feat: add 7 new skills from GitHub repo analysis

New skills: - prompt-library: Curated role-based and task-specific prompt templates - javascript-mastery: 33+ essential JavaScript concepts - llm-app-patterns: RAG pipelines, agent architectures, LLMOps - workflow-automation: Multi-step automation and API integration - autonomous-agent-patterns: Tool design, permissions, browser automation - bun-development: Bun runtime, testing, bundling, Node.js migration - github-workflow-automation: AI PR reviews, issue triage, CI/CD Sources: n8n, awesome-chatgpt-prompts, dify, gemini-cli, bun, 33-js-concepts, cline, codex Total skills: 62 → 69
2026-01-16 16:09:39 +01:00
parent c86c93582e
commit f8eaf7bd50
9 changed files with 4824 additions and 45 deletions
--- a/skills/javascript-mastery/SKILL.md
+++ b/skills/javascript-mastery/SKILL.md
@@ -0,0 +1,645 @@
+---
+name: javascript-mastery
+description: "Comprehensive JavaScript reference covering 33+ essential concepts every developer should know. From fundamentals like primitives and closures to advanced patterns like async/await and functional programming. Use when explaining JS concepts, debugging JavaScript issues, or teaching JavaScript fundamentals."
+---
+
+# 🧠 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 coercion
+"5" == 5; // true  (coerces)
+"5" === 5; // false (strict)
+```
+
+**Falsy values** (8 total):
+`false`, `0`, `-0`, `0n`, `""`, `null`, `undefined`, `NaN`
+
+### 1.3 Equality Operators
+
+```javascript
+// == (loose equality) - coerces types
+null == undefined; // true
+"1" == 1; // true
+
+// === (strict equality) - no coercion
+null === undefined; // false
+"1" === 1; // false
+
+// Object.is() - handles edge cases
+Object.is(NaN, NaN); // true (NaN === NaN is false!)
+Object.is(-0, 0); // false (0 === -0 is true!)
+```
+
+**Rule**: Always use `===` unless you have a specific reason not to.
+
+---
+
+## 2. Scope & Closures
+
+### 2.1 Scope Types
+
+```javascript
+// Global scope
+var globalVar = "global";
+
+function outer() {
+  // Function scope
+  var functionVar = "function";
+
+  if (true) {
+    // Block scope (let/const only)
+    let blockVar = "block";
+    const alsoBlock = "block";
+    var notBlock = "function"; // var ignores blocks!
+  }
+}
+```
+
+### 2.2 Closures
+
+A closure is a function that remembers its lexical scope:
+
+```javascript
+function createCounter() {
+  let count = 0; // "closed over" variable
+
+  return {
+    increment() {
+      return ++count;
+    },
+    decrement() {
+      return --count;
+    },
+    getCount() {
+      return count;
+    },
+  };
+}
+
+const counter = createCounter();
+counter.increment(); // 1
+counter.increment(); // 2
+counter.getCount(); // 2
+```
+
+**Common use cases**:
+
+- Data privacy (module pattern)
+- Function factories
+- Partial application
+- Memoization
+
+### 2.3 var vs let vs const
+
+```javascript
+// var - function scoped, hoisted, can redeclare
+var x = 1;
+var x = 2; // OK
+
+// let - block scoped, hoisted (TDZ), no redeclare
+let y = 1;
+// let y = 2; // Error!
+
+// const - like let, but can't reassign
+const z = 1;
+// z = 2; // Error!
+
+// BUT: const objects are mutable
+const obj = { a: 1 };
+obj.a = 2; // OK
+obj.b = 3; // OK
+```
+
+---
+
+## 3. Functions & Execution
+
+### 3.1 Call Stack
+
+```javascript
+function first() {
+  console.log("first start");
+  second();
+  console.log("first end");
+}
+
+function second() {
+  console.log("second");
+}
+
+first();
+// Output:
+// "first start"
+// "second"
+// "first end"
+```
+
+Stack overflow example:
+
+```javascript
+function infinite() {
+  infinite(); // No base case!
+}
+infinite(); // RangeError: Maximum call stack size exceeded
+```
+
+### 3.2 Hoisting
+
+```javascript
+// Variable hoisting
+console.log(a); // undefined (hoisted, not initialized)
+var a = 5;
+
+console.log(b); // ReferenceError (TDZ)
+let b = 5;
+
+// Function hoisting
+sayHi(); // Works!
