Написать функцию, которая будет разбивать строку на символы и сравнивать с другими символами из другой строки.

/**
 * Given two strings, first and second. Find all letters (ignoring case)
 * which are present in both strings. Ignore whitespaces.
 */
function getOverlappingLetters(first, second) {
  // Only change code below this line
  // Only change code above this line
}
 
 
// Tests
test(getOverlappingLetters('a', 'b'), '', 'empty overlap');
test(getOverlappingLetters('aa', 'a'), 'a', 'duplicated string in overlap');
test(getOverlappingLetters('A', 'a'), 'a', 'case insensitive overlap');
test(getOverlappingLetters('Happy New Year', 'nyhz'), 'hyn', 'simple phrase');
 
function test(actual, expected, testName = '') {
  if (actual === undefined || expected === undefined) {
    console.error(`Test ${testName} failed!`);
    return;
  }
  
  let actualArr = actual.split('').sort();
  let expectedArr = expected.split('').sort();
  if (actualArr.length != expectedArr.length) {
    console.error(`Test ${testName} failed: "${actual}" length is not equal to expected "${expected}" length`);
    return;
  }
  for (let i = 0; i < expectedArr.length; i++) {
    if (actualArr[i] != expectedArr[i]) {
      console.error(`Test ${testName} failed: "${actual}" is not equal to expected "${expected}"`);
      return;
    }
  }
  console.log(`Test ${testName} passed!`);
}

        /**
         * Given two strings, first and second. Find all letters (ignoring case)
         * which are present in both strings. Ignore whitespaces.
         */
        function getOverlappingLetters(first, second) {
            // Only change code below this line
            let set1 = new Set(first.toLowerCase()),
                set2 = new Set(second.toLowerCase()),
                result = "";
 
            for (let item of set1) {
                if (item !== " " && set2.has(item)) {
                    result += item;
                }
            }
            return result;
            // Only change code above this line
        }

        /**
         * Given two strings, first and second. Find all letters (ignoring case)
         * which are present in both strings. Ignore whitespaces.
         */
        function getOverlappingLetters(first, second) {
            // Only change code below this line
            let set1 = new Set(first.toLowerCase()),
                set2 = new Set(second.toLowerCase());
 
            return [...set1].flatMap(e => e !== " " && set2.has(e) ? [e] : []).join("");
            // Only change code above this line
        }