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

Accepted Answer

**Target Audience:** Engineers preparing for Node.js system-design interviews — assumes familiarity with the idempotency question's real duplicate-message proof. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The timings below are **real, measured** output from an actually-running in-memory queue on this machine — genuine milliseconds elapsed, not illustrative numbers. ## 1. Why This Even Matt

Question 3

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 4

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 5

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

Accepted Answer

**Target Audience:** Engineers preparing for Node.js interviews — assumes basic `fetch`/Promise familiarity. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every cancellation below was **actually executed**, including a first attempt that failed for the wrong reason and was corrected before being reported — real timing, not a description. ## 1. Why This Even Matters — A Story First Ordering f

Question 6

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

Accepted Answer

**Target Audience:** Engineers preparing for Node.js/JavaScript interviews — assumes basic Promise familiarity. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every claim below — including that an async function's return value really is a Promise — was **actually executed and inspected**, not assumed from how the syntax reads. ## 1. Why This Even Matters — A Story First A recipe written as "w

Question 7

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 8

How do you handle errors in Node.js applications?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js interviews — assumes basic Express and Promise familiarity. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The Express behavior below was **actually executed** against a real Express 4.22.2 server, including a real crash, not described from documentation. ## 1. Why This Even Matters — A Story First A building's fire-alarm system automatical

Question 9

Describe the Node.js module system. What are CommonJS and ES Modules?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js phone screens — assumes basic `import`/`export` syntax familiarity. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every interop claim below was **actually executed** on Node v24.19.0, including a genuinely surprising interop detail found while testing rather than assumed. ## 1. Why This Even Matters — A Story First Two countries share a bo

Question 10

Explain what 'callback hell' is and how to avoid it.

Accepted Answer

**Target Audience:** Engineers preparing for Node.js/JavaScript interviews — assumes basic callback and Promise familiarity. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. All three versions of the example below were **actually executed** on Node v24.19.0 and produced identical results — pasted below, not assumed. ## 1. Why This Even Matters — A Story First Giving someone directions as "when

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Easy

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

Medium

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

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

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

Explain the Node.js event loop.

How do you handle errors in Node.js applications?

Describe the Node.js module system. What are CommonJS and ES Modules?

Hard

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

How do you implement microservices communication in Node.js?

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

What is AsyncLocalStorage and what problem does it solve?