Just bought this - can't wait to try it out!  Will post updates :)

Here's a fun interaction:
As per the Lightning Bolt function, the Javelin of Lightning literally turns into a bolt of lightning.
This means that anyone hit by that lightning, even if they make the save, is being hit by the weapon itself, and probably taking damage from it.
Anything that would activate on hit or on dealing damage with the javelin therefore carries over to the lightning, because the lightning is the javelin.

If nothing else, Weapon Mastery allows you to Slow all targets in a 120ft Line, without having to worry about rolling to hit or making saving throws.
I'm sure there are plenty of interesting things you can stack that require hitting or dealing damage with a weapon.

Situational? Yep. Broken? Probably not.
Kinda cool though.

I would question that working as you state.   D&D is usually pretty specific about terminology,  you cannot hit something unless you roll an attack on it, which is why the descriptions says "If you hit a creature with this weapon and deal damage to it" this would imply that first you need to roll a hit, not the enemy failing a save, and do damage, so if there is immunity for some reason it doesn't work either.   I looked a bit for a clarification out there and didn't see one. Also if there is a debate at your table it is going to be up to the DM to decide that.   Whichever way you go with that ruling, seems like a fun weapon.

Fair point!
Typical problem with ambiguity between 'hit' meaning a 'roll to hit' or the regular interpretation. Would be straight-forward if the Slow mastery property used wording like Cleave or Graze, which specify an "attack roll" to trigger.

Is this intended to return to the users hand/reappear if you just throw it as a normal attack?
Seems pretty useless to throw it once, then have to go get it every time, and only when you use its special property does it automatically return to the hand. 

Quote from TelxoF >>

Here's a fun interaction:
As per the Lightning Bolt function, the Javelin of Lightning literally turns into a bolt of lightning.
This means that anyone hit by that lightning, even if they make the save, is being hit by the weapon itself, and probably taking damage from it.
Anything that would activate on hit or on dealing damage with the javelin therefore carries over to the lightning, because the lightning is the javelin.

If nothing else, Weapon Mastery allows you to Slow all targets in a 120ft Line, without having to worry about rolling to hit or making saving throws.
I'm sure there are plenty of interesting things you can stack that require hitting or dealing damage with a weapon.

Situational? Yep. Broken? Probably not.
Kinda cool though.

this is not correct. the lightning bolt does not "hit" anything. there is no attack roll. it is a dexterity saving throw. all of the examples you provide (weapon damage, weapon mastery, other weapon based properties) are contingent on hitting with a weapon. 

I believe you are correct. Typically, a "hit" necessitates some form of "attack" or "attack roll," but I cannot find a rule that specifies a "hit" must be tied to those things.  Looking around a bit, I've found a similar interaction to the one you are describing.

The Monk Warrior of the Open Hand subclass has the level 3 feature Open Hand Technique that states, "Whenever you hit a creature with an attack granted by your Flurry of Blows." Flurry of Blows makes Unarmed Strikes, which are melee attacks where the Grapple and Shove options require saving throws instead of attack rolls. The design intention seems to be that you can use Open Hand Technique in combination with Grapple and Shove. 

Hitting with Flurry of Blows as a result of a failed Dex save against Shove or Grapple seems very similar to hitting with the Javelin of Lightning as a result of a failed Dex save against its Lightning Bolt feature.