Vue $router.push in Stripe cannot work.

Here is my code where I want to do a redirect after charge finish.

This handler is called when a button is pressed.

However, I found that $router.push cannot be recognised, while all my other $router.push work fine.

            this.handler = StripeCheckout.configure({

                key: Stripe.key,
                image: "https://irp-cdn.multiscreensite.com/af471d46/dms3rep/multi/thumbnail/SUMMIT-LOGO-768x739.png",
                locale: 'au',
                email: this.email,
                token: function(token) {
                    this.stripeToken = token.id;
                    this.stripeEmail = token.email;

                    let formData = {
                        email : this.email,
                        stripeToken : token.id,
                        stripeEmail : token.email
                    };

                    let self = this;
                    axios.post('/api/form/purchase',formData).then(response => {
                        console.log(self.$router); // this gives undefined warning
                        self.$router.push({name: 'form'}) // this gives error 'push' of undefined
                    }).catch(error => {
                        console.log("errrrrr: ", error); // the error get printed through this statement
                    })
                }
            });
        },

The above piece of code is called here:

        methods:{
            buy(){
                this.handler.open({
                    name: 'GCC',
                    description: 'The online purchase channel',
                    currency: 'AUD',
                    amount: 10000
                });
            }
        }

So I'm wondering any one can help on this? How can I make an AJAX redirect after purchase.

It's Laravel 5.5