Top 42 Node JS Coding Interview Questions and Answers

Node JS has evolved, seamlessly integrating with various front-end technologies in today’s time. It also plays an important role as the back-end of fast web applications that require the management of complicated server-side business logic. With more companies adopting Node JS for scalable and efficient systems, there has been a surge in the demand for skilled Node JS developers. If you are preparing for a Node JS coding interview, learn basic aspects like event-driven architecture, relational database management, the concept of RESTful, etc., to ace the interview.
This blog presents the top Node JS coding interview questions and answers for all aspiring developers. The guide also lists sample answers drafted to assist you in preparing for a Node JS development job interview and help you showcase your technical competencies.

Node JS Coding Interview Questions and Answers for Freshers

Freshers starting in Node JS development are frequently asked basic principles-based interview questions to check their mastery of the programming language. This section answers common Node JS coding interview questions and answers for 0 to 1 year of experience candidates, allowing the candidate to gain confidence in essential concepts such as event handling, modules, and asynchronous programming.

Q1. What do you mean by Node JS, and how can it be distinguished from JavaScript?

Sample Answer: Node JS is a runtime environment that allows JavaScript to be executed on the server side, enabling back-end development. In contrast, JavaScript is primarily a client-side scripting language used to create dynamic web content and runs in web browsers. Node JS extends JavaScript’s capabilities by providing built-in modules like fs and HTTP for file handling and networking, making it suitable for server-side applications. 

Here’s an example that shows the difference between Node JS and JavaScript:

// Node JS example
const fs = require('fs'); // Node JS Module
fs.writeFileSync('example.txt', 'Hello World');
// JavaScript example
console.log('Hello World'); // Browser Console

Q2. Explain how to import and export modules in Node JS.

Sample Answer: In Node JS, modules are used to organize and reuse code across files. You can export functionality using module.exports and import it in other files using require(). 

Here’s a code for importing and exporting modules in Node JS:

// myModule.js
module.exports = function() { console.log("Hello, World!"); };
// app.js
const myModule = require('./myModule');
myModule();

Q3. What do you mean by callback hell and how does one get rid of it?

Sample Answer: Callback hell is a programming situation when several asynchronous functions are called one inside the other in a single program, making it complex to code and understand. This condition can be solved using promises and async / await or by dividing the functions in such a way that nesting is minimal.

Q4. How do you import and export modules in Node JS?

Sample Answer: In Node JS, modules enable code reuse by exporting and importing functionalities between files. You can export a function or object using module.exports and include it in another file using require(). In greet.js, the greet function is exported using module.exports:

// greet.js
module.exports = function greet(name) {
return 'Hello, ${name}!`;
};
// app.js
const greet = require('./greet');
console.log(greet('John')); // Output: Hello, John!

Q5. Describe the FS module in Node JS.

Sample Answer: In Node JS, the FS module enables developers to perform file system-based operations such as create, read and delete files. The operations can be performed in both synchronous and asynchronous modes, where asynchronous mode is non-blocking and hence is preferred for efficiency.

Q6. What is an event in Node JS? Give an example.

Sample Answer: An event in Node JS is a signal indicating something has occurred, enabling asynchronous interaction. The EventEmitter module allows for creating and handling events. In the example, the greet event is triggered, and the listener responds with a message. 

Below is the code for the same:

const EventEmitter = require('events');
const eventEmitter = new EventEmitter();
eventEmitter.on('greet', (name) =>
=>{
console.log("Hello, ${name}!');
});
eventEmitter.emit('greet', 'Alice'); // Output: Hello, Alice!

Q7. Explain middleware in Node JS.

Sample Answer: Middleware is a function of a Node JS application that can access the request and response objects. Middleware functions are common in such frameworks as Express.js when some manipulations need to be done with the incoming requests before the response is sent, or there are other things to manage such as error, authentication, and logging among others.

Q8. What is a callback function in Node JS?

Sample Answer: A callback refers to a function supplied as an argument within another function which gets called after the latter has completed executing. The code for the function is:

function greet(name, callback) {
console.log("Hello, ${name}!');
callback();
}
greet('Alice', () => {
| });
console.log('Callback executed!');

Q9. What is Express.js? How does it relate to Node JS?

Sample Answer: Express.js is a back-end web application framework for Node JS. It is designed for building web applications and APIs. The framework enables a more structured and efficient approach to routing, middleware, and handling of HTTP requests compared to a scenario where only Node JS is employed.

Q10. How can one approach the problem of asynchronous programming in Node JS using async/await?

