To generate the APK go to the root of the project in the terminal and run the below command:
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
Go to android directory:
cd android
Now in this android folder, run this command
./gradlew assembleDebug
You should find the apk file in the following:
yourProject/android/app/build/outputs/apk/debug/app-debug.apk
The thing is, this will generate a "debug apk" to generate a "Release APK" you will have to setup a signature for the APK, but this is only necessary when publishing to the stores, not to create a debug apk version.

## React Version 0.62.1 ## In your root project directory
Make sure you have already directory **android/app/src/main/assets/**, if not create directory, after that create new file and save as `index.android.bundle` and put your file in like this **android/app/src/main/assets/index.android.bundle**
After that run this
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/ cd android && ./gradlew assembleDebug
Then you can get `apk` in **app/build/outputs/apk/debug/app-debug.apk**