@ngrx/signals - Why can't DeepSignals handle nullability?

[james-vista]

We've recently migrated our app's state management from @ngrx/store to @ngrx/signals, and it's been great. However, there's one pain point that we keep encountering, and it appears other teams in our org and also some GitHub users have been running into it as well.

Issue

We commonly use optional or undefined state in our app, mostly in cases where data is not immediately available. As others have mentioned, this poses an issue with signal stores, as deep signals can't be made from undefined state:

type ExampleState = {
    user?: {
        name: string;
        email: string;
    };
    userLoaded: boolean;
};

const ExampleStore = signalStore(withState<ExampleState>({ userLoaded: false }));

// ...

inject(ExampleStore).user.name(); // Throws, as `user` is undefined

The same issue also applies to indexed types. (@ngrx/signals/entities exists for this use-case but is overkill a lot of the time.)

type ExampleState = {
    users: Record<string, {
        name: string;
        email: string;
    }>;
};

const ExampleStore = signalStore(withState<ExampleState>({ users: {} }));

// ...

inject(ExampleStore).users[id].name(); // Throws, as `users[id]` is undefined

Our workaround for this at the moment has been to invoke the signal before doing optional chaining, the obvious downside of which is losing the benefit of signals.

inject(ExampleStore).user()?.name;
inject(ExampleStore).users()[id]?.name;

Discussion

These limitations are due to the fact that toDeepSignal returns a regular signal instead of a DeepSignal when its value isn't a record at the time of creation.

I wrote a naive alternative implementation that acts more like building a lens over a state slice using optional chaining, shown below.
The main differences in this approach are:

• DeepSignal won't take the value of the property into account, so accessing a property on a DeepSignal will always return another DeepSignal
  • Means that nullable properties always return DeepSignal, but also means that any arbitrary property access returns a DeepSignal too.
• Keys won't be enumerable since they aren't defined on the target Signal

function toDeepSignal<T>(signal: Signal<T>): DeepSignal<T> {
    const cache = {};
    return new Proxy(signal, {
        get(target, prop) {
            // For Angular's `SIGNAL` symbol, there's probably a nicer way to do this
            if (typeof prop === 'symbol') {
                return target[prop];
            }

            return (cache[prop] ??= toDeepSignal(computed(() => target()?.[prop])));
        }
    }) as DeepSignal<T>;
}

Questions that I want to bring up:

• What's the rationale behind the current approach?
• What are the shortcomings with my naive approach?
• What's the intended way to handle nullable state objects (asynchronously loaded or otherwise) using signal stores?

Related to #4305