Nuxt/docs/content/3.docs/1.usage/2.state.md

# State

Nuxt provides `useState` to create a globally shared state within the context.

`useState` is SSR-friendly `ref` replacement in that its value will be hydrated (preserved) after server-side rendering and is shared across all components using a unique key.

::alert{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!

✅ Instead use `const useX = () => useState('x')`
::

## Usage

Within your pages, components and plugins you can use `useState`.

```js
const state = useState<T>(key: string, init?: () => T): Ref<T>
```

* **key**: A unique key ensuring that data fetching can be properly de-duplicated across requests
* **init**: A function that provides initial value for the state when it's not initiated
* **T**: (typescript only) Specify type of state

::alert{icon=👉}
**`useState` only works during `setup` or `Lifecycle Hooks`**
::

## Shared State

Using [auto-imported composables](/docs/directory-structure/composables) we can define global type-safe states.

::code-group

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

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

```

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

::

## Example

In this example, we use a [server-only plugin](/docs/directory-structure/plugins) to find about request locale.

::code-group

```vue [app.vue]
<script setup>
const locale = useLocale()
</script>

<template>
  Current locale: {{ locale }}
</template>
```

```ts [composables/states.ts]
export const useLocale = () => useState<string>('locale', () => 'en')
```

```ts [plugins/locale.server.ts]
export default defineNuxtPlugin((nuxtApp) => {
  const locale = useLocale()
  locale.value = locale.value ?? nuxtApp.ssrContext?.req.headers['accept-language']?.split(',')[0]
})
```

::

:button-link[Full example on StackBlitz]{href="https://stackblitz.com/github/nuxt/framework/tree/main/examples/use-state?terminal=dev" blank}