Basic JavaScript: Replace Loops using` Recursion

Can someone help me with this code please? I don’t undestand how it works:
Write a recursive function, sum(arr, n) , that returns the sum of the first n elements of an array arr .

function sum(arr, n) {
  if(n <= 0) {
    return 0;
  } else {
    return sum(arr, n - 1) + arr[n - 1];

I know this is the answer, but I can notunderstand the way of testing it.
sum([2, 3, 4, 5], 3) should equal 9.
sum([2, 3, 4], 1) should equal 2.
sum([1], 0) should equal 0.
Thank you in advance!

I’ve edited your post for readability. When you enter a code block into a forum post, please precede it with a separate line of three backticks and follow it with a separate line of three backticks to make it easier to read.

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