JavaScript/Conversions/RomanToDecimal.js

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

function romanToDecimal(romanNumber){
  let prev = ' ';

  let sum = 0;

  let newPrev = 0;
  for(let i = romanNumber.length - 1; i >= 0; i--){
    let c = romanNumber.charAt(i);

    if(prev !== ' '){
      newPrev = values[prev] > newPrev ? values[prev] : newPrev;
    }

    let currentNum = values[c];
    if(currentNum >= newPrev){
      sum += currentNum;
    } else {
      sum -= currentNum;
    }
    
    prev = c;
  }
  return sum;
}

console.log(romanToDecimal('XXIIVV'));
console.log(romanToDecimal('MDCCCIV'));
console.log(romanToDecimal('XXIVI'));