Sample Answer: Asynchronous programming in Node JS allows non-blocking operations but can lead to callback hell when sequencing tasks. Using async/await simplifies this by enabling sequential execution of promises with readable syntax. The async keyword defines an asynchronous function, while await pauses execution until the promise resolves, ensuring clean and manageable workflows, as demonstrated in the example code. 

The following code can be used for solving asynchronous:

const fetchData = async () => {
const data = await new Promise((resolve) => {
});
setTimeout(() => resolve('Data fetched!'), 1000);
console.log(data);
fetchData(); // Output: Data fetched!

Pro Tip: Candidates for entry-level coding jobs are asked a number of common Node JS coding interview questions to evaluate their technical and conceptual abilities. Read our guide on basic coding interview questions and answers to prepare for the interview efficiently.

Q11. How do you perform reading/writing of files with Node JS?

Sample Answer: In Node JS, the fs module allows reading and writing files. Using writeFileSync, you can write data to a file synchronously, and readFileSync lets you read file contents synchronously. 

The example below demonstrates writing to a file and then reading its contents:

const fs = require('fs');
//Writing to a file
fs.writeFileSync('example.txt', 'Hello, Node JS!');
// Reading from a file
const data = fs.readFileSync('example.txt', 'utf8'); console.log(data); // Output: Hello, Node JS!

Q12. Explain the importance of the process object in Node JS.

Sample Answer: Process is a built-in global variable in Node JS that provides attributes on the currently running process, such as command line parameters and environmental variables. 

Below is the code for the process object in Node JS:

console.log(process.argv); // Outputs command-line arguments

Q13. How does one write a simple HTTP server in Node JS?

Sample Answer: The module HTTP provided by Node JS is used for creating a server that handles requests and responses. 

The following code is used to write the same:

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

Q14. What is middleware in Node JS? Provide an express.js example.

Sample Answer: In Express.js, middleware functions are executed during the request-response cycle and can modify the request or response, terminate the cycle, or pass control to the next middleware. For example, in the provided code, the middleware logs a message and calls next() to continue to the next middleware or route handler. 

Below is an example of the code for middleware in Node JS:

const express = require('express');
const app = express();
app.use((req, res, next) => {
});
console.log('Middleware executed');
next();
app.get('/', (req, res) => {
});
res.send('Hello, Express.js!');
app.listen(3000, () => {
});
console.log('Server running on http://localhost:3000');

Node JS Coding Interview Questions for Intermediate Professional

At the intermediate professional level, Node JS coding interview questions often focus on advanced topics like optimizing application performance, implementing robust security measures, and strategies for scaling applications effectively. In this segment of the blog, you will find Node JS coding interview questions with answers for senior applicants that should help demonstrate your ability to create optimized and well-performing applications.

Q15. What is the functionality of the Node JS cluster module?

Sample Answer: The cluster module is used by Node JS to create child processes (workers) that listen to the same server port – thus allowing the application to make use of multiple CPU cores and achieve parallelism. This helps appease the demand for a particular application through efficient division of workload between the CPU cores for speed enhancement.

Q16. What methods can be applied to optimize a Node JS application?

Sample Answer: To optimize a Node JS application, you can implement strategies such as clustering to utilize multiple CPU cores, caching data to reduce redundant computations, and streamlining database queries. Additionally, using asynchronous programming helps handle concurrent requests more efficiently. 

The following code demonstrates how to use the Node JS Cluster module to improve application performance:

const cluster = require('cluster');
const http = require('http');
const os = require('os');
if (cluster.isMaster) {
const numCPUs = os.cpus().length;
for (let i = 0; i < numCPUs; i++) {
cluster.fork();
}
} else {
http.createServer((req, res) => {
res.writeHead(200);
res.end('Hello, World!');
}).listen(3000);

Q17. How can you optimize the performance of a Node JS application?

Sample Answer: Optimizing the performance of a Node JS application is essential for ensuring scalability, reliability, and faster response times. 

Here are key techniques to enhance performance:

• Caching with Redis 
• Implementing Load Balancing
• Optimise Database Queries 
• Monitor and scale with PM2
• Use Asynchronous Programming

Q18. How can we manage failures due to unhandled exceptions in a Node JS application?

Sample Answer: Unhandled exceptions can be managed using process.on(‘uncaughtException’, callback) to catch and log unexpected errors. However, it’s recommended to log the error and gracefully shut down the application, as continuing after an unhandled exception may lead to unpredictable behavior or instability. 

The code given below can be used for this function:

process.on('uncaughtException', (err) => {
});
console.error('There was an uncaught error:', err);
process.exit(1); // Gracefully shut down the application
throw new Error('Oops!');

Q19. Illustrate what process.nextTick() is and its application.

