dataReducer.js

// initial app state can be supplied (or bootstrapped) here
const defaultState = {
  items: [],
};

/**
 * @params
 *   state {object} - this is the current state of this branch of the store (store.data)
 *   action {object} - the instructions from the component, commonly { type, data }
 */
export default function(state = defaultState, action) {
  switch (action.type) {
    case ‘ADD_DATA’:
      // we're changing the value of store.data here
      // to add the new data to the list
      return {
        items: state.items.concat(action.data)
      };
      break;
    case default:
      return state; // don't change
}