alpacato

const declOfNum = (number, titles) => {  
  let cases = [2, 0, 1, 1, 1, 2];  
  return titles[(number % 100 > 4 && number % 100 < 20) ? 2 : cases[(number % 10 < 5) ? number % 10 : 5]];  
}

// use:
const number = 100;
declOfNum(number, ['найдена', 'найдено', 'найдены']);