What is partial hydration in astro

AstroConceptBeginner · 4 min read

Partial Hydration in Astro: What It Is and How It Works

Partial hydration in Astro means loading only the interactive parts of a webpage with JavaScript, while the rest stays static HTML. This approach improves performance by sending less JavaScript to the browser and hydrating components only when needed.

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How It Works

Imagine a webpage as a car. The engine is the interactive parts that need JavaScript to run, while the body is the static content that just sits there. Partial hydration in Astro means you only start the engine for the parts that need it, instead of the whole car. This saves energy and makes the page load faster.

Astro renders most of the page as plain HTML on the server, which the browser can show immediately. Then, it adds JavaScript only to specific components that require interaction, like buttons or forms. This selective activation is called partial hydration.

This method reduces the amount of JavaScript the browser downloads and runs, leading to faster page loads and better user experience, especially on slow networks or devices.

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Example

This example shows an Astro component that partially hydrates a button only when the user interacts with it.

astro

---
import Counter from '../components/Counter.jsx';
---
<html lang="en">
  <head>
    <title>Partial Hydration Example</title>
  </head>
  <body>
    <h1>Welcome to Astro Partial Hydration</h1>
    <Counter client:idle />
  </body>
</html>

Output

<h1>Welcome to Astro Partial Hydration</h1> <button>Count: 0</button>

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When to Use

Use partial hydration when you want fast page loads but still need some interactive parts on your site. For example, a blog page can be mostly static text, but the comment form or like button can be interactive.

This approach is great for improving performance on content-heavy sites, e-commerce product pages, or landing pages where only a few elements need JavaScript.

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Key Points

Partial hydration loads JavaScript only for interactive components.
It improves page speed by reducing JavaScript sent to the browser.
Astro supports partial hydration with directives like client:load, client:idle, and client:visible.
This technique balances static content with dynamic interactivity.

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Key Takeaways

Partial hydration in Astro loads JavaScript only for interactive parts of a page.

It helps pages load faster by sending less JavaScript to the browser.

Use it when you want mostly static content with some interactive components.

Astro provides easy directives to control when components hydrate.

This technique improves user experience on slow devices and networks.