capri

Capri (React)

Capri is a static site generator with island architecture. Only components with .island.tsx suffix ship JavaScript to the browser. Everything else renders to static HTML with zero JS.

Coming from Next.js?

• No getStaticProps - fetch data in main.server.tsx or use useFetch in components
• No <Head> component - render full documents via prerenderToNodeStream() or use CSS selector injection
• No API routes - this is pure static generation
• getStaticPaths exists but lives in main.server.tsx

Coming from Astro?

• Islands use .island.tsx file suffix instead of client:* directives
• Hydration timing via export const options = { loading: "visible" }
• No frontmatter - use React components throughout

Project Structure

├── main.tsx            # Client entry - hydrates islands
├── main.server.tsx     # SSR entry - renders static HTML, exports getStaticPaths
├── router.tsx          # File-based routing logic
├── pages/              # Page components (auto-discovered)
│   └── root.tsx        # Home page (/)
├── components/
│   ├── islands/        # Interactive components (.island.tsx)
│   └── ui/             # Static components (no JS shipped)
├── index.html          # HTML template
└── vite.config.ts      # Capri plugin config

Islands (Interactive Components)

Any component with .island.tsx suffix becomes interactive:

// components/islands/counter.island.tsx
import { useState } from "react";

export default function Counter({ start = 0 }) {
  const [count, setCount] = useState(start);
  return <button onClick={() => setCount((c) => c + 1)}>{count}</button>;
}

Hydration Options

Control when islands hydrate by exporting an options object:

export const options = {
  loading: "visible", // "eager" (default) | "idle" | "visible"
  media: "(max-width: 768px)", // Only hydrate when media query matches
};

• eager: Hydrate immediately (default)
• idle: Hydrate when browser is idle (requestIdleCallback)
• visible: Hydrate when scrolled into view (IntersectionObserver)
• media: Only hydrate when media query matches (e.g., mobile-only islands)

Data Fetching

In entry files (for CMS/API data)

Fetch data at the top level of main.server.tsx:

// main.server.tsx
const posts = await fetchPosts(); // Runs once at build time

export async function getStaticPaths() {
  return ["/", ...posts.map((p) => `/blog/${p.slug}`)];
}

In components (using React 19's use())

import { use } from "react";

const cache = new Map();
function fetchJson(url: string) {
  if (!cache.has(url)) {
    cache.set(
      url,
      fetch(url).then((r) => r.json()),
    );
  }
  return cache.get(url);
}

export function useFetch<T>(url: string): T {
  return use(fetchJson(url));
}

Routing

File-based routing

Files in pages/ map to routes:

• pages/root.tsx → /
• pages/about.tsx → /about
• pages/blog/post.tsx → /blog/post

Dynamic routes

Export getStaticPaths from main.server.tsx:

export async function getStaticPaths() {
  const posts = await fetchAllPosts();
  return ["/", "/about", ...posts.map((p) => `/blog/${p.slug}`)];
}

The router in router.tsx handles URL matching.

Head/Meta Management

Using prerenderToNodeStream (recommended)

React can render full HTML documents including <head>:

// main.server.tsx
import { prerenderToNodeStream } from "react-dom/static";

export async function render(url: string) {
  return {
    "#app": prerenderToNodeStream(<App path={url} />),
  };
}

CSS selector injection

Return an object with CSS selectors as keys:

export async function render(url: string) {
  return {
    "#app": renderToString(<App path={url} />),
    title: getPageTitle(url),
    "meta[name=description]": { content: getPageDescription(url) },
  };
}

Common Tasks

Commands

npm run dev      # Start dev server
npm run build    # Build static site (vite build && vite build --ssr)
npm run preview  # Preview production build

Vite Config

import capri from "@capri-js/react";
import react from "@vitejs/plugin-react";

export default {
  plugins: [
    react(),
    capri({
      prerender: ["/", "/about"], // Explicit paths to render
      followLinks: true, // Auto-discover by crawling links
      spa: "/preview", // Client-only SPA routes
      islandGlobPattern: "**/*.island.*",
      sitemap: { origin: "https://example.com" },
    }),
  ],
};

Key Files

• main.server.tsx - SSR render function, getStaticPaths
• main.tsx - Client hydration entry
• router.tsx - URL to component mapping
• vite.config.ts - Build configuration
• components/async/use-fetch.ts - Data fetching utility