I really need a feed back on my explanations especially on 2) and 3). I just want to confirm if I understood it correctly or no since I am a newbie.

This is a freeCodeCamp challenge which was really challenging for me because I have no experience with JS before. It goes as below.

Write a recursive function, sum(arr, n), that returns the sum of the first n elements of an array arr.

function sum(arr, n) { if(n <= 0) { return 0; } else { return sum(arr, n - 1) + arr[n - 1]; } } /* 1) sum([1], 0) should equal 0. 2) sum([2, 3, 4], 1) should equal 2. 3) sum([2, 3, 4, 5], 3) should equal 9. */ /* My explanations are down below */ /* Explanation 1) sum([1], 0) should equal 0. n is less or equal to 0 so line 2 works and returns 0 at line 3. */ /* Explanation 2) sum([2, 3, 4], 1) should equal 2 n is not less or equal to 0 so it will not return 0 according to line 2. We move to line 5. *return sum(arr, n - 1) + arr[n - 1]; => return sum([2, 3, 4], 1 - 1) + arr[1 - 1]; => return sum([2, 3, 4], 0) + arr[0] => n is less or equal to 0 so it will return zero according to line 2. => return 0 + arr[0] => Since arr[0] is equals to 2 => return 0 + 2; => 2 */ /* Explanation 3) sum([2, 3, 4, 5], 3) should equal 9 n is not less or equal to 0 so it will not return 0 according to line 2. We move to line 5. *return sum(arr, n - 1) + arr[n - 1]; => return sum([2, 3, 4, 5], 3 - 1) + arr[3 - 1]; => return sum([2, 3, 4, 5], 2) + arr[2]; => n is 2, not less or equal to 0 so go back to line 5 + arr[2]. => return sum(arr, n - 1) + arr[n - 1] + arr[2]; => return sum([2, 3, 4, 5], 2 - 1) + arr[2 - 1] + arr[2]; => return sum([2, 3, 4, 5], 1) + arr[1] + arr[2]; => n is 1, not less or equal to 0 so goes back to line 5 arr[1] + arr[2]. => return sum(arr, n - 1) + arr[n - 1] + arr[1] + arr[2]; => return sum([2, 3, 4, 5], 1 - 1) + arr[1 - 1] + arr[1] + arr[2]; => return sum([2, 3, 4, 5], 0) + arr [0] + arr[1] + arr[2]; => return 0 + arr[0] + arr[1] + arr[2]; => in our array, arr[0] is 2, arr[1] is 3, arr[2] is 4. => 0 + 2 + 3 + 4 => 5 + 4 => returns 9 */

## Answer

I don’t think we can make it shorter:

const sum = (arr,n) => --n<0 ? 0 : sum(arr,n) +arr[n] console.log ( sum([1], 0) ) console.log ( sum([2, 3, 4], 1) ) console.log ( sum([2, 3, 4, 5], 3) )

.as-console-wrapper { max-height: 100% !important; top: 0; }