Sum identical elements within one array in JavaScript

We are required to write a JavaScript function that takes in an array of Numbers.

The array might contain some repeating / duplicate entries within it. Our function should add all the duplicate entries and return the new array thus formed.

Example

The code for this will be −

const arr = [20, 20, 20, 10, 10, 5, 1];
const sumIdentical = (arr = []) => {
   let map = {};
   for (let i = 0; i < arr.length; i++) {
      let el = arr[i];
      map[el] = map[el] ? map[el] + 1 : 1;
   };
   const res = [];
   for (let count in map) {
      res.push(map[count] * count);
   };
   return res;
};
console.log(sumIdentical(arr));

Output

And the output in the console will be −

[ 1, 5, 20, 60 ]