Question 1

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 2

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 3

What does Object.hasOwn() do differently from Object.prototype.hasOwnProperty.call(), and why is it the recommended modern replacement?

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 Imagine asking someone "do you personally own this book, or did you just borrow it

Question 4

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 5

What are modules in JavaScript?

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 module-caching claim below was actually run and confirmed inside a real CommonJS script. ## 1. Why This Even Matters — A Story First A shared office supply closet, restocked once at the start of the day, is used identically by every single department that visits it

Question 6

How would you combine multiple AbortSignals or add a timeout to a fetch request using AbortSignal.any() and AbortSignal.timeout()?

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 which signal genuinely wins each race — was actually run and timed against a real network endpoint. ## 1. Why This Even Matters — A Story First A race with two separate finish-line judges — one who blows a whistle if a runner crosses the

Question 7

What is the difference between ES modules and CommonJS?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript/Node.js fundamentals interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The caching and environment claims below were actually run in Node, directly inside a real CommonJS script. ## 1. Why This Even Matters — A Story First CommonJS's `require()` is like asking a librarian in person for a book by a title you might decide on the s

Question 8

What does Promise.withResolvers() solve and how does it replace the deferred anti-pattern?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript/async interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every resolve/reject path below was actually run in Node (ES2024, Node 20.12+). ## 1. Why This Even Matters — A Story First A `new Promise` executor is like a sealed room where only whoever is standing inside at that exact moment can press the "done" button. Sometimes the a

Question 9

Explain how prototypal inheritance differs from classical inheritance.

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript object-model interview questions that expect a comparison to classical OOP. **Difficulty:** Hard > **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 leads with the CONCEPTUAL DIFFERENCE from classical inheritance; for how the J

Question 10

How do Stage 3 JavaScript Decorators work — class, method and field decorators?

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 about what DOES and does NOT run natively was actually tested — in both Node v24.19.0 and a real, current Chrome browser — not assumed. **Honesty note:** unlike every other example in this bank, the code example below is genuinely NOT runnable in this playg

JAVASCRIPTDECODED.

Easy

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

What are static methods and properties?

What does Object.hasOwn() do differently from Object.prototype.hasOwnProperty.call(), and why is it the recommended modern replacement?

How does instanceof work?

Medium

What are modules in JavaScript?

How would you combine multiple AbortSignals or add a timeout to a fetch request using AbortSignal.any() and AbortSignal.timeout()?

What is the difference between ES modules and CommonJS?

What does Promise.withResolvers() solve and how does it replace the deferred anti-pattern?

Hard

Explain how prototypal inheritance differs from classical inheritance.

How do Stage 3 JavaScript Decorators work — class, method and field decorators?

What are tagged template literals?

How does top-level await change how an ES module and its importers load, and what are the risks of using it carelessly?

How would you use Symbol.hasInstance and Symbol.toPrimitive to customize instanceof checks and type coercion for a class?