chore: release v4.0.0 - sync 550+ skills and restructure docs

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+---
+name: modern-javascript-patterns
+description: Master ES6+ features including async/await, destructuring, spread operators, arrow functions, promises, modules, iterators, generators, and functional programming patterns for writing clean, efficient JavaScript code. Use when refactoring legacy code, implementing modern patterns, or optimizing JavaScript applications.
+---
+
+# Modern JavaScript Patterns
+
+Comprehensive guide for mastering modern JavaScript (ES6+) features, functional programming patterns, and best practices for writing clean, maintainable, and performant code.
+
+## Use this skill when
+
+- Refactoring legacy JavaScript to modern syntax
+- Implementing functional programming patterns
+- Optimizing JavaScript performance
+- Writing maintainable and readable code
+- Working with asynchronous operations
+- Building modern web applications
+- Migrating from callbacks to Promises/async-await
+- Implementing data transformation pipelines
+
+## Do not use this skill when
+
+- The task is unrelated to modern javascript patterns
+- You need a different domain or tool outside this scope
+
+## Instructions
+
+- Clarify goals, constraints, and required inputs.
+- Apply relevant best practices and validate outcomes.
+- Provide actionable steps and verification.
+- If detailed examples are required, open `resources/implementation-playbook.md`.
+
+## Resources
+
+- `resources/implementation-playbook.md` for detailed patterns and examples.
--- a/skills/modern-javascript-patterns/resources/implementation-playbook.md
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+# Modern JavaScript Patterns Implementation Playbook
+
+This file contains detailed patterns, checklists, and code samples referenced by the skill.
+
+# Modern JavaScript Patterns
+
+Comprehensive guide for mastering modern JavaScript (ES6+) features, functional programming patterns, and best practices for writing clean, maintainable, and performant code.
+
+## When to Use This Skill
+
+- Refactoring legacy JavaScript to modern syntax
+- Implementing functional programming patterns
+- Optimizing JavaScript performance
+- Writing maintainable and readable code
+- Working with asynchronous operations
+- Building modern web applications
+- Migrating from callbacks to Promises/async-await
+- Implementing data transformation pipelines
+
+## ES6+ Core Features
+
+### 1. Arrow Functions
+
+**Syntax and Use Cases:**
+```javascript
+// Traditional function
+function add(a, b) {
+  return a + b;
+}
+
+// Arrow function
+const add = (a, b) => a + b;
+
+// Single parameter (parentheses optional)
+const double = x => x * 2;
+
+// No parameters
+const getRandom = () => Math.random();
+
+// Multiple statements (need curly braces)
+const processUser = user => {
+  const normalized = user.name.toLowerCase();
+  return { ...user, name: normalized };
+};
+
+// Returning objects (wrap in parentheses)
+const createUser = (name, age) => ({ name, age });
+```
+
+**Lexical 'this' Binding:**
+```javascript
+class Counter {
+  constructor() {
+    this.count = 0;
+  }
+
+  // Arrow function preserves 'this' context
+  increment = () => {
+    this.count++;
+  };
+
+  // Traditional function loses 'this' in callbacks
+  incrementTraditional() {
+    setTimeout(function() {
+      this.count++;  // 'this' is undefined
+    }, 1000);
+  }
+
+  // Arrow function maintains 'this'
+  incrementArrow() {
+    setTimeout(() => {
+      this.count++;  // 'this' refers to Counter instance
+    }, 1000);
+  }
+}
+```
+
+### 2. Destructuring
+
+**Object Destructuring:**
+```javascript
+const user = {
+  id: 1,
+  name: 'John Doe',
