Node js Interview Questions & Answers

Top frequently asked interview questions with detailed answers, code examples, and expert tips.

51 Questions All Difficulty Levels Updated Mar 2026

What is Node.js? Easy

Node.js is a JavaScript runtime built on Chrome V8 engine that allows running JavaScript on the server-side.

Key Point: Non-blocking, event-driven architecture.

nodejs basics architecture

What is the Event Loop in Node.js? Easy

The Event Loop handles asynchronous callbacks and ensures non-blocking execution.

event-loop async

What is npm? Easy

npm (Node Package Manager) manages dependencies and packages in Node.js applications.

npm package-manager

Difference between CommonJS and ES Modules? Medium

CommonJS uses require(). ES Modules use import/export syntax.

modules commonjs esm

What is middleware in Express? Medium

Middleware functions execute during request-response lifecycle.

express middleware

Explain callback vs Promise vs async/await. Medium

Callbacks are traditional async functions. Promises improve readability. Async/await simplifies promise usage.

async promise

What is JWT? Medium

JSON Web Token is used for authentication between client and server.

jwt authentication

How does Node handle concurrency? Medium

Node uses single-threaded event loop with non-blocking I/O.

concurrency event-loop

What is clustering in Node? Hard

Cluster module allows Node to use multi-core systems.

cluster scaling

What are streams in Node.js? Medium

Streams process data in chunks, improving memory efficiency.

streams performance

Explain process.nextTick(). Hard

Executes callback after current operation completes.

event-loop nexttick

How to secure Node.js app? Hard

Use HTTPS, helmet, rate limiting, input validation.

security production

What is CORS? Easy

Cross-Origin Resource Sharing controls resource access between domains.

cors http

What is Buffer? Easy

Buffer handles binary data in Node.

buffer binary

What is REST API? Easy

Representational State Transfer architecture style for APIs.

api rest

How to handle errors in Express? Medium

Use centralized error handling middleware.

error-handling express

What is Redis used for? Medium

Redis is used for caching and session management.

redis caching

How to improve Node.js performance? Hard

Use clustering, caching, async operations.

performance scaling

What is PM2? Medium

PM2 is a production process manager for Node.js apps.

pm2 devops

Explain microservices in Node. Hard

Microservices break application into small independent services.

microservices architecture

What is package.json? Easy

package.json is a configuration file that stores metadata about a Node.js project including dependencies, scripts, version, and project details.

package json npm

What is npx? Easy

npx is a package runner tool that executes Node packages without globally installing them.

npx npm

What is __dirname in Node.js? Easy

__dirname is a global variable that returns the directory path of the current module.

globals nodejs

What is __filename? Easy

__filename returns the absolute path of the currently executing file.

globals nodejs

What is dotenv? Easy

dotenv is a package used to load environment variables from a .env file into process.env.

environment config

What is the difference between spawn() and exec()? Medium

spawn() launches a new process with a stream interface. exec() runs a command and buffers the output.

child-process system

What is child_process module? Medium

The child_process module allows Node.js to spawn subprocesses to execute system commands.

child-process

What is Helmet in Express? Medium

Helmet is middleware that helps secure Express apps by setting HTTP headers.

security express

What is rate limiting? Medium

Rate limiting restricts the number of requests a client can make in a given time to prevent abuse.

security rate-limit

What is Morgan? Medium

Morgan is an HTTP request logger middleware for Node.js.

logging express

What is the difference between synchronous and asynchronous APIs? Medium

Synchronous APIs block execution, while asynchronous APIs allow non-blocking operations.

async nodejs

What is process.env? Medium

process.env is an object containing environment variables for the Node.js process.

environment

What is WebSocket? Medium

WebSocket enables real-time two-way communication between client and server.

realtime socket

What is Socket.io? Medium

Socket.io is a library that enables real-time bidirectional event-based communication.

socketio realtime

What is Sequelize? Medium

Sequelize is an ORM for Node.js used to interact with SQL databases.

sequelize orm

What is Mongoose? Medium

Mongoose is an ODM for MongoDB in Node.js applications.

mongoose mongodb

What is API throttling? Medium

API throttling limits the number of API requests to maintain system stability.

api security

What is load balancing? Medium

Load balancing distributes traffic across multiple servers to improve reliability and scalability.

scaling architecture

What is message queue? Medium

A message queue allows asynchronous communication between services.

queue architecture

What is Bull queue? Medium

Bull is a Redis-based queue library for handling background jobs.

queue redis

Explain Node.js internal architecture. Hard

Node.js uses V8 engine, libuv for async I/O, event loop, and single-threaded architecture.

architecture nodejs

What is libuv? Hard

libuv is a multi-platform C library that provides asynchronous I/O operations in Node.js.

libuv event-loop

How does Node.js handle CPU intensive tasks? Hard

Node.js uses worker threads or child processes to handle CPU-intensive tasks.

worker-threads performance

What are Worker Threads? Hard

Worker threads allow running JavaScript in parallel threads.

worker-thread parallel

What is backpressure in streams? Hard

Backpressure is a mechanism to control data flow between readable and writable streams.

streams performance

Explain garbage collection in Node.js. Hard

Node.js uses V8 garbage collector to manage memory automatically.

memory v8

What is event emitter pattern? Hard

EventEmitter allows objects to emit named events and attach listeners.

event-emitter design-pattern

What is Zero-downtime deployment? Hard

Zero-downtime deployment updates application without interrupting service.

deployment devops

How to implement circuit breaker in Node.js? Hard

Circuit breaker prevents repeated failures by temporarily blocking calls to failing service.

resilience architecture

What is API gateway? Hard

API Gateway acts as single entry point for multiple backend services.

microservices architecture

How to scale Node.js application horizontally? Hard

Use clustering, load balancers, container orchestration (Docker + Kubernetes).

scaling kubernetes

📊 Questions Breakdown

🟢 Easy 11

🟡 Medium 24

🔴 Hard 16

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