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 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 3

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 4

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 5

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 6

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 7

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

Accepted Answer

**Target Audience:** Engineers preparing for the single most common JavaScript async-ordering interview question. **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 mechanics behind WHY this ordering happens (the call stack and the event loop), see

Question 8

What is the event loop?

Accepted Answer

**Target Audience:** Engineers who need a clear, concise definition of the event loop for a definitional interview question. **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. This is deliberately the OVERVIEW angle -- for the call-stack mechanics in depth, see

Question 9

What is the difference between microtasks and macrotasks?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript event loop and async-queue 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 MEMBERSHIP -- an exhaustive list of what actually lives in each queue; for the underlying cal

Question 10

How do you implement a simple Promise library from scratch?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript async/Promise interview questions. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every test result shown below is **real, captured output** from actually running this doc's MyPromise implementation on Node.js v24.19.0 through 10 real test groups, including a direct interleaved comparison against native Promise — not illustrative sample outp

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