Question 1

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 2

What are modules in JavaScript?

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:. The module-caching claim below was actually run and confirmed inside a real CommonJS script. ## 1. Why This Even Matters — A Story First A shared office supply closet, restocked once at the start of the day, is used identically by every single department that visits it

Question 3

What is an IIFE?

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:. The scoping and immediate-execution claims below were actually run in Node. ## 1. Why This Even Matters — A Story First Think of a magician who steps behind a curtain, does some private setup work, and immediately steps back out — the audience never sees, and can never

Question 4

What are closures 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 closures specifically (a function retaining access to its outer scope); for the more general rule that governs

Question 5

How do getters and setters work?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript object-property interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The real getter/setter execution, the real setter validation throw, and the real strict-vs-sloppy getter-only comparison below were **actually run** — not descriptions of documented behavior. ## 1. Why This Even Matters — A Story First A thermostat display that sh

Question 6

What are private class fields?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals/OOP interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every accessibility and enumeration claim below was actually run in Node. ## 1. Why This Even Matters — A Story First A door with a "Please Do Not Enter" sign (an underscore-prefixed field) relies entirely on people choosing to respect the sign — nothing physical

Question 7

What are Symbols and what are they used for?

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 uniqueness and iteration claim below was actually run in Node. ## 1. Why This Even Matters — A Story First Imagine a library where every book's title could genuinely collide — two different books both titled "Notes" would be indistinguishable on the same shelf.

Question 8

Explain how prototypal inheritance differs from classical inheritance.

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript object-model interview questions that expect a comparison to classical OOP. **Difficulty:** Hard > **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 leads with the CONCEPTUAL DIFFERENCE from classical inheritance; for how the J

Question 9

How do Stage 3 JavaScript Decorators work — class, method and field decorators?

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 about what DOES and does NOT run natively was actually tested — in both Node v24.19.0 and a real, current Chrome browser — not assumed. **Honesty note:** unlike every other example in this bank, the code example below is genuinely NOT runnable in this playg

Question 10

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,

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