Nuxt/docs/1.getting-started/7.state-management.md

---
navigation.icon: uil:database
description: Nuxt provides powerful state management libraries and the useState composable to create a reactive and SSR-friendly shared state.
---

# State Management

Nuxt provides the `useState` composable to create a reactive and SSR-friendly shared state across components.

`useState` is an SSR-friendly [`ref`](https://vuejs.org/api/reactivity-core.html#ref) replacement. Its value will be preserved after server-side rendering (during client-side hydration) and shared across all components using a unique key.

::ReadMore{link="/docs/api/composables/use-state"}
::

::alert{icon=👉}
`useState` only works during `setup` or [`Lifecycle Hooks`](https://vuejs.org/api/composition-api-lifecycle.html#composition-api-lifecycle-hooks).
::
::alert{type=warning}
Because the data inside `useState` will be serialized to JSON, it is important that it does not contain anything that cannot be serialized, such as classes, functions or symbols.
::

## Best Practices

::alert{type=danger icon=🚨}
Never define `const state = ref()` outside of `<script setup>` or `setup()` function.<br>
Such state will be shared across all users visiting your website and can lead to memory leaks!
::
::alert{type=success icon=✅}
Instead use `const useX = () => useState('x')`
::

## Examples

### Basic Usage

In this example, we use a component-local counter state. Any other component that uses `useState('counter')` shares the same reactive state.

```vue [app.vue]
<script setup>
const counter = useState('counter', () => Math.round(Math.random() * 1000))
</script>

<template>
  <div>
    Counter: {{ counter }}
    <button @click="counter++">
      +
    </button>
    <button @click="counter--">
      -
    </button>
  </div>
</template>
```

::LinkExample{link="/docs/examples/composables/use-state"}
::

::ReadMore{link="/docs/api/composables/use-state"}
::

::alert{icon=📘}
To globally invalidate cached state, see [`clearNuxtState`](/docs/api/utils/clear-nuxt-state).
::

### Advanced Usage

In this example, we use a composable that detects the user's default locale from the HTTP request headers and keeps it in a `locale` state.

```ts [composables/locale.ts]
import type { Ref } from 'vue'

export const useLocale = () => useState<string>('locale', () => useDefaultLocale().value)

export const useDefaultLocale = (fallback = 'en-US') => {
  const locale = ref(fallback)
  if (process.server) {    
    const reqLocale = useRequestHeaders()['accept-language']?.split(',')[0]
    if (reqLocale) {
      locale.value = reqLocale
    }
  } else if (process.client) {
    const navLang = navigator.language
    if (navLang) {
      locale.value = navLang
    }
  }
  return locale
}

export const useLocales = () => {
  const locale = useLocale()
  const locales = ref([
    'en-US',
    'en-GB',
    ...
    'ja-JP-u-ca-japanese'
  ])
  if (!locales.value.includes(locale.value)) {
    locales.value.unshift(locale.value)
  }
  return locales
}

export const useLocaleDate = (date: Ref<Date> | Date, locale = useLocale()) => {
  return computed(() => new Intl.DateTimeFormat(locale.value, { dateStyle: 'full' }).format(unref(date)))
}
```

```vue [app.vue]
<script setup>
const locales = useLocales()
const locale = useLocale()
const date = useLocaleDate(new Date('2016-10-26'))
</script>

<template>
  <div>
    <h1>Nuxt birthday</h1>
    <p>{{ date }}</p>    
    <label for="locale-chooser">Preview a different locale</label>
    <select id="locale-chooser" v-model="locale">
      <option v-for="locale of locales" :key="locale" :value="locale">
        {{ locale }}
      </option>
    </select>
  </div>
</template>
```

::LinkExample{link="/docs/examples/other/locale"}
::

## Shared State

By using [auto-imported composables](/docs/guide/directory-structure/composables) we can define global type-safe states and import them across the app.

```ts [composables/states.ts]
export const useCounter = () => useState<number>('counter', () => 0)
export const useColor = () => useState<string>('color', () => 'pink')
```

```vue [app.vue]
<script setup>
const color = useColor() // Same as useState('color')
</script>

<template>
  <p>Current color: {{ color }}</p>
</template>
```

## Using third-party libraries

Nuxt **used to rely** on the Vuex library to provide global state management. If you are migrating from Nuxt 2, please head to [the migration guide](/docs/migration/configuration#vuex).

Nuxt is not opiniated about state management, so feel free to choose the right solution for your needs. There are multiple integrations with the most popular state management libraries, including:

- [Pinia](/modules/pinia) - the official Vue recommendation
- [Harlem](/modules/harlem) - immutable global state management
- [XState](/modules/xstate) - state machine approach with tools for visualising and testing your state logic