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 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 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 the difference between == and ===?

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 comparison below was actually run in Node — not recited from memory. ## 1. Why This Even Matters — A Story First Imagine two shipping labels: one says "5 kg" and the other says "5000 g." A loose inspector (==) converts both to the same unit and says they match. A

Question 5

How do you implement infinite currying in JavaScript?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript closures and functional-programming 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 Think of a vending machine that keeps a running tab open: each coin you insert (each f

Question 6

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 7

How do you correctly check for NaN?

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 coercion and comparison result below was actually run in Node. ## 1. Why This Even Matters — A Story First Asking a bouncer "is this person over 21?" and getting "yes" back sounds definitive — until you learn the bouncer first tried to convert the person's PET in

Question 8

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

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 and coercion claim below was actually run in Node (v21+ required; verified on v24.19.0). ## 1. Why This Even Matters — A Story First Sorting mail into pigeonholes labeled only with a HANDWRITTEN NAME (a coerced string) genuinely cannot tell two differe

Question 9

When would you reach for Map.groupBy() instead of Object.groupBy(), and what problem does it solve with non-string 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:. Every claim below was actually run with real, distinct object keys — not asserted from documentation. ## 1. Why This Even Matters — A Story First Sorting mail into pigeonholes labeled by the SENDER'S NAME works fine when every sender's name is unique text. But if you

Question 10

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

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript metaprogramming/coercion interview questions. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every hint value shown below is **real, captured output** from actually running the code in this doc on Node.js v24.19.0 — including one claim this doc's own draft got wrong on the first attempt, caught only by actually running it, exactly the fail

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