I was just wondering something - a magazine is composed of parts - outer shell, spring, follower, base plate, etc...

If you dont have all the parts for magazine - is it still a magazine?  Say you dont have the follower, spring and base plate. Is it still a magazine?

A flute with no holes, is not a flute. A donut with no hole, is a Danish.

In New Jersey I believe it is “Constructive Intent”. As in if you have all the parts in your possession then you have it, whatever that it may be. ??

No - because the definition of a high-capacity magazine includes it being able to feed ammunition continuously into a semi-automatic firearm.  There is nothing in statute that says having all of the parts to readily assemble a magazine is illegal, as there is for assault firearms.

A magazine without the spring is just a box...  it doesn't meet the NJ legal definition of ammunition magazine.

So that means if I have some of the parts but not all the parts its no longer a magazine as it cant be assembled into one?

That's the way I read the statute:

2C:39-1y "Large capacity ammunition magazine" means a box, drum, tube or other container which is capable of holding more than 10 rounds of ammunition to be fed continuously and directly therefrom into a semi-automatic firearm.

And that definition is why, on appeal, Brian Aikens's conviction of possession of a large capacity ammunition magazine was overturned - the state proved that he was in possession of it, and they proved that the magazine held 16 rounds, but they did not prove (or even verify themselves) that the magazine was capable of feeding the ammunition "continuously and directly therefrom into a semi-automatic firearm".

* not a lawyer, not legal advice, just a lay opinion, yada yada etc.