Sample Answer: ‘process.nextTick()’ schedules a function to run after the current operation, before the event loop continues. It’s used for deferring execution until the current task is completed, handling errors in asynchronous tasks, splitting long-running processes into smaller chunks, and ensuring immediate execution for critical initialization logic.

Q20. How can you prevent exposure to sensitive data while writing a Node JS application?

Sample Answer: To avoid the hard coding of sensitive data such as API tokens and database passwords, tools like dotenv that help in using environment variables can be used. 

To prevent exposure of data, the below code is usually used:

require('dotenv').config();
console.log('Your secret key is: ${process.env. SECRET_KEY}');

Pro Tip: To improve your coding skills further, practice Node JS projects. Check out our guide on Node JS projects with source code from beginners to advanced level. Practicing the projects will help you prepare for the Node JS coding job interview.

Q21. In the file systems- fs module, how does the readFileSync differ from readFile?

Sample Answer: The ‘readFileSync’ method reads a file in a blocking way, meaning it stops other tasks from running until the file is fully read. On the other hand, ‘readFile’ reads the file in a non-blocking way, allowing other tasks to continue while the file is being read.

Q22. What measures can be taken against SQL Injection in a Node JS Application?

Sample Answer: To prevent SQL injection in a Node JS application, use parameterized queries or prepared statements. These techniques ensure that user input is treated as data, not executable SQL code, effectively blocking injection attacks. 

The following code is used to prevent SQL injection:

const mysql = require('mysql2');
const connection = mysql.createConnection({
});
host: 'localhost',
user: 'root',
password: ",
database: 'test',
const username = 'user1';
const query = 'SELECT * FROM users WHERE username = ?';
connection.execute(query, [username], (err, results) => {
});
if (err) throw err;
console.log(results);

Q23. How can you implement rate limiting in a Node JS application?

Sample Answer: Rate limiting controls the number of requests a user can make to a server within a specific time, protecting applications from abuse like DDoS attacks and ensuring fair resource usage. In Node JS, it can be implemented using express-rate-limit. For scalable setups, integrating Redis allows distributed rate limiting across multiple instances, ensuring consistent enforcement.

The following code can also be used to implement rate limiting:

const express = require('express');
const rateLimit = require('express-rate-limit');
const app = express();
const limiter = rateLimit({
});
windowMs: 15 * 60 * 1000, // 15 minutes
max: 100, // limit each IP to 100 requests per windowMs
app.use(limiter);
app.get('/', (req, res) => res.send('Rate limited!'));
app.listen(3000);

Q24. How do we deal with file uploads in a Node JS application?

Sample Answer: In the Express framework, file uploads are managed using the Multer middleware, which handles file storage in specified folders while also capturing and processing file details. 

The following is the code file uploads in Nodes JS:

const express = require('express');
const multer = require('multer');
const upload = multer({ dest: 'uploads/' });
const app = express();
app.post('/upload', upload.single('file'), (req, res) => {
});
res.send('File uploaded successfully!');
app.listen(3000);

Q25. How do we perform caching in Node JS?

Sample Answer: Caching in Node JS involves storing frequently accessed data temporarily to reduce redundant processing and improve application performance. This can be done in memory or using external caching systems like Redis. The example below demonstrates simple in-memory caching using a JavaScript object. 

The following code can be used to perform caching in Node JS:

const express = require('express');
const cache = {};
const app = express();
app.get('/data', (req, res) => {
const key = 'uniqueKey';
if (cache[key]) {
}
return res.send(cache[key]);
const data = { message: 'This is cached data' };
cache[key] = data;
res.send(data);
});
app.listen(3000);

Q26. How can you enforce different levels of async execution in Node JS?

Sample Answer: In Node JS, different levels of asynchronous execution can be enforced using async/await to handle sequential execution of promises. In the code, tasks are processed one by one using await to ensure each task is completed before moving to the next. 

The following is the code to enforce async:

const express = require('express');
const cache = {};
async function process Tasks() {
const task1 = await new Promise((resolve) => setTimeout(() => resolve('Task 1 completed"), 1000));
console.log(task1);
const task2 = await new Promise((resolve) => setTimeout(() => resolve('Task 2 completed'), 1000));
console.log(task2);
processTasks();

Q27. How can you monitor and debug in Node JS applications?

Sample Answer: Tools like PM2 and Node JS debugging flags (-–inspect) and monitoring applications with console.time() are helpful. Here is the code that can be used to monitor and debug in Node JS application:

const http = require('http');
const server = http.createServer((req, res) => {
});
console.time('Request duration');
res.end('Hello, Debugging!');
console.timeEnd('Request duration');
server.listen(3000, () => console.log('Server running on port 3000'));

Q28. How can you compose a WebSocket server using Node JS?

