Critical Patterns

Async/Await (REQUIRED)

// ✅ ALWAYS: Use async/await over .then() chains async function fetchUserData(userId) { try { const response = await fetch(/api/users/${userId}); if (!response.ok) throw new Error('User not found'); return await response.json(); } catch (error) { console.error('Failed to fetch user:', error); throw error; } }

// ❌ NEVER: Nested .then() chains fetch(/api/users/${userId}) .then(res => res.json()) .then(user => fetch(/api/orders/${user.id})) .then(res => res.json()) .then(orders => { ... });

Destructuring (REQUIRED)

// ✅ ALWAYS: Use destructuring for cleaner code const { name, email, role = 'user' } = user; const [first, second, ...rest] = items;

// Function parameters function createUser({ name, email, role = 'user' }) { return { id: uuid(), name, email, role }; }

Optional Chaining (REQUIRED)

// ✅ ALWAYS: Use optional chaining for safe access const street = user?.address?.street ?? 'Unknown';

// ❌ NEVER: Long && chains const street = user && user.address && user.address.street;

Decision Tree

Need default value? → Use ?? (nullish coalescing) Need safe property? → Use ?. (optional chaining) Need array transform? → Use map/filter/reduce Need unique values? → Use Set Need key-value pairs? → Use Map Need immutable update? → Use spread operator

Code Examples

Array Methods

// Map, filter, reduce const activeUsers = users .filter(u => u.active) .map(u => ({ id: u.id, name: u.name }));

const total = items.reduce((sum, item) => sum + item.price, 0);

// Find and includes const admin = users.find(u => u.role === 'admin'); const hasAdmin = users.some(u => u.role === 'admin');

Object Patterns

// Spread for immutable updates const updated = { ...user, name: 'New Name' };

// Computed property names const key = 'dynamicKey'; const obj = { [key]: 'value' };

// Object shorthand const name = 'John'; const user = { name, active: true };

Commands

node script.js # Run script npm init -y # Initialize package.json npm install package # Install dependency npx eslint . # Lint code

Resources

• Express API: express-api.md