Universal Patterns

┌─────────────────────────────┐
│     Presentation Layer      │  UI, API handlers, CLI
├─────────────────────────────┤
│     Application Layer       │  Use cases, services
├─────────────────────────────┤
│       Domain Layer          │  Business logic, entities
├─────────────────────────────┤
│    Infrastructure Layer     │  DB, cache, external APIs
└─────────────────────────────┘

         ┌─────────────────┐
         │   Frameworks    │  (outermost)
         │   & Drivers     │
     ┌───┴─────────────────┴───┐
     │    Interface Adapters    │
     │   (Controllers, Gateways)│
 ┌───┴─────────────────────────┴───┐
 │        Application Business      │
 │           Rules (Use Cases)      │
 ┌─────────────────────────────────┐
 │    Enterprise Business Rules     │  (innermost)
 │         (Entities)               │
 └─────────────────────────────────┘

src/
├── components/
│   ├── common/         # Shared UI components
│   ├── layout/         # Layout components
│   └── features/       # Feature-specific components
├── hooks/              # Custom hooks
├── stores/             # State management
├── services/           # API services
└── utils/              # Utilities

// BAD: Multiple responsibilities
function processUser(user) {
  validateUser(user);
  saveToDatabase(user);
  sendEmail(user);
  logAnalytics(user);
}

// GOOD: Single responsibility
function validateUser(user) { /* validation only */ }
function saveUser(user) { /* persistence only */ }
function notifyUser(user) { /* notification only */ }

// BAD: Hard dependency
class UserService {
  constructor() {
    this.db = new Database(); // Hard-coded
  }
}

// GOOD: Injected dependency
class UserService {
  constructor(db) {
    this.db = db; // Injected
  }
}

// BAD: Fat interface
interface Worker {
  work();
  eat();
  sleep();
}

// GOOD: Segregated interfaces
interface Workable { work(); }
interface Eatable { eat(); }
interface Sleepable { sleep(); }

function processOrder(order) {
  // Validate early
  if (!order) throw new Error('Order required');
  if (!order.items?.length) throw new Error('Order must have items');
  if (!order.customerId) throw new Error('Customer ID required');

  // Process only after validation passes
  return executeOrder(order);
}

// API boundary - catch and format errors
async function apiHandler(req, res) {
  try {
    const result = await processRequest(req);
    res.json({ success: true, data: result });
  } catch (error) {
    res.status(error.statusCode || 500).json({
      success: false,
      error: error.message
    });
  }
}

type Result<T, E> = { ok: true; value: T } | { ok: false; error: E };

function parseConfig(input: string): Result<Config, ParseError> {
  try {
    return { ok: true, value: JSON.parse(input) };
  } catch (e) {
    return { ok: false, error: new ParseError(e.message) };
  }
}

Action → Dispatcher → Store → View → Action

// Publisher
eventBus.emit('user.created', { userId: '123' });

// Subscriber
eventBus.on('user.created', async (event) => {
  await sendWelcomeEmail(event.userId);
});

// Query - returns data, no side effects
function getUser(id) { return db.users.find(id); }

// Command - mutates data, returns void/status
function updateUser(id, data) { db.users.update(id, data); }