Fiction. The book has elements that are clearly the product of creative writing. For example, the unique voice of Holden Caulfield, his adventures in New York City which are filled with strange encounters and his rather extreme reactions to the people and situations around him. All these aspects point to it being a work of fiction rather than a non - fiction account of real events or people.
It's fiction. The story is a fictional account of Holden Caulfield's experiences.