Question 1

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 2

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 3

What is event delegation?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript DOM/events interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The bubbling proof and listener-count comparison below were actually run with jsdom. ## 1. Why This Even Matters — A Story First A single security guard stationed at a building's only entrance can identify and log every visitor who walks in, no matter which office they

Question 4

Why would you choose a WeakSet over a Set to track a group of DOM elements, and what happens to its entries once those elements are removed?

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 DOM-tracking claims below were verified directly against a real jsdom document with real button elements — not simulated with plain objects standing in for DOM nodes. ## 1. Why This Even Matters — A Story First A guest list written on a sticky note taped to a doo

Question 5

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

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript iteration-protocol and generator interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every behavior below was **actually run** — a genuine `TypeError` for a non-iterable class, real `for...of`/spread/destructuring/`Array.from` output over a real generator-based iterable, and a real, observed `finally`-block cleanup on early exit — n

Question 6

What are WeakMap and WeakSet?

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 constraint below, including the exact TypeErrors, was actually run in Node. ## 1. Why This Even Matters — A Story First Think of a regular `Map` as a filing cabinet that keeps a lock on every folder it holds — even if the original owner throws their copy away,

Question 7

What problem do WeakRef and FinalizationRegistry solve, and why should they be used carefully?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript memory-management interview questions. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every result below is **real, captured output** from actually forcing garbage collection with Node's --expose-gc flag on this machine — not illustrative sample output, and the non-determinism described below was observed directly across repeated real runs

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