because it's and addition in the end.

although I would use

const arr = [2,3,4,5];

console.log(sum(arr, arr.length));

 -> sum: [2,3,4,5], 3
  - 3 > 0 
  - return sum([2,3,4,5], 2) + 4
 -> sum: ...., 2
  - 2 > 0 
  - return sum(....., 1) + 3;
 -> sum: ....., 1
  - 1 > 0
  - return sum(...., 0) + 2
 -> sum: ....., 0
  - 0 == 0
  - return 0

which means:

first call (4) + second call (3) + third call (2) + forth call(0)
4+3+2+0 -> 9;

does this make sense?

a more visual approach would be if you just pop the value before passing

const val = arr.pop();
return sum(arr, arr.length) + val;

but since JS an array is just a specialized form of an object you actually would need to

const newArr = [...arr];
const val = newArr.pop();
return sum(newArr, newArr.length) + val;

to have no sideeffects. But this maybe is more confusing. it's just so the array you pass in as a reference does not get changed but you still see that the recursion your wrote basically is a stack call that removes one element after another and passes it to the previous call.

a question of taste so to say.