stream-saga/src/js/reducers/index.js

import { ADD_ARTICLE } from '../constants/action-types'

const initialState = {
  articles: []
}

function rootReducer(state = initialState, action) {
  if (action.type === ADD_ARTICLE) {
		return Object.assign({}, state, {
      articles: state.articles.concat(action.payload)
    })
	}

  return state
}

export default rootReducer