Keeping only redundant words in a string in 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 = 'this is a is this string that contains that some repeating words';

Output

Then the output should be −

const output = 'this is that';

Let’s write the code for this function −

Example

The code for this will be −

const str = 'this is a is this string that contains that some repeating
words';
const keepDuplicateWords = 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(keepDuplicateWords(str));

Output

The output in the console −

this is that