacks

It continues in this cycle as long as there are callbacks or I/O operations to handle.

Que 52. What is the difference between process.nextTick() and setImmediate()?

Feature	process.nextTick()	setImmediate()
Execution	Before the next event loop tick	On the check phase of the loop
Priority	Higher	Lower
Use Case	Immediate microtask scheduling	Deferring execution until current I/O completes

Que 53. How can you prevent callback hell in Node.js?

Answer:

• Use named functions instead of anonymous ones
• Utilize Promises and async/await
• Modularize code by breaking into smaller functions
• Use control flow libraries like async

// Before (Callback Hell)
fs.readFile('file.txt', function(err, data){
  if(err) throw err;
  processFile(data, function(err, result){
    saveFile(result, function(err){
      console.log('Done');
    });
  });
});

// After (Promise/Async)
async function handleFile() {
  const data = await fs.promises.readFile('file.txt');
  const result = await processFile(data);
  await saveFile(result);
  console.log('Done');
}

Que 54. How is middleware used in Express.js?

Answer:
Middleware functions in Express.js are functions that have access to the request, response, and next objects. They are used to:

• Modify the request or response
• Perform logging, authentication, validation, etc.
• End the request-response cycle or pass control to the next middleware

Example:

app.use((req, res, next) => {
  console.log('Request received');
  next();
});

Que 55. How do you handle file uploads in Node.js?

Answer:

• Use the multer middleware for Express
• Configure disk or memory storage
• Define limits for file size and type

const multer = require('multer');
const upload = multer({ dest: 'uploads/' });

app.post('/upload', upload.single('file'), (req, res) => {
  res.send('File uploaded');
});

Que 56. What are environment variables in Node.js and how are they managed?

Answer:
Environment variables are used to store config outside the code. They are accessed using process.env.

Commonly managed using:

• .env files with dotenv package
• System or CI/CD environment settings

require('dotenv').config();
console.log(process.env.DB_HOST);

Que 57. How do you manage dependencies in Node.js?

Answer:

• Using package.json for declaring dependencies
• Install with npm install or yarn
• Use devDependencies for dev tools like linters/test frameworks
• Lock files (package-lock.json or yarn.lock) ensure consistency

Que 58. How would you implement logging in a Node.js application?

Answer:

• Use logging libraries like winston, pino, or bunyan
• Set up log levels (info, warn, error)
• Support log rotation and external logging services

const winston = require('winston');
const logger = winston.createLogger({
  transports: [new winston.transports.Console()]
});
logger.info('Server started');

Que 59. What is a memory leak in Node.js and how do you detect it?

Answer:
A memory leak occurs when memory that is no longer needed isn’t released. It causes performance issues or crashes.

Detection tools:

• Chrome DevTools (heap snapshots)
• clinic.js
• heapdump for capturing and analyzing memory usage

Que 60. What is the purpose of the cluster module in Node.js?

Answer:
The cluster module enables spawning child processes (workers) that share the same server port. It helps to:

• Utilize multiple CPU cores
• Improve application throughput

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

if (cluster.isMaster) {
  cluster.fork();
  cluster.fork();
} else {
  http.createServer((req, res) => res.end('Hello')).listen(3000);
}

Que 61. How do you handle cross-origin requests (CORS) in Node.js?

Answer:

• Use cors middleware in Express
• Manually set CORS headers

const cors = require('cors');
app.use(cors({
  origin: 'https://example.com',
  methods: ['GET', 'POST']
}));

Que 62. What is the difference between require() and import in Node.js?

Answer:

Feature	require()	import
Syntax	CommonJS	ES Modules
Support	Default in Node.js	Enabled via "type": "module"
Dynamic Usage	Can be used dynamically	Static only
File Extensions	Optional	Often required

Que 63. How do you create and use a custom event in Node.js?

Answer:

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

emitter.on('userLoggedIn', (user) => {
  console.log(`Welcome ${user}`);
});

emitter.emit('userLoggedIn', 'Alice');

Also Check: Java OOPs Interview Questions

NodeJS Interview Questions PDF

Want to study offline or save these questions for later?

This PDF includes, most asked interview questions and detailed answers with examples. Perfect for last-minute revision and in-depth preparation.

FAQs: NodeJS Interview Questions

Conclusion

Understanding the concepts of Node.js is essential for performing well in interviews. These questions are designed not only to help you revise key topics but also to simulate the kind of in-depth discussions that often occur in real technical interviews. Be sure to practice writing code, optimizing performance, and designing robust APIs, these are the areas interviewers love to explore.