When developing in stateless node you don't want data structures' data in memory. You want your arrays read from and written directly to non-volatile storage. The mongodb driver for node.js supports this, with some limitations. You can't do this with SQL. In addition, with replication, mongodb scales like a dream. However, you can write only to the primary mongo node:
https://docs.mongodb.com/manual/replication/
https://github.com/williamkapke/mongo-mock