Partial sum in array of arrays JavaScript

We are required to write a JavaScript function that takes in an array of arrays of numbers. For each subarray, the function such create a partial sum subarray (an array in which a particular value is the sum of itself and the previous value).

For example −

If the input array is −

const arr = [
[1, 1, 1, -1],
[1, -1, -1],
[1, 1]
];

Then the output should be −

const output = [
[1, 2, 3, 2],
[1, 0, -1],
[1, 2]
];

Example

const arr = [ [1, 1, 1, -1], [1, -1, -1], [1, 1] ]
   const partialSum = (arr = []) => {
      const res = [] arr.forEach(sub => {
         let accu = 0
         const nestedArr = [] sub.forEach(n => {
            accu += n;
            nestedArr.push(accu);
      });
      res.push(nestedArr);
   });
   return res;
};
console.log(partialSum(arr));

Output

And the output in the console will be −

[ [ 1, 2, 3, 2 ], [ 1, 0, -1 ], [ 1, 2 ] ]