Q: How do you enable gzip/Brotli compression for HTTP responses?

**TL;DR.** Compress responses based on the client's Accept-Encoding header using zlib (gzip/brotli) or Express's compression middleware. **Brotli** compresses smaller but costs more CPU; you typically **skip** already-compressed assets (images, video) and tiny payloads, and often offload compression to a reverse proxy/CDN. <svg class='iq-diagram' viewBox='0 0 460 150' role='img' aria-label='Server picks an encoding from Accept-Encoding and compresses the b

Q: What is the difference between dependencies, devDependencies, and peerDependencies?

**TL;DR.** dependencies are needed at **runtime** (shipped). devDependencies are needed only for **development/build/test** (not installed in production with --omit=dev ). peerDependencies declare a package your **host app must provide** (e.g. a React plugin expecting react ), avoiding duplicate copies. <rect class='d

Q: Explain the concept of 'middleware' in Express.js.

**TL;DR.** **Middleware** are functions with the signature (req, res, next) that run **in order** for each request. Each can inspect/modify req / res , end the response, or call next() to pass control onward. They form a **pipeline** for cross-cutting concerns: logging, auth, parsing, validation, and error handling.

Q: What are stream piping and the .pipe() method in Node.js?

**TL;DR.** readable.pipe(writable) connects a source stream to a destination, **forwarding chunks automatically** and applying **backpressure** (pausing the source when the destination's buffer fills). It replaces manual 'data' / 'end' wiring — but for production, prefer pipeline() , which also propagates errors and cleans up. <r

Q: What does the Node.js --watch flag do and how does it replace nodemon?

**TL;DR.** node --watch app.js **restarts the process automatically** when any imported file changes — built into Node, zero dependencies. --watch-path scopes which directories to watch. It covers nodemon's core "restart on save" use case; nodemon still offers more configuration (debounce, custom exec, extension maps). <rect class='d-box' x='20' y='55' wid

Q: What is the difference between readFile and createReadStream in Node.js?

**TL;DR.** fs.readFile reads the **whole file into memory** and gives you one Buffer/string when done — simple, but memory grows with file size. fs.createReadStream returns a **Readable stream** that emits the file in chunks, using constant memory and letting you start processing before the file is fully read. <rect class='d-box-muted' x='20' y='30' width=

Q: What is the role of the path module and why is it preferred over manual string concatenation?

**TL;DR.** The node:path module builds and manipulates file paths **correctly across operating systems**, which use different separators ( / on POSIX, \ on Windows). path.join / path.resolve pick the right separator, normalize redundant slashes and .. segments, and avoid the subtle bugs of hand-built strings. <svg class='iq-diagram' viewBox='0 0 460 160' role='img' aria-label='path.join handles separators that manual con

Question 1

How do you enable gzip/Brotli compression for HTTP responses?

Accepted Answer

**TL;DR.** Compress responses based on the client's Accept-Encoding header using zlib (gzip/brotli) or Express's compression middleware. **Brotli** compresses smaller but costs more CPU; you typically **skip** already-compressed assets (images, video) and tiny payloads, and often offload compression to a reverse proxy/CDN.

Question 2

What is the difference between dependencies, devDependencies, and peerDependencies?

Accepted Answer

**TL;DR.** dependencies are needed at **runtime** (shipped). devDependencies are needed only for **development/build/test** (not installed in production with --omit=dev). peerDependencies declare a package your **host app must provide** (e.g. a React plugin expecting react), avoiding duplicate copies.

Question 3

Explain the difference between require() and import in Node.js.

Accepted Answer

`require()` is the synchronous, runtime loading mechanism used by the CommonJS module system, while `import` is the static, compile-time loading mechanism used by ECMAScript Modules (ESM). Because `require()` is dynamic, it can be called conditionally inside `if` statements, whereas standard `import` statements must reside at the top-level of the file. To use `import` in Node.js, you must either use the `.mjs` extension or set `"type": "module"` in `package.json`.

Question 4

Explain the concept of 'middleware' in Express.js.

Accepted Answer

**TL;DR.** **Middleware** are functions with the signature (req, res, next) that run **in order** for each request. Each can inspect/modify req/res, end the response, or call next() to pass control onward. They form a **pipeline** for cross-cutting concerns: logging, auth, parsing, validation, and error handling.

Question 5

How do you handle CORS (Cross-Origin Resource Sharing) in Node.js?

Accepted Answer

**TL;DR.** **CORS** is a browser security mechanism: the browser blocks cross-origin requests unless the server returns the right Access-Control-Allow-* headers. In Node you set those headers — usually via the cors middleware in Express — to **allowlist** specific origins, methods, headers, and credentials.

Question 6

What are stream piping and the .pipe() method in Node.js?

Accepted Answer

**TL;DR.** readable.pipe(writable) connects a source stream to a destination, **forwarding chunks automatically** and applying **backpressure** (pausing the source when the destination's buffer fills). It replaces manual 'data'/'end' wiring — but for production, prefer pipeline(), which also propagates errors and cleans up.

Question 7

What does the Node.js --watch flag do and how does it replace nodemon?

Accepted Answer

**TL;DR.** node --watch app.js **restarts the process automatically** when any imported file changes — built into Node, zero dependencies. --watch-path scopes which directories to watch. It covers nodemon's core "restart on save" use case; nodemon still offers more configuration (debounce, custom exec, extension maps).

Question 8

What is the difference between readFile and createReadStream in Node.js?

