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

What is the difference between find and filter?

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 call-counter proof and the real no-match comparison below were **actually run** — not descriptions of documented behavior. ## 1. Why This Even Matters — A Story First Looking for one specific named folder in a filing cabinet — you genuinely stop searching the

Question 3

What do Array.some and Array.every 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 call-counter proof and the real empty-array edge cases below were **actually run** — not descriptions of documented behavior. ## 1. Why This Even Matters — A Story First A security guard checking IDs at a door who genuinely stops the moment they spot ONE under

Question 4

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 5

How does instanceof work?

Accepted Answer

**Target Audience:** Engineers preparing for precise JavaScript object-model 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 — the printed output is real, captured output, not illustrative sample text. instanceof is built directly on the prototype chain covered in

Question 6

Explain 'this' keyword in JavaScript.

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals 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. This doc covers the CONCEPT of this (why it behaves the way it does); for the exact binding-rule priority order with verified o

Question 7

What is lexical scoping and the scope chain?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals 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. This doc covers the GENERAL rule for how any variable lookup resolves in JavaScript; for the specific case where a function out

Question 8

Explain prototypal inheritance.

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript object-model and inheritance 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. This doc covers the MECHANISM (how prototypal inheritance actually works); for how it conceptually differs from

Question 9

What is the prototype chain?

Accepted Answer

**Target Audience:** Engineers preparing for precise JavaScript object-model 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. This is a focused, definitional look at the lookup mechanism itself; for the broader concept of how prototypal inherita

Question 10

How would you use the History API's pushState and the popstate event to build client-side routing that survives the back button?

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 exact restored state after a real back-button navigation — was verified live in a real browser, calling the same `history.back()` method the browser's own UI button triggers. ## 1. Why This Even Matters — A Story First A choose-your-

JAVASCRIPTDECODED.

Easy

What is the difference between includes and indexOf?

What is the difference between find and filter?

What do Array.some and Array.every do?

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

How does instanceof work?

Medium

Explain 'this' keyword in JavaScript.

What is lexical scoping and the scope chain?

Explain prototypal inheritance.

What is the prototype chain?

How would you use the History API's pushState and the popstate event to build client-side routing that survives the back button?

Hard

How would you use the new Iterator helper methods (.map, .filter, .take, .drop) to process a large or infinite iterator lazily?

How would you write a deep-clone function that correctly handles a self-referencing (circular) object without infinite recursion?

How would you implement Symbol.asyncIterator on a class to transparently page through a paginated API with for await...of?