Great!

Overriding the prototype is definitely not a good practice.

But other than that, this code is simple and that is a good thing.

Sure, it won't be optimal for running anagramm checks for millions of items, but that was not a requirement. Usually if something has to handle big data you know either because it is obvious from the context, or you are told that.

In this case premature optimization would only made the code more complex and thus harder to read and understand. And that can be bad.

Can you explain the math behind it?

It is best practice because people voted for it. Always, not, but very often top best practice voted solution are not good practice at all... Take it easy, and vote for the real best practice according to you ;)

I'm new here, anyone could explain me why this solution is considered best practices?
Maybe I'm worng but:

1. I think we should not be messing around with prototype.
2. If the words array would have 1M words, it would take too much time to check it.

less code !== (better code || best practices)

Is this always like that? :(