surreal.js

function convertToRoman (num) {
  let romans = [['I', 'V', 'X'], ['X', 'L', 'C'], ['C', 'D', 'M'], ['M']]
  function getOutput (number, one, five, ten) {
    var romanMap = {
      0: () => ‘’,
      1: one => `${one}`,
      2: one => romanMap[1](one) + romanMap[1](one),
      3: one => romanMap[2](one) + romanMap[1](one),
      4: (one, five) => romanMap[1](one) + `${five}`,
      5: (one, five) => `${five}`,
      6: (one, five) => romanMap[5](one, five) + romanMap[1](one),
      7: (one, five) => romanMap[5](one, five) + romanMap[2](one),
      8: (one, five) => romanMap[5](one, five) + romanMap[3](one),
      9: (one, five, ten) => romanMap[1](one) + `${ten}`
    }
    return romanMap[number](one, five, ten)
  }
  return ('' + num)
  .split('')
  .reverse()
  .reduce((acc, curr, ix) => {
  return getOutput(curr, ...romans[ix]) + acc
  }, '')
}
convertToRoman(997);