Convert decimal number to Roman numeral

roman-to-decimal.js

const NUMERALS = {
  I: 1,
  V: 5,
  X: 10,
  L: 50,
  C: 100,
  D: 500,
  M: 1000,
  IV: 4,
  IX: 9,
  XL: 40,
  XC: 90,
  CD: 100,
  CM: 900,
};

const romanToDecimal =  (numerals) => {
  // validate here
  
  let value = 0;

  // loop through the roman numeral string
  for (let i = 0; i < numerals.length; i += 1) {
    // create a combo numeral with this index and it's
    // next sibling
    let combo = `${numerals[i]}${numerals[i + 1]}`;

    // check to see if the combo has a value
    if (NUMERALS[combo]) {
      // if it does, add to the total value, and increment the
      // loop by an extra 1, so that the next sibling doesn't
      // get counted again
      value += NUMERALS[combo];
      i += 1;
      continue;
    }

    // combo didn't have a value, so let's just try the single numeral
    if (NUMERALS[numerals[i]]) {
      value += NUMERALS[numerals[i]];
      continue;
    } else {
      // hmm, combo and single numeral did not have an associated
      // decimal value, so let's set `value` to false and break out
      // of the loop, cause this looks to be an invalid roman numeral
      value = false;
      break;
    }
  }

  return value;
};