pinia@4.0.3

Source citations use package-relative paths from the prepared pinia 4.0.3 source: package.json, README.md, dist/pinia.js (runtime), dist/pinia.d.ts (types). Other citations are official documentation URLs.

Requirements

Vue ^3.5.11 peer [package.json:62]. Vue 2 is unsupported since v3.
@vue/devtools-api ^8.1.5 is a required, non-optional peer [package.json:60-68]. Install it alongside pinia: npm install pinia @vue/devtools-api [README.md:108-111], v4.0.0 release notes.
TypeScript >=5.6.0 optional peer [package.json:69-71].
ESM-only package: "type": "module", single export . to dist/pinia.js [package.json:42,47-50]. v4 removed CJS resolution; a require() fallback breaks.
@pinia/nuxt 1.x and @pinia/testing 2.x are the companion versions for pinia 4 (https://github.com/vuejs/pinia/releases).

Install and setup

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

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

From [README.md:119-130]. In dev on the client, createPinia() registers the devtools plugin itself [dist/pinia.js:832,847].

Define a store

Two syntaxes. The id is a unique string, first argument in both [dist/pinia.d.ts:645,653].

import { defineStore } from 'pinia'

// Option store: state/getters/actions
export const useCounterStore = defineStore('counter', {
  state: () => ({ count: 0 }),
  getters: {
    doubleCount: (state) => state.count * 2,
  },
  actions: {
    increment() {
      this.count++
    },
  },
})

// Setup store: refs are state, computed are getters, functions are actions
export const useTodos = defineStore('todos', () => {
  const todos = ref<Todo[]>([])
  const done = computed(() => todos.value.filter((t) => t.done))
  function add(todo: Todo) {
    todos.value.push(todo)
  }
  return { todos, done, add }
})

Rules for setup stores [dist/pinia.js:1178-1202]:

ref() and reactive() become state.
computed() becomes a getter.
Functions become actions, wrapped automatically for $onAction.
Return every state property. Pinia only picks up returned state, so private state breaks SSR hydration and devtools.

The setup function receives helpers: defineStore('id', ({ action }) => { ... }) [dist/pinia.js:1177, dist/pinia.d.ts:628-638]. action(fn, name?) makes a helper function trackable by $onAction when called within the store. Rarely needed; intended for advanced cases like Pinia Colada.

Use a store

Call useStore() inside setup() (or any composable). Every call returns the same store instance.

const counter = useCounterStore()

counter.count++            // direct state mutation
counter.doubleCount        // getter
counter.increment()        // action

const { count, doubleCount } = storeToRefs(counter) // refs for destructuring

storeToRefs() converts state and getters to refs. It ignores actions and non-reactive properties [dist/pinia.js:1475-1489]. Destructure with it, never plain toRefs() or ...store, when you need reactivity.

Common operations:

Group changes into one devtools entry with store.$patch(obj) or $patch(fn). The function form must be synchronous [dist/pinia.d.ts:219-225] and suits array, Map, and Set edits [dist/pinia.js:964-975].
$reset() exists only on option stores. On a setup store it throws in dev [dist/pinia.js:1085-1087]; implement your own $reset in the returned object.
Assigning store.$state = {...} does not replace state; it calls $patch internally [dist/pinia.js:1207-1216].
Declare every state key in state() (or return it from setup), even when the initial value is undefined. New keys added later are not reactive.
Outside a component, pass the pinia instance: useStore(pinia). On the server this is mandatory; skipping it risks cross-request state pollution [dist/pinia.js:37-39].

Details: stores, subscriptions.

Version notes and migration

v4.0.0, technically breaking only (https://github.com/vuejs/pinia/releases/tag/v4.0.0):

ESM-only distribution.
@vue/devtools-api upgraded to v8 and now a required install.
Errors and dev warnings refactored onto Nostics diagnostics, codes PINIA_R1001 to PINIA_R1007 [dist/pinia.js:18-56]. See diagnostics.
piniaSymbol is now part of the public exports [dist/pinia.js:1491].
storeToRefs() skips nullish values gracefully; $subscribe() ignores a duplicate callback instead of stacking a second watcher.

Still relevant from v3.0.0 (https://pinia.vuejs.org/cookbook/migration-v2-v3.html):

defineStore({ id: 'x', ... }) object-only signature removed. Use defineStore('x', { ... }).
PiniaStorePlugin type removed. Use PiniaPlugin.
Vue 2 support dropped. Vue 2 users stay on pinia v2.

Reference files

API surface: every public export with signatures and type helpers.
Stores: state, getters, actions, typing, composing stores, HMR.
Subscriptions: $subscribe, $onAction, MutationType, detached and flush options.
Plugins: pinia.use(), context, typing extensions, new state and options.
SSR: hydration, skipHydrate, per-request pinia, Nuxt.
Testing: setActivePinia, @pinia/testing 2.x usage.
Options API: map helpers without setup().
Diagnostics: dev-only PINIA_R100x catalog with fixes.

Official docs: https://pinia.vuejs.org

pinia-skilld