Merge two reducers in NGRX

Question

I am using @[email protected] and I have a reducer that is created in a factory as it is the same for several entities in my project.

function createEntityReducer(initialState, actions) {
  const reducer = createReducer(
    initialState,
    on(actions.loadAll.success, (state, action) => {
      ...
    })
  ); 
  return reducer;
}

const generalReducer = createEntityReducer(initialState, actions);

But I would like to extend this basic reducer for specific actions of a certain entity.

const specificReducer = createReducer(
        initialState,
        on(actions.loadChildren.success, (state, action) => {
          ...
        })
      );

How can I merge the two reducers, without having to create nested features, like this?

const reducer = mergeReducers(generalReducer, specificReducer);

Answer 1

After checking how the @ngrx/store/createReducer function works, I compose the below that seems to work correctly.

function mergeReducers<TState>(
  ...reducers: ActionReducer<TState, Action>[]
): ActionReducer<TState, Action> {
  const mergedReducers = (
    state: TState | undefined,
    action: Action
  ): TState => {
    if (reducers.length === 0) {
      throw new Error('At least one reducer was expected');
    }
    const newState = reducers.reduce((initialState, reducer) => {
      const intermediaryState = reducer(initialState, action);
      return intermediaryState;
    }, state);

    return newState as TState;
  };
  return mergedReducers;
}