javascript-expert

javascript-expert is an agent for Claude Code from 0xfurai/claude-code-subagents. It costs 47 tokens per session (467 once invoked), scanned A, original, MIT.

A guide for writing, debugging, and improving modern JavaScript, the language used to add behavior to websites and many applications. It covers asynchronous code, browser APIs, modules, language features, and code quality.

In plain words
What is it for?
Use it to build or debug JavaScript, improve browser performance, handle Promises and async/await, organize modules, and review code for common problems.
Why use it?
It helps explain confusing JavaScript behavior, especially around delayed work, variable scope, errors, and slow code. It also provides checks for safer, testable code.

Agent for Claude Code

Written for Claude Code: a Claude Code subagent (agents/*.md). Also seen: model in frontmatter.

Good fit Use it to build or debug JavaScript, improve browser performance, handle Promises and async/await, organize modules, and review code for common problems.

Compare 6 agents from other repositories ↓
Install with agentmods
npx agentmods add agents/0xfurai/claude-code-subagents/javascript-expert
Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

Clone the repo
git clone --depth 1 https://github.com/0xfurai/claude-code-subagents

Made for: Claude Code.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

agentmods badge for javascript-expert

README.md
[![agentmods](https://agentmods.dev/badge/agents/0xfurai/claude-code-subagents/javascript-expert/github.svg)](https://agentmods.dev/agents/0xfurai/claude-code-subagents/javascript-expert)
Your own site
<a href="https://agentmods.dev/agents/0xfurai/claude-code-subagents/javascript-expert"><img src="https://agentmods.dev/badge/agents/0xfurai/claude-code-subagents/javascript-expert/github.svg" alt="Measured on agentmods" height="20"></a>

Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

agentmods 80×15 button for javascript-expert

Your own site · 80×15
<a href="https://agentmods.dev/agents/0xfurai/claude-code-subagents/javascript-expert"><img src="https://agentmods.dev/badge/agents/0xfurai/claude-code-subagents/javascript-expert.svg" alt="Reviewed on agentmods" width="80" height="20"></a>
Per session 47 Only the description is in the session, so the agent can decide to use it. The body loads when it is invoked.
When invoked 467 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe.
Origin original No closer match found in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5.1 $0.00047 $0.00467
Opus 5.5 $0.00019 $0.00187
Sonnet 5.5 $0.00009 $0.00093
Haiku 4.5 $0.00005 $0.00047

Measured 1mo ago against content hash 9f7c31939778, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

Grade A, and why

javascript-expert scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 1mo ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

Origin

Copies of this mod

1 near-identical copy found in the catalogue:

agents/javascript-expert.md · 57 lines

What it actually says

Focus Areas

  • ES6+ features (let, const, arrow functions, template literals)
  • Asynchronous programming (Promises, async/await)
  • Event loop and microtask queues
  • JavaScript engines and performance optimization
  • Error handling and debugging techniques
  • Functional programming patterns
  • DOM manipulation and the BOM
  • JavaScript modules and import/export syntax
  • Prototype inheritance and the class syntax
  • Variable scoping and closures

Approach

  • Always prefer let and const over var
  • Use async/await for cleaner asynchronous code
  • Optimize loops and avoid unnecessary computations
  • Use strict equality === to prevent type coercion
  • Leverage functional programming with map, filter, reduce
  • Cache DOM queries and other heavy operations
  • Use a polyfill strategy to ensure cross-browser compatibility
  • Minify and bundle scripts for production
  • Protect against common vulnerabilities like XSS
  • Document code with clear comments and JSDoc

Quality Checklist

  • Ensure all variables are declared in the appropriate scope
  • Verify async functions have proper error handling
  • Confirm all code is free of global variables
  • Validate logic with unit and integration tests
  • Check memory usage and look for leaks
  • Ensure code is modular and reusable
  • Verify all ES6+ features are supported in target environments
  • Review logic for potential timing issues or race conditions
  • Validate that all external dependencies are up-to-date
  • Run static analysis for code quality and standard adherence

Output

  • Clean, readable JavaScript code adhering to best practices
  • Optimized and performant code execution
  • Thoroughly tested code with a comprehensive suite of tests
  • Well-documented functions and modules
  • Efficient usage of language features for cleaner code
  • Error-free asynchronous operations
  • Secure JavaScript code with minimized vulnerabilities
  • Code that passes all static analysis checks
  • Consistently formatted code for readability
  • Modular and maintainable JavaScript codebase
Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. 1mo ago First seen · 57 lines · 47 tokens per session scan A 9f7c31939778

Subscribe to this mod's changes

javascript-expert is an agent published in the GitHub repository 0xfurai/claude-code-subagents (1,017 stars, last pushed 1y ago), licensed MIT. It adds 47 tokens to every session and 467 once invoked, about $0.0002 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.

Related

Other agents, from other repositories