TypeScript: combineReducers returns reducer that accepts partial state

[OliverJAsh]

combineReducers returns a reducer that accepts partial state, not the whole state.

[OliverJAsh]

It seems a shame to break apart from the Reducer type here, but without adding an extra generic to Reducer, I don't see a way around it?

[aikoven]

We could consider adding a type

type CombinedReducer<S, A extends Action = AnyAction> = 
    (state: Partial<S> | undefined, action: A) => S

It would be assignable to Reducer<S, A>, so you'd be able to use it anywhere like a regular Reducer.

Although when you pass it to a generic function with S as a type parameter, it is inferred to Reducer<Partial<S>>. Particularly, when nesting combineReducers:

declare const reducer: Reducer<number>;

const combined = combineReducers({
  parent: combineReducers({
    child: reducer,
  }),
});

typeof combined === (
  state: Partial<{parent: Partial<{ child: number }>}> | undefined,
  action: AnyAction
) => {parent: Partial<{ child: number }>}

combined(undefined, {type: ''}).parent.child  // number | undefined, but expected number

[OliverJAsh]

Thinking about this some more, I think the state parameter type needs to use something like DeepPartial, to allow partial state to be passed in when using nested combineReducers.

Although DeepPartial is problematic in itself: #2808

[timdorr]

Since this stalled, I'm going to close it out.