feat(signals)!: freeze state in dev mode

BREAKING CHANGE: the state of `signalStore` and `signalState`
is frozen during dev mode.

Mutable changes - not just in `patchState` - will throw an error.

BEFORE:

```typescript
const userState = signalState(initialState);
patchState(userState, (state) => {
  state.user.firstName = 'mutable change'; // mutable change went through
  return state;
});

getState(userState).user.firstName = 'mutable change'; // mutable change went through
```

AFTER:

```
const userState = signalState(initialState);
patchState(userState, (state) => {
  state.user.firstName = 'mutable change'; // throws in dev mode
  return state;
});

getState(userState).user.firstName = 'mutable change'; // throws in dev mode
```

modules/signals/spec/deep-freeze.spec.ts

@@ -0,0 +1,115 @@
+import { getState, patchState } from '../src/state-source';
+import { signalState } from '../src/signal-state';
+import { signalStore } from '../src/signal-store';
+import { TestBed } from '@angular/core/testing';
+import { withState } from '../src/with-state';
+
+describe('deepFreeze', () => {
+  const initialState = {
+    user: {
+      firstName: 'John',
+      lastName: 'Smith',
+    },
+    foo: 'bar',
+    numbers: [1, 2, 3],
+    ngrx: 'signals',
+  };
+
+  for (const { stateFactory, name } of [
+    {
+      name: 'signalStore',
+      stateFactory: () => {
+        const Store = signalStore(
+          { protectedState: false },
+          withState(initialState)
+        );
+        return TestBed.configureTestingModule({ providers: [Store] }).inject(
+          Store
+        );
+      },
+    },
+    { name: 'signalState', stateFactory: () => signalState(initialState) },
+  ]) {
+    describe(name, () => {
+      it(`throws on a mutable change`, () => {
+        const state = stateFactory();
+        expect(() =>
+          patchState(state, (state) => {
+            state.ngrx = 'mutable change';
+            return state;
+          })
+        ).toThrowError("Cannot assign to read only property 'ngrx' of object");
+      });
+
+      it('throws on a nested mutable change', () => {
+        const state = stateFactory();
+        expect(() =>
+          patchState(state, (state) => {
+            state.user.firstName = 'mutable change';
+            return state;
+          })
+        ).toThrowError(
+          "Cannot assign to read only property 'firstName' of object"
+        );
+      });
+      describe('mutable changes outside of patchState', () => {
+        it('throws on reassigned a property of the exposed state', () => {
+          const state = stateFactory();
+          expect(() => {
+            state.user().firstName = 'mutable change 1';
+          }).toThrowError(
+            "Cannot assign to read only property 'firstName' of object"
+          );
+        });
+
+        it('throws when exposed state via getState is mutated', () => {
+          const state = stateFactory();
+          const s = getState(state);
+
+          expect(() => (s.ngrx = 'mutable change 2')).toThrowError(
+            "Cannot assign to read only property 'ngrx' of object"
+          );
+        });
+
+        it('throws when mutable change happens for', () => {
+          const state = stateFactory();
+          const s = { user: { firstName: 'M', lastName: 'S' } };
+          patchState(state, s);
+
+          expect(() => {
+            s.user.firstName = 'mutable change 3';
+          }).toThrowError(
+            "Cannot assign to read only property 'firstName' of object"
+          );
+        });
+      });
+    });
+  }
+
+  describe('special tests', () => {
+    for (const { name, mutationFn } of [
+      {
+        name: 'location',
+        mutationFn: (state: { location: { city: string } }) =>
+          (state.location.city = 'Paris'),
+      },
+      {
+        name: 'user',
+        mutationFn: (state: { user: { firstName: string } }) =>
+          (state.user.firstName = 'Jane'),
+      },
+    ]) {
+      it(`throws on concatenated state (${name})`, () => {
+        const UserStore = signalStore(
+          { providedIn: 'root' },
+          withState(initialState),
+          withState({ location: { city: 'London' } })
+        );
+        const store = TestBed.inject(UserStore);
+        const state = getState(store);
+
+        expect(() => mutationFn(state)).toThrowError();
+      });
+    }
+  });
+});

