One of my players approached me with this custom trait and I honestly don't know how to deal with it. I do not know if this, or even if it is too strong or underwhelming. So I have popped on here to ask for people's opinion on this custom trait and whether it is balanced, or what changes I could make to it to bring it back into balance.

Javelin Master:
- If you score a critical hit with a melee javelin attack, you can reroll the damage dice. If you reroll the dice, you can use either result.
- If you move at least 10 feet toward a target before making a ranged javelin attack, you make a Str check contested by the opponents Str or Dex. If they fail, they are knocked prone
- Attacking at a long range with a javelin does not impose a disadvantage on the attack roll.

I'm not sure what you mean by "trait," but as a feat, this seems balanced.  For the prone ability, I would limit it to size Large or smaller creatures.

The main problem here is the contest. If you're subjecting an enemy to a harmful effect, that should be a saving throw. It should also have a size restriction; you won't be knocking ancient dragons prone with that toothpick.

For whatever it's worth I'm not feeling the second bullet point at all; doesn't seem that thematic. I'd consider giving it a small damage bonus instead. I'd even go so far as to make that particular benefit work with any thrown melee weapon. Most niche feats have one benefit that's more broadly applicable.

I feel like the ability to knock them prone with a javelin seems a little odd. If I am remembering correctly a javelin is a piercing weapon, so throwing it harder and with more buildup should only improve its piercing, not risk knocking the target prone. 

I would maybe consider replacing this part of the feature with something else that is still helpful, but maybe makes more sense with how it works. 

IDK, I could be completely wrong, but I haven't heard about anyone being pushed to the ground by a javelin before.