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hardMachine Coding

Build a Virtualized List

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01

Understand the problem

Build a list that renders only the visible rows for huge datasets.

ReactPerformanceVirtualization
02

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Building a Virtualized List in React

Mental model: Rendering 10,000 DOM rows is slow; instead render only the handful visible in the viewport. A tall spacer (count * rowHeight) preserves the scrollbar; on scroll you compute the first visible index from scrollTop and render that window, offset into place wit

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04

React solution

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import React from 'react';
import VirtualList from './src/VirtualList';

const items = Array.from({ length: 10000 }, (_, i) => ({ id: i, label: `Row #${i + 1}` }));

export default function App() {
  return (
    <div style={{ fontFamily: 'sans-serif', padding: 20 }}>
      <h1>Virtualized List Demo</h1>
      <p style={{ color: '#666' }}>10,000 rows — only the visible ones are in the DOM.</p>
      <VirtualList items={items} rowHeight={36} height={300} />
    </div>
  );
}
05

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