Vue + Pinia

Pinia is the standard state management library for Vue 3, and setup stores built with the Composition API give shared application state the same ergonomics as local ref/computed state.

Workflow for Adding a Pinia Store

1. Decide if Pinia is the right tool — Confirm the state is shared across routes/components, not server cache data, and not purely local UI state (see State Ownership below).
2. Create the store file — Add one file per domain under stores/, e.g. stores/cart.ts, using defineStore('cart', () => { ... }).
3. Define state, getters, and actions — Declare state with ref/reactive, derive values with computed, and put all writes/side effects in functions (actions).
4. Return everything from the setup function — Return every piece of state, every getter, and every action so Pinia devtools, SSR, and plugins can track them.
5. Consume in components — Call the store at the top of <script setup>, destructure reactive state with storeToRefs(), and destructure actions directly.
6. Handle SSR/router-guard usage — Call the store from inside setup, a getter, an action, or pass the active Pinia instance explicitly outside setup contexts.
7. Add persistence and tests — Allowlist persisted fields, validate rehydrated data, and test actions directly with @pinia/testing.

State Ownership

• Keep component-only state in the component with ref, reactive, or computed.
• Use Pinia for shared client state that spans routes, layouts, or unrelated component trees.
• Use route params and query strings for shareable navigation state — don't duplicate it into a store.
• Use Nuxt useFetch / useAsyncData, TanStack Query for Vue, or the existing API layer for server state — Pinia is for client state, not a data-fetching cache.
• Do not copy server cache data into Pinia unless it is an intentional editable draft, offline cache, or workflow snapshot with its own lifecycle.

Store Structure

• Prefer setup stores with defineStore('name', () => { ... }) for Vue 3 Composition API projects over the older options-API defineStore({ state, getters, actions }) form.
• Use one store file per domain under stores/, such as stores/cart.ts or stores/session.ts.
• Keep state, computed getters, and actions together when they represent one cohesive domain.
• Return every state property from setup stores so Pinia can track it for devtools, SSR, and plugins — an un-returned ref is invisible to the rest of the system.
• Keep getters pure and side-effect free; put writes, I/O, and orchestration in actions.

// stores/cart.ts
import { computed, ref } from 'vue'
import { defineStore } from 'pinia'

interface CartLine {
  id: string
  name: string
  quantity: number
  unitPrice: number
}

export const useCartStore = defineStore('cart', () => {
  const lines = ref<CartLine[]>([])

  const itemCount = computed(() =>
    lines.value.reduce((total, line) => total + line.quantity, 0),
  )

  const subtotal = computed(() =>
    lines.value.reduce((total, line) => total + line.quantity * line.unitPrice, 0),
  )

  function addLine(line: CartLine) {
    const existing = lines.value.find((item) => item.id === line.id)
    if (existing) {
      existing.quantity += line.quantity
      return
    }
    lines.value.push(line)
  }

  function removeLine(id: string) {
    lines.value = lines.value.filter((line) => line.id !== id)
  }

  function clearCart() {
    lines.value = []
  }

  return {
    lines,
    itemCount,
    subtotal,
    addLine,
    removeLine,
    clearCart,
  }
})

Component Usage

• Call stores at the top of <script setup> or inside setup functions, getters, and actions — never conditionally, and never inside a plain .js/.ts module loaded at import time.
• Use storeToRefs() when destructuring store state or getters in components, so reactivity is preserved.
• Destructure actions directly when useful; actions remain bound to the store and don't need storeToRefs().
• Avoid writing large business workflows in components; move them to store actions or composables so they're testable and reusable.
• Prefer computed values over watchers when deriving state — a watch that just recomputes a value should usually be a computed.

