Bruce doesn't know about Joker's failed comedian backstory. All he knows is that the Joker was the Red Hood guy that fell into the chemical lake in Ace Chemicals, but that's it. That's very little information on who the Joker was prior to his transformation. Bruce has even said he knows very little about the Joker's origin in Batman: Hush Issue #7.
Originally posted by Galan007
Ah, so Bruce has always known Joker's true identity ever since they first met...cuz Batman... but just never told anyone.Nice? srsly
I honestly took it to be Johns trying to show that, as Batman's identity doesn't matter to the Joker, Joker's identity doesn't really matter to Batman either.
Bar protecting the family at least.