How do you implement a custom event sourcing system?

Question

Accepted Answer

**Event sourcing** — store state as sequence of events.

```javascript
class EventStore {
    constructor() {
        this.events = [];
        this.projections = new Map();
    }

append(aggregateId, event) {
        const entry = {
            id: this.events.length,
            aggregateId,
            type: event.type,
            data: event.data,
            timestamp: Date.now(),
        };
        this.events.push(entry);
        this.project(entry);
    }

getEvents(aggregateId) {
        return this.events.filter(e => e.aggregateId === aggregateId);
    }

project(event) {
        this.projections.forEach(projection => projection(event));
    }

addProjection(name, handler) {
        this.projections.set(name, handler);
    }
}

// Usage
const store = new EventStore();

// Add projection
store.addProjection("orderSummary", (event) => {
    if (event.type === "OrderCreated") {
        console.log(`Order ${event.data.id} created`);
    }
});

// Append events
store.append("order-123", {
    type: "OrderCreated",
    data: { id: "order-123", items: ["item1"] },
});

store.append("order-123", {
    type: "OrderPaid",
    data: { amount: 99.99 },
});

// Rebuild state
function rebuildState(aggregateId) {
    return store.getEvents(aggregateId).reduce((state, event) => {
        switch (event.type) {
            case "OrderCreated": return { ...state, ...event.data, status: "created" };
            case "OrderPaid": return { ...state, status: "paid", amount: event.data.amount };
            default: return state;
        }
    }, {});
}
```

Node.js: How do you implement a custom event sourcing system?