React Redux -- can I make mapStateToProps only take in part of the state?

Question

I want to make reusable modules that could be plugged in to any react-redux application. Ideally, my module would have a container component, actions, and reducer at the top level (and then any presentational components below the container). I would want the module to only work off its own slice of the app's state, and ideally to not have to know anything about the rest of the app state (so it's truly modular).

Reducers only work off of part of the state (using combineReducers), so I'm happy there. However, with container components, it seems like mapStateToProps always takes in the full state of the app.

I'd like it if mapStateToProps only took in the same "state slice" that I am handling in my module (like the reducer does). That way my module would truly be modular. Is this possible? I guess I could just pass that slice of the state down to be the props of this component (so I could just use the second argument of mapStateToProps, ownProps), but am not sure if this would have the same effect.

Answer 1

That is actually something of a complicated topic. Because Redux is a single global store, the idea of a completely encapsulated, fully reusable plug-and-play set of logic does become rather difficult. In particular, while the reducer logic can be fairly generic and ignorant of where it lives, the selector functions need to know where in the tree to find that data.

The specific answer to your question is "no, mapState is always given the complete state tree".

I do have links to a number of relevant resources, which may possibly help with your situation:

• There's several existing libraries that try to implement "per-component state in Redux". I have a list of them in my Redux addons catalog, in the Component State category.
• A group of devs have been discussing and prototyping various approaches to the "reusable logic module in Redux" concept. Their work is at https://github.com/slorber/scalable-frontend-with-elm-or-redux .
• Randy Coulman recently posted a three-part blog series related to state encapsulation and modularity in Redux. He didn't come up with definitive answers, but the posts are worth reading: Encapsulating the Redux State Tree, Redux Reducer Asymmetry, and Modular Reducers and Selectors.