SOURCE

// make sure to call Vue.use(Vuex) if using a module system

const store = new Vuex.Store({
  state: {
    count: 0
  },
  mutations: {
  	increment: state => state.count++,
    decrement: state => state.count--
  }
})

new Vue({
  el: '#app',
  computed: {
    count () {
	    return store.state.count
    }
  },
  methods: {
    increment () {
      store.commit('increment')
    },
    decrement () {
    	store.commit('decrement')
    }
  }
})

<div id="app" class="app">
  <p>{{ count }}</p>
  <p>
    <button @click="increment">+</button>
    <button @click="decrement">-</button>
  </p>
</div>

.app{
  background-color:#ffffff
}