Question 1

What is the difference between for...of and for...in?

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 loop's real output below was actually run in Node. ## 1. Why This Even Matters — A Story First Imagine a numbered coat-check rack (an array) that also has a sticky note taped to its SIDE reading "capacity: 50" — a piece of metadata about the rack itself, not on

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

When should you use Map and Set over objects and arrays?

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 coercion and dedup claim below was actually run in Node. ## 1. Why This Even Matters — A Story First A plain object's keys are like index cards in a box that only accepts cards labeled with PLAIN TEXT — hand it an actual physical object instead of a label, and

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Easy

What is the difference between for...of and for...in?

What is the difference between includes and indexOf?

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

Medium

When should you use Map and Set over objects and arrays?