Using a store outside of a component

Pinia stores rely on the pinia instance to share the same store instance across all calls. Most of the time, this works out of the box by just calling your useStore() function. For example, in setup(), you don't need to do anything else. But things are a bit different outside of a component.
Behind the scenes, useStore() injects the pinia instance you gave to your app. This means that if the pinia instance cannot be automatically injected, you have to manually provide it to the useStore() function.
You can solve this differently depending on the kind of application you are writing.

Single Page Applications

If you are not doing any SSR (Server Side Rendering), any call of useStore() after installing the pinia plugin with app.use(pinia) will work:

import { useUserStore } from '@/stores/user'
import { createApp } from 'vue'
import App from './App.vue'

// ❌  fails because it's called before the pinia is created
const userStore = useUserStore()

const pinia = createPinia()
const app = createApp(App)
app.use(pinia)

// ✅ works because the pinia instance is now active
const userStore = useUserStore()

The easiest way to ensure this is always applied is to defer calls of useStore() by placing them inside functions that will always run after pinia is installed.

Let's take a look at this example of using a store inside of a navigation guard with Vue Router:

import { createRouter } from 'vue-router'
const router = createRouter({
  // ...
})

// ❌ Depending on the order of imports this will fail
const store = useStore()

router.beforeEach((to, from, next) => {
  // we wanted to use the store here
  if (store.isLoggedIn) next()
  else next('/login')
})

router.beforeEach((to) => {
  // ✅ This will work because the router starts its navigation after
  // the router is installed and pinia will be installed too
  const store = useStore()

  if (to.meta.requiresAuth && !store.isLoggedIn) return '/login'
})

SSR Apps

When dealing with Server Side Rendering, you will have to pass the pinia instance to useStore(). This prevents pinia from sharing global state between different application instances.

There is a whole section dedicated to it in the SSR guide, this is just a short explanation: