[Mr_Po]

Всё решил, долго думал конечно, но итог таков, Спасибо за подсказки))))

function recommendRoom(adultsCount = 0, childrenCount = 0, babiesCount = 0) {
  const totalCount = adultsCount + childrenCount + babiesCount;

  if (totalCount === 5 && babiesCount > 0) {
    return 'small room + extra bed';
  }

  if (totalCount === 9 && babiesCount > 0) {
    return 'big room + extra bed';
  }

  if (adultsCount + childrenCount <= 4) {
    return 'small room';
  }

  if (adultsCount + childrenCount <= 8) {
    return 'big room';
  }
}