What was th e joker's true name?

Answer 1

Short answer: he doesn't have one. Long answer:

It's changed throughout the years, but for the post Denny O'Neil era, he didn't have one. A major part of Alan Moore's The Killing Joke is Joker flashing back to memories of himself as a struggling comedian who had "one bad day," but Joker himself discounts the flashbacks at the end with the statement "sometimes i remember it one way, sometimes it's another. If I'm going to have a past, i prefer it be multiple choice." Whilst there have been events in recent years to establish the past given in TKJ as cannon, the general understanding is that Joker himself doesn't know who he was before anymore. The Dark Knight references Joker's shifting memories with the various stories about his scars.

Answer 2

In most Batman comic book and movie portrayals, the Joker's true name remains unknown or ambiguous. There have been various origin stories proposed, but the character's creators have intentionally left his true identity mysterious to maintain his enigmatic and unpredictable nature.