Nuxt.js firebase GoogleAuth

plugins/firebase.js

import firebase from "firebase";

if (!firebase.apps.length) {
  const firebaseConfig = {
    apiKey: "<apiKey>",
    authDomain: "<authDomain>",
    databaseURL: "<databaseURL>",
    projectId: "<projectId>",
    storageBucket: "<storageBucket>",
    messagingSenderId: "<messagingSenderId>",
    appId: "<appId>",
    measurementId: "<measurementId>"
  };

  firebase.initializeApp(firebaseConfig);
}

export default firebase;

/pages/login.vue

<template>
  <div>
    <button type="button" @click="signIn()" class="btn btn-primary">Sign in with Google</button>
    <div><router-link to="/mypage" >マイページ</router-link></div>
  </div>
</template>

<script>
import firebase from "@/plugins/firebase";

export default {
  created: function() {
    const user = firebase.auth().currentUser;
    console.log('user', user);
    if (user) {
      this.$router.push('/mypage'); 
    }
  },
  methods: {
    signIn() {
      const provider = new firebase.auth.GoogleAuthProvider();
      firebase.auth().signInWithPopup(provider).then((result) => {
        console.log('result', result);
        this.$router.push('/mypage');
      }).catch((error) => {
        console.error(error.code);
      });
    }
  }
}
</script>

pages/mypage.vue

<template>
  <div>
    <div>{{email}}</div>
    <button type="button" @click="signOut()" class="btn btn-primary">ログアウト</button>
    <div><router-link to="/login" >ログイン画面</router-link></div>
  </div>
</template>

<script>

import firebase from "@/plugins/firebase";

export default {
  data: function() {
    return { email: null };
  },  
  created: function() {
    const user = firebase.auth().currentUser;
    console.log('user', user);
    if (user) {
      this.email = user.email; 
    } else {
      this.$router.push('/login');
    }
  },
  methods: {
    async signOut(){
      await firebase.auth().signOut();
      this.$router.push('/login');
    },
  }
}
</script>