Finding duplicate "words" in a string - JavaScript

We are required to write a JavaScript function that takes in a string and returns a new string with only the words that appeared for more than once in the original string.

For example: If the input string is −

const str = "big black bug bit a big black dog on his big black nose";

Then the output should be −

const output = "big black";

Example

Let’s write the code for this function −

const str = "big black bug bit a big black dog on his big black nose";
const findDuplicateWords = str => {
   const strArr = str.split(" ");
   const res = [];
   for(let i = 0; i < strArr.length; i++){
      if(strArr.indexOf(strArr[i]) !== strArr.lastIndexOf(strArr[i])){
         if(!res.includes(strArr[i])){
            res.push(strArr[i]);
         };
      };
   };
   return res.join(" ");
};
console.log(findDuplicateWords(str));

Output

The output in the console: −

big black