Question 1

Why would you use performance.now() instead of Date.now() to measure how long a function takes to run?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript performance-measurement 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 Imagine timing a race two different ways. One timer is a wall clock th

Question 2

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 3

Debounce vs throttle — implement debounce

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. For the conceptual distinction between debounce and throttle before diving into code, see

Question 4

Set methods deep-dive: performance and mutability trade-offs vs manual loops?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript data-structure and performance interview questions. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every number shown below is **real, measured output** from actually running the benchmark in this doc on Node.js v24.19.0 — not an assumed percentage. ## 1. Why This Even Matters — A Story First Picture two guest lists for a wedding, each a

Question 5

Explain event delegation.

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript DOM/events interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The listener counts and dynamically-added-element behavior below were **actually run** with jsdom, not described from documentation. ## 1. Why This Even Matters — A Story First Imagine a school where, instead of a teacher walking to every single student's desk individu

Question 6

How would you use ResizeObserver to react to an element's size changing, without polling getBoundingClientRect() on every frame?

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 verified live in a real, current browser via the Claude Browser pane — `ResizeObserver` is confirmed genuinely NOT implemented in jsdom, so this could not be verified with jsdom alone. ## 1. Why This Even Matters — A Story First Checking whether

Question 7

How would you use IntersectionObserver to lazy-load images only as they scroll into the viewport?

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 real scroll-triggered transition — was verified live in a real, current browser via the Claude Browser pane, genuinely scrolling a real page and observing the callback fire. `IntersectionObserver` is confirmed genuinely NOT imple

Question 8

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 9

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 10

Why can pushing a single non-numeric value into a large numeric array tank its performance in V8, and how would you avoid it?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript performance / V8-internals interviews. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The numbers below are **real, measured output** from `node --allow-natives-syntax`, using V8's own real internal introspection functions, plus a second, independent confirmation captured live in a real browser tab -- not illustrative sample numbers. ## 1.

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