---

name: deno description: > Use this skill when building with Deno — TypeScript runtime, Deno Deploy, Fresh framework, Deno std library. This skill enforces: permission-based security, URL import system, std lib usage over npm, Fresh island architecture. Do NOT use for: Node.js projects, Bun projects, browser-only TypeScript. version: "1.0.0" author: "j4flmao" license: "MIT" compatibility: claude-code: true cursor: true codex: true windsurf: true tags: [backend, deno, phase-10]

Deno

Purpose

Build secure-by-default TypeScript applications with Deno runtime — permissions model, module system, standard library, Fresh framework, and deployment.

Agent Protocol

Trigger

User request includes: Deno, deno deploy, deno fresh, deno std, deno run, deno compile, deno test, deno fmt, deno lint, deno task, deno.json, import_map.json, TypeScript runtime, Fresh framework, deno.land.

Input Context

• Runtime (Deno, Deno Deploy, self-hosted)
• Framework (Fresh, Oak, Hono, bare)
• Permissions required (net, read, write, env, ff, run)
• Module strategy (deno.land/x, npm, JSR)

Output Artifact

Project setup, permission flags, module imports, Fresh route structure, deployment config.

Response Format

Produce artifact directly. No preamble, no postamble, no explanations. No filler, no hedging, no transitions. Strip articles a/an/the where unambiguous. Compress output — why use many token when few do trick.

Completion Criteria

• Deno.json config with tasks, lint, fmt, permissions
• Import map or deno.json imports configured
• Fresh project structure with routes and islands
• Deno Deploy or Docker deployment config

Max Response Length

4096 tokens

Architecture Decision Trees

Framework Selection: Fresh vs Oak vs Hono vs Bare

Criterion	Fresh	Oak	Hono	Bare (std/http)
SSR + Islands	Yes (Preact)	No	No	No
REST API	Heavy (file-based)	Best	Best	Minimal
Edge deployment	Deno Deploy native	Compatible	Compatible	Compatible
Middleware model	Route-level	Context pipeline	Express-like	Manual
Type safety	Partial	Generic params	Full (TypeBox/Zod)	Manual
npm compat	Via JSR	Via JSR	Built-in	Via JSR

Decision: routes/ file-based routing → Fresh. REST API w/ middleware → Oak. TypeScript-first w/ validation → Hono. Minimal/single-purpose → std/http.

Module Import Strategy: deno.land/x vs JSR vs npm

Source	Pros	Cons	Best For
deno.land/x	Native Deno, no conversion	Slower resolution, no semver enforcement	Std lib, Oak, Djwt
JSR	Fast resolution, semver, TypeScript native	Smaller registry	Published modules with TS types
npm	Largest ecosystem	CJS/ESM conversion overhead, perf cost	Browser compat packages (React, Preact)

Decision: Std lib → deno.land/x. New TS-first modules → JSR. Only use npm for packages unavailable on JSR/deno.land/x.

Permission Model Strategy

Minimal permissions per deployment target:

• Deno Deploy: --allow-net, --allow-env (no fs by default)
• Self-hosted API: --allow-net --allow-read --allow-env
• Fresh app: --allow-net --allow-read --allow-env --allow-write (dev only)
• Background worker: --allow-net --allow-read --allow-env

Never use -A in production. Group permissions in deno.json tasks. Use --deny-* to explicitly block capabilities.

Workflow

Step 1: Project Configuration

// deno.json
{
  "name": "@myapp/api",
  "version": "1.0.0",
  "tasks": {
    "dev": "deno run --watch --allow-net --allow-read --allow-env src/main.ts",
    "start": "deno run --allow-net --allow-read --allow-env src/main.ts",
    "test": "deno test --allow-net --allow-read --allow-env",
    "lint": "deno lint",
    "fmt": "deno fmt",
    "check": "deno check src/main.ts",
    "compile": "deno compile --allow-net --allow-read --allow-env --output dist/api src/main.ts"
  },
  "compilerOptions": {
    "strict": true,
    "noUnusedLocals": true,
    "noUnusedParameters": true
  },
  "imports": {
    "std/": "https://deno.land/std@0.220.0/",
    "oak/": "https://deno.land/x/oak@v12.6.2/",
    "hono/": "https://deno.land/x/hono@v4.0.0/",
    "zod/": "https://deno.land/x/zod@v3.22.0/",
    "djwt/": "https://deno.land/x/djwt@v3.0.1/"
  },
  "lint": {
    "rules": {
      "tags": ["recommended"],
      "exclude": ["no-explicit-any"]
    }
  },
  "fmt": {
    "indentWidth": 2,
    "lineWidth": 100,
    "semiColons": true,
    "singleQuote": false
  }
}

