State

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The state is, most of the time, the central part of your store. People often start by defining the state that represents their app. In Pinia the state is defined as a function that returns the initial state. This allows Pinia to work in both Server and Client Side.

import { defineStore } from 'pinia'

export const useStore = defineStore('storeId', {
  // arrow function recommended for full type inference
  state: () => {
    return {
      // all these properties will have their type inferred automatically
      count: 0,
      name: 'Eduardo',
      isAdmin: true,
      items: [],
      hasChanged: true,
    }
  },
})

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If you are using Vue 2, the data you create in state follows the same rules as the data in a Vue instance, i.e. the state object must be plain and you need to call Vue.set() when adding new properties to it. See also: Vue#data.

TypeScript

You don't need to do much in order to make your state compatible with TS: make sure strict, or at the very least, noImplicitThis, is enabled and Pinia will infer the type of your state automatically! However, there are a few cases where you should give it a hand with some casting:

export const useUserStore = defineStore('user', {
  state: () => {
    return {
      // for initially empty lists
      userList: [] as UserInfo[],
      // for data that is not yet loaded
      user: null as UserInfo | null,
    }
  },
})

interface UserInfo {
  name: string
  age: number
}

If you prefer, you can define the state with an interface and type the return value of state():

interface State {
  userList: UserInfo[]
  user: UserInfo | null
}

export const useUserStore = defineStore('user', {
  state: (): State => {
    return {
      userList: [],
      user: null,
    }
  },
})

interface UserInfo {
  name: string
  age: number
}

Accessing the state

By default, you can directly read and write to the state by accessing it through the store instance:

const store = useStore()

store.count++

Note you cannot add a new state property if you don't define it in state(). It must contain the initial state. e.g.: we can't do store.secondCount = 2 if secondCount is not defined in state().

Resetting the state

In Option Stores, you can reset the state to its initial value by calling the $reset() method on the store:

const store = useStore()

store.$reset()

Internally, this calls the state() function to create a new state object and replaces the current state with it.

In Setup Stores, you need to create your own $reset() method:

export const useCounterStore = defineStore('counter', () => {
  const count = ref(0)

  function $reset() {
    count.value = 0
  }

  return { count, $reset }
})

Usage with the Options API

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For the following examples, you can assume the following store was created:

// Example File Path:
// ./src/stores/counter.js

import { defineStore } from 'pinia'

export const useCounterStore = defineStore('counter', {
  state: () => ({
    count: 0,
  }),
})

If you are not using the Composition API, and you are using computed, methods, ..., you can use the mapState() helper to map state properties as readonly computed properties:

import { mapState } from 'pinia'
import { useCounterStore } from '../stores/counter'

export default {
  computed: {
    // gives access to this.count inside the component
    // same as reading from store.count
    ...mapState(useCounterStore, ['count'])
    // same as above but registers it as this.myOwnName
    ...mapState(useCounterStore, {
      myOwnName: 'count',
      // you can also write a function that gets access to the store
      double: store => store.count * 2,
      // it can have access to `this` but it won't be typed correctly...
      magicValue(store) {
        return store.someGetter + this.count + this.double
      },
    }),
  },
}

Modifiable state

If you want to be able to write to these state properties (e.g. if you have a form), you can use mapWritableState() instead. Note you cannot pass a function like with mapState():

import { mapWritableState } from 'pinia'
import { useCounterStore } from '../stores/counter'

export default {
  computed: {
    // gives access to this.count inside the component and allows setting it
    // this.count++
    // same as reading from store.count
    ...mapWritableState(useCounterStore, ['count']),
    // same as above but registers it as this.myOwnName
    ...mapWritableState(useCounterStore, {
      myOwnName: 'count',
    }),
  },
}

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You don't need mapWritableState() for collections like arrays unless you are replacing the whole array with cartItems = [], mapState() still allows you to call methods on your collections.

Mutating the state

Apart from directly mutating the store with store.count++, you can also call the $patch method. It allows you to apply multiple changes at the same time with a partial state object:

store.$patch({
  count: store.count + 1,
  age: 120,
  name: 'DIO',
})

However, some mutations are really hard or costly to apply with this syntax: any collection modification (e.g. pushing, removing, splicing an element from an array) requires you to create a new collection. Because of this, the $patch method also accepts a function to group these kinds of mutations that are difficult to apply with a patch object:

store.$patch((state) => {
  state.items.push({ name: 'shoes', quantity: 1 })
  state.hasChanged = true
})

The main difference here is that $patch() allows you to group multiple changes into one single entry in the devtools. Note that both direct changes to state and $patch() appear in the devtools and can be time traveled (not yet in Vue 3).

Replacing the state

You cannot exactly replace the state of a store as that would break reactivity. You can however patch it:

// this doesn't actually replace `$state`
store.$state = { count: 24 }
// it internally calls `$patch()`:
store.$patch({ count: 24 })

You can also set the initial state of your whole application by changing the state of the pinia instance. This is used during

pinia.state.value = {}

Subscribing to the state

You can watch the state and its changes through the $subscribe() method of a store, similar to Vuex's subscribe method. The advantage of using $subscribe() over a regular watch() is that subscriptions will trigger only once after patches (e.g. when using the function version from above).

cartStore.$subscribe((mutation, state) => {
  // import { MutationType } from 'pinia'
  mutation.type // 'direct' | 'patch object' | 'patch function'
  // same as cartStore.$id
  mutation.storeId // 'cart'
  // only available with mutation.type === 'patch object'
  mutation.payload // patch object passed to cartStore.$patch()

  // persist the whole state to the local storage whenever it changes
  localStorage.setItem('cart', JSON.stringify(state))
})

By default, state subscriptions are bound to the component where they are added (if the store is inside a component's setup()). Meaning, they will be automatically removed when the component is unmounted. If you also want to keep them after the component is unmounted, pass { detached: true } as the second argument to detach the state subscription from the current component:

<script setup>
const someStore = useSomeStore()

// this subscription will be kept even after the component is unmounted
someStore.$subscribe(callback, { detached: true })
</script>

TIP

You can watch the whole state on the pinia instance with a single watch():

watch(
  pinia.state,
  (state) => {
    // persist the whole state to the local storage whenever it changes
    localStorage.setItem('piniaState', JSON.stringify(state))
  },
  { deep: true }
)