What is Codegen?

Codegen is a tool to avoid writing a lot of repetitive code. Using Codegen is not mandatory: you can write all the generated manually. However, Codegen generates scaffolding code that could save you a lot of time.

React Native invokes Codegen automatically every time an iOS or Android app is built. Occasionally, you would like to manually run the Codegen scripts to know which types and files are actually generated: this is a common scenario when developing Turbo Native Modules and Fabric Native Components.

How does Codegen works

Codegen is a process that is tightly coupled with a React Native app. The Codegen scripts live inside the react-native NPM package and the apps calls those scripts at build time.

Codegen crawls the folders in your project, starting form a directory you specify in your package.json, looking for some specific JS files that contains the specificiation (or specs) for your custom modules and components. Spec files are JS files written in a typed dialect: React Native currently supports Flow and TypeScript.

Every time Codegen finds a spec files, it generates the boilerplate code associated to it. Codegen generates some C++ glue-code and then it generates platform-specific code, using Java for Android and Objective-C++ for iOS.