islands/Counter.tsx 
import type { Signal } from "@preact/signals";
import { Button } from "../components/Button.tsx";

interface CounterProps {
  count: Signal<number>;
}

export default function Counter(props: CounterProps) {
  return (
    <div class="flex gap-8 py-6">
      <Button onClick={() => props.count.value -= 1}>-1</Button>
      <p class="text-3xl tabular-nums">{props.count}</p>
      <Button onClick={() => props.count.value += 1}>+1</Button>
    </div>
  );
}

Fresh knows this file is an island because it lives in the /islands directory. This means Fresh will render the file on the front end. It’ll ship the minimum amount of front-end JavaScript to handle just this “island” of interactivity. Then, it can be used in a variety of places, even by elements that are fully rendered on the server, where they can be optimized, pre-rendered, and shipped in a compact form. In theory, at least, this setup gives you the best of both worlds. Incorporating the island concept into file routing is a compelling idea.

If we return to the main index.tsx file, you’ll see how the island is used:

/routes/index.tsx 
import { useSignal } from "@preact/signals";
import Counter from "../islands/Counter.tsx";

export default function Home() {
  const count = useSignal(3);
  return (
    <div class="px-4 py-8 mx-auto bg-[#86efac]">
      <div class="max-w-screen-md mx-auto flex flex-col items-center justify-center">
        <img
          class="my-6"
          src="/logo.svg"
          width="128"
          height="128"
          alt="the Fresh logo: a sliced lemon dripping with juice"
        />
        <h1 class="text-4xl font-bold">Welcome to Fresh</h1>
        <p class="my-4">
          Try updating this message in the
          <code class="mx-2">./routes/index.tsx</code> file, and refresh.
        </p>
        <Counter count={count} />
      </div>
    </div>
  );
}

The main thing to notice here is the inclusion of the Counter component (import Counter from "../islands/Counter.tsx") and its use in the normal JSX markup. This is a simple and direct means of combining server-side rendered code with front-end island functionality.