Writes, debugs, and refactors JavaScript code using modern ES2023+ features, async/await patterns, ESM module systems, and Node.js APIs. Use when building vanilla JavaScript applications, implementing Promise-based async flows, optimising browser or Node.js performance, working with Web Workers or F

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: Topic Reference Load When Modern Syntax references/modern syntax.md ES2023+ features, optional chaining, private fields Async Patterns references/async patterns.md Promises, async/await, error handling, event loop Modules references/modules.md ESM vs CJS, dynamic imports, package.json exports Browser APIs references/browser apis.md Fetch, Web Workers, Storage, IntersectionObserver Node Essentials references/node essentials.md fs/promises, streams, EventEmitter, worker threads 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 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](https://jeffallan.github.io/claude skills/skills/language/javascript pro/)

javascript-pro