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Node.js Interview Questions

1. Core Concepts and Architecture​

  1. What is Node.js, and what makes it different from other server-side technologies?

    • How does Node.js's single-threaded nature impact its performance under high concurrency?
    • What kind of workloads is Node.js NOT well-suited for, and why?
    • How does Node.js compare to Go or Rust for backend services?

  2. What is the event loop in Node.js?

    • Can you explain the phases of the event loop in detail (timers, I/O, idle, poll, check, close)?
    • What is the difference between the microtask queue and the macrotask queue?
    • How do process.nextTick and Promise.resolve interact with the event loop phases?

  3. What is the difference between synchronous and asynchronous code in Node.js?

    • How do you handle CPU-intensive synchronous tasks without blocking the event loop?
    • What happens to other requests while a synchronous operation is running?
    • How do you offload synchronous work to Worker Threads?

  4. What is the role of V8 in Node.js?

    • How does V8 optimize JavaScript execution using JIT compilation?
    • What is hidden class optimization in V8 and how does it affect performance?
    • What is the difference between V8's "young generation" and "old generation" memory spaces?

  5. What are the benefits of using Node.js for backend development?

    • When would you choose Node.js over Django, Spring, or Laravel?
    • How does sharing code between frontend and backend (isomorphic JS) benefit a team?
    • What is the npm ecosystem's role in Node.js's popularity?

  6. What is the difference between Node.js and traditional multi-threaded servers?

    • How does Node.js handle concurrency without threads using the event loop?
    • What is the C10K problem and how does Node.js address it?
    • What are the downsides of the single-threaded model (e.g., unhandled exceptions crashing the process)?

  7. What is a REPL in Node.js?

    • How do you use the Node.js REPL for quick debugging and prototyping?
    • How do you load a module or file into the REPL session?
    • What is the _ variable in the REPL and what does it store?

  8. What is the purpose of the process object in Node.js?

    • How do you access environment variables using process.env?
    • What is process.argv and how do you use it to parse CLI arguments?
    • How do you gracefully shut down a Node.js process using process.exit and signal handlers?

  9. What are streams in Node.js, and why are they useful?

    • What are the four types of streams (Readable, Writable, Duplex, Transform)?
    • Can you give an example of using a Transform stream to compress data on the fly?
    • What is backpressure in streams and how do you handle it?

  10. What is the difference between Node.js's CommonJS and ES Modules?

    • How do you migrate a CommonJS project to ES Modules?
    • What is the difference between require() (synchronous) and import (asynchronous evaluation)?
    • How do you use ES Modules in Node.js without a bundler (.mjs, "type": "module")?

2. Modules and Packages​

  1. What is the require function in Node.js?

    • How does require resolve module paths (node_modules lookup, relative paths, core modules)?
    • What happens if require cannot find a module?
    • How does require differ from dynamic import() in ES Modules?

  2. What is the purpose of module.exports?

    • How do you export multiple functions or classes from a module?
    • What is the difference between module.exports and exports?
    • What happens if you reassign exports directly instead of module.exports?

  3. What is the Node Package Manager (npm), and how does it work?

    • What is the difference between npm install and npm ci?
    • What does npm install --save-exact do and when would you use it?
    • What is the purpose of package-lock.json and why should it be committed?

  4. What is a package.json file, and what are its key fields?

    • How do you manage dependencies vs devDependencies correctly?
    • What is the engines field and how does it enforce Node.js version requirements?
    • What is the scripts field and how do you chain npm scripts?

  5. What is the difference between dependencies and peerDependencies?

    • When would you use optionalDependencies?
    • What happens if a peer dependency is not installed?
    • What is bundledDependencies and when is it used?

  6. How does Node.js handle module caching?

    • How can you bypass module caching in development (delete from require.cache)?
    • What are the implications of module caching on singleton patterns?
    • How does module caching behave differently with ES Modules vs CommonJS?

  7. What is Yarn, and how does it differ from npm?

    • What is the advantage of Yarn's Plug'n'Play (PnP) feature?
    • How does Yarn workspaces help manage monorepos?
    • What is pnpm and how does it differ from both Yarn and npm in disk usage?

  8. What is the purpose of the node_modules folder?

    • How do you optimize node_modules size in production (pruning, npm prune --production)?
    • What is the problem with deeply nested node_modules and how did npm v3+ fix it?
    • What is the .npmrc file and what configurations can it hold?

