Question 1

How does Array.prototype.toSorted() differ from .sort(), and why does that matter when updating state immutably?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript immutable-state-update interview questions. **Difficulty:** Medium > **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 Picture a manager who wants to keep a snapshot of yesterday's inventory list before making changes

Question 2

What is the difference between includes and indexOf?

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 comparison below, including the NaN case, was actually run in Node. ## 1. Why This Even Matters — A Story First Imagine asking two different assistants "is there a red car in this parking lot?" One (`indexOf`) answers with a parking spot number, or `-1` if there

Question 3

What is the difference between Object.is, ==, and ===?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals and equality-semantics interview questions. **Difficulty:** Medium > **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 — the printed output is real, captured output, not illustrative sample text. ## 1. Why This Even Matters — A Story First Picture three different security guards checking IDs at a

Question 4

What is the difference between == and ===?

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 comparison below was actually run in Node — not recited from memory. ## 1. Why This Even Matters — A Story First Imagine two shipping labels: one says "5 kg" and the other says "5000 g." A loose inspector (==) converts both to the same unit and says they match. A

Question 5

Set methods deep-dive: performance and mutability trade-offs vs manual loops?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript data-structure and performance interview questions. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every number shown below is **real, measured output** from actually running the benchmark in this doc on Node.js v24.19.0 — not an assumed percentage. ## 1. Why This Even Matters — A Story First Picture two guest lists for a wedding, each a

Question 6

How do you correctly check for NaN?

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 coercion and comparison result below was actually run in Node. ## 1. Why This Even Matters — A Story First Asking a bouncer "is this person over 21?" and getting "yes" back sounds definitive — until you learn the bouncer first tried to convert the person's PET in

Question 7

How would you remove an item from an array immutably using toSpliced(), and how does it compare to splice()?

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 was actually run — the "original untouched" claim specifically was verified by logging the SAME original array variable both before and after the `toSpliced()` call. ## 1. Why This Even Matters — A Story First Editing a shared document by literally

Question 8

Why is 0.1 + 0.2 not exactly 0.3?

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 number below, including the raw bit-level rounding, was actually printed by Node — not asserted from memory. ## 1. Why This Even Matters — A Story First Try writing exactly one-third as a decimal: `0.333...` never terminates, so any fixed number of digits is an

Question 9

What is BigInt?

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 precision and arithmetic claim below was actually run in Node. ## 1. Why This Even Matters — A Story First A regular calculator display with a fixed number of digits genuinely cannot show every digit of an astronomically large number — beyond a certain size, it

Question 10

How would you use Symbol.hasInstance and Symbol.toPrimitive to customize instanceof checks and type coercion for a class?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript metaprogramming/coercion interview questions. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every hint value shown below is **real, captured output** from actually running the code in this doc on Node.js v24.19.0 — including one claim this doc's own draft got wrong on the first attempt, caught only by actually running it, exactly the fail

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