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 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 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 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 5

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 6

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 7

What is short-circuit evaluation?

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 call-count claim below was actually run in Node, using a real counter, not an assumption about laziness. ## 1. Why This Even Matters — A Story First A security checkpoint that requires two separate approvals in sequence does not bother calling the second approver

Question 8

What are the quirks of the typeof operator?

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 result below, including the real thrown error, was actually run in Node. ## 1. Why This Even Matters — A Story First Asking "does this specific book exist anywhere in the library?" is a genuinely safe question to ask even if the book's title was never catalogued

Question 9

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 10

What is destructuring?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript ES6+ syntax interviews. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The real swap, the real nested/renamed/defaulted extraction, and the real thrown error on `null` below were **actually run** — not descriptions of documented behavior. ## 1. Why This Even Matters — A Story First Unpacking a delivered grocery bag by reaching in and pulli

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

What does Array.filter do?

What are optional chaining and nullish coalescing?

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

What is the difference between null and undefined?

What is short-circuit evaluation?

What are the quirks of the typeof operator?

Medium

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

What does Object.create do?

What is the difference between `Object.groupBy()` and `Map.groupBy()` — keys, prototypes and ordering?

What are the basics of regular expressions in JavaScript?

Explain prototypal inheritance.

What are the logical assignment operators?

What is the prototype chain?

When would you reach for Map.groupBy() instead of Object.groupBy(), and what problem does it solve with non-string keys?

Hard

How would you write a deep-clone function that correctly handles a self-referencing (circular) object without infinite recursion?

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