+  email: 'john@example.com',
+  address: {
+    city: 'New York',
+    country: 'USA'
+  }
+};
+
+// Basic destructuring
+const { name, email } = user;
+
+// Rename variables
+const { name: userName, email: userEmail } = user;
+
+// Default values
+const { age = 25 } = user;
+
+// Nested destructuring
+const { address: { city, country } } = user;
+
+// Rest operator
+const { id, ...userWithoutId } = user;
+
+// Function parameters
+function greet({ name, age = 18 }) {
+  console.log(`Hello ${name}, you are ${age}`);
+}
+greet(user);
+```
+
+**Array Destructuring:**
+```javascript
+const numbers = [1, 2, 3, 4, 5];
+
+// Basic destructuring
+const [first, second] = numbers;
+
+// Skip elements
+const [, , third] = numbers;
+
+// Rest operator
+const [head, ...tail] = numbers;
+
+// Swapping variables
+let a = 1, b = 2;
+[a, b] = [b, a];
+
+// Function return values
+function getCoordinates() {
+  return [10, 20];
+}
+const [x, y] = getCoordinates();
+
+// Default values
+const [one, two, three = 0] = [1, 2];
+```
+
+### 3. Spread and Rest Operators
+
+**Spread Operator:**
+```javascript
+// Array spreading
+const arr1 = [1, 2, 3];
+const arr2 = [4, 5, 6];
+const combined = [...arr1, ...arr2];
+
+// Object spreading
+const defaults = { theme: 'dark', lang: 'en' };
+const userPrefs = { theme: 'light' };
+const settings = { ...defaults, ...userPrefs };
+
+// Function arguments
+const numbers = [1, 2, 3];
+Math.max(...numbers);
+
+// Copying arrays/objects (shallow copy)
+const copy = [...arr1];
+const objCopy = { ...user };
+
+// Adding items immutably
+const newArr = [...arr1, 4, 5];
+const newObj = { ...user, age: 30 };
+```
+
+**Rest Parameters:**
+```javascript
+// Collect function arguments
+function sum(...numbers) {
+  return numbers.reduce((total, num) => total + num, 0);
+}
+sum(1, 2, 3, 4, 5);
+
+// With regular parameters
+function greet(greeting, ...names) {
+  return `${greeting} ${names.join(', ')}`;
+}
+greet('Hello', 'John', 'Jane', 'Bob');
+
+// Object rest
+const { id, ...userData } = user;
+
+// Array rest
+const [first, ...rest] = [1, 2, 3, 4, 5];
+```
+
+### 4. Template Literals
+
+```javascript
+// Basic usage
+const name = 'John';
+const greeting = `Hello, ${name}!`;
+
+// Multi-line strings
+const html = `
+  <div>
+    <h1>${title}</h1>
+    <p>${content}</p>
+  </div>
+`;
+
+// Expression evaluation
+const price = 19.99;
+const total = `Total: $${(price * 1.2).toFixed(2)}`;
+
+// Tagged template literals
+function highlight(strings, ...values) {
+  return strings.reduce((result, str, i) => {
+    const value = values[i] || '';
+    return result + str + `<mark>${value}</mark>`;
+  }, '');
+}
+
+const name = 'John';
+const age = 30;
+const html = highlight`Name: ${name}, Age: ${age}`;
+// Output: "Name: <mark>John</mark>, Age: <mark>30</mark>"
+```
+
+### 5. Enhanced Object Literals
+
+```javascript
+const name = 'John';
+const age = 30;
+
+// Shorthand property names
+const user = { name, age };
+
+// Shorthand method names
+const calculator = {
+  add(a, b) {
+    return a + b;
+  },
+  subtract(a, b) {
+    return a - b;
+  }
+};
+
+// Computed property names
+const field = 'email';
+const user = {
+  name: 'John',
+  [field]: 'john@example.com',
+  [`get${field.charAt(0).toUpperCase()}${field.slice(1)}`]() {
+    return this[field];
+  }
+};
+
+// Dynamic property creation
+const createUser = (name, ...props) => {
+  return props.reduce((user, [key, value]) => ({
+    ...user,
+    [key]: value
+  }), { name });
+};
+
+const user = createUser('John', ['age', 30], ['email', 'john@example.com']);
+```
+
+## Asynchronous Patterns
+
+### 1. Promises
+
+**Creating and Using Promises:**
+```javascript
+// Creating a promise
+const fetchUser = (id) => {
