# useState
> The useState composable creates a reactive and SSR-friendly shared state.
## Usage
```ts
// Create a reactive state and set default value
const count = useState('counter', () => Math.round(Math.random() * 100))
```
Because the data inside `useState` will be serialized to JSON, it is important that it does not contain anything that cannot be serialized, such as classes, functions or symbols.
`useState` is a reserved function name transformed by the compiler, so you should not name your own function `useState`.
## 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#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
```
## Type
```ts [Signature]
export function useState (init?: () => T | Ref): Ref
export function useState (key: string, init?: () => T | Ref): Ref
```
- `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/4.x/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
## Troubleshooting
### `Cannot stringify arbitrary non-POJOs`
This error occurs when you try to store a non-serializable payload with `useState`, such as class instances.
If you want to store class instances with `useState` that are not supported by Nuxt, you can use [`definePayloadPlugin`](/docs/4.x/api/composables/use-nuxt-app#custom-reducerreviver) to add a custom serializer and deserializer for your classes.
---
- [Source](https://github.com/nuxt/nuxt/blob/main/packages/nuxt/src/app/composables/state.ts)