Wire Up Debounced Resize and Throttled Scroll Handlers on a Real Component

Q: Wire Up Debounced Resize and Throttled Scroll Handlers on a Real Component

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Medium > **How to read this doc:** the specific real, measured invocation counts below (0 immediately after 10 rapid resize dispatches, then exactly 1; and exactly 2 for 10 rapid scroll dispatches) were confirmed against a real jsdom window, not assumed from the debounce/throttle implementations alone. ## 1. The problem, restated Attach a debounced `resize` handler and a throttled `scroll` handler to `window`, ensuring: (1) rapid resize events collapse into one real recalculation, (2) scroll updates happen at a steady, bounded rate rather than on every pixel, and (3) both are genuinely, correctly removed on cleanup. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | Create the wrapper once, or fresh on every attach? | ONCE — this bank's own debounce/throttle questions cover why a fresh wrapper on every render would break the whole mec

Browser window scroll resize handlers | JS Coding Challenge

The problem

Problem, as an interviewer would state it: "Attach a debounced resize listener and a throttled scroll listener to window, correctly clean them up, and explain why a naive attach-in-render approach breaks."

Examples:

Code

useEffect(() => {
  const onResize = debounce(() => recalcLayout(), 150);
  const onScroll = throttle(() => updateScrollProgress(), 100);
  window.addEventListener("resize", onResize);
  window.addEventListener("scroll", onScroll);
  return () => {
    window.removeEventListener("resize", onResize);
    window.removeEventListener("scroll", onScroll);
    onResize.cancel?.();
  };
}, []);

Clarifying questions expected:

Should the debounced/throttled wrapper be created ONCE and reused, or freshly created on every attach?
Does removeEventListener need the EXACT same function reference used in addEventListener?
What real cleanup, beyond removeEventListener, is needed on unmount?

Code / implementation expected: Yes — real, direct proof via a real jsdom window that rapid dispatched events produce exactly the expected number of real handler invocations, and that removal genuinely stops further ones.

js-coding
utility
algorithm

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Engineers preparing for practical JavaScript coding interviews. Difficulty: Medium

How to read this doc: the specific real, measured invocation counts below (0 immediately after 10 rapid resize dispatches, then exactly 1; and exactly 2 for 10 rapid scroll dispatches) were confirmed against a real jsdom window, not assumed from the debounce/throttle implementations alone.

1. The problem, restated

Attach a debounced resize handler and a throttled scroll handler to window, ensuring: (1) rapid resize events collapse into one real recalculation, (2) scroll updates happen at a steady, bounded rate rather than on every pixel, and (3) both are genuinely, correctly removed on cleanup.

2. Clarifying questions a strong candidate asks

Question	Why it matters
Create the wrapper once, or fresh on every attach?	ONCE — this bank's

The solution is hidden so you can try first

5 min read · 9 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

JavaScriptReal, direct proof via jsdom: debounced resize and throttled scroll handlers fire the expected number of times, and cleanup genuinely stops them