+  return new Promise((resolve, reject) => {
+    setTimeout(() => {
+      if (id > 0) {
+        resolve({ id, name: 'John' });
+      } else {
+        reject(new Error('Invalid ID'));
+      }
+    }, 1000);
+  });
+};
+
+// Using promises
+fetchUser(1)
+  .then(user => console.log(user))
+  .catch(error => console.error(error))
+  .finally(() => console.log('Done'));
+
+// Chaining promises
+fetchUser(1)
+  .then(user => fetchUserPosts(user.id))
+  .then(posts => processPosts(posts))
+  .then(result => console.log(result))
+  .catch(error => console.error(error));
+```
+
+**Promise Combinators:**
+```javascript
+// Promise.all - Wait for all promises
+const promises = [
+  fetchUser(1),
+  fetchUser(2),
+  fetchUser(3)
+];
+
+Promise.all(promises)
+  .then(users => console.log(users))
+  .catch(error => console.error('At least one failed:', error));
+
+// Promise.allSettled - Wait for all, regardless of outcome
+Promise.allSettled(promises)
+  .then(results => {
+    results.forEach(result => {
+      if (result.status === 'fulfilled') {
+        console.log('Success:', result.value);
+      } else {
+        console.log('Error:', result.reason);
+      }
+    });
+  });
+
+// Promise.race - First to complete
+Promise.race(promises)
+  .then(winner => console.log('First:', winner))
+  .catch(error => console.error(error));
+
+// Promise.any - First to succeed
+Promise.any(promises)
+  .then(first => console.log('First success:', first))
+  .catch(error => console.error('All failed:', error));
+```
+
+### 2. Async/Await
+
+**Basic Usage:**
+```javascript
+// Async function always returns a Promise
+async function fetchUser(id) {
+  const response = await fetch(`/api/users/${id}`);
+  const user = await response.json();
+  return user;
+}
+
+// Error handling with try/catch
+async function getUserData(id) {
+  try {
+    const user = await fetchUser(id);
+    const posts = await fetchUserPosts(user.id);
+    return { user, posts };
+  } catch (error) {
+    console.error('Error fetching data:', error);
+    throw error;
+  }
+}
+
+// Sequential vs Parallel execution
+async function sequential() {
+  const user1 = await fetchUser(1);  // Wait
+  const user2 = await fetchUser(2);  // Then wait
+  return [user1, user2];
+}
+
+async function parallel() {
+  const [user1, user2] = await Promise.all([
+    fetchUser(1),
+    fetchUser(2)
+  ]);
+  return [user1, user2];
+}
+```
+
+**Advanced Patterns:**
+```javascript
+// Async IIFE
+(async () => {
+  const result = await someAsyncOperation();
+  console.log(result);
+})();
+
+// Async iteration
+async function processUsers(userIds) {
+  for (const id of userIds) {
+    const user = await fetchUser(id);
+    await processUser(user);
+  }
+}
+
+// Top-level await (ES2022)
+const config = await fetch('/config.json').then(r => r.json());
+
+// Retry logic
+async function fetchWithRetry(url, retries = 3) {
+  for (let i = 0; i < retries; i++) {
+    try {
+      return await fetch(url);
+    } catch (error) {
+      if (i === retries - 1) throw error;
+      await new Promise(resolve => setTimeout(resolve, 1000 * (i + 1)));
+    }
+  }
+}
+
+// Timeout wrapper
+async function withTimeout(promise, ms) {
+  const timeout = new Promise((_, reject) =>
+    setTimeout(() => reject(new Error('Timeout')), ms)
+  );
+  return Promise.race([promise, timeout]);
+}
+```
+
+## Functional Programming Patterns
+
+### 1. Array Methods
+
+**Map, Filter, Reduce:**
+```javascript
+const users = [
+  { id: 1, name: 'John', age: 30, active: true },
+  { id: 2, name: 'Jane', age: 25, active: false },
+  { id: 3, name: 'Bob', age: 35, active: true }
+];
+
+// Map - Transform array
+const names = users.map(user => user.name);
