React Native how to add index.android.js manually?

For projects created with react-native init, an index.js file is still generated, and looks something like this initially:

import { AppRegistry } from 'react-native';
import App from './App';

AppRegistry.registerComponent('YourProjectName', () => App);

Manually adding App.ios.js and App.android.js in the same directory will allow you to include unique code for each platform, as it follows the pattern for platform-specific extensions. However, it won't be enough to have either one or the other. You need to include both otherwise the code splitting won't take effect (and will default to whatever is present in App.js).

So to answer your question, yes you need to delete App.js and replace it with android.App.js and ios.App.js. Then restart the React Native packager terminal window for your simulator or device to detect the change.