javascript-pro

JavaScript Pro

When to Use This Skill

• Building vanilla JavaScript applications
• Implementing async/await patterns and Promise handling
• Working with modern module systems (ESM/CJS)
• Optimizing browser performance and memory usage
• Developing Node.js backend services
• Implementing Web Workers, Service Workers, or browser APIs

Core Workflow

1. Analyze requirements — Review package.json, module system, Node version, browser targets; confirm .js/.mjs/.cjs conventions
2. Design architecture — Plan modules, async flows, and error handling strategies
3. Implement — Write ES2023+ code with proper patterns and optimisations
4. Validate — Run linter (eslint --fix); if linter fails, fix all reported issues and re-run before proceeding. Check for memory leaks with DevTools or --inspect, verify bundle size; if leaks are found, resolve them before continuing
5. Test — Write comprehensive tests with Jest achieving 85%+ coverage; if coverage falls short, add missing cases and re-run. Confirm no unhandled Promise rejections

Reference Guide

Load detailed guidance based on context:

Constraints

MUST DO

• Use ES2023+ features exclusively
• Use X | null or X | undefined patterns
• Use optional chaining (?.) and nullish coalescing (??)
• Use async/await for all asynchronous operations
• Use ESM (import/export) for new projects
• Implement proper error handling with try/catch
• Add JSDoc comments for complex functions
• Follow functional programming principles

MUST NOT DO

• Use var (always use const or let)
• Use callback-based patterns (prefer Promises)
• Mix CommonJS and ESM in the same module
• Ignore memory leaks or performance issues
• Skip error handling in async functions
• Use synchronous I/O in Node.js
• Mutate function parameters
• Create blocking operations in the browser

Key Patterns with Examples

Async/Await Error Handling

// ✅ Correct — always handle async errors explicitly
async function fetchUser(id) {
  try {
    const response

Optional Chaining & Nullish Coalescing

ESM Module Structure

Avoid var / Prefer const

Output Templates

When implementing JavaScript features, provide:

1. Module file with clean exports
2. Test file with comprehensive coverage
3. JSDoc documentation for public APIs
4. Brief explanation of patterns used

Documentation