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 Array.prototype.with() let you replace one array element immutably, and where would that pattern actually help?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript immutable-data-pattern 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 Imagine a printed photograph versus a photo-editing app with an undo hist

Question 3

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 4

How does Object.groupBy() work in ES2024 and when should you use it over reduce?

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 grouping result and prototype check below was actually run in Node (v21+ required; this project verified on v24.19.0). ## 1. Why This Even Matters — A Story First A grouping bucket labeled with a plain sticky note is a regular object — perfectly fine, but it si

Question 5

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 6

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 7

How would you use the Page Visibility API to pause a setInterval-based poller when a user switches to a different browser tab?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript browser-API 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 against a real jsdom document 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 delivery app polling a restaurant every fe

Question 8

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 9

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 10

What is event bubbling and capturing?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript DOM/events interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The exact firing order below was actually reproduced with jsdom, not recited from documentation. ## 1. Why This Even Matters — A Story First Think of a click event as a security guard doing a walk-through: first the guard walks INWARD from the building's front gate all

JAVASCRIPTDECODED.

Easy

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

How does Array.prototype.with() let you replace one array element immutably, and where would that pattern actually help?

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

How does Object.groupBy() work in ES2024 and when should you use it over reduce?

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

What are static methods and properties?

How would you use the Page Visibility API to pause a setInterval-based poller when a user switches to a different browser tab?

What is the difference between null and undefined?

Medium

Set methods deep-dive: performance and mutability trade-offs vs manual loops?

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

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

What does Object.create do?

What is a Promise and what are its states?

Explain event delegation.

What is the difference between call, apply, and bind?

How do you implement a custom Promise.all polyfill?

Hard

Explain the event loop, call stack, and task queues.

What does the RegExp `v` flag (Unicode Sets) add over the `u` flag?

What is the difference between Promise.all, allSettled, race, and any?

How does `Atomics.waitAsync()` enable non-blocking waits on SharedArrayBuffer?

Implement an async task queue with a concurrency limit.

What does yield* do in a generator?

How does SharedArrayBuffer let two Web Workers share memory directly, and what race conditions do you need to guard against?

What is the Reflect API?