I need help understanding this function works

## Basic JavaScript: Replace Loops using Recursion

Recursion is the concept that a function can be expressed in terms of itself. To help understand this, start by thinking about the following task: multiply the first `n`

elements of an array to create the product of those elements. Using a `for`

loop, you could do this:

```
function multiply(arr, n) {
var product = 1;
for (var i = 0; i < n; i++) {
product *= arr[i];
}
return product;
}
```

However, notice that `multiply(arr, n) == multiply(arr, n - 1) * arr[n - 1]`

. That means you can rewrite `multiply`

in terms of itself and never need to use a loop.

```
function multiply(arr, n) {
if (n <= 0) {
return 1;
} else {
return multiply(arr, n - 1) * arr[n - 1];
}
}
```

The recursive version of `multiply`

breaks down like this. In the base case, where `n <= 0`

, it returns 1. For larger values of `n`

, it calls itself, but with `n - 1`

. That function call is evaluated in the same way, calling `multiply`

again until `n <= 0`

. At this point, all the functions can return and the original `multiply`

returns the answer.

**Note:** Recursive functions must have a base case when they return without calling the function again (in this example, when `n <= 0`

), otherwise they can never finish executing.

Write a recursive function, `sum(arr, n)`

, that returns the sum of the first `n`

elements of an array `arr`

.