function toRoman(num) {
  return 'I'
    .repeat(num)
    .replace(/IIIII/g, 'V')
    .replace(/VV/g, 'X')
    .replace(/XXXXX/g, 'L')
    .replace(/LL/g, 'C')
    .replace(/CCCCC/g, 'D')
    .replace(/DD/g, 'M')
    .replace(/VIIII/g, 'IX')
    .replace(/LXXXX/g, 'XC')
    .replace(/XXXX/g, 'XL')
    .replace(/DCCCC/g, 'CM')
    .replace(/CCCC/g, 'CD')
    .replace(/IIII/g, 'IV');
}

console.log(toRoman(1666));