Node.js Job Interview Questions and Answers

Here’s how you can confidently answer these Node.js interview questions in a face-to-face interview:

Beginner or Fresher Node.js Interview Questions and Answers

1. What is Node.js, and how does it work?

“Node.js is a runtime environment that allows us to run JavaScript on the server-side using the V8 engine, which is also used in Chrome. It is asynchronous, event-driven, and non-blocking, making it highly efficient for scalable applications. Node.js operates on a single-threaded event loop, which helps handle multiple connections concurrently.”

2. How is Node.js different from JavaScript in the browser?

“JavaScript in the browser is mainly used for client-side scripting, while Node.js is used for server-side development. In the browser, JavaScript interacts with the DOM, while in Node.js, it interacts with the file system, databases, and network requests. Additionally, Node.js has built-in modules like fs, http, and path, which are not available in the browser.”

3. What is the event loop in Node.js, and why is it important?

“The event loop is the heart of Node.js, allowing it to handle asynchronous operations efficiently. Since Node.js runs on a single-threaded, non-blocking architecture, the event loop ensures that tasks like I/O operations, timers, and network requests don’t block the execution of other tasks. This is what makes Node.js highly scalable.”

4. What is npm, and how is it used in Node.js?

“npm (Node Package Manager) is the default package manager for Node.js. It helps in installing, managing, and sharing JavaScript packages. We use commands like npm install <package-name> to install dependencies, and we can also publish our own packages using npm publish. It’s essential for managing project dependencies.”

5. Explain the difference between require() and import.

“require() is used in CommonJS modules, whereas import is used in ES Modules (ESM). require() is synchronous and works in older versions of Node.js, while import is asynchronous and requires "type": "module" in package.json. Modern Node.js applications prefer import for better compatibility with ES6.”

6. What are modules in Node.js, and how do you create one?

*”Modules are reusable pieces of code that help in structuring large applications. Node.js provides built-in modules (fs, http, path), third-party modules via npm, and custom modules. To create a module, we use module.exports:

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

And then import it:

const math = require('./math');
console.log(math.add(5, 3));  

This keeps our code modular and maintainable.”*

7. What is the difference between synchronous and asynchronous programming in Node.js?

“Synchronous code executes line by line, blocking further execution until the current task completes. Asynchronous code, on the other hand, executes non-blocking operations using callbacks, promises, or async/await. This is crucial in Node.js to handle multiple tasks without performance issues.”

8. How do you handle errors in Node.js applications?

*”Error handling is crucial in Node.js. We commonly use:

• Try-catch blocks for synchronous errors
• Callbacks with error-first parameters (function(err, data))
• Promises with .catch()
• Async/await with try-catch

Example:

async function fetchData() {
  try {
    let data = await fetch("https://api.example.com");
    return await data.json();
  } catch (error) {
    console.error("Error:", error);
  }
}

This ensures our applications don’t crash due to unhandled errors.”*

9. What are streams in Node.js, and how do they work?

*”Streams handle large amounts of data efficiently by processing them in chunks instead of loading everything into memory. There are four types of streams:

• Readable (e.g., fs.createReadStream())
• Writable (e.g., fs.createWriteStream())
• Duplex (both read and write, e.g., TCP sockets)
• Transform (modifies data in transit, e.g., zlib compression)

Example of reading a file using streams:

const fs = require('fs');
const readStream = fs.createReadStream('file.txt', 'utf8');

readStream.on('data', (chunk) => {
  console.log(chunk);
});

This improves performance by handling data efficiently.”*

10. Explain the difference between process.nextTick() and setImmediate().

*”Both are used for scheduling tasks in the event loop:

• process.nextTick() runs before the next event loop iteration, giving it higher priority.
• setImmediate() runs after the current event loop iteration.

