Invariant Violation: “main” has not been registered

Open the index.js, the content of the file should be like this:

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

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

if (Platform.OS === 'web') {
    const rootTag = document.getElementById('root') || document.getElementById('X');
    AppRegistry.runApplication('X', { rootTag });
}

If you have this error Invariant Violation: “main” has not been registered you have to replace the 'X' by 'main'.

Another example :

If you have this error Invariant Violation: “app” has not been registered you have to replace the 'X' by 'app'.

For android :

Open ./android/app/src/main/java/[multiple folders]/MainActivity.java

/**
   * Returns the name of the main component registered from JavaScript.
   * This is used to schedule rendering of the component.
   */
  @Override
  protected String getMainComponentName() {
    return "X";
  }

The 'X' of MainActivity.java and index.js must match.