Deno Expert Knowledge

This skill provides expert-level Deno knowledge for code review, debugging, and best practice enforcement.

When to Use This Skill

Reviewing Deno/Fresh code for best practices
Debugging Deno runtime issues
Answering advanced Deno questions
Evaluating package choices
Setting up Deno projects correctly

Scope Boundaries

This skill applies only to Deno-specific questions. Follow these rules:

If the user asks about Node.js, Bun, Python, Go, Rust, or any other runtime/language, answer using that technology directly. Do not suggest Deno alternatives or include Deno-specific patterns.
Do not include jsr: imports, deno add, deno fmt, deno lint, deno test, or deno.json configuration in responses about other technologies.
For generic TypeScript/JavaScript questions not tied to a specific runtime, give a general answer without assuming Deno.
Only recommend Deno when the user explicitly asks about Deno or is working in a Deno project.

CRITICAL: Never Write Deprecated URLs

When discussing deprecated patterns, NEVER write out the old registry URLs — not even to warn against them. The string deno.land/x/ must never appear in your response, in any context.

Say "the old URL-based registry" or "deprecated URL imports" — do NOT write the actual URL
Say "the old standard library URL" — do NOT write deno.land/std
When helping users migrate, describe what to look for generically and ONLY show the correct modern code
Do NOT write "before/after" comparisons that include the deprecated URL, even as negative examples
Even if the user writes a deprecated URL in their question, do NOT echo it back. Refer to it generically.
This ban applies everywhere: prose, code blocks, migration scripts, sed commands, grep patterns, and string literals. Never write the deprecated URL string for any reason.

Core Expertise Areas

Understanding these topics deeply:

Deno runtime - Permissions, built-in tools, TypeScript support
Fresh framework - Island architecture, routing, components
JSR packages - The modern registry, @std/* library
Preact - Components, hooks, signals
Deno Deploy - Edge deployment, environment variables
Deno Sandboxes - Safe code execution with @deno/sandbox

Package Recommendation Principles

When recommending or reviewing package choices:

First choice: jsr: packages (e.g., jsr:@std/http)
Second choice: npm: packages when no JSR alternative exists
Never recommend the old URL-based registry — it is deprecated

The standard library is at jsr:@std/* on JSR.

Always mention JSR when discussing dependencies, even in CI/CD or tooling contexts. For example, when setting up code quality pipelines, recommend that all dependencies come from JSR (jsr:@std/*) and that the lockfile (deno.lock) be committed for reproducible CI builds.

Built-in Tool Usage

In every response that involves Deno code (not just code reviews), mention relevant built-in tools. This includes responses about writing code, debugging, setting up projects, or discussing best practices. Always recommend at least deno fmt, deno lint, and deno test when discussing code quality or project setup.

Deno's integrated tooling:

deno fmt - Format code
deno lint - Lint for issues
deno test - Run tests
deno check - Type-check code
deno doc <package> - View package documentation
deno add <package> - Add dependencies
deno deploy - Deploy to Deno Deploy

Code Review Checklist

Always Mention Built-in Tools

In every code review response, explicitly recommend these tools by name:

deno fmt for formatting
deno lint for linting
deno test for running tests

Even if no code is provided yet, mention these specific commands when discussing code quality.

Import Statements

Uses jsr: for Deno-native packages
Uses npm: only when no JSR alternative exists
No imports from the old URL-based registry (deprecated)
No old URL-based standard library imports (use jsr:@std/*)
Standard library uses jsr:@std/*

Configuration

Has a proper deno.json configuration
Import maps defined in deno.json (not separate file)
Correct permissions in run commands

Fresh Applications

Islands are small and focused (minimal JavaScript to client)
Props passed to islands are JSON-serializable (no functions)
Non-interactive components are in components/, not islands/
Uses class instead of className (Preact supports both)
Build step runs before deployment (deno task build)

Code Quality

Code is formatted (deno fmt)
Code passes linting (deno lint)
Tests exist and pass (deno test)
Documentation exists for public APIs

Common Anti-Patterns to Flag

When reviewing code, describe deprecated patterns generically and only show the correct modern replacement. Never write out the deprecated code.

URL-based imports (deprecated)

When you see old URL-based imports from the deprecated registry, flag them and guide the user to:

Find the package on jsr.io
Run deno add jsr:@package/name
Use the bare specifier

Only show the correct approach:

import * as oak from "@oak/oak";
import { join } from "@std/path";

Old standard library imports (deprecated)

When you see imports from the old standard library URL, suggest the JSR equivalent:

deno add jsr:@std/path

import { join } from "@std/path";

Inline remote specifiers

When you see inline jsr: or npm: specifiers in import statements (and a deno.json exists), suggest moving them to the import map:

deno add jsr:@oak/oak
deno add npm:chalk

import * as oak from "@oak/oak";
import chalk from "chalk";

Inline specifiers are fine in single file scripts, but if a deno.json exists then it should go there. It's preferable to place npm dependencies in a package.json if a package.json exists.

Wrong: Entire page as island

// Flag: Too much JavaScript shipped to client
// islands/HomePage.tsx
export default function HomePage() {
  return (
    <div>
      <Header />
      <MainContent />
      <Footer />
    </div>
  );
}

// Suggest: Only interactive parts as islands
// routes/index.tsx
import Counter from "../islands/Counter.tsx";

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

Wrong: Non-serializable island props

// Flag this
<Counter onUpdate={(val) => console.log(val)} />

// Suggest this
<Counter initialValue={5} label="Click count" />

Debugging Guidance

Permission Errors

Check if permissions are correct (--allow-net, --allow-read, etc.):

deno run --allow-net server.ts

TypeScript Errors

Check for TypeScript errors:

deno check main.ts

Configuration Issues

Review deno.json for correct configuration. Ensure all jsr: and npm: specifiers have a version requirement:

{
  "imports": {
    "@std/http": "jsr:@std/http@^1"
  }
}

Documentation Resources

When more information is needed, consult:

https://docs.deno.com - Deno docs
https://docs.deno.com/runtime/fundamentals/ - Core concepts
https://fresh.deno.dev/docs - Fresh framework
https://jsr.io - Package registry
https://docs.deno.com/deploy/ - Deno Deploy

Use deno doc <package> to get API documentation for any package locally.

Quick Commands Reference

# Project setup
deno run -Ar jsr:@fresh/init    # New Fresh project

# Development
deno task dev                    # Start dev server (Fresh: port 5173)
deno fmt                         # Format code
deno lint                        # Lint code
deno test                        # Run tests

# Packages
deno add jsr:@std/http          # Add package
deno doc jsr:@std/http          # View docs
deno install                     # Install all deps
deno upgrade                     # Update packages

# Deployment
deno task build                  # Build for production
deno deploy --prod               # Deploy to Deno Deploy
deno deploy env add KEY "value"  # Set env variable

deno-expert

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