Reaction fanfiction is basically fanfiction where the focus is on the reactions of the characters. It can be really interesting as it gives a new perspective on the existing characters and their relationships. Writers create scenarios where the characters are put in unexpected situations and then write about how they would respond. This could be in response to seeing their future selves, or something completely out of the ordinary in their fictional world.
Reaction fanfiction is a type of fan - made story. It often involves characters from a particular fandom reacting to something new, like a new plot development in the original work, a different version of their story, or even modern things like pop culture references. For example, if it's a Harry Potter reaction fanfiction, the characters might react to a 'what if' scenario such as what if Voldemort had won the war.
Reaction fanfiction is a form of fan - created writing. In it, the author imagines how the characters from a certain franchise would react to various things. It could be as simple as how they'd react to a new character being introduced, or more complex like their reaction to a change in the laws of their world. It allows fans to explore different aspects of the characters' personalities that might not have been shown in the original work.