fromRoman

const mapper = {
  '': 0,
  'I': 1,
  'II': 2,
  'III': 3,
  'IV': 4,
  'V': 5,
  'VI': 6,
  'VII': 7,
  'VIII': 8,
  "IX": 9,
  'X': 10,
  'XX': 20,
  'XXX': 30,
  'XL': 40,
  'L': 50,
  'LX': 60,
  'LXX': 70,
  'LXXX': 80,
  'XC': 90,
  'C': 100,
  'CC': 200,
  'CCC': 300,
  'CD': 400,
  'D': 500,
  'DC': 600,
  'DCC': 700,
  'DCCC': 800,
  'CM': 900,
  'M': 1000,
  'MM': 2000,
  'MMM': 3000
}

const fromRoman = (roman) => {
  const pattern = /^(MMM|MM|M)*(C|CC|CCC|CD|D|DC|DCC|DCCC|CM)?(X|XX|XXX|XL|L|LX|LXX|LXXX|XC)?(I|II|III|IV|V|VI|VII|VIII|IX)?$/gi;

  const arrayOfRoman = pattern.exec(roman);
  const [_, hilyadni, stotici, desetici, edinici] = [...arrayOfRoman]

  return [hilyadni, stotici, desetici, edinici].map(el => el ? el : '').reduce((result, el) => result += mapper[el], 0)
}


console.log(fromRoman('M'));
console.log(fromRoman('CMXCIX'));
console.log(fromRoman('IV'));
console.log(fromRoman('I'));
console.log(fromRoman('MCMXCI'));
console.log(fromRoman('MMVI'));
console.log(fromRoman('MMXX'));

console.log(fromRoman('XXI'));
console.log(fromRoman('I'));
console.log(fromRoman("III"));
console.log(fromRoman('IV'));
console.log(fromRoman('MMVII'));
console.log(fromRoman('MDCLXIX'));