Example:

process.nextTick(() => console.log("Next Tick"));
setImmediate(() => console.log("Set Immediate"));
console.log("Main Thread");

Output:

Main Thread  
Next Tick  
Set Immediate  

This shows nextTick executes first, making it useful for urgent tasks.”*

11. What is the purpose of package.json in a Node.js project?

*”package.json is the configuration file for a Node.js project. It contains metadata about the project, dependencies, scripts, and configurations. Example:

{
  "name": "my-app",
  "version": "1.0.0",
  "dependencies": {
    "express": "^4.17.1"
  },
  "scripts": {
    "start": "node app.js"
  }
}

It helps in dependency management, version control, and automation.”*

12. How do you install dependencies locally and globally in Node.js?

*”We use npm install to install dependencies:

• Locally (inside the project): npm install express Stores dependencies in node_modules.
• Globally (accessible system-wide): npm install -g nodemon Useful for CLI tools.”*

13. What is middleware in Express.js?

*”Middleware functions in Express.js process requests before they reach the final route handler. They can be used for logging, authentication, error handling, etc.

Example:

app.use((req, res, next) => {
  console.log("Middleware executed");
  next();
});

They improve modularity and code reusability.”*

14. What is the purpose of Buffer in Node.js?

*”Buffer allows handling binary data directly in Node.js, especially useful for file handling and network communication.

Example:

const buf = Buffer.from("Hello");
console.log(buf); // Outputs: <Buffer 48 65 6c 6c 6f>

Buffers help manage raw data efficiently, especially in stream-based applications.”*

15. Explain the difference between fs.readFileSync() and fs.readFile().

*”fs.readFileSync() is blocking and halts execution until the file is read.

const data = fs.readFileSync("file.txt", "utf8");
console.log(data);

fs.readFile() is non-blocking and works asynchronously.

fs.readFile("file.txt", "utf8", (err, data) => {
  console.log(data);
});

The async version is preferred for better performance in large applications.”

Intermediate or Experienced Level Node.js Interview Questions and Answers

Here’s how you can confidently answer these Node.js interview questions in a face-to-face interview:

1. How does Node.js handle asynchronous operations internally?

*”Node.js handles asynchronous operations using the event-driven, non-blocking architecture powered by the event loop. It uses:

• Libuv: A C++ library that handles I/O operations asynchronously.
• Thread pool: For expensive tasks like file system access or cryptographic functions.
• Event loop: Manages the execution of JavaScript callbacks, ensuring non-blocking behavior.

Example: When we use fs.readFile(), Node.js delegates the task to the OS, allowing it to continue executing other code while waiting for the file to be read.”*

2. Explain the difference between callbacks, promises, and async/await.

*”These are different ways to handle asynchronous code in Node.js:

• Callbacks: Functions passed as arguments to be executed later. fs.readFile("file.txt", (err, data) => { if (err) console.error(err); console.log(data.toString()); }); Issue: Callback Hell (nested callbacks become hard to manage).
• Promises: Provide .then() and .catch() for handling async results. fs.promises.readFile("file.txt", "utf8").then(console.log).catch(console.error);
• Async/Await: Syntactic sugar for promises, making async code look synchronous. async function readFile() { try { let data = await fs.promises.readFile("file.txt", "utf8"); console.log(data); } catch (err) { console.error(err); } } It improves readability and error handling.”*

3. What is the difference between process.env and dotenv in Node.js?

*”process.env is a built-in object that stores environment variables.

Example:

console.log(process.env.NODE_ENV);

dotenv is a third-party package used to load environment variables from a .env file.

Example:

require("dotenv").config();
console.log(process.env.API_KEY);

dotenv is useful for managing secrets securely instead of hardcoding them.”*

4. How do you implement authentication in a Node.js application?

*”Common authentication methods include:

• JWT (JSON Web Token) for stateless auth
• OAuth for third-party logins (Google, Facebook)
• Session-based authentication using express-session

Example using JWT:

const jwt = require("jsonwebtoken");
const token = jwt.sign({ userId: 1 }, "secret_key", { expiresIn: "1h" });
console.log(token);

On login, we generate a JWT token, store it on the client, and validate it in protected routes.”*

5. What are WebSockets, and how can you implement them in Node.js?

*”WebSockets enable real-time, bidirectional communication between client and server.

Using Socket.io in Node.js:

const io = require("socket.io")(server);
io.on("connection", (socket) => {
  console.log("User connected");
  socket.on("message", (msg) => io.emit("message", msg));
});

WebSockets are used in chat apps, live notifications, and multiplayer games.”*

6. How do you secure a Node.js application against common vulnerabilities?

*”To secure a Node.js app:

• Use environment variables for secrets (dotenv).
• Validate user input (avoid SQL Injection).
• Sanitize inputs using libraries like validator.js.
• Use Helmet.js to set secure HTTP headers.
• Rate limiting to prevent brute force attacks.
• Use HTTPS for encrypted communication.

