BackAndroid.addEventListener('hardwareBackPress', function() {
    if (!this.onMainScreen()) {
        this.goBack();
        return true;
    }
    return false;
});

Note: this.onMainScreen() and this.goBack() are not built in functions, you also need to implement those. (https://github.com/immidi/react-native/commit/ed7e0fb31d842c63e8b8dc77ce795fac86e0f712)

This is an example on how to use React Native's BackAndroid along with the Navigator.

componentWillMount registers an event listener to handle the taps on the back button. It checks if there is another view in the history stack, and if there is one, it goes back -otherwise it keeps the default behaviour.

More information on the BackAndroid docs and the Navigator docs.

import React, { Component } from 'react'; // eslint-disable-line no-unused-vars

import {
  BackAndroid,
  Navigator,
} from 'react-native';

import SceneContainer from './Navigation/SceneContainer';
import RouteMapper from './Navigation/RouteMapper';

export default class AppContainer extends Component {

  constructor(props) {
    super(props);

    this.navigator;
  }

  componentWillMount() {
    BackAndroid.addEventListener('hardwareBackPress', () => {
      if (this.navigator && this.navigator.getCurrentRoutes().length > 1) {
        this.navigator.pop();
        return true;
      }
      return false;
    });
  }

  renderScene(route, navigator) {
    this.navigator = navigator;

    return (
      <SceneContainer
        title={route.title}
        route={route}
        navigator={navigator}
        onBack={() => {
          if (route.index > 0) {
            navigator.pop();
          }
        }}
        {...this.props} />
    );
  }

  render() {
    return (
      <Navigator
        initialRoute={<View />}
        renderScene={this.renderScene.bind(this)}
        navigationBar={
          <Navigator.NavigationBar
            style={{backgroundColor: 'gray'}}
            routeMapper={RouteMapper} />
        } />
    );
  }
};

Since BackAndroid is deprecated. Use BackHandler instead of BackAndroid.

import { BackHandler } from 'react-native';

{...}
  ComponentWillMount(){
    BackHandler.addEventListener('hardwareBackPress',()=>{   
      if (!this.onMainScreen()) {
        this.goBack();
        return true;
      }
      return false;
    });
  }    

This example will show you back navigation which is expected generally in most of the flows. You will have to add following code to every screen depending on expected behavior. There are 2 cases:

1. If there are more than 1 screen on stack, device back button will show previous screen.
2. If there is only 1 screen on stack, device back button will exit app.

Case 1: Show previous screen

import { BackHandler } from 'react-native';

constructor(props) {
    super(props)
    this.handleBackButtonClick = this.handleBackButtonClick.bind(this);
}

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

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

handleBackButtonClick() {
    this.props.navigation.goBack(null);
    return true;
}

Important: Don't forget to bind method in constructor and to remove listener in componentWillUnmount.

Case 2: Exit App

In this case, no need to handle anything on that screen where you want to exit app.

Important: This should be only screen on stack.