Deno Frontend Development

Overview

This skill covers frontend development in Deno using Fresh (Deno's web framework), Preact (a lightweight React alternative), and Tailwind CSS. Fresh uses "island architecture" where pages render on the server and only interactive parts ship JavaScript to the browser.

When to Use This Skill

•Creating a new Fresh web application
•Building interactive UI components (islands)
•Adding server-rendered pages and routes
•Integrating Tailwind CSS for styling
•Choosing between islands (client-side) vs components (server-only)
•Working with Preact hooks and signals

Apply these practices when building web applications in Deno.

Fresh Framework

Reference: https://fresh.deno.dev/docs

Fresh is Deno's web framework. It uses island architecture - pages are rendered on the server, and only interactive parts ("islands") get JavaScript on the client.

Creating a Fresh Project

bash

deno run -Ar jsr:@fresh/init
cd my-project
deno task dev    # Runs at http://127.0.0.1:5173/

Project Structure

code

my-project/
├── deno.json           # Config and dependencies
├── main.ts             # Entry point
├── vite.config.ts      # Vite configuration
├── routes/             # Pages and API routes
│   ├── _app.tsx        # App layout wrapper
│   ├── index.tsx       # Home page (/)
│   └── (group)/        # Route groups with layouts
│       ├── _layout.tsx # Group layout
│       └── _middleware.ts # Route middleware
├── islands/            # Interactive components (hydrated on client)
│   └── Counter.tsx
├── components/         # Server-only components (no JS shipped)
│   └── Button.tsx
├── libs/               # Shared utilities and logic
├── types/              # TypeScript type definitions
├── data/               # Static data files (JSON, etc.)
├── assets/             # CSS and other assets
│   └── styles.css
├── static/             # Static files served directly
│   └── fonts/
├── scripts/            # Build/generation scripts
└── _fresh/             # Auto-generated build output (don't edit)

Key Concepts

Routes (routes/ folder)

•File-based routing: routes/about.tsx → /about
•Dynamic routes: routes/blog/[slug].tsx → /blog/my-post
•API routes: routes/api/users.ts exports handlers

Layouts (_app.tsx)

tsx

import { PageProps } from "jsr:@fresh/core";

export default function App({ Component }: PageProps) {
  return (
    <html>
      <head>
        <title>My App</title>
      </head>
      <body>
        <Component />
      </body>
    </html>
  );
}

Async Server Components

tsx

export default async function Page() {
  const data = await fetchData(); // Runs on server only
  return <div>{data.title}</div>;
}

Islands (Interactive Components)

Islands are components that get hydrated (made interactive) on the client. Place them in the islands/ folder.

When to Use Islands

•User interactions (clicks, form inputs)
•Client-side state (counters, toggles)
•Browser APIs (localStorage, geolocation)

Island Example

tsx

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

export default function Counter() {
  const count = useSignal(0);

  return (
    <div>
      <p>Count: {count.value}</p>
      <button onClick={() => count.value++}>
        Increment
      </button>
    </div>
  );
}

Island Rules

•Props must be serializable - No functions, only JSON-compatible data
•Keep islands small - Less JavaScript shipped to client
•Prefer server components - Only use islands when you need interactivity
•Check version first - To make sure solutions applied are using current convention

Preact

Preact is a 3KB alternative to React. Fresh uses Preact instead of React.

Preact vs React Differences

Preact	React
`class` works	`className` required
`@preact/signals`	`useState`
3KB bundle	~40KB bundle

Hooks (Same as React)

tsx

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

function MyComponent() {
  const [value, setValue] = useState(0);
  const inputRef = useRef<HTMLInputElement>(null);

  useEffect(() => {
    console.log("Component mounted");
  }, []);

  return <input ref={inputRef} value={value} />;
}

Signals (Preact's Reactive State)

Signals are Preact's more efficient alternative to useState:

tsx

import { signal, computed } from "@preact/signals";

const count = signal(0);
const doubled = computed(() => count.value * 2);

function Counter() {
  return (
    <div>
      <p>Count: {count}</p>
      <p>Doubled: {doubled}</p>
      <button onClick={() => count.value++}>+1</button>
    </div>
  );
}

Benefits of signals:

•More granular updates (only re-renders what changed)
•Can be defined outside components
•Cleaner code for shared state

Tailwind CSS in Fresh

Fresh 2.0 uses Vite for builds, which means Tailwind integrates via the Vite plugin.

