Fix unknown action type handling

[msafi]

As discussed in #1054, and if I understand correctly, the private action handling test is not really possible if the user has access ActionTypes.INIT.

So, I changed the way combineReducers inits reducers by making it do a random probe and undefined state checks simultaneously.

I modified the tests to make sure combineReducers implementation throws if the given reducer could potentially return undefined either for an undefined state or an unknown action type.

[gaearon]

Hey, sorry for not getting back to this sooner. This change still kinda eludes me sometimes. Can you clarify:

1. What would be the meaning of this? What are the scenarios it protects against, and why is it better than what we have right now?
2. Is this a breaking change? Can it possibly break any projects?
3. Does it have any impact on Redux DevTools or related tooling? Can you try searching @@redux/INIT in GitHub projects to check that?

Thanks.

[msafi]

Hey Dan,

1. So, in the current code, Redux does two checks separately. First, it checks that if undefined state is passed, the reducer knows to return a value. Second, it checks that if unknown action type is passed, the reducer knows to return a value. My change combines these checks, makes the error message clearer, and simplifies the code. It doesn't protect against anything.
2. I don't see how it could be a breaking change. It's the same functionality, only simplified.
3. Regarding Redux DevTools, yes, it would break it! And possibly other tooling projects? 😢 To solve this, I could restore PROBE_UNKNOWN_ACTION, and have combineReducers use it to assert reducer sanity.

All in all, this isn't an important change. I just made it as a quick first contribution and it is turning out to be more confusing than it's worth, so feel free to close it! 😄

[timdorr]

To solve this, I could restore PROBE_UNKNOWN_ACTION, and have combineReducers use it to assert reducer sanity.

This sounds like the more sane solution to me. Having a separate probe action from the core init action avoids problems when trying to overload its purpose like this.