What is the React Native New Architecture?

The New Architecture (introduced in React Native 0.68, stable in 0.76) fundamentally changes how JavaScript communicates with native code, replacing the legacy "bridge" with a more efficient system. Old Architecture problems: the bridge was asynchronous — JS and native were separated, all communication was serialized to JSON and passed asynchronously. This caused: dropped frames (bridge got congested), inability to call native code synchronously, limited native-to-JS synchronous callbacks. New Architecture components: (1) JSI (JavaScript Interface): replaces the async JSON bridge with direct synchronous calls. JS objects can hold references to C++ objects and call methods directly — like calling a native function from JS without serialization. This enables synchronous communication; (2) Fabric (new rendering engine): reimplements the rendering system in C++. UI rendering can happen on any thread, enabling synchronous layout and concurrent features (React 18 Concurrent Mode). Shadow tree is now C++ instead of Java/ObjC; (3) TurboModules: lazy loading of native modules — only loaded when first used (not all upfront at startup). Typed interface via CodeGen; (4) CodeGen: generates type-safe native code from TypeScript/Flow type annotations — eliminates type mismatches between JS and native. Benefits: faster startup (TurboModules lazy load), synchronous native calls possible via JSI, React 18 Concurrent Mode support, better performance on complex UIs. Interop layer: Old and New Architecture components can work together during migration.