function _countChanger(countsByChar, isIncrement = false) {
  const incrementer = (char) => {
    if (char === ' ') {
      return;
    }
    var currentCount = countsByChar[char] || 0;
    if (isIncrement) {
      countsByChar[char] = currentCount + 1;
    } else {
      countsByChar[char] = currentCount - 1;
    }
  }
  return incrementer;
}

// returns: true if the two strings, s1 and s2, are anagrams; false otherwise
function areAnagrams(s1, s2) {
  if (!s1 || !s2) {
    return false;
  }
  const countsByChar = {};
  s1.split('').forEach(_countChanger(countsByChar, true));
  s2.split('').forEach(_countChanger(countsByChar));

  const counts = Object.values(countsByChar);
  return counts.every(num => num === 0);
}

console.log('false?: ' + areAnagrams('abc', null));
console.log('false?: ' + areAnagrams(null, 'abc'));
console.log('false?: ' + areAnagrams('anagram', 'not a gram'));
console.log('true?: ' + areAnagrams('ana gram', 'nag a ram'));
console.log('true?: ' + areAnagrams('anagram', 'nag a ram')); // spaces don't matter
console.log('false?: ' + areAnagrams('aa', 'bb'));