Question 1

Why would you add { passive: true } to a scroll or touchstart listener, and what problem does it solve for scroll performance?

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:. The core "does preventDefault() do anything in a passive listener" claim below was actually run and checked, not assumed. ## 1. Why This Even Matters — A Story First A crossing guard who must wait for every single car in a long line to individually confirm "I am not tu

Question 2

What causes layout thrashing when you read and write DOM properties in a loop, and how would you batch those operations to avoid it?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals interviews. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The ~85x slowdown figure below is a real, direct measurement from this project's own verification session — timed with `performance.now()` against 200 real DOM elements in a real, current browser — not an estimate or a commonly-cited approximation taken on faith. ## 1.

Question 3

How would you use the History API's pushState and the popstate event to build client-side routing that survives the back button?

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 restored state after a real back-button navigation — was verified live in a real browser, calling the same `history.back()` method the browser's own UI button triggers. ## 1. Why This Even Matters — A Story First A choose-your-

Question 4

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 5

What is a pure function?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals interview questions. **Difficulty:** Easy-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. The Core Idea 📌 **Interview term:** a **pure function** satisfies two conditions at once: (1) given the same inpu

Question 6

What is the difference between client-side and server-side rendering?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript/web-architecture interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The real HTML-content difference below was actually demonstrated with jsdom, not just described. ## 1. Why This Even Matters — A Story First A restaurant handing you a fully-plated meal the moment you sit down is server-side rendering — the "content" (food) is al

Question 7

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

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