this is NOT best practice, it modifies given data

!! is used to convert a truthy/falsy value to its boolean equivalent.

let num = 5; (truthy)
!num // false
!!num // true

let num = 0; (falsy)
!num // true
!!num // false

comparing 5 with !!5 would be the equivalent of comparing 5 with true (which is false)
5 === !!5
5 === true

The only reason people still use it over Boolean (other than wanting to look like elite hackers) is because it's something like 10-15% faster (last time I checked)

Nice idea. But it doesn't work with all values in JS. For example it returns true with 1 and 2

From description:
""xor" only returns true if EXACTLY one of the two expressions evaluate to true."

What's that? :b

Yes, you are right. It sorts in place.

My first attempt was to try and return l.reverse(). I kept getting "None" as a result. Can someone explain why you have to do l.reverse() then return l instead of return l.reverse()?

This solution certainly solves the kata.

I only commented because this solution has a number of "best practices" votes.

Per the kata description:
"...a function that takes in a list and returns a list with the revese order."

In a function with that description modifying the passed in list is not "best practices"

If I could cast a down vote for "best practices" I would have done that instead of leaving a comment.

Which solution is "much better" depends on the purpose of reversing the list.

If you're done with the old list, this method is probably better as it doesn't leave the old list in memory. The kata instructions weren't clear about whether the old one needed to be preserved, so either approach should be fine.

This one has the side effect of modifying the passed in list.

I would be much better to copy the list before reversing it.