Javelin of Lightning

• WOTC Official Reminder Textofficial rules
  Equip — Equip [cost] ([cost]: Attach to target creature you control. Equip only as a sorcery.)
• WOTC Official Reminder Textofficial rules
  Flash — You may cast this spell any time you could cast an instant.

• WWizards of the Coast2022-06-10
  Official Ruling
  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.
• WWizards of the Coast2022-06-10
  Official Ruling
  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.
• WWizards of the Coast2022-06-10
  Official Ruling
  If the target creature becomes an illegal target, the Equipment remains on the battlefield unattached.

Rulings via Scryfall