FreeCodeCamp: Roman Numeral Converter

toRoman.js

'use strict';

const getRange = (num, list) => {
  return [
      list.reduce((previous, current) => {
        return (previous < num && num < current) ? previous : current;
      }),
      list.reduce((previous, current) => {
        return (previous < num && num < current) ? current : previous;
      })
    ]
}

const isInRange = (num, range) => {
  return range.reduce((previous, current) => previous < num && num < current);
}

const convertToRoman = (num) => {
  let romanNumber = '';

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

  const romanList = Object.keys(romanDigits).map(n => Number(n));
  
  if (romanList.includes(num)) {
    romanNumber = romanDigits[num];
  }
  else {
    let [floor, ceil] = getRange(num, romanList);
    let quotient = 0;
    let remainder = 0;

    if (isInRange(num, [floor, ceil-floor])) {
      quotient = parseInt(num / floor);
      remainder = num % floor;
      romanNumber = romanDigits[floor].repeat(quotient);
    }
    else if (num > 1000 && num < 4000) {
      quotient = parseInt(num / 1000);
      remainder = num % 1000;
      romanNumber = romanDigits[floor].repeat(quotient);
    }
    else {
      if (/^5/.test(floor) && (ceil - romanList[romanList.indexOf(floor) - 1]) <= num) {
        quotient = parseInt(num / romanList[romanList.indexOf(floor) - 1])
        remainder = num % romanList[romanList.indexOf(floor) - 1];
        romanNumber = romanDigits[romanList[romanList.indexOf(floor) - 1]] + romanDigits[ceil];
      }
      else {
        remainder = num % (ceil-floor);
        romanNumber += romanDigits[floor] + ((/^5/.test(floor)) ? '' : romanDigits[ceil]);
      }
    }
    if (remainder > 0) {
      romanNumber += convertToRoman(remainder);
    }
  }
  return romanNumber;
}