Like the weapon.  How do we get the 4D6 lightning damage to show in the damage roll?  (It just shows the javelin damage) 

Why are people assuming this does any regular bonus damage? Where does it say that?

Its a magical javelin that does normal javelin damage. Once per rest you can use its properties to conjure a mini-ligtning bolt and as a part of that action you can make a ranged attack with bonus damage.

There is no added damage on regular hits, unless you homebrew it.

Path of the Giant barbarian pairs great with this, because you can just chuck it and it immediately reappears in your hand.

if I say the command word then cast catapult on it would it still do the extra damage and AOE affect

If the target is more than 30 feet away, remember that you need to roll the ranged weapon attack with disadvantage since it's at long range.

No, there is no disadvantage. As it says that it transforms into a bolt of lighting and only materializes back into a javelin when it reaches the target. So the original javelin range doesn't apply

It says to make a “ranged weapon attack,” which means you make an attack with the javelin itself. Anything in the line when it’s a lightning bolt doesn’t apply, but the target does. 

I think you are correct. But would also be DMs decision. I mean you don't exactly throw the javelin 120ft because of the transformation.

I'm giving this to my players for their next quest rewards.

Each of them will get a Magic Item. I'm giving this to the Paladin or the Fighter. Not sure yet, one is getting Adamantine Chain Shirt and the other is getting the Javelin.

I'm adding a return property to the player, but it requires attunement to use as a payout.

Otherwise, I'll see how this goes. The session is sometime next month (May), so I'll update how they perform with this later on.

What command word have y'all used to make it transform?

Would this work with or against a Path of the Giant's Elemental Cleaver feature? Have a character in the works that if I could get my hands on one of these, I could have some fun.

Mastery is on the 2014 Version. This is a failure of implementation.

My players use "Spark on" :D

It's in the Phandelver & Below campaign module, so it's good at fifth/sixth level.   Could probably be lower as well.