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Every component is rendered on the server at build time. Hydration directives _optionally_ add client-side hydration at runtime.
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| layout | title |
|---|---|
| ~/layouts/Main.astro | React, Svelte, Vue, etc. |
By default, Astro generates your site with zero client-side JavaScript. If you use any frontend UI components (React, Svelte, Vue, etc.) Astro will automatically render them on the server, generating only HTML and CSS without any client-side JavaScript. This makes your site as fast as possible by default.
---
import MyReactComponent from '../components/MyReactComponent.jsx';
---
<!-- By default: Astro renders this on the server,
generating HTML and CSS, but no client-side JavaScript. -->
<MyReactComponent />
However, there are plenty of cases where you might like to include an interactive component on your page:
- An image carousel
- An auto-complete search bar
- A mobile sidebar open/close button
- A "Buy Now" button
With Astro, you can hydrate these components individually, without forcing the rest of the page to ship any other unnecesary JavaScript. This technique is called partial hydration.
Hydrate Frontend Components
Astro renders every component on the server at build time. To hydrate your components on the client at runtime, you may use any of the following techniques:
<MyComponent:load />will hydrate the component on page load.<MyComponent:idle />will use [requestIdleCallback()][mdn-ric] to hydrate the component as soon as main thread is free.<MyComponent:visible />will use an [IntersectionObserver][mdn-io] to hydrate the component when the element enters the viewport.
Hydrate Astro Components
Astro components (.astro) are HTML-only templating languages with no client-side runtime. You cannot hydrate an Astro component to run on the client (because the JavaScript front-matter only ever runs at build time).
If you want to make your Astro component interactive on the client, you should convert it to React, Svelte, or Vue. Otherwise, you can consider adding a <script> tag to your Astro component that will run JavaScript on the page.