Deno Core Best Practices

When to Use This Skill

Use this skill for ALL Deno TypeScript development:

Setting up new Deno projects
Writing Deno applications or libraries
Configuring build, test, and deployment
Working with dependencies and imports

Core Deno Philosophy

One Tool, Zero Dependencies

Deno is the only tool you need for TypeScript development
Built-in tooling: typecheck, lint, format, test, coverage, benchmark
Avoid node_modules at all costs - reduce supply chain attack surface
No need for: tsc, eslint, prettier, jest, vitest, webpack, etc.

TypeScript Excellence

Strict TypeScript adherence - not just "TS support"
Bleeding-edge TypeScript features by default - no flags, no config needed
No compilation step - just run your code
Target ES2024+ with Stage 3 TC39 proposals

Security First

Explicit permissions model (no implicit file system or network access)
Supply chain security through minimal external dependencies
First-class support for modern security patterns

Language & Compiler

TypeScript Configuration

Do not use tsconfig.json - Deno uses deno.json(c) as the single source of truth
Type-checking powered by deno check / deno test - do not rely on external tsc
Default module format is ESM only - no CommonJS interop
Prefer Deno's runtime-provided types (Deno.*, Web APIs, Fetch, URLPattern) over polyfills

Strictest Compiler Settings

Always use the strictest possible settings in deno.json:

{
  "compilerOptions": {
    "strict": true,
    "noImplicitAny": true,
    "noImplicitReturns": true,
    "noImplicitThis": true,
    "noUnusedLocals": true,
    "noUnusedParameters": true,
    "exactOptionalPropertyTypes": true
  }
}

Configuration & Tasks

deno.json - Single Source of Truth

Use deno.json or deno.jsonc as the single configuration file for:

Compiler options
Linting and formatting rules
Tasks (script aliases)
Import maps (dependency management)
Exclusions

Complete Configuration Example:

{
  "compilerOptions": {
    "strict": true,
    "noImplicitAny": true,
    "noImplicitReturns": true,
    "noImplicitThis": true,
    "noUnusedLocals": true,
    "noUnusedParameters": true
  },
  "tasks": {
    "dev": "deno run --watch --allow-net --allow-read --allow-env src/main.ts",
    "test": "deno test --allow-net --allow-read --allow-env --coverage=coverage src/",
    "test:unit": "deno test --allow-net --allow-read --allow-env --coverage=coverage src/",
    "test:e2e": "deno test --allow-net --allow-read --allow-env tests/e2e/",
    "test:watch": "deno test --allow-net --allow-read --allow-env --watch --fail-fast",
    "coverage": "deno coverage coverage --html",
    "check": "deno check $(find src -name '*.ts' -not -name '*.sql')",
    "lint": "deno lint",
    "fmt": "deno fmt"
  },
  "imports": {
    "@/": "./src/",
    "@/domain/": "./src/domain/",
    "@/infrastructure/": "./src/infrastructure/",
    "@/application/": "./src/application/",
    "@std/assert": "jsr:@std/assert@^1.0.14",
    "@std/fs": "jsr:@std/fs@^1.0.19",
    "@std/testing": "jsr:@std/testing@^1.0.15",
    "@std/ulid": "jsr:@std/ulid@1",
    "zod": "npm:zod@^3.23.8"
  },
  "exclude": [
    "coverage/",
    "node_modules/"
  ],
  "lock": true
}

Essential Tasks

Define these deno task aliases at minimum:

dev - Development with watch mode
test, test:watch - Testing
coverage - Generate coverage reports
check - Type-check all files
lint, fmt - Code quality

Lockfile Management

Always commit deno.lock to version control
Run with --lock=deno.lock --lock-write=false in CI
Update lockfile: deno cache --lock=deno.lock --lock-write

Imports & Module Resolution

Import Strategy

CRITICAL: Never use direct JSR/npm imports in source files. All external dependencies MUST be declared in deno.json import map.

