Deno Frontend

Deno Frontend Development

Overview

This skill covers frontend development in Deno using Fresh 2.x (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.

Scope Boundaries

This skill applies only to Fresh/Deno frontend questions. Follow these rules:

• If the user asks about Next.js, React Native, Vue, Svelte, Angular, or any other framework, answer using that framework directly. Do not suggest Fresh alternatives.
• Do not include Fresh imports, useSignal, island patterns, or Fresh project structure in responses about other frameworks.
• Only recommend Fresh when the user explicitly asks about Fresh or is working in a Fresh/Deno project.

CRITICAL: Never Show Deprecated Syntax

When helping users migrate from Fresh 1.x, describe old patterns generically and ONLY show correct Fresh 2.x code. Never write out old dollar-sign import paths or deprecated syntax, even in "before/after" comparisons.

• Say "Replace the old dollar-sign import paths with stable Fresh 2.x imports" — then show only the correct from "fresh" approach
• Do NOT write ❌ Old: import { App } from "$fresh/server.ts" — this is never acceptable, even as a negative example
• The strings _404.tsx and _500.tsx must never appear in your response, even when comparing Fresh 2.x to 1.x. Say "the old separate error pages" instead.

Only demonstrate Fresh 2.x patterns.

CRITICAL: Fresh 2.x vs 1.x

Always use Fresh 2.x patterns. Fresh 1.x is deprecated. Key differences:

• Fresh 2.x uses import { App } from "fresh" — the old dollar-sign import paths are deprecated
• Fresh 2.x has no manifest file — the old auto-generated manifest is no longer needed
• Fresh 2.x uses vite.config.ts for dev — the old dev.ts entry point is gone
• Fresh 2.x configures via new App() — the old config file is no longer used
• Fresh 2.x handlers take a single (ctx) parameter — the old two-parameter signature is deprecated
• Fresh 2.x uses a unified _error.tsx — the old separate error pages are replaced

Always use Fresh 2.x stable imports:

// ✅ CORRECT - Fresh 2.x stable
import { App, staticFiles } from "fresh";
import { define } from "./utils/state.ts"; // Project-local define helpers

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

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

Project Structure (Fresh 2.x)

my-project/
├── deno.json           # Config, dependencies, and tasks
├── main.ts             # Server entry point
├── client.ts           # Client entry point (CSS imports)
├── vite.config.ts      # Vite configuration
├── routes/             # Pages and API routes
│   ├── _app.tsx        # App layout wrapper (outer HTML)
│   ├── _layout.tsx     # Layout component (optional)
│   ├── _error.tsx      # Unified error page (404/500)
│   ├── index.tsx       # Home page (/)
│   └── api/            # API routes
├── islands/            # Interactive components (hydrated on client)
│   └── Counter.tsx
├── components/         # Server-only components (no JS shipped)
│   └── Button.tsx
├── static/             # Static assets
└── utils/
    └── state.ts        # Define helpers for type safety

Note: Fresh 2.x does not use a manifest file, a separate dev entry point, or a separate config file.

main.ts (Fresh 2.x Entry Point)

import { App, fsRoutes, staticFiles, trailingSlashes } from "fresh";

const app = new App()
  .use(staticFiles())
  .use(trailingSlashes("never"));

await fsRoutes(app, {
  dir: "./",
  loadIsland: (path) => import(`./islands/${path}`),
  loadRoute: (path) => import(`./routes/${path}`),
});

if (import.meta.main) {
  await app.listen();
}

vite.config.ts

import { defineConfig } from "vite";
import { fresh } from "@fresh/plugin-vite";
import tailwindcss from "@tailwindcss/vite";

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

deno.json Configuration

A Fresh 2.x project's deno.json looks like this (created by jsr:@fresh/init):

{
  "tasks": {
    "dev": "vite",
    "build": "vite build",
    "preview": "deno serve -A _fresh/server.js"
  },
  "imports": {
    "fresh": "jsr:@fresh/core@^2",
    "fresh/runtime": "jsr:@fresh/core@^2/runtime",
    "@fresh/plugin-vite": "jsr:@fresh/plugin-vite@^1",
    "@preact/signals": "npm:@preact/signals@^2",
    "preact": "npm:preact@^10",
    "preact/hooks": "npm:preact@^10/hooks",
    "@/": "./"
  }
}

Adding dependencies: Use deno add to add new packages:

deno add jsr:@std/http          # JSR packages
deno add npm:@tailwindcss/vite  # npm packages

Import Reference (Fresh 2.x)

// Core Fresh imports
import { App, staticFiles, fsRoutes } from "fresh";
import { trailingSlashes, cors, csp } from "fresh";
import { createDefine, HttpError } from "fresh";
import type { PageProps, Middleware, RouteConfig } from "fresh";

