roman.js

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

const ARABIC_MAP = new Map([
  [ 1000, 'M' ],
  [ 900, 'CM' ],
  [ 500, 'D' ],
  [ 400, 'CD' ],
  [ 100, 'C' ],
  [ 90, 'XC' ],
  [ 50, 'L' ],
  [ 40, 'XL' ],
  [ 10, 'X' ],
  [ 9, 'IX' ],
  [ 5, 'V' ],
  [ 4, 'IV' ],
  [ 1, 'I' ]
]);

function toArabic (numberString) {
  numberString = numberString.split('').map(digit => digit.toUpperCase());
  return numberString.reduce((tally, digit, i) => {
    const nextDigit = ROMAN_MAP[numberString[i + 1]];
    digit = ROMAN_MAP[digit];
    if (nextDigit > digit) {
      tally = tally - digit;
    } else {
      tally = tally + digit;
    }
    return tally;
  }, 0);
}

function toRoman (integer) {
  let romanString = '';
  let remainder = integer;
  ARABIC_MAP
    .forEach((romanLetter, decimalValue) => {
      const count = parseInt(remainder / decimalValue, 10);
      if (count > 0) {
        romanString = romanString.split('').concat(Array(count).fill(romanLetter)).join('');
        remainder = remainder - (decimalValue * count);
      }
    }, integer);
  return romanString;
}

console.log(toArabic('MCMV'));
console.log(toRoman(1905));
console.log(toRoman(8));
console.log(toRoman(328));