ok, thanks

Where did you get that last 20?

If there are 5 or more courses and all courses has a grade of B or above, additional 20 points are awarded.

...

Read the info on that link please, you need to write print(whatever_value_you_want_to_inspect). In this case, print(arr). And that works in both, you need to write it inside your function find_hack, not in another function that doesn't get called in the tests.

log outputs only on the 'test' button, not on the 'attempt'

Print the input yourself to debug your code: https://docs.codewars.com/training/troubleshooting#print-input

what are the inputs 'Fixed tests 3'???

Fixed tests 3:
['name3', 'name4'] should equal ['name2', 'name3', 'name4']???
Why???,,,
the code should go perfectly

all other 204 tests have been passed

When you mutate the input, unexpected results could happen. I've changed the tests so it doesn't matter anymore, but keep that in mind.

So please forgive me if I'm missing something basic and this is a moronic question,

but it's a key part of this not to mutate the input?

That happened because you mutated the input in your code. Now it doesn't matter.

Tests pass, but I get "Cannot read property 'reduce' of undefined
at arr.filter" on attempt. Can anyone explain what this means? I'm not using reduce or filter in my code. Is this a bug or a user error?

First of all, thank you! Also I found a mistake in my own understanding of the task. I didn't take account that any other letters except 'C' and 'D' lead to absence of additional 20 points. Briefly, we get 20 points if an array consists only of 'A' and 'B'. Unfortunately, i didn't understand it at first.

Print the input to debug your code.

Everything else: 0 points

Your code marks students having any other mark as hacked.