Firebase Vuex Auth Module

Auth.js

import firebase from 'firebase';

const state = {
  userId: '',
};

const actions = {
  login({ commit, state }, { email, password }) {
    return new Promise((resolve, reject) => {
      firebase.auth().signInWithEmailAndPassword(email, password)
        .then((user) => {
          resolve(user);
        })
        .catch((err) => {
          reject(err);
        });
    });
  },
  logout({ commit, state }) {
    return new Promise((resolve, reject) => {
      firebase.auth().signOut()
        .then(() => {
          commit('LOGOUT');
          resolve();
        })
        .catch((err) => {
          reject(err);
        });
    });
  },
  checkUserStatus({ commit, state }) {
    return new Promise((resolve, reject) => {
      firebase.auth().onAuthStateChanged((user) => {
        if (user) {
          commit('SET_USER_ID', user.uid);
          resolve(user.uid);
        } else {
          reject('User is not logged in.');
        }
      });
    });
  },
};

const mutations = {
  LOGOUT(state, userId) {
    state.userId = '';
  },
  SET_USER_ID(state, userId) {
    state.userId = userId;
  },
};

export default {
  state,
  actions,
  mutations,
};

Index.vue

<template>
  <div>
    <span>{{ userId }}</span>
  </div>
</template>

<script>
import { mapState } from 'vuex';

export default {
  name: 'index',
  computed: {
    ...mapState({
      userId: state => state.userId,
    }),
  }
};
</script>

Login.vue

<template>
  <form @submit="handleLogin">
    <input type="text" placeholder="Email" v-model="email"/>
    <input type="password" placeholder="Password" v-model="password"/>
    <button type="submit">Login</button>
  </form>
</template>
<script>
export default {
  name: 'login',
  data() {
    return {
      email: '',
      password: '',
    };
  },
  methods: {
    handleLogin() {
      const email = this.email;
      const password = this.password;
      this.$store.dispatch('login', { email, password })
      .then((user) => {
        // do something with user object
      })
      .catch((err) => {
        console.log(err);
      })
    }
  }
};
</script>