reducer.js

import { ADD_PHOTOS } from './actions';

function getInitialState() {
  return {
    photos: [],
  };
}

export default function reducer(state = getInitialState(), action) {
  switch (action.type) {
    case ADD_PHOTOS:
      return Object.assign({}, { photos: [...state.photos, ...action.payload] });
    default:
      return state;
  }
}