Vue 3 State Management Patterns

Same intuition as react-state-management. Pinia replaces Vuex for Vue 3.

Decision tree

Data	Tool
Local component state (count, isOpen)	ref, reactive
Shared between siblings	Lift to common parent + props
Composable shared logic	composable returning { refs, actions }
App-wide state (current user, theme)	Pinia or provide/inject
Complex domain state	Pinia
Server data with caching	TanStack Query Vue (@tanstack/vue-query)
Form state	vee-validate (see vue-forms)
URL-synced state	vue-router useRoute().query + useRouter().push

ref and reactive (covered in vue-conventions)

For component-local state, prefer ref (works for primitives + objects, no destructure footgun).

Composables for shared logic

// src/composables/useCounter.ts
import { ref, computed } from 'vue';

export function useCounter(initial = 0) {
  const count = ref(initial);
  const double = computed(() => count.value * 2);
  function increment() { count.value++; }
  function reset() { count.value = initial; }
  return { count, double, increment, reset };
}

Each useCounter() call creates a fresh state — composables don't share state by default. For shared state across components, use Pinia.

Pinia (Vue 3 default)

pnpm add pinia. Setup once in main.ts:

import { createApp } from 'vue';
import { createPinia } from 'pinia';
import App from './App.vue';

const app = createApp(App);
app.use(createPinia());
app.mount('#app');

Setup syntax (preferred for Vue 3)

// src/stores/users.ts
import { defineStore } from 'pinia';
import { ref, computed } from 'vue';
import type { User } from '@/types/user';

export const useUserStore = defineStore('users', () => {
  // state
  const items = ref<User[]>([]);
  const currentUser = ref<User | null>(null);
  const loading = ref(false);

  // getters (computed)
  const count = computed(() => items.value.length);
  const isAuthenticated = computed(() => currentUser.value !== null);

  // actions
  async function fetchUsers() {
    loading.value = true;
    try {
      const res = await fetch('/api/users');
      items.value = await res.json();
    } finally {
      loading.value = false;
    }
  }

  function setCurrentUser(user: User | null) {
    currentUser.value = user;
  }

  return { items, currentUser, loading, count, isAuthenticated, fetchUsers, setCurrentUser };
});

Usage in components:

<script setup lang="ts">
import { useUserStore } from '@/stores/users';
import { storeToRefs } from 'pinia';

const userStore = useUserStore();

// Direct access to reactive state — but DESTRUCTURING loses reactivity
const { items, count, loading } = storeToRefs(userStore);  // ✅ keeps reactivity

// Actions can be destructured directly (they're plain functions)
const { fetchUsers, setCurrentUser } = userStore;          // ✅ OK

onMounted(() => fetchUsers());
</script>

<template>
  <p v-if="loading">Loading...</p>
  <p v-else>Total users: {{ count }}</p>
</template>

storeToRefs is the Pinia equivalent of toRefs — preserves reactivity when destructuring state and getters. Don't use it for actions.

Options syntax (alternative)

export const useUserStore = defineStore('users', {
  state: () => ({
    items: [] as User[],
    currentUser: null as User | null,
    loading: false,
  }),
  getters: {
    count: (state) => state.items.length,
    isAuthenticated: (state) => state.currentUser !== null,
  },
  actions: {
    async fetchUsers() {
      this.loading = true;
      try {
        const res = await fetch('/api/users');
        this.items = await res.json();
      } finally {
        this.loading = false;
      }
    },
  },
});

Setup syntax is preferred for TS friendliness (better inference) and Composition API consistency. Options syntax is fine if the team prefers it.

Persistence

pnpm add pinia-plugin-persistedstate

// main.ts
import { createPinia } from 'pinia';
import piniaPluginPersistedstate from 'pinia-plugin-persistedstate';

const pinia = createPinia();
pinia.use(piniaPluginPersistedstate);

export const useUserStore = defineStore('users', () => { /* ... */ }, {
  persist: { storage: sessionStorage },
});

Never persist auth tokens — XSS-readable. Use httpOnly cookies (server-set) or in-memory state cleared on logout.

Reset

const userStore = useUserStore();
userStore.$reset();              // works only with Options syntax automatically
// Setup syntax: define a reset action manually

For Setup syntax stores, define a reset function:

function $reset() {
  items.value = [];
  currentUser.value = null;
  loading.value = false;
}
return { /* ... */, $reset };

provide / inject

Dependency injection for app-wide deps that don't justify a Pinia store. Common: theme, locale, current authenticated user.