  9. How do you create a custom Node.js module?

    • How do you publish a module to the npm registry?
    • What is semantic versioning (semver) and how does it apply to npm packages?
    • How do you create a scoped package (e.g., @myorg/mypackage)?

  10. What is the difference between global and local npm packages?

    • When would you install a package globally vs locally?
    • What is npx and how does it avoid the need for global installs?
    • How do you manage multiple global Node.js versions with tools like nvm or volta?

3. Asynchronous Programming​

  1. What are callbacks in Node.js, and what is callback hell?

    • How do you avoid callback hell using Promises or async/await?
    • What is the error-first callback convention and why is it used?
    • How do you convert a callback-based library to Promises using util.promisify?

  2. What are Promises in Node.js, and how do they work?

    • How do you handle Promise rejections globally using process.on('unhandledRejection')?
    • What is Promise chaining and what are its pitfalls?
    • What is the difference between Promise.resolve() and new Promise(resolve => resolve())?

  3. What is async/await, and how does it simplify asynchronous code?

    • How do you handle errors in async/await using try/catch?
    • What happens if you forget the await keyword before a Promise?
    • How do you run multiple async operations in parallel using async/await?

  4. What is the difference between setTimeout, setInterval, and setImmediate?

    • When would you use setImmediate over setTimeout(fn, 0)?
    • How do you cancel a setInterval and why is it important to do so?
    • What is queueMicrotask and how does it differ from setImmediate?

  5. What is the async module, and when is it useful?

    • How does async.waterfall compare to native Promise chaining?
    • What is async.parallel and async.series and when do you use each?
    • How does the async module compare to native Promise.all?

  6. How do you handle parallel asynchronous operations in Node.js?

    • What is the difference between Promise.all and Promise.allSettled?
    • What is Promise.race and when is it useful?
    • How do you limit concurrency when running many async operations in parallel (e.g., p-limit)?

  7. What is an EventEmitter in Node.js?

    • How do you create a custom EventEmitter class?
    • What is the once method vs on on an EventEmitter?
    • What is a memory leak risk with EventEmitters and how do you detect it (maxListeners)?

  8. How do you handle uncaught exceptions in asynchronous code?

    • What is the role of process.on('uncaughtException') and when should you use it?
    • What is the difference between uncaughtException and unhandledRejection?
    • Why is it dangerous to continue running after an uncaughtException?

  9. What are async hooks in Node.js?

    • How can async hooks be used for performance monitoring and request tracing?
    • What is AsyncLocalStorage and how does it replace async hooks for context propagation?
    • What is the performance overhead of enabling async hooks?

  10. How do you convert callback-based code to Promises?

    • What is the util.promisify function and how does it work?
    • How do you manually wrap a callback function in a Promise?
    • How do you handle callbacks that call callback(result) instead of callback(err, result)?

6. Performance and Optimization​

  1. How do you optimize a Node.js application for performance?

    • What tools do you use to profile a Node.js app (--prof, clinic.js, 0x)?
    • What is flame graph analysis and how do you read one?
    • How do you identify and resolve hot code paths?

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

    • How do you manage inter-process communication (IPC) in clusters?
    • How does the cluster module distribute incoming connections across workers?
    • What is the difference between clustering and using Worker Threads?

  3. What is the pm2 process manager, and how does it help?

    • How do you configure pm2 for zero-downtime deployments (pm2 reload)?
    • How do you use pm2 in cluster mode vs fork mode?
    • How do you monitor memory and CPU usage with pm2 monit?

  4. How do you implement caching in Node.js?

    • What is the difference between in-memory caching and Redis caching?
    • How do you implement cache-aside pattern in a Node.js API?
    • What is cache invalidation and what strategies exist (TTL, event-driven)?

  5. What is the Worker Threads module, and when is it useful?

    • How do Worker Threads differ from child processes in terms of memory and communication?
    • How do you share memory between Worker Threads using SharedArrayBuffer?
    • What types of tasks are best suited for Worker Threads (CPU-bound work)?

  6. How do you handle memory leaks in Node.js?

    • What tools help identify memory leaks (node --inspect, heapdump, clinic.js)?
    • What are the most common causes of memory leaks in Node.js (global variables, event listeners, closures)?
    • How do you take and analyze a heap snapshot?