Sample Answer: A WebSocket server in Node JS can be created using the ws library. Start by initializing a server on a specific port and listening for client connections. Handle connection and message events to establish real-time communication, enabling the server to send and receive messages dynamically with connected clients. 

To compose a WebSocket server, the code given below is used:

const WebSocket = require('ws');
const server = new WebSocket.Server({ port: 8080 });
server.on('connection', (socket) => {
console.log('Client connected');
socket.on('message', (message) => {
console.log('Received: ${message}');
socket.send('Message received!');
});
});

Node JS Coding Interview Questions and Answers for Experienced Professionals

For experienced professionals, Node JS coding interview questions focus on advanced topics like microservices architecture, application scaling, and debugging complex issues. This section provides Node JS coding interview questions and answers designed to evaluate the ability to build efficient, scalable, and maintainable applications.

Q29. What is the difference between horizontal and vertical scaling in Node JS?

Sample Answer: Horizontal scaling involves running the application on multiple servers or CPU cores by creating additional instances, while vertical scaling enhances a single instance by adding more resources, such as CPUs or RAM. In a Node JS environment, horizontal scaling is often preferred because it is more reliable and easier to manage, especially for handling high traffic and ensuring fault tolerance.

Q30. How can you create a simple microservice using Node JS and Express?

Sample Answer: A simple microservice in Node JS can be created using the Express framework. Define specific routes for the service (e.g., /service1) and use the app.listen to run the microservice on a designated port, enabling lightweight and efficient functionality. 

The code given below can also be used for this process:

const express = require('express');
const app = express();
app.get('/service1', (req, res) => res.send('Microservice 1 Response'));
app.listen(3001, () => console.log('Microservice 1 running on port 3001'));

Q31. What shall be done to combat memory leaks in a Node JS program?

Sample Answer: To combat memory leaks in a Node JS program, use tools like Chrome DevTools or Node JS built-in diagnostics to monitor memory usage. Avoid global variables, properly handle event listeners, and ensure efficient garbage collection by managing object references carefully.

Q32. In what specific way can a message queue be a solution for service decoupling?

Sample Answer: A message queue enables service decoupling by acting as an intermediary for communication between services. It allows one service to publish messages asynchronously, while other services consume them independently. This ensures scalability, fault tolerance, and reduced dependency between services.

Q33. Give a brief on microservices and how to incorporate them in Node JS applications. 

Sample Answer: Microservices is an architectural style wherein applications are broken into independent services, each handling a specific functionality. In Node JS, microservices can be built using frameworks like Express or Koa, communicate via REST or gRPC, and deployed using Docker or Kubernetes. This ensures scalability, flexibility, and easier maintenance of applications.

Q34. What is the load balancing technique in a Node JS application?

Sample Answer: In a Node JS application, load balancing is achieved using the cluster module. It distributes incoming requests across multiple worker processes. This technique ensures efficient utilization of all CPU cores, improving performance and handling higher traffic loads effectively. 

The code for the load balancing technique is:

const cluster = require('cluster');
const http = require('http');
const os = require('os');
if (cluster.isMaster) {
const numCPUs = os.cpus().length;
for (let i = 0; i < numCPUs; i++) {
cluster.fork();
}
} else {
}
http.createServer((req, res) => {
res.writeHead (200);
res.end('Handled by Worker Process');
}).listen(3000);

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Q35. What is load-balancing in Node JS, and how can it be achieved?

Sample Answer: Load balancing in Node JS resolves performance issues by distributing traffic across multiple server processes or instances. This is achieved using reverse proxy servers like NGINX, the Node JS cluster module, or cloud-based solutions like AWS Elastic Load Balancer for scalability. 

Q36. What advanced logging management tools does a Node JS application use?

Sample Answer: Advanced logging tools for Node.js applications include:

• Winston: Flexible, supports multiple transports, and customizable formats.
• Pino: High-performance, lightweight, and developer-friendly.
• Morgan: Specialized for HTTP request logging in Express.js.
• Log4js: Pattern-based logging inspired by Log4j.
• Elastic Stack (ELK): Centralized logging with powerful visualization.
• Graylog: Centralized log management for high-volume logs.
• Datadog: Cloud-based real-time log monitoring.
• Fluentd: Log aggregation and forwarding.

These tools enhance debugging, monitoring, and scaling applications effectively.

Q37. What role does Redis play in implementing caching as a service in Node JS applications?

