Lang Javascript

Activate this skill when the user mentions any of:

File Extensions: .js, .mjs, .cjs, package.json, bun.lockb

JavaScript Core: JavaScript, ES2024, ES2025, ESM, CommonJS, import/export, async/await, Promise, Set methods, Object.groupBy, Map.groupBy, toSorted, toReversed, toSpliced

Runtime: Bun, Bun.serve, Bun.file, Bun.write, bun:sqlite, bun:test, Node.js alternative

Web Frameworks: Hono, hono/cors, hono/jwt, edge-first, Cloudflare Workers, c.json, c.text, middleware

Validation: Valibot, @hono/valibot-validator, v.object, v.string, v.pipe, schema validation

Testing: bun test, describe, it, expect, mock, beforeEach, afterEach, --coverage

Linting: Biome, biomejs, biome.json, linter, formatter, biome check

Database: bun:sqlite, Drizzle ORM, SQLite, prepared statements

⛔ Forbidden Patterns

1. NO var: Always use const or let. Block scope is mandatory.
2. NO require in ESM: Use import/export syntax unless explicitly in a CJS context or interacting with legacy CJS-only packages in a compatible way.
3. NO Promise Constructors for Async/Await: Avoid new Promise() wrappers around functions that already return promises. Use async/await directly.
4. NO Loose Equality: Always use === and !== instead of == and !=.
5. NO Ignoring Floating Point Precision: Be aware of 0.1 + 0.2 !== 0.3. Use libraries like decimal.js for financial calculations if needed.

🤖 Agent Tool Strategy

1. Discovery: Check for justfile first. If it exists, use just -l to list recipes and prefer just commands over language-specific CLIs (npm, cargo, poetry, etc.). Then, read package.json to understand the project type (type: "module" vs "commonjs"), dependencies, and scripts.
2. File Operations:
   • Use bun init or npm init (via Bash) only if creating a new project.
   • Use Read to inspect existing code before modifying.
3. Testing: Prefer running tests via bun test or npm test after changes to verify correctness.
4. Linting: Check for biome.json or eslint.config.js and respect the project's linting rules.

Quick Reference (30 seconds)

JavaScript ES2024+ Development Specialist - Modern JavaScript with Bun runtime and Hono framework.

Auto-Triggers: .js, .mjs, .cjs files, package.json, JavaScript projects

Core Stack:

• ES2024+: Set methods, Promise.withResolvers, immutable arrays, import attributes
• Bun: Fast all-in-one runtime, bundler, test runner, package manager
• Hono: Ultrafast, edge-first web framework
• Testing: Bun's built-in test runner
• Linting: Biome (linter + formatter)
• Validation: Valibot (tree-shakable)

Quick Commands:

# Create new project
bun init

# Install dependencies
bun install
bun add hono valibot
bun add -d @biomejs/biome

# Run development server
bun run --watch src/index.js

# Run tests
bun test

# Bundle for production
bun build ./src/index.js --outdir=./dist --target=bun

Implementation Guide (5 minutes)

See patterns.md for detailed implementation patterns covering:

• ES2024 Key Features (Set Operations, Promise.withResolvers)
• ES2025 Features (Import Attributes, RegExp.escape)
• Bun Runtime (File I/O, HTTP Server, SQLite)
• Hono Web Framework (Basic Setup, Validation, JWT, Error Handling)
• Testing with Bun
• Biome (Linter + Formatter)

Quick Troubleshooting

Bun Issues:

# Check version
bun --version

# Upgrade Bun
bun upgrade

# Clear cache
bun pm cache rm

Module System:

# Check package.json type
# (Use the Read tool to inspect package.json)

# ESM: "type": "module" - use import/export
# CommonJS: "type": "commonjs" or omitted - use require/module.exports

Common Fixes:

# Delete cache and reinstall
rm -rf node_modules bun.lockb && bun install

# Check for outdated packages
bun outdated

Advanced Patterns

For comprehensive documentation including advanced async patterns, module system details, performance optimization, and production deployment configurations, see:

• patterns.md - Implementation patterns (Bun, Hono, Biome)
• reference.md - Complete API reference, Context7 library mappings, Bun APIs
• examples.md - Production-ready code examples, full-stack patterns, testing templates

Context7 Integration

// Bun - mcp__context7__get_library_docs("/oven-sh/bun", "runtime bundler test", 1)
// Hono - mcp__context7__get_library_docs("/honojs/hono", "middleware validators routing", 1)
// Valibot - mcp__context7__get_library_docs("/fabian-hiller/valibot", "schema validation", 1)
// Drizzle - mcp__context7__get_library_docs("/drizzle-team/drizzle-orm", "queries migrations", 1)

Quick Troubleshooting

Bun Issues:

# Check version
bun --version

# Upgrade Bun
bun upgrade

# Clear cache
bun pm cache rm

Module System:

# Check package.json type
# (Use the Read tool to inspect package.json)

# ESM: "type": "module" - use import/export
# CommonJS: "type": "commonjs" or omitted - use require/module.exports

Common Fixes:

# Delete cache and reinstall
rm -rf node_modules bun.lockb && bun install

# Check for outdated packages
bun outdated