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.claude/skills/languages/javascript/SKILL.md

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+# JavaScript
+
+## Description
+
+Modern JavaScript (ES6+) patterns and best practices for Node.js and browser environments.
+
+## When to Use
+
+- Working with JavaScript files (.js, .mjs)
+- Browser scripting
+- Node.js applications without TypeScript
+
+---
+
+## Core Patterns
+
+### Modern Syntax
+
+```javascript
+// Destructuring
+const { name, email } = user;
+const [first, ...rest] = items;
+
+// Spread operator
+const merged = { ...defaults, ...options };
+const combined = [...array1, ...array2];
+
+// Template literals
+const message = `Hello, ${name}!`;
+
+// Optional chaining and nullish coalescing
+const city = user?.address?.city ?? 'Unknown';
+```
+
+### Async Patterns
+
+```javascript
+// Async/await
+async function fetchData(url) {
+  const response = await fetch(url);
+  if (!response.ok) throw new Error('Fetch failed');
+  return response.json();
+}
+
+// Promise.all for parallel
+const results = await Promise.all([
+  fetchData(url1),
+  fetchData(url2),
+]);
+
+// Error handling
+try {
+  const data = await fetchData(url);
+} catch (error) {
+  console.error('Failed:', error.message);
+}
+```
+
+### Array Methods
+
+```javascript
+// Map, filter, reduce
+const names = users.map(u => u.name);
+const active = users.filter(u => u.active);
+const total = items.reduce((sum, i) => sum + i.price, 0);
+
+// Find and includes
+const user = users.find(u => u.id === id);
+const hasAdmin = users.some(u => u.role === 'admin');
+```
+
+### Classes
+
+```javascript
+class UserService {
+  #db; // Private field
+
+  constructor(database) {
+    this.#db = database;
+  }
+
+  async findById(id) {
+    return this.#db.users.find(u => u.id === id);
+  }
+}
+```
+
+## Best Practices
+
+1. Use `const` by default, `let` when needed
+2. Avoid `var` - use block-scoped declarations
+3. Use arrow functions for callbacks
+4. Handle all promise rejections
+5. Use ESLint for consistent style
+
+## Common Pitfalls
+
+- **Implicit type coercion**: Use `===` instead of `==`
+- **Callback hell**: Use async/await
+- **Mutating objects**: Create new objects with spread
+- **Not handling errors**: Always catch promise rejections