Node.js Backend Developer Interview Prep (2025)

Hi are you Preparing for Senior Backend Developer role at your dream company? Here are very frequently asked NodeJS Interview question that will let interviewer know that you have very well knowledge in NodeJS/JavaScript core foundation. We’ve organized them into key areas: Core Node.js Concepts, API Design, and Authentication & Security. This are most important concept that is definingly 100% asked in an JavaScript/NodeJS Backend Developer interview role.

Core Node.js Concepts

1. Explain the Node.js Event Loop and How It Works

The event loop is a fundamental concept in Node.js. It allows Node.js to perform non-blocking I/O operations by offloading operations to the system kernel whenever possible. When operations are completed, their callbacks are added to the event queue and executed in a loop. This makes Node.js efficient and suitable for I/O-heavy tasks.

2. Difference Between process.nextTick(), setImmediate(), and setTimeout()

• process.nextTick(): Executes a callback immediately after the current operation completes, before the event loop continues.
• setTimeout(): Executes a callback after a minimum delay in the timer phase.
• setImmediate(): Executes a callback in the check phase, after I/O events are processed.

These methods help control the order of execution in asynchronous operations.

3. How Node.js Handles Asynchronous Operations

Node.js uses the event loop and non-blocking I/O, managed by the libuv library. Developers can write asynchronous code using callbacks, Promises, or the async/await syntax. This allows handling thousands of requests concurrently without blocking the main thread.

4. What Is the Role of libuv in Node.js?

libuv is a multi-platform C library that provides support for asynchronous I/O operations such as file system, DNS, and TCP/UDP. It is the backbone for Node.js’s event loop, allowing Node.js to abstract and perform non-blocking tasks.

5. Difference Between CommonJS and ES Modules

• CommonJS: Uses require() and module.exports, synchronous in nature. It’s the default in older Node.js versions.
• ES Modules: Use import and export, support asynchronous loading, and are becoming the standard in modern JavaScript development.

6. What Is Clustering in Node.js?

Clustering allows Node.js to utilize multiple CPU cores. The cluster module can spawn child processes (workers) that share the same server port. It’s beneficial for high-performance applications that handle numerous concurrent requests.

API Design & RESTful Services

1. How Do You Design Scalable RESTful APIs?

Scalable REST APIs should follow resource-oriented design, use appropriate HTTP methods (GET, POST, PUT, DELETE), implement pagination, caching, proper error handling, and validation. Consider stateless communication and use reverse proxies/load balancers for distributing traffic.

2. Key Principles of REST Architecture

• Stateless: No client context stored on the server.
• Client-Server: Separation of concerns.
• Cacheable: Responses must define themselves as cacheable or not.
• Uniform Interface: Standard methods and resource URIs.
• Layered System: Supports scalability via layered architecture.
• Code on Demand (optional): Servers can provide executable code.

3. How Do You Handle API Versioning?

API versioning allows backward compatibility. Strategies include:

• URI versioning: /api/v1/resource
• Query parameters: /resource?version=1
• HTTP headers: Accept: application/vnd.api+json;version=1

URI versioning is the most common and easily maintainable.

4. How Would You Implement Rate Limiting?

Rate limiting prevents abuse by restricting how many requests a user can make within a certain time frame. Use middleware like express-rate-limit or Redis-backed implementations for distributed environments.

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);

5. Explain the Importance of Idempotency in HTTP Methods

Idempotency means that making multiple identical requests has the same effect as making a single request. Methods like GET, PUT, and DELETE should be idempotent. This ensures reliability and safe retries in distributed systems.

Authentication & Security

1. How Do You Implement Authentication and Authorization in Node.js?

Use libraries like Passport.js for modular authentication strategies, or implement custom middleware. JWT (JSON Web Tokens) are common for stateless authentication in APIs. For authorization, manage user roles and permissions at route levels.

2. What Is the Difference Between JWT and Session-Based Authentication?

• JWT: Token stored on the client side (usually in localStorage), stateless, ideal for APIs and mobile apps.
• Session-Based: Session data stored on the server with session ID in a cookie. Better suited for traditional web apps.

3. How Do You Protect Node.js Apps from Common Security Threats?

• XSS: Use output encoding, CSP headers.
• CSRF: Implement CSRF tokens, SameSite cookies.
• SQL Injection: Use ORM libraries or parameterized queries.

Always validate and sanitize input from users.

4. What Is helmet.js and How Does It Improve Security?

Helmet is a middleware that sets various HTTP headers to secure your app:

• Prevents clickjacking
• Adds Content Security Policy
• Hides X-Powered-By header

Install with:

npm install helmet

Use it in Express:

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

5. How Do You Securely Store and Manage API Keys and Secrets?

• Use environment variables (process.env) and keep them out of source control.
• Use secret managers like AWS Secrets Manager, Azure Key Vault, or HashiCorp Vault.
• Never hardcode credentials in your codebase.

Conclusion

This article gives you the top interview Question with Answers related to NodeJS Interview for the role of senior backend interview, This article especially is focused on JavaScript concept and Node.js, Yes, Code skill is required but having basic foundation knowledge is more necessary to clear any interview. Use this article to revision fundamental concepts of JS & NodeJS and align your answers with what hiring managers are looking for: clarity, scalability, and security.

Note: Keep practicing with real-world code examples and stay updated with Node.js improvements. Good luck!