Nuxt/docs/3.api/2.composables/on-prehydrate.md

---
title: "onPrehydrate"
description: "Use onPrehydrate to run a callback on the client immediately before
Nuxt hydrates the page."
links:
  - label: Source
    icon: i-simple-icons-github
    to: https://github.com/nuxt/nuxt/blob/main/packages/nuxt/src/app/composables/ssr.ts
    size: xs
---

::important
This composable is available in Nuxt v3.12+.
::

`onPrehydrate` is a composable lifecycle hook that allows you to run a callback on the client immediately before
Nuxt hydrates the page.

::note
This is an advanced utility and should be used with care. For example, [`nuxt-time`](https://github.com/danielroe/nuxt-time/pull/251) and [`@nuxtjs/color-mode`](https://github.com/nuxt-modules/color-mode/blob/main/src/script.js) manipulate the DOM to avoid hydration mismatches.
::

## Usage

`onPrehydrate` can be called directly in the setup function of a Vue component (for example, in `<script setup>`), or in a plugin.
It will only have an effect when it is called on the server, and it will not be included in your client build.

## Parameters

- `callback`: A function that will be stringified and inlined in the HTML. It should not have any external
dependencies (such as auto-imports) or refer to variables defined outside the callback. The callback will run
before Nuxt runtime initializes so it should not rely on the Nuxt or Vue context.

## Example

```vue twoslash [app.vue]
<script setup lang="ts">
declare const window: Window
// ---cut---
// onPrehydrate is guaranteed to run before Nuxt hydrates
onPrehydrate(() => {
  console.log(window)
})

// As long as it only has one root node, you can access the element
onPrehydrate((el) => {
  console.log(el.outerHTML)
  // <div data-v-inspector="app.vue:15:3" data-prehydrate-id=":b3qlvSiBeH:"> Hi there </div>
})

// For _very_ advanced use cases (such as not having a single root node) you
// can access/set `data-prehydrate-id` yourself
const prehydrateId = onPrehydrate((el) => {})
</script>

<template>
  <div>
    Hi there
  </div>
</template>
```