Example:

const helmet = require("helmet");
app.use(helmet());

Security best practices prevent exploits like XSS and CSRF.”*

7. What is clustering in Node.js, and how does it improve performance?

*”Node.js runs on a single thread, but clustering allows it to use multiple CPU cores.

Example using cluster:

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

if (cluster.isMaster) {
  for (let i = 0; i < require("os").cpus().length; i++) {
    cluster.fork();
  }
} else {
  http.createServer((req, res) => res.end("Hello")).listen(3000);
}

This improves performance by spawning multiple worker processes.”*

8. How do you manage environment variables in Node.js applications?

*”Use dotenv for storing env variables:

1. Create a .env file: API_KEY=your_secret_key
2. Load it in Node.js: require("dotenv").config(); console.log(process.env.API_KEY);

This keeps sensitive information secure.”*

9. Explain the purpose of process.exit() in Node.js.

*”process.exit() terminates the Node.js process.

Example:

if (error) process.exit(1);

It is useful for gracefully shutting down applications.”*

10. How does Node.js handle multi-threading?

*”Node.js is single-threaded, but it can handle multi-threading using:

• Worker threads (for CPU-intensive tasks).
• Clustering (to utilize multiple cores).

Example using worker threads:

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

This improves performance for CPU-heavy tasks.”*

11. What are the advantages and disadvantages of using Node.js for backend development?

Advantages:
✅ Non-blocking I/O → Handles many connections efficiently.
✅ Fast execution → Uses the V8 engine.
✅ JavaScript-based → One language for frontend and backend.

Disadvantages:
❌ Single-threaded → Not ideal for CPU-intensive tasks.
❌ Callback Hell → Need to use Promises or async/await.

12. How does Node.js handle child processes, and when would you use them?

*”child_process module allows running external processes in Node.js.

Example:

const { exec } = require("child_process");
exec("ls", (err, stdout) => console.log(stdout));

Used for background tasks, automation, and shell commands.”*

13. What is the difference between microservices and monolithic architecture in Node.js?

*”Monolithic: One large app, tightly coupled.
Microservices: Small independent services communicating via APIs.

Microservices advantages:
✅ Scalable
✅ Easy to maintain
✅ Fault isolation

Example using Express microservices:

app.get("/user", (req, res) => res.json({ name: "John" }));

Microservices improve scalability and maintainability.”*

14. What are event emitters in Node.js, and how do they work?

*”Event emitters handle custom events in Node.js.

Example:

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

emitter.on("event", () => console.log("Event triggered"));
emitter.emit("event");

Useful for real-time notifications.”*

15. What are worker threads in Node.js, and when should you use them?

*”Worker threads run CPU-intensive tasks in parallel.

Example:

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

Used for image processing, AI models, and data crunching.”*

Advanced-Level Node.js Interview Questions

Here are the answers to your Node.js interview questions, using concepts from the previous notes:

1. How does Node.js handle garbage collection, and how can you optimize it?

*”Node.js uses the V8 engine’s garbage collector, which follows a generational garbage collection model:

• Young Generation: Short-lived objects are stored here.
• Old Generation: Long-lived objects move here.
• Mark-and-Sweep Algorithm: Identifies and removes unused objects.

Optimization Strategies:
✅ Use --expose-gc and global.gc() to trigger manual garbage collection (use cautiously).
✅ Optimize memory usage by closing database connections and clearing unused variables.
✅ Use WeakMaps for managing objects without preventing garbage collection.
✅ Profile memory usage using node --inspect.”*

2. What is the purpose of the V8 engine in Node.js?

*”The V8 engine is Google’s open-source JavaScript engine that:

• Compiles JavaScript into machine code for faster execution.
• Uses Just-In-Time (JIT) Compilation for speed.
• Provides built-in garbage collection and memory management.

Node.js leverages V8 to execute JavaScript outside the browser efficiently.”*

3. Explain the concept of backpressure in Node.js streams.

*”Backpressure occurs when the readable stream produces data faster than the writable stream can consume, causing a memory overflow.