// Runtime imports (for client-side checks)
import { IS_BROWSER } from "fresh/runtime";

// Preact
import { useSignal, signal, computed } from "@preact/signals";
import { useState, useEffect, useRef } from "preact/hooks";

Key Concepts

Routes (routes/ folder)

• File-based routing: routes/about.tsx → /about
• Dynamic routes: routes/blog/[slug].tsx → /blog/my-post
• Optional segments: routes/docs/[[version]].tsx → /docs or /docs/v2
• Catch-all routes: routes/old/[...path].tsx → /old/foo/bar
• Route groups: routes/(marketing)/ for shared layouts without URL path changes

Layouts (_app.tsx)

import type { PageProps } from "fresh";

export default function App({ Component }: PageProps) {
  return (
    <html>
      <head>
        <meta charset="utf-8" />
        <meta name="viewport" content="width=device-width, initial-scale=1" />
        <title>My App</title>
      </head>
      <body>
        <Component />
      </body>
    </html>
  );
}

Async Server Components

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

Data Fetching Patterns

Fresh 2.x provides two approaches for fetching data on the server. The handler pattern is the recommended default because it demonstrates the full Fresh 2.x architecture and provides the most flexibility.

Approach A: Handler with Data Object (Recommended)

Use this as the default for data fetching. It uses the full Fresh 2.x handler pattern with typed data passing. Always show the complete setup including utils/state.ts when demonstrating this pattern.

// utils/state.ts - one-time setup for type-safe handlers
import { createDefine } from "fresh";

export interface State {
  user?: { id: string; name: string };
}

export const define = createDefine<State>();

// routes/posts.tsx
import { define } from "@/utils/state.ts";

// Handler fetches data and returns it via { data: {...} }
export const handler = define.handlers(async (ctx) => {
  const response = await fetch("https://jsonplaceholder.typicode.com/posts");
  const posts = await response.json();
  return { data: { posts } };
});

// Page receives typed data
export default define.page<typeof handler>(({ data }) => {
  return (
    <div>
      <h1>Posts</h1>
      <ul>
        {data.posts.map((post) => <li key={post.id}>{post.title}</li>)}
      </ul>
    </div>
  );
});

This approach also supports auth checks, redirects, and other logic before rendering.

Approach B: Async Server Components (Shorthand)

For the simplest cases where you just need to fetch and display data with no auth or redirects:

// routes/servers.tsx
export default async function ServersPage() {
  const servers = await db.query("SELECT * FROM servers");

  return (
    <div>
      <h1>Servers</h1>
      <ul>
        {servers.map((s) => <li key={s.id}>{s.name}</li>)}
      </ul>
    </div>
  );
}

Decision Guide

Need to fetch data on server?
├─ Yes → Use handler with { data: {...} } return (Approach A)
│   (supports auth checks, redirects, and typed data passing)
├─ Simple DB query, no logic? → Async page component is also fine (Approach B)
└─ No → Just use a regular page component

Handlers and Define Helpers (Fresh 2.x)

Fresh 2.x uses a single context parameter pattern for handlers. Always use (ctx) as the only parameter.

Important: When demonstrating any handler pattern (data fetching, form handling, API routes, auth), always show or reference the utils/state.ts setup which imports createDefine from "fresh". This ensures the complete Fresh 2.x architecture is visible.

Route Handlers

Always use define.handlers() for type-safe route handlers in file-based routes:

// routes/api/users.ts
import { define } from "@/utils/state.ts";

// Single function handles all methods
export const handler = define.handlers((ctx) => {
  return new Response(`Hello from ${ctx.req.method}`);
});

// Or method-specific handlers
export const handler = define.handlers({
  GET(ctx) {
    return Response.json({ users: [] });
  },
  async POST(ctx) {
    const body = await ctx.req.json();
    return Response.json({ created: true }, { status: 201 });
  },
});

Note: Bare handler exports (export const handler = (ctx) => {...}) also work but lose TypeScript type safety. Prefer define.handlers().