+const upperNames = users.map(user => user.name.toUpperCase());
+
+// Filter - Select elements
+const activeUsers = users.filter(user => user.active);
+const adults = users.filter(user => user.age >= 18);
+
+// Reduce - Aggregate data
+const totalAge = users.reduce((sum, user) => sum + user.age, 0);
+const avgAge = totalAge / users.length;
+
+// Group by property
+const byActive = users.reduce((groups, user) => {
+  const key = user.active ? 'active' : 'inactive';
+  return {
+    ...groups,
+    [key]: [...(groups[key] || []), user]
+  };
+}, {});
+
+// Chaining methods
+const result = users
+  .filter(user => user.active)
+  .map(user => user.name)
+  .sort()
+  .join(', ');
+```
+
+**Advanced Array Methods:**
+```javascript
+// Find - First matching element
+const user = users.find(u => u.id === 2);
+
+// FindIndex - Index of first match
+const index = users.findIndex(u => u.name === 'Jane');
+
+// Some - At least one matches
+const hasActive = users.some(u => u.active);
+
+// Every - All match
+const allAdults = users.every(u => u.age >= 18);
+
+// FlatMap - Map and flatten
+const userTags = [
+  { name: 'John', tags: ['admin', 'user'] },
+  { name: 'Jane', tags: ['user'] }
+];
+const allTags = userTags.flatMap(u => u.tags);
+
+// From - Create array from iterable
+const str = 'hello';
+const chars = Array.from(str);
+const numbers = Array.from({ length: 5 }, (_, i) => i + 1);
+
+// Of - Create array from arguments
+const arr = Array.of(1, 2, 3);
+```
+
+### 2. Higher-Order Functions
+
+**Functions as Arguments:**
+```javascript
+// Custom forEach
+function forEach(array, callback) {
+  for (let i = 0; i < array.length; i++) {
+    callback(array[i], i, array);
+  }
+}
+
+// Custom map
+function map(array, transform) {
+  const result = [];
+  for (const item of array) {
+    result.push(transform(item));
+  }
+  return result;
+}
+
+// Custom filter
+function filter(array, predicate) {
+  const result = [];
+  for (const item of array) {
+    if (predicate(item)) {
+      result.push(item);
+    }
+  }
+  return result;
+}
+```
+
+**Functions Returning Functions:**
+```javascript
+// Currying
+const multiply = a => b => a * b;
+const double = multiply(2);
+const triple = multiply(3);
+
+console.log(double(5));  // 10
+console.log(triple(5));  // 15
+
+// Partial application
+function partial(fn, ...args) {
+  return (...moreArgs) => fn(...args, ...moreArgs);
+}
+
+const add = (a, b, c) => a + b + c;
+const add5 = partial(add, 5);
+console.log(add5(3, 2));  // 10
+
+// Memoization
+function memoize(fn) {
+  const cache = new Map();
+  return (...args) => {
+    const key = JSON.stringify(args);
+    if (cache.has(key)) {
+      return cache.get(key);
+    }
+    const result = fn(...args);
+    cache.set(key, result);
+    return result;
+  };
+}
+
+const fibonacci = memoize((n) => {
+  if (n <= 1) return n;
+  return fibonacci(n - 1) + fibonacci(n - 2);
+});
+```
+
+### 3. Composition and Piping
+
+```javascript
+// Function composition
+const compose = (...fns) => x =>
+  fns.reduceRight((acc, fn) => fn(acc), x);
+
+const pipe = (...fns) => x =>
+  fns.reduce((acc, fn) => fn(acc), x);
+
+// Example usage
+const addOne = x => x + 1;
+const double = x => x * 2;
+const square = x => x * x;
+
+const composed = compose(square, double, addOne);
+console.log(composed(3));  // ((3 + 1) * 2)^2 = 64
+
+const piped = pipe(addOne, double, square);
+console.log(piped(3));  // ((3 + 1) * 2)^2 = 64
+
+// Practical example
+const processUser = pipe(
+  user => ({ ...user, name: user.name.trim() }),
+  user => ({ ...user, email: user.email.toLowerCase() }),
+  user => ({ ...user, age: parseInt(user.age) })
+);
+
+const user = processUser({
+  name: '  John  ',
+  email: 'JOHN@EXAMPLE.COM',
+  age: '30'
+});
