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

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 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 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 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 7

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 8

What is the difference between includes and indexOf?

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 comparison below, including the NaN case, was actually run in Node. ## 1. Why This Even Matters — A Story First Imagine asking two different assistants "is there a red car in this parking lot?" One (`indexOf`) answers with a parking spot number, or `-1` if there

Question 9

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 10

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

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Easy

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

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

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 Array.filter do?

What are optional chaining and nullish coalescing?

What is the difference between includes and indexOf?

Medium

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

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

Implement a robust Event Emitter class.

How do you implement a deepFreeze function recursively?

How does Array.reduce work?

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

How do you implement a custom Promise.all polyfill?

What do flat and flatMap do?

Hard

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

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

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

Implement an async task queue with a concurrency limit.

What does yield* do in a generator?

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

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

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