Step 2: Oak REST API Setup

// src/main.ts
import { Application, Router } from 'oak/mod.ts';
import { oakCors } from 'https://deno.land/x/cors@v1.2.2/mod.ts';
import { logger } from 'std/log/mod.ts';
import { load } from 'std/dotenv/mod.ts';
import { router as userRouter } from './routes/users.ts';
import { errorHandler } from './middleware/error-handler.ts';

const env = await load({ export: true });

const app = new Application();

// Middleware
app.use(oakCors({ origin: env.CORS_ORIGIN || '*' }));
app.use(errorHandler);
app.use(loggerMiddleware);

// Routes
const api = new Router({ prefix: '/api/v1' });
api.use('/users', userRouter.routes(), userRouter.allowedMethods());
app.use(api.routes());
app.use(api.allowedMethods());

// Start
const port = parseInt(env.PORT || '8000');
logger.info(`Server running on http://localhost:${port}`);
await app.listen({ port });

// src/routes/users.ts
import { Router } from 'oak/mod.ts';
import { userController } from '../controllers/user.controller.ts';

const router = new Router();

router.get('/', userController.list);
router.get('/:id', userController.getById);
router.post('/', userController.create);
router.put('/:id', userController.update);
router.delete('/:id', userController.remove);

export { router };

// src/middleware/error-handler.ts
import { Context, isHttpError, Status } from 'oak/mod.ts';
import { logger } from 'std/log/mod.ts';

export async function errorHandler(ctx: Context, next: () => Promise<unknown>) {
  try {
    await next();
  } catch (err) {
    if (isHttpError(err)) {
      ctx.response.status = err.status;
      ctx.response.body = {
        success: false,
        error: { code: err.name, message: err.message },
      };
    } else {
      logger.error(`Unhandled error: ${err}`);
      ctx.response.status = Status.InternalServerError;
      ctx.response.body = {
        success: false,
        error: { code: 'INTERNAL_ERROR', message: 'An unexpected error occurred' },
      };
    }
  }
}

Step 3: Hono REST API Pattern (Alternative)

// src/main.ts
import { Hono } from 'hono/mod.ts';
import { cors } from 'hono/cors.ts';
import { logger } from 'hono/logger.ts';
import { z } from 'zod/mod.ts';
import { zValidator } from 'hono/zod-validator.ts';

const app = new Hono();

app.use('/*', cors());
app.use('/*', logger());

const userSchema = z.object({
  name: z.string().min(2),
  email: z.string().email(),
});

const routes = app
  .get('/users', async (c) => {
    const users = await db.list({ prefix: ['users'] });
    return c.json({ data: users });
  })
  .post('/users', zValidator('json', userSchema), async (c) => {
    const body = await c.req.valid('json');
    const id = crypto.randomUUID();
    await db.set(['users', id], body);
    return c.json({ data: { id, ...body } }, 201);
  })
  .get('/users/:id', async (c) => {
    const id = c.req.param('id');
    const user = await db.get(['users', id]);
    if (!user) return c.json({ error: 'Not found' }, 404);
    return c.json({ data: user });
  });

export default app;

// main entry
Deno.serve(app.fetch);

Step 4: Fresh Project Setup

deno run -A -r https://fresh.deno.dev my-app
cd my-app
deno task start

my-app/
  main.ts                       # Entry point
  deno.json                     # Config, tasks, imports
  fresh.config.ts               # Fresh configuration
  fresh.gen.ts                  # Auto-generated routes manifest
  static/
    favicon.ico
  routes/
    _app.tsx                    # App wrapper (layout)
    _layout.tsx                 # Route group layout
    index.tsx                   # GET /
    [slug].tsx                  # Dynamic route
    api/
      users.ts                  # API handler
      users/
        [id].ts                 # /api/users/:id
  islands/
    Counter.tsx                 # Interactive island
    SearchBar.tsx               # Client-side interactivity
  components/
    Button.tsx                  # Shared components
    Layout.tsx
  dev.ts                        # Dev server entry

