ReactJS Program to Throttled Scroll Progress with Explanation
ReactJS
Hard
Performance & Patterns
19 views
1 min read
85 words
This problem helps you practice core ReactJS fundamentals in a practical way. It builds intuition around scroll, progress, update. Let’s break it down step by step so you can implement it confidently.
Problem Statement
Update scroll progress smoothly without setting state on every scroll event.
Input Format
No input.
Output Format
Render a React component.
Constraints
Use requestAnimationFrame to throttle state updates.
Code Solution
This explanation is written for learning purposes and to help beginners understand the concept clearly.
import React, { useRef, useState } from 'react';
export default function App() {
const [pct, setPct] = useState(0);
const raf = useRef(0);
function onScroll(e) {
const el = e.currentTarget;
const max = el.scrollHeight - el.clientHeight;
const nextPct = max <= 0 ? 0 : Math.round((el.scrollTop / max) * 100);
if (raf.current) cancelAnimationFrame(raf.current);
raf.current = requestAnimationFrame(() => setPct(nextPct));
}
const blocks = Array.from({ length: 40 }, (_, i) => i + 1);
return (
<div style={{ padding: 16, width: 520 }}>
<h2 style={{ marginTop: 0 }}>meetcode reading</h2>
<div style={{ height: 10, borderRadius: 999, background: '#eee', overflow: 'hidden' }}>
<div style={{ height: '100%', width: pct + '%', background: '#0b5' }} />
</div>
<div style={{ marginTop: 8, color: '#555' }}>Progress: {pct}%</div>
<div onScroll={onScroll} style={{ marginTop: 10, height: 240, overflow: 'auto', border: '1px solid #eee', borderRadius: 14, padding: 12 }}>
{blocks.map((n) => (
<div key={n} style={{ padding: '10px 0', borderBottom: '1px solid #f3f4f6' }}>
Block {n} on meetcode
</div>
))}
</div>
</div>
);
}
Output Example
No sample I/O is provided for this question.
Common Mistakes
- Misreading input/output format.
- Not handling constraints and edge cases.
- Off-by-one errors in loops.
- Forgetting to reset variables between test cases (if any).
Solution Guide
Problem
Update scroll progress smoothly without setting state on every scroll event.
Input / Output
Output
Render a React component.
Constraints
Use requestAnimationFrame to throttle state updates.
Details
Common Mistakes
- Misreading input/output format.
- Not handling constraints and edge cases.
- Off-by-one errors in loops.
- Forgetting to reset variables between test cases (if any).
Official Solution
import React, { useRef, useState } from 'react';
export default function App() {
const [pct, setPct] = useState(0);
const raf = useRef(0);
function onScroll(e) {
const el = e.currentTarget;
const max = el.scrollHeight - el.clientHeight;
const nextPct = max <= 0 ? 0 : Math.round((el.scrollTop / max) * 100);
if (raf.current) cancelAnimationFrame(raf.current);
raf.current = requestAnimationFrame(() => setPct(nextPct));
}
const blocks = Array.from({ length: 40 }, (_, i) => i + 1);
return (
<div style={{ padding: 16, width: 520 }}>
<h2 style={{ marginTop: 0 }}>meetcode reading</h2>
<div style={{ height: 10, borderRadius: 999, background: '#eee', overflow: 'hidden' }}>
<div style={{ height: '100%', width: pct + '%', background: '#0b5' }} />
</div>
<div style={{ marginTop: 8, color: '#555' }}>Progress: {pct}%</div>
<div onScroll={onScroll} style={{ marginTop: 10, height: 240, overflow: 'auto', border: '1px solid #eee', borderRadius: 14, padding: 12 }}>
{blocks.map((n) => (
<div key={n} style={{ padding: '10px 0', borderBottom: '1px solid #f3f4f6' }}>
Block {n} on meetcode
</div>
))}
</div>
</div>
);
}
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