redux-define | Define action constants for Redux | State Container library

So there are at least two issues here, I'll try to explain them one by one, I'm not sure how these frameworks you're using interact but here are a few points that will at least fix your issue.

1. Your reducer is trying to extract { baseCurrency } but this is not a property of your action. You instead need to extract the payload here, like this: { payload }, this payload value will contain your baseCurrency, and to properly save it in the reducer you should return { ...state, baseCurrency: payload }

2. Your selectors are trying to read directly from the state variable, but this one contains your reducers under the keys you sent to combineReducers, in your case you called your reducer reducer, thus you need to select state like this state => state.reducer.baseCurrency

See my fork of your Sandbox where I've fixed the baseCurrency case for you: https://codesandbox.io/s/todoapp-with-redux-and-normalized-store-ih79q