Question 1

What are stream piping and the .pipe() method in Node.js?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js phone screens — assumes very basic stream familiarity. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The pipe below was **actually run** on Node v24.19.0, delivering real data end to end, not described from documentation. ## 1. Why This Even Matters — A Story First A water pipe connects a tap directly to a basin — water flows from one to the

Question 2

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 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

Explain the concept of 'event-driven architecture' in Node.js.

Accepted Answer

**Target Audience:** Engineers preparing for Node.js system-design interviews — assumes `EventEmitter` basics from its own dedicated question. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The subscriber-addition demonstration below was **actually run** on Node v24.19.0 — the fourth subscriber genuinely started receiving events with zero changes to the publisher, not a description of expected

Question 5

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 6

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 7

What is the 12-factor app methodology as applied to Node.js services?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js system-design and deployment-practices interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The config demo below was **actually run twice**, with and without environment variables set — real, different output, not a description of intended behavior. ## 1. Why This Even Matters — A Story First A recipe that says "add the amount of sal

Question 8

Explain how the EventEmitter class works under the hood.

Accepted Answer

**Target Audience:** Engineers preparing for Node.js interviews — assumes familiarity with EventEmitter's public API (see the dedicated basics question), going one level deeper. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The internal `_events` inspection below was **actually run** on Node v24.19.0 — real property values, not a description of the source code. ## 1. Why This Even Matters — A

Question 9

What is Helmet and which HTTP security headers should a Node.js API set?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js/Express security interviews. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The header diff below is **real, measured** output from two actually-running Express servers — genuine response headers, not a description of what Helmet's documentation says it does. ## 1. Why This Even Matters — A Story First A house with no lock on the door, no pee

Question 10

What are `EventEmitter`s in Node.js?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js interviews — assumes very basic callback familiarity, no prior EventEmitter knowledge required. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every behavior below was **actually executed** on Node v24.19.0, including the special `'error'`-event case, verified as a real thrown and caught exception. ## 1. Why This Even Matters — A Story First

NODE.JSDECODED.

Easy

What are stream piping and the .pipe() method in Node.js?

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

Medium

How does the Node.js event loop work?

Explain the concept of 'event-driven architecture' in Node.js.

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

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

What is the 12-factor app methodology as applied to Node.js services?

Explain how the EventEmitter class works under the hood.

What is Helmet and which HTTP security headers should a Node.js API set?

What are `EventEmitter`s in Node.js?

Hard

Explain how memory management and Garbage Collection work in the V8 engine.

What is diagnostics_channel and when would you use it?

How do you implement a circuit breaker for unreliable downstream services?

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

How do you implement microservices communication in Node.js?

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

What do `unref()` and `ref()` do on a Node.js Timeout, and why do timers drift?

How do you add OpenTelemetry distributed tracing to a Node.js service?