App.vue

<template>
  <div id="app">
		{{ $store.state.a.name }}
		{{ $store.state.a.initData }}
  </div>
</template>

<script>
export default {
  name: 'app',
}
</script>

<style>
</style>

main.js

import Vue from 'vue'
import App from './App.vue'
import store from './store'

new Vue({
  el: '#app',
	store,
  render: h => h(App)
})

store.js

import Vue from 'vue'
import Vuex from 'vuex'

Vue.use(Vuex)

const moduleA = {
	state: {
		name: 'module A',
		initData: ''
	},
	mutations: {
		INIT (state, payload) {
			state.initData = 'Initialized A'
		}
	}
}

const moduleB = {
	state: {
		name: 'module B',
		initData: ''
	},
	mutations: {
		INIT (state, payload) {
			state.initData = 'Initialized B'
		}
	}
}

const initPlugin = store => {
	store.commit('INIT')
}

export default new Vuex.Store({
	state: {},
	mutations: {},
	modules: {
		a: moduleA,
		b: moduleB
	},
	plugins: [initPlugin]
})