next.js/examples/with-redux/store.js

import { createStore, applyMiddleware } from 'redux'
import thunkMiddleware from 'redux-thunk'

export const reducer = (state = { lastUpdate: 0, light: false }, action) => {
  switch (action.type) {
    case 'TICK': return { lastUpdate: action.ts, light: !!action.light }
    default: return state
  }
}

export const startClock = () => dispatch => {
  setInterval(() => dispatch({ type: 'TICK', light: true, ts: Date.now() }), 800)
}

export const initStore = (reducer, initialState, isServer) => {
  if (isServer && typeof window === 'undefined') {
    return createStore(reducer, initialState, applyMiddleware(thunkMiddleware))
  } else {
    if (!window.store) {
      window.store = createStore(reducer, initialState, applyMiddleware(thunkMiddleware))
    }
    return window.store
  }
}