Import Order in Source Files:

// 1. Standard library imports (via import map)
import { assertEquals } from "@std/assert";

// 2. Third-party imports (via import map)
import { z } from "zod";

// 3. Internal imports (absolute paths using import map)
import { Agent } from "@/domain/agent.ts";

// 4. Relative imports (only within same module/context)
import { validatePrompt } from "./validation.ts";

Dependency Source Priority

Use sources in this order:

jsr: registry (first choice for TypeScript modules)
```
"@std/assert": "jsr:@std/assert@^1.0.14"
```
npm: specifier (when needed; prefer ESM-compatible)
```
"zod": "npm:zod@^3.23.8"
```
URL imports (rarely needed with import maps)

Version Pinning

CRITICAL: Version pin all external imports. No floating @latest in committed code.

{
  "imports": {
    "zod": "npm:zod@^3.23.8",           // GOOD - pinned
    "zod": "npm:zod",                    // BAD - no version
    "@std/assert": "jsr:@std/assert@1"   // GOOD - pinned
  }
}

Internal Path Aliases

Use import map aliases for clean internal imports:

{
  "imports": {
    "@/": "./src/",
    "@/domain/": "./src/domain/",
    "@/infrastructure/": "./src/infrastructure/"
  }
}

// GOOD - Clean, refactor-safe
import { Agent } from "@/domain/agent.ts";

// BAD - Brittle relative paths
import { Agent } from "../../../domain/agent.ts";

Type-Only Imports

Use type-only imports when importing types:

import type { Agent } from "@/domain/agent.ts";
import type { z } from "zod";

Testing

Test Organization

Unit Tests - Co-located with Source:

src/
└── domain/
    ├── agent.ts
    └── agent.test.ts          # Unit tests next to code

Integration & E2E Tests - Separate Directory:

tests/
├── integration/
│   └── openai_provider.test.ts
└── e2e/
    └── workflow.test.ts

Why Co-location:

Discoverability - tests next to code
Maintenance - easy to keep in sync
Deno convention - follows deno test discovery

Coverage Requirements

Non-Negotiable:

Line coverage: 80%+ - MUST be met
Branch coverage: 60-80% - MUST be met

deno test --coverage=coverage
deno coverage coverage --html

Test Structure

Always use explicit AAA (Arrange-Act-Assert):

import { assertEquals } from "@std/assert";

Deno.test("agent should process valid input", () => {
  // Arrange
  const agent = new Agent({ name: "TestAgent" });
  const input = "Hello, world!";

  // Act
  const result = agent.process(input);

  // Assert
  assertEquals(result.status, "success");
});

Test Development

Red-Green-Refactor with fast feedback:

# Watch mode with fail-fast
deno test --watch --fail-fast

# Run specific file
deno test src/domain/agent.test.ts --watch

Deterministic Tests

CRITICAL: All tests must be deterministic.

Test Flakiness Policy:

Flakiness = highest priority bug
Never ignore, retry, or "fix" with delays
Action: investigate, quarantine, fix
Do NOT merge flaky tests

Use stable seeds and fixtures:

import { FakeTime } from "@std/testing/time";

Deno.test("timer test", () => {
  using time = new FakeTime();
  // Deterministic time control
  time.tick(1000);
});

Test File Naming

All test files must end with .test.ts:

agent.test.ts     # GOOD
agent_test.ts     # BAD
agent.spec.ts     # BAD

Testing Tools

Use @std/assert for assertions
Use @std/testing/mock for test doubles
Use @std/testing/time for time control
Do NOT use Jest - use Deno's built-in runner

Permissions & Security

Principle of Least Privilege

Default to minimum required permissions:

# BAD
deno run --allow-all script.ts

# GOOD
deno run --allow-read=./data --allow-net=api.example.com script.ts

Common Permission Flags

--allow-read[=<path>]      # File system read
--allow-write[=<path>]     # File system write
--allow-net[=<domain>]     # Network access
--allow-env[=<var>]        # Environment variables
--allow-run[=<program>]    # Subprocess execution

