How can save data using cloud functions to firestore? - google-cloud-firestore

Question

I saw some other answers and I tried some of those unsuccessfully.
In my case, I created an iOS app and integrated