@ngrx/signal 20 patchState removes deepSignal properties when partial object is passed

[obraud]

Which @ngrx/* package(s) are the source of the bug?

signals

Minimal reproduction of the bug/regression with instructions

Hello!
In NgRx 20, using patchState on a deepSignal with an object missing required properties will actually remove those properties from state, resulting in errors at runtime when reading values from those missing signals.

Example:

interface State {
  loader: boolean;
  config: {
    isAdmin: boolean;
    name: string | undefined;
  };
}

MyStore = signalStore(
  withState<State>({
    loader: true,
    config: {
      isAdmin: false,
      name: 'World',
    },
  }),
  withMethods((store) => ({
    patch() {
      const config = { isAdmin: true } as State['config'];
      patchState(store, { config });
    },
  }))
)

Here, calling store.patch() will actually remove name from config. Then any call to store.config.name() will result in the following error TypeError: ctx.store.config.name is not a function. In NgRx 19, store.config.name() would juste return undefined.

Here is a stackblitz for reproduction with NgRX 20. For comparison, the same repo with NgRx 19.

Expected behavior

I understand this is an edge case as it should not happen with strict typing. I am also not sure if this is intended or not,
I would expect deepSignal property structure not to evolve depending on how its patched at runtime?

Versions of NgRx, Angular, Node, affected browser(s) and operating system(s)

NgRx: 20.0.0
Angular: 20.1.3
Node: 22.17.1
Browser: Firefox, Chrome
Windows

Other information

No response

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

• Yes
• No

[rainerhahnekamp]

I can take a look at it.

[markostanimirovic]

Hi @obraud,

{ isAdmin: boolean } is not assignable to { isAdmin: boolean; name: string | undefined }:

const config: State['config'] = { isAdmin: true }; // => compilation error

There is a compilation error because State['config'] has a required property name of type string | undefined. Therefore, when we provide name property, it works as expected:

const config: State['config'] = { isAdmin: true, name: undefined };
patchState(store, { config });

Stackblitz: https://stackblitz.com/edit/stackblitz-starters-jkpqnyh5?file=src%2Fmain.store.ts

[obraud]

Hi!
Thanks for answering 😄
I understand that the code is invalid typewise. In the example project, I casted the object to illustrate the state mutation that results if such partial object is encountered at runtime (for example, coming from an API that does not send an object property if undefined ).

patchState() is normally able to work with partial object when patching regular state properties. So one could assume patching a deepSignal state with partial data in NGRX 20 would work the same without actually removing existing properties, as it would in NGRX 19?