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

navigation.icon	description
uil:database	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 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. :: ::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.
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.

<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"} ::

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.

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)))
}

<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 we can define global type-safe states and import them across the app.

export const useCounter = () => useState<number>('counter', () => 0)
export const useColor = () => useState<string>('color', () => 'pink')

<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.

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 - the official Vue recommendation
• Harlem - immutable global state management
• XState - state machine approach with tools for visualising and testing your state logic