A practical guide to rebuilding current entity state from events in Event Sourcing using the Stream Aggregation pattern. Covers the three-step process: fetching ordered stream events, creating an empty entity, and applying each event sequentially. Demonstrates both object-oriented (C# with a When/Apply pattern using EventStoreDB and Marten) and functional (TypeScript with reduce and Partial types) approaches, comparing their trade-offs. Also debunks the myth that Snapshots are mandatory for performance.
