Question 1

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 2

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 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 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 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 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 7

How do you implement a custom Promise.all polyfill?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript async/Promise interview questions. **Difficulty:** Medium > **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 Imagine ordering three packages online on the same day and ask

Question 8

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 9

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

Question 10

What does `using` / `await using` do for Explicit Resource Management (ES2024)?

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 disposal order and error-path claim below was actually run natively in Node — this feature genuinely runs without a transpiler. ## 1. Why This Even Matters — A Story First Checking several items out of a library one at a time and being trusted to return each on

JAVASCRIPTDECODED.

Easy

What is the difference between synchronous and asynchronous code?

What is the difference between map and forEach?

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

Medium

How does the fetch API work?

Implement a robust Event Emitter class.

How do you handle asynchronous operations in JavaScript?

How do you implement a custom Promise.all polyfill?

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

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

What does `using` / `await using` do for Explicit Resource Management (ES2024)?

How do you make an AJAX request?

Hard

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

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

Implement an async task queue with a concurrency limit.

What are async iterators and for await...of?

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 custom Promise.any polyfill?

What is the event loop?

How do you combine Stage 3 decorators with `using` for auto-disposable resources?