getAccessToken.js

// [GET USER FROM SERVER]
export const getAccessToken = ({ commit, dispatch }, { req }) => {
  if (!req || !req.headers.cookie) return Promise.reject('no-cookie')
  // [get new tokens]
  return axios
    .post(`https://securetoken.googleapis.com/v1/token?key=${projectID}`, {
      grant_type: 'refresh_token',
      refresh_token: Cookie.parse(req.headers.cookie).refresh
    })
    .then(res => {
      if (res.data.access_token && res.data.user_id && res.data.refresh_token) {
        return Promise.resolve({
          NEW_TOKEN: res.data.access_token,
          NEW_UID: res.data.user_id
        })
      }
      return Promise.reject('no-cookie')
    })
    .catch(error => Promise.reject(error))
}

getUserData.js

// [GET USER DATA FROM FIREBASE DB]
export const getUserData = ({ state, commit }, { NEW_TOKEN, NEW_UID }) => {
  return axios
    .get(
      database.child('users').child(NEW_UID).toString() +
        `.json?auth=${NEW_TOKEN}`
    )
    .then(res => {
      commit('SET_USER', res.data)
      return Promise.resolve(state.authUser)
    })
    .catch(error => Promise.reject(error))
}

middleware.js

export default ({ store, req, isServer }) => {
  if (isServer) {
    return store
      .dispatch('getAccessToken', { req })
      .then(tokens => {
        return store.dispatch('getUserData', tokens)
      })
      .catch(console.log)
  }
  return store.dispatch('getUser')
}

template.vue

<template>
  <h1>
    {{ user }}
  </h1>
</template>
<script>
  export default {
    name: 'vueheader',
    data () {
      return {
        user: this.$store.state.authUser
      }
    }
  }
</script>