From ed985d469f33733a80c057f17e8f72f0233a2228 Mon Sep 17 00:00:00 2001
From: Rainer Hahnekamp <rainer.hahnekamp@gmail.com>
Date: Wed, 18 Sep 2024 15:43:23 +0200
Subject: [PATCH] feat: apply `Object.freeze` in `patchState` on state in dev
 mode

---
 modules/signals/spec/signal-state.spec.ts | 10 ++++++++++
 modules/signals/src/state-source.ts       | 14 +++++++++++++-
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/modules/signals/spec/signal-state.spec.ts b/modules/signals/spec/signal-state.spec.ts
index c509d07f04..81b3416818 100644
--- a/modules/signals/spec/signal-state.spec.ts
+++ b/modules/signals/spec/signal-state.spec.ts
@@ -194,4 +194,14 @@ describe('signalState', () => {
       expect(stateCounter).toBe(3);
       expect(userCounter).toBe(1);
     }));
+
+  it('throws on a mutable change', () => {
+    const userState = signalState(initialState);
+    expect(() =>
+      patchState(userState, (state) => {
+        state.ngrx = 'mutable change';
+        return state;
+      })
+    ).toThrowError("Cannot assign to read only property 'ngrx' of object");
+  });
 });
diff --git a/modules/signals/src/state-source.ts b/modules/signals/src/state-source.ts
index 940e2d5f81..62dcc7dc12 100644
--- a/modules/signals/src/state-source.ts
+++ b/modules/signals/src/state-source.ts
@@ -10,6 +10,8 @@ import {
 import { SIGNAL } from '@angular/core/primitives/signals';
 import { Prettify } from './ts-helpers';
 
+declare const ngDevMode: boolean;
+
 const STATE_WATCHERS = new WeakMap<object, Array<StateWatcher<any>>>();
 
 export const STATE_SOURCE = Symbol('STATE_SOURCE');
@@ -40,7 +42,9 @@ export function patchState<State extends object>(
     updaters.reduce(
       (nextState: State, updater) => ({
         ...nextState,
-        ...(typeof updater === 'function' ? updater(nextState) : updater),
+        ...(typeof updater === 'function'
+          ? updater(freezeInDevMode(nextState))
+          : updater),
       }),
       currentState
     )
@@ -49,6 +53,14 @@ export function patchState<State extends object>(
   notifyWatchers(stateSource);
 }
 
+function freezeInDevMode<State extends object>(value: State): State {
+  if (ngDevMode) {
+    Object.freeze(value);
+  }
+
+  return value;
+}
+
 export function getState<State extends object>(
   stateSource: StateSource<State>
 ): State {