Build an iOS Simulator .app in a React Native project

1. Generate the bundle file for ios:

npx react-native bundle --entry-file='index.js' --bundle-output='./ios/main.jsbundle' --dev=false --platform='ios'

2. Launch Xcode and open the workspace for your application.
3. Follow these steps to generate the .app file:
4. Validate your .app file using a simulator:

Build an Android emulator .apk in a React Native project

1. Create the assets folder under ./android/app/src/main/

mkdir android/app/src/main/assets

2. Generate the bundle file for android:

npx react-native bundle --platform android --dev false --entry-file index.js --bundle-output android/app/src/main/assets/index.android.bundle --assets-dest android/app/src/main/res

3. Go to the android folder:

cd android

4. Build the .apk file:

./gradlew assembleDebug

5. Your .apk file will be located in ./android/app/build/outputs/apk/debug/app-debug.apk. You can use an Android emulator to validate it.