Basic JS Replace Loops using Recursion not passing

JavaScript
1

Tell us what’s happening:
I couldn’t get why this condition is true in the challenge:


return sum(arr,n-1) + arr[n-1];

I thought that it can be written as:


return sum(arr,n-1) + arr[n];

or


return sum(arr,n) + arr[n-1];

Your code so far


function sum(arr, n) {
// Only change code below this line
if(n<=0){
return 0;
} else{
return sum(arr,n-1) + arr[n-1];
}
// Only change code above this line
}

var a = [1,23,3,4,5];
console.log(sum(a,5))

Your browser information:

User Agent is: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.130 Safari/537.36.

Challenge: Replace Loops using Recursion

Link to the challenge:
https://www.freecodecamp.org/learn/javascript-algorithms-and-data-structures/basic-javascript/replace-loops-using-recursion

2

this will make a stack overflow error

with this you never sum arr[n]

this should be the correct one

but there is an other issue:

with this you are never summing arr[0] as when n is 0 you just add 0 instead of the first item in the array