React Native's new architecture replaces the old JSON-serializing Bridge with JSI (JavaScript Interface), a C++ abstraction layer that allows JavaScript and native code to share memory references directly. Instead of copying objects as JSON across a bridge, JSI lets C++ hold GC-aware handles (jsi::Object) pointing into the JS engine's memory, enabling synchronous native calls where appropriate. The reverse also works via HostObjects — C++ objects exposed to JS that route method calls directly into native code. On iOS, this is seamless via Objective-C++ (.mm files) since both runtimes share memory. On Android, the JVM boundary requires JNI, which React Native handles cleanly through Meta's fbjni library. The result is direct, low-overhead access to device APIs through TurboModules and Fabric, making the old 'minimize bridge crossings' mental model obsolete.
