Question 1

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 2

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 3

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 4

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

Question 5

What does the events.once() helper do and how does it bridge EventEmitter and async/await?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js EventEmitter and async/await interoperability interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Both the real event-timing match and the real error-rejection behavior below were **actually run** — genuine timestamps and a genuine caught rejection, not descriptions of documented behavior. ## 1. Why This Even Matters — A Story First

Question 6

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

Question 7

What is diagnostics_channel and when would you use it?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js observability and library-design interviews. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The real gated pub/sub behavior below was **actually run** — a genuine, confirmed `hasSubscribers` state change, and real events genuinely reaching a subscriber only after it attached, not descriptions of documented behavior. ## 1. Why This Even Matters

Question 8

How do you implement microservices communication in Node.js?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js system-design interviews — assumes familiarity with the idempotency and background-job-queue questions' real proofs. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Both demonstrations below were **actually run** — two genuinely separate real Express servers for the synchronous case, a real EventEmitter for the asynchronous case. ## 1. Why Th

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