app.js

var app = new Vue({
    el: '#app',
    data: {
        user: null,
        registerUser: {
            email: "",
            password: ""
        },
        loginUser: {
            email: "",
            password: ""
        }
    },

    methods: {

        checkUser: async function() {
            const res = await fetch('/auth/status', {
                credentials: 'include',
            })
            const body = await res.json()
            if (body.status === 'error') {
                alert(body.message)
                return
            }
            Vue.set(this, 'user', body.user)
        },

        register: async function(e) {
            const email = this.registerUser.email
            const password = this.registerUser.password
            const res = await fetch('/auth/local/register', {
                method: 'POST',
                headers: {
                    'Content-Type': 'application/json'
                },
                body: JSON.stringify({
                    email,
                    password,
                }),
                credentials: 'include',
            })
            const body = await res.json()
            if (body.status === 'error') {
                alert(body.message)
                return
            }
            await this.loginWithEmailPassword(email, password)
        },

        loginWithEmailPassword: async function(username, password) {
            const res = await fetch('/auth/local/login', {
                method: 'POST',
                headers: {
                    'Content-Type': 'application/json'
                },
                body: JSON.stringify({
                    username,
                    password,
                }),
                credentials: 'include',
            })
            const body = await res.json()
            if (body.status === 'error') {
                alert(body.message)
                return
            }
            Vue.set(this, 'user', body.user)
            this.clearForm()
        },

        logout: async function() {
            const res = await fetch('/auth/logout', {
                method: 'POST',
                credentials: 'include',
            })
            const body = await res.json()
            if (body.status === 'error') {
                alert(body.message)
                return
            }
            Vue.set(this, 'user', null)
        },

        clearForm: function() {
            this.registerUser.email = ""
            this.registerUser.password = ""
            this.loginUser.email = ""
            this.loginUser.password = ""
        }
    },

    mounted: function() {
        this.$nextTick(function() {
            this.checkUser()
        })
    }
})