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Platform Specific Code

When building a cross-platform app, you'll want to re-use as much code as possible. Scenarios may arise where it makes sense for the code to be different, for example you may want to implement separate visual components for iOS and Android.
React Native provides two ways to easily organize your code and separate it by platform:
Certain components may have properties that work on one platform only. All of these props are annotated with @platform and have a small badge next to them on the website.

React Native provides a module that detects the platform in which the app is running. You can use the detection logic to implement platform-specific code. Use this option when only small parts of a component are platform-specific.
import { Platform, StyleSheet } from 'react-native';

const styles = StyleSheet.create({
  height: Platform.OS === 'ios' ? 200 : 100,
});
Platform.OS will be ios when running on iOS and android when running on Android.
There is also a Platform.select method available, that given an object containing Platform.OS as keys, returns the value for the platform you are currently running on.
import { Platform, StyleSheet } from 'react-native';

const styles = StyleSheet.create({
  container: {
    flex: 1,
    ...Platform.select({
      ios: {
        backgroundColor: 'red',
      },
      android: {
        backgroundColor: 'blue',
      },
    }),
  },
});
This will result in a container having flex: 1 on both platforms, a red background color on iOS, and a blue background color on Android.
Since it accepts any value, you can also use it to return platform specific component, like below:
const Component = Platform.select({
  ios: () => require('ComponentIOS'),
  android: () => require('ComponentAndroid'),
})();

<Component />;

On Android, the Platform module can also be used to detect the version of the Android Platform in which the app is running:
import { Platform } from 'react-native';

if (Platform.Version === 25) {
  console.log('Running on Nougat!');
}

On iOS, the Version is a result of -[UIDevice systemVersion], which is a string with the current version of the operating system. An example of the system version is "10.3". For example, to detect the major version number on iOS:
import { Platform } from 'react-native';

const majorVersionIOS = parseInt(Platform.Version, 10);
if (majorVersionIOS <= 9) {
  console.log('Work around a change in behavior');
}

When your platform-specific code is more complex, you should consider splitting the code out into separate files. React Native will detect when a file has a .ios. or .android. extension and load the relevant platform file when required from other components.
For example, say you have the following files in your project:
BigButton.ios.js
BigButton.android.js
You can then require the component as follows:
import BigButton from './BigButton';
React Native will automatically pick up the right file based on the running platform.

You can also use the .native.js or .web.js extensions when a module needs to be shared between NodeJS/Web and React Native but it has no Android/iOS differences, or vice versa. This is especially useful for projects that have common code shared among React Native and ReactJS.
For example, say you have the following files in your project:
Container.web.js # picked up by Webpack, Rollup or any other Web bundler
Container.native.js # picked up by the React Native bundler for both Android and iOS (Metro)
You can still require it without the .native / .web extension, as follows:
import Container from './Container';
Pro tip: Configure your Web bundler to ignore .native.js extensions in order to avoid having unused code in your production bundle, thus reducing the final bundle size.