I like this idea. I've also written a couple of chat apps over the years and converged on a JSON flat-file for storing data as well. I believe I used node-json-db [0] as a wrapper on top of the file, though it's pretty cool this author seems to have written their own from scratch. I think this is an excellent solution for projects like these.

https://www.npmjs.com/package/node-json-db

It's not better. It's just different.

I think SQLite would have been a more practical choice.