state-management

What this skill does

# State Management

## Philosophy

- **Server State** → TanStack Query
- **Client State** → Zustand
- **Form State** → React Hook Form + Zod
- **URL State** → nuqs or searchParams

## TanStack Query

### Query Keys Factory
```typescript
export const userKeys = {
  all: ['users'] as const,
  lists: () => [...userKeys.all, 'list'] as const,
  list: (filters: Filters) => [...userKeys.lists(), filters] as const,
  details: () => [...userKeys.all, 'detail'] as const,
  detail: (id: string) => [...userKeys.details(), id] as const,
};
```

### Hooks Pattern
```typescript
export function useUsers(filters?: Filters) {
  return useQuery({
    queryKey: userKeys.list(filters ?? {}),
    queryFn: () => getUsers(filters),
  });
}

export function useUser(id: string) {
  return useQuery({
    queryKey: userKeys.detail(id),
    queryFn: () => getUser(id),
    enabled: !!id,
  });
}

export function useCreateUser() {
  const queryClient = useQueryClient();
  return useMutation({
    mutationFn: createUser,
    onSuccess: () => {
      queryClient.invalidateQueries({ queryKey: userKeys.lists() });
    },
  });
}

export function useUpdateUser() {
  const queryClient = useQueryClient();
  return useMutation({
    mutationFn: ({ id, data }: { id: string; data: UpdateUserInput }) =>
      updateUser(id, data),
    onSuccess: (_, { id }) => {
      queryClient.invalidateQueries({ queryKey: userKeys.detail(id) });
      queryClient.invalidateQueries({ queryKey: userKeys.lists() });
    },
  });
}
```

## Zustand

### Typed Store
```typescript
interface UIStore {
  sidebarOpen: boolean;
  theme: 'light' | 'dark';
  toggleSidebar: () => void;
  setTheme: (theme: 'light' | 'dark') => void;
}

export const useUIStore = create<UIStore>((set) => ({
  sidebarOpen: true,
  theme: 'light',
  toggleSidebar: () => set((s) => ({ sidebarOpen: !s.sidebarOpen })),
  setTheme: (theme) => set({ theme }),
}));
```

### Performance: Use Selectors
```typescript
// CORRECT - Only re-renders when sidebarOpen changes
const sidebarOpen = useUIStore((s) => s.sidebarOpen);

// WRONG - Re-renders on ANY state change
const { sidebarOpen } = useUIStore();
```

### Persist Middleware
```typescript
import { persist } from 'zustand/middleware';

export const useSettingsStore = create<SettingsStore>()(
  persist(
    (set) => ({
      language: 'en',
      setLanguage: (language) => set({ language }),
    }),
    {
      name: 'settings-storage',
    }
  )
);
```

### Computed Values with Selectors
```typescript
// Create a selector
const selectFilteredItems = (state: Store) =>
  state.items.filter(item => item.active);

// Use in component
const filteredItems = useStore(selectFilteredItems);
```

## Form State: React Hook Form + Zod

```typescript
import { useForm } from 'react-hook-form';
import { zodResolver } from '@hookform/resolvers/zod';
import { z } from 'zod';

const schema = z.object({
  name: z.string().min(1, 'Required'),
  email: z.string().email('Invalid email'),
});

type FormData = z.infer<typeof schema>;

export function UserForm() {
  const { register, handleSubmit, formState: { errors } } = useForm<FormData>({
    resolver: zodResolver(schema),
  });

  const onSubmit = (data: FormData) => {
    // data is typed and validated
  };

  return (
    <form onSubmit={handleSubmit(onSubmit)}>
      <input {...register('name')} />
      {errors.name && <span>{errors.name.message}</span>}

      <input {...register('email')} />
      {errors.email && <span>{errors.email.message}</span>}

      <button type="submit">Submit</button>
    </form>
  );
}
```