Nuxt/docs/3.api/3.utils/call-once.md

title	description	navigation	links
callOnce	Run a given function or block of code once during SSR or CSR.	badge New	label icon to size Source i-simple-icons-github https://github.com/nuxt/nuxt/blob/main/packages/nuxt/src/app/composables/once.ts xs

::callout{icon="i-ph-info-duotone" color="blue"} This utility is available since Nuxt 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

<script setup lang="ts">
const websiteConfig = useState('config')

await callOnce(async () => {
  console.log('This will only be logged once')
  websiteConfig.value = await $fetch('https://my-cms.com/api/website-config')
})
</script>

::callout{to="/docs/getting-started/state-management#usage-with-pinia"} callOnce is useful in combination with the Pinia module to call store actions. ::

:read-more{to="/docs/getting-started/state-management"}

::callout{color="info" icon="i-ph-warning-duotone"} Note that callOnce doesn't return anything. You should use useAsyncData or useFetch if you want to do data fetching during SSR. ::

::callout 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

callOnce(fn?: () => any | Promise<any>): Promise<void>
callOnce(key: string, fn?: () => any | Promise<any>): Promise<void>

• 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. This function can also return a Promise and a value.