Question 1

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 2

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 3

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 4

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

Accepted Answer

**Target Audience:** Engineers preparing for Node.js system-design interviews — assumes familiarity with the event loop and the libuv thread pool from their own dedicated questions. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Both the blocking main-thread run and the Worker Thread run below used the **identical** computation, executed for real on Node v24.19.0 — the heartbeat contrast is mea

Question 5

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

Accepted Answer

**Target Audience:** Engineers preparing for Node.js interviews — assumes basic event-loop and thread-pool familiarity. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Both the frozen and the free heartbeat below were **actually measured** on Node v24.19.0, using the identical computation in both cases. ## 1. Why This Even Matters — A Story First A single cashier who personally walks to a back

Question 6

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

Accepted Answer

While the JavaScript execution engine (V8) runs on a single thread, Node.js offloads heavy asynchronous operations to C++ APIs and libuv. Libuv maintains a thread pool (default size of 4) to execute blocking tasks like cryptography, compression, and file system I/O in parallel. Once a background thread finishes a task, it notifies the main thread to execute its callback via the event loop queue.

Question 7

How does the thread pool work in Node.js?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js interviews — assumes basic event-loop familiarity. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every timing number below came from **actually running real `crypto.pbkdf2` calls** on Node v24.19.0, not an estimate. ## 1. Why This Even Matters — A Story First A small kitchen with exactly four stovetop burners can cook four dishes simultane

Question 8

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 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

Describe the Node.js architecture.

Accepted Answer

**Target Audience:** Engineers preparing for Node.js system-design interviews — assumes familiarity with V8's role and libuv's role from their own dedicated questions. **Difficulty:** Hard > **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, confirming genuinely separate components, not a single combined runtime version. ## 1. Why This Even Matters

NODE.JSDECODED.

Easy