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vitepress/docs/guide/ssr-compat.md

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---
outline: deep
---
# SSR Compatibility
VitePress pre-renders the app in Node.js during the production build, using Vue's Server-Side Rendering (SSR) capabilities. This means all custom code in theme components are subject to SSR Compatibility.
The [SSR section in official Vue docs](https://vuejs.org/guide/scaling-up/ssr.html) provides more context on what is SSR, the relationship between SSR / SSG, and common notes on writing SSR-friendly code. The rule of thumb is to only access browser / DOM APIs in `beforeMount` or `mounted` hooks of Vue components.
## `<ClientOnly>`
If you are using or demoing components that are not SSR-friendly (for example, contain custom directives), you can wrap them inside the built-in `<ClientOnly>` component:
```md
<ClientOnly>
<NonSSRFriendlyComponent />
</ClientOnly>
```
## Libraries that Access Browser API on Import
Some components or libraries access browser APIs **on import**. To use code that assumes a browser environment on import, you need to dynamically import them.
### Importing in Mounted Hook
```vue
<script setup>
import { onMounted } from 'vue'
onMounted(() => {
import('./lib-that-access-window-on-import').then((module) => {
// use code
})
})
</script>
```
### Conditional Import
You can also conditionally import a dependency using the `import.meta.env.SSR` flag (part of [Vite env variables](https://vitejs.dev/guide/env-and-mode.html#env-variables)):
```js
if (!import.meta.env.SSR) {
import('./lib-that-access-window-on-import').then((module) => {
// use code
})
}
```
Since [`Theme.enhanceApp`](./custom-theme#theme-interface) can be async, you can conditionally import and register Vue plugins that access browser APIs on import:
```js
// .vitepress/theme/index.js
/** @type {import('vitepress').Theme} */
export default {
// ...
async enhanceApp({ app }) {
if (!import.meta.env.SSR) {
const plugin = await import('plugin-that-access-window-on-import')
app.use(plugin)
}
}
}
```
If you're using TypeScript:
```ts
// .vitepress/theme/index.ts
import type { Theme } from 'vitepress'
export default {
// ...
async enhanceApp({ app }) {
if (!import.meta.env.SSR) {
const plugin = await import('plugin-that-access-window-on-import')
app.use(plugin)
}
}
} satisfies Theme
```
### `defineClientComponent`
VitePress provides a convenience helper for importing Vue components that access browser APIs on import.
```vue
<script setup>
import { defineClientComponent } from 'vitepress'
const ClientComp = defineClientComponent(() => {
return import('component-that-access-window-on-import')
})
</script>
<template>
<ClientComp />
</template>
```
You can also pass props/children/slots to the target component:
```vue
<script setup>
import { ref } from 'vue'
import { defineClientComponent } from 'vitepress'
const clientCompRef = ref(null)
const ClientComp = defineClientComponent(
() => import('component-that-access-window-on-import'),
// args are passed to h() - https://vuejs.org/api/render-function.html#h
[
{
ref: clientCompRef
},
{
default: () => 'default slot',
foo: () => h('div', 'foo'),
bar: () => [h('span', 'one'), h('span', 'two')]
}
],
// callback after the component is loaded, can be async
() => {
console.log(clientCompRef.value)
}
)
</script>
<template>
<ClientComp />
</template>
```
The target component will only be imported in the mounted hook of the wrapper component.