function closestNumbers(arr) {
  const sortedArray = [...arr].sort((a, b) => a - b);
  const startingPoint = sortedArray.splice(0, 2);

  return sortedArray.reduce((acc, element, index) => {
    const previosElement = index === 0 ? acc[1] : sortedArray[index - 1];
    const currentDifference = Math.abs(element - previosElement);
    const existingDifference = Math.abs(acc[0] - acc[1]);

    if (currentDifference < existingDifference) return [previosElement, element];
    if (currentDifference === existingDifference) return acc.concat(previosElement, element);

    return acc;
  }, startingPoint);
}