getStateForAction.js

// Our Navigator:
const AppNavigator = StackNavigator({
  login: {
     screen: Login
  },
  screen1: {
    screen: Screen1
  },
  screen2: {
    screen: Logout
  }
});

// Suppose the current navigation state of the Navigator is:
currentState = {
    index: 1, // represents the current screen from routes array, routes[1]
    routes: [
        {
          routeName:"login",
          key:"123456",
        },
        {
          routeName:"screen1",
          key:"100000"
        }
    ]
}

// Now we want to navigate to screen2, first get the action needed to naivigate to screen2
const ActionForScreen2 = AppNavigator.router.getActionForPathAndParams(
    "screen2"
);

// Now we will get the new state for the navigator using ActionForScreen2 and currentState
const NewStateOfNavigator = AppNavigator.router.getStateForAction(ActionForScreen2, currentState)

// The new updated state of the navigator, given the action and currentState:
console.log(JSON.stringify(NewStateOfNavigator))

// The new updated navigation state, We will see further on how to pass this state to Navigator.
/*
NewStateOfNavigator = {
    index: 2, // represents the current screen from routes array, routes[2]
    routes: [
        {
          routeName:"login",
          key:"123456",
        },
        {
          routeName:"screen1",
          key:"100000"
        },
        {
          routeName:"screen2",
          key:"7268828"
        }
    ]
}
*/