Question 1

What is the difference between readFile and createReadStream in Node.js?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js phone screens — assumes very basic `fs` module familiarity. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The numbers below came from **actually reading a real 20MB file** both ways on Node v24.19.0, not a description of expected behavior. ## 1. Why This Even Matters — A Story First Moving a house's contents in one giant crate needs a crane

Question 2

What is the purpose of the util.promisify function in Node.js?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js phone screens — assumes basic Promise and callback familiarity. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Both the success and rejection paths below were **actually executed** on Node v24.19.0, not assumed from the function's name. ## 1. Why This Even Matters — A Story First An old appliance with a two-prong plug still works perfectly —

Question 3

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 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

What is AbortSignal.timeout and how do you cancel async work cleanly?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js async-cancellation interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Both the successful and the aborted operation below were **actually run** — a real, measured abort at ~304ms for a configured 300ms timeout, and a real, specific error, not descriptions of documented behavior. ## 1. Why This Even Matters — A Story First Setting a

Question 6

What are 'Event Emitting' patterns and how does error-first callback convention relate to them?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js interviews — assumes familiarity with both `EventEmitter` basics and error-first callbacks individually, connecting the two. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The silent-swallow-versus-throw contrast below was **actually run** on Node v24.19.0 — a real absence of a crash in one case, a real caught exception in the other. ## 1. W

Question 7

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 8

What are some best practices for structuring a Node.js project?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js system-design interviews — assumes familiarity with the dependency-injection question's real, verified demonstration. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. This is a structural/organizational judgment question; the technical claim it rests on (that layered structure enables real dependency injection) is grounded in the real, verified

Question 9

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 10

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

NODE.JSDECODED.

Easy

What is the difference between readFile and createReadStream in Node.js?

What is the purpose of the util.promisify function in Node.js?

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

Medium

How does the Node.js event loop work?

What is AbortSignal.timeout and how do you cancel async work cleanly?

What are 'Event Emitting' patterns and how does error-first callback convention relate to them?

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

What are some best practices for structuring a Node.js project?

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

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?

Hard

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

How does CommonJS `require()` an ES Module in modern Node.js?

What is backpressure in Node.js streams and how do you handle it?

How do you implement microservices communication in Node.js?

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

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?