33 JavaScript Concepts is a learning guide that explains core JavaScript ideas through practical examples and selected resources. It helps developers understand topics such as primitive values, objects, coercion, equality, scope, closures, and the call stack. The catalogue add-ons provide skills related to studying or working with the guide.
Borrowing it
Nothing to install: this file belongs to leonardomso/33-js-concepts. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.
curl -O https://raw.githubusercontent.com/leonardomso/33-js-concepts/master/.claude/skills/write-concept/SKILL.mdgit clone --depth 1 https://github.com/leonardomso/33-js-conceptsWrote 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/leonardomso/33-js-concepts/write-concept)<a href="https://agentmods.dev/skills/leonardomso/33-js-concepts/write-concept"><img src="https://agentmods.dev/badge/skills/leonardomso/33-js-concepts/write-concept/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/leonardomso/33-js-concepts/write-concept"><img src="https://agentmods.dev/badge/skills/leonardomso/33-js-concepts/write-concept.svg" alt="Reviewed on agentmods" width="80" height="20"></a>- Snyk pass
- NVIDIA SkillSpector warn
SkillSpector: 3 findings, 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 644 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.
- medium Data Exfiltration · line 1054 Data is being sent to an external URL. This could be legitimate telemetry or data exfiltration. Manual review is recommended.Fix: Verify the destination URL is trusted and necessary. Remove or replace with documented APIs. Ensure no secrets, tokens, or PII are transmitted.
- medium Data Exfiltration · line 1063 Data is being sent to an external URL. This could be legitimate telemetry or data exfiltration. Manual review is recommended.Fix: Verify the destination URL is trusted and necessary. Remove or replace with documented APIs. Ensure no secrets, tokens, or PII are transmitted.
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 | $0.00027 | $0.12405 |
| Opus 5.5 | $0.00011 | $0.04962 |
| Sonnet 5.5 | $0.00005 | $0.02481 |
| Haiku 4.5 | $0.00003 | $0.01240 |
Grade A, and why
write-concept 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 2mo 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 — 1,445 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Skill: Write JavaScript Concept Documentation
Use this skill when writing or improving concept documentation pages for the 33 JavaScript Concepts project.
When to Use
- Creating a new concept page in
/docs/concepts/ - Rewriting or significantly improving an existing concept page
- Reviewing an existing concept page for quality and completeness
- Adding explanatory content to a concept
Target Audience
Remember: the reader might be someone who has never coded before or is just learning JavaScript. Write with empathy for beginners while still providing depth for intermediate developers. Make complex topics feel approachable and never assume prior knowledge without linking to prerequisites.
Writing Guidelines
Voice and Tone
- Conversational but authoritative: Write like you're explaining to a smart friend
- Encouraging: Make complex topics feel approachable
- Practical: Focus on real-world applications and use cases
- Concise: Respect the reader's time; avoid unnecessary verbosity
- Question-driven: Open sections with questions the reader might have
Avoiding AI-Generated Language
Your writing must sound human, not AI-generated. Here are specific patterns to avoid:
Words and Phrases to Avoid
| ❌ Avoid | ✓ Use Instead |
|---|---|
| "Master [concept]" | "Learn [concept]" |
| "dramatically easier/better" | "much easier" or "cleaner" |
| "one fundamental thing" | "one simple thing" |
| "one of the most important concepts" | "This is a big one" |
| "essential points" | "key things to remember" |
| "understanding X deeply improves" | "knowing X well makes Y easier" |
| "To truly understand" | "Let's look at" or "Here's how" |
| "This is crucial" | "This trips people up" |
| "It's worth noting that" | Just state the thing directly |
| "It's important to remember" | "Don't forget:" or "Remember:" |
| "In order to" | "To" |
| "Due to the fact that" | "Because" |
| "At the end of the day" | Remove entirely |
| "When it comes to" | Remove or rephrase |
| "In this section, we will" | Just start explaining |
| "As mentioned earlier" | Remove or link to the section |
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.
- 2mo ago First seen · 1,445 lines · 27 tokens per session scan A 8ed8542de8c8
write-concept is a skill published in the GitHub repository leonardomso/33-js-concepts (66,529 stars, last pushed 16d ago), licensed MIT. It adds 27 tokens to every session and 12,405 once invoked, about $0.0001 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.
write-concept compared with skills from other repositories
| Mod | Type | Stars | Last push | Licence | Scan | Tokens | Works with |
|---|---|---|---|---|---|---|---|
| write-conceptleonardomso/33-js-concepts | Skill | 67k | 16d ago | MIT | A | 27 | Claude Code |
| guided-tourwasp-lang/open-saas | Skill | 16k | 9d ago | MIT | A | 29 | Claude Code |
| image-studio-ui-testsAutomattic/wp-calypso | Skill | 13k | today | GPL-2.0 | A | 52 | Claude Code, Codex |
| typescript-expertpersonamanagmentlayer/pcl | Skill | 44 | 11d ago | Apache-2.0 | A | 40 | Claude Code |
| netsuite-suitescript-learningoracle/netsuite-suitecloud-sdk | Skill | 370 | 7d ago | UPL-1.0 | A | 71 | Claude Code, Codex |
| add-form-managerbishopZ/2026-Boilerplate | Skill | 24 | 3mo ago | MIT | A | 22 | Claude Code, Codex |
| migrate-api-to-tanstack-querybishopZ/2026-Boilerplate | Skill | 24 | 3mo ago | MIT | A | 32 | Claude Code, Codex |