plain-recursion-incrementNestedNumbers.js

// Fetched from server
const nestedNumbers = [
  [[0], [[[[[[[1, 2]]]]]]], [3]],
  [[[4], [[5]]], [[[6, 7, 8]]]],
  [9]
];

const incrementNestedNumbers = (arrayWithNums) => {
  for (let i = 0; i < arrayWithNums.length; i++) {
    if (Array.isArray(arrayWithNums[i])) { // if array
      incrementNestedNumbers(arrayWithNums[i]);
    } else { // if number
      arrayWithNums[i] = arrayWithNums[i] + 1;
    }
  }
};

incrementNestedNumbers(nestedNumbers);

/* nestedNumbers now look like this:
  [[1], [[[[[[[2, 3]]]]]]], [4]],
  [[[5], [[6]]], [[[7, 8, 9]]]],
  [10]
*/