Looks good! I need to give more ateention to tricks like ||

Why do you use Array.prototype.reverse.call instead v.split('').reverse()?

Yeah! Now, I see that I could use "prev" variable instead "char" to return final result. It would be more sophisticated solution. Thanks for help!

You are not making use of the functionality of reduce (the accumulator or initial value is left unused). But it's nice to see someone else using reduce for this :-) Feel free to look at my solution how to use the accumulator.

I am so upvoting this comment! On my machine my script works excellent upon many tests, but here...

You should notice about "undefine" condition in kata description.