How to calculate sum of matched number in array includes string

let userInput = ["1 6 7 9 12 15", 12,16,15,34,28,2];

I’ve given array like above.

The result I want is to sum the elements of an array by index matching string elements and return the sum arrays.

The output:

[ 13, 22, 22, 43, 40, 17 ]

More optimised results are better.

let userInput = ["1 6 7 9 12 15", 12,16,15,34,28,2];

let stringArray = userInput[0].split(' ');
let stringTonum = stringArray.map(item => parseInt(item));


let sumArray = stringTonum.map((item, index)=>{return item + userInput[index+1]});

console.log(sumArray)