Refactor languageSlice: improve type safety, update logic to avoid unnecessary i18n calls, add comprehensive tests, ensure full coverage and lint compliance

Author: rgilks

app/store/languageSlice.test.ts

@@ -0,0 +1,67 @@
+import { create } from 'zustand';
+import { immer } from 'zustand/middleware/immer';
+import { createLanguageSlice, type LanguageSlice } from './languageSlice';
+import type { Language } from '@/lib/domain/language';
+import { vi } from 'vitest';
+import type { TextGeneratorState } from './textGeneratorStore';
+
+global.window = Object.create(window);
+const mockReload = vi.fn();
+Object.defineProperty(window, 'location', {
+  value: { reload: mockReload, search: '?foo=bar' },
+  writable: true,
+});
+
+vi.mock('../i18n.client', () => ({
+  __esModule: true,
+  default: { changeLanguage: vi.fn() },
+}));
+import i18n from '../i18n.client';
+
+const minimalTextGeneratorState: Omit<TextGeneratorState, keyof LanguageSlice> = {} as any;
+type Store = TextGeneratorState;
+const createStore = () =>
+  create<Store>()(
+    immer((...a) => ({ ...minimalTextGeneratorState, ...createLanguageSlice(...a) }))
+  );
+
+describe('languageSlice', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    (i18n.changeLanguage as any).mockResolvedValue(undefined);
+  });
+
+  it('initializes with default language and languages', () => {
+    const store = createStore();
+    expect(store.getState().language).toBe('en');
+    expect(store.getState().languages).toBeDefined();
+  });
+
+  it('setLanguage updates language and calls i18n', async () => {
+    const store = createStore();
+    await store.getState().setLanguage('fr' as Language, undefined, '/en/page');
+    expect(store.getState().language).toBe('fr');
+    expect(i18n.changeLanguage).toHaveBeenCalledWith('fr');
+  });
+
+  it('setLanguage does not update if language is the same', async () => {
+    const store = createStore();
+    await store.getState().setLanguage('en' as Language, undefined, '/en/page');
+    expect(i18n.changeLanguage).not.toHaveBeenCalled();
+  });
+
+  it('reloads page if i18n.changeLanguage throws', async () => {
+    (i18n.changeLanguage as any).mockRejectedValue(new Error('fail'));
+    const store = createStore();
+    await store.getState().setLanguage('fr' as Language, undefined, '/en/page');
+    expect(mockReload).toHaveBeenCalled();
+  });
+
+  it('router.push is called with new path if router is provided', async () => {
+    const push = vi.fn();
+    const router = { push };
+    const store = createStore();
+    await store.getState().setLanguage('fr' as Language, router, '/en/page');
+    expect(push).toHaveBeenCalledWith('/fr/page?foo=bar');
+  });
+});

app/store/languageSlice.ts

@@ -32,10 +32,13 @@ export const createLanguageSlice: StateCreator<
   language: 'en',
   languages: UI_LANGUAGES,
   setLanguage: async (lang, router, pathname) => {
+    let prevLang: Language | undefined;
     set((state) => {
+      prevLang = state.language;
       if (state.language === lang) return;
       state.language = lang;
     });
+    if (prevLang === lang) return;
     try {
       await i18n.changeLanguage(lang);
     } catch (e) {