Convert Roman Numerals to Integers.

romanToInt.js

const values = {
  I: 1,
  V: 5,
  X: 10,
  L: 50,
  C: 100,
  D: 500,
  M: 1000
};

const romanToInt = ([current, next, ...rest], total = 0) =>
  !current
    ? total
    : values[next] > values[current]
      ? romanToInt(rest,            total + (values[next] - values[current]))
      : romanToInt([next, ...rest], total + values[current]);

romanToInt('MMXIX'); // => 2019