The Context Object

The ctx parameter provides everything you need:

export const handler = (ctx) => {
  ctx.req          // The Request object
  ctx.url          // URL instance with pathname, searchParams
  ctx.params       // Route parameters { slug: "my-post" }
  ctx.state        // Request-scoped data for middlewares
  ctx.config       // Fresh configuration
  ctx.route        // Matched route pattern
  ctx.error        // Caught error (on error pages)

  // Methods
  ctx.render(<JSX />)           // Render JSX to Response (JSX only, NOT data objects!)
  ctx.render(<JSX />, { status: 201, headers: {...} })  // With response options
  ctx.redirect("/other")        // Redirect (302 default)
  ctx.redirect("/other", 301)   // Permanent redirect
  ctx.next()                    // Call next middleware
};

Define Helpers (Type Safety)

Create a utils/state.ts file for type-safe handlers:

// utils/state.ts
import { createDefine } from "fresh";

// Define your app's state type
export interface State {
  user?: { id: string; name: string };
}

// Export typed define helpers
export const define = createDefine<State>();

Use in routes:

// routes/profile.tsx
import { define } from "@/utils/state.ts";
import type { PageProps } from "fresh";

// Typed handler with data
export const handler = define.handlers((ctx) => {
  if (!ctx.state.user) {
    return ctx.redirect("/login");
  }
  return { data: { user: ctx.state.user } };
});

// Page receives typed data
export default define.page<typeof handler>(({ data }) => {
  return <h1>Welcome, {data.user.name}!</h1>;
});

Middleware (Fresh 2.x)

// routes/_middleware.ts
import { define } from "@/utils/state.ts";

export const handler = define.middleware(async (ctx) => {
  // Before route handler
  console.log(`${ctx.req.method} ${ctx.url.pathname}`);

  // Call next middleware/route
  const response = await ctx.next();

  // After route handler
  return response;
});

API Routes

// routes/api/posts/[id].ts
import { define } from "@/utils/state.ts";
import { HttpError } from "fresh";

export const handler = define.handlers({
  async GET(ctx) {
    const post = await getPost(ctx.params.id);
    if (!post) {
      throw new HttpError(404); // Uses _error.tsx
    }
    return Response.json(post);
  },

  async DELETE(ctx) {
    if (!ctx.state.user) {
      throw new HttpError(401);
    }
    await deletePost(ctx.params.id);
    return new Response(null, { status: 204 });
  },
});

Islands (Interactive Components)

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

When to Use Islands

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

Island Example

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

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

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

Client-Only Code with IS_BROWSER

// islands/LocalStorageCounter.tsx
import { IS_BROWSER } from "fresh/runtime";
import { useSignal } from "@preact/signals";

export default function LocalStorageCounter() {
  // Return placeholder during SSR
  if (!IS_BROWSER) {
    return <div>Loading...</div>;
  }

  // Client-only code
  const stored = localStorage.getItem("count");
  const count = useSignal(stored ? parseInt(stored) : 0);

  return (
    <button => {
      count.value++;
      localStorage.setItem("count", String(count.value));
    }}>
      Count: {count.value}
    </button>
  );
}

Island Props (Serializable Types)

Islands can receive these prop types:

• Primitives: string, number, boolean, bigint, undefined, null
• Special values: Infinity, -Infinity, NaN, -0
• Collections: Array, Map, Set
• Objects: Plain objects with string keys
• Built-ins: URL, Date, RegExp, Uint8Array
• Preact: JSX elements, Signals (with serializable values)
• Circular references are supported

Functions cannot be passed as props.

Island Rules

1. Props must be serializable - No functions, only JSON-compatible data
2. Keep islands small - Less JavaScript shipped to client
3. Prefer server components - Only use islands when you need interactivity

Preact

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

Preact vs React Differences

Hooks (Same as React)

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:

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 => 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 (Optional)

Tailwind CSS is optional—you don't need it to build a great Fresh app. However, many developers prefer it for rapid styling. Fresh 2.x uses Vite for builds, so Tailwind integrates via the Vite plugin.

Setup

Install both Tailwind packages:

deno add npm:@tailwindcss/vite npm:tailwindcss

Important: You need both packages. @tailwindcss/vite is the Vite plugin, and tailwindcss is the core library it depends on.

Configure Vite in vite.config.ts:

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

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

Add the Tailwind import to your CSS file (e.g., assets/styles.css):

@import "tailwindcss";

Then import this CSS file in your client.ts:

import "./assets/styles.css";

Usage

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

1. Prefer utility classes over @apply
2. Use class not className (Preact supports both, but class is simpler)
3. Dark mode: Use class strategy in tailwind.config.js

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

Building and Deploying

Development

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

Production Build

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

Deploy to Deno Deploy

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

Quick Reference

Common Mistakes

Using Fresh 1.x Patterns (Most Common LLM Error)

Using old import specifiers

The old dollar-sign Fresh import paths and alpha version imports are deprecated. Always use the stable fresh package:

// ✅ CORRECT - Fresh 2.x stable imports
import { App, staticFiles } from "fresh";
import type { PageProps } from "fresh";

Using two-parameter handlers

The old two-parameter handler signature is deprecated. Fresh 2.x uses a single context parameter:

