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62 lines
2.1 KiB
Markdown
62 lines
2.1 KiB
Markdown
---
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title: "onPrehydrate"
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description: "Use onPrehydrate to run a callback on the client immediately before
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Nuxt hydrates the page."
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links:
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- label: Source
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icon: i-simple-icons-github
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to: https://github.com/nuxt/nuxt/blob/main/packages/nuxt/src/app/composables/ssr.ts
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size: xs
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---
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::important
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This composable will be available in Nuxt v3.12+ or in [the nightly release channel](/docs/guide/going-further/nightly-release-channel).
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::
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`onPrehydrate` is a composable lifecycle hook that allows you to run a callback on the client immediately before
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Nuxt hydrates the page.
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::note
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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.
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::
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## Usage
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`onPrehydrate` can be called directly in the setup function of a Vue component (for example, in `<script setup>`), or in a plugin.
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It will only have an effect when it is called on the server, and it will not be included in your client build.
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## Parameters
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- `callback`: A function that will be stringified and inlined in the HTML. It should not have any external
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dependencies (such as auto-imports) or refer to variables defined outside the callback. The callback will run
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before Nuxt runtime initializes so it should not rely on the Nuxt or Vue context.
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## Example
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```vue twoslash [app.vue]
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<script setup lang="ts">
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declare const window: Window
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// ---cut---
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// onPrehydrate is guaranteed to run before Nuxt hydrates
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onPrehydrate(() => {
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console.log(window)
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})
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// As long as it only has one root node, you can access the element
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onPrehydrate((el) => {
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console.log(el.outerHTML)
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// <div data-v-inspector="app.vue:15:3" data-prehydrate-id=":b3qlvSiBeH:"> Hi there </div>
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})
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// For _very_ advanced use cases (such as not having a single root node) you
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// can access/set `data-prehydrate-id` yourself
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const prehydrateId = onPrehydrate((el) => {})
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</script>
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<template>
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<div>
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Hi there
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</div>
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</template>
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```
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