Solution: Use pipe() with throttling to handle backpressure:

const fs = require("fs");
const readable = fs.createReadStream("large-file.txt");
const writable = fs.createWriteStream("output.txt");

// Automatically handles backpressure
readable.pipe(writable);

This ensures smooth data flow without overwhelming memory.”*

4. How does Node.js handle database connections efficiently?

*”Node.js optimizes database connections using:
✅ Connection pooling → Reuses existing database connections.
✅ Asynchronous queries → Uses async/await or Promises to prevent blocking.
✅ ORMs like Sequelize or Mongoose → Manage connections efficiently.

Example using a MySQL connection pool:

const mysql = require("mysql2");
const pool = mysql.createPool({ host: "localhost", user: "root", database: "test", connectionLimit: 10 });

pool.query("SELECT * FROM users", (err, results) => {
  console.log(results);
});

This ensures efficient resource management.”*

5. What are some strategies for optimizing performance in a large-scale Node.js application?

*”To optimize performance:
✅ Use clustering → Utilize multiple CPU cores (cluster module).
✅ Optimize queries → Use indexed database queries and connection pools.
✅ Implement caching → Use Redis for frequently accessed data.
✅ Compress responses → Use compression middleware in Express.
✅ Optimize memory usage → Avoid memory leaks and monitor garbage collection.

Example of compression in Express:

const compression = require("compression");
app.use(compression());

These strategies improve scalability and response time.”*

6. How do you implement caching in a Node.js application?

*”Use Redis for caching frequently used data:

Example:

const redis = require("redis");
const client = redis.createClient();

client.set("key", "value", "EX", 3600); // Expires in 1 hour
client.get("key", (err, data) => console.log(data));

Caching reduces database load and speeds up responses.”*

7. What is the difference between process.on('uncaughtException') and process.on('unhandledRejection')?

*”Both handle errors but for different scenarios:

• uncaughtException → Catches synchronous errors not handled in try-catch. process.on("uncaughtException", (err) => { console.error("Unhandled Error:", err); }); throw new Error("Crash!");
• unhandledRejection → Catches Promise rejections without .catch(). process.on("unhandledRejection", (reason) => { console.error("Unhandled Rejection:", reason); }); Promise.reject("Promise failed!");

Use these handlers only for logging; restarting the process is recommended.”*

8. How can you debug a Node.js application effectively?

“Use built-in tools and external debuggers:
✅ Node Inspector → Run with node --inspect index.js
✅ Chrome DevTools → Open chrome://inspect
✅ Debugger in VS Code → Set breakpoints and debug visually
✅ Console logging → console.log() for quick checks
✅ Error handling middleware → Log errors centrally in Express.”

9. Explain how middleware works in Express.js with an example.

*”Middleware functions execute between request and response in Express.js.

Example:

const express = require("express");
const app = express();

app.use((req, res, next) => {
  console.log("Request received");
  next(); // Pass control to the next middleware
});

app.get("/", (req, res) => res.send("Hello, World!"));
app.listen(3000);

Middleware is used for logging, authentication, and error handling.”*

10. How do you prevent memory leaks in Node.js applications?

*”To prevent memory leaks:
✅ Close database connections after use.
✅ Avoid global variables that retain references.
✅ Use WeakMap for caching objects to allow garbage collection.
✅ Monitor memory usage with process.memoryUsage().
✅ Profile heap memory using Chrome DevTools.

Example:

setInterval(() => console.log(process.memoryUsage()), 5000);

Regular monitoring helps detect and fix memory leaks.”*

Here are the solutions to your Node.js interview questions, using concepts from the previous notes:

11. How does Node.js integrate with message queues like RabbitMQ or Kafka?

*”Node.js integrates with message queues like RabbitMQ and Kafka to handle asynchronous communication between services.

