🚀 Node.js Interview Quiz (100 Questions with Answers)

1️⃣ Node.js Fundamentals (1–15)

1) What is Node.js?
Answer: A JavaScript runtime built on Chrome’s V8 engine.
Explanation: Node.js allows JS to run outside the browser, mainly for servers and tooling.

2) Is Node.js single-threaded?
Answer: Yes (but it uses background threads).
Explanation: JS runs on one thread, but Node offloads I/O to a thread pool.

3) What is V8?
Answer: Google’s JavaScript engine written in C++.
Explanation: Compiles JS to machine code for fast execution.

4) What is npm?
Answer: Node Package Manager.
Explanation: Manages third-party libraries and dependencies.

5) What is the purpose of package.json?
Answer: Metadata, scripts, dependencies, versioning.
Explanation: Central config file for Node projects.

6) What does node index.js do?
Answer: Executes the file in the Node runtime.
Explanation: Entry point for most Node applications.

7) What is process in Node.js?
Answer: A global object providing info about the current Node process.
Explanation: Used for env vars, exit codes, signals.

8) How do you access environment variables?

process.env.PORT

Explanation: Often used for config and secrets.

9) What is __dirname?
Answer: Absolute path of the current file’s directory.
Explanation: Useful for file system operations.

10) Difference between Node.js and browser JS?
Answer: Node has no DOM, but has filesystem, networking, OS access.
Explanation: Different runtimes, same language.

11) What is REPL?
Answer: Read–Eval–Print Loop.
Explanation: Interactive Node shell.

12) What is a Node module?
Answer: A reusable block of code.
Explanation: Encapsulated via CommonJS or ES Modules.

13) What file extension enables ES Modules in Node?
Answer: .mjs or “type”: “module”
Explanation: Enables import/export.

14) What does require() do?
Answer: Imports CommonJS modules.
Explanation: Synchronous loading.

15) What is npx?
Answer: Runs packages without installing globally.
Explanation: Useful for CLIs.

2️⃣ Event Loop & Async Internals (16–30)

16) What is the Node.js event loop?
Answer: Handles async operations and callback execution.
Explanation: Enables non-blocking I/O.

17) Is Node good for CPU-heavy tasks?
Answer: No.
Explanation: CPU work blocks the event loop.

18) What is non-blocking I/O?
Answer: Operations don’t stop JS execution.
Explanation: Core Node performance feature.

19) What is the libuv library?
Answer: C library handling async I/O.
Explanation: Manages event loop & thread pool.

20) What is the thread pool size?
Answer: Default = 4 threads.
Explanation: Configurable via UV_THREADPOOL_SIZE.

21) What goes into the thread pool?
Answer: fs, crypto, DNS, compression.
Explanation: CPU-like async tasks.

22) Order of execution?

setTimeout(()=>console.log(“A”));

Promise.resolve().then(()=>console.log(“B”));

Answer: B, then A
Explanation: Microtasks run before timers.

23) What is backpressure?
Answer: When data arrives faster than it’s processed.
Explanation: Streams handle this.

24) What are microtasks?
Answer: Promise callbacks.
Explanation: Executed before next event loop phase.

25) What is starvation?
Answer: Tasks never execute due to others dominating.
Explanation: Too many microtasks can delay timers.

26) How do you block the event loop?
Answer: Long loops, sync fs, CPU-heavy logic.
Explanation: Avoid at all costs.

27) What is setImmediate()?
Answer: Executes after I/O callbacks.
Explanation: Different timing than setTimeout(0).

28) What is process.nextTick()?
Answer: Executes before Promise microtasks.
Explanation: Can starve the event loop.

29) Should nextTick be used often?
Answer: No.
Explanation: Easy to misuse and block execution.

30) Why is Node fast?
Answer: Non-blocking async + V8 + event loop.
Explanation: Efficient I/O handling.

3️⃣ Modules, FS, Streams (31–45)

31) CommonJS vs ES Modules?
Answer: require vs import.
Explanation: ES Modules are standard.

32) Is require() synchronous?
Answer: Yes.
Explanation: Loaded at runtime.

33) What is fs.readFileSync() risk?
Answer: Blocks event loop.
Explanation: Avoid in servers.

34) Preferred file reading method?
Answer: fs.promises.readFile()
Explanation: Async and non-blocking.

35) What is a stream?
Answer: Data processed in chunks.
Explanation: Efficient memory usage.

36) Types of streams?
Answer: Readable, Writable, Duplex, Transform.
Explanation: Core Node concept.

37) Why use streams?
Answer: Handle large data efficiently.
Explanation: Avoid loading everything into memory.

38) What does .pipe() do?
Answer: Connects streams.
Explanation: Handles backpressure automatically.

39) What is a buffer?
Answer: Raw binary data.
Explanation: Used for file/network data.

40) Are buffers mutable?
Answer: Yes.
Explanation: Careful with shared buffers.

41) How do you check file existence safely?
Answer: fs.access()
Explanation: Avoid race conditions.

42) What is path traversal risk?
Answer: User accessing unintended files.
Explanation: Validate paths.

43) How do you delete a file async?
Answer: fs.unlink()
Explanation: Non-blocking.

44) What is process.cwd()?
Answer: Current working directory.
Explanation: Can differ from __dirname.

45) When to use streams over buffers?
Answer: Large files or continuous data.
Explanation: Better scalability.

4️⃣ Express & APIs (46–65)

46) What is Express.js?
Answer: Minimal Node web framework.
Explanation: Routing, middleware, HTTP handling.

47) What is middleware?
Answer: Functions executed in request lifecycle.
Explanation: Modify req/res or end cycle.

