Justice League #42, the latest chapter in Geoff Johns and Jason Fabok's' Multiverse-spanning "Darkseid War," has turned Batman into a god-like being who can learn the answer to anything — and as such, he knows the secret identity of the Joker.
The new "Bat-God" would actually be most accurately described as a New God, as he took over the Mobius Chair from Metron. The Mobius Chair is playing a significant role in "Darkseid War," since Johns has now revamped the Anti-Monitor's history to make him the original Mobius who occupied the chair.
The Mobius Chair, which has been part of New Gods mythology since Metron first appeared in 1971, is a time-space vehicle that apparently stores vast knowledge, and when Batman took over the chair from Metron in Justice League #42, he learned answers to questions from throughout the Multiverse.
To test its knowledge, Batman asks the chair, who killed his parents? Yep, the chair knew it was Joe Chill.
Then he asked, what's the Joker's true name? (We can only imagine that, if the Joker reading this issue, he'd say, "Oh, Bruce… you care. You really care.") But when Batman learns his name, he doesn't believe it and denies that it can be true.
And Batman doesn't share the name he learned and apparently doubts its accuracy. Will Johns or another writer follow-up on this in the Bat-titles or somewhere else? It remains to be seen.
Outside of the emergence of "Bat-God," the issue also showed Superman helping Lex Luthor and informing him that his sister was the one who shot him. The two, who are now on Apokolips, will presumably be teaming up as the story continues for an as-yet-unnamed period of time in Justice League. After all, Darkseid has unleashed his slaves to kill Superman.
Meanwhile, Mister Miracle is getting to know Myrina Black, and although they'd both like to see Darkseid destroyed, the New God doesn't agree with Myrina's planned method of taking down the dark god. He doesn't want to see the Earth destroyed in a war between the Anti-Monitor ("Mobius") and Darkseid, but Myrina is obviously obsessed with Darkseid's downfall.
The issue concludes with the League being saved from Grail (Darkseid and Myrina's daughter) by the entrance of Metron. Although he stops the battle, he tells the Justice League that they should give up and abandon their planet. Of course, the League isn't willing to do that.
Instead, Wonder Woman wraps her lasso around Metron to get the truth, and she ends up removing him from his chair. After it's discovered that Metron doesn't have the answers — the chair does — Batman jumps into the chair and becomes Bat-God.
Batman always was prepared. Now he knows everything.