Quick thoughts on composing parts of Redux

redux-composition.md

Redux reducers can compose in a couple of different ways:

combineReducers creates a new state tree with the composed reducers owning/controlling leaves (subtrees).

Or a straight merge, something like:

const reducer = (state, action) => ({
    ...reducerA(state, action),
    ...reducerB(state, action)
});

Or sequential ("traditional"?) composition:

const reducer = (state, action) => 
    reducerB(reducerA(state, action), action);

Both of the merge styles are disaster-prone, as we could imagine...

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Actions/Action-creators (ignoring middleware) compose as a straight union of the set of all actions.

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A possible composition of Redux stores into a new store:

• The composed store's dispatch sends all actions to all composed stores.

• getState's value composes in the same way as reducers, above.

• subscribe would act as if you passed the same subscription callback to each store individually.