function maxChar(str) {
  let count = {};
  let max = 0;
  let maxChar = "";

  for (let char of str) {
    // count[char] = count[char] ? count[char] + 1 : 1;
    count[char] = count[char] + 1 || 1;
  }

  for (let n in count) {
    if (max < count[n]) {
      max = count[n];
      maxChar = n;
    }
  }
  return maxChar;
}

console.log(maxChar("Hello there!"));