Astro Islands Architecture Skill

When to Use This Skill

Use this skill when:

• •Adding interactive components to Astro pages
• •Deciding between React, vanilla JS, or static HTML
• •Troubleshooting hydration errors
• •Optimizing JavaScript bundle size

Core Principle

React is for composition and layout. Vanilla JS is for interactivity.

In Astro, the best architecture ships minimal JavaScript:

• •Use React/components for SSR composition (zero JS shipped)
• •Use vanilla JS for simple interactions (theme toggle, mobile menu)
• •Use client:only sparingly for complex widgets that truly need React state

Astro Client Directives

Architecture Patterns

Pattern 1: Pure SSR (Recommended for most content)

---
// index.astro
import { HomePage } from '../components/HomePage';
const data = await fetchData();
---
<HomePage data={data} />  <!-- NO client directive = static HTML -->

What you get:

• •React components render to static HTML at build time
• •Zero JavaScript shipped
• •Fully testable in isolation
• •Fast page loads

Pattern 2: Vanilla JS for Simple Interactivity

---
// Layout.astro
---
<button id="theme-toggle">Toggle Theme</button>

<script is:inline>
  document.getElementById('theme-toggle').addEventListener('click', () => {
    const current = document.documentElement.getAttribute('data-theme');
    const next = current === 'dark' ? 'light' : 'dark';
    document.documentElement.setAttribute('data-theme', next);
    localStorage.setItem('theme', next);
  });
</script>

Use for:

• •Theme toggles (~20 lines)
• •Mobile menu open/close (~15 lines)
• •Simple form validation
• •Scroll-triggered animations

Benefits:

• •~0.5kb vs ~5kb+ for React hydration
• •No hydration bugs
• •Works immediately (no waiting for React to load)

Pattern 3: client:only for Complex Widgets (Last Resort)

---
// page.astro
import { PhotoGallery } from '../components/PhotoGallery';
---
<!-- Only when vanilla JS isn't sufficient -->
<PhotoGallery client:only="react" photos={photos} />

Use for:

• •Complex state management
• •Heavy user interaction (drag-drop, canvas)
• •Third-party React libraries

Trade-offs:

• •Ships React runtime (~5kb+)
• •Renders empty until JS loads
• •Cannot be nested inside SSR React

Critical Constraint

You CANNOT Nest Client Islands Inside SSR React

<!-- THIS DOES NOT WORK -->
<HomePage data={data} />  <!-- SSR React -->
<script>
  // Trying to mount React inside a div that HomePage rendered
  createRoot(document.getElementById('widget-root')).render(<Widget />);
</script>

Error: @vitejs/plugin-react can't detect preamble

Why: Astro's React integration doesn't support split-brain React:

• •Part server-rendered
• •Part client-mounted into server-rendered placeholders

Solution: Use vanilla JS for the interaction, or restructure so the interactive part is a sibling, not a child.

Decision Tree

Need to render HTML?
├── Yes → Use React component (no directive)
│   └── Need interactivity?
│       ├── Simple (toggle, menu) → Vanilla JS script
│       └── Complex (gallery, forms) → Separate client:only component
└── No → Why are you here?

Performance Comparison

Common Pitfalls

DON'T: Use client:load on Layout Components

<!-- Bad: Ships React for static content -->
<Layout client:load>
  <Content />
</Layout>

DON'T: Over-engineer Simple Interactions

<!-- Bad: React for a toggle -->
<ThemeToggle client:only="react" />

<!-- Good: Vanilla JS -->
<button id="theme-toggle">Toggle</button>
<script is:inline>/* 10 lines */</script>

DON'T: Mix Astro Components Inside React

// This DOES NOT WORK
export const Page = () => {
  return <AstroComponent />;  // Can't import .astro into .tsx
};

DO: Keep React Containers Pure SSR

// Good: No useState, no useEffect
export const HomePage: React.FC<Props> = ({ articles }) => {
  return (
    <>
      <Header />
      <ArticleGrid articles={articles} />
      <Footer />
    </>
  );
};

Script Directive Reference

Use is:inline for:

• •Theme initialization (must run before first paint)
• •Tiny scripts where bundling overhead isn't worth it
• •Scripts that need to run synchronously

Testing Strategy

You cannot test vanilla JS interactions in component isolation tools like Ladle - they need the full Astro page context.