deno

Deno Best Practices

Deno is our runtime of choice for its security, built-in tooling, and first-class TypeScript support. This guide outlines the definitive best practices for writing Deno applications, ensuring consistency, performance, and maintainability across our projects.

1. Project Configuration: deno.json is King

Always use deno.json (or deno.jsonc) as the single source of truth for project configuration. This file unifies TypeScript compiler options, Deno-specific flags, tasks, linting, and formatting. Avoid tsconfig.json for Deno-first projects.

✅ GOOD: Centralized deno.json

{
  "compilerOptions": {
    "target": "es2022",
    "lib": ["deno.window", "deno.unstable"],
    "strict": true,
    "checkJs": true,
    "jsx": "react-jsx"
  },
  "lint": {
    "rules": {
      "tags": ["recommended"]
    }
  },
  "fmt": {
    "options": {
      "useTabs": false,
      "lineWidth": 100,
      "indentWidth": 2,
      "singleQuote": true
    }
  },
  "imports": {
    "std/": "https://deno.land/[email protected]/",
    "oak": "jsr:@oak/oak@16"
  },
  "tasks": {
    "dev": "deno run --allow-net --watch main.ts",
    "test": "deno test --allow-read",
    "start": "deno run --allow-net main.ts"
  }
}

❌ BAD: Fragmented Configuration

{
  "compilerOptions": {
    "target": "es2022",
    "strict": true
  }
}

{
  "scripts": {
    "dev": "deno run --allow-net --watch main.ts"
  }
}

2. Module Organization: ES Modules & Explicit Imports

Deno embraces ES modules and URL-based dependencies. Always use explicit file extensions and prefer JSR or deno.land/x for external modules.

2.1. Explicit URL Imports

Import directly from URLs or use deno.json's imports map for cleaner paths.

✅ GOOD: URL Imports with Import Map

import { serve } from "std/http/server.ts"; // Uses import map
import { Application } from "oak"; // Uses import map

const app = new Application();
app.listen({ port: 8000 });
console.log("Server running on http://localhost:8000");