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

How does 'use strict' change what `this` refers to inside a regular function called without a receiver, compared to non-strict mode?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript this-binding 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 Imagine handing someone a form that says "sign here as: ___" with the blank left empty. In a lenient office,

Question 3

What does Array.filter 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 non-mutation proof, the real chaining, and the real hole-skipping demonstration below were **actually run** — not descriptions of documented behavior. ## 1. Why This Even Matters — A Story First Sifting flour through a sieve produces a genuinely NEW, separate

Question 4

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 5

What are static methods and properties?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals/OOP interviews. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every shared-state and accessibility claim below was actually run in Node. ## 1. Why This Even Matters — A Story First Think of a factory's total-units-produced counter mounted on the FACTORY BUILDING itself, versus each individual product coming off the line havin

Question 6

What are optional chaining and nullish coalescing?

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 throw-vs-no-throw and call-count claim below was actually run in Node. ## 1. Why This Even Matters — A Story First Walking down a hallway of doors, checking behind each one for a specific box: optional chaining is like a careful walker who, upon finding any door

Question 7

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 8

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 9

What is the difference between null and undefined?

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 check below was actually run in Node. ## 1. Why This Even Matters — A Story First Think of a form field on a job application. A field that was never shown to the applicant at all is like `undefined` — nobody had the chance to fill it in. A field that was shown, a

Question 10

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

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Easy

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

How does 'use strict' change what `this` refers to inside a regular function called without a receiver, compared to non-strict mode?

What does Array.filter do?

What is hoisting in JavaScript?

What are static methods and properties?

What are optional chaining and nullish coalescing?

Describe the concept of 'scope' in JavaScript.

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

Medium

How do you convert a callback-based function to a Promise?

What does Object.create do?

How does the 'this' keyword work?

How would you use ResizeObserver to react to an element's size changing, without polling getBoundingClientRect() on every frame?

What is the difference between arrow and regular functions?

How do you deep clone an object in JavaScript?

What is the purpose of the `bind`, `call`, and `apply` methods?

What are the logical assignment operators?

Hard

What is ShadowRealm and how does it provide true JavaScript sandboxing?

How does ArrayBuffer transfer and detach work with `transfer()` vs `transferToFixedLength()`?

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?

How do iterators and the iterable protocol work?

What does the 'new' keyword do?

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

What problem do WeakRef and FinalizationRegistry solve, and why should they be used carefully?