// define ADD, addMessage(), messageReducer(), and store here:
const ADD = "ADD";
const addMessage = message => {
  return {
    type: ADD,
    message
  };
};

// Use ES6 default paramter to give the 'previousState' parameter an initial value.
const messageReducer = (previousState = [], action) => {
  // Use switch statement to lay out the reducer logic in response to different action type
  switch (action.type) {
    case ADD:
      // Use ES6 spread operator to return a new array where the new message is added to previousState
      return [...previousState, action.message];
      break;

    default:
      // A default case to fall back on in case if the update to Redux store is not for this specific state.
      return previousState;
  }
};

const store = Redux.createStore(messageReducer);