const maxStringCounter = str => {
  let obj = {};

  str.split("").forEach(el => {
    if (obj[el]) {
      return (obj[el] += 1);
    } else {
      return (obj[el] = 1);
    }
  });

  const values = Object.values(obj);
  const keys = Object.keys(obj); //?
  const maxValue = Math.max(...values);
  const indexOfLargestValue = values.indexOf(maxValue);

  return keys[indexOfLargestValue];
};

maxStringCounter("44123"); // 4
maxStringCounter("441111123"); // 1