possibleSolution.js

const initialState = {
    activeIndex: null,
    flights: []
}

export default function reducer (state = initialState, action){
  switch(action.type) {
    // lets create first. 
    // This means we create a flight then add it to a *copy* of the list.
    case CREATE_NEW_FLIGHT:
        const newFlight = {
            departure: action.departure,
            arrival: action.arrival
        }

        return { 
            ...state, // spread out the original state
            // update just the flights object. It's a list so we spread it out then add one
            flights: [...state.flights, newFlight]
        }

    case SET_ACTIVE:
        return {
            ...state,
            // the setting active
            activeIndex: action.activeIndex; 
        }

    case UPDATE_ACTIVE_FLIGHT:

        const idx = action.flightNumber;
        
        const updatedFlight = {
            fromLocation: action.from.code,
            fromLat: action.from.lat,
            fromLng: -action.from.lng,
            fromCity: action.from.city,
            fromValue: action.from.value,
            toLocation: action.destination.code,
            toLat: action.destination.lat,
            toLng: -action.destination.lng,
            toCity: action.destination.city,
            toValue: action.destination.value
        }

        return {
            ...state,
            flights: state.flights.map(flight => (idx === state.activeIndex) ? updatedFlight : flight)
        }
      }

    default: 
        return state;
  }
}