Code snippet from ReactNav application as featured in http://platypus.is/posts/4

createTabNav.js

# App.js

...
const TabNav = TabNavigator(
  {
    Green: { screen: GreenScreen },
    Red: { screen: RedScreen },
    Blue: { screen: BlueScreen },
    Purple: { screen: PurpleScreen }
  },
  {
    tabBarOptions: {
      labelStyle: {
        fontSize: 20,
        marginBottom: 10
      }
    }
  }
)
...