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81 lines
2.5 KiB
Markdown
81 lines
2.5 KiB
Markdown
# State
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Nuxt provides `useState` composable to create a reactive and ssr-friendly shared state across components.
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`useState` is a 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.
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## Signature
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```ts
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useState<T>(key: string, init?: () => T): Ref<T>
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```
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* **key**: A unique key ensuring that data fetching can be properly de-duplicated across requests
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* **init**: A function that provides initial value for the state when it's not initiated
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* **T**: (typescript only) Specify type of state
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::alert{icon=👉}
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`useState` only works during `setup` or [`Lifecycle Hooks`](https://v3.vuejs.org/guide/composition-api-lifecycle-hooks.html#lifecycle-hooks).
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::
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## Best practices
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::alert{type=danger icon=🚨}
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Never define `const state = ref()` outside of `<script setup>` or `setup()` function.<br>
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Such state will be shared across all users visiting your website and can lead to memory leaks!
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::
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::alert{type=success icon=✅}
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Instead use `const useX = () => useState('x')`
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::
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## Examples
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### Basic usage
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In this example, we use a component-local counter state. Any other component that uses `useState('counter')` shares the same reactive state.
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```vue [app.vue]
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<script setup>
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const counter = useState('counter', () => Math.round(Math.random() * 1000))
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</script>
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<template>
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<div>
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Counter: {{ counter }}
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<button @click="counter++">
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+
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</button>
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<button @click="counter--">
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-
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</button>
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</div>
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</template>
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```
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:button-link[Open on StackBlitz]{href="https://stackblitz.com/github/nuxt/framework/tree/main/examples/use-state?terminal=dev" blank}
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### Advanced
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In this example, we use a composable that detects the user's default locale from the request HTTP headers and keeps in `locale` state.
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:button-link[Open on StackBlitz]{href="https://stackblitz.com/github/nuxt/framework/tree/main/examples/locale?terminal=dev" blank}
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## Shared state
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By using [auto-imported composables](/docs/directory-structure/composables) we can define global type-safe states and import them across the app.
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```ts [composables/states.ts]
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export const useCounter = () => useState<number>('counter', () => 0)
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export const useColor = () => useState<string>('color', () => 'pink')
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```
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```vue [app.vue]
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<script setup>
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const color = useColor() // Same as useState('color')
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</script>
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<template>
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<p>Current color: {{ color }}</p>
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</template>
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```
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