Setup

In deno.json:

json

{
  "imports": {
    "@tailwindcss/vite": "npm:@tailwindcss/vite"
  }
}

In vite.config.ts:

typescript

import { defineConfig } from "vite";
import tailwindcss from "@tailwindcss/vite";

export default defineConfig({
  plugins: [tailwindcss()],
});

Usage

tsx

export default function Button({ children }) {
  return (
    <button class="px-4 py-2 bg-blue-500 text-white rounded hover:bg-blue-600">
      {children}
    </button>
  );
}

Best Practices

•Prefer utility classes over @apply
•Use class not className (Preact supports both, but class is simpler)
•Dark mode: Use class strategy in tailwind.config.js
•Use daisyui class names as it is convenient and prefer less javascript

tsx

<div class="bg-white dark:bg-gray-900">
  <p class="text-gray-900 dark:text-white">Hello</p>
</div>

Building and Deploying

Development

bash

deno task dev      # Start dev server with hot reload (http://127.0.0.1:5173/)

Production Build

bash

deno task build    # Build for production
deno task preview  # Preview production build locally

Deploy to Deno Deploy

bash

deno task build           # Build first
deno deploy --prod        # Deploy to production

Quick Reference

Task	Command/Pattern
Create Fresh project	`deno run -Ar jsr:@fresh/init`
Start dev server	`deno task dev` (port 5173)
Build for production	`deno task build`
Add a page	Create `routes/pagename.tsx`
Add an API route	Create `routes/api/endpoint.ts`
Add interactive component	Create `islands/ComponentName.tsx`
Add static component	Create `components/ComponentName.tsx`

Common Mistakes

Putting too much JavaScript in islands

tsx

// ❌ Wrong - entire page as an island (ships all JS to client)
// islands/HomePage.tsx
export default function HomePage() {
  return (
    <div>
      <Header />
      <MainContent />  {/* Most of this doesn't need interactivity */}
      <Footer />
    </div>
  );
}

// ✅ Correct - only interactive parts are islands
// routes/index.tsx (server component)
import Counter from "../islands/Counter.tsx";

export default function HomePage() {
  return (
    <div>
      <Header />
      <MainContent />
      <Counter />  {/* Only this needs to be an island */}
      <Footer />
    </div>
  );
}

Passing non-serializable props to islands

tsx

// ❌ Wrong - functions can't be serialized
<Counter onUpdate={(val) => console.log(val)} />

// ✅ Correct - only pass JSON-serializable data
<Counter initialValue={5} label="Click count" />

Using className instead of class

tsx

// ❌ Works but unnecessary in Preact
<div className="container">

// ✅ Preact supports native HTML attribute
<div class="container">

Forgetting to build before deploying Fresh 2.0

bash

# ❌ Wrong - Fresh 2.0 requires a build step
deno deploy --prod

# ✅ Correct - build first, then deploy
deno task build
deno deploy --prod

Creating islands for non-interactive content

tsx

// ❌ Wrong - this doesn't need to be an island (no interactivity)
// islands/StaticCard.tsx
export default function StaticCard({ title, body }) {
  return <div class="card"><h2>{title}</h2><p>{body}</p></div>;
}

// ✅ Correct - use a regular component (no JS shipped)
// components/StaticCard.tsx
export default function StaticCard({ title, body }) {
  return <div class="card"><h2>{title}</h2><p>{body}</p></div>;
}