Advent of Code - adventofcode.com

I think your problem is here:

Object.keys(strNumMap).reduce((res, key) => {
  const index = str.indexOf(key);
  if (index > -1) {
    res[key] = index;
  }
  return res;
}

It's not handling strings with multiple of the same number correctly. E.g. 121 is getting mapped to 12 instead of 11.