# Mapped Sub States

NGXS provides the ability to merge multiple dynamic selectors into one.

Let's look at the code below:

```ts
interface Animal {
  type: string;
  age: string;
  name: string;
}

@State<Animal[]>({
  name: 'animals',
  defaults: [
    { type: 'zebra', age: 'old', name: 'Ponny' },
    { type: 'panda', age: 'young', name: 'Jimmy' }
  ]
})
@Injectable()
export class ZooState {
  static getPandas(age: string) {
    return createSelector([ZooState], (state: Animal[]) => {
      return state.filter(animal => animal.type === 'panda' && animal.age === age);
    });
  }

  static getZebras(age: string) {
    return createSelector([ZooState], (state: Animal[]) => {
      return state.filter(animal => animal.type === 'zebra' && animal.age === age);
    });
  }

  static getPandasAndZebras(age: string) {
    return createSelector(
      [ZooState.pandas(age), ZooState.zebras(age)],
      (pandas: Animal[], zebras: Animal[]) => {
        return [pandas, zebras];
      }
    );
  }
}
```

This construct will merge 2 dynamic selectors and memoize the result.

Another example could be multiple Zoos in our application:

```ts
interface Animal {
  type: string;
  age: string;
  name: string;
}

interface ZooStateModel {
  [id: string]: {
    animals: Animal[];
    ageFilter: string;
  };
}

@State<ZooStateModel>({
  name: 'animals',
  defaults: {
    zoo1: {
      ageFilter: 'young',
      animals: [
        { type: 'zebra', age: 'old', name: 'Ponny' },
        { type: 'panda', age: 'young', name: 'Jimmy' }
      ]
    }
  }
})
@Injectable()
export class ZooState {
  static getZooAnimals(zooName: string) {
    return createSelector([ZooState], (state: ZooStateModel) => state[zooName].animals);
  }

  static getPandas(zooName: string) {
    return createSelector([ZooState.getZooAnimals(zooName)], (state: Animal[]) => {
      return state.filter(animal => animal.type === 'panda' && animal.age === 'young');
    });
  }

  static getPandasWithoutMemoize(zooName: string) {
    return createSelector([ZooState], (state: ZooStateModel) => {
      return state[zooName].animals.filter(
        animal => animal.type === 'panda' && animal.age === 'young'
      );
    });
  }
}
```

In that example merging is required to avoid unnecessary store events. When we subscribe to `Zoo.getPandasWithoutMemoize` store will dispatch event whenever `ZooState` will change (even `ZooState.getAgeFilter`), but when subscribing to `Zoo.getPandas` store will dispatch event only if result has been changed.


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