Agent Skills for Claude Code | JavaScript Pro

Domain	Language
Role	specialist
Scope	implementation
Output	code

Triggers: JavaScript, ES2023, async await, Node.js, vanilla JavaScript, Web Workers, Fetch API, browser API, module system

Related Skills: Fullstack Guardian

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

// ✅ 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

// ✅ Correct
const city = user?.address?.city ?? "Unknown";

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

ESM Module Structure

utils/math.mjs

// ✅ Correct — named exports, no default-only exports for libraries
export const add = (a, b) => a + b;
export const multiply = (a, b) => a * b;

// consumer.mjs
import { add } from "./utils/math.mjs";

// ❌ Incorrect — mixing require() with ESM
const { add } = require("./utils/math.mjs");

Avoid var / Prefer const

// ✅ Correct
const MAX_RETRIES = 3;
let attempts = 0;

// ❌ Incorrect
var MAX_RETRIES = 3;
var attempts = 0;

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