Angular NgRx Selector returns undefined

Question

Here is my app.state.ts

export interface AppState {
  getCards: Card[];
  getFlippedCards: Card[];
  currentPlayer: boolean;
  firstPlayerScore: number;
  secondPlayerScore: number;
  flipped: boolean;
  error: string;
}
export const initialState: AppState = {
  getCards: [],
  getFlippedCards: [],
  currentPlayer: false,
  firstPlayerScore: 0,
  secondPlayerScore: 0,
  flipped: false,
  error: '',
};
const appState = (State: AppState) => State;


export const getCards = createSelector(appState, (state) => state.getCards);
export const getFlippedCards = createSelector(
  appState,
  (state) => state.getFlippedCards
);
export const currentPlayer = createSelector(
  appState,
  (state) => state.currentPlayer
);
export const firstPlayerScore = createSelector(
  appState,
  (state) => state.firstPlayerScore
);
export const secondPlayerScore = createSelector(
  appState,
  (state) => state.secondPlayerScore
);
export const flipped = createSelector(appState, (state) => state.flipped);
export const error = createSelector(appState, (state) => state.error);

app.module.ts:

imports: [
    BrowserModule,
    AppRoutingModule,
    StoreModule.forRoot({ AppReducer }),
  ],

app.reducer.ts

export function AppReducer(state = initialState, action: AppActions): AppState {
  switch (action.type) {
    case ActionTypes.Load:
      return { ...state, getCards: action.payload };
    case ActionTypes.ToggleFlip:
....

app.actions.ts

export enum ActionTypes {
  Load = 'Load Cards',
  ToggleFlip = 'Flip a Card',
}
export class ToggleFlip implements Action {
  readonly type = ActionTypes.ToggleFlip;
  constructor(public payload: Card) {}
}
export class Load implements Action {
  readonly type = ActionTypes.Load;
  constructor(public payload: Card[]) {
    console.log('load app.actions ', payload);
  }
}
export type AppActions = ToggleFlip | Load;

However, I cannot retrieve any information from the store, this.cards$ is undefined from the selector:

app.component.ts

this.store.dispatch(new fromActions.Load(this.cards));
this.cards$ = this.store.pipe(select(fromState.getCards));

The store is being populated with the array just fine. So there is no problem there, I think my app.state.ts selectors are not set up properly (or maybe the part in app.module.ts). Where did I do wrong?

Andrei Gătej Andrei Gătej · Accepted Answer · 2020-05-15T04:48:16

By declaring your store like this: StoreModule.forRoot({ AppReducer }), as far as I can tell, your state object would look like this:

{
 AppReducer: {
   getCards: ...,
   getFlippedCards: ...,
 }
}

Meaning that const appState = (State: AppState) => State; will return the above object, which does not have any other properties, except AppReducer.

As a rule of thumb, each property(slice) of the store must be associated with a reducer. So, if you have: StoreModule.forRoot({ a: aReducer, b: bReducer, c: cReducer }), your state would look like this:

{
 a: ...,
 b: ...,
 c: ...,
}

I think of the entire state as a pie and each slice is a feature.

A quick fix would be this:

export interface AppState {
  featureName: {
    getCards: Card[];
    getFlippedCards: Card[];
    currentPlayer: boolean;
    firstPlayerScore: number;
    secondPlayerScore: number;
    flipped: boolean;
    error: string;
  }
}

const featureState = createFeatureSelector<AppState>('featureName');

export const getCards = createSelector(featureState, (state) => state.getCards);
/* ... */

Angular NgRx Selector returns undefined

1 Answers