react-native/docs/PlatformSpecificInformation.md

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platform-specific-code	Platform Specific Code	docs	Guides	docs/platform-specific-code.html	native-modules-ios

When building a cross-platform app, the need to write different code for different platforms may arise. This can always be achieved by organizing the various components in different folders:

/common/components/   
/android/components/   
/ios/components/

Another option may be naming the components differently depending on the platform they are going to be used in:

BigButtonIOS.js
BigButtonAndroid.js

But React Native provides two alternatives to easily organize your code separating it by platform:

Platform specific extensions

React Native will detect when a file has a .ios. or .android. extension and load the right file for each platform when requiring them from other components.

For example, you can have these files in your project:

BigButton.ios.js
BigButton.android.js

With this setup, you can just require the files from a different component without paying attention to the platform in which the app will run.

var BigButton = require('./components/BigButton');

React Native will import the correct component for the running platform.

Platform module

A module is provided by React Native to detect what is the platform in which the app is running. This piece of functionality can be useful when only small parts of a component are platform specific.

var {Platform} = React;

var styles = StyleSheet.create({
  height: (Platform.OS === 'ios') ? 200 : 100,
});

Platform.OS will be ios when running in iOS and android when running in an Android device or simulator.

###Detecting Android version On Android, the Platform module can be also used to detect which is the version of the Android Platform in which the app is running

var {Platform} = React;

if(Platform.Version === '5.0'){
  console.log('Running on Lollipop!');
}