// ✅ CORRECT - Fresh 2.x uses single context parameter
export const handler = {
  GET(ctx) {  // Single ctx param
    return ctx.render(<MyPage />);
  }
};

Creating legacy files

Fresh 2.x does not use a manifest file, a separate dev entry point, or a separate config file. The correct Fresh 2.x file structure is:

main.ts           # Server entry
client.ts         # Client entry
vite.config.ts    # Vite config

Using deprecated context methods

The old renderNotFound(), bare render() without JSX, and basePath patterns are deprecated. Use these Fresh 2.x patterns instead:

// ✅ CORRECT - Fresh 2.x patterns
throw new HttpError(404)
ctx.render(<MyComponent />)
ctx.config.basePath

Passing data from handlers to pages (VERY COMMON MISTAKE)

This is a frequent error. In Fresh 2.x, you cannot pass data through ctx.render(). Instead, return an object with a data property from the handler:

// ✅ CORRECT - Return object with data property from handler
export const handler = define.handlers(async (ctx) => {
  const servers = await getServers();
  return { data: { servers } };  // Return { data: {...} } object
});

// ✅ CORRECT - Link page to handler type with typeof
export default define.page<typeof handler>(({ data }) => {
  return <ul>{data.servers.map((s) => <li>{s.name}</li>)}</ul>;
});

// ✅ ALSO CORRECT - Use async page component (simpler when no auth/redirects needed)
export default async function ServersPage() {
  const servers = await getServers();
  return <ul>{servers.map((s) => <li>{s.name}</li>)}</ul>;
}

Old task commands in deno.json

The old task commands that reference dev.ts are deprecated. Use the Vite-based tasks:

// ✅ CORRECT - Fresh 2.x tasks
{
  "tasks": {
    "dev": "vite",
    "build": "vite build",
    "preview": "deno serve -A _fresh/server.js"
  }
}

Island Mistakes

Putting too much JavaScript in islands

// ❌ Wrong - entire page as an island (ships all JS to client)
// islands/HomePage.tsx
export default function HomePage() {
  return (
    <div>
      <Header />
      <MainContent />
      <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 />
      <Footer />
    </div>
  );
}

Passing non-serializable props to islands

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

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

Other Common Mistakes

Using className instead of class

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

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

Forgetting to build before deploying Fresh 2.x

# ❌ Wrong - Fresh 2.x requires a build step
deno deploy --prod

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

Creating islands for non-interactive content

// ❌ 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>;
}

Using old Tailwind plugin

The old Fresh 1.x Tailwind plugin is deprecated. Fresh 2.x uses the Vite Tailwind plugin:

// ✅ CORRECT - Fresh 2.x uses Vite Tailwind plugin
import tailwindcss from "@tailwindcss/vite";

Missing tailwindcss package

# Error: Can't resolve 'tailwindcss' in '/path/to/assets'

# ❌ WRONG - Only installed the Vite plugin
deno add npm:@tailwindcss/vite

# ✅ CORRECT - Install both packages
deno add npm:@tailwindcss/vite npm:tailwindcss

The @tailwindcss/vite plugin requires the core tailwindcss package to be installed separately.

Fresh Alpha Versions (2.0.0-alpha.*)

Some projects use Fresh 2.x alpha releases (e.g., @fresh/core@2.0.0-alpha.29). These are not Fresh 1.x but use a different setup than stable Fresh 2.x:

IMPORTANT: If you see dev.ts in a project with @fresh/core@2.0.0-alpha.* in deno.json, do NOT treat it as a Fresh 1.x artifact. It is the correct entry point for alpha versions. Check the deno.json imports to determine which version is in use before suggesting changes.

Alpha projects also use the handler pattern define.handlers({ GET(ctx) { ... } }) which returns { data: {...} } - this is the same as stable 2.x.

Migrating from Fresh 1.x to 2.x

If you have an existing Fresh 1.x project, run the migration tool:

deno run -Ar jsr:@fresh/update

This tool automatically:

• Converts old import paths to the new fresh package
• Updates handler signatures to use the single (ctx) parameter
• Removes legacy generated and config files
• Creates vite.config.ts and client.ts
• Updates deno.json tasks to use Vite
• Merges separate error pages into unified _error.tsx
• Updates deprecated context method calls to Fresh 2.x equivalents

Manual Migration Checklist

If the tool misses anything:

1. Imports: All Fresh imports should come from fresh or fresh/runtime
2. Handlers: Should use single (ctx) parameter, access request via ctx.req
3. Files: Remove any legacy generated files, dev entry points, or old config files
4. Tasks: Update to vite, vite build, deno serve -A _fresh/server.js
5. Error pages: Use a single unified _error.tsx
6. Tailwind: Use @tailwindcss/vite (the Vite plugin)

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