I prefer it this way to be extra sure. I doubt it would slow it down much

bad practice to reassign arguments

Length cannot be smaller than the number of keys. When setting a length, any elements beyond the new length are deleted.

I just try '010.013.037.01', the regex return false.
In the description, '010.013.037.01' expect return false too.

The easier solution is String.prototype.reduce = Array.prototype.reduce (well, watch out for enumerability).

In any case, this is not an issue but more of a suggestion, so closing.

Different application environments use different code.
if this is a very high performance requirements of the application environment, we should try to optimize the code to improve performance.
If not, try to streamline the code, to give the job to CPU, don't let yourself exhausted ;-)
So my conclusion is that, in codewars, as long as the code can pass all the tests will be enough.
If the author is concerned about the performance of the code, he will add some testcases about the performance.

Yea, it's cool, but it not so inefficient ;-)

I thought that each number in an IP address could not be preceeded with 0? In which case that IP should indeed fail.

I was just focused on the javascript solution looking exactly the same as the javasolution, really there was no need.. sorry

Hey thanks for reporting the issue. I had a look a the js translation of the kata and I believe I found the problem with the random tests. Could you please try again and let me know.
As per your first question maybe Matt, who provided the translation, could shade some light. I am in no position to discuss javascript best practices.

I don't know if you can link Katas together (step 1, step 2, step 3, etc) or not

Codewars doesn't provide you that functionality (yet), but you can do it by hand. I usually use a self-written banner, see http://www.codewars.com/kata/552679ea44a9e400b600124f for an example. Feel free to use the HTML/CSS:

<div style="border:1px solid;position:relative;padding:1ex 1ex 1ex 11.1em;">
<div style="position:absolute;left:0;top:0;bottom:0; width:10em; padding:1ex;text-align:center;border:1px solid;margin:0 1ex 0 0;color:#000;background-color:#eee;font-variant:small-caps">
Part of Series x/y
</div><p>This kata is part of a series on !TOPIC!. Make sure you solve the [previous part][!LINK_TO_PREV!] before you try this one.</p>
</div>

Make sure to replace x, y, !TOPIC! and !LINK_TO_PREV!.