我正在使用 firebase 登录用户，但控制台抱怨 firebase 未定义。 我正在导入 firebase，我已根据 firebase 提供的文档正确设置了配置，但仍然没有成功。

import Vue from 'vue'
import App from './App.vue'
import VeeValidate from 'vee-validate';
import router from './router';
import firebase from 'firebase';
Vue.use(VeeValidate);
Vue.config.productionTip = false
// Initialize Firebase
 var config = {
   apiKey: "<my api key>",
   authDomain: "<my auth domain>",
   databaseURL: "<my database url>",
   projectId: "todo-bb5d3",
   storageBucket: "",
   messagingSenderId: "638496899966"
 };
 firebase.initializeApp(config);


new Vue({
  router,
  render: h => h(App)
}).$mount('#app')

导致问题的文件：

<template>
    <div class="LogIn">
      <h3>Sign Up</h3>
      <input type="email" v-model="email" placeholder="email" name="" value="">
      <input type="passeord" v-model="password" placeholder="password" name="" value="">
      <button v-on:click="signUp" type="button" name="button">Sign Up</button>
    </div>
</template>


<script>
  export default{
    name: 'LogIn',
    data: function() {
      return{
        email: '',
        password: ''
      }
    },
    methods: {
      signUp: function(){
        firebase.auth().createUserWithEmailAndPassword(this.email, this.password).then(
          function(user){
            alert('Your account has been created')
          },
          function(err) {
            alert('Opps! ' + err.message)
          }
        )
      }
    }
  }
</script>