router.js

import React from "react";
import {
  createStackNavigator,
  createBottomTabNavigator,
  createSwitchNavigator
} from "react-navigation";

import Signup from "./screens/Signup";
import Login from "./screens/Login";
import Home from "./screens/Home";
import Profile from "./screens/Profile";

export const Unauthenticated = createStackNavigator({
  Signup: {
    screen: Signup,
    navigationOptions: {
      title: "Sign up"
    }
  },
  Login: {
    screen: Login,
    navigationOptions: {
      title: "Login"
    }
  }
});

export const Authenticated = createBottomTabNavigator({
  Home: {
    screen: Home,
    navigationOptions: {
      title: "Home"
    }
  },
  Profile: {
    screen: Profile,
    navigationOptions: {
      title: "Profile"
    }
  }
});

export const createRootNavigator = isAuthenticated => {
  return createSwitchNavigator(
    {
      Authenticated: {
        screen: Authenticated
      },
      Unauthenticated: {
        screen: Unauthenticated
      }
    },
    {
      initialRouteName: isAuthenticated ? "Authenticated" : "Unauthenticated"
    }
  );
};