5.f4 ? is just bad. After 1.e4 e5 2.Nf3 Nc6 3.Nc3 Nf6 4.Nxe5 Nxe5 5.f4 Nc6 6.e5 Ng8 7.d4 d5 White has nothing for the piece.
thoughts on Halloween Gambit with 5.f4!?
#2
5 f4 or 5 d4 do not matter: after 4 Nxe5? white has nothing for the piece.
This is not true. It is not because the gambit is unsound that White has no compensation. He gets the center and a long time pressure with dangerous attacking possibilities.

White has nothing worth talking about after both 5...Nc6 6.e5 Ng8 7.d4 d5, AND 5...Ng6 5.e5 Ng8.
Methinks that the Halloween is playable only with a tempo less as Black: 1.e4 e5 2.Nf3 Nc6 3.Nc3 Nf6 4.g3 Nxe4!? although white can transpose to a regular g3 Vienna by returning the piece: 5.Nxe4 d5 6.Nc3 d4 7.Bg2 dxc3 8.bxc3 etc.
Note that in the 5. .. Nc6 line White has a terrible attack with 9.Qe2 instead of the classic move 9.d6.
1.e4 e5 2.Nf3 Nc6 3.Nc3 Nf6 4.Nxe5 Nxe5 5.d4 Nc6? 6.d5 Ne5 7.f4 Ng6 8.e5 Ng8 9.Qe2
Stockfish suggests 9. .. Bb4 but after 10.f5 N6e7 11. Bg5 he has nothing better than 11. .. Kf8
Then after 12. 0-0-0 White has complete compensation. He controls the center, has more development, and the Black King will stay in the middle for a long time.
because sometimes my opponent and even stockfish will play 4. ..., Ng6, i think f5 has a goal of harrasing the f6 Knight... but it opens up the kingside extra early and if 5. ..., Ng8 black could attack the white king similar to the king's gambit except the g1 knight doesnt exist. thoughts?