---
title: "useState"
description: The useState composable creates a reactive and SSR-friendly shared state.
---

# `useState`

```ts
useState<T>(init?: () => T | Ref<T>): Ref<T>
useState<T>(key: string, init?: () => T | Ref<T>): Ref<T>
```

* **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.
* **init**: A function that provides initial value for the state when not initiated. This function can also return a `Ref`.
* **T**: (typescript only) Specify the type of state

::alert{type=warning}
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.
::

::alert{type=warning}
[`useState`](/docs/api/composables/use-state) is a reserved function name transformed by the compiler, so you should not name your own function `useState`.
::

::ReadMore{link="/docs/getting-started/state-management"}
::

## Using `shallowRef`

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.

```ts
const state = useState('my-shallow-state', () => shallowRef({ deep: 'not reactive' }))
// isShallow(state) === true
```