Sum of distinct elements of an array - JavaScript

We are required to write a JavaScript function that takes in one such array and counts the sum of all distinct elements of the array.

For example: Suppose, we have an array of numbers like this −

const arr = [1, 5, 2, 1, 2, 3, 4, 5, 7, 8, 7, 1];

The output for the array mentioned above will be 20.

Example

Following is the code −

const arr = [1, 5, 2, 1, 2, 3, 4, 5, 7, 8, 7, 1];
const distinctSum = arr => {
   let res = 0;
   for(let i = 0; i < arr.length; i++){
      if(i === arr.lastIndexOf(arr[i])){
         res += arr[i];
      };
      continue;
   };
   return res;
};
console.log(distinctSum(arr));

Output

Following is the output in the console −

30