Question 1

How would you use Object.groupBy() to group an array of orders by status, and how does it compare to a manual reduce()?

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 claim below was actually run — both implementations were executed against the identical real orders array and their outputs directly compared. ## 1. Why This Even Matters — A Story First Sorting a pile of mail into labeled bins by recipient is a task every office

Question 2

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 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 Array.prototype.toSorted() differ from .sort(), and why does that matter when updating state immutably?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript immutable-state-update 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 Picture a manager who wants to keep a snapshot of yesterday's inventory list before making changes

Question 5

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 6

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 7

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 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 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

JAVASCRIPTDECODED.

Easy

How would you use Object.groupBy() to group an array of orders by status, and how does it compare to a manual reduce()?

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?

How does Array.prototype.toSorted() differ from .sort(), and why does that matter when updating state immutably?

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

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

Medium

Debounce vs throttle — implement debounce

How do you implement infinite currying in JavaScript?

What is the difference between Object.freeze, seal, and preventExtensions?

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

Explain 'this' keyword in JavaScript.

How does the fetch API work?

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

How does WeakMap allow garbage collection of keys?

Hard

Why can pushing a single non-numeric value into a large numeric array tank its performance in V8, and how would you avoid it?

What are async iterators and for await...of?

How would you make a custom class iterable with for...of by implementing Symbol.iterator as a generator method?

What is the Reflect API?

What are Symbols and what are they used for?

What is a generator function?

Explain how prototypal inheritance differs from classical inheritance.

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