Counting Duplicates

DESCRIPTION:
Count the number of Duplicates
Write a function that will return the count of distinct case-insensitive alphabetic characters and numeric digits that occur more than once in the input string. The input string can be assumed to contain only alphabets (both uppercase and lowercase) and numeric digits.

Example
"abcde" -> 0 # no characters repeats more than once
"aabbcde" -> 2 # 'a' and 'b'
"aabBcde" -> 2 # 'a' occurs twice and 'b' twice (b and B)
"indivisibility" -> 1 # 'i' occurs six times
"Indivisibilities" -> 2 # 'i' occurs seven times and 's' occurs twice
"aA11" -> 2 # 'a' and '1'
"ABBA" -> 2 # 'A' and 'B' each occur twice

function duplicateCount(text){
 
  text = text.toLowerCase();
  
  // 각 문자의 출현 횟수를 저장할 객체
  let charCount = {};
  
  // 중복된 문자의 개수를 세기 위한 변수
  let duplicates = 0;
  
  // 문자열을 순회하면서 각 문자의 출현 횟수를 센다
  for (let char of text) {
    if (charCount[char]) {
      charCount[char]++;
    } else {
      charCount[char] = 1;
    }
  }
  
  // 출현 횟수가 2 이상인 문자들을 찾아 개수를 센다
  for (let key in charCount) {
    if (charCount[key] > 1) {
      duplicates++;
    }
  }
  
  return duplicates;
}

문제