Thank you for the very nice explanation.

You could refactor your code a little more by scoping the variables closer to where they are used.

Also you could use Array.reduce to accumulate the total amount.

const map = new Map([
  ['I', 1],
  ['V', 5],
  ['X', 10],
  ['L', 50],
  ['C', 100],
  ['D', 500],
  ['M', 1000],
]);
function romanToInt(s) {
  return s.split('').reduce((accumulator, _currentValue, index, arr) => {
    const current = map.get(arr[index]) || 0;
    const next = map.get(arr[index + 1]) || 0;
if (current < next) {
      return accumulator - current;
    }
return accumulator + current;
  }, 0);
}

https://jsfiddle.net/marcobiedermann/kgbx4sof/