test.html

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
</head>
<body>
<div id="app">
  <p>{{ count }}</p>
  <p>
    <button @click="increment">+</button>
    <button @click="decrement">-</button>
  </p>
</div>
<script>
  var Vue = require('Vue');
  var Vuex = require('Vuex');
  Vue.use(Vuex)

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

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


</script>
</body>
</html>