+```
+
+### 4. Pure Functions and Immutability
+
+```javascript
+// Impure function (modifies input)
+function addItemImpure(cart, item) {
+  cart.items.push(item);
+  cart.total += item.price;
+  return cart;
+}
+
+// Pure function (no side effects)
+function addItemPure(cart, item) {
+  return {
+    ...cart,
+    items: [...cart.items, item],
+    total: cart.total + item.price
+  };
+}
+
+// Immutable array operations
+const numbers = [1, 2, 3, 4, 5];
+
+// Add to array
+const withSix = [...numbers, 6];
+
+// Remove from array
+const withoutThree = numbers.filter(n => n !== 3);
+
+// Update array element
+const doubled = numbers.map(n => n === 3 ? n * 2 : n);
+
+// Immutable object operations
+const user = { name: 'John', age: 30 };
+
+// Update property
+const olderUser = { ...user, age: 31 };
+
+// Add property
+const withEmail = { ...user, email: 'john@example.com' };
+
+// Remove property
+const { age, ...withoutAge } = user;
+
+// Deep cloning (simple approach)
+const deepClone = obj => JSON.parse(JSON.stringify(obj));
+
+// Better deep cloning
+const structuredClone = obj => globalThis.structuredClone(obj);
+```
+
+## Modern Class Features
+
+```javascript
+// Class syntax
+class User {
+  // Private fields
+  #password;
+
+  // Public fields
+  id;
+  name;
+
+  // Static field
+  static count = 0;
+
+  constructor(id, name, password) {
+    this.id = id;
+    this.name = name;
+    this.#password = password;
+    User.count++;
+  }
+
+  // Public method
+  greet() {
+    return `Hello, ${this.name}`;
+  }
+
+  // Private method
+  #hashPassword(password) {
+    return `hashed_${password}`;
+  }
+
+  // Getter
+  get displayName() {
+    return this.name.toUpperCase();
+  }
+
+  // Setter
+  set password(newPassword) {
+    this.#password = this.#hashPassword(newPassword);
+  }
+
+  // Static method
+  static create(id, name, password) {
+    return new User(id, name, password);
+  }
+}
+
+// Inheritance
+class Admin extends User {
+  constructor(id, name, password, role) {
+    super(id, name, password);
+    this.role = role;
+  }
+
+  greet() {
+    return `${super.greet()}, I'm an admin`;
+  }
+}
+```
+
+## Modules (ES6)
+
+```javascript
+// Exporting
+// math.js
+export const PI = 3.14159;
+export function add(a, b) {
+  return a + b;
+}
+export class Calculator {
+  // ...
+}
+
+// Default export
+export default function multiply(a, b) {
+  return a * b;
+}
+
+// Importing
+// app.js
+import multiply, { PI, add, Calculator } from './math.js';
+
+// Rename imports
+import { add as sum } from './math.js';
+
+// Import all
+import * as Math from './math.js';
+
+// Dynamic imports
+const module = await import('./math.js');
+const { add } = await import('./math.js');
+
+// Conditional loading
+if (condition) {
+  const module = await import('./feature.js');
+  module.init();
+}
+```
+
+## Iterators and Generators
+
+```javascript
+// Custom iterator
+const range = {
+  from: 1,
+  to: 5,
+
+  [Symbol.iterator]() {
+    return {
+      current: this.from,
+      last: this.to,
+
+      next() {
+        if (this.current <= this.last) {
+          return { done: false, value: this.current++ };
+        } else {
+          return { done: true };
+        }
+      }
+    };
+  }
+};
+
+for (const num of range) {
+  console.log(num);  // 1, 2, 3, 4, 5
+}
+
+// Generator function
+function* rangeGenerator(from, to) {
+  for (let i = from; i <= to; i++) {
+    yield i;
+  }
+}
+
+for (const num of rangeGenerator(1, 5)) {
+  console.log(num);
+}
+
+// Infinite generator
+function* fibonacci() {
+  let [prev, curr] = [0, 1];
+  while (true) {
+    yield curr;
+    [prev, curr] = [curr, prev + curr];
+  }
+}
+
+// Async generator
+async function* fetchPages(url) {
