Not sure if you're still interested in the hint, but try to think about what will happen if your function is called with these parameters:
nextItem([1,2,3,4], 4);
Good luck!

Duplicate unfortuantely.. http://www.codewars.com/kata/5262119038c0985a5b00029f/train/javascript

I would suggest trying to make this kata unique to all the others, I don't know how that would work but it would solve the fact that this is a duplicate kata..

You need to have some random tests in there so that solutions like the one @mattc did (i.e. cheating) won't be valid.

already exist, frankly there is so much wrong it's difficult to point all of them

Needs a lot more fixed tests and random tests

Duplicate of many others.

sometimes a generator is passed into xs so you have to invoke the next value from it using xs.next().value I believe.

I have no idea what this is asking me to do. The instructions are far too vague!
Some further help would be awesome.