Написать калькулятор

const checkNumber = (str) => {
  if (+str > 10 || +str <= 0) return false;
  return +str;
}
 
const transformRomantoArabic = (str) => {
  if (str[0] === '-') return 'neg';
  if (str === 'I') {
    return 1;
  } else if (str === 'II') {
    return 2;
  } else if (str === 'III') {
    return 3;
  } else if (str === 'IV') {
    return 4;
  } else if (str === 'V') {
    return 5;
  } else if (str === 'VI') {
    return 6;
  } else if (str === 'VII') {
    return 7;
  } else if (str === 'VIII') {
    return 8;
  } else if (str === 'IX') {
    return 9;
  } else if (str === 'X') {
    return 10;
  } else {
    return 'other'
  }
}
 
const calc = {
  '+': (a,b) => +parseInt(a+b).toFixed(2),
  '-': (a,b) => +parseInt(a-b).toFixed(2),
  '*': (a,b) => +parseInt(a*b).toFixed(2),
  '/': (a,b) => +a === 0 || +b === 0 ? 0 : +parseInt(a/b)
}
 
const parseString = (dirtyString) => {
  const tmpArray = Array(3).fill('');                               // creating array to store values
  let arabic = false, roman = false;
  const arr = Array.from(dirtyString.replace(/\s*/g, ''));          // clear from spaces
  const idx = arr.findIndex((_,i) => i !== 0 && /[-+*/]/.test(_));  // find operator [-+*/] value
  if (idx === -1) {
    return null;
  } else {
    tmpArray[1] = arr[idx];
  }
 
  //  before operator
  for (let i = 0; i < idx; i++) {
    tmpArray[0] += arr[i];
    if (/[0-9]/.test(arr[i])) {
      arabic = true;
    } else if (/[XVI]/.test(arr[i])) {
      roman = true;
    }
  }
 
  //  after operator
  for (let i = idx + 1; i < arr.length; i++) {
    tmpArray[2] += arr[i];
    if (/[0-9]/.test(arr[i])) {
      arabic = true;
    } else if (/[XVI]/.test(arr[i])) {
      roman = true;
    }
  }
 
  // tmpArray = ['III', '+', 'X']
 
 
  // here check operands
  if (arabic && roman) {
    return {result: false, type: null, tmpArr: null};
  } else if (arabic) {
    return {result: true, type: 'arabic', tmpArr: tmpArray};
  } else {
    return {result: true, type: 'roman', tmpArr: tmpArray};
  }
}
 
const calcArabic = (array) => {
  const oper1 = checkNumber(array.tmpArr[0]);
  const oper2 = checkNumber(array.tmpArr[2]);
  if (oper1 === false || oper2 === false) {
    console.error(`Use numbers from 1 to 10`);
    return null;
  } else {
    return calc[array.tmpArr[1]](oper1, oper2);
  }
}
 
const calcRoman = (array) => {
  const oper1 = transformRomantoArabic(array.tmpArr[0]);
  const oper2 = transformRomantoArabic(array.tmpArr[2]);
  if (oper1 === 'neg' || oper2 === 'neg') {
    console.error(`You don't use negative number in roman numbers`);
    return null;
  } else if (oper1 === 'other' || oper2 === 'other') {
    console.error(`Use numbers from 1 to 10`);
    return null;
  } else {
    const result = calc[array.tmpArr[1]](oper1, oper2);
    return result <= 0 ? '' : result;
  }
}
 
const calculate = (str) => {
  const array = parseString(str);
 
  if (array === null) {
    console.error('Unknown error!');
    return null;
  } else if (array.result === false) {
    console.error('You must be use only arabic or roman numbers!');
    return null;
  } else if (array.type === 'arabic') {
    return calcArabic(array);
  } else if (array.type === 'roman') {
    return calcRoman(array);
  }
}
 
 
const string = 'III + X';
const result = calculate(string);
console.log(result);