next.js/examples/with-apollo-and-redux/lib/initRedux.js

import { createStore, combineReducers, applyMiddleware, compose } from 'redux'
import reducers from './reducers'

let reduxStore = null

// Get the Redux DevTools extension and fallback to a no-op function
let devtools = f => f
if (process.browser && window.__REDUX_DEVTOOLS_EXTENSION__) {
  devtools = window.__REDUX_DEVTOOLS_EXTENSION__()
}

function create (apollo, initialState = {}) {
  return createStore(
    combineReducers({ // Setup reducers
      ...reducers
    }),
    initialState, // Hydrate the store with server-side data
    compose(
      // Add additional middleware here
      devtools
    )
  )
}

export default function initRedux (initialState) {
  // Make sure to create a new store for every server-side request so that data
  // isn't shared between connections (which would be bad)
  if (!process.browser) {
    return create(initialState)
  }

  // Reuse store on the client-side
  if (!reduxStore) {
    reduxStore = create(initialState)
  }

  return reduxStore
}