# callOnce
> Run a given function or block of code once during SSR or CSR.
This utility is available since [Nuxt v3.9](/blog/v3-9).
## Purpose
The `callOnce` function is designed to execute a given function or block of code only once during:
- server-side rendering but not hydration
- client-side navigation
This is useful for code that should be executed only once, such as logging an event or setting up a global state.
## Usage
The default mode of `callOnce` is to run code only once. For example, if the code runs on the server it won't run again on the client. It also won't run again if you `callOnce` more than once on the client, for example by navigating back to this page.
```vue [app/app.vue]
```
It is also possible to run on every navigation while still avoiding the initial server/client double load. For this, it is possible to use the `navigation` mode:
```vue [app/app.vue]
```
`navigation` mode is available since [Nuxt v3.15](/blog/v3-15).
`callOnce` is useful in combination with the [Pinia module](/modules/pinia) to call store actions.
Note that `callOnce` doesn't return anything. You should use [`useAsyncData`](/docs/4.x/api/composables/use-async-data) or [`useFetch`](/docs/4.x/api/composables/use-fetch) if you want to do data fetching during SSR.
`callOnce` is a composable meant to be called directly in a setup function, plugin, or route middleware, because it needs to add data to the Nuxt payload to avoid re-calling the function on the client when the page hydrates.
## Type
```ts [Signature]
export function callOnce (key?: string, fn?: (() => any | Promise), options?: CallOnceOptions): Promise
export function callOnce (fn?: (() => any | Promise), options?: CallOnceOptions): Promise
type CallOnceOptions = {
/**
* Execution mode for the callOnce function
* @default 'render'
*/
mode?: 'navigation' | 'render'
}
```
## Parameters
- `key`: A unique key ensuring that the code is run once. If you do not provide a key, then a key that is unique to the file and line number of the instance of `callOnce` will be generated for you.
- `fn`: The function to run once. It can be asynchronous.
- `options`: Setup the mode, either to re-execute on navigation (`navigation`) or just once for the lifetime of the app (`render`). Defaults to `render`.
- `render`: Executes once during initial render (either SSR or CSR) - Default mode
- `navigation`: Executes once during initial render and once per subsequent client-side navigation
---
- [Source](https://github.com/nuxt/nuxt/blob/main/packages/nuxt/src/app/composables/once.ts)