error 'Stripe' is not defined no-undef

It's only a problem of getting it compiled. The proper way to fix it is to ask compiler to not look for Stripe and it will be available globally. Add this comment /* global Stripe */ before you initialize Stripe:

/* global Stripe */
const stripe = Stripe('STRIPE_KEY');

I figured it out! I was being a bit of a dumby.

I changed Stripe to window.Stripe in my code, and it works now!

Thanks everyone.