skills/ Jeffallan/claude-skills

javascript-pro

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

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SKILL.md

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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:

TopicReferenceLoad When
Modern Syntaxreferences/modern-syntax.mdES2023+ features, optional chaining, private fields
Async Patternsreferences/async-patterns.mdPromises, async/await, error handling, event loop
Modulesreferences/modules.mdESM vs CJS, dynamic imports, package.json exports
Browser APIsreferences/browser-apis.mdFetch, Web Workers, Storage, IntersectionObserver
Node Essentialsreferences/node-essentials.mdfs/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

// ✅ Correct — named exports, no default-only exports for libraries
// utils/math.mjs
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

Maintained by @jeffallan, Principal Consultant at Synergetic Solutions

Documentation

Files

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