  7. What is the purpose of the --max-old-space-size flag?

    • How do you determine the right memory limit for your Node.js process?
    • What happens when Node.js exceeds its memory limit?
    • How do you monitor heap usage in production?

  8. How do you scale a Node.js application horizontally?

    • What is the role of a load balancer (Nginx, AWS ALB) in horizontal scaling?
    • What is sticky session and when is it needed for stateful Node.js apps?
    • How do you handle session sharing across multiple Node.js instances using Redis?

  9. What is stream piping, and how does it improve performance?

    • How do you pipe a file read stream directly to an HTTP response?
    • What happens if the writable stream errors during piping?
    • What is the difference between pipe and pipeline in Node.js streams?

  10. How do you optimize database queries in Node.js?

    • What is query batching and how does it reduce database round trips?
    • How do you use database connection pooling to improve throughput?
    • What is the N+1 query problem and how do you solve it in Node.js?

9. Microservices​

  1. How do you implement microservices in Node.js?

    • What communication protocols work best (REST, gRPC, message queues)?
    • How do you handle service versioning in a microservices architecture?
    • What is the single responsibility principle applied to microservice design?

  2. What is a message broker, and how do you use it in Node.js?

    • How does RabbitMQ compare to Kafka for Node.js microservices?
    • How do you use amqplib to publish and consume messages from RabbitMQ?
    • What is at-least-once delivery and how do you handle message idempotency?

  3. How do you handle service discovery in Node.js microservices?

    • What tools support service discovery (Consul, Kubernetes DNS, Eureka)?
    • What is the difference between client-side and server-side service discovery?
    • How do you implement health checks for service discovery?

  4. What is an API Gateway, and how do you implement it in Node.js?

    • How does an API Gateway improve security (auth, rate limiting, input validation)?
    • What is the BFF (Backend for Frontend) pattern?
    • What are the trade-offs of a custom Node.js API gateway vs managed solutions (AWS API Gateway)?

  5. How do you monitor microservices in Node.js?

    • What metrics are most important to track (request rate, error rate, latency)?
    • How do you implement distributed tracing with OpenTelemetry in Node.js?
    • What is a correlation ID and how do you propagate it across services?

  6. What is the circuit breaker pattern and how do you implement it in Node.js?

    • What are the three states of a circuit breaker (closed, open, half-open)?
    • How does the opossum library implement circuit breaking in Node.js?
    • What is the retry pattern and how do you combine it with a circuit breaker?

  7. How do you implement gRPC in Node.js?

    • What is Protocol Buffers (protobuf) and how do you define a .proto file?
    • How does gRPC compare to REST for inter-service communication in Node.js?
    • What are the four types of gRPC communication (unary, server streaming, client streaming, bidirectional)?

  8. How do you handle distributed transactions in Node.js microservices?

    • What is the SAGA pattern and how do you implement choreography vs orchestration?
    • What is a compensating transaction?
    • How do you handle partial failures in a distributed transaction?

  9. How do you implement event-driven architecture in Node.js?

    • What is event sourcing and how does it differ from CRUD?
    • How do you use Kafka with Node.js for event streaming?
    • What is CQRS (Command Query Responsibility Segregation) and when is it applicable?

  10. How do you secure inter-service communication in Node.js microservices?

    • What is mutual TLS (mTLS) and how do you implement it between Node.js services?
    • How do you use API keys or service tokens for internal service authentication?
    • What is a service mesh (Istio, Linkerd) and when would you use one?

11. Advanced Topics​

  1. What is the child_process module, and when is it useful?

    • What is the difference between fork, spawn, exec, and execFile?
    • How do you communicate between parent and child processes using IPC?
    • What are the security risks of using exec with user input?

  2. What is the fs module, and how do you use it?

    • How do you handle large file reads without loading the entire file into memory?
    • What is the difference between fs.readFile and fs.createReadStream?
    • How do you watch files for changes using fs.watch or chokidar?

  3. What is the http module in Node.js?

    • How do you create a custom HTTP server without Express?
    • How do you manually parse request body in a raw HTTP server?
    • What is the difference between http.Server and https.Server?

