Joker: Folie à Deux is set in the same universe as Batman, technically. It takes place in Gotham City, and obviously the Joker is one of Batman's most famous enemies. But Batman himself doesn't really appear in these Joker movies, save a young Bruce Wayne turning up in the first film. Also, at the end of Folie à Deux, Arthur Fleck, aka the Joker, is stabbed to death in prison by a man identified in the credits only as "Young Inmate," played by Connor Storrie. So obviously Joaquin Phoenix isn't going to be fighting Batman anytime soon.

One explaination is that these Joker movies borrow things from the Batman mythos but don't actually take place in that same universe. Maybe this is a world where the Joker rises and falls without ever actually encountering the Caped Crusader. But there might be a way for the movies to have their cake and eat it,...