Question 1

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 2

How do the new Set methods `union()`, `intersection()`, `difference()` and `symmetricDifference()` work?

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 operation's real result below was actually run in Node (Baseline-shipped across all major browsers since 2024). ## 1. Why This Even Matters — A Story First Two overlapping guest lists for two different parties: union is the combined list of everyone invited to

Question 3

What is `Error.isError()` and why is `instanceof Error` unreliable across realms?

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:. The cross-realm failure below was actually reproduced using Node's own real `vm` module — not merely described. ## 1. Why This Even Matters — A Story First Two separate countries can each mint their own, genuinely different "$20 bill" — both are real, valid currency

Question 4

What is the difference between map and forEach?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript array-method and async-pattern interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The real return-value proof and the real async-forEach bug demonstration below were **actually run** — not descriptions of documented behavior. ## 1. Why This Even Matters — A Story First Handing a stack of letters to a mail carrier who genuinely wa

Question 5

What is the difference between slice and splice?

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 before/after array state below was actually run in Node. ## 1. Why This Even Matters — A Story First Photocopying a few pages out of a physical book (slice) leaves the original book completely intact on the shelf — you walk away with a separate, independent copy.

JAVASCRIPTDECODED.

Easy

What is the difference between includes and indexOf?

How do the new Set methods `union()`, `intersection()`, `difference()` and `symmetricDifference()` work?

What is `Error.isError()` and why is `instanceof Error` unreliable across realms?

What is the difference between map and forEach?

Medium

What is the difference between slice and splice?