+  let page = 1;
+  while (true) {
+    const response = await fetch(`${url}?page=${page}`);
+    const data = await response.json();
+    if (data.length === 0) break;
+    yield data;
+    page++;
+  }
+}
+
+for await (const page of fetchPages('/api/users')) {
+  console.log(page);
+}
+```
+
+## Modern Operators
+
+```javascript
+// Optional chaining
+const user = { name: 'John', address: { city: 'NYC' } };
+const city = user?.address?.city;
+const zipCode = user?.address?.zipCode;  // undefined
+
+// Function call
+const result = obj.method?.();
+
+// Array access
+const first = arr?.[0];
+
+// Nullish coalescing
+const value = null ?? 'default';      // 'default'
+const value = undefined ?? 'default'; // 'default'
+const value = 0 ?? 'default';         // 0 (not 'default')
+const value = '' ?? 'default';        // '' (not 'default')
+
+// Logical assignment
+let a = null;
+a ??= 'default';  // a = 'default'
+
+let b = 5;
+b ??= 10;  // b = 5 (unchanged)
+
+let obj = { count: 0 };
+obj.count ||= 1;  // obj.count = 1
+obj.count &&= 2;  // obj.count = 2
+```
+
+## Performance Optimization
+
+```javascript
+// Debounce
+function debounce(fn, delay) {
+  let timeoutId;
+  return (...args) => {
+    clearTimeout(timeoutId);
+    timeoutId = setTimeout(() => fn(...args), delay);
+  };
+}
+
+const searchDebounced = debounce(search, 300);
+
+// Throttle
+function throttle(fn, limit) {
+  let inThrottle;
+  return (...args) => {
+    if (!inThrottle) {
+      fn(...args);
+      inThrottle = true;
+      setTimeout(() => inThrottle = false, limit);
+    }
+  };
+}
+
+const scrollThrottled = throttle(handleScroll, 100);
+
+// Lazy evaluation
+function* lazyMap(iterable, transform) {
+  for (const item of iterable) {
+    yield transform(item);
+  }
+}
+
+// Use only what you need
+const numbers = [1, 2, 3, 4, 5];
+const doubled = lazyMap(numbers, x => x * 2);
+const first = doubled.next().value;  // Only computes first value
+```
+
+## Best Practices
+
+1. **Use const by default**: Only use let when reassignment is needed
+2. **Prefer arrow functions**: Especially for callbacks
+3. **Use template literals**: Instead of string concatenation
+4. **Destructure objects and arrays**: For cleaner code
+5. **Use async/await**: Instead of Promise chains
+6. **Avoid mutating data**: Use spread operator and array methods
+7. **Use optional chaining**: Prevent "Cannot read property of undefined"
+8. **Use nullish coalescing**: For default values
+9. **Prefer array methods**: Over traditional loops
+10. **Use modules**: For better code organization
+11. **Write pure functions**: Easier to test and reason about
+12. **Use meaningful variable names**: Self-documenting code
+13. **Keep functions small**: Single responsibility principle
+14. **Handle errors properly**: Use try/catch with async/await
+15. **Use strict mode**: `'use strict'` for better error catching
+
+## Common Pitfalls
+
+1. **this binding confusion**: Use arrow functions or bind()
+2. **Async/await without error handling**: Always use try/catch
+3. **Promise creation unnecessary**: Don't wrap already async functions
+4. **Mutation of objects**: Use spread operator or Object.assign()
+5. **Forgetting await**: Async functions return promises
+6. **Blocking event loop**: Avoid synchronous operations
+7. **Memory leaks**: Clean up event listeners and timers
+8. **Not handling promise rejections**: Use catch() or try/catch
+
+## Resources
+
+- **MDN Web Docs**: https://developer.mozilla.org/en-US/docs/Web/JavaScript
+- **JavaScript.info**: https://javascript.info/
+- **You Don't Know JS**: https://github.com/getify/You-Dont-Know-JS
+- **Eloquent JavaScript**: https://eloquentjavascript.net/
+- **ES6 Features**: http://es6-features.org/