Build your first Astro island

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Use a Preact component to greet your visitors with a randomly-selected welcome message!

Preparati a…

Add Preact to your Astro project
Include Astro islands (Preact .jsx components) on your home page
Use client: directives to make islands interactive

Add Preact to your Astro project

If it’s running, quit the dev server to have access to the terminal (keyboard shortcut: Ctrl + C).
Add the ability to use Preact components in your Astro project with a single command:
- npm
- pnpm
- Yarn
Terminal window
npx astro add preact
Terminal window
pnpm astro add preact
Terminal window
yarn astro add preact
Follow the command line instructions to confirm adding Preact to your project.

This component will take an array of greeting messages as a prop and randomly select one of them to show as a welcome message. The user can click a button to get a new random message.

Create a new file in src/components/ named Greeting.jsx

Note the .jsx file extension. This file will be written in Preact, not Astro.

Add the following code to Greeting.jsx:

import { useState } from 'preact/hooks';

export default function Greeting({messages}) {

  const randomMessage = () => messages[(Math.floor(Math.random() * messages.length))];

  const [greeting, setGreeting] = useState(messages[0]);

  return (
    <div>
      <h3>{greeting}! Thank you for visiting!</h3>
      <button onClick={() => setGreeting(randomMessage())}>
        New Greeting
      </button>
    </div>
  );
}

Import and use this component on your Home page index.astro.

---
import BaseLayout from '../layouts/BaseLayout.astro';
import Greeting from '../components/Greeting';
const pageTitle = "Home Page";
---
<BaseLayout pageTitle={pageTitle}>
  <h2>My awesome blog subtitle</h2>
  <Greeting messages={["Hi", "Hello", "Howdy", "Hey there"]} />
</BaseLayout>

Check the preview in your browser: you should see a random greeting, but the button won’t work!

Add a second <Greeting /> component with the client:load directive.

---
import BaseLayout from '../layouts/BaseLayout.astro';
import Greeting from '../components/Greeting';
const pageTitle = "Home Page";
---
<BaseLayout pageTitle={pageTitle}>
  <h2>My awesome blog subtitle</h2>
  <Greeting messages={["Hi", "Hello", "Howdy", "Hey there"]} />
  <Greeting client:load messages={["Hej", "Hallo", "Hola", "Habari"]} />
</BaseLayout>

Revisit your page and compare the two components. The second button works because the client:load directive tells Astro to send and rerun its JavaScript on the client when the page loads, making the component interactive. This is called a hydrated component.

Once the difference is clear, remove the non-hydrated Greeting component.

---
import BaseLayout from '../layouts/BaseLayout.astro';
import Greeting from '../components/Greeting';
const pageTitle = "Home Page";
---
<BaseLayout pageTitle={pageTitle}>
  <h2>My awesome blog subtitle</h2>
  <Greeting messages={["Hi", "Hello", "Howdy", "Hey there"]} />
  <Greeting client:load messages={["Hej", "Hallo", "Hola", "Habari"]} />
</BaseLayout>

Analyze the Pattern

There are other client: directives to explore. Each sends the JavaScript to the client at a different time. client:visible, for example, will only send the component’s JavaScript when it is visible on the page.

Consider an Astro component with the following code:

---
import BaseLayout from '../layouts/BaseLayout.astro';
import AstroBanner from '../components/AstroBanner.astro';
import PreactBanner from '../components/PreactBanner';
import SvelteCounter from '../components/SvelteCounter.svelte';
---
<BaseLayout>
  <AstroBanner />
  <PreactBanner />
  <PreactBanner client:load />
  <SvelteCounter />
  <SvelteCounter client:visible />
</BaseLayout>

Which of the five components will be hydrated islands, sending JavaScript to the client?

<PreactBanner client:load /> and <SvelteCounter client:visible /> will be hydrated islands.
In what way(s) will the two <PreactBanner /> components be the same? In what way(s) will they be different?

Same: They both show the same HTML elements and look the same initially. Different: The component with the client:load directive will rerender after the page is loaded, and any interactive elements that it has will work.
Assume the SvelteCounter component shows a number and has a button to increase it. If you couldn’t see your website’s code, only the live published page, how would you tell which of the two <SvelteCounter /> components used client:visible?

Try clicking the button, and see which one is interactive. If it responds to your input, it must have had a client: directive.