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:

| 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
```js
// ✅ Correct — always handle async errors explicitly
async function fetchUser(id) {
  try {
    const response = await fetch(`/api/users/${id}`);
    if (!response.ok) throw new Error(`HTTP ${response.status}`);
    return await response.json();
  } catch (err) {
    console.error("fetchUser failed:", err);
    return null;
  }
}

// ❌ Incorrect — unhandled rejection, no null guard
async function fetchUser(id) {
  const response = await fetch(`/api/users/${id}`);
  return response.json();
}
```

### Optional Chaining & Nullish Coalescing
```js
// ✅ Correct
const city = user?.address?.city ?? "Unknown";

// ❌ Incorrect — throws if address is undefined
const city = user.address.city || "Unknown";
```

### ESM Module Struct