title: How to use `$axios` in utility function in Vuex of Nuxt.js
body: <p>I am configuring VUEX of Nuxt.js as follows.<br></p>
  
  <pre><code>store
  ├── README.md
  ├── posts
  │   ├── actions.js
  │   ├── getters.js
  │   ├── index.js
  │   ├── mutations.js
  │   └── utils
  │       └── getPostById.js
  └── index.js
  </code></pre>
  
  <p>I added <code>@nuxtjs/axios</code> to modules in nuxt.config.js and make it possible to use <code>this.$axios.$get</code> in actions.<br>
  However, you can not use A in store/posts/utils/getPostById.js.<br>
  An error of <code>Cannot read property '$axios' of undefined</code> will occur.<br>
  <br>
  Each code is described as follows.<br>
  <br>
  ・ store/index.js</p>
  
  <pre><code>import Vuex from 'vuex'
  import postsModule from './posts'
  
  new Vuex.Store({
    modules: {
      posts: postsModule
    }
  })
  </code></pre>
  
  <p>・store/posts/index.js</p>
  
  <pre><code>import actions from './actions'
  import getters from './getters'
  import mutations from './mutations'
  
  export const state = () =&gt; ({
    posts: [],
    post: {}
  })
  
  export default {
    namespaced: true,
    state,
    actions,
    getters,
    mutations
  }
  </code></pre>
  
  <p>・ store/posts/actions.js</p>
  
  <pre><code>import getPostById from './utils/getPostById'
  
  export default {
    async fetchPost({ commit }, count = 10) {
      const params = { count: count }
      // Here `$axios` can be used normally
      const result = await this.$axios.$get("ApiUrl")
      commit('setPosts', result)
    },
    async fetchPostById({ commit }, category_ids, count = 10) {
      const topCommunities = {}
      const result = await getPostById(id)
      commit('setPost', result)
    }
  }
  </code></pre>
  
  <p>・ store/posts/utils/getPostById.js</p>
  
  <pre><code>export default async function(post_id) {
    // Here `$axios` can not use
    const result = await this.$axios.$get(`${ApiUrl}/${post_id}`)
    return result
  }
  </code></pre>
  
  <p>How can I use <code>this.$axios.$get</code> inside <code>getPostById.js</code>?<br></p>