Files on disk are the original database.
NodeJS is good at files.
path libraries have some slick and efficient ways of reading and writing files.
Rather than go through the full API, it's best to look at sample code like that inside the Express Blog Codealong (github repo)
Remember that most of Node's I/O functions are asynchronous, so you may need to use callbacks, Promises, and/or async-await.
Heroku's Ephemeral Filesystem
you can write files to the
they will stay there for a while, but will disappear when your server restarts
...so for apps on Heroku at least, we need a better solution for storing data long-term