Question 1

What is the difference between var, let, 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 scoping and TDZ claim below was actually run in Node — including the classic loop-closure bug. ## 1. Why This Even Matters — A Story First Imagine a shared office whiteboard (`var`) versus a sticky note stuck to one specific desk (`let`/`const`). Anyone in the wh

Question 2

What is hoisting in JavaScript?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals 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. ## 1. Why This Even Matters — A Story First Think of a scope like a conference room with name tags laid out before the meeting

Question 3

Describe the concept of 'scope' in JavaScript.

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals 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. ## 1. Why This Even Matters — A Story First Picture a program as rooms inside rooms: a house (global scope) contains a kitchen

Question 4

What are `let`, `const`, and `var`? What are their differences?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals 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. ## 1. Why This Even Matters — A Story First Think of three different ways to reserve a locker. `var` reserves a locker for the

Question 5

What is globalThis?

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:. The identity claim below was actually run in Node. ## 1. Why This Even Matters — A Story First Imagine three different countries calling the same building "City Hall," "Rathaus," and "Ayuntamiento" — the same physical building, three different local names, and any trav

Question 6

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 7

What do Object.keys, values, and entries return?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript object-enumeration interviews. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The real inclusion/exclusion output and the real key-ordering demonstration below were **actually run** — not descriptions of documented behavior. ## 1. Why This Even Matters — A Story First A guest list for a party that only includes people who both RSVP'd AND l

Question 8

Debounce vs throttle — implement debounce

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript / frontend-implementation 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. For the conceptual distinction between debounce and throttle before diving into code, see

Question 9

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 10

How would you write a Proxy 'set' trap that validates a value before allowing an assignment, and what happens if you return false?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript meta-programming and data-validation interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The real rejection, the real accepted write, and the real strict-mode throw below were **actually run** — not descriptions of documented behavior. See also this bank's own dedicated

JAVASCRIPTDECODED.

Easy

What is the difference between var, let, and const?

What is hoisting in JavaScript?

Describe the concept of 'scope' in JavaScript.

What are `let`, `const`, and `var`? What are their differences?

What is globalThis?

What do Array.some and Array.every do?

What do Object.keys, values, and entries return?

Medium

Debounce vs throttle — implement debounce

Explain 'this' keyword in JavaScript.

How would you write a Proxy 'set' trap that validates a value before allowing an assignment, and what happens if you return false?

What are modules in JavaScript?

What is lexical scoping and the scope chain?

What is an IIFE?

How does the 'this' keyword work?

What is the difference between arrow and regular functions?

Hard

How do you implement a custom Promise.any polyfill?

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

What is the temporal dead zone?

What is debouncing and throttling?