Question 1

What is structuredClone and when is it useful in Node.js?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js/JavaScript data-handling interviews. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every comparison below was **actually run** — the real type losses through JSON, the real cyclic-reference crash and success, and the real function-cloning error, are genuine, observed output, not descriptions of documented behavior. ## 1. Why This Even Matters

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 node:util parseArgs helper for building CLIs?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js CLI-tooling interviews. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Both the successful parse and the strict-mode rejection below were **actually run** — real, structured output from a real CLI invocation, and a real thrown error, not descriptions of documented behavior. ## 1. Why This Even Matters — A Story First Reaching for a full-featu

Question 4

How do you mock modules, timers, and network calls in Node.js tests?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js testing interviews — assumes familiarity with the `node:test` runner question's real pass/fail proof. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. All three mocking mechanisms below were **actually run** — a real 10-second timer genuinely fired in under 1ms, and a real method mock genuinely avoided a real network call, not descriptions of i

Question 5

When would you use `child_process` module in Node.js?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js system-design interviews — assumes basic `child_process` familiarity (see the dedicated fork/spawn/exec question for the full mechanical detail). **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The command below was **actually executed** on Node v24.19.0 — a real external process, real returned output. ## 1. Why This Even Matters — A Story Firs

Question 6

What is prototype pollution and how do you prevent it in Node.js?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js security interviews. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The pollution and its process-wide persistence below were **actually run** — a real, unrelated object genuinely gaining a real property it never had, not a description of documented behavior. ## 1. Why This Even Matters — A Story First Vandalizing one specific book in a libra

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