The Joker's Real Name

Jack Nicholson as Jack Napier

The 1989 Tim Burton Batman film uses the name Jack Napier but, that's never been used in the comics or in anything since.

Officially nobody knows his name, probably not even himself.

Aliases:
Clown Prince of Crime  Harlequin of Hate Public Enemy #1 Jack Napier Mr. Rekoj Nemesis of The Knight The Bandit Mr. Face Paint Patient 0 Patient 223 Red Hood Joe White Jocund Jack of  Jocund Jack of All Crimes Eric Border Oberon Sexton Red Hood One

The Joker was created by Jerry Robinson

If you have a better answer please do tell me in the comments box bellow. Have a great day in Gotham City