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

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 3

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

Accepted Answer

**Target Audience:** Engineers preparing for Node.js system-design interviews — assumes familiarity with Node's concurrency model (see the dedicated question). **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. This is a judgment-and-trade-off question; the technical claims it rests on are grounded in real measurements verified in their own dedicated questions, cited here rather than re-derived. #

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

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

Accepted Answer

**Target Audience:** Engineers preparing for Node.js ESM and async-architecture interviews — assumes familiarity with the module-type question's real require(ESM) boundary. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Both timing claims below were **actually measured** with real `Date.now()` deltas around real `setTimeout`-based delays — genuine milliseconds, not illustrative numbers. ## 1.

Question 7

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

Accepted Answer

**Target Audience:** Engineers preparing for Node.js interviews — assumes basic `child_process` familiarity. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. All three functions below were **actually run** on Node v24.19.0, including a real `fork()` IPC message round-trip. ## 1. Why This Even Matters — A Story First Three ways to get help from someone in another room: shout a short instruction

Question 8

Explain the Node.js event loop.

Accepted Answer

The **event loop** is the heart of Node's concurrency: a single thread that repeatedly runs queued callbacks across ordered **phases** (timers → pending → poll → check → close). Between phases (and after each callback) it drains the **microtask** queues (process.nextTick, then Promises). This lets one thread juggle many async operations without blocking.

Question 9

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

Accepted Answer

**Target Audience:** Engineers preparing for Node.js async-patterns and API-integration interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Both the limited and unbounded concurrency measurements below were **actually run** — a real, measured maximum-concurrency count in each case, not descriptions of documented behavior. ## 1. Why This Even Matters — A Story First A coffee shop with o

Question 10

How do you handle database connection pooling in a Node.js application?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js system-design interviews — assumes basic database-client familiarity. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The timing comparison below was **actually measured** on Node v24.19.0, using a small, real, hand-built pool — a genuine before/after difference, not an estimate. ## 1. Why This Even Matters — A Story First A courier who driv

NODE.JSDECODED.

Easy

What is Node.js and why is it popular?

Medium

How does the Node.js event loop work?

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

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

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

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

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

Explain the Node.js event loop.

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

Hard

When would you use `child_process` module in Node.js?

What changes for a Node.js app running on AWS Lambda vs a long-running server?

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

How do you build a worker-thread pool (e.g. Piscina) for CPU-bound work?

What is AsyncLocalStorage and what problem does it solve?