+function sayHi() {
+  console.log("Hi!");
+}
+
+// Function expressions don't hoist
+sayBye(); // TypeError
+var sayBye = function () {
+  console.log("Bye!");
+};
+```
+
+### 3.3 this Keyword
+
+```javascript
+// Global context
+console.log(this); // window (browser) or global (Node)
+
+// Object method
+const obj = {
+  name: "Alice",
+  greet() {
+    console.log(this.name); // "Alice"
+  },
+};
+
+// Arrow functions (lexical this)
+const obj2 = {
+  name: "Bob",
+  greet: () => {
+    console.log(this.name); // undefined (inherits outer this)
+  },
+};
+
+// Explicit binding
+function greet() {
+  console.log(this.name);
+}
+greet.call({ name: "Charlie" }); // "Charlie"
+greet.apply({ name: "Diana" }); // "Diana"
+const bound = greet.bind({ name: "Eve" });
+bound(); // "Eve"
+```
+
+---
+
+## 4. Event Loop & Async
+
+### 4.1 Event Loop
+
+```javascript
+console.log("1");
+
+setTimeout(() => console.log("2"), 0);
+
+Promise.resolve().then(() => console.log("3"));
+
+console.log("4");
+
+// Output: 1, 4, 3, 2
+// Why? Microtasks (Promises) run before macrotasks (setTimeout)
+```
+
+**Execution order**:
+
+1. Synchronous code (call stack)
+2. Microtasks (Promise callbacks, queueMicrotask)
+3. Macrotasks (setTimeout, setInterval, I/O)
+
+### 4.2 Callbacks
+
+```javascript
+// Callback pattern
+function fetchData(callback) {
+  setTimeout(() => {
+    callback(null, { data: "result" });
+  }, 1000);
+}
+
+// Error-first convention
+fetchData((error, result) => {
+  if (error) {
+    console.error(error);
+    return;
+  }
+  console.log(result);
+});
+
+// Callback hell (avoid this!)
+getData((data) => {
+  processData(data, (processed) => {
+    saveData(processed, (saved) => {
+      notify(saved, () => {
+        // 😱 Pyramid of doom
+      });
+    });
+  });
+});
+```
+
+### 4.3 Promises
+
+```javascript
+// Creating a Promise
+const promise = new Promise((resolve, reject) => {
+  setTimeout(() => {
+    resolve("Success!");
+    // or: reject(new Error("Failed!"));
+  }, 1000);
+});
+
+// Consuming Promises
+promise
+  .then((result) => console.log(result))
+  .catch((error) => console.error(error))
+  .finally(() => console.log("Done"));
+
+// Promise combinators
+Promise.all([p1, p2, p3]); // All must succeed
+Promise.allSettled([p1, p2]); // Wait for all, get status
+Promise.race([p1, p2]); // First to settle
+Promise.any([p1, p2]); // First to succeed
+```
+
+### 4.4 async/await
+
+```javascript
+async function fetchUserData(userId) {
+  try {
+    const response = await fetch(`/api/users/${userId}`);
+    if (!response.ok) throw new Error("Failed to fetch");
+    const user = await response.json();
+    return user;
+  } catch (error) {
+    console.error("Error:", error);
+    throw error; // Re-throw for caller to handle
+  }
+}
+
+// Parallel execution
+async function fetchAll() {
+  const [users, posts] = await Promise.all([
+    fetch("/api/users"),
+    fetch("/api/posts"),
+  ]);
+  return { users, posts };
+}
+```
+
+---
+
+## 5. Functional Programming
+
+### 5.1 Higher-Order Functions
+
+Functions that take or return functions:
+
+```javascript
+// Takes a function
+const numbers = [1, 2, 3];
+const doubled = numbers.map((n) => n * 2); // [2, 4, 6]
+
+// Returns a function
+function multiply(a) {
+  return function (b) {
+    return a * b;
+  };
+}
+const double = multiply(2);
+double(5); // 10
+```
+
+### 5.2 Pure Functions
+
+```javascript
+// Pure: same input → same output, no side effects
+function add(a, b) {
+  return a + b;
+}
+
+// Impure: modifies external state
+let total = 0;
+function addToTotal(value) {
+  total += value; // Side effect!
+  return total;
+}
+
+// Impure: depends on external state
+function getDiscount(price) {
+  return price * globalDiscountRate; // External dependency
+}
+```
+
+### 5.3 map, filter, reduce
+
+```javascript
+const users = [
+  { name: "Alice", age: 25 },
+  { name: "Bob", age: 30 },
+  { name: "Charlie", age: 35 },
+];
+
+// map: transform each element
+const names = users.map((u) => u.name);
+// ["Alice", "Bob", "Charlie"]
+
+// filter: keep elements matching condition
+const adults = users.filter((u) => u.age >= 30);
+// [{ name: "Bob", ... }, { name: "Charlie", ... }]
+
+// reduce: accumulate into single value
+const totalAge = users.reduce((sum, u) => sum + u.age, 0);
+// 90
+
+// Chaining
+const result = users
+  .filter((u) => u.age >= 30)
+  .map((u) => u.name)
+  .join(", ");
+// "Bob, Charlie"
+```
+
+### 5.4 Currying & Composition
+
+```javascript
+// Currying: transform f(a, b, c) into f(a)(b)(c)
