How do I exit/shut down a React Native app?

For Android, Use BackHandler to exit the App:

import React, { BackHandler } from 'react-native';

BackHandler.exitApp();

I am answering the question too late, but i thought the way i have chosen might help someone, so I am answering this question.

componentWillMount() {
   BackHandler.addEventListener('hardwareBackPress', this.backPressed);
}

componentWillUnmount() {
   BackHandler.removeEventListener('hardwareBackPress', this.backPressed);
}

backPressed = () => {
  Alert.alert(
    'Exit App',
    'Do you want to exit?',
    [
      {text: 'No', onPress: () => console.log('Cancel Pressed'), style: 'cancel'},
      {text: 'Yes', onPress: () => BackHandler.exitApp()},
    ],
    { cancelable: false });
    return true;
}