convertToRoman.js

//jshint esversion:6

function convertToRoman(num) {
  const UNITS = {
    1: 'I',
    4: 'IV',
    5: 'V',
    9: 'IX',
    10: 'X',
    40: 'XL',
    50: 'L',
    90: 'XC',
    100: 'C',
    400: 'CD',
    500: 'D',
    900: 'CM',
    1000: 'M'
  };
  if (num === 0) {
    return '';
  }

  let value = Object.keys(UNITS).map(i => parseInt(i, 10)).reduce((a, b) => {
    if (b <= num) {
      return b;
    }
    return a;
  }, 0);
  
  return UNITS[value] + convertToRoman(num - value);
}

convertToRoman(36);