Flash
When this Equipment enters, attach it to target creature you control.
During your turn, equipped creature gets +2/+0 and has first strike.
Equip (: Attach to target creature you control. Equip only as a sorcery.)
(10/06/2022)
Javelin of Lightning doesn't enter the battlefield attached to a creature. Instead, the Equipment enters the battlefield and then a triggered ability attaches it to a creature. You may cast Javelin of Lightning even if you don't control any creatures.
(10/06/2022)
Attaching an Equipment with its enters-the-battlefield triggered ability isn't the same as using its equip ability. You don't pay mana for the attachment, and the timing restrictions for equip abilities don't apply.
(10/06/2022)
If the target creature becomes an illegal target, the Equipment remains on the battlefield unattached.