State Management Skill

When to Activate

• Deciding where to put state in a React/Vue/Svelte app
• Server data is going stale or showing inconsistent values
• Global state causing unnecessary re-renders
• Form state is getting complex
• URL not reflecting application state (back button broken)
• Migrating server data out of Zustand into TanStack Query to eliminate manual cache sync
• Adding optimistic updates to a mutation so the UI responds instantly before the server confirms

---

State Type Decision Tree

Is this data from the server?
  YES → TanStack Query (not a state library)
  NO  → Is it needed in multiple components far apart in the tree?
          YES → Zustand (global client state)
          NO  → Is it navigation/filter/search state that should survive a refresh?
                  YES → URL state (search params)
                  NO  → Is it form input?
                          YES → React Hook Form
                          NO  → useState / useReducer (local)

The most common mistake: putting server data in Zustand. Use TanStack Query instead.

---

Server State: TanStack Query

// queries/users.ts
import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';

// Query keys: structured, predictable, invalidation-friendly
export const userKeys = {
  all: ['users'] as const,
  lists: () => [...userKeys.all, 'list'] as const,
  list: (filters: UserFilters) => [...userKeys.lists(), filters] as const,
  details: () => [...userKeys.all, 'detail'] as const,
  detail: (id: string) => [...userKeys.details(), id] as const,
};

// Fetch a single user
export function useUser(id: string) {
  return useQuery({
    queryKey: userKeys.detail(id),
    queryFn: () => api.get<User>(`/users/${id}`),
    staleTime: 5 * 60 * 1000,  // Consider fresh for 5 minutes
    gcTime: 10 * 60 * 1000,    // Keep in cache 10 minutes after unused
  });
}

// Fetch list with filters
export function useUsers(filters: UserFilters) {
  return useQuery({
    queryKey: userKeys.list(filters),
    queryFn: () => api.get<User[]>('/users', { params: filters }),
    placeholderData: keepPreviousData,  // Don't flash empty on filter change
  });
}

// Mutation with optimistic update
export function useUpdateUser() {
  const queryClient = useQueryClient();

  return useMutation({
    mutationFn: (data: { id: string; update: Partial<User> }) =>
      api.patch<User>(`/users/${data.id}`, data.update),

    onMutate: async ({ id, update }) => {
      // Cancel in-flight queries for this user
      await queryClient.cancelQueries({ queryKey: userKeys.detail(id) });

      // Snapshot previous value for rollback
      const previous = queryClient.getQueryData<User>(userKeys.detail(id));

      // Optimistically update
      queryClient.setQueryData<User>(userKeys.detail(id), (old) =>
        old ? { ...old, ...update } : old
      );

      return { previous };
    },

    onError: (_, { id }, context) => {
      // Rollback on error
      if (context?.previous) {
        queryClient.setQueryData(userKeys.detail(id), context.previous);
      }
    },

    onSettled: (_, __, { id }) => {
      // Always refetch after mutation (source of truth from server)
      queryClient.invalidateQueries({ queryKey: userKeys.detail(id) });
      queryClient.invalidateQueries({ queryKey: userKeys.lists() });
    },
  });
}

// Usage in component
function UserProfile({ id }: { id: string }) {
  const { data: user, isLoading, error } = useUser(id);
  const { mutate: updateUser, isPending } = useUpdateUser();

  if (isLoading) return <Skeleton />;
  if (error) return <ErrorMessage error={error} />;

  return (
    <form onSubmit={() => updateUser({ id, update: { name: 'New Name' } })}>
      {/* ... */}
    </form>
  );
}

---

Global Client State: Zustand

Only for state that is truly client-side and needed across the app (UI state, user preferences, shopping cart before checkout).

