how to find repeated characters in a string in javascript

1/* Time complexity of solution: O(n) */
2const getRepeatedChars = (str) => {
3 	const chars = {};
4    for (const char of str) {
5        chars[char] = (chars[char] || 0) + 1;
6    }
7    return Object.entries(chars).filter(char => char[1] > 1).map(char => char[0]);
8}
9
10getRepeatedChars("aabbkdndiccoekdczufnrz"); // ["a", "b", "c", "d", "k", "n", "z"]

1const str = "big black bug bit a big black dog on his big black nose";
2const findDuplicateWords = str => {
3   const strArr = str.split(" ");
4   const res = [];
5   for(let i = 0; i < strArr.length; i++){
6      if(strArr.indexOf(strArr[i]) !== strArr.lastIndexOf(strArr[i])){
7         if(!res.includes(strArr[i])){
8            res.push(strArr[i]);
9         };
10      };
11   };
12   return res.join(" ");
13};
14console.log(findDuplicateWords(str));