Fix combineReducers typings.

[gilbsgilbs]

PR Type

Does this PR add a new feature, or fix a bug?

Fixes a bug.

Why should this PR be included?

It fixes a typing issue I and some other folks have with the combineReducers function: #2709

Checklist

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Is there an existing issue for this PR?
  • combineReducers is broken with TypeScript 2.6.1 and Redux 3.7.2 #2709 (it is closed, but many people are still affected)
• Have the files been linted and formatted?
• Have the docs been updated to match the changes in the PR?
• Have the tests been updated to match the changes in the PR?
• Have you run the tests locally to confirm they pass?

Bug Fixes

What is the current behavior, and the steps to reproduce the issue?

Unfortunately, I failed to create a simple reproduction example. Still, it happens that TypeScript would fail to infer the proper generic type to combineReducers function. In my case, I'm doing something like this:

interface FooState {…}
interface AppAction extends Redux.Action<string> {}
interface FooAction0 extends AppAction {…}
interface FooAction1 extends AppAction {…}
type FooActions = FooAction0 | FooAction1;

function fooReducer(state: FooState, action: FooActions): FooState {
  return state;
}

// Type error:
// Argument of type 'Reducer<CombinedState<{ fooReducer: FooState; }>, FooActions>' is not assignable to parameter of type 'Reducer<CombinedState<{ fooReducer: FooState; }>, AnyAction>'.
combineReducers({ fooReducer: fooReducer });

// works
combineReducers<{fooReducer: FooState}>({ fooReducer: fooReducer });

What is the expected behavior?

I would expect TypeScript to infer a generic type automatically.

How does this PR fix the problem?

It removes explicit anys which appears to help TypeScript to infer a type. I'm not sure why though 🤷‍♂️ . Could this be a bug in TypeScript?

More specifically, the first generic type of ReducersMapObject is a state which is already any by default, and the second generic type is A extends Action which is Action by default and not any. So the fact that the second generic type is any confuses TS for some reason.

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[timdorr]

Should ReducersMapObject's second arg default be AnyAction instead?

[gilbsgilbs]

Should ReducersMapObject's second arg default be AnyAction instead?

It sounds reasonable to me. I have checked against my local project though and it doesn't improve the type inference in this case.

I also see another usage of ReducersMapObject<any, any> in StateFromReducersMapObject. Should I also update this one to ReducerMapObject without specifiying generics?

[gilbsgilbs]

@timdorr I don't want to be "that guy" you know, but is there anything I can do to help getting this merged and released anytime soon? 😉

[jonjaques]

Can confirm that this fixes a regression we experienced recently. Specifically, our thunk dispatch type was conflicting w/ a third party reducer (TS complaining about actions missing type & payload.

Thanks @gilbsgilbs!

Erm, edit: Unfortunately this is still conflicting w/ one less property "type"

[timdorr]

Only thing this needs is some tests!

[timdorr]

I guess I'll (finally) merge this in without tests. Thanks!