How does multiply(arr,n) == multiply(arr, n - 1) * arr[n - 1] how does the two statemtents above equal each , n and n - 1??

function sum(arr, n) {
// Only change code below this line
if (n <= 0) {
return 0;
} else {
return sum(arr, n - 1) + arr[n - 1]
}
**Challenge:** Basic JavaScript - Replace Loops using Recursion
**Link to the challenge:**
https://www.freecodecamp.org/learn/javascript-algorithms-and-data-structures/basic-javascript/replace-loops-using-recursion

If you are asking about recursion, I’ll do my best to explain it.
let’s say that n=3:

function sum(arr, n) {
// Only change code below this line
if (n <= 0) {
return 0;
} else {
return sum(arr, n - 1) + arr[n - 1];
}
sum([5,2,2,5,3],3);// ans = 9
// The first recursive will return sum(arr, 2) + arr[2];
// The second recursive will return sum(arr, 1) + arr[1];
// The third recursive will return sum(arr, 0) + arr[0];
// Because n = 0, it will return 0 this time.
// Now, we can combine all the 'RETURNS' : arr[0] + arr[1] + arr[2] + 0;

I wanted to know why does we subtract 1 from n, and i understand it now. It’s to take the first element of an array, since the first element index < last element index and thanks for your time!