Question 1

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 2

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 3

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

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Medium

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

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

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