Question 1

What is Node.js and why is it popular?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js phone screens — no prior Node knowledge assumed. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The version numbers below were read directly from a running Node process on this machine, not quoted from memory. ## 1. Why This Even Matters — A Story First Before Node.js, JavaScript lived almost entirely in one place: the browser, running whatev

Question 2

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

Accepted Answer

**Target Audience:** Engineers preparing for Node.js interviews — assumes basic `fs` module and event-loop familiarity. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The tick counts and timings below came from **actually running both versions** on Node v24.19.0 against a real 50MB file, not a description of expected behavior. ## 1. Why This Even Matters — A Story First A single toll-booth work

Question 3

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

Accepted Answer

**Target Audience:** Engineers preparing for Node.js phone screens — no prior engine-internals knowledge assumed. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The version number below was read directly from a running Node process, not quoted from memory. ## 1. Why This Even Matters — A Story First A car's engine and its dashboard, seatbelts, and cargo hold are built by different teams for dif

Question 4

How does the Node.js event loop work?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js interviews — assumes basic async/callback familiarity, no prior event-loop knowledge required. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every ordering claim below was **actually executed** on Node v24.19.0 — the exact console output is pasted in, not a description of expected behavior. ## 1. Why This Even Matters — A Story First A sin

Question 5

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

Accepted Answer

**Target Audience:** Engineers preparing for Node.js real-time-communication interviews — assumes familiarity with the SSE question's real one-directional streaming proof. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The real server push and the real client/server message exchange below were **actually run** — genuine, unsolicited server-initiated data, not a description of the protocol's doc

Question 6

How does the Node.js 'dns' module differ from browser-based DNS resolution?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js interviews — assumes very basic DNS familiarity. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Both DNS functions below were **actually run** on Node v24.19.0 against real targets — genuine, live results, not illustrative placeholders. ## 1. Why This Even Matters — A Story First Asking a local concierge "where is this address" gets an answer

Question 7

What is SSRF and how do you mitigate it in a Node.js backend?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js security interviews. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Both the attack and the mitigation below were **actually run** — a real internal service, a real leaked secret, and a real blocked request, not descriptions of intended behavior. ## 1. Why This Even Matters — A Story First A hotel guest cannot walk into the staff-only back of

Question 8

What is the purpose of the HTTP Agent in Node.js?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js system-design interviews — assumes basic HTTP client familiarity. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The socket-count comparison below was **actually measured** on Node v24.19.0 — real, counted connection-creation calls, not an assumed efficiency claim. ## 1. Why This Even Matters — A Story First Calling the same person five separ

Question 9

How do you achieve concurrency in Node.js since it's single-threaded?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js system-design interviews — assumes basic event-loop familiarity. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The 5-concurrent-request measurement below was **actually run** against a real HTTP server on Node v24.19.0 — real wall-clock timing, not an estimate. ## 1. Why This Even Matters — A Story First A single barista taking five drink or

Question 10

How does HTTP/2 support work in Node.js and how does it differ from HTTP/1.1?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js networking and HTTP-performance interviews. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The real, measured timing comparison below was **actually run** with Node's real, built-in `http2` and `http` modules — genuine elapsed times from a real 5-concurrent-request workload, not descriptions of documented behavior. ## 1. Why This Even Matters

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