Deno

Overview

Deno is a secure JavaScript/TypeScript runtime with built-in tooling including a formatter, linter, test runner, and bundler. It features a permissions model that restricts file, network, and environment access by default, native TypeScript support without transpilation, and full web standards compatibility.

Instructions

• When creating servers, use Deno.serve() for high-performance HTTP handling with Web Standards Request/Response, and enable parallel workers with deno serve --parallel for multi-core utilization.
• When configuring security, specify permissions explicitly (--allow-read, --allow-net, --allow-env) scoped to specific paths, hosts, or variable names. Never deploy with --allow-all.
• When managing dependencies, use JSR (jsr:) for versioned, type-checked packages, npm: specifier for npm packages, and configure import maps in deno.json for clean paths.
• When writing tests, use Deno.test() with @std/assert assertions, @std/testing for mocking, and deno test --coverage for coverage reports. Deno's sanitizers detect resource leaks automatically.
• When building CLI tools, use deno compile to produce standalone executables that cross-compile for Linux, macOS, and Windows with no runtime dependency.
• When deploying to the edge, use Deno Deploy with Deno KV for key-value storage, Deno.cron() for scheduled tasks, and queues for background processing.
• When using Deno KV, structure keys hierarchically (["users", id, "profile"]), use atomic() for transactions, and configure TTL with expireIn for automatic expiration.

Examples

Example 1: Build a REST API with Deno KV

User request: "Create an API with Deno that stores data in Deno KV"

Actions:

1. Create HTTP server with Deno.serve() and route matching
2. Open KV store with Deno.openKv() and define key structure
3. Implement CRUD operations using kv.get(), kv.set(), and kv.atomic()
4. Set explicit permissions in deno.json task definitions

Output: A secure API with embedded key-value storage, ready for Deno Deploy.

Example 2: Compile a CLI tool for distribution

User request: "Create a Deno CLI tool that can be distributed as a single binary"

Actions:

1. Build the CLI with argument parsing using @std/cli
2. Add file and network permissions scoped to required resources
3. Write tests with Deno.test() and run with deno test
4. Compile to standalone binaries with deno compile --target for each platform

Output: Cross-platform standalone executables with no runtime dependency.

Guidelines

• Always specify permissions explicitly in production; never deploy with --allow-all.
• Use deno.json imports map for clean import paths instead of raw URLs.
• Prefer JSR (jsr:) over URL imports for versioned, type-checked, immutable packages.
• Use npm: specifier for npm packages instead of CDN URLs.
• Run deno fmt and deno lint in CI for zero-config formatting and linting.
• Use Deno.serve() over third-party frameworks for simple APIs; it is faster and lighter.
• Compile to standalone binary with deno compile for distribution with no runtime dependency.