// stores/ui.ts
import { create } from 'zustand';
import { persist, devtools } from 'zustand/middleware';

interface UIStore {
  sidebarOpen: boolean;
  theme: 'light' | 'dark' | 'system';
  toggleSidebar: () => void;
  setTheme: (theme: UIStore['theme']) => void;
}

export const useUIStore = create<UIStore>()(
  devtools(
    persist(
      (set) => ({
        sidebarOpen: true,
        theme: 'system',
        toggleSidebar: () => set(state => ({ sidebarOpen: !state.sidebarOpen })),
        setTheme: (theme) => set({ theme }),
      }),
      { name: 'ui-store' }   // persisted to localStorage
    )
  )
);

// Cart store (not persisted to server yet)
interface CartStore {
  items: CartItem[];
  addItem: (item: CartItem) => void;
  removeItem: (id: string) => void;
  clear: () => void;
  total: () => number;
}

export const useCartStore = create<CartStore>()((set, get) => ({
  items: [],
  addItem: (item) =>
    set(state => {
      const existing = state.items.find(i => i.id === item.id);
      if (existing) {
        return {
          items: state.items.map(i =>
            i.id === item.id ? { ...i, quantity: i.quantity + 1 } : i
          ),
        };
      }
      return { items: [...state.items, item] };
    }),
  removeItem: (id) =>
    set(state => ({ items: state.items.filter(i => i.id !== id) })),
  clear: () => set({ items: [] }),
  total: () => get().items.reduce((sum, i) => sum + i.price * i.quantity, 0),
}));

---

URL State (nuqs / useSearchParams)

Use for: filters, pagination, search queries, active tabs — anything that should be bookmarkable.

import { useQueryState, parseAsInteger, parseAsString } from 'nuqs';

function UserList() {
  const [page, setPage] = useQueryState('page', parseAsInteger.withDefault(1));
  const [search, setSearch] = useQueryState('q', parseAsString.withDefault(''));
  const [status, setStatus] = useQueryState('status');

  const { data } = useUsers({ page, search, status });

  return (
    <>
      <input value={search} onChange={e => setSearch(e.target.value)} />
      <select value={status ?? ''} onChange={e => setStatus(e.target.value || null)}>
        <option value="">All</option>
        <option value="active">Active</option>
      </select>
      {data?.map(user => <UserRow key={user.id} user={user} />)}
      <Pagination current={page} onChange={setPage} />
    </>
  );
}
// URL: /users?page=2&q=alice&status=active
// Back button works. Sharing link works. Refresh works.

---

Form State: React Hook Form

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

const schema = z.object({
  name: z.string().min(2, 'At least 2 characters'),
  email: z.string().email('Invalid email'),
  role: z.enum(['admin', 'member']),
});

type FormData = z.infer<typeof schema>;

function UserForm({ onSuccess }: { onSuccess: () => void }) {
  const { mutate: createUser, isPending } = useCreateUser();

  const {
    register,
    handleSubmit,
    formState: { errors },
    setError,
  } = useForm<FormData>({
    resolver: zodResolver(schema),
    defaultValues: { role: 'member' },
  });

  const onSubmit = (data: FormData) => {
    createUser(data, {
      onSuccess,
      onError: (err) => {
        // Map server errors back to fields
        if (err.field === 'email') {
          setError('email', { message: err.message });
        }
      },
    });
  };

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

---

Anti-Patterns

Anti-pattern	Problem	Fix
Server data in Zustand	Manual sync, stale data, double fetch	TanStack Query
useEffect to sync state	Infinite loops, race conditions	Derive from source of truth
Huge single global store	All components re-render on any change	Multiple small stores
Filters/search in local state	Back button breaks, can't share URL	URL state
Form state in useState	Manual validation, reset logic	React Hook Form
Prop drilling 5+ levels	Brittle, painful refactoring	Zustand or Context

---

Checklist

• Server data in TanStack Query (not Zustand)
• Query keys are structured and hierarchical (for precise invalidation)
• Mutations invalidate the right query keys in onSettled
• Optimistic updates implemented for snappy UX on common mutations
• URL reflects all filterable/paginated state
• Forms use React Hook Form + Zod schema validation
• Zustand stores are small and single-purpose (not one mega-store)
• DevTools enabled in development (TanStack Query DevTools, Zustand DevTools)