is this considered a collapsible stock on a  AR15
i want one (NJ LEGALITY QUESTION)
or i might get it for my 870 shotty build

I would say this maybe in the gray areas of NJ law....Hell, all of the gun laws in NJ are gray to me but, from what I read the law only addresses stocks in 2 ways:

1. 3)A semi-automatic shotgun with either a magazine capacity exceeding six rounds, a pistol grip, or a folding stock.

2.  The NJAG guidelines on AW say one type of banned feature is a folding or telescoping stock.

Being neither an expert or lawyer I would say from looking at it, that the PRS is not folding or telescoping, thus OK to have, but you never know what some gung ho prosecutor may say, PK90 may be a good person to ask. I am sure he will chime in here. I would get official verification before purchasing just to CYA.

well i know for a shotgun it would be OK as you can have aq collapsible in NJ on a pump shotgun

In the closed position, it is about the same length as an A1 stock.  It's range of adjustment is about 1.5", IIRC.  I do not see any reason why this would considered a collapsible stock unless competition space guns are illegal in NJ.  

It looks legal to me.  I've seen shotguns with more adjustment than that.  The deciding factor on it being considered collapsible is probably the speed in which it can be changed.