mastodon/app/javascript/mastodon/components/animated_number.tsx

import React, { useCallback, useState } from 'react';
import ShortNumber from './short_number';
import { TransitionMotion, spring } from 'react-motion';
import { reduceMotion } from '../initial_state';

const obfuscatedCount = (count: number) => {
  if (count < 0) {
    return 0;
  } else if (count <= 1) {
    return count;
  } else {
    return '1+';
  }
};

type Props = {
  value: number;
  obfuscate?: boolean;
}
export const AnimatedNumber: React.FC<Props> = ({
  value,
  obfuscate,
})=> {
  const [previousValue, setPreviousValue] = useState(value);
  const [direction, setDirection] = useState<1|-1>(1);

  if (previousValue !== value) {
    setPreviousValue(value);
    setDirection(value > previousValue ? 1 : -1);
  }

  const willEnter = useCallback(() => ({ y: -1 * direction }), [direction]);
  const willLeave = useCallback(() => ({ y: spring(1 * direction, { damping: 35, stiffness: 400 }) }), [direction]);

  if (reduceMotion) {
    return obfuscate ? <>{obfuscatedCount(value)}</> : <ShortNumber value={value} />;
  }

  const styles = [{
    key: `${value}`,
    data: value,
    style: { y: spring(0, { damping: 35, stiffness: 400 }) },
  }];

  return (
    <TransitionMotion styles={styles} willEnter={willEnter} willLeave={willLeave}>
      {items => (
        <span className='animated-number'>
          {items.map(({ key, data, style }) => (
            <span key={key} style={{ position: (direction * style.y) > 0 ? 'absolute' : 'static', transform: `translateY(${style.y * 100}%)` }}>{obfuscate ? obfuscatedCount(data) : <ShortNumber value={data} />}</span>
          ))}
        </span>
      )}
    </TransitionMotion>
  );
};