nested-array-sum.js

const findArraySum = (array, index, sum) => {
  if (index === array.length) {
    return sum;
  }

  let element = array[index];

  if (Array.isArray(element)) {
    element = findArraySum(element, 0, 0);
  }

  sum += element;

  return findArraySum(array, index + 1, sum);
};

/* 
example input
const array = [1,[11,42,[8, 1], 4, [22,21]]]; 
*/