// src/keys.ts
import type { InjectionKey, Ref } from 'vue';

export interface ThemeContext {
  theme: Ref<'light' | 'dark'>;
  setTheme: (t: 'light' | 'dark') => void;
}

export const ThemeKey: InjectionKey<ThemeContext> = Symbol('theme');

<!-- App.vue — provider -->
<script setup lang="ts">
import { provide, ref } from 'vue';
import { ThemeKey } from '@/keys';

const theme = ref<'light' | 'dark'>('light');
function setTheme(t: 'light' | 'dark') {
  theme.value = t;
}
provide(ThemeKey, { theme, setTheme });
</script>

<!-- Any descendant — consumer -->
<script setup lang="ts">
import { inject } from 'vue';
import { ThemeKey } from '@/keys';

const themeCtx = inject(ThemeKey);
if (!themeCtx) throw new Error('ThemeKey not provided');
const { theme, setTheme } = themeCtx;
</script>

<template>
  <p>Current theme: {{ theme }}</p>
  <button @click="setTheme(theme === 'light' ? 'dark' : 'light')">Toggle</button>
</template>

InjectionKey<T> provides type safety — inject(key) returns T | undefined.

Use provide/inject for: theme, locale, current user, feature flags shared across the tree. For everything else (frequent updates, complex state), prefer Pinia.

TanStack Query Vue (@tanstack/vue-query)

Server state with caching, deduplication, refetching.

// main.ts
import { VueQueryPlugin } from '@tanstack/vue-query';
app.use(VueQueryPlugin);

// src/composables/useUsers.ts
import { useQuery, useMutation, useQueryClient } from '@tanstack/vue-query';
import { ref } from 'vue';
import type { User } from '@/types/user';

export function useUsers() {
  return useQuery<User[]>({
    queryKey: ['users'],
    queryFn: async () => {
      const res = await fetch('/api/users');
      if (!res.ok) throw new Error('Failed to fetch');
      return res.json();
    },
  });
}

export function useUser(id: string) {
  return useQuery<User>({
    queryKey: ['users', id],
    queryFn: () => fetch(`/api/users/${id}`).then((r) => r.json()),
    enabled: !!id,
  });
}

export function useCreateUser() {
  const qc = useQueryClient();
  return useMutation({
    mutationFn: (input: Omit<User, 'id'>) =>
      fetch('/api/users', { method: 'POST', body: JSON.stringify(input) }).then((r) => r.json()),
    onSuccess: () => {
      qc.invalidateQueries({ queryKey: ['users'] });
    },
  });
}

Usage:

<script setup lang="ts">
import { useUsers, useCreateUser } from '@/composables/useUsers';

const { data: users, isLoading, error } = useUsers();
const { mutate: createUser, isPending } = useCreateUser();
</script>

<template>
  <p v-if="isLoading">Loading...</p>
  <p v-else-if="error">Error: {{ error }}</p>
  <ul v-else>
    <li v-for="user in users" :key="user.id">{{ user.name }}</li>
  </ul>
</template>

Same query key conventions as React Query — hierarchical arrays.

VueUse useStorage

Reactive localStorage / sessionStorage wrapper. NEVER for auth tokens.

import { useStorage } from '@vueuse/core';

const theme = useStorage('theme', 'light');     // Ref<string>, syncs to localStorage
theme.value = 'dark';                            // updates storage automatically

For SSR-safety and serialization options, see VueUse docs.

URL-synced state

Use vue-router:

import { useRoute, useRouter } from 'vue-router';

const route = useRoute();
const router = useRouter();

// Read query param
const filter = computed(() => (route.query.filter as string) ?? 'all');

// Update
function setFilter(value: string) {
  router.push({ query: { ...route.query, filter: value } });
}

Use for filters, pagination, sort orders, search queries — anything users want to bookmark/share.

Vuex (Vue 2 legacy — pointer only)

For Vue 2 codebases:

// store/index.ts (Vue 2 + Vuex 3)
import Vuex from 'vuex';

export default new Vuex.Store({
  state: { count: 0 },
  mutations: { increment(state) { state.count++; } },
  actions: { incrementAsync({ commit }) { setTimeout(() => commit('increment'), 100); } },
  getters: { double: (state) => state.count * 2 },
});

Migrating Vuex 3 → Pinia: typically rewrites all store modules. Don't migrate as part of feature work — only with explicit BA spec.

Anti-patterns

• ❌ Destructuring Pinia store state without storeToRefs — breaks reactivity.
• ❌ Storing auth tokens in useStorage (localStorage XSS risk).
• ❌ Pinia store for transient UI state (modal open, sidebar collapsed) — use ref/reactive in component.
• ❌ Forgetting app.use(createPinia()) in main.ts — stores throw "no active pinia" error.
• ❌ Mutating Pinia store state from outside actions — direct mutations work but bypass devtools tracking.
• ❌ Reaching for Vuex in a Vue 3 project — use Pinia.
• ❌ Forgetting invalidateQueries after mutations in TanStack Query — stale UI.
• ❌ Long composables that "do everything" — split by concern.
• ❌ Mixing multiple state libs without a clear boundary (Pinia + custom Context + Vuex shim).