how to solve error “this is undefine” in vue 3

Question

I have create a vue component like bellow:

<template>
    <div class="login">
        <h3>Sign in</h3>
        <input type="text" placeholder="Email" v-model="email"/><br>
        <input type="password" placeholder="Password" v-model="password"/><br>
        <button v-on:click="login()">Login</button>
    </div>
</template>

<script>
    import firebase from 'firebase'

    export default {
        name: "Login",
        data: function () {
            return {
                email: '',
                password: ''
            }
        },
        methods: {
            login: function () {
                firebase.auth().signInWithEmailAndPassword(this.email, this.password).then(function (user) {
                    this.$router.replace('home')
                })
            }
        }
    }
</script>

It's still ok until I enter username and password and click Login, firebase auth success and console show error this is undefined. How can I solve this error? I cannot use router.

Answer 1

Try with an arrow function instead:

firebase.auth().signInWithEmailAndPassword(this.email, this.password).then((user) => {
    this.$router.replace('home')
})

Answer 2

login: function () {
   let vm=this
   firebase.auth().signInWithEmailAndPassword(this.email,  this.password).then(function (user) {
                    vm.$router.replace('home')
                })
            }

Answer 3

The callback function with the then does not have the context of login function available to it. If you're able to use ES6, use an arrow function instead :

 login: function () {
            firebase.auth().signInWithEmailAndPassword(this.email, this.password).then((user) => {
                this.$router.replace('home')
            })
        }

However, if you for some reason are not able to use ES6, try storing a reference to this and use it inside the function :

 login: function () {
           var self = this;
            firebase.auth().signInWithEmailAndPassword(this.email, this.password).then(function (user) {
                self.$router.replace('home')
            })
        }

Answer 4

Using arrow function or bind() method Example:

foo().then(() => {
     // access this
})
// or
foo().then(function() {
     // access this
}.bind(this))