Islands architecture in Fresh in Deno

Introduction

Islands architecture helps build fast web pages by only loading small interactive parts instead of the whole page. This makes websites quicker and easier to use.

When you want a mostly static page with some interactive buttons or forms.

When you want to improve page load speed by sending less JavaScript to the browser.

When you want to keep your website simple but still have parts that react to user actions.

When you want to build a web app that feels fast without complex client-side frameworks.

Syntax

Deno

import { useState } from "preact/hooks";

export const handler = {
  async GET(req, ctx) {
    return ctx.render();
  },
};

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

export default function Page() {
  return (
    <div>
      <Counter />
    </div>
  );
}

Islands are small interactive components inside mostly static pages.

Components using useState or event handlers like onClick are automatically islands.

Examples

This example shows a simple button island that changes text when clicked.

Deno

import { useState } from "preact/hooks";

const Button = () => {
  const [clicked, setClicked] = useState(false);
  return <button onClick={() => setClicked(true)}>{clicked ? "Clicked!" : "Click me"}</button>;
};

This example shows a component that receives props and renders a greeting.

Deno

export default function Page() {
  return (
    <main>
      <Greeting name="Alice" />
    </main>
  );
}

const Greeting = ({ name }) => {
  return <p>Hello, {name}!</p>;
};

Sample Program

This is a complete Fresh page using islands architecture. The page shows a heading and a button that updates its count when clicked. Only the button is interactive and runs JavaScript on the client, making the page fast and accessible.

Deno

import { useState } from "preact/hooks";

export const handler = {
  async GET(req, ctx) {
    return ctx.render();
  },
};

const Counter = () => {
  const [count, setCount] = useState(0);
  return (
    <button onClick={() => setCount(count + 1)} aria-label="Increment count">
      Count: {count}
    </button>
  );
};

export default function Home() {
  return (
    <main>
      <h1>Welcome to Fresh Islands</h1>
      <Counter />
    </main>
  );
}

Important Notes

Islands run JavaScript only where needed, improving performance.

Use semantic HTML and ARIA labels inside islands for accessibility.

Islands can receive props to customize their behavior.

Summary

Islands architecture splits pages into static and interactive parts.

Components with client-side JavaScript like useState are automatically islands in Fresh.

This approach makes web pages faster and easier to maintain.

Practice

1. What is the main purpose of the Islands architecture in Fresh?

easy

A. To split pages into static and interactive parts for better performance

B. To make all parts of the page fully interactive

C. To load the entire page on the client side only

D. To disable JavaScript on the page

Solution

1. Step 1: Understand Islands architecture concept

   Islands architecture divides a web page into static parts and small interactive components called islands.

2. Step 2: Identify the main benefit

   This split improves page speed and maintainability by only hydrating interactive parts.

3. Final Answer:

   To split pages into static and interactive parts for better performance -> Option A

4. Quick Check:

   Islands architecture = split static + interactive [OK]

Hint: Islands means splitting page into static and interactive parts [OK]

Common Mistakes:

• Thinking all page parts are interactive
• Believing Islands disables JavaScript
• Assuming full client-side rendering

2. Which syntax correctly marks an interactive island component in Fresh?

easy

A. <ISLAND>MyComponent</ISLAND>

B. <Island>MyComponent</Island>

C. <island>MyComponent</island>

D. <ISLAND />

Solution

1. Step 1: Recall correct tag casing

   Fresh uses uppercase ISLAND tag to mark interactive components.

2. Step 2: Check tag usage

   The ISLAND tag wraps the component, so it needs opening and closing tags.

3. Final Answer:

   <ISLAND>MyComponent</ISLAND> -> Option A

4. Quick Check:

   Correct tag is uppercase ISLAND with open/close [OK]

Hint: Use uppercase <ISLAND> tags to mark interactive parts [OK]

Common Mistakes:

• Using lowercase or mixed case tags
• Using self-closing without children
• Confusing with other component tags

3. Given this Fresh page snippet, what will be interactive on the rendered page?

export default function Page() {
  return (
    <main>
      <h1>Welcome</h1>
      <ISLAND><Counter /></ISLAND>
      <p>Static text</p>
    </main>
  );
}

medium

A. Nothing is interactive because ISLAND is not a valid tag

B. The entire page including the heading and paragraph is interactive

C. Only the Counter component inside the ISLAND tag is interactive

D. Only the heading is interactive

Solution

1. Step 1: Identify ISLAND usage

   The Counter component is wrapped inside the ISLAND tag, marking it interactive.

2. Step 2: Understand static vs interactive parts

   The heading and paragraph are outside ISLAND, so they remain static and non-interactive.

3. Final Answer:

   Only the Counter component inside the ISLAND tag is interactive -> Option C

4. Quick Check:

   ISLAND wraps interactive parts only [OK]

Hint: Only components inside <ISLAND> are interactive [OK]

Common Mistakes:

• Assuming whole page is interactive
• Thinking ISLAND tag is invalid
• Believing static parts become interactive

4. What is wrong with this Fresh island component usage?

export default function Page() {
  return (
    <main>
      <ISLAND><Button /></ISLAND>
      <ISLAND><Button /></ISLAND>
    </main>
  );
}

medium

A. ISLAND tags must be self-closing

B. Using multiple ISLAND tags with the same component can cause duplicate hydration issues

C. Button component cannot be used inside ISLAND

D. ISLAND tags must wrap only one element, not components

Solution

1. Step 1: Analyze multiple ISLAND usage

   Using the same interactive component multiple times inside separate ISLAND tags can cause hydration conflicts if keys are missing.

2. Step 2: Understand hydration uniqueness

   Each island needs a unique key or identifier to avoid duplicate hydration errors.

3. Final Answer:

   Using multiple ISLAND tags with the same component can cause duplicate hydration issues -> Option B

4. Quick Check:

   Duplicate islands need unique keys to avoid errors [OK]

Hint: Multiple identical islands need unique keys to avoid errors [OK]

Common Mistakes:

• Thinking ISLAND tags must be self-closing
• Believing components can't be inside ISLAND
• Assuming ISLAND wraps only HTML elements

5. You want to optimize a Fresh page with many interactive buttons. Which approach best uses Islands architecture to improve performance?

hard

A. Use <ISLAND> tags only on static content to reduce JavaScript

B. Wrap all buttons together in a single <ISLAND> to hydrate them all at once

C. Do not use <ISLAND> tags and hydrate the entire page client-side

D. Wrap each button in its own <ISLAND> with unique keys to hydrate only needed parts

Solution

1. Step 1: Understand hydration granularity

   Wrapping each button in its own ISLAND allows independent hydration, improving performance by loading only needed parts.

2. Step 2: Importance of unique keys

   Assigning unique keys to each island prevents hydration conflicts and ensures smooth interactivity.

3. Final Answer:

   Wrap each button in its own <ISLAND> with unique keys to hydrate only needed parts -> Option D

4. Quick Check:

   Separate islands with keys = better performance [OK]

Hint: Use separate islands with unique keys for many interactive parts [OK]

Common Mistakes:

• Grouping all buttons in one island causing unnecessary hydration
• Not using ISLAND tags and hydrating whole page
• Misusing ISLAND on static content