Hard to say. Sometimes stories like this are a blend of real experiences and creative imagination. You might need to research the background of the book or its author to find out for sure.
I'd recommend 'Dark Love Pursuit'. In this novel, the heroine's stalker is someone from her past, and it leads to a series of intense and thrilling events. The author does a great job of building tension.
In such a fanfiction, it would be a very strange and chaotic scenario. Harry Potter comes from a world of magic and the Joker from a world of dark, madcap crime. If the Joker were to stalk Harry, Harry would likely be extremely confused at first. He might try to use magic to fend off the Joker, but the Joker's unpredictability would make it difficult. It could lead to a lot of psychological warfare as Harry tries to figure out the Joker's motives and protect himself and his friends.