javascript-pro

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Summary

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 Fetch API, or reviewing .js/.mjs/.cjs files for correctness and best practices.

SKILL.md

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

Analyze requirements — Review package.json, module system, Node version, browser targets; confirm .js/.mjs/.cjs conventions
Design architecture — Plan modules, async flows, and error handling strategies
Implement — Write ES2023+ code with proper patterns and optimisations
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
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

// ✅ 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();
}

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

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

// ✅ 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");

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

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

javascript-pro

Summary

SKILL.md

JavaScript Pro

When to Use This Skill

Core Workflow

Reference Guide

Constraints

MUST DO

MUST NOT DO

Key Patterns with Examples

Async/Await Error Handling

Optional Chaining & Nullish Coalescing

ESM Module Structure

Avoid var / Prefer const

Output Templates

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