+const curry = (fn) => {
+  return function curried(...args) {
+    if (args.length >= fn.length) {
+      return fn.apply(this, args);
+    }
+    return (...moreArgs) => curried(...args, ...moreArgs);
+  };
+};
+
+const add = curry((a, b, c) => a + b + c);
+add(1)(2)(3); // 6
+add(1, 2)(3); // 6
+add(1)(2, 3); // 6
+
+// Composition: combine functions
+const compose =
+  (...fns) =>
+  (x) =>
+    fns.reduceRight((acc, fn) => fn(acc), x);
+
+const pipe =
+  (...fns) =>
+  (x) =>
+    fns.reduce((acc, fn) => fn(acc), x);
+
+const addOne = (x) => x + 1;
+const double = (x) => x * 2;
+
+const addThenDouble = compose(double, addOne);
+addThenDouble(5); // 12 = (5 + 1) * 2
+
+const doubleThenAdd = pipe(double, addOne);
+doubleThenAdd(5); // 11 = (5 * 2) + 1
+```
+
+---
+
+## 6. Objects & Prototypes
+
+### 6.1 Prototypal Inheritance
+
+```javascript
+// Prototype chain
+const animal = {
+  speak() {
+    console.log("Some sound");
+  },
+};
+
+const dog = Object.create(animal);
+dog.bark = function () {
+  console.log("Woof!");
+};
+
+dog.speak(); // "Some sound" (inherited)
+dog.bark(); // "Woof!" (own method)
+
+// ES6 Classes (syntactic sugar)
+class Animal {
+  speak() {
+    console.log("Some sound");
+  }
+}
+
+class Dog extends Animal {
+  bark() {
+    console.log("Woof!");
+  }
+}
+```
+
+### 6.2 Object Methods
+
+```javascript
+const obj = { a: 1, b: 2 };
+
+// Keys, values, entries
+Object.keys(obj); // ["a", "b"]
+Object.values(obj); // [1, 2]
+Object.entries(obj); // [["a", 1], ["b", 2]]
+
+// Shallow copy
+const copy = { ...obj };
+const copy2 = Object.assign({}, obj);
+
+// Freeze (immutable)
+const frozen = Object.freeze({ x: 1 });
+frozen.x = 2; // Silently fails (or throws in strict mode)
+
+// Seal (no add/delete, can modify)
+const sealed = Object.seal({ x: 1 });
+sealed.x = 2; // OK
+sealed.y = 3; // Fails
+delete sealed.x; // Fails
+```
+
+---
+
+## 7. Modern JavaScript (ES6+)
+
+### 7.1 Destructuring
+
+```javascript
+// Array destructuring
+const [first, second, ...rest] = [1, 2, 3, 4, 5];
+// first = 1, second = 2, rest = [3, 4, 5]
+
+// Object destructuring
+const { name, age, city = "Unknown" } = { name: "Alice", age: 25 };
+// name = "Alice", age = 25, city = "Unknown"
+
+// Renaming
+const { name: userName } = { name: "Bob" };
+// userName = "Bob"
+
+// Nested
+const {
+  address: { street },
+} = { address: { street: "123 Main" } };
+```
+
+### 7.2 Spread & Rest
+
+```javascript
+// Spread: expand iterable
+const arr1 = [1, 2, 3];
+const arr2 = [...arr1, 4, 5]; // [1, 2, 3, 4, 5]
+
+const obj1 = { a: 1 };
+const obj2 = { ...obj1, b: 2 }; // { a: 1, b: 2 }
+
+// Rest: collect remaining
+function sum(...numbers) {
+  return numbers.reduce((a, b) => a + b, 0);
+}
+sum(1, 2, 3, 4); // 10
+```
+
+### 7.3 Modules
+
+```javascript
+// Named exports
+export const PI = 3.14159;
+export function square(x) {
+  return x * x;
+}
+
+// Default export
+export default class Calculator {}
+
+// Importing
+import Calculator, { PI, square } from "./math.js";
+import * as math from "./math.js";
+
+// Dynamic import
+const module = await import("./dynamic.js");
+```
+
+### 7.4 Optional Chaining & Nullish Coalescing
+
+```javascript
+// Optional chaining (?.)
+const user = { address: { city: "NYC" } };
+const city = user?.address?.city; // "NYC"
+const zip = user?.address?.zip; // undefined (no error)
+const fn = user?.getName?.(); // undefined if no method
+
+// Nullish coalescing (??)
+const value = null ?? "default"; // "default"
+const zero = 0 ?? "default"; // 0 (not nullish!)
+const empty = "" ?? "default"; // "" (not nullish!)
+
+// Compare with ||
+const value2 = 0 || "default"; // "default" (0 is falsy)
+```
+
+---
+
+## Quick Reference Card
+
+| Concept        | Key Point                         |
+| :------------- | :-------------------------------- |
+| `==` vs `===`  | Always use `===`                  |
+| `var` vs `let` | Prefer `let`/`const`              |
+| Closures       | Function + lexical scope          |
+| `this`         | Depends on how function is called |
+| Event loop     | Microtasks before macrotasks      |
+| Pure functions | Same input → same output          |
+| Prototypes     | `__proto__` → prototype chain     |
+| `??` vs `\|\|` | `??` only checks null/undefined   |
+
+---
+
+## Resources
+
+- [33 JS Concepts](https://github.com/leonardomso/33-js-concepts)
+- [JavaScript.info](https://javascript.info/)
+- [MDN JavaScript Guide](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide)
+- [You Don't Know JS](https://github.com/getify/You-Dont-Know-JS)