<script setup lang="ts">
import { storeToRefs } from 'pinia'
import { useCartStore } from '@/stores/cart'

const cart = useCartStore()
const { itemCount, subtotal } = storeToRefs(cart)
const { clearCart } = cart
</script>

<template>
  <div>
    <p>{{ itemCount }} items — ${{ subtotal.toFixed(2) }}</p>
    <button type="button" :disabled="itemCount === 0" @click="clearCart">
      Clear cart
    </button>
  </div>
</template>

TypeScript

• Type store state, action payloads, and API responses explicitly.
• Avoid any; use unknown and narrow external inputs (API responses, persisted storage) before committing them to state.
• Use interfaces for object state that is shared across components or API boundaries.
• Prefer discriminated unions for workflow status and error state, e.g. { status: 'idle' } | { status: 'loading' } | { status: 'error'; error: string }.
• Keep store IDs (the first argument to defineStore) stable and descriptive because they appear in devtools and persistence keys.

Actions and Side Effects

• Actions may be sync or async; keep each action focused on one user or domain workflow.
• Validate action inputs at the boundary before mutating store state.
• Represent async workflows with explicit status, error, and lastUpdatedAt fields when the UI depends on them.
• Reset stale errors before retrying an async action so a previous failure doesn't linger in the UI.
• Keep subscriptions, intervals, sockets, and browser listeners outside stores unless the store owns their lifecycle and cleanup (e.g. an explicit connect()/disconnect() action pair).

SSR, Nuxt, and Router Guards

• In SSR contexts, use the store inside setup, getters, or actions so Pinia can resolve the active app instance.
• When using a store outside setup, such as in a router guard, pass the active Pinia instance if the framework requires it (Nuxt's auto-imports handle this automatically).
• Do not read browser-only storage (localStorage, window) during server rendering — guard with import.meta.client (Nuxt) or an onMounted check.
• In Nuxt, prefer the @pinia/nuxt integration and SSR-safe composables for data fetching instead of manually wiring a Pinia instance.
• Avoid singleton state leaks across requests by relying on the framework-created Pinia instance per request rather than a module-level singleton.

// router guard example (outside setup)
router.beforeEach((to) => {
  const auth = useAuthStore(pinia) // pass the active Pinia instance explicitly
  if (to.meta.requiresAuth && !auth.isAuthenticated) {
    return { name: 'login' }
  }
})

Persistence

• Persist only the fields that must survive reloads, such as preferences or incomplete local drafts.
• Never persist secrets, access tokens, refresh tokens, raw PII, or authorization decisions in browser storage.
• Use field allowlists and versioned migrations for persisted store schemas so old persisted shapes don't crash a new store version.
• Treat persisted state as untrusted input and validate it before using it for critical workflows — a user can edit localStorage directly.
• Account for hydration timing before rendering UI that depends on persisted values, especially under SSR where the server has no access to browser storage.

Testing and Tooling

• Use @pinia/testing (createTestingPinia()) for component tests that need stores, with actions stubbed by default.
• Test store actions directly for domain behavior and edge cases, independent of any component.
• Reset Pinia between tests to avoid shared state leakage across test cases.
• Add HMR support with acceptHMRUpdate() in stores when the project uses Vite HMR patterns.
• Keep stores easy to inspect in Vue Devtools by using clear state names and focused, single-domain stores.

import { setActivePinia, createPinia } from 'pinia'
import { describe, it, expect, beforeEach } from 'vitest'
import { useCartStore } from '@/stores/cart'

describe('cart store', () => {
  beforeEach(() => {
    setActivePinia(createPinia())
  })

  it('merges quantities for duplicate line items', () => {
    const cart = useCartStore()
    cart.addLine({ id: '1', name: 'Widget', quantity: 1, unitPrice: 10 })
    cart.addLine({ id: '1', name: 'Widget', quantity: 2, unitPrice: 10 })
    expect(cart.itemCount).toBe(3)
  })
})

Anti-Patterns

• Do not use Pinia as a dumping ground for every reactive value — most component state should stay local.
• Do not destructure state directly from a store without storeToRefs(); this breaks reactivity silently.
• Do not mutate props or route objects through store actions.
• Do not put server-only objects, request instances, DOM nodes, or timers in store state — they don't serialize and break SSR/devtools.
• Do not create circular reads between stores in setup functions; compose stores through actions or computed values instead of having two stores read each other at module scope.