Node.js Interview Questions: Basic to Advanced

1What is Node.js actually made of, and which parts are V8, libuv, and the standard library?Basicconceptual35 minIn writing
2How does V8 execute your JavaScript, and what is its garbage collector doing while your server serves traffic?Basicconceptual40 minIn writing
3Which globals does Node give you that the browser does not, and what is the process object actually for?Basicconceptual35 minIn writing
4How do you pick and pin a Node version for a project, and which runtime flags are worth knowing?Basicpractical30 minIn writing
5What makes an operation blocking in Node, and how do you spot a blocking call during a code review?Basicconceptual40 minIn writing
6How would you explain the difference between Node, the browser runtime, and newer runtimes like Deno or Bun to a team choosing one?Intermediateconceptual35 minIn writing
7An interviewer asks why a company should run its API on Node.js at all. How do you answer without hand-waving about speed?Scenarioscenario25 minIn writing
8Check your understanding of the Node runtime, V8, and blocking versus non-blocking work.Basicconceptual15 minIn writing

9How does require() resolve a specifier, and what does the module cache do to the singletons you export?Basicconceptual40 minIn writing
10What actually changes when a file is loaded as ESM instead of CommonJS?Intermediateconceptual45 minIn writing
11Which package.json fields decide how your code is loaded, and how do type, main, and exports interact?Intermediatepractical40 minIn writing
12How do you ship a library that works for both require() and import consumers without shipping two subtly different copies?Advancedpractical45 minIn writing
13What does a lockfile actually guarantee, and how do you decide when to upgrade a dependency?Intermediatepractical35 minIn writing
14How do workspaces change dependency resolution, and what tends to break first in a monorepo?Advancedpractical40 minIn writing
15A build fails with 'require() of an ES Module is not supported'. Walk an interviewer through your diagnosis and the fixes you would consider.Scenarioscenario30 minIn writing
16Check your understanding of module resolution, ESM/CJS interop, and package metadata.Basicconceptual15 minIn writing

17How does Node.js handle concurrency with a single JavaScript thread?Basicconceptual5 min
18Walk through the phases of a libuv event loop tick and name what runs in each one.Intermediateconceptual50 minIn writing
19Predict the output order of setTimeout, setImmediate, process.nextTick, and a resolved promise, and explain why that order holds.Intermediateconceptual45 minIn writing
20Why does setTimeout(fn, 100) rarely fire at exactly 100ms, and when does that inaccuracy actually matter?Intermediatepractical35 minIn writing
21When should you reach for setImmediate, and why is process.nextTick starvation a real production bug?Advancedconceptual40 minIn writing
22Which operations use the libuv thread pool, and what does raising UV_THREADPOOL_SIZE actually change?Advancedpractical45 minIn writing
23Explain the event loop to an interviewer well enough that they believe you have actually debugged one.Scenarioscenario30 minIn writing
24Check your understanding of loop phases, timer ordering, and microtask scheduling.Basicconceptual15 minIn writing

25Compare callbacks, Promises, and async/await for handling asynchronous work in Node.js, and when each is still the right tool.Intermediatepractical5 min
26What is the error-first callback convention, and what goes wrong when a callback is invoked twice?Basicconceptual35 minIn writing
27When do you reach for Promise.all, allSettled, race, or any, and what does each one do with errors?Intermediatepractical40 minIn writing
28How do you wrap a callback-based API in a promise without leaking an unhandled rejection?Intermediatepractical35 minIn writing
29How do you run ten thousand async tasks without opening ten thousand sockets at once?Advancedpractical45 minIn writing
30What problem do async iterators and for await...of solve that Promise.all does not?Advancedpractical45 minIn writing
31You inherit a service that mixes callbacks, promises, and async/await. How do you argue for a migration order?Scenarioscenario30 minIn writing
32Check your understanding of async control flow, combinators, and concurrency limits.Basicconceptual15 minIn writing

33How do you build an error a caller can actually act on, and what are custom error classes and Error.cause for?Basicconceptual35 minIn writing
34Why does try/catch fail to catch an error thrown inside a callback, and what catches it instead?Intermediateconceptual40 minIn writing
35What happens to an unhandled promise rejection in modern Node, and how should a long-running service react to one?Intermediatepractical40 minIn writing
36How do you decide whether to retry an error, surface it to the client, or let the process crash?Advancedconceptual40 minIn writing
37How do you shut a Node service down on SIGTERM without dropping in-flight requests?Advancedpractical45 minIn writing
38How do you add timeouts, retries, and backoff to an outbound call without amplifying an outage?Advancedpractical45 minIn writing
39A service restarts a few times an hour and nobody knows why. How do you investigate, and what do you change first?Scenarioscenario30 minIn writing
40Check your understanding of error propagation, rejection handling, and shutdown behaviour.Basicconceptual15 minIn writing