48) Order of middleware matters?
Answer: Yes.
Explanation: Executed top-down.

49) What is next()?
Answer: Moves to next middleware.
Explanation: Required to continue pipeline.

50) What is REST?
Answer: Architectural style for APIs.
Explanation: Stateless, resource-based.

51) Difference between PUT and PATCH?
Answer: PUT replaces; PATCH updates partially.
Explanation: Important REST distinction.

52) What is JSON middleware?
Answer: express.json()
Explanation: Parses request body.

53) How to handle errors in Express?
Answer: Error-handling middleware (err, req, res, next)
Explanation: Centralized error logic.

54) What is CORS?
Answer: Cross-Origin Resource Sharing.
Explanation: Browser security policy.

55) How do you enable CORS?
Answer: cors middleware.
Explanation: Server-controlled.

56) What is rate limiting?
Answer: Prevents abuse.
Explanation: Protects APIs.

57) What is HTTP status 401 vs 403?
Answer: 401 = unauthenticated; 403 = unauthorized.
Explanation: Common interview question.

58) What is JWT?
Answer: JSON Web Token.
Explanation: Stateless auth mechanism.

59) Where should JWT be stored?
Answer: HTTP-only cookies (preferred).
Explanation: Safer than localStorage.

60) What is CSRF?
Answer: Cross-Site Request Forgery.
Explanation: Mitigated with tokens.

61) What is input validation?
Answer: Ensuring correct data.
Explanation: Prevents injection attacks.

62) What is SQL injection risk?
Answer: Unsanitized queries.
Explanation: Use parameterized queries.

63) How do you structure large Express apps?
Answer: Routes, controllers, services, middleware.
Explanation: Separation of concerns.

64) What is MVC in Node?
Answer: Model–View–Controller.
Explanation: Common backend pattern.

65) Why avoid fat controllers?
Answer: Hard to test and maintain.
Explanation: Business logic belongs in services.

5️⃣ Performance, Security, Production (66–85)

66) What is clustering?
Answer: Using multiple Node processes.
Explanation: Utilizes multiple CPU cores.

67) What module supports clustering?
Answer: cluster
Explanation: Built-in.

68) What is PM2?
Answer: Process manager.
Explanation: Restarts crashes, clustering, logs.

69) How do you handle uncaught exceptions?
Answer: Log and restart process.
Explanation: App may be unstable.

70) What is memory leak in Node?
Answer: Objects not released.
Explanation: Leads to crashes.

71) How do you detect memory leaks?
Answer: Heap snapshots, profiling.
Explanation: Use Chrome DevTools.

72) What is graceful shutdown?
Answer: Finish requests before exit.
Explanation: Important in production.

73) How do you listen for shutdown signals?
Answer: process.on(“SIGTERM”)
Explanation: Clean up resources.

74) What is load balancing?
Answer: Distributing traffic.
Explanation: Improves reliability.

75) What is horizontal scaling?
Answer: More instances.
Explanation: Node-friendly.

76) Vertical scaling?
Answer: Bigger server.
Explanation: Limited growth.

77) What is logging best practice?
Answer: Structured logs (JSON).
Explanation: Easier monitoring.

78) What is dotenv?
Answer: Loads env vars from .env.
Explanation: Dev convenience.

79) Why not commit .env?
Answer: Security risk.
Explanation: Contains secrets.

80) What is helmet?
Answer: Security middleware.
Explanation: Sets safe HTTP headers.

81) What is HTTPS importance?
Answer: Encrypts traffic.
Explanation: Required for auth & security.

82) What is N+1 query problem?
Answer: Excessive DB queries.
Explanation: Performance killer.

83) How to optimize DB access?
Answer: Indexing, batching, caching.
Explanation: Backend fundamentals.

84) What is caching?
Answer: Store frequent results.
Explanation: Improves performance.

85) What tools monitor Node apps?
Answer: PM2, New Relic, Datadog.
Explanation: Production readiness.

6️⃣ Advanced & Architecture (86–100)

86) What is event-driven architecture?
Answer: Systems react to events.
Explanation: Node fits naturally.

87) What is message queue?
Answer: Async task processing.
Explanation: Redis, RabbitMQ, Kafka.

88) Why use workers?
Answer: Offload CPU work.
Explanation: Prevent event loop blocking.

89) What is worker_threads?
Answer: Multi-threaded JS execution.
Explanation: For CPU-heavy tasks.

90) Difference: workers vs cluster?
Answer: Workers share memory; cluster uses processes.
Explanation: Different trade-offs.

91) What is monolith vs microservices?
Answer: Single app vs distributed services.
Explanation: Architecture choice.

92) Node best use cases?
Answer: APIs, real-time apps, tooling.
Explanation: Not heavy computation.

93) What is WebSocket?
Answer: Persistent bi-directional connection.
Explanation: Real-time apps.

94) What is SSE?
Answer: Server-Sent Events.
Explanation: One-way streaming.

95) What is GraphQL vs REST?
Answer: Client-defined queries vs fixed endpoints.
Explanation: Different trade-offs.

96) What is middleware chaining risk?
Answer: Hard debugging.
Explanation: Keep middleware small.

97) What is API versioning?
Answer: Supporting multiple API versions.
Explanation: Avoid breaking clients.

98) What is idempotency?
Answer: Same request → same result.
Explanation: Important for retries.

99) What is a health check endpoint?
Answer: Status endpoint.
Explanation: Used by load balancers.

100) What makes a Node.js dev “senior”?
Answer: Understanding internals, async, performance, architecture.
Explanation: Not just syntax.

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