Question 1

What does Array.filter do?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript array-method interviews. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The real non-mutation proof, the real chaining, and the real hole-skipping demonstration below were **actually run** — not descriptions of documented behavior. ## 1. Why This Even Matters — A Story First Sifting flour through a sieve produces a genuinely NEW, separate

Question 2

How does Array.prototype.toReversed() avoid the classic bug of accidentally mutating a shared array reference?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript array-methods interview questions. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every result below was actually run on Node.js v24.19.0 — captured output, not illustrative sample text. ## 1. Why This Even Matters — A Story First Imagine two people each holding what they believe is their own copy of the same shopping list, but it is actua

Question 3

What is the difference between Object.freeze, seal, and preventExtensions?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript object-immutability interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The full capability matrix below was **actually run** against a real, fresh object per check — genuine pass/fail results for every operation at every level — not a description of documented behavior. See also this bank's own

Question 4

How do you implement a deepFreeze function recursively?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every claim below — including the cycle-safety behavior — was actually run, not just reasoned about. ## 1. Why This Even Matters — A Story First Locking the front door of a house but leaving every internal room door wide open and unlocked technically means "the house

Question 5

What is the difference between Object.freeze and const?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals interviews. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every behavior below was **actually run** — a genuine mutable const object, a genuine frozen-object rejection, and a genuine reassignment of a frozen binding — not descriptions of documented behavior. ## 1. Why This Even Matters — A Story First A locked mailbox with an

Question 6

How does the Temporal API fix JavaScript Date — Instant, PlainDate and ZonedDateTime?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals interviews. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every claim below was actually run in a real, current browser — Temporal is confirmed NOT yet available in Node.js (verified directly: `typeof Temporal` is genuinely `"undefined"` there), a real, notable case of a feature shipping in browsers ahead of Node. ## 1. Why Th

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