How do you implement a custom CQRS (Command Query Responsibility Segregation) pattern?

Question

Accepted Answer

**CQRS** — separate read and write models.

```javascript
// Command side
class OrderCommandHandler {
    constructor(eventStore) {
        this.eventStore = eventStore;
    }

async createOrder(data) {
        const orderId = generateId();
        await this.eventStore.append(orderId, {
            type: "OrderCreated",
            data: { ...data, orderId },
        });
        return orderId;
    }

async addItem(orderId, item) {
        await this.eventStore.append(orderId, {
            type: "ItemAdded",
            data: { orderId, item },
        });
    }
}

// Query side
class OrderQueryHandler {
    constructor(readModel) {
        this.readModel = readModel;
    }

async getOrder(orderId) {
        return this.readModel.getOrder(orderId);
    }

async listOrders(userId) {
        return this.readModel.getOrdersByUser(userId);
    }
}

// Read model (projection)
class OrderReadModel {
    constructor(db) {
        this.db = db;
    }

async project(event) {
        switch (event.type) {
            case "OrderCreated":
                await this.db.insert("orders", event.data);
                break;
            case "ItemAdded":
                await this.db.addItem(event.data.orderId, event.data.item);
                break;
        }
    }

async getOrder(id) {
        return this.db.query("SELECT * FROM orders WHERE id = $1", [id]);
    }
}
```

Node.js: How do you implement a custom CQRS (Command Query Responsibility Segregation) pattern?