41How does EventEmitter work, and when is it a better fit than passing a callback around?Basicconceptual35 minIn writing
42Why does an unhandled 'error' event crash the process, and how do you handle it correctly?Intermediateconceptual35 minIn writing
43What is the MaxListenersExceededWarning telling you, and how do you find the leak behind it?Intermediatetroubleshooting40 minIn writing
44How do events.once and events.on let you consume an emitter with async/await?Intermediatepractical35 minIn writing
45How do you use AbortController and AbortSignal to cancel in-flight work in Node?Advancedpractical40 minIn writing
46When does an in-process EventEmitter stop being enough and a real message broker start earning its operational cost?Advanceddesign45 minIn writing
47Design the event surface of a job-processing library and defend which events you chose to expose.Scenarioscenario30 minIn writing
48Check your understanding of emitters, error events, and cancellation.Basicconceptual15 minIn writing

49Design a Node.js API endpoint that exports a large CSV report without loading the entire dataset into memory.Advanceddesign5 min
50What are readable, writable, duplex, and transform streams, and when do you reach for each?Intermediateconceptual40 minIn writing
51What is backpressure, and what actually happens if you ignore the return value of writable.write()?Advancedconceptual45 minIn writing
52Why should you prefer stream.pipeline over .pipe(), and what does it fix about error handling and cleanup?Intermediatepractical40 minIn writing
53How do you write a Transform stream that parses newline-delimited JSON without buffering the whole file?Advancedpractical45 minIn writing
54How do Node streams and Web Streams interoperate, and when does that conversion actually matter?Advancedpractical40 minIn writing
55A file upload endpoint works fine locally and runs the pod out of memory in production. Diagnose it out loud.Scenarioscenario30 minIn writing
56Check your understanding of stream types, backpressure, and pipeline error handling.Basicconceptual15 minIn writing

57What is a Buffer, how does it relate to a TypedArray, and when do you actually need one?Basicconceptual35 minIn writing
58What goes wrong when you slice a UTF-8 buffer at an arbitrary byte offset?Intermediateconceptual35 minIn writing
59How do the callback, promise, and sync fs APIs differ, and when is the sync version defensible?Basicpractical35 minIn writing
60How do path.join, path.resolve, and __dirname behave, and why does the last one differ under ESM?Basicpractical30 minIn writing
61How do you read and process a file that is larger than the memory available to the process?Intermediatepractical40 minIn writing
62How do you safely turn a user-supplied filename into a path on disk?Advancedtroubleshooting40 minIn writing
63Design the storage layer for user-uploaded files in a Node service and defend the trade-offs you made.Scenarioscenario30 minIn writing
64Check your understanding of buffers, encodings, and file-system APIs.Basicconceptual15 minIn writing

65Build a server with the core http module and explain what happens to a request from socket to response.Basicpractical40 minIn writing
66How do you read a request body in core Node, and which limits must you enforce while doing it?Intermediatepractical40 minIn writing
67What do keep-alive, chunked encoding, and Content-Length change about how you write a response?Intermediateconceptual35 minIn writing
68How do outbound HTTP requests pool connections in Node, and what does an Agent control?Intermediatepractical40 minIn writing
69Which timeouts does the Node HTTP server expose, and what does each one protect you from?Advancedpractical40 minIn writing
70What changes when your Node server sits behind a proxy that terminates TLS and speaks HTTP/2?Advancedconceptual40 minIn writing
71Requests to your service hang until they time out while CPU sits idle. Walk through what you check and in what order.Scenarioscenario30 minIn writing
72Check your understanding of the HTTP request lifecycle, agents, and timeouts.Basicconceptual15 minIn writing

73How do Express routing and middleware ordering work, and what does next() actually do?Basicpractical40 minIn writing
74What belongs in middleware, and what should never be allowed to live there?Intermediatedesign40 minIn writing
75How do Express and Fastify differ in performance, validation, and plugin model, and how would you choose between them?Intermediateconceptual40 minIn writing
76Why does a rejected promise in an Express handler silently hang the request, and how do you fix that globally?Intermediatetroubleshooting40 minIn writing
77How do you structure a Node API so route handlers stay thin and business rules stay testable?Advanceddesign50 minIn writing
78How does AsyncLocalStorage let you carry a request id through a call stack without threading it through every function?Advancedpractical45 minIn writing
79Design the module boundaries for a Node API that three teams will work on at once, and defend your layering.Scenarioscenario35 minIn writing
80Check your understanding of middleware, framework choice, and service layering.Basicconceptual15 minIn writing

