index.js

const functions = require('firebase-functions');
const stripe = require("./lib/stripe");

exports.createCheckoutSession = functions.https.onCall(async (data, context) => {
  console.log("Checkout session request incoming", data);
  const res = await stripe.createSubscriptionCheckout(data);
  return res;
});

stripe.js

const functions = require('firebase-functions');
const publicKey = functions.config().stripe.public_key;
const secretKey = functions.config().stripe.secret_key;
var stripe = require('stripe')(secretKey)
const root_url = functions.config().app.root_url;

const createSession = async (payload) => {
  
  const planId = payload.plan === "Monthly" ? functions.config().stripe.monthly : functions.config().stripe.yearly

  //do we have a customer?
  let customer = null;
  const res = await stripe.customers.list({email: payload.email, limit: 1});

  if(res.data.length > 0) customer = res.data[0];
  if(!customer){
    customer = await stripe.customers.create({
      email: payload.email
    });
  }
  const stripeData = {
    success_url: payload.success_url || `${root_url}/thanks-for-signing-up`,
    cancel_url: payload.cancel_url || `${root_url}/maybe-next-time`,
    payment_method_types: ['card'],
    customer: customer.id,
    allow_promotion_codes: true,
    subscription_data: {
      trial_from_plan: true,
      items: [{
          plan: planId
      }]
    }
  }
  if(payload.coupon){
    stripeData.subscription_data.coupon = payload.coupon;
  }
  try{
    const session = await stripe.checkout.sessions.create(stripeData);
    return {
      success: true,
      sessionId: session.id,
      public_key: publicKey
    };
  }catch(err){
    return {
      success: false,
      error: err.message
    }
  }

}

exports.createSubscriptionCheckout = function(payload){
  return createSession(payload);
}