javascript-mastery
33+ essential JavaScript concepts every developer should know, inspired by [33-js-concepts](https://github.com/leonardomso/33-js-concepts).
Profile →npx skills add sickn33/agentic-awesome-skills --skill javascript-mastery --agent claude-code
Same command for any agent — swap --agent for codex, cursor, copilot.
Weekly change comes from our own snapshots, not the repository page — it measures attention, not adoption.
# 🧠 JavaScript Mastery > 33+ essential JavaScript concepts every developer should know, inspired by [33-js-concepts](https://github.com/leonardomso/33-js-concepts). ## When to Use This Skill Use this skill when: - Explaining JavaScript concepts - Debugging tricky JS behavior - Teaching JavaScript fundamentals - Reviewing code for JS best practices - Understanding language quirks --- ## 1. Fundamentals ### 1.1 Primitive Types JavaScript has 7 primitive types: ```javascript // String const str = "hello"; // Number (integers and floats) const num = 42; const float = 3.14; // BigInt (for large integers) const big = 9007199254740991n; // Boolean const bool = true; // Undefined let undef; // undefined // Null const empty = null; // Symbol (unique identifiers) const sym = Symbol("description"); ``` **Key points**: - Primitives are immutable - Passed by value - `typeof null === "object"` is a historical bug ### 1.2 Type Coercion JavaScript implicitly converts types: ```javascript // String coercion "5" + 3; // "53" (number → string) "5" - 3; // 2 (string → number) // Boolean coercion Boolean(""); // false Boolean("hello"); // true Boolean(0); // false Boolean([]); // true (!) // Equality
- When to Use This Skill
- 1. Fundamentals
- 1.1 Primitive Types
- 1.2 Type Coercion
- 1.3 Equality Operators
- 2. Scope & Closures
- 2.1 Scope Types
- 2.2 Closures
- 2.3 var vs let vs const
- 3. Functions & Execution
- 3.1 Call Stack
- 3.2 Hoisting
- 3.3 this Keyword
- 4. Event Loop & Async
What does the javascript-mastery skill do?
33+ essential JavaScript concepts every developer should know, inspired by [33-js-concepts](https://github.com/leonardomso/33-js-concepts).
How do I install it?
Run `npx skills add sickn33/agentic-awesome-skills --skill javascript-mastery --agent claude-code` — it drops the skill into your project so the agent can pick it up. Swap the --agent value for codex, cursor or copilot if you use one of those.
Where does this skill come from?
From sickn33/agentic-awesome-skills, a repository with 44,414 stars. We read it straight from the repository tree rather than a submitted listing, so what you see here is what is actually published.
Is a popular skill a good skill?
Not necessarily. Stars measure attention, not adoption — a repository can trend for a week and be abandoned. That is why we show the weekly change from our own snapshots next to the total, instead of a single flattering number.