Sample Answer: Redis serves as a distributed in-memory caching system that helps improve the performance of Node JS applications. By storing frequently accessed data, Redis reduces the number of direct database queries, enabling faster data retrieval. This approach enhances application speed and scalability, making Redis an essential tool for caching as a service in Node JS.

Q38. How can you stop a Node JS app that is currently running without forcing it to close?

Sample Answer: When shutting an application down, it is important to take care of signals that allow termination (SIGINT). This will allow for the closing of all resources (database connections) properly. The following is the code used for this process:

process.on('SIGINT', () => {
});
console.log('Closing application...');
process.exit();
setInterval(() => console.log('Running...'), 1000);

Q39. How does one handle API versioning in an application written in Node JS?

Sample Answer: Several techniques can be used to implement API versioning including the use of versioned endpoints as provided by the application. To handle API through a code, the following is used:

const express = require('express');
const app = express();
app.get('/v1/resource', (req, res) => res.send('Response from API v1'));
app.get('/v2/resource', (req, res) => res.send('Response from API v2')); app.listen(3000, () => console.log('API versioning in action'));

Q40. What are the approaches for debugging without terminating the mobile application process written in Node JS?

Sample Answer: This can be done using the –inspect-brk flag, which lets you start a Node JS application in debugging mode along with speech tools such as Chrome dev tools for detailed debugging. The following is the code used for the same:

// Run using: node --inspect-brk app.js
const http = require('http');
http.createServer((req, res) => {
debugger; //Debugging breakpoint
res.end('Debugging in action');
}).listen(3000);

Q41. How can you implement rate limiting in a Nodes JS application using Redis for scalability?

Sample Answer: Rate limiting in a Node JS application helps prevent excessive requests to a server, ensuring stability and fair usage of resources. Redis enables distributed rate limiting across multiple instances, making it highly scalable. 

To implement this, use a package like express-rate-limit with Redis as the backend. Redis stores request counts and enforce limits across distributed instances to ensure throttling policies are applied consistently. The following code is used to implement rate limiting:

const rateLimit = require('express-rate-limit');
const RedisStore = require('rate-limit-redis');
const limiter = rateLimit({
});
store: new RedisStore({}),
max: 100,
windowMs: 15* 60 * 1000, // 15 minutes
app.use(limiter);

Q42. What is the use of  WebSockets?

Sample Answer: WebSocket allows users to exploit bi-directional instantaneous communications, which are ideal for chat applications, notifications, and active dashboards. The following is the code for using WebSockets using Node JS:

const WebSocket = require('ws');
const wss = new WebSocket.Server({ port: 8080 });
wss.on('connection', (ws) => {
ws.on('message', (message) => {
});
console.log('Received: ${message}');
ws.send('Acknowledged!');
});

Tips to Prepare for Node JS Coding Interview

To prepare for Node JS coding interview questions, focus on mastering key concepts and applying them to real-world scenarios. Practice interview questions in advance to handle technical and behavioral questions confidently while showcasing your skills and knowledge.

Here are some tips to prepare for Node JS coding interview questions:

1. Examine the Job’s Specifications: Read the job specification and find what Node JS skills or technologies the position stresses. 
2. Focus on the Foundational Node JS Concepts: Learn the essential Node JS topics, such as event loops, asynchronous programming, streams, error handling, and so on. Be prepared to put your skill or knowledge into practice through coding exercises and explaining concepts thoroughly.
3. Be Familiar with Widely Used Technologies: If you are applying for a senior position, knowledge of frameworks. This includes Express JS and authentication libraries such as JSON. Be prepared to tell how you used them and how they influenced project development and performance.
4. Perform Coding Integrating Real-Life Experience: Compete with practice coding problems concerning Node JS such as designing RESTful interfaces, processing HTTP requests, working with persistent storage databases, etc. Concentrate on providing well-formatted codes and solving problems under time restrictions.
5. Prepare Behavioral Interview Questions: Companies value teamwork and problem-solving skills. Prepare answers around teamwork efforts, project deadlines, and overcoming challenges using the STAR technique (situation, task, action, and result).
6. Be Aware of Security Best Practices: Know about security best practices on input validation, rate limiting, and HTTPS. One should also know techniques for improving Node JS performance, including caching and load balancing, etc.

Conclusion

The selection process for the Node JS developer tests one’s technical skills, problem-solving abilities, and knowledge of the best practices. The selected candidates must have practical experience and showcase teamwork capabilities. Prepare by practising additional Node JS coding interview questions and answers to showcase your experience and technical skills effectively. For more elaborative interview preparation, check out our guide on the Node JS interview questions.

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