JavaScript Pro

What it does

When your agent uses it

• Writing vanilla JavaScript without a framework
• Converting callback-based code to Promises and async/await
• Migrating a CommonJS package to ES modules
• Tracking down memory leaks or slow code in the browser
• Moving heavy work into Web Workers or Service Workers

Example prompts

• “Rewrite this callback-based file loader with fs/promises and async/await.”
• “Convert our utils folder from CommonJS to ES modules.”
• “Move the image hashing code into a Web Worker so the page stays responsive.”
• “Write Jest tests for the date helpers and check that we reach 85% coverage.”

Requirements

• Node.js with `eslint` and Jest for the validate and test steps

Workflow steps

5 steps, taken from the first numbered list in SKILL.md.

1. 1Analyze requirements — Review package.json, module system, Node version, browser targets; confirm .js/.mjs/.cjs conventions
2. 2Design architecture — Plan modules, async flows, and error handling strategies
3. 3Implement — Write ES2023+ code with proper patterns and optimisations
4. 4Validate — Run linter (eslint --fix); if linter fails, fix all reported issues and re-run before proceeding. Check for memory leaks with…
5. 5Test — Write comprehensive tests with Jest achieving 85%+ coverage; if coverage falls short, add missing cases and re-run. Confirm no…

What it can do on your machine

Read from SKILL.md and the folder at commit 1be15d8. It shows what the files ask for, not the result of running them.

• Tool permissions

  Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.

  From allowed-tools in the SKILL.md frontmatter.

• Runs code

  Shell commands in SKILL.md call:

  • eslint

  From the folder's file list and the shell code blocks in SKILL.md.

• Network

  Links to these hosts (documentation or services it may open):

  • github.com
  • synergetic.solutions
  • jeffallan.github.io

  From URLs in SKILL.md, links to its own repository left out.

• Credentials

  Names no API keys, tokens, secrets or passwords.

  From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

Context cost

JavaScript Pro loads about 1.2k tokens when it runs, and up to ~12k if it reads all its reference files. Until then it costs about 98 tokens; SKILL.md has 348 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it

~98

When it runs · the whole SKILL.md, loaded when a task matches

~1.2k

With references · SKILL.md plus every file in references/, read only if the agent opens them

~12k

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.

SKILL.md

The full file from Jeffallan/claude-skills at commit 1be15d8, republished under its MIT licence (© Jeffallan). 348 words, ~1,241 tokens.

name

javascript-pro

description

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.

license

MIT

metadata.author

https://github.com/Jeffallan

metadata.company

https://synergetic.solutions

metadata.version

1.1.0

metadata.domain

language

metadata.triggers

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

metadata.role

specialist

metadata.scope

implementation

metadata.output-format

code

metadata.related-skills

fullstack-guardian

// ✅ 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;

© Jeffallan, MIT. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file

Files

SKILL.md and 5 other files (references) in skills/javascript-pro of Jeffallan/claude-skills.

• SKILL.md
• references/async-patterns.md
• references/browser-apis.md
• references/modern-syntax.md
• references/modules.md
• references/node-essentials.md

Open the folder on GitHubat commit 1be15d8