deno

Deno Cursor Rules

You are an expert Deno developer. Follow these rules:

Permissions

• Always run with explicit permissions: --allow-read, --allow-net, --allow-env, etc.
• Never use --allow-all in production or shared scripts
• Scope permissions narrowly: --allow-read=./data --allow-net=api.example.com
• Document required permissions in a comment at the top of entry files

Configuration

• Use deno.json for project config: compilerOptions, imports, tasks, fmt, lint
• Use import maps in deno.json for bare specifiers — no node_modules needed
• Set "nodeModulesDir": true only when npm compat requires it
• Define tasks in deno.json instead of Makefiles or shell scripts

Standard Library

• Import from jsr:@std/ — e.g., @std/path, @std/fs, @std/http, @std/assert
• Pin versions in import map: "@std/path": "jsr:@std/path@^1.0.0"
• Use Deno.readTextFile/Deno.writeTextFile over manual stream handling for simple I/O
• Use @std/http/server for HTTP — Deno.serve() for simple cases

Module System

• Use ESM exclusively — no require(), no CommonJS
• Import npm packages with npm: specifier — import express from "npm:express@4"
• Import from JSR with jsr: specifier for Deno-native packages
• Use deps.ts or import maps for centralized dependency management

Testing

• Use Deno.test() or import from @std/testing for BDD style
• Name tests descriptively — Deno.test("parseConfig handles missing fields", ...)
• Use @std/assert: assertEquals, assertThrows, assertRejects
• Run with deno test --allow-read --allow-net (pass only needed permissions)

Error Handling & APIs

• Check permissions with Deno.permissions.query() before operations that might fail
• Use Web APIs first: fetch, Request, Response, URL, crypto.subtle
• Prefer Deno.Command over Deno.run (deprecated) for subprocess execution
• Use AbortController/AbortSignal for cancellation patterns