A simple gist to convert roman numbers to decimal using reduceRight

roman-to-decimal.ts

const VALUES = {
  I: 1,
  V: 5,
  X: 10,
  L: 50,
  C: 100,
  D: 500,
  M: 1000,
}
const INIT = { value: 0, prev: 0 }
const romanToDecimal = (roman: string) =>
  roman
    .split('')
    .map((s) => VALUES[s] || 0) // SUPER EDGE CASE
    .reduceRight(
      (acc, value) => ({ value: value >= acc.prev ? value + acc.value : acc.value - value, prev: value }),
      INIT
    ).value

romanToDecimal('XXI') // 21
romanToDecimal('VXXI') // 16 (but wrong)
romanToDecimal('IV') // 4
romanToDecimal('XVI') // 16 (but right)