React is a great fit for Leporello.js. There is an example of React TODO app that you can write and debug in Leporello. It is showcased in the video. You can play with it yourself if follow the link https://app.leporello.tech/?example=todos-preact
Speaking about backend frameworks, basically you code is a function Request -> Response. How do you organize your code is up to you. You can code it in a functional manner. What is great about Leporello, is that it remembers all the calls your app made to databases, other microservices and external APIs and allows to debug them in a time-travel manner, seeing requests and responses. You can run your code once, and then debug and navigate it forward and backward, seeing runtime values that were generated when your code was executed. It a huge time saving, especially when external resources are slow and may require complex setup or teardown before or after being called.
>> It feels like leporello has to store a lot of possible branch-outs of the state
Could you please clarify, what do you mean by branch-outs of the state?