如何在字符串行中输出重复的字母？

function findDuplicateCharacters(str) {
  var charCount = {};
  var duplicates = "";

  for (var i = 0; i < str.length; i++) {
    var char = str[i];
    if (charCount[char]) {
      charCount[char]++;
    } else {
      charCount[char] = 1;
    }
  }

  for (var char in charCount) {
    if (charCount[char] > 1) {
      duplicates += char;
    }
  }

  return duplicates;
}

var inputString = "hello world";
var result = findDuplicateCharacters(inputString);
console.log(result); // 输出 "lo"