feat(signals store): infer type from custom feature

[EinfachHans]

Which @ngrx/* package(s) are relevant/related to the feature request?

signals

Information

We have a quire complex custom feature which we apply to different stores to be able to gave them everything they need for a specific flow.

I have components / pages which needs a store, that have this feature set. Everything else doesn't matter.

So i need to infer the type of the store feature function, to correctly have this "part" of the final store, to access it's state, props, methods, ...

I can create a PR, but would need assistance!

Describe any alternatives/workarounds you're currently using

No response

I would be willing to submit a PR to fix this issue

• Yes
• No

[markostanimirovic]

Can you provide more details and a code example of what you're trying to achieve?

[EinfachHans]

Sure, let's say we have a custom feature like:

const initialState = {
  someStateProp: 1,
  _somePrivateProp: 0
};

export function withMyFeature<_>() {
  return signalStoreFeature(
    {
      props: type<{ requiredProp: Signal<boolean> }>(),
      methods: type<{
        someRequiredMethod(): string;
        _reset(): void;
      }>()
    },
    withState(initialState),
    withMethods((store) => ({
      someNewMethod() {
        // ...
      },
      reset() {
        // ...
      }
    }))
  );
}

I would then need to be able to infer the type of this feature that the store "has" when this feature is applied so basically:

{
  requiredProp: Signal<boolean>;
  someStateProp: Signal<number>;
  someRequiredMethod(): string;
  someNewMethod(): void;
  reset(): void;
}

When the store feature has non-private props or computed they should be included as well, so just everything that this feature brings to the table.

[EinfachHans]

@rainerhahnekamp Thank you! This definitely goes into the right direction imo, but for example private props/methods are also included in the type.

[rainerhahnekamp]

Is this better?

type SignalStoreFeatureType<
  Feature extends () => SignalStoreFeature<any, any>
> = ReturnType<Feature> extends SignalStoreFeature<infer Input, infer Output>
  ? Input & Output
  : EmptyFeatureResult;

type WithoutPrivates<Store extends object> = {
  [K in keyof Store as K extends `_${infer Name}` ? never : K]: Store[K];
};

type InferredFeature = WithoutPrivates<
  SignalStoreFeatureType<typeof withMyFeature>['methods'] &
    SignalStoreFeatureType<typeof withMyFeature>['props'] &
    SignalStoreFeatureType<typeof withMyFeature>['state']
>;

[EinfachHans]

@rainerhahnekamp state is not typed as a signal like that 🤔

[rainerhahnekamp]

Well, then one last try.

type SignalStoreFeatureType<
  Feature extends () => SignalStoreFeature<any, any>
> = ReturnType<Feature> extends SignalStoreFeature<infer Input, infer Output>
  ? Input & Output
  : EmptyFeatureResult;

type WithoutPrivates<Store extends object> = {
  [K in keyof Store as K extends `_${infer Name}` ? never : K]: Store[K];
};

type InferredFeature = WithoutPrivates<
  SignalStoreFeatureType<typeof withMyFeature>['methods'] &
    SignalStoreFeatureType<typeof withMyFeature>['props'] &
    StateSignals<SignalStoreFeatureType<typeof withMyFeature>['state']>
>;

StateSignals is exposed via @ngrx/signals.

[EinfachHans]

@rainerhahnekamp This seems to work quite good in my short test! Thank you for your effort so far!

Do you think this makes sense to be exported from the core package, so it's a one liner in applications. We have several cases where we use larger features to prepare stores's for different flows and in the flow don't care which store it is, it's only important that the store has everything that the feature brings with.