Node.js Interview Questions

Question 1

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

Accepted Answer

Node.js is a JavaScript runtime built on V8 that runs on servers, with access to file system, network, and OS — unlike browser JS. Node.js uses Chrome's V8 engine but adds server-side APIs: fs (file system), http (web server), crypto, path, os. No DOM, window, or document objects. Uses CommonJS modules (require) alongside ES modules. Single-threaded event loop handles concurrent I/O. npm is the world's largest package registry. Used for APIs, microservices, real-time apps, CLI tools, and build tools.

Question 2

What is the event loop in Node.js?

Accepted Answer

The event loop is Node's mechanism for handling non-blocking I/O by offloading operations to the system kernel or thread pool. Phases: (1) Timers: execute setTimeout/setInterval callbacks. (2) Pending callbacks: system-level callbacks. (3) Idle/Prepare: internal. (4) Poll: retrieve new I/O events, execute I/O callbacks. (5) Check: setImmediate callbacks. (6) Close callbacks: socket.on('close'). process.nextTick() runs between phases (microtask). Promises run in microtask queue. libuv provides the thread pool (default 4 threads) for blocking operations like DNS lookup and file I/O.

Question 3

What is middleware in Express.js?

Accepted Answer

Middleware functions have access to request, response, and next() function, executing code in a pipeline for each request. Middleware runs sequentially: app.use((req, res, next) => { ... next(); }). Types: application-level, router-level, error-handling (4 args), built-in (express.json()), third-party (cors, helmet). Can modify req/res, end the request cycle, or call next(). Error middleware: (err, req, res, next). Common uses: authentication, logging, CORS, body parsing, rate limiting, input validation.

Question 4

What are streams in Node.js?

Accepted Answer

Streams are objects for reading/writing data in chunks rather than loading everything into memory at once. Types: Readable (fs.createReadStream), Writable (fs.createWriteStream), Duplex (TCP socket), Transform (zlib compression). Use pipe() to connect streams: readStream.pipe(writeStream). Events: data, end, error, finish. Streams handle large files efficiently — process 10GB file with constant memory. Backpressure mechanism prevents overwhelming slow consumers. Pipeline utility handles errors: pipeline(source, transform, dest, callback).

Question 5

What is npm and what is package.json?

Accepted Answer

npm is Node's package manager; package.json defines project metadata, dependencies, scripts, and configuration. npm install downloads packages to node_modules. Key package.json fields: dependencies (production), devDependencies (development), scripts (custom commands), engines (Node version). package-lock.json locks exact versions for reproducible builds. Semantic versioning: ^1.2.3 (minor updates), ~1.2.3 (patch only), 1.2.3 (exact). npx executes packages without installing. npm workspaces support monorepos.

Question 6

How do you handle errors in Node.js?

Accepted Answer

Use try-catch for synchronous code, .catch() for promises, error-first callbacks, and process.on('uncaughtException'). Strategies: (1) Synchronous: try-catch blocks. (2) Callbacks: error-first pattern (err, data). (3) Promises: .catch() or try-catch with async/await. (4) Express: error middleware (err, req, res, next). (5) Global handlers: process.on('uncaughtException'), process.on('unhandledRejection'). (6) Custom error classes extending Error for typed errors. Best practice: fail fast, log everything, return appropriate HTTP status codes, never swallow errors silently.

Question 7

What is the cluster module and how does it work?

Accepted Answer

The cluster module creates child processes (workers) that share the same server port, utilizing multiple CPU cores. Node.js is single-threaded. cluster.fork() creates worker processes, each running a copy of the server. The master process distributes incoming connections (round-robin on Linux). Workers share the same port. If a worker crashes, the master can restart it. PM2 abstracts this: pm2 start app.js -i max. Worker threads (worker_threads module) are different — they share memory via SharedArrayBuffer for CPU-intensive tasks.

Question 8

Explain the difference between SQL and NoSQL databases in the context of Node.js.

Accepted Answer

SQL databases use structured tables with relations (PostgreSQL); NoSQL uses flexible documents (MongoDB) — Node.js works with both. SQL (PostgreSQL, MySQL): ACID compliance, joins, structured schemas, ORMs like Sequelize/Prisma. Best for: complex queries, transactions, relational data. NoSQL (MongoDB): flexible schemas, horizontal scaling, native JSON, ODM like Mongoose. Best for: rapid prototyping, unstructured data, high write throughput. Node.js ORMs: Prisma (type-safe, both SQL/NoSQL), Sequelize (SQL), Mongoose (MongoDB). Choose based on data relationships, scale needs, and query patterns.

Question 9

How do you implement authentication with JWT in Node.js?

Accepted Answer

Generate JWT on login with jsonwebtoken, send in response, verify on protected routes via middleware. Flow: (1) User sends credentials. (2) Server validates, creates JWT: jwt.sign({ userId }, secret, { expiresIn: '1h' }). (3) Client stores token (httpOnly cookie preferred). (4) Auth middleware: jwt.verify(token, secret). (5) Attach decoded user to req. (6) Refresh tokens: long-lived token to get new access tokens. Security: use strong secrets, short expiration, HTTPS only, httpOnly cookies. Libraries: passport.js for strategy-based auth, bcrypt for password hashing.

Question 10

What is rate limiting and how do you implement it?

Accepted Answer

Rate limiting restricts the number of requests from a client within a time window to prevent abuse and DDoS attacks. Implementation: (1) express-rate-limit middleware: windowMs (time window), max (request limit). (2) Redis-based for distributed systems (rate-limit-redis). (3) Algorithms: Fixed window, Sliding window, Token bucket, Leaky bucket. (4) Apply per IP, per user, or per API key. (5) Return 429 Too Many Requests with Retry-After header. (6) Different limits for different endpoints (stricter for login, lenient for reads). Also consider: API key tiers, cost-based limiting.

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Express.js Framework

Database Integration

Authentication & Security

API Development & Documentation

Testing & Deployment

Real-time Applications

Microservices & Architecture

Advanced Node.js Patterns

Advanced Microservices & Scaling

DevOps & Deployment

Advanced Patterns & Best Practices

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