// routes/api/users.ts
import { Handlers } from '$fresh/server.ts';
import { db } from '../../db/kv.ts';

export const handler: Handlers = {
  async GET(req, ctx) {
    const users = await db.list({ prefix: ['users'] });
    return new Response(JSON.stringify({ data: users }), {
      headers: { 'content-type': 'application/json' },
    });
  },

  async POST(req, ctx) {
    const body = await req.json();
    const id = crypto.randomUUID();
    await db.set(['users', id], body);
    return new Response(JSON.stringify({ data: { id, ...body } }), {
      status: 201,
      headers: { 'content-type': 'application/json' },
    });
  },
};

// islands/SearchBar.tsx
import { IS_BROWSER } from '$fresh/runtime.ts';
import { useState } from 'preact/hooks';

export default function SearchBar() {
  const [query, setQuery] = useState('');
  const [results, setResults] = useState<string[]>([]);

  const search = async () => {
    const res = await fetch(`/api/search?q=${query}`);
    const data = await res.json();
    setResults(data.results);
  };

  return (
    <div>
      <input
        value={query}
        onChange={(e) => setQuery(e.currentTarget.value)}
        onKeyDown={(e) => e.key === 'Enter' && search()}
        placeholder='Search...'
      />
      <ul>
        {results.map((r) => <li key={r}>{r}</li>)}
      </ul>
    </div>
  );
}

Step 5: Deno KV for State

// db/kv.ts
const kv = await Deno.openKv();

export async function saveUser(user: User) {
  const key = ['users', user.id];
  const res = await kv.set(key, user);
  return res.ok;
}

export async function getUser(id: string) {
  const res = await kv.get(['users', id]);
  return res.value as User | null;
}

export async function listUsers() {
  const iter = kv.list({ prefix: ['users'] });
  const users: User[] = [];
  for await (const entry of iter) {
    users.push(entry.value as User);
  }
  return users;
}

Step 6: Testing Patterns

// tests/app_test.ts
import { assertEquals, assertExists } from 'std/testing/asserts.ts';
import { createApp } from '../src/app.ts';

Deno.test('GET /health returns 200', async () => {
  const app = createApp();
  const listener = app.listen({ port: 0 });
  const { port } = await listener;
  const res = await fetch(`http://localhost:${port}/health`);
  assertEquals(res.status, 200);
  const body = await res.json();
  assertEquals(body.status, 'ok');
  listener.close();
});

Deno.test('POST /users validates input', async () => {
  const app = createApp();
  const listener = app.listen({ port: 0 });
  const { port } = await listener;
  const res = await fetch(`http://localhost:${port}/api/v1/users`, {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({ name: '' }),
  });
  assertEquals(res.status, 400);
  listener.close();
});

Step 7: Deployment Configuration

# Dockerfile — self-hosted
FROM denoland/deno:alpine-2.0.2
WORKDIR /app
COPY deno.json .
COPY src/ src/
RUN deno cache src/main.ts
EXPOSE 8000
CMD ["deno", "run", "--allow-net", "--allow-read", "--allow-env", "src/main.ts"]

# deno Deploy — project config
# Set in deno deploy dashboard or via deployctl
name: my-api
entrypoint: src/main.ts
include: src/
env:
  - name: PORT
    value: "8080"
  - name: LOG_LEVEL
    value: "info"

Implementation Patterns

Pattern: Typed State with Oak Context

interface AppState {
  userId: string;
  role: string;
  requestId: string;
}

const app = new Application<AppState>();

// Set state via middleware
app.use(async (ctx, next) => {
  ctx.state.userId = '...';
  ctx.state.role = 'admin';
  ctx.state.requestId = crypto.randomUUID();
  await next();
});

// Access in controller
async function listOrders(ctx: RouterContext<'/', AppState>) {
  const userId = ctx.state.userId;
  const orders = await orderService.findByUser(userId);
  ctx.response.body = orders;
}

Pattern: Composable Middleware

// src/middleware/compose.ts
type Middleware = (ctx: Context, next: () => Promise<unknown>) => Promise<unknown>;

export function compose(...middleware: Middleware[]): Middleware {
  return async (ctx, next) => {
    let index = -1;
    const dispatch = async (i: number): Promise<void> => {
      if (i <= index) throw new Error('next() called multiple times');
      index = i;
      const fn = middleware[i] || next;
      if (fn) await fn(ctx, () => dispatch(i + 1));
    };
    await dispatch(0);
  };
}

