How do I hide the shadow under react-navigation headers?

Question

How do I hide the shadow under react-navigation headers?
They look like this.

Answer 1

As of 2019 this answer doesn't work in version 3.

The new way of doing it is:

const AppNavigation = StackNavigator(
  {
    'The First Screen!': { screen: FirstScreen },
  },
  {
    defaultNavigationOptions: {
      headerStyle: {
        elevation: 0,
        shadowOpacity: 0,
      },
    },
  },
);