modules/signals/src/deep-freeze.ts

@@ -0,0 +1,49 @@
+declare const ngDevMode: boolean;
+
+export function deepFreeze<T>(target: T): T {
+  Object.freeze(target);
+
+  const targetIsFunction = typeof target === 'function';
+
+  Object.getOwnPropertyNames(target).forEach((prop) => {
+    // Ignore Ivy properties, ref: https://github.com/ngrx/platform/issues/2109#issuecomment-582689060
+    if (prop.startsWith('ɵ')) {
+      return;
+    }
+
+    if (
+      hasOwnProperty(target, prop) &&
+      (targetIsFunction
+        ? prop !== 'caller' && prop !== 'callee' && prop !== 'arguments'
+        : true)
+    ) {
+      const propValue = target[prop];
+
+      if (
+        (isObjectLike(propValue) || typeof propValue === 'function') &&
+        !Object.isFrozen(propValue)
+      ) {
+        deepFreeze(propValue);
+      }
+    }
+  });
+
+  return target;
+}
+
+export function freezeInDevMode<T>(target: T): T {
+  return ngDevMode ? deepFreeze(target) : target;
+}
+
+function hasOwnProperty(
+  target: unknown,
+  propertyName: string
+): target is { [propertyName: string]: unknown } {
+  return isObjectLike(target)
+    ? Object.prototype.hasOwnProperty.call(target, propertyName)
+    : false;
+}
+
+function isObjectLike(target: unknown): target is object {
+  return typeof target === 'object' && target !== null;
+}

modules/signals/src/signal-state.ts

@@ -1,14 +1,15 @@
 import { signal } from '@angular/core';
 import { STATE_SOURCE, WritableStateSource } from './state-source';
 import { DeepSignal, toDeepSignal } from './deep-signal';
+import { freezeInDevMode } from './deep-freeze';
 
 export type SignalState<State extends object> = DeepSignal<State> &
   WritableStateSource<State>;
 
 export function signalState<State extends object>(
   initialState: State
 ): SignalState<State> {
-  const stateSource = signal(initialState as State);
+  const stateSource = signal(freezeInDevMode(initialState as State));
   const signalState = toDeepSignal(stateSource.asReadonly());
   Object.defineProperty(signalState, STATE_SOURCE, {
     value: stateSource,

modules/signals/src/state-source.ts

@@ -8,6 +8,7 @@ import {
   WritableSignal,
 } from '@angular/core';
 import { Prettify } from './ts-helpers';
+import { freezeInDevMode } from './deep-freeze';
 
 const STATE_WATCHERS = new WeakMap<Signal<object>, Array<StateWatcher<any>>>();
 
@@ -37,10 +38,11 @@ export function patchState<State extends object>(
 ): void {
   stateSource[STATE_SOURCE].update((currentState) =>
     updaters.reduce(
-      (nextState: State, updater) => ({
-        ...nextState,
-        ...(typeof updater === 'function' ? updater(nextState) : updater),
-      }),
+      (nextState: State, updater) =>
+        freezeInDevMode({
+          ...nextState,
+          ...(typeof updater === 'function' ? updater(nextState) : updater),
+        }),
       currentState
     )
   );

modules/signals/src/with-state.ts

@@ -9,6 +9,7 @@ import {
   SignalStoreFeature,
   SignalStoreFeatureResult,
 } from './signal-store-models';
+import { freezeInDevMode } from './deep-freeze';
 
 export function withState<State extends object>(
   stateFactory: () => State
@@ -35,10 +36,12 @@ export function withState<State extends object>(
 
     assertUniqueStoreMembers(store, stateKeys);
 
-    store[STATE_SOURCE].update((currentState) => ({
-      ...currentState,
-      ...state,
-    }));
+    store[STATE_SOURCE].update((currentState) =>
+      freezeInDevMode({
+        ...currentState,
+        ...state,
+      })
+    );
 
     const stateSignals = stateKeys.reduce((acc, key) => {
       const sliceSignal = computed(