반복 대체 루프를 사용하는 방법

function sum(arr, n) {
    var product = 0;
    for (var i = 0; i < n; i++) {
        product += arr[i];
    }
    return product;
  }

console.log(sum([2, 3, 4, 5], 3)); will display 9

function sum(arr, n) {
if (n <= 0) {
  return 0;
} else {
  return sum(arr, n - 1) + arr[n - 1]; //return sum(arr, n - 1) <—-calls itself
}
}

console.log(sum([2, 3, 4, 5], 3)); will display 9

// sum([2, 3, 4, 5], 3 - 1) + arr[3 - 1];
// sum([2, 3, 4 ,5], 2) + arr[2];
// sum([2, 3, 4, 5], 2) + 4;
//        n = 2

// sum([2, 3, 4, 5], 2 - 1) + arr[2 - 1];
// sum([2, 3, 4, 5], 1) + arr[1];
// sum([2, 3, 4, 5], 1) + 3; 
//      n = 1             3

// sum([2, 3, 4, 5], 1 - 1) + arr[1 - 1];
// sum([2  3, 4, 5], 0) + arr[0];
// sum([2, 3, 4, 5], 0) + 2;
//      n = 0             2

//      we hit our base case so now n = 0

// 0 + 2 + 3 + 4 = 9
// we want it to return 9 because 4 + 3 + 2 = 9;