Minimal Vuex from scratch (no package.json)

index.html

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.5.17/vue.js"></script>
<script src="store.js"></script>

store.js

class Store {
  static install(Vue) {
    Vue.mixin({
      beforeCreate() {
        this.$store = this.$options.store 
      }
    })
  }

  constructor({ state, mutations }) {
    this.mutations = mutations
    this.vm = new Vue({
      data() {
        return { $$state: state }
      }
    })
  }

  get state() {
    return this.vm.$data.$$state
  }

  commit(handler, payload) {
    this.mutations[handler](this.state, payload)
  }
}

document.addEventListener("DOMContentLoaded", () => {
  const el = document.createElement("div")
  el.id = "app"
  document.body.appendChild(el)

  Vue.use(Store)

  const mutations = {
    increment(state, { amount }) {
      state.count = state.count + amount
    }
  }

  const store = new Store({ state: { count: 0 }, mutations })

  window.app = new Vue({ 
    el, 
    store,
    template: `
      <div>
        {{ $store.state.count }}
        <button @click="$store.commit('increment', { amount: 1 })">
          Increment
        </button>
      </div>
    `
  })
})