Create a new site

npm init capri my-capri-site -- -e preact

This will download and install capri-js/capri/examples/preact.

You can view a deployed version of the demo on GitHub pages, including the preview SPA.

Entry files

The client entry file is a regular Preact single page app. We render a <PreviewBanner> on top of the app, so users know that they are viewing the SPA version, which is usually the case when they are editing the site in a CMS.

// src/main.tsx

import { render } from "preact";
import { Router } from "wouter-preact";

import { App } from "./App";
import { PreviewBanner } from "./Preview.jsx";

    <PreviewBanner />
    <App />

We use wouter-preact in this demo but you could as well use preact-router or even react-router together with preact/compat.

On the server, we use renderToString and a staticLocationHook hook:

// src/main.server.tsx

import { renderToString } from "@capri-js/preact/server";
import { Router } from "wouter-preact";
import staticLocationHook from "wouter-preact/static-location";

import { App } from "./App";

export async function render(url: string) {
  const hook = staticLocationHook(url);
  const res = await renderToString(
    <Router hook={hook}>
      <App />

  return {
    "#app": res.html,


You can define interactive islands by naming your components *.island.tsx:

// src/Counter.island.tsx

import { useState } from "preact/hooks";

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

Media queries

You can export an options object to hydrate an island as soon as a media query matches. The following example will hydrate once the viewport width gets below 700px:

import { useEffect, useState } from "preact/hooks";

export const options = {
  media: "(max-width:700px)",

export default function MediaQuery() {
  const [content, setContent] = useState(
    "Resize your browser below 700px to hydrate this island."
  useEffect(() => {
    setContent("The island has been hydrated.");
  }, []);
  return <div>{content}</div>;

Data fetching

Components can throw a promise and will get re-rendered once the promise is resolved. You can use a library like @urql/preact that supports suspense or write your own custom hook:

// src/hooks/useFetch.ts

const promises = new Map();
const response = new Map();

export function useFetch(url: string) {
  const data = response.get(url);
  if (data) return data;
  let promise = promises.get(url);
  if (!promise) {
    promise = fetch(url).then((res) => res.json().then(data => response.set(url, data));
    promises.set(url, promise);
  throw promise;
// src/Profile.tsx
import { useFetch } from "./hooks/useFetch.js";

export function Profile() {
  const user = useFetch("");
  return <div>Hello {}!</div>;

Add a <Suspense> boundary somewhere above in your component tree where you want to render a fallback while the data is loading. The fallback will only be shown in the preview SPA. When generating static pages, Capri will wait until all data is loaded.

// src/App.tsx

export function App() {
  return (
    <Suspense fallback={<div>loading...</div>}>
      <Profile />
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