The new rules language (which was already used by Cloud Storage) is a full programming language. It's not turing complete, but that's by design. Still, you ca

The new rules language (which was already used by Cloud Storage) is a full programming language. It's not turing complete, but that's by design. Still, you can declare functions and do other complex things that were never possible with the Firebase Realtime Database JSON rules. We're working on opening up the tools to work with the rules language so you can test and iterate more easily.