kwego.js

const toPairs = roman => roman.map((v, i, array) => array.slice(i, i + 2))
const toValues = pairs => pairs.map(([p, n = 0]) => ([romanChars[p], romanChars[n]]))
const toParts = pairs => pairs.map(([p, n = 0]) => p >= n ? p : -p)
const toSum = parts => parts.reduce((part, sum) => sum + part, 0)

const romanChars = {
  I: 1,
  V: 5,
  X: 10,
  L: 50,
  C: 100,
  D: 500,
  M: 1000,
  0: 0
}

const toInt = roman => toSum(toParts(toValues(toPairs(roman.split('')))))

console.log(toInt('XXVI'));
console.log(toInt('CI'));
console.log(toInt('IX'));