main.tsx

import React from 'react'
import ReactDOM from 'react-dom'

import './index.css'

import {
  elementScroll,
  useVirtualizer,
  VirtualizerOptions,
} from '@tanstack/react-virtual'

function easeInOutQuint(t) {
  return t < 0.5 ? 16 * t * t * t * t * t : 1 + 16 * --t * t * t * t * t
}

function App() {
  const parentRef = React.useRef<HTMLDivElement>()
  const scrollingRef = React.useRef<number>()

  const scrollToFn: VirtualizerOptions<any, any>['scrollToFn'] =
    React.useCallback((offset, canSmooth, instance) => {
      const duration = 1000
      const start = parentRef.current.scrollTop
      const startTime = (scrollingRef.current = Date.now())

      const run = () => {
        if (scrollingRef.current !== startTime) return
        const now = Date.now()
        const elapsed = now - startTime
        const progress = easeInOutQuint(Math.min(elapsed / duration, 1))
        const interpolated = start + (offset - start) * progress

        if (elapsed < duration) {
          elementScroll(interpolated, canSmooth, instance)
          requestAnimationFrame(run)
        } else {
          elementScroll(interpolated, canSmooth, instance)
        }
      }

      requestAnimationFrame(run)
    }, [])

  const rowVirtualizer = useVirtualizer({
    count: 10000,
    getScrollElement: () => parentRef.current,
    estimateSize: () => 35,
    overscan: 5,
    scrollToFn,
  })

  const randomIndex = Math.floor(Math.random() * 10000)

  return (
    <div>
      <p>
        This smooth scroll example uses the <code>`scrollToFn`</code> to
        implement a custom scrolling function for the methods like{' '}
        <code>`scrollToIndex`</code> and <code>`scrollToOffset`</code>
      </p>

      <br />
      <br />

      <div>
        <button onClick={() => rowVirtualizer.scrollToIndex(randomIndex)}>
          Scroll To Random Index ({randomIndex})
        </button>
      </div>

      <br />
      <br />

      <div
        ref={parentRef}
        className="List"
        style={{
          height: `200px`,
          width: `400px`,
          overflow: 'auto',
        }}
      >
        <div
          style={{
            height: `${rowVirtualizer.getTotalSize()}px`,
            width: '100%',
            position: 'relative',
          }}
        >
          {rowVirtualizer.getVirtualItems().map((virtualRow) => (
            <div
              key={virtualRow.index}
              className={virtualRow.index % 2 ? 'ListItemOdd' : 'ListItemEven'}
              style={{
                position: 'absolute',
                top: 0,
                left: 0,
                width: '100%',
                height: `${virtualRow.size}px`,
                transform: `translateY(${virtualRow.start}px)`,
              }}
            >
              Row {virtualRow.index}
            </div>
          ))}
        </div>
      </div>
      <br />
      <br />
      {process.env.NODE_ENV === 'development' ? (
        <p>
          <strong>Notice:</strong> You are currently running React in
          development mode. Rendering performance will be slightly degraded
          until this application is build for production.
        </p>
      ) : null}
    </div>
  )
}

ReactDOM.render(
  <React.StrictMode>
    <App />
  </React.StrictMode>,
  document.getElementById('root'),
)