Must there be a type assertion in non-initial reducers?

Question

Since the type of of a reducer is defined as

export type Reducer<T> = (state: T | undefined) => T | undefined;

In my reducers that are not the initial reducer, I must declare

state = state as State

Am I missing something, or is this considered a minor inconvenience, please?

Answer 1

Non-initial reducers can be typed (in TypeScript) as (state: T) => T and these will be compatible with the Reducer<T> type found in the library. Here is an example from my codebase, the first snippet is an initial reducer that needs to treat the case for undefined:

const initReducer$ = xs.of(function initReducer(prev?: State): State {
  if (prev) {
    return prev;
  } else {
    return {
      selfFeedId: '',
      currentTab: 0,
    };
  }
});

This second snippet is a non-initial reducer where I am sure the previous state is not undefined:

const setSelfFeedId$ = ssbSource.selfFeedId$.map(
  selfFeedId =>
    function setSelfFeedId(prev: State): State {
      return {...prev, selfFeedId};
    },
);

Notice that when these streams are merged, the resulting type can be Stream<Reducer<State>> with no casting involved:

const reducer$: Stream<Reducer<State>> = xs.merge(initReducer$, setSelfFeedId$);