【v2対応】Nuxt.jsとFirebaseを組み合わせて爆速でWebアプリケーションを構築する ref: https://qiita.com/potato4d/items/cfddeb8732fec63cb29c

auth.js

import firebase from '~/plugins/firebase'

function auth () {
  return new Promise((resolve, reject) => {
    firebase.auth().onAuthStateChanged((user) => {
      resolve(user || false)
    })
  })
}
export default auth

file0.txt

$ npm i -g create-nuxt-app

file1.js

import Vue from 'vue'
import Vuex from 'vuex'
import firebase from 'firebase'
import { firebaseMutations, firebaseAction } from 'vuexfire'
const db = firebase.database()
const postsRef = db.ref('/posts')

Vue.use(Vuex)

return new Vuex.Store({
  state: {
    posts: []
  },
  mutations: {
    ...firebaseMutations
  },
  actions: {
    INIT_POSTS: firebaseAction(({ bindFirebaseRef }) => {
      bindFirebaseRef('posts', postsRef)
    })
  }
}

file1.txt

$ vue init nuxt/starter

file2.js

import Vue from 'vue'
import Vuex from 'vuex'
import firebase from 'firebase'
import { firebaseMutations, firebaseAction } from 'vuexfire'
const db = firebase.database()
const postsRef = db.ref('/posts')

Vue.use(Vuex)

return new Vuex.Store({
  state: {
    posts: []
  },
  mutations: {
    ...firebaseMutations
  },
  actions: {
    INIT_POSTS: firebaseAction(({ bindFirebaseRef }) => {
      bindFirebaseRef('posts', postsRef)
    })
  }
}

firebase.js

import firebase from 'firebase'

if (!firebase.apps.length) {
  firebase.initializeApp(
    {
      apiKey: process.env.APIKEY,
      authDomain: process.env.AUTHDOMAIN,
      databaseURL: process.env.DATABASEURL,
      projectId: process.env.PROJECTID,
      storageBucket: process.env.STORAGEBUCKET,
      messagingSenderId: process.env.MESSAGINGSENDERID
    }
  )
}

export default firebase

OAuthサンプル

import firebase from 'firebase'
const provider = new firebase.auth.GoogleAuthProvider()
firebase.auth().signInWithRedirect(provider)