I'm not entirely sure specifically what 'konosuba ahead of light novel' is about without more context. It could potentially be a spin - off, an adaptation that goes ahead of the original light novel's story, or some sort of special project related to the Konosuba series.
The Konosuba light novel is not a fanfic. It has its own unique plot, characters, and world-building developed by the author's imagination and creativity.
The Konosuba light novel is good. It has a charming mix of fantasy elements and comedic situations that make it highly enjoyable. The character development is also well done.