Several ways to do Rome numbers converter

roman-number.js

const romanNumerals = [
  [1000, "M"], [500, "D"], [100, "C"],
  [50, "L"], [10, "X"], [5, "V"], [1, "I"]
];

const toRomeRecursion = (function convert(index, number) {
  if (number === 0) return '';

  let [romanNumber, romanLiteral] = romanNumerals[index];
  if (number < romanNumber) return convert(index+1, number);

  let [minRomanNumber, minRomanLiteral] = romanNumerals[index+index%2];
  let [prevRomanNumber, prevRomanLiteral] = romanNumerals[index-1] || [romanNumber*10, ''];
  
  return (number + minRomanNumber) >= prevRomanNumber ?
    minRomanLiteral + prevRomanLiteral + convert(index+1+index%2, number%minRomanNumber) :
    romanLiteral.repeat(number / romanNumber) + convert(index+1, number%romanNumber)
}).bind(null, 0);

function toRomeImperative(number) {
  let index = 0, value = '';
  while(number > 0) {
    let [romanNumber, romanLiteral] = romanNumerals[index];
    if (number >= romanNumber) {
      let [minRomanNumber, minRomanLiteral] = romanNumerals[index+index%2];
      let [prevRomanNumber, prevRomanLiteral] = romanNumerals[index-1] || [romanNumber*10, ''];
      if ((number + minRomanNumber) >= prevRomanNumber) {
        value += minRomanLiteral + prevRomanLiteral;
        number %= minRomanNumber;
        index += index%2;
      } else {
        value += romanLiteral.repeat(number / romanNumber);
        number %= romanNumber;
      }
    } 
    index++;
  }
  return value;
}

function toRomeReducer(number) {
  return romanNumerals.reduce(([result, number], [romanNumber, romanLiteral], index, numbers) => {
    if (number >= romanNumber) {
      let [minRomanNumber, minRomanLiteral] = numbers[index+index%2];
      let [prevRomanNumber, prevRomanLiteral] = numbers[index-1] || [romanNumber*10, ''];
      return (number + minRomanNumber) >= prevRomanNumber ?
          [result + minRomanLiteral + prevRomanLiteral, number%minRomanNumber] :
          [result + romanLiteral.repeat(number / romanNumber), number%romanNumber];
    } 
    return [result, number];
  }, ['', number])[0];
}

for (let i = 1; i < 4000; i++) {
  console.log(i, toRomeImperative(i), toRomeRecursion(i), toRomeReducer(i));
}