Level 3
Looks like you haven't defined callback function stripeTokenHandler. Here is a sample:
checkout(){
Stripe.setPublishableKey('your_publishable_key');
Stripe.card.createToken({
number: this.cardNumber,
cvc: this.cvc,
exp_month: this.month,
exp_year: this.year
}, this.stripeTokenHandler);
},
stripeTokenHandler(status, response) {
// validate payment
}