Question 1

How would you use the Page Visibility API to pause a setInterval-based poller when a user switches to a different browser tab?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript browser-API 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 against a real jsdom document 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 delivery app polling a restaurant every fe

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 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 4

Why would you use performance.now() instead of Date.now() to measure how long a function takes to run?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript performance-measurement 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 Imagine timing a race two different ways. One timer is a wall clock th

Question 5

What is destructuring?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript ES6+ syntax interviews. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The real swap, the real nested/renamed/defaulted extraction, and the real thrown error on `null` below were **actually run** — not descriptions of documented behavior. ## 1. Why This Even Matters — A Story First Unpacking a delivered grocery bag by reaching in and pulli

Question 6

What is JSON and why is it commonly used?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals and data-interchange 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. ## 1. Why This Even Matters — A Story First Imagine shipping a package overseas. A JS backend, a Python se

Question 7

Debounce vs throttle — implement debounce

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript / frontend-implementation 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. For the conceptual distinction between debounce and throttle before diving into code, see

Question 8

How would you use the BroadcastChannel API to sync application state across multiple open browser tabs?

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 self-exclusion behavior — was verified live in a real, current browser via the Claude Browser pane. `BroadcastChannel` is confirmed genuinely NOT implemented in jsdom. ## 1. Why This Even Matters — A Story First A group text message

Question 9

How does the fetch API work?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript/browser-API interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every request below was actually made against a real local HTTP server spun up for this verification, not simulated. ## 1. Why This Even Matters — A Story First Calling a delivery company to ask "did my package arrive?" — the phone call itself succeeding (someone answ

Question 10

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

JAVASCRIPTDECODED.

Easy

How would you use the Page Visibility API to pause a setInterval-based poller when a user switches to a different browser tab?

What is the difference between null and undefined?

What is the difference between synchronous and asynchronous code?

Why would you use performance.now() instead of Date.now() to measure how long a function takes to run?

What is destructuring?

What is JSON and why is it commonly used?

Medium

Debounce vs throttle — implement debounce

How would you use the BroadcastChannel API to sync application state across multiple open browser tabs?

How does the fetch API work?

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

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

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

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

What is the Reflect API?

How would you implement an async generator function to stream paginated API results, and how does a consumer use it with for await...of?

How do you implement a throttle function?

What is ShadowRealm and how does it provide true JavaScript sandboxing?

How does ArrayBuffer transfer and detach work with `transfer()` vs `transferToFixedLength()`?

What are tagged template literals?

How does the Temporal API fix JavaScript Date — Instant, PlainDate and ZonedDateTime?