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

How does `try...catch` work in JavaScript?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript error-handling 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 Think of a factory floor with a trip wire around one risky machine. If nothing go

Question 4

How would you combine multiple AbortSignals or add a timeout to a fetch request using AbortSignal.any() and AbortSignal.timeout()?

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 which signal genuinely wins each race — was actually run and timed against a real network endpoint. ## 1. Why This Even Matters — A Story First A race with two separate finish-line judges — one who blows a whistle if a runner crosses the

Question 5

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

Question 6

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 7

How would you implement a debounce utility that also exposes a .cancel() method to cancel a pending call?

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. This doc assumes the base debounce mechanism from

Question 8

What is `scheduler.yield()` and how does it prevent main-thread blocking?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript/browser-performance interviews. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every ordering claim below was actually run live in a real, current browser — `scheduler.yield()` is confirmed genuinely UNAVAILABLE in Node.js. ## 1. Why This Even Matters — A Story First A cashier mid-transaction who briefly steps aside to let someone with an

Question 9

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

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript event loop and async-mechanics 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 focuses on the MECHANICS (the call stack and how the loop drives the queues); for an exhaustive list

Question 10

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

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript concurrency and SharedArrayBuffer interview questions. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every result shown below is **real, captured output** from actually running the code in this doc on Node.js v24.19.0 — not illustrative sample output. ## 1. Why This Even Matters — A Story First Waiting for a delivery by standing at the do

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Easy

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

What is the difference between synchronous and asynchronous code?

How does `try...catch` work in JavaScript?

Medium

How would you combine multiple AbortSignals or add a timeout to a fetch request using AbortSignal.any() and AbortSignal.timeout()?

What does setTimeout with 0 delay actually do?

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

How would you implement a debounce utility that also exposes a .cancel() method to cancel a pending call?

What is `scheduler.yield()` and how does it prevent main-thread blocking?

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?

What is debouncing and throttling?