Some people use low-difficulty kata like this one as a way to practice implementing the fundamentals rather than using the standard library methods. :-)

No problem, we've all been :)

Thank you for your answer. I'm quite new as you can see :)

This is how you should read the error logs: "expected YOUR ANSWER to equal THE CORRECT ANSWER". Here it means your function returns [1,NaN,3] (an array) while you should have returned a single number (13). See there for a general help and how to print the input: https://docs.codewars.com/training/troubleshooting/#print-input

Given that the kata's description says "Only positive integers will be passed to the function (> 0 ), no negatives or zeros". Why do I get this error?
"expected [ 1, NaN, 3 ] to equal 13".