Here "magic weapon" is described as a spell, and the effect it has on the weapon is decided through what spell slot you used to cast the spell.
You touch a nonmagical weapon. Until the spell ends, that weapon becomes a magic weapon with a +1 bonus to Attack rolls and Damage Rolls.
At Higher Levels: When you cast this spell using a spell slot of 4th level or higher, the bonus increases to +2. When you use a spell slot of 6th level or higher, the bonus increases to +3.
Here it says
This Javelin is a Magic Weapon. (.....) The javelin's property can't be used again until the next dawn. In the meantime, the Javelin can still be used as a Magic Weapon.`
But how is the javalin used as a magic weapon when there's no text to describe it?
Should we assume it is a magic weapon lvl 1, and I should just add +1 to my attack and damage rolls?
This answer on stackexchange: What is the casting level of magic items? eg, Javelin of lightning
Explains that the spells from a weapon should be cast on the lowest possible level, but I'm not sure if that answers my question well enough