arabicToRoman.js

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

const DescendingBase10Values = Object.keys(RomanNumerals).sort((a, b) => b - a);

const LongToShort = {
    'DCCCC': 'CM',  // 900
    'CCCC': 'CD',  // 400
    'LXXXX': 'XC',  // 90
    'XXXX': 'XL',  // 40
    'VIIII': 'IX',  // 9
    'IIII': 'IV' // 4
};

const ShortToLong = Object.keys(LongToShort).reduce((accumulator, currentKey) => {
  return Object.assign(accumulator, {
    [LongToShort[currentKey]]: currentKey
  });
  
}, {});

console.log(ShortToLong);

const convertToAdditiveForm = (value) => {
  let result = '';
  
  DescendingBase10Values.reduce((toBeConverted, currentBase10Value) => {
    const [numberOfLetters, toBeConvertedNext] = [toBeConverted / currentBase10Value, toBeConverted % currentBase10Value];
    result += RomanNumerals[currentBase10Value].repeat(numberOfLetters);
    
    return toBeConvertedNext;
  },
  value);
  
  return result;
};

console.log(convertToAdditiveForm(14));

const convert = (map) => (roman) => {
  return Object.keys(map).reduce((result, currentAlternateForm) => {
    return result.replace(currentAlternateForm, map[currentAlternateForm]);
  }, 
  roman);
};

const additiveToSubtractive = convert(LongToShort);
const subtractiveToAdditive = convert(ShortToLong);

console.log(subtractiveToAdditive(additiveToSubtractive(convertToAdditiveForm(14))));