In mobile development, there is a trade-off between portability code and fast execution. A new feature of React Native called javascript Native Interface (JSI) removes that trade-off by allowing developers to write portable code that executes as binary code on the native device. javascript Interface (JSI) is a lightweight, general-purpose API provided by Facebook for interacting with javascript data types and functions in C++ code. Introduced in the context of React Native, JSI offers a way to call C++ functions directly from javascript and vice versa. Before JSI, React Native used a “Native Bridge” feature, which required that developers write native code in the device-native language, for example Swift on iOS and Java or Kotlin on Android. React Native developers who wanted to have the performance of a native app therefore needed to maintain two code-bases for the same features. JSI solves this by allowing developers to maintain high-performance code in only one language: C++. While JSI brings numerous advantages, especially for complex and performance-critical applications, it's worth noting that it requires a good understanding of both C++ and javascript, as well as the internals of React Native. As such, it's often used for building advanced features, or optimizing performance-critical paths. This book is intended for programmers, engineers, and technology enthusiasts who want to learn about mobile development.