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 ccheney/robust-skills --skill modern-javascriptgit clone --depth 1 https://github.com/ccheney/robust-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/ccheney/robust-skills/modern-javascript)<a href="https://agentmods.dev/skills/ccheney/robust-skills/modern-javascript"><img src="https://agentmods.dev/badge/skills/ccheney/robust-skills/modern-javascript/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/ccheney/robust-skills/modern-javascript"><img src="https://agentmods.dev/badge/skills/ccheney/robust-skills/modern-javascript.svg" alt="Reviewed on agentmods" width="80" height="20"></a>- NVIDIA SkillSpector pass
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 | $0.00047 | $0.00759 |
| Opus 5.5 | $0.00019 | $0.00304 |
| Sonnet 5.5 | $0.00009 | $0.00152 |
| Haiku 4.5 | $0.00005 | $0.00076 |
Grade A, and why
modern-javascript 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.
How it starts
The opening of the file, as written. The whole thing — 46 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Modern JavaScript
Keep specification status, runtime support, and toolchain support separate. The user's explicit instructions take precedence over this skill's guidelines.
Work against the target
Use the relevant JavaScript, required behavior, and declared runtime as input. Inspect engines, browser targets, lockfiles, and build configuration as needed for the feature. Do not invent a baseline or upgrade runtimes merely to use newer syntax.
Prefer a supported standardized feature. A proposal can fit when the user/project chooses it and a maintained transform or polyfill implements the required revision. Bundled edition and support tables are dated snapshots; verify changing status against the sources below.
Preserve semantics
- Keep mutation, property access, iterator consumption, evaluation order, error propagation, and concurrency behavior intact when modernizing.
- Distinguish syntax transforms from runtime polyfills. TypeScript declarations do not supply missing runtime APIs.
- Treat iterator helpers as lazy and single-pass;
Array.fromAsyncconsumes sequentially, whilePromise.allobserves already-started work concurrently. - Do not use
forEach(async ...)when completion must be awaited. Rejection and timeouts do not cancel underlying work by themselves. - Distinguish host APIs such as
fetchandstructuredClonefrom ECMA-262 features, andIntlfrom the separate ECMA-402 standard. - Do not introduce withdrawn Records & Tuples syntax or silently switch between legacy and standard-style decorator semantics.
Deliver the requested code or explanation, noting a required compatibility path when relevant. Verify changed behavior and any fallback that ships; select checks proportional to the change rather than requiring every project check for a syntax answer.
References
Load the semantic or compatibility topic needed, not every edition.
| Task | Reference |
|---|---|
| Runtime targets, Babel, TypeScript, polyfills | COMPATIBILITY.md |
| Async functions and early annual additions | ES2016-ES2017.md |
| Object spread, async iteration, RegExp, flattening | ES2018-ES2019.md |
| Classes, property checks, change-by-copy | ES2022-ES2023.md |
| Grouping, promise resolvers, buffers, Unicode sets | ES2024.md |
| Set/iterator helpers, import attributes, RegExp | ES2025.md |
ES2026 additions such as Array.fromAsync |
ES2026.md |
| Temporal, resource management, proposals, withdrawn syntax | UPCOMING.md |
| Promise combinators and error behavior | PROMISES.md |
| Pools, retries, cancellation, async iteration | CONCURRENCY.md |
| Compact syntax lookup | CHEATSHEET.md |
What ships with it
11 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.
- references/CHEATSHEET.md 13 KB
- references/COMPATIBILITY.md 10 KB
- references/CONCURRENCY.md 7.7 KB
- references/ES2016-ES2017.md 5.0 KB
- references/ES2018-ES2019.md 8.7 KB
- references/ES2022-ES2023.md 8.5 KB
- references/ES2024.md 7.5 KB
- references/ES2025.md 11 KB
- references/ES2026.md 7.7 KB
- references/PROMISES.md 7.0 KB
- references/UPCOMING.md 9.9 KB
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 · -136 lines · -64 tokens per session a2cc69874bc5
- 1mo ago First seen · 182 lines · 111 tokens per session scan A 9ef7c2398ee4
modern-javascript is a skill published in the GitHub repository ccheney/robust-skills (61 stars, last pushed 21d ago), licensed MIT. It adds 47 tokens to every session and 759 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.
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