Reducer helper functions

reducerhelpers.ts

export type Action = { type: string };
export type Reducer<S, A extends Action> = (state: S, action: A) => S;


export type ReducerByAction<S, A extends Action> = A extends any
  ? Partial<Record<A["type"], Reducer<S, A>>>
  : never;

export type ReducerByKey<S, A extends Action> = Partial<{
  [SK in keyof S]: Reducer<S[SK], A>
}>;

export const reducerByAction = <S, A extends Action>(
  handlers: ReducerByAction<S, A>
): Reducer<S, A> => (state, action) => {
  const handler: Reducer<S, A> = handlers[action.type] ?? identity;
  return handler(state, action);
};

export const reducerByKey = <S, A extends Action>(
  handlers: ReducerByKey<S, A>
): Reducer<S, A> => {
  const keys = Object.keys(handlers) as (keyof S)[];
  return (state, action) => {
    return keys.reduce((s, k) => {
      const skReducer = handlers[k]!;
      const sk = skReducer(s[k], action);
      return { ...s, [k]: sk };
    }, state);
  };
};

  
// Example
  
type S = { foo: { bar: number } };
type A = { type: "inc" };

reducerByKey<S, A>({
  foo: reducerByKey({
    bar: reducerByAction({
      inc: (bar, inc) => bar + 1
    })
  })
});