The Joker is purely fictional. It's a creation of the imagination in the world of comics and movies. There's no real-life basis for it. The character and the plotlines are crafted to engage and entertain the audience through imaginative storytelling.
Well, it's sort of based on a true story. The movie draws on the concept of how society can marginalize people, which is a real - life phenomenon. It takes this general idea and creates the character of the Joker, who is a product of a cruel and neglectful society. It's not a direct retelling of one specific true story, but rather a fictional exploration of real - world problems.