Writing epics with redux-observable

epic.js

import * as ajax from '../../utils/ajax';
import 'rxjs/add/operator/catch'
import {
  Observable
} from 'rxjs/Observable';
import queryString from 'query-string'
import _ from 'lodash'

import {
  PRODUCTS_REQUEST_START,
  CANCEL_PRODUCTS_REQUEST
} from '../actions/actionTypes';

import {
  doFetchProductsFulfilled
} from '../actions/doFetchProducts';
// import { push } from 'react-router-redux';


export default function epicFetchProducts(action$, store) {
  return action$.ofType(PRODUCTS_REQUEST_START)
    .map(function(action) {

      // Get existing selections/filters attributes from store
      const currentSelections = store.getState().catalogReducer.currentSelections
      // Now extend/override old atts with new attributes coming through action
      const selections = Object.assign({}, currentSelections, action.newSelections)
      const searchString = queryString.stringify(selections, {
       arrayFormat: 'bracket'
      })
      // Push new search query by excluding default params
      window.appHistory.push({
       search: queryString.stringify(selections)
      })
      // Return new `action` object with new key `searchString` to call API
      return Object.assign({}, action, {
       searchString
      })

    })
    .mergeMap(action =>
      ajax.get(`/products?${action.searchString}`)
      .map(response => doFetchProductsFulfilled(response))
      .catch(error => doFetchProductsError(error))
      .takeUntil(action$.ofType(CANCEL_PRODUCTS_REQUEST))
    );
}