Pattern: Structured Logger

// src/shared/logger.ts
import { getLogger, setup } from 'std/log/mod.ts';

await setup({
  handlers: {
    console: new ConsoleHandler('DEBUG', {
      formatter: (r) => JSON.stringify({
        level: r.levelName,
        msg: r.msg,
        time: r.datetime.toISOString(),
        logger: r.loggerName,
        ...r.args[0],
      }),
    }),
  },
  loggers: {
    default: { level: 'DEBUG', handlers: ['console'] },
  },
});

export const logger = getLogger();

Production Considerations

Permission Hardening

• Audit permissions with deno info --json to list all used URLs and files
• Use --deny-env=AWS_SECRET_KEY to block specific dangerous env access
• In Docker, run as non-root user: USER deno after copy
• For Fresh, only require --allow-net --allow-read --allow-env

Module Pinning and Integrity

• Pin versions in deno.json imports: use @v1.2.3 not @latest
• Add integrity checks via --check=all in CI
• Run deno cache --lock=lock.json --lock-write to generate lockfile
• Commit lock.json and verify in CI with deno cache --lock=lock.json

Performance Tuning

• Use std/http for max throughput (no framework overhead)
• Oak adds ~5-10% overhead vs bare std/http
• Hono benchmarks near std/http for simple routes
• Enable --v8-flags=--max-old-space-size=512 for memory limits
• Use Deno.serve (Deno 2) instead of std/http server

Cold Start Optimization (Deno Deploy)

• Minimize top-level await — use lazy imports
• Keep bundle under 1MB
• Use static file serving from static/ not computed
• Avoid dynamic imports in hot paths

Anti-Patterns

Anti-Pattern	Why	Fix
-A in production	Grants all permissions	List exact --allow-* flags
Raw URLs in source	Version drift, no caching	Use import map in deno.json
npm-only packages	Performance penalty	Prefer deno.land/x or JSR
std/node polyfill	Slows startup, compatibility bugs	Use Deno-native APIs
init() pattern	No lifecycle guarantee	Use explicit setup in main.ts
Dynamic import for routes (non-Fresh)	Hurts tree-shaking, cold start	Static import with barrel file
Relying on self global	Not in strict mode	Use globalThis

Security Considerations

• Deno's permission system is the primary security boundary — never bypass with --allow-all
• Validate Origin headers in CORS middleware: oakCors({ origin: /^https:\/\/myapp\.com$/ }) over '*'
• Use djwt for JWT — verify algorithm is HS256 or RS256, never none
• Rate limiting via oak-rate-limit or manual IP tracking in Deno KV
• Input validation with Zod at the boundary — never trust raw ctx.request.body
• SQL injection prevention: use parameterized queries via deno_postgres or deno_mongo
• Content-Type sniffing protection: set X-Content-Type-Options: nosniff
• Secrets via environment variables only, never hardcoded

Testing Strategies

Unit Tests

Deno.test('user validation - rejects short names', () => {
  const result = validateUser({ name: 'A' });
  assertEquals(result.success, false);
  assertExists(result.error);
});

Integration Tests with KV

Deno.test('CRUD operations against KV', async () => {
  const kv = await Deno.openKv(':memory:');
  try {
    await kv.set(['users', '1'], { name: 'Test' });
    const res = await kv.get(['users', '1']);
    assertEquals(res.value, { name: 'Test' });
  } finally {
    kv.close();
  }
});

Load test with autocannon or wrk against the compiled binary. Use deno bench for microbenchmarks.

Rules

• Always specify minimum permissions with --allow-* flags. Never use -A in production.
• Use deno.land/x or JSR imports via deno.json import map. Avoid raw URLs in source files.
• Standard library (std/) preferred over npm equivalents (std/http, std/log, std/testing).
• Fresh islands for client-side interactivity. Preact components for server-only rendering.
• Deno KV for simple state, PostgreSQL driver for complex persistence.
• deno fmt and deno lint in CI. Check before commit.
• Compile binaries with deno compile for Docker-less deployment.
• All env vars loaded via std/dotenv in dev, env vars in production.
• Generate lockfile: deno cache --lock=lock.json --lock-write.
• Type AppState in Application constructor for type-safe context.

References

• references/deno-advanced.md — Deno Advanced Topics
• references/deno-deployment.md — Deno Deployment
• references/deno-essentials.md — Deno Essentials
• references/deno-fundamentals.md — Deno Fundamentals
• references/deno-runtime.md — Deno Runtime Deep Dive
• references/fresh-framework.md — Fresh Framework

Handoff

Hand off to backend/universal/api-response/SKILL.md for API response formatting or backend-testing skill for test patterns.