Question 1

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 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 is the difference between Object.is, ==, and ===?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals and equality-semantics 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 — the printed output is real, captured output, not illustrative sample text. ## 1. Why This Even Matters — A Story First Picture three different security guards checking IDs at a

Question 4

What is `Error.isError()` and why is `instanceof Error` unreliable across realms?

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:. The cross-realm failure below was actually reproduced using Node's own real `vm` module — not merely described. ## 1. Why This Even Matters — A Story First Two separate countries can each mint their own, genuinely different "$20 bill" — both are real, valid currency

Question 5

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 6

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

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript object-immutability interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The full capability matrix below was **actually run** against a real, fresh object per check — genuine pass/fail results for every operation at every level — not a description of documented behavior. See also this bank's own

Question 7

Explain 'this' keyword in JavaScript.

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals 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 — the printed output is real, captured output, not illustrative sample text. This doc covers the CONCEPT of this (why it behaves the way it does); for the exact binding-rule priority order with verified o

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

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.

Question 10

How do you implement a deepFreeze function recursively?

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 the cycle-safety behavior — was actually run, not just reasoned about. ## 1. Why This Even Matters — A Story First Locking the front door of a house but leaving every internal room door wide open and unlocked technically means "the house

JAVASCRIPTDECODED.

Easy

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 Object.is, ==, and ===?

What is `Error.isError()` and why is `instanceof Error` unreliable across realms?

What is JSON and why is it commonly used?

Medium

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

Explain 'this' keyword in JavaScript.

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

How does WeakMap allow garbage collection of keys?

How do you implement a deepFreeze function recursively?

How do you implement a custom Promise.all polyfill?

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

What are the basics of regular expressions in JavaScript?

Hard

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?

What is ShadowRealm and how does it provide true JavaScript sandboxing?

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

How does the Temporal API fix JavaScript Date — Instant, PlainDate and ZonedDateTime?

How do iterators and the iterable protocol work?

What are WeakMap and WeakSet?

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