Roman numeral to integer parser

rom2int.js

function romanToNum(str) {
    const symbols = 'IVXLCDM';

    let acc = '';
    // Reverse iterate through the provided characters
    for (let i = str.length - 1; i >= 0; i--) {
        // Get the associated roman character symbol's order in the list
        let index = symbols.indexOf(str[i]);
        // Calculate the value for the symbol 1 || 5 ^ 10 based on the previous index value
        let value = (index % 2  === 0 ? 1 : 5) * (10**(index>>1));
      
        // Append the value to the math equation that we'll eval at the end
        acc += value;

        // Add either a + or a - based on the index value of the following character
        if (i !== 0) {
          if (i > 0 && symbols.indexOf(str[i-1]) < index) acc += '-';
          else acc += '+';
        }
    }
    return Number(eval(acc));
}