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

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

Accepted Answer

**Target Audience:** Engineers preparing for Node.js testing-strategy interviews — assumes familiarity with the `node:test` runner and mocking questions' real proofs. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The timing comparison below is **real, measured** output from actually running both a unit and an integration test with `node --test` — genuine milliseconds, not illustrative numbers.

Question 5

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 6

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 7

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 8

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 9

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

Accepted Answer

**Target Audience:** Engineers preparing for Node.js/JavaScript interviews — assumes basic callback familiarity, no prior Promise knowledge required. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every timing and chaining claim below was **actually executed** on Node v24.19.0. ## 1. Why This Even Matters — A Story First A tracking number for a package in transit is a small, honest object: ri

Question 10

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

Accepted Answer

**Target Audience:** Engineers preparing for Node.js interviews — assumes very basic file I/O familiarity, no prior stream-specific knowledge required. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The Transform stream below was **actually run** on Node v24.19.0, producing the exact output pasted in. ## 1. Why This Even Matters — A Story First A factory assembly line processes each item as i

NODE.JSDECODED.

Easy

What is Node.js and why is it popular?

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

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

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

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

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

How does the thread pool work in Node.js?

What is `node:sqlite`, Node's built-in SQLite module?

Hard

What is `process.nextTick()` and `setImmediate()` and when to use them?

How do you profile CPU usage with --prof, 0x, or clinic.js flame graphs?

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

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

What are some common ways to improve performance in a Node.js application?

Explain the concept of 'Event Loop Pollution' and how to avoid it.

Describe the Node.js architecture.

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