How to find the single different character between two strings in JavaScript

// Assumptions:
// - All strings are lowercase
// - inputs might be null
// - strings fits in memory
// - strings are in ASCII
// - strings can be in different orders
// - there will be some difference between the two strings
function singleDifferentChar(str1, str2) {
  if (typeof str1 !== 'string' || typeof str2 !== 'string') {
    throw new Error(`One or both of your inputs is not a string. Please use two strings - str1: ${str1} str2: ${str2} `);
  }
  
  const str1CharCount = getCharCountMap(str1);
  const str2CharCount = getCharCountMap(str2);
  
  return findDifference(str1CharCount, str2CharCount)
}
// returns a character count map given a string
function getCharCountMap(string) {
  return string.split('').reduce((map, char) => {
    map[char] = map[char] + 1 || 1;
    return map;
  }, {})
}
// find the difference between two character count maps
function findDifference(map1, map2) {
  const largestMap = Object.keys(map1).length >= Object.keys(map2).length ? map1 : map2;
  
  for (let key in largestMap) {
    const isDifferent = map1[key] !== map2[key];
    if (isDifferent) return key; 
  }
}
// A simpler xor implementation
function findDiff(str1, str2) {
    if (str1 === null || str2 === null) {
        throw new Error('str1 or str2 cannot be null');
    }
    let result = 0;
    for (let i = 0; i < str1.length; i++) {
        result ^= str1.charCodeAt(i)
    }
    for (let i = 0; i < str2.length; i++) {
        result ^= str2.charCodeAt(i)
    }
    return String.fromCharCode(result);
}
// Uncomment to use findDiff
// singleDifferentChar = findDiff
// ==== Tests
function runTests(func) {
  const inputOutput = [['abcd', 'abcde', 'e'], ['aaabbcdd', 'abdbacade', 'e']];
  
  for (let i = 0; i < inputOutput.length; i++) {
    const input = inputOutput[i].slice(0, inputOutput[i].length-1);
  
    const expected = inputOutput[i][inputOutput[i].length-1];
    const result = func(...input);
    
    if (result !== expected) {
      return [false, result, expected];
    }
  }
  return [true];
}
let throwsErrorCorrectly = false;
let runsTestSuccessfully = false;
let result, expected;
try {
  singleDifferentChar(null, 'input'); // This should throw an error
} catch (e) {
  [runsTestSuccessfully, result, expected]  = runTests(singleDifferentChar); 
  throwsErrorCorrectly = true;
} finally {
  const message = runsTestSuccessfully && throwsErrorCorrectly ? 'All tests passed' : `Expected: ${expected}. Got ${result}`;
  console.log(message);
}
// Test cases
// singleDifferentChar(null, 'input'); // Error!
// singleDifferentChar('abcd', 'abcde') // 'e'
// singleDifferentChar('aaabbcdd', 'abdbacade', 'e') // 'e'