javascript-pro skill

by Jeffallan·Jeffallan/claude-skills·12k stars·MIT

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.

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Install the javascript-pro skill

A skill is a folder. Copy it into your agent's skills folder and the agent loads it when the task matches its description.

git clone --depth 1 https://github.com/Jeffallan/claude-skills.git /tmp/claude-skills
mkdir -p ~/.claude/skills
cp -r /tmp/claude-skills/skills/javascript-pro ~/.claude/skills/javascript-pro

available in every project

In the Claude apps, zip the folder and upload it from the Skills settings. The folder on GitHub

The instructions your agent would load

SKILL.md as published, without the frontmatter. Read it on GitHub

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:

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:

Module file with clean exports
Test file with comprehensive coverage
JSDoc documentation for public APIs
Brief explanation of patterns used

Documentation

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