Roman numerals to Arabic numerals

romanArabic.ts

const romanMap: { [key: string]: number } = {
  I: 1,
  V: 5,
  X: 10,
  L: 50,
  C: 100,
  D: 500,
  M: 1000,
}

export function romanArabic(roman: string): number {
  const chars = roman.toUpperCase().split("")
  return chars.reduce((acc: number, char: string, i: number) => {
    const previousChar = chars[i - 1]
    const isGreater = romanMap[char] > romanMap[previousChar]

    return isGreater
      ? acc -
          romanMap[previousChar] +
          (romanMap[char] - romanMap[previousChar])
      : acc + romanMap[char]
  }, 0)
}

Another method

const romanList: { [key: string]: number } = { CM: 900, M: 1000, CD: 400, D: 500, XC: 90, C: 100, XL: 40, L: 50, IX: 9, X: 10, IV: 4, V: 5, I: 1 }

export function romanArabic(romanNum: string): number {
  let index = 0
  let number = 0
  romanNum = romanNum.toUpperCase()

  for (const key in romanList) {
    index = romanNum.indexOf(key)
    while (index != -1) {
      number += romanList[key]
      romanNum = romanNum.replace(key, "*")
      index = romanNum.indexOf(key)
    }
  }
  return number
}