To make the challenges more challenging for me I'm aiming for minimal code instead of product quality code ;)

supper dupper hard to read, could not you use some meaningful names

This one knows golf.

Length of sequence times average of sequence equals sum of sequence.

true, there should be a test case for that

What about if null is passed?

First time i tried something like this, but have Maximum call stack size exceeded

Excuse me, could you explain the same stuff you wrote, but for dummies who are not into math?

Genius

True, replacing nums with [...nums] would fix it if that is needed

Be careful. This solution will change the initial array.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort
"Note that the array is sorted in place, and no copy is made."

Should be fixed

Are there any time- & spacecomplexity Kata-Stats to checkout ?

It looks so good but why does it work?