How do i make a TabNavigator button push a modal screen with React Navigation

Not sure if this is still relevant for you, but i've managed to find away to achieve this.

So i've managed to get it working by using the tabBarComponent inside the tabNavigatorConifg, you can stop the tab navigation from navigating depending on the index.

   tabBarComponent: ({jumpToIndex, ...props, navigation}) => (
        <TabBarBottom
            {...props}
            jumpToIndex={index => {
                if (index === 2) {
                    navigation.navigate('camera')
                }
                else {
                    jumpToIndex(index)
                }
            }}
        />

    )

Once you've done this, my method of showing the view modally on top of the tab views was to put the tabnavigator inside of a stacknavigatior and then just navigate to a new screen inside of the stacknavigator.

Actually, there is no support in react-navigation to change the way of presentation on the fly from default to modal (see the discussion about this here). I ran into the same issue and solved it by using a very top StackNavigator with headerMode set to none and mode set to modal:

const MainTabNavigator = TabNavigator(
    {
        Tab1Home: { screen: Tab1Screen },
        Tab2Home: { screen: Tab2Screen }
    }
);

const LoginRegisterStackNavigator = StackNavigator({
    Login: { screen: LoginScreen }
});

const ModalStackNavigator = StackNavigator({
    MainTabNavigator:          { screen: MainTabNavigator            },
    LoginScreenStackNavigator: { screen: LoginRegisterStackNavigator }
}, {
    headerMode: 'none',
    mode:       'modal'
});

This allows me to do the following (using redux) in Tab1Screen and Tab2Screen to bring up the modal view from wherever I want:

this.props.navigation.navigate('LoginScreenStackNavigator');