fCC - Basic JavaScript: Replace Loops using Recursion

I don't understand this. I got through this exercise using the hints given by fCC but I still don't really get it

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.

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
}
console.log(sum([1], -1))
console.log(sum([2, 3, 4], 1))