Question 1

How would you use Object.groupBy() to group an array of orders by status, and how does it compare to a manual reduce()?

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 claim below was actually run — both implementations were executed against the identical real orders array and their outputs directly compared. ## 1. Why This Even Matters — A Story First Sorting a pile of mail into labeled bins by recipient is a task every office

Question 2

What is the difference between var, let, and const?

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 scoping and TDZ claim below was actually run in Node — including the classic loop-closure bug. ## 1. Why This Even Matters — A Story First Imagine a shared office whiteboard (`var`) versus a sticky note stuck to one specific desk (`let`/`const`). Anyone in the wh

Question 3

What is the difference between preventDefault and stopPropagation?

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:. Both isolated tests below were actually run with jsdom. ## 1. Why This Even Matters — A Story First Imagine a letter mailed through a chain of post offices, each one an ancestor listener along the route. `stopPropagation()` is telling the current post office "do not fo

Question 4

What is event bubbling and capturing?

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 exact firing order below was actually reproduced with jsdom, not recited from documentation. ## 1. Why This Even Matters — A Story First Think of a click event as a security guard doing a walk-through: first the guard walks INWARD from the building's front gate all

Question 5

Implement a robust Event Emitter class.

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript design-pattern / low-level API implementation interview questions. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every behavior shown below is **real, captured output** from actually running the code in this doc on Node.js v24.19.0, including a direct comparison against Node's actual built-in `node:events` module — not illustrative sample

Question 6

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

Question 7

Explain 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 listener counts and dynamically-added-element behavior below were **actually run** with jsdom, not described from documentation. ## 1. Why This Even Matters — A Story First Imagine a school where, instead of a teacher walking to every single student's desk individu

Question 8

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 9

Why is navigator.sendBeacon() the right tool for sending analytics data when a user closes a tab, instead of a normal fetch call?

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 synchronous, near-instant return behavior below was actually timed, not assumed. ## 1. Why This Even Matters — A Story First Shouting a quick, complete sentence to someone as they're already closing the door behind them works — a complete thought, delivered insta

Question 10

What does setTimeout with 0 delay actually do?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript timer and event loop 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. For the broader microtask-vs-macrotask picture this fits into, see

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Easy

How would you use Object.groupBy() to group an array of orders by status, and how does it compare to a manual reduce()?

What is the difference between var, let, and const?

What is the difference between preventDefault and stopPropagation?

What is event bubbling and capturing?

Medium

Implement a robust Event Emitter class.

How do you implement a deepFreeze function recursively?

Explain event delegation.

What is event delegation?

Why is navigator.sendBeacon() the right tool for sending analytics data when a user closes a tab, instead of a normal fetch call?

What does setTimeout with 0 delay actually do?

What is the difference between Object.freeze and const?

Why must you pass the same function reference to removeEventListener?

Hard

Explain the event loop, call stack, and task queues.

What is the difference between microtasks and macrotasks?

What is debouncing and throttling?