Document Required Permissions

/**
 * Fetches data from API and caches locally.
 *
 * Required permissions:
 * - --allow-net=api.example.com
 * - --allow-read=./cache
 * - --allow-write=./cache
 */
export async function fetchData(): Promise<Data> {
  // ...
}

Secrets Management

// BAD - Hardcoded
const apiKey = "sk-1234";

// GOOD - From environment
const apiKey = Deno.env.get("API_KEY");
if (!apiKey) {
  throw new Error("API_KEY required");
}

Run with: deno run --allow-env=API_KEY script.ts

Anti-Patterns to Avoid

Import Anti-Patterns

// BAD - Direct JSR/npm imports in source
import { z } from "npm:zod@^3.23.8";

// GOOD - Use import map
import { z } from "zod";

// BAD - Floating versions
"zod": "npm:zod"

// GOOD - Pinned versions
"zod": "npm:zod@^3.23.8"

Node.js Anti-Patterns

// BAD - Node.js APIs
const fs = require("fs");
import * as fs from "node:fs";

// GOOD - Deno APIs
await Deno.readTextFile("file.txt");

Testing Anti-Patterns

// BAD - Unnecessary delay
await new Promise(r => setTimeout(r, 100));

// GOOD - Deterministic time
import { FakeTime } from "@std/testing/time";
using time = new FakeTime();
time.tick(100);

Permission Anti-Patterns

# BAD - Overly broad
deno run --allow-all script.ts

# GOOD - Specific
deno run --allow-read=./data script.ts

Async Anti-Patterns

// BAD - Unnecessary async
async function validate(input: string): Promise<boolean> {
  return input.length > 0;
}

// GOOD - Remove async if no await
function validate(input: string): boolean {
  return input.length > 0;
}

Quick Command Reference

Development

# Run with watch
deno run --watch src/main.ts

# Type-check
deno check src/**/*.ts

# Format
deno fmt

# Lint
deno lint

Testing

# Run all tests
deno test

# With coverage
deno test --coverage=coverage
deno coverage coverage --html

# Watch mode
deno test --watch --fail-fast

Dependencies

# Update dependencies
deno cache --reload

# Update lockfile
deno cache --lock=deno.lock --lock-write

Tasks

# Run tasks from deno.json
deno task dev
deno task test
deno task coverage

Key Principles Summary

One tool - Deno replaces tsc, eslint, prettier, jest
Security first - Explicit permissions, minimal dependencies
Import maps - All deps in deno.json, never direct imports
Version pinning - No floating versions
Co-located tests - Unit tests next to source
80%/60% coverage - Line/branch, non-negotiable
No flakiness - Highest priority, never ignore
AAA pattern - Explicit in every test
Least privilege - Minimal permissions
ESM only - No CommonJS

Additional Resources

Deno Manual: https://docs.deno.com/
Deno Standard Library: https://jsr.io/@std
JSR Registry: https://jsr.io/

deno-core

Deno Core Best Practices

When to Use This Skill

Core Deno Philosophy

One Tool, Zero Dependencies

TypeScript Excellence

Security First

Language & Compiler

TypeScript Configuration

Strictest Compiler Settings

Configuration & Tasks

deno.json - Single Source of Truth

Essential Tasks

Lockfile Management

Imports & Module Resolution

Import Strategy

Dependency Source Priority

Version Pinning

Internal Path Aliases

Type-Only Imports

Testing

Test Organization

Coverage Requirements

Test Structure

Test Development

Deterministic Tests

Test File Naming

Testing Tools

Permissions & Security

Principle of Least Privilege

Common Permission Flags

Document Required Permissions

Secrets Management

Anti-Patterns to Avoid

Import Anti-Patterns

Node.js Anti-Patterns

Testing Anti-Patterns

Permission Anti-Patterns

Async Anti-Patterns

Quick Command Reference

Development

Testing

Dependencies

Tasks

Key Principles Summary

Additional Resources

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