Sum all similar elements in one array - JavaScript

We are required to write a JavaScript function that takes in an array of Numbers and sums all the identical numbers together at one index

For example −

If the input array is −

const arr = [20, 10, 15, 20, 15, 10];

Then the output should be −

const output = [40, 20, 30];

Example

Following is the code −

const arr = [20, 10, 15, 20, 15, 10];
const addSimilar = arr => {
   for(let i = 0; i < arr.length; i++){
      while(i !== arr.lastIndexOf(arr[i])){
         const ind = arr.lastIndexOf(arr[i]);
         arr[i] += arr.splice(ind, 1)[0];
      };
   };
};
addSimilar(arr);
console.log(arr);

Output

This will produce the following output in console −

[ 40, 20, 30 ]