deno-guidance

# Deno Development Guidance

## Overview

This skill provides foundational knowledge for building modern Deno applications. Deno is a secure JavaScript/TypeScript runtime that runs TypeScript directly, has built-in tools (formatter, linter, test runner), and uses modern package management through JSR.

## When to Use This Skill

- Starting a new Deno project
- Adding dependencies to a project
- Configuring `deno.json` settings
- Running Deno CLI commands (fmt, lint, test)
- Setting up import maps
- Understanding Deno's permission system

Apply these practices whenever working in a Deno project (identified by the presence of `deno.json`).

## Package Management Priority

When adding dependencies, follow this priority order:

1. **JSR packages (`jsr:`)** - Preferred for Deno-native packages
   - Better TypeScript support (types are built-in)
   - Faster to resolve and install
   - Example: `jsr:@std/http`, `jsr:@fresh/core`

2. **npm packages (`npm:`)** - Fully supported, use when no JSR alternative exists
   - Deno has full npm compatibility
   - Example: `npm:express`, `npm:zod`

3. **AVOID: `https://deno.land/x/`** - Deprecated registry
   - This is the old package registry
   - Many LLMs incorrectly default to this
   - Always use `jsr:` instead

### Standard Library

The Deno standard library lives at `@std/` on JSR:

```jsonc
// deno.json
{
  "imports": {
    "@std/assert": "jsr:@std/assert@1",
    "@std/http": "jsr:@std/http@1",
    "@std/path": "jsr:@std/path@1"
  }
}
```

```typescript
import { serve } from "@std/http";
import { join } from "@std/path";
import { assertEquals } from "@std/assert";
```

Never use `https://deno.land/std/` - always use `jsr:@std/*`.

## Understanding Deno

Reference: https://docs.deno.com/runtime/fundamentals/

Key concepts:
- **Native TypeScript** - No build step needed, Deno runs TypeScript directly
- **Explicit permissions** - Use flags like `--allow-net`, `--allow-read`, `--allow-env`
- **deno.json** - The config file (simila