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.
npx skills add PracticalSwan/agent-skills --skill javascript-developmentgit clone --depth 1 https://github.com/PracticalSwan/agent-skillsWrote 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.
[](https://agentmods.dev/skills/practicalswan/agent-skills/javascript-development)<a href="https://agentmods.dev/skills/practicalswan/agent-skills/javascript-development"><img src="https://agentmods.dev/badge/skills/practicalswan/agent-skills/javascript-development/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.
<a href="https://agentmods.dev/skills/practicalswan/agent-skills/javascript-development"><img src="https://agentmods.dev/badge/skills/practicalswan/agent-skills/javascript-development.svg" alt="Reviewed on agentmods" width="80" height="20"></a>- NVIDIA SkillSpector warn
SkillSpector: 1 finding, up to high
These are SkillSpector’s own severities. On a checked sample its high-severity flags on skills were ~96% false positives — a documented command, a public API, a “never do X” rule — so we show them as a caution to read, not a verdict. Why →
- high Prompt Injection · line 1012 Hidden instructions were detected in comments or invisible text. These could contain malicious directives. Manual review is recommended.Fix: Audit all comments and invisible characters. Remove any instructions that direct the agent to perform unauthorized actions. Use plain, reviewable content.
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.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00052 | $0.06864 |
| Opus 5.5 | $0.00021 | $0.02746 |
| Sonnet 5.5 | $0.00010 | $0.01373 |
| Haiku 4.5 | $0.00005 | $0.00686 |
Grade A, and why
javascript-development scanned grade A with 1 finding 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.
Makes network callslowCapability
Not a fault in itself. Listed so you know the mod talks to something, and to what.
endpoints.map(url => fetch(url).then(r => r.json())) How it starts
The opening of the file, as written. The whole thing — 1,042 lines — stays where its author put it; the contents beside it link to each section on GitHub.
JavaScript Development
Optimized for ECMAScript 2024+, Node.js 22+, TypeScript 5.5+, and modern browser or server-first JavaScript runtimes.
Expert guidance for writing modern JavaScript code with ES2024+ features, async programming patterns, DOM manipulation, API integration, and best practices following official JavaScript resources at https://developer.mozilla.org/en-US/docs/Web/JavaScript.
- Leverage native parallel subagent dispatch and 200k+ context windows where available.
Cross-Client Portability
This skill is written to stay usable across GitHub Copilot, Claude Code, and Codex.
- GitHub Copilot: keep the folder in a Copilot-visible skill path or wrap the workflow in project instructions when folder discovery is unavailable.
- Claude Code: keep the folder in a local skills directory or a compatible plugin source.
- Codex: install or sync the folder into
$CODEX_HOME/skills/javascript-developmentand restart Codex after major changes.
MCP Availability And Fallback
Preferred MCP Server: None required
- Fallback prompt: "Use the JavaScript Development skill without MCP. Rely on its local instructions, bundled resources, standard shell or editor tools, and direct verification. Show the evidence used before concluding."
- Do not claim an MCP operation was used when the active host does not expose it.
- Treat local files, tests, rendered outputs, logs, or screenshots as the fallback evidence path.
Anti-Patterns
- Copying outdated browser or framework patterns: Deprecated APIs and old workarounds add complexity immediately.
- Skipping abort, timeout, or response checks in async code: Network paths fail at the edges first, not on the happy path.
- Treating accessibility as a final polish pass: Markup and state shape are harder to fix after the component contract is set.
Verification Protocol
Before claiming "skill applied successfully":
- Pass/fail: The Javascript Development implementation names the target runtime, framework version, and affected files.
- Pass/fail: Build, lint, test, or equivalent local validation is run for the changed surface.
- Pass/fail: Edge cases for errors, dependency drift, and environment differences are addressed or explicitly out of scope.
- Pressure-test scenario: Apply the workflow to a change that passes happy-path tests but fails one boundary condition.
- Success metric: Zero untested success claims; every implementation claim maps to a command or artifact.
What ships with it
5 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.
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.
- 1mo ago Changed · -1 lines 0e29b3622a36
- 1mo ago Changed 495014956533
- 1mo ago First seen · 1,043 lines · 52 tokens per session scan A bedf1b64f163
javascript-development is a skill published in the GitHub repository PracticalSwan/agent-skills (21 stars, last pushed yesterday), licensed MIT. It adds 52 tokens to every session and 6,864 once invoked, about $0.0002 per session on Opus 5.5. A static security scan graded it A with 1 finding (makes network calls). No closer match exists in the catalogue, so it is treated as the original; first seen 2026-09-03.
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