An example of calling the same code from index.android.js and index.ios.js

App.js

import React, { Component } from 'react';
import { View, Text } from 'react-native';

class App extends Component {
  render () {
    return (
      <View>
        <Text>App here</Text>
      </View>
    )
  }
}

export default App;

index.android.js

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

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

index.ios.js

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

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