81Where should request validation live, and what should a validation failure return to the client?Basicpractical35 minIn writing
82How do schema libraries like Zod give you a runtime check and a TypeScript type from a single definition?Intermediatepractical40 minIn writing
83What does JSON.stringify do to dates, BigInt, undefined, and circular references?Intermediateconceptual35 minIn writing
84Why is JSON serialization often the hottest function in a Node API, and what do you do about it?Advancedpractical40 minIn writing
85How do you evolve a JSON API without breaking clients you cannot deploy?Advanceddesign45 minIn writing
86How do you keep an OpenAPI document honest about what the server actually accepts and returns?Advancedpractical40 minIn writing
87A client reports intermittent 500s on a field they insist they always send. How do you find the truth?Scenarioscenario30 minIn writing
88Check your understanding of validation boundaries, JSON behaviour, and contract evolution.Basicconceptual15 minIn writing

89How do spawn, exec, execFile, and fork differ, and which one runs an external command safely?Intermediatepractical40 minIn writing
90When does a worker thread beat a child process, and how do you move data between them cheaply?Advancedpractical45 minIn writing
91What does the cluster module do, and why is one process per core the usual answer?Intermediateconceptual40 minIn writing
92Where does shared state live once you run more than one Node process?Advanceddesign45 minIn writing
93A request needs 400ms of pure CPU. What are your options, and how do you choose between them?Advanceddesign45 minIn writing
94How do you run background work in a Node deployment without a job silently dying unnoticed?Advanceddesign45 minIn writing
95Your service pins one core at 100% while the rest of the box is idle. What do you propose, and in what order?Scenarioscenario35 minIn writing
96Check your understanding of workers, clustering, and CPU-bound work.Basicconceptual15 minIn writing

97A production Node.js service's event loop is blocked for multiple seconds under load, causing request timeouts. Walk through how you'd find and fix the cause.Scenarioscenario5 min
98How do you capture and read a CPU profile from a running Node process?Advancedpractical50 minIn writing
99How do you use heap snapshots to prove where a memory leak is actually coming from?Advancedtroubleshooting50 minIn writing
100What are the usual causes of a Node memory leak, and which one do you check first?Intermediatetroubleshooting40 minIn writing
101How do you benchmark a Node function without fooling yourself?Intermediatepractical40 minIn writing
102What do --max-old-space-size and the garbage collector actually control, and when is tuning them the right move?Advancedpractical45 minIn writing
103p99 latency tripled after a deploy while p50 did not move. How do you find the cause?Scenarioscenario35 minIn writing
104Check your understanding of profiling, heap analysis, and Node performance limits.Basicconceptual15 minIn writing

105How should a Node service load its secrets, and why is a committed .env such a common cause of a breach?Basicpractical35 minIn writing
106How do SQL injection and command injection actually reach a Node service, and what stops each of them?Intermediateconceptual40 minIn writing
107How do you decide whether a new npm dependency is worth the risk it adds?Intermediatepractical40 minIn writing
108How do you store and verify sessions or tokens in a Node API without rolling your own crypto?Advanceddesign45 minIn writing
109How do you stop a single client from trivially overwhelming a Node API?Advancedpractical45 minIn writing
110What is prototype pollution, and how does untrusted JSON turn into it?Advancedtroubleshooting40 minIn writing
111A dependency you ship publishes a compromised version. Walk through what you do over the next hour.Scenarioscenario35 minIn writing
112Check your understanding of secrets, injection, and dependency risk.Basicconceptual15 minIn writing

113What does the built-in node:test runner give you, and when would you still reach for Vitest or Jest?Basicpractical35 minIn writing
114Where do you draw the line between unit and integration tests in a Node API?Intermediateconceptual40 minIn writing
115What should you mock in a Node service, and what should you refuse to mock?Intermediatepractical40 minIn writing
116How do you test asynchronous code without flaky timeouts and leaked handles keeping the process alive?Intermediatetroubleshooting40 minIn writing
117How do you test HTTP handlers and database access without a shared, dirty test database?Advancedpractical45 minIn writing
118What should a Node CI pipeline run on every push, and what does a coverage number actually tell you?Advancedpractical40 minIn writing
119A suite passes locally and fails one run in ten in CI. How do you make it trustworthy again?Scenarioscenario30 minIn writing
120Check your understanding of test boundaries, mocking, and async test reliability.Basicconceptual15 minIn writing

121What makes a log line useful in production, and why structured JSON instead of console.log?Basicpractical35 minIn writing
122What do metrics, logs, and traces each answer, and what does OpenTelemetry give a Node service?Intermediateconceptual45 minIn writing
123What should a liveness probe check, and how is that different from readiness?Intermediatepractical35 minIn writing
124What can you inspect on a live Node process in production without restarting it?Advancedtroubleshooting50 minIn writing
125How do you configure, containerize, and deploy a Node service so that restarts are boring?Advancedpractical45 minIn writing
126Design, instrument, and defend a production Node.js service end to end.Scenariodesign240 minIn writing
127How would you convince a sceptical reviewer that your Node service is ready for production traffic?Scenarioscenario30 minIn writing
128Check your understanding of logging, observability, and production operations.Basicconceptual15 minIn writing