Definitely fiction. 'Dear Dumb Diary' is classified as fiction because it builds a fictional world and presents fictional events and characters to captivate the readers' imagination.
It's not historical fiction. The book mainly presents the typical life and problems of a current-day youngster, rather than having any historical context or elements.
Diary fiction is a literary genre where the narrative is structured like a diary. It has a unique charm as it mimics real - life journal - keeping. The language used in diary fiction is usually quite personal and can range from simple, everyday expressions to more complex and emotional outpourings. It's a great way for authors to explore the psychological depths of their characters in a more natural - seeming way compared to other narrative forms.
The 18 Diary was a form of literature that could record one's thoughts, feelings, and experiences. We can find some content related to the diary, but there is no specific mention of "18 Diaries". Therefore, based on the information provided, I am unable to answer this question.