# State Nuxt provides `useState` composable to create a reactive and SSR-friendly shared state across components. `useState` is an SSR-friendly [`ref`](https://v3.vuejs.org/api/refs-api.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. ## Signature ```ts 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 the type of state ::alert{icon=👉} `useState` only works during `setup` or [`Lifecycle Hooks`](https://v3.vuejs.org/guide/composition-api-lifecycle-hooks.html#lifecycle-hooks). :: ## 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> ``` :button-link[Open on StackBlitz]{href="https://stackblitz.com/github/nuxt/framework/tree/main/examples/use-state?terminal=dev" blank} ### Advanced 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. :button-link[Open on StackBlitz]{href="https://stackblitz.com/github/nuxt/framework/tree/main/examples/locale?terminal=dev" blank} ## Shared state By using [auto-imported composables](/docs/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> ```