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36 lines
1.5 KiB
Markdown
36 lines
1.5 KiB
Markdown
---
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title: "useState"
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description: The useState composable creates a reactive and SSR-friendly shared state.
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---
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# `useState`
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```ts
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useState<T>(init?: () => T | Ref<T>): Ref<T>
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useState<T>(key: string, init?: () => T | Ref<T>): Ref<T>
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```
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* **key**: A unique key ensuring that data fetching is properly de-duplicated across requests. If you do not provide a key, then a key that is unique to the file and line number of the instance of [`useState`](/docs/api/composables/use-state) will be generated for you.
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* **init**: A function that provides initial value for the state when not initiated. This function can also return a `Ref`.
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* **T**: (typescript only) Specify the type of state
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::alert{type=warning}
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Because the data inside [`useState`](/docs/api/composables/use-state) will be serialized to JSON, it is important that it does not contain anything that cannot be serialized, such as classes, functions or symbols.
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::
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::alert{type=warning}
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[`useState`](/docs/api/composables/use-state) is a reserved function name transformed by the compiler, so you should not name your own function `useState`.
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::
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::ReadMore{link="/docs/getting-started/state-management"}
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::
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## Using `shallowRef`
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If you don't need your state to be deeply reactive, you can combine `useState` with [`shallowRef`](https://vuejs.org/api/reactivity-advanced.html#shallowref). This can improve performance when your state contains large objects and arrays.
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```ts
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const state = useState('my-shallow-state', () => shallowRef({ deep: 'not reactive' }))
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// isShallow(state) === true
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```
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