roman_numerals1.js

const RomanDictionary = {
  '1': 'I',
  '5': 'V',
  '10': 'X',
  '50': 'L',
  '100': 'C',
  '500': 'D',
  '1000': 'M',
};

function toRoman(num) {
  var digits;
  var last_idx
  
  digits = num.toString().split('');
  last_idx = digits.length - 1;

  function toRomanNumeral(digit, idx) {
    var place_val = 10 **(last_idx - idx);
    var toRomanSym = num => RomanDictionary[num.toString()];

    digit = Number(digit);

    if (digit === 0) return '';
    if (digit < 4) return toRomanSym(place_val).repeat(digit);
    if (digit === 4 || digit === 9) {
      return toRomanSym(place_val) + toRomanSym((digit + 1) *  place_val);
    }

    // Default: for digits 6 to 8
    return toRomanSym(5 * place_val) + toRomanSym(place_val).repeat(digit - 5);
  }
  
  return digits.map(toRomanNumeral).join('');
}