ReactJS Program to React.memo With Custom Compare with Explanation
ReactJS
Hard
Performance & Patterns
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1 min read
89 words
This problem helps you practice core ReactJS fundamentals in a practical way. It builds intuition around react, memo, custom. Let’s break it down step by step so you can implement it confidently.
Problem Statement
Memoize rows so only the selected row re-renders when selection changes.
Input Format
No input.
Output Format
Render a React component.
Constraints
Use React.memo with a custom compare function.
Code Solution
This explanation is written for learning purposes and to help beginners understand the concept clearly.
import React, { memo, useCallback, useMemo, useState } from 'react';
const items = Array.from({ length: 600 }, (_, i) => ({ id: i + 1, name: 'meetcode item ' + (i + 1) }));
const Row = memo(
function Row({ item, selected, onSelect }) {
return (
<button
type='button'
onClick={() => onSelect(item.id)}
style={{
width: '100%',
textAlign: 'left',
padding: '8px 12px',
borderRadius: 12,
border: '1px solid ' + (selected ? '#0b5' : '#eee'),
background: selected ? '#eafaf1' : '#fff',
cursor: 'pointer'
}}
>
{item.name}
</button>
);
},
(prev, next) => prev.item.id === next.item.id && prev.item.name === next.item.name && prev.selected === next.selected
);
export default function App() {
const [selectedId, setSelectedId] = useState(1);
const onSelect = useCallback((id) => setSelectedId(id), []);
const shown = useMemo(() => items.slice(0, 120), []);
return (
<div style={{ padding: 16, width: 520 }}>
<h2 style={{ marginTop: 0 }}>Memo rows</h2>
<div style={{ color: '#555' }}>Selected: {selectedId}</div>
<div style={{ marginTop: 10, display: 'grid', gap: 8, height: 300, overflow: 'auto' }}>
{shown.map((item) => (
<Row key={item.id} item={item} selected={item.id === selectedId} onSelect={onSelect} />
))}
</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
Memoize rows so only the selected row re-renders when selection changes.
Input / Output
Output
Render a React component.
Constraints
Use React.memo with a custom compare function.
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, { memo, useCallback, useMemo, useState } from 'react';
const items = Array.from({ length: 600 }, (_, i) => ({ id: i + 1, name: 'meetcode item ' + (i + 1) }));
const Row = memo(
function Row({ item, selected, onSelect }) {
return (
<button
type='button'
onClick={() => onSelect(item.id)}
style={{
width: '100%',
textAlign: 'left',
padding: '8px 12px',
borderRadius: 12,
border: '1px solid ' + (selected ? '#0b5' : '#eee'),
background: selected ? '#eafaf1' : '#fff',
cursor: 'pointer'
}}
>
{item.name}
</button>
);
},
(prev, next) => prev.item.id === next.item.id && prev.item.name === next.item.name && prev.selected === next.selected
);
export default function App() {
const [selectedId, setSelectedId] = useState(1);
const onSelect = useCallback((id) => setSelectedId(id), []);
const shown = useMemo(() => items.slice(0, 120), []);
return (
<div style={{ padding: 16, width: 520 }}>
<h2 style={{ marginTop: 0 }}>Memo rows</h2>
<div style={{ color: '#555' }}>Selected: {selectedId}</div>
<div style={{ marginTop: 10, display: 'grid', gap: 8, height: 300, overflow: 'auto' }}>
{shown.map((item) => (
<Row key={item.id} item={item} selected={item.id === selectedId} onSelect={onSelect} />
))}
</div>
</div>
);
}
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