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React Native

A framework for building native applications using React.

Repo structure

React Native is a monorepo: the react-native package, the packages published alongside it, and the apps and tooling used to develop them.

Path Contents
packages/react-native/Libraries JavaScript source (Flow) — the legacy location, with code gradually moving to src/private
packages/react-native/src/private JavaScript source (Flow)
packages/react-native/ReactCommon Shared C++ — Fabric renderer, TurboModules, JSI, Yoga, jsinspector-modern
packages/react-native/ReactAndroid Android runtime (Kotlin, Java, JNI)
packages/react-native/{React,ReactApple} Apple runtime (Objective-C++, Swift)
packages/rn-tester RNTester — test app showcasing each core component and API, plus a Playground scratch surface
packages/* Other published packages — Metro config, Codegen, ESLint config, dev-middleware, React Native DevTools frontend
private/* Unpublished — the helloworld sample app, the react-native-fantom test runner
scripts/* Repository tooling — build, test, release, and CI scripts

Architecture notes live in __docs__ directories beside the code they describe, indexed by __docs__/README.md. Treat them as reference for the subsystem you are working in, not as required reading.

Environment

  • Yarn v1, pinned via packageManager. Run commands from the repository root.
  • JavaScript work — lint, type checks, Jest, Metro — needs only Node and Yarn, on any platform.
  • Native builds need a platform toolchain: Xcode with CocoaPods or Swift Package Manager for iOS (macOS only), and the Android SDK and NDK with Gradle for Android. See Building from source.

Common commands

Command Purpose
yarn test <path> Jest unit tests, found in __tests__ directories
yarn fantom <path> Fantom integration tests, named *-itest.js — builds a native tester on first run
yarn lint ESLint (--max-warnings 0)
yarn flow-check Flow
yarn format Prettier and clang-format (yarn format-check for Prettier only)

JavaScript CI is the lint, test_js, and build_js_types jobs in .github/workflows/test-all.yml; Fantom and the native platforms have their own jobs.

Verification: Running RNTester

Needed only for changes that have to be seen running, such as user interface behavior.

yarn start serves RNTester over Metro at http://localhost:8081; yarn android builds and installs it on Android. Check the bundler with curl "http://localhost:8081/js/RNTesterApp.bundle?platform=ios&dev=true".

Gotchas

  • yarn install dirties the working tree: a preinstall hook (scripts/try-set-hermes-compiler-prebuilt.js) resolves the hermes-compiler placeholder in packages/react-native/package.json (0.0.0 → a real version) and touches yarn.lock. Expected — do not commit it.
  • Metro cannot bundle until codegen is built once — yarn --cwd packages/react-native-codegen build. Without it, bundling fails with Cannot find module '@react-native/codegen/lib/parsers/flow/parser'. Every other package runs from source — yarn build is not needed for development (see scripts/build/README.md).

Generated code

Never hand-edit generated output — change the source and regenerate. CI validates the committed snapshots.

  • Native modules and components are declared by JavaScript specs (Native*.js, *NativeComponent.js); their native counterparts are generated at build time.
  • Feature flags are declared in packages/react-native/scripts/featureflags/ReactNativeFeatureFlags.config.js; yarn featureflags regenerates the JavaScript, Java, and C++ accessors.
  • JavaScript sources are typed with Flow, and the public API is exported from packages/react-native/index.js. TypeScript types are generated from those sources, and packages/react-native/ReactNativeApi.d.ts is a committed snapshot of that API — run yarn build-types to regenerate both whenever the public API changes, then yarn test-generated-typescript to type-check the result.
  • The public native API is snapshotted as well: C++ under scripts/cxx-api (yarn cxx-api-build), and Android in packages/react-native/ReactAndroid/api/ReactAndroid.api.
  • CHANGELOG.md is compiled at release time from pull request descriptions.

Contributing guidelines

  • Keep each change focused — no unrelated refactors, formatting, or dependency updates.
  • Complete the pull request template — the motivation and the user-visible effect, and a changelog entry with its category and type tags.
  • In the test plan, give the exact commands you ran and their results, plus screenshots or a video for user-interface changes. Say which checks you could not run.

The full process is on reactnative.dev.