Example: Using amqplib for RabbitMQ in Node.js:

const amqp = require('amqplib');

async function sendMessage() {
  const connection = await amqp.connect('amqp://localhost');
  const channel = await connection.createChannel();
  await channel.assertQueue('task_queue', { durable: true });
  channel.sendToQueue('task_queue', Buffer.from('Hello, World!'));
  console.log("Message sent to queue");
}

sendMessage();

For Kafka, Node.js can use KafkaJS or node-rdkafka for high-performance streaming. These allow scalability and event-driven architectures.”*

12. What is the difference between GraphQL and REST APIs in Node.js?

*”GraphQL and REST are used for building APIs, but they differ in:

Example using Express + GraphQL:

const { graphqlHTTP } = require("express-graphql");
const { buildSchema } = require("graphql");

const schema = buildSchema(`
  type Query { hello: String }
`);
const root = { hello: () => "Hello World!" };

app.use("/graphql", graphqlHTTP({ schema, rootValue: root, graphiql: true }));

GraphQL is best for complex data relationships, while REST is simpler for CRUD APIs.”*

13. How do you handle file uploads efficiently in Node.js?

*”File uploads should be handled efficiently to prevent memory overload.

✅ Use Multer middleware with Express.js:

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

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

✅ Store files in cloud storage (AWS S3, Google Cloud) instead of local servers.
✅ Use streaming to process large files instead of loading them into memory.
✅ Implement rate limiting to prevent excessive uploads.”*

14. What are JWT tokens, and how do you use them for authentication in Node.js?

*”JWT (JSON Web Token) is used for secure, stateless authentication.

✅ Generating a JWT token:

const jwt = require("jsonwebtoken");
const token = jwt.sign({ userId: 123 }, "secretKey", { expiresIn: "1h" });
console.log(token);

✅ Verifying JWT in Express.js:

app.use((req, res, next) => {
  const token = req.headers.authorization?.split(" ")[1];
  if (!token) return res.status(401).send("Unauthorized");

  jwt.verify(token, "secretKey", (err, decoded) => {
    if (err) return res.status(403).send("Invalid Token");
    req.user = decoded;
    next();
  });
});

JWT is lightweight and stateless, but token expiration and refresh mechanisms should be implemented.”*

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

*”Use express-rate-limit to prevent DDoS attacks and abuse.

✅ Example:

const rateLimit = require("express-rate-limit");

const limiter = rateLimit({
  windowMs: 15 * 60 * 1000, // 15 minutes
  max: 100, // Limit each IP to 100 requests per windowMs
});

app.use(limiter);

This ensures API stability and security.”*

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

✅ Use fork() for CPU-intensive tasks (creates a new Node.js process).
✅ Use spawn() for continuous data streams.
✅ Use exec() when output is small and you need a simple buffer.”*

17. How does Node.js handle TLS and HTTPS connections?

*”Node.js provides built-in TLS/SSL support using the https module.

✅ Example of HTTPS Server:

const https = require("https");
const fs = require("fs");

const options = {
  key: fs.readFileSync("server.key"),
  cert: fs.readFileSync("server.crt"),
};

https.createServer(options, (req, res) => {
  res.writeHead(200);
  res.end("Secure Connection!");
}).listen(443);

TLS is essential for encrypting sensitive data like passwords and API keys.”*

18. What is the purpose of the ‘domain’ module in Node.js, and why is it deprecated?

*”The domain module was used for error handling across asynchronous tasks, but it was deprecated because:
❌ It had inconsistent behavior.
❌ Modern error-handling methods (like try/catch in async functions) are better.

✅ Use proper error handling instead:

process.on("uncaughtException", (err) => {
  console.error("Unhandled Error:", err);
});

Developers should use async/await and Promises instead of domain.”*

19. How would you scale a Node.js application to handle high traffic?

*”To scale Node.js applications:
✅ Use clustering → Leverage multiple CPU cores.
✅ Implement load balancing → Use NGINX or PM2.
✅ Use caching → Implement Redis for performance.
✅ Optimize database queries → Use indexes and connection pooling.
✅ Use microservices architecture → Split app into independent services.

Example: Clustering with PM2

pm2 start app.js -i max  # Runs app on all available cores

This improves performance and fault tolerance.”*

20. What are some best practices for writing scalable and maintainable Node.js code?

*”Follow these best practices:
✅ Use asynchronous patterns → Avoid blocking operations.
✅ Follow MVC architecture → Organize files properly.
✅ Implement proper logging → Use winston or pino.
✅ Use environment variables → Store secrets in .env file.
✅ Write unit tests → Use Jest or Mocha for testing.

Example of logging with Winston:

const winston = require("winston");
const logger = winston.createLogger({
  transports: [new winston.transports.Console()],
});

logger.info("Application started");

Following these principles ensures maintainability and scalability.”*