fix(store): add root store initializer guard

In this commit, we are adding the root store initializer guard, which ensures that
`provideStore()` is not accidentally called multiple times (e.g., in feature-level
providers such as route providers).

CHANGELOG.md

@@ -6,7 +6,7 @@ $ npm install @ngxs/store@dev
 
 ### To become next patch version
 
-- ...
+- Fix(store): Add root store initializer guard [#2278](https://github.com/ngxs/store/pull/2278)
 
 ### 19.0.0 2024-12-3
 

packages/store/src/standalone-features/initializers.ts

@@ -7,6 +7,7 @@ import { InitState, UpdateState } from '../plugin_api';
 import { FEATURE_STATE_TOKEN, ROOT_STATE_TOKEN } from '../symbols';
 import { StateFactory } from '../internal/state-factory';
 import { StatesAndDefaults } from '../internal/internals';
+import { assertRootStoreNotInitialized } from './root-guard';
 import { SelectFactory } from '../decorators/select/select-factory';
 import { InternalStateOperations } from '../internal/state-operations';
 import { LifecycleStateManager } from '../internal/lifecycle-state-manager';
@@ -18,6 +19,10 @@ import { installOnUnhandhedErrorHandler } from '../internal/unhandled-rxjs-error
  * same initialization functionality.
  */
 export function rootStoreInitializer(): void {
+  if (typeof ngDevMode !== 'undefined' && ngDevMode) {
+    assertRootStoreNotInitialized();
+  }
+
   // Override the RxJS `config.onUnhandledError` within the root store initializer,
   // but only after other code has already executed.
   // If users have a custom `config.onUnhandledError`, we might overwrite it too

packages/store/src/standalone-features/root-guard.ts

@@ -0,0 +1,14 @@
+import { inject, InjectionToken } from '@angular/core';
+
+export const ROOT_STORE_GUARD = /* @__PURE__ */ new InjectionToken('ROOT_STORE_GUARD', {
+  providedIn: 'root',
+  factory: () => ({ initialized: false })
+});
+
+export function assertRootStoreNotInitialized(): void {
+  const rootStoreGuard = inject(ROOT_STORE_GUARD);
+  if (rootStoreGuard.initialized) {
+    throw new Error('provideStore() should only be called once.');
+  }
+  rootStoreGuard.initialized = true;
+}

packages/store/tests/issues/issue-2277-provide-store-twice.spec.ts

@@ -0,0 +1,43 @@
+import { ApplicationConfig, Component } from '@angular/core';
+import { bootstrapApplication } from '@angular/platform-browser';
+import { provideRouter, Router } from '@angular/router';
+import { provideStore } from '@ngxs/store';
+import { freshPlatform, skipConsoleLogging } from '@ngxs/store/internals/testing';
+
+describe('provideStore() being called twice', () => {
+  @Component({ selector: 'app-root', template: '' })
+  class TestComponent {}
+
+  @Component({ template: '' })
+  class RouteComponent {}
+
+  const appConfig: ApplicationConfig = {
+    providers: [
+      provideStore(),
+      provideRouter([
+        { path: 'route', loadComponent: () => RouteComponent, providers: [provideStore()] }
+      ])
+    ]
+  };
+
+  it(
+    'should throw an error when provideStore() is called twice',
+    freshPlatform(async () => {
+      // Arrange
+      expect.hasAssertions();
+
+      // Act
+      const { injector } = await skipConsoleLogging(() =>
+        bootstrapApplication(TestComponent, appConfig)
+      );
+      const router = injector.get(Router);
+
+      try {
+        await router.navigateByUrl('/route');
+      } catch (error) {
+        // Assert
+        expect(error.message).toEqual('provideStore() should only be called once.');
+      }
+    })
+  );
+});