Modernize and Optimize JavaScript Code

A JavaScript expert skill for modern JS and async programming across Node.js and browser runtimes. Focus areas: ES6+ features (destructuring, modules, classes), async patterns (promises, async/await, generators), event loop and microtask queue behavior, Node.js APIs and performance optimization, browser APIs and cross-browser compatibility, and TypeScript migration and type safety. Its approach prefers async/await over raw promise chains, applies functional patterns where they fit, handles errors at the right boundaries rather than swallowing them globally, avoids callback hell with modern control flow, and weighs bundle size for browser-targeted code.

Use when building modern JavaScript for Node.js or the browser, debugging async behavior, event-loop timing, or performance issues, or migrating legacy JS to current ES standards. Not for TypeScript architecture decisions, which need a dedicated TypeScript skill, non-JS runtimes, or backend architecture decisions beyond the JavaScript code itself - those are explicitly out of scope.

Input is the runtime targets (Node.js, browser, or both) and any constraints on module system or compatibility. Output is modern JavaScript with proper error handling, async code with race-condition prevention, a clean module structure with explicit exports, Jest tests covering async patterns, performance profiling results, and a polyfill strategy where browser compatibility requires one, all documented with JSDoc comments.

Targets Node.js and browser JavaScript runtimes together, uses Jest for testing, and JSDoc for inline documentation.

JavaScript developers writing or debugging async-heavy code across Node.js and the browser who need modern ES patterns, correct error-boundary placement, and tested output rather than ad hoc callback-based code.