catstodon/app/javascript/mastodon/utils/numbers.js

import React, { Fragment } from 'react';
import { FormattedNumber } from 'react-intl';

export const shortNumberFormat = number => {
  if (number < 1000) {
    return <FormattedNumber value={number} />;
  } else {
    return <Fragment><FormattedNumber value={number / 1000} maximumFractionDigits={1} />K</Fragment>;
  }
};