function sumArrayIndex(arr, index = 0) {
  // Base case: when index is greater than or equal to the length of the array return 0
  if (index >= arr.length) {
    return 0;
  }
  // Recursive case: add the current element to the sum of the rest of the array
  return arr[index] + sumArrayIndex(arr, index + 1);
}

// Test cases
console.log(sumArrayIndex([1, 2, 3, 4, 5])); // => 15
console.log(sumArrayIndex([10, -2, 33, 47])); // => 88
console.log(sumArrayIndex([])); // => 0