Basic Roman Numerals

readme.txt

Have the function BasicRomanNumerals(str) read str which will be a string of Roman numerals. 
The numerals being used are: I for 1, V for 5, X for 10, L for 50, C for 100, D for 500 and M for 1000. 
In Roman numerals, to create a number like 11 you simply add a 1 after the 10, so you get XI. 
But to create a number like 19, you use the subtraction notation which is to add an I before an X or V 
(or add an X before an L or C). So 19 in Roman numerals is XIX.

The goal of your program is to return the decimal equivalent of the Roman numeral given.
For example: if str is "XXIV" your program should return 24

BasicRomanNumerals.js

function BasicRomanNumerals(str) {
  let arr = []

  str.split('').forEach(num => {
    switch (num) {
      case 'I':
        arr.push(1);
        break;
      case 'V':
        arr.push(5);
        break;
      case 'X':
        arr.push(10);
        break;
      case 'L':
        arr.push(50);
        break;
      case 'C':
        arr.push(100);
        break;
      case 'D':
        arr.push(500);
        break;
      case 'M':
        arr.push(1000);
        break;
    }
  });

  let res = 0;
  let x;
  let y;

  //For Even Roman digits
  if (arr.length % 2 === 0) {
    for (let i = 0, j = 1; i < arr.length; i += 2, j += 2) {
      x = arr[i];
      y = arr[j];

      if (arr[i] < arr[j]) {
        res += (y - x)
      } else {
        res += (y + x)
      }
    }
  } // For Odd Roman digits 
  else {
    // Case 1: when first two digits are ascending
    if (arr[0] > arr[1]) {
      res += arr[0];
      arr.shift();

      for (let i = 0, j = 1; i < arr.length; i += 2, j += 2) {
        x = arr[i];
        y = arr[j];

        if (arr[i] < arr[j]) {
          res += (y - x)
        } else {
          res += (y + x)
        }
      }
    } else {
      // Case 2: when first two digits are descending
      res += arr[arr.length - 1];
      arr.pop();
      for (let i = 0, j = 1; i < arr.length; i += 2, j += 2) {
        x = arr[i];
        y = arr[j];

        if (arr[i] < arr[j]) {
          res += (y - x)
        } else {
          res += (y + x)
        }
      }
    }
  }
  return res;
}