Accepted Answer

**TL;DR.** fs.readFile reads the **whole file into memory** and gives you one Buffer/string when done — simple, but memory grows with file size. fs.createReadStream returns a **Readable stream** that emits the file in chunks, using constant memory and letting you start processing before the file is fully read.

Question 9

What is the role of the path module and why is it preferred over manual string concatenation?

Accepted Answer

**TL;DR.** The node:path module builds and manipulates file paths **correctly across operating systems**, which use different separators (/ on POSIX, \ on Windows). path.join/path.resolve pick the right separator, normalize redundant slashes and .. segments, and avoid the subtle bugs of hand-built strings.

Question 10

Explain how modules work in Node.js.

Accepted Answer

**TL;DR.** A **module** is a file with its **own scope**: variables are private unless explicitly exported. Node supports two systems — **CommonJS** (require/module.exports, loaded synchronously) and **ES Modules** (import/export, statically analyzed). Each file is loaded once and **cached**, so repeated requires return the same instance.

Node.js Interview Library

Question Catalog

How do you enable gzip/Brotli compression for HTTP responses?

What is the difference between dependencies, devDependencies, and peerDependencies?

Explain the difference between require() and import in Node.js.

Explain the concept of 'middleware' in Express.js.

How do you handle CORS (Cross-Origin Resource Sharing) in Node.js?

What are stream piping and the .pipe() method in Node.js?

What does the Node.js --watch flag do and how does it replace nodemon?

What is the difference between readFile and createReadStream in Node.js?

What is the role of the path module and why is it preferred over manual string concatenation?

Explain how modules work in Node.js.

What are the main security risks of using the 'eval()' function in Node.js?

What is the difference between string concatenation and path.join/path.resolve?

How do you run Node.js processes as a background service?

What is npx and how does it differ from a global install?

What does NODE_ENV=production actually change and why does it matter?

What is Node.js and why is it popular?

What is structuredClone and when is it useful in Node.js?

What is the purpose of the assert module in Node.js?

How do you replicate __dirname in ES Modules using import.meta.url?

What is the purpose of the util.promisify function in Node.js?

What is the difference between blocking and non-blocking I/O in Node.js?

What is the use of the 'os' module in Node.js?

What is `npm` and what are its core functionalities?

How can you securely store and verify passwords in a Node.js application?

What is the role of the V8 JavaScript engine in Node.js?

Explain the concept of REPL in Node.js

What is the purpose of `__dirname` and `__filename` in Node.js?

Explain the purpose of `package.json` and `npm`.

What is semantic versioning and how do ^, ~, and exact versions behave?

What is the node:util parseArgs helper for building CLIs?

What is the difference between unit, integration, and end-to-end tests for a Node.js API?

What is the purpose of `package-lock.json`?

How do you handle environment variables in Node.js?

How does the Node.js event loop work?

What is AbortSignal.timeout and how do you cancel async work cleanly?

How do you mock modules, timers, and network calls in Node.js tests?

How would you implement caching in a Node.js application?

Explain the concept of 'event-driven architecture' in Node.js.

What are 'Event Emitting' patterns and how does error-first callback convention relate to them?

When would you choose Node.js over other backend technologies?

How do you manage configuration in a Node.js application for different environments?

How do you implement a custom Transform stream?

What are npm workspaces and how do you manage a monorepo with them?

What are some best practices for structuring a Node.js project?

What are Worker Threads in Node.js and when would you use them?

How do you manage sessions and authentication in a Node.js web application?

What is the difference between spawn() and exec() in Node.js?

What is a background job queue (BullMQ/Redis) and when do you need one?

What is the 12-factor app methodology as applied to Node.js services?

How does Node.js handle crypto operations efficiently without blocking the event loop?

Explain how the EventEmitter class works under the hood.

How does Node.js handle multi-threading behind the scenes if JavaScript is single-threaded?

What are Promises in Node.js and how do they improve async code?

What is Helmet and which HTTP security headers should a Node.js API set?

What is the difference between "type": "module", .mjs, and .cjs?

What is top-level await and what are its caveats in Node.js ESM?

What is the purpose of the AbortController in modern Node.js?

What are the differences between npm, Yarn, and pnpm?

How does `async/await` work and what are its benefits?

What are `EventEmitter`s in Node.js?

How do you design liveness and readiness health-check endpoints?

What does the events.once() helper do and how does it bridge EventEmitter and async/await?

Explain how the 'crypto' module secures sensitive data in Node.js

Explain the differences between fork(), spawn(), and exec() in the child_process module.

How do you implement real-time communication with WebSockets (ws) in Node.js?

What is the Repository pattern and how does it decouple business logic from the database?

Explain the Node.js event loop.

How do you monitor a Node.js application's memory usage in production?

How do you containerize a Node.js app with a small, secure Docker image (multi-stage, distroless)?

How do you implement retries with exponential backoff and jitter?

How do you manage database schema migrations in a Node.js project?

How do you handle errors in Node.js applications?

What is the perf_hooks module and how do you measure performance?

How do you implement Server-Sent Events (SSE) in Node.js?

How do you limit the concurrency of many async operations (p-limit / promise pool)?

How do you protect a Node.js API against Denial of Service (DoS) and brute-force attacks?

What are Node.js Streams and when would you use them?

What is the difference between an ORM, a query builder, and a raw driver?

What is the purpose of `dirname` and `filename` in Node.js?