Question 1

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 2

What is the difference between synchronous and asynchronous code?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals/event-loop interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The exact ordering below was actually run in Node — not recited from a diagram. ## 1. Why This Even Matters — A Story First Ordering food at a counter is synchronous: you stand there, place your order, and nothing else happens until the cashier finishes ri

Question 3

What is the difference between map and forEach?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript array-method and async-pattern interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The real return-value proof and the real async-forEach bug demonstration below were **actually run** — not descriptions of documented behavior. ## 1. Why This Even Matters — A Story First Handing a stack of letters to a mail carrier who genuinely wa

Question 4

How do you handle asynchronous operations in JavaScript?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals/async interviews. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. All three approaches below were actually run against the identical real delayed operation. ## 1. Why This Even Matters — A Story First Ordering food can be described three different ways in English — "call me when it's ready," "I'll check back with you and you'll

Question 5

How would you use ResizeObserver to react to an element's size changing, without polling getBoundingClientRect() on every frame?

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 verified live in a real, current browser via the Claude Browser pane — `ResizeObserver` is confirmed genuinely NOT implemented in jsdom, so this could not be verified with jsdom alone. ## 1. Why This Even Matters — A Story First Checking whether

Question 6

What is the difference between arrow and regular functions?

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. ## 1. The Core Distinction Arrow functions are not just shorter syntax for regular functions -- they deliberately omit severa

Question 7

What is the purpose of the `bind`, `call`, and `apply` methods?

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 detached-method failure and its real fix below were actually run in Node. ## 1. Why This Even Matters — A Story First An employee's ID badge only works at the specific building it was issued for — carry it to a different building's door reader, and it fails, even

Question 8

How would you use IntersectionObserver to lazy-load images only as they scroll into the viewport?

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 exact real scroll-triggered transition — was verified live in a real, current browser via the Claude Browser pane, genuinely scrolling a real page and observing the callback fire. `IntersectionObserver` is confirmed genuinely NOT imple

Question 9

What does queueMicrotask() do, and how does its callback ordering compare to a resolved Promise .then() and setTimeout(fn, 0)?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript event loop and async-ordering 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 a barista mid-pour on a drink. Right next to the registe

Question 10

How would you use MutationObserver to detect when a third-party script injects new nodes into the DOM?

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 against a real jsdom document — jsdom, unlike ResizeObserver/IntersectionObserver (covered in this bank's own dedicated questions), genuinely does implement MutationObserver, confirmed directly. ## 1. Why This Even Matters — A Story

JAVASCRIPTDECODED.

Easy

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

What is the difference between synchronous and asynchronous code?

What is the difference between map and forEach?

Medium

How do you handle asynchronous operations in JavaScript?

How would you use ResizeObserver to react to an element's size changing, without polling getBoundingClientRect() on every frame?

What is the difference between arrow and regular functions?

What is the purpose of the `bind`, `call`, and `apply` methods?

How would you use IntersectionObserver to lazy-load images only as they scroll into the viewport?

What does queueMicrotask() do, and how does its callback ordering compare to a resolved Promise .then() and setTimeout(fn, 0)?

How would you use MutationObserver to detect when a third-party script injects new nodes into the DOM?

Hard

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

How does `Atomics.waitAsync()` enable non-blocking waits on SharedArrayBuffer?

In what order do console.log, setTimeout, and Promise.then run?

What is the event loop?

What is the difference between microtasks and macrotasks?

How do you implement a simple Promise library from scratch?