  4. What is the crypto module, and how do you use it?

    • How do you generate secure random tokens for password reset links?
    • What is HMAC and how do you use it for request signing?
    • What is the difference between crypto.createHash and crypto.createHmac?

  5. What is the zlib module, and when is it useful?

    • How do you compress HTTP responses using gzip or Brotli in Express?
    • What is the trade-off between compression level and CPU usage?
    • How do you pipe a file through zlib for on-the-fly compression?

  6. What is the net module in Node.js?

    • How do you create a raw TCP server using the net module?
    • What is the difference between a TCP socket and a WebSocket?
    • When would you use the net module directly instead of HTTP?

  7. What are Buffers in Node.js?

    • What is the difference between a Buffer and a regular JavaScript array?
    • How do you convert between Buffers and strings with different encodings?
    • What is a zero-copy operation and how do Buffers enable it?

  8. What is the cluster module and how does it work internally?

    • How does the master process distribute connections to workers?
    • What happens when a cluster worker crashes — does the master restart it automatically?
    • How do you share state between cluster workers (you generally cannot — how do you work around it)?

  9. What are native addons in Node.js (N-API)?

    • When would you write a native C++ addon for Node.js?
    • What is N-API and how does it provide ABI stability across Node.js versions?
    • What is node-gyp and what does it do?

13. TypeScript with Node.js​

  1. Why use TypeScript with Node.js?

    • What are the main benefits of TypeScript in a Node.js backend (type safety, IDE support, refactoring)?
    • What are the trade-offs (compilation step, learning curve, type definition maintenance)?
    • How does TypeScript catch bugs at compile time that JavaScript would only catch at runtime?

  2. How do you set up a Node.js project with TypeScript?

    • What are the key tsconfig.json options for a Node.js backend (target, module, strict, outDir)?
    • What is the difference between ts-node and compiling with tsc for production?
    • How do you use ts-node-dev for hot reloading during development?

  3. What are TypeScript types and interfaces, and how do you use them in Node.js?

    • What is the difference between type and interface in TypeScript?
    • How do you type Express Request and Response objects with custom properties?
    • What is a generic type and when would you use one in a Node.js backend?

  4. How do you handle TypeScript declaration files (.d.ts)?

    • What is @types/node and why do you need it?
    • How do you install type definitions for third-party packages (@types/express)?
    • What do you do when a package has no type definitions available?

  5. What are decorators in TypeScript and how are they used in Node.js?

    • How do decorators work in frameworks like NestJS (@Controller, @Injectable)?
    • What is the difference between experimental decorators and the TC39 decorator proposal?
    • How do you write a custom class decorator?

  6. What is NestJS and how does it use TypeScript?

    • How does NestJS's module system organize application code?
    • What is dependency injection in NestJS and how does it work?
    • How does NestJS compare to Express for large TypeScript backend projects?

  7. How do you enforce strict TypeScript in a Node.js project?

    • What does "strict": true enable in tsconfig.json?
    • What is noImplicitAny and why is it important?
    • How do you handle any types when integrating with untyped third-party code?

  8. How do you type asynchronous code in TypeScript?

    • How do you type a function that returns a Promise<T>?
    • What is Awaited<T> utility type and when do you use it?
    • How do you type error handling in async/await with TypeScript?

  9. How do you migrate an existing Node.js JavaScript project to TypeScript?

    • What is the allowJs flag and how does it enable gradual migration?
    • What is checkJs and how does it add type checking to .js files?
    • What is the recommended incremental migration strategy (file by file vs all at once)?

15. Authentication and Authorization​

  1. What is the difference between authentication and authorization?

    • What is the difference between session-based auth and token-based auth?
    • What is the difference between OAuth 2.0 (authorization) and OpenID Connect (authentication)?
    • How do you implement role-based access control (RBAC) in a Node.js API?

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

    • What are the three parts of a JWT (header, payload, signature)?
    • How do you verify and decode a JWT using jsonwebtoken?
    • What is the difference between jwt.sign with a secret vs an RSA private key?

  3. What is OAuth 2.0 and how do you implement it in Node.js?

    • What are the OAuth 2.0 grant types (authorization code, client credentials, implicit, device)?
    • How do you implement the authorization code flow with PKCE in Node.js?
    • How do you use Passport.js to handle OAuth 2.0 strategies?

  4. What is Passport.js and how does it work?

    • What is a Passport strategy and how do you configure one (Local, JWT, Google OAuth)?
    • How does passport.initialize() and passport.session() work in Express?
    • How do you serialize and deserialize a user with Passport sessions?

  5. How do you implement refresh token rotation in Node.js?

    • What is refresh token rotation and why does it improve security?
    • How do you store refresh tokens securely (database, httpOnly cookie)?
    • How do you detect and handle refresh token reuse attacks?

  6. What is session management in Node.js and how do you implement it securely?

    • How does express-session work and where should sessions be stored (Redis, database)?
    • What is session fixation and how do you prevent it?
    • What is the difference between a session cookie and a persistent cookie?

  7. What is RBAC (Role-Based Access Control) and how do you implement it?

    • How do you attach roles to a JWT payload and enforce them in middleware?
    • What is the difference between RBAC and ABAC (Attribute-Based Access Control)?
    • How do you implement resource-level permissions (user can only edit their own posts)?

  8. What is multi-factor authentication (MFA) and how do you implement it in Node.js?

    • How do you implement TOTP (Time-based One-Time Password) using speakeasy or otplib?
    • How do you generate and validate backup codes for MFA recovery?
    • What is WebAuthn and how does it enable passwordless authentication?

  9. How do you implement social login (Google, GitHub) in a Node.js application?

    • How does Passport.js GoogleStrategy work step by step?
    • How do you link a social account to an existing email/password account?
    • What user data do you store after a successful OAuth login?

  10. How do you protect API endpoints from unauthorized access?

    • How do you write an authentication middleware that validates JWTs on every protected route?
    • What is API key authentication and when is it appropriate vs JWT?
    • How do you implement IP whitelisting for internal APIs?

17. Scenario-Based and Behavioral Questions​

  1. Describe a challenging Node.js project you worked on.

    • What was the most difficult technical issue and how did you resolve it?
    • What would you do differently if you started that project today?
    • What Node.js-specific constraints shaped your architectural decisions?

  2. How would you debug a Node.js application crashing in production?

    • What steps do you take first — logs, metrics, or traces?
    • How do you use node --inspect or clinic.js to diagnose the crash?
    • What steps do you implement to prevent future crashes (process manager, error monitoring)?

  3. What would you do if a Node.js API is slow to respond?

    • How do you identify whether the bottleneck is CPU, I/O, database, or external API?
    • What profiling tools do you reach for first?
    • How do you prioritize and implement optimization tasks?

  4. How do you ensure code quality in a Node.js project?

    • How do you configure ESLint and Prettier for a Node.js/TypeScript project?
    • How do you enforce code standards in CI (failing builds on lint errors)?
    • What is a pre-commit hook and how do you set one up with husky and lint-staged?

  5. How do you handle a memory leak in a Node.js application?

    • What symptoms indicate a memory leak in production (growing heap, OOM restarts)?
    • What tools do you use to diagnose memory leaks (heapdump, Chrome DevTools, clinic.js)?
    • Walk me through a real or hypothetical memory leak investigation step by step.

  6. How do you onboard a new developer to a Node.js project?

    • What documentation is critical for a Node.js project (README, API docs, architecture diagrams)?
    • How do you use Docker Compose to make local setup a single command?
    • How do you share coding standards and patterns with new team members?

  7. What would you do if a third-party package breaks your application after an update?

    • How do you identify which package caused the regression?
    • How do you pin package versions and set up npm audit in CI?
    • How do you evaluate whether to replace a package vs patch it?

  8. How do you stay updated with Node.js advancements?

    • What resources do you rely on (Node.js changelog, NodeWeekly, official blog)?
    • How do you evaluate whether to adopt a new Node.js feature in production?
    • How do you manage Node.js version upgrades across a team?

  9. Describe a time you optimized a Node.js application.

    • What metrics did you use to measure success (p99 latency, throughput, error rate)?
    • What was the biggest single win and what was the approach?
    • How did you validate the optimization did not introduce regressions?

  10. How do you collaborate with frontend developers on a Node.js backend?

    • How do you handle API contract changes without breaking the frontend?
    • How do you use tools like OpenAPI/Swagger or GraphQL schema to document your API?
    • How do you involve frontend developers in API design decisions early?