RomanToDecimal.js

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

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

 let sum = 0

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

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

 const currentNum = values[c]
 if (currentNum >= newPrev) {
 sum += currentNum
 } else {
 sum -= currentNum
 }

 prev = c
 }
 return sum
}