Question 1

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 2

What is a Promise and what are its states?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals/async interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every state-transition claim below was actually run in Node. ## 1. Why This Even Matters — A Story First A verdict read aloud in a courtroom is genuinely final the moment it's spoken — the judge cannot walk back in five minutes later and announce a completely d

Question 3

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 4

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 5

How do you make an AJAX request?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript/browser-API interviews. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The environment-availability claims below were actually run in Node. ## 1. Why This Even Matters — A Story First Before smartphone apps had built-in instant messaging, checking for a new message meant reloading the entire webpage — a full-page refresh just to see if any

Question 6

What is `Array.fromAsync()` and how does it handle async iterables differently from `Array.from()`?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals/async interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every collection result below was actually run in Node (Chrome 121+/Firefox 115+/Safari 16.4+/Node 22+). ## 1. Why This Even Matters — A Story First Handing a clerk a stack of sealed envelopes: the plain, quick clerk (`Array.from`) genuinely just stacks the sea

Question 7

What does `Promise.try()` do and why is it better than `Promise.resolve().then(fn)`?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript/async interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every throw and catch path below was actually run in Node. ## 1. Why This Even Matters — A Story First Calling a function directly and hoping any error gets funneled into a `.catch()` somewhere downstream is like shouting a warning INTO a mailbox and hoping the mail carrier

Question 8

What does Promise.withResolvers() solve and how does it replace the deferred anti-pattern?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript/async interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every resolve/reject path below was actually run in Node (ES2024, Node 20.12+). ## 1. Why This Even Matters — A Story First A `new Promise` executor is like a sealed room where only whoever is standing inside at that exact moment can press the "done" button. Sometimes the a

Question 9

How does async/await work?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals/async interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every execution-order and timing claim below was actually run and measured in Node. ## 1. Why This Even Matters — A Story First `await` is like standing at a ticket counter and genuinely waiting your turn before walking up to the NEXT counter — you cannot be at

Question 10

How do you abort a Fetch API request using AbortController?

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 shape of the rejection value in each case — was actually run against a real, live network endpoint, not simulated. ## 1. Why This Even Matters — A Story First Placing a takeout order over the phone and then calling back moments

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Medium

Implement a robust Event Emitter class.

What is a Promise and what are its states?

How do you handle asynchronous operations in JavaScript?

How do you implement a custom Promise.all polyfill?

How do you make an AJAX request?

What is `Array.fromAsync()` and how does it handle async iterables differently from `Array.from()`?

What does `Promise.try()` do and why is it better than `Promise.resolve().then(fn)`?

What does Promise.withResolvers() solve and how does it replace the deferred anti-pattern?

Hard

What is the difference between Promise.all, allSettled, race, and any?

Implement an async task queue with a concurrency limit.

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

How do you implement a custom Promise.any polyfill?

How would you implement Symbol.asyncIterator on a class to transparently page through a paginated API with for await...of?

How do you implement a simple Promise library from scratch?