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 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 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

How do the new Set methods `union()`, `intersection()`, `difference()` and `symmetricDifference()` work?

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 operation's real result below was actually run in Node (Baseline-shipped across all major browsers since 2024). ## 1. Why This Even Matters — A Story First Two overlapping guest lists for two different parties: union is the combined list of everyone invited to

Question 5

What is the difference between map and forEach?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript array-method and async-pattern interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The real return-value proof and the real async-forEach bug demonstration below were **actually run** — not descriptions of documented behavior. ## 1. Why This Even Matters — A Story First Handing a stack of letters to a mail carrier who genuinely wa

Question 6

What do Object.keys, values, and entries return?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript object-enumeration interviews. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The real inclusion/exclusion output and the real key-ordering demonstration below were **actually run** — not descriptions of documented behavior. ## 1. Why This Even Matters — A Story First A guest list for a party that only includes people who both RSVP'd AND l

Question 7

What is JSON and why is it commonly used?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals and data-interchange 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 shipping a package overseas. A JS backend, a Python se

Question 8

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

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 coercion and dedup claim below was actually run in Node. ## 1. Why This Even Matters — A Story First A plain object's keys are like index cards in a box that only accepts cards labeled with PLAIN TEXT — hand it an actual physical object instead of a label, and

Question 9

What is memoization?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals / performance interview questions. **Difficulty:** Easy-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 — the printed output is real, captured output, not illustrative sample text. ## 1. Why This Even Matters — A Story First Picture a student working through a long worksheet of math pro

Question 10

How does WeakMap allow garbage collection of keys?

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:. This answer does something most documentation on this topic does not: it does not merely ASSERT that WeakMap allows key collection — it directly, actually OBSERVES a real collection event happen, via `node --expose-gc` and a real `FinalizationRegistry` callback. ## 1.

JAVASCRIPTDECODED.

Easy

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?

How do the new Set methods `union()`, `intersection()`, `difference()` and `symmetricDifference()` work?

What is the difference between map and forEach?

What do Object.keys, values, and entries return?

What is JSON and why is it commonly used?

Medium

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

What is memoization?

How does WeakMap allow garbage collection of keys?

Implement a robust Event Emitter class.

What does Object.create do?

How does Array.reduce work?

What do flat and flatMap do?

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

Hard

Implement an async task queue with a concurrency limit.

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

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

How do iterators and the iterable protocol work?

How does V8 Array copy-on-write optimization work for `toSorted`/`toReversed`/`toSpliced`/`with`?

What are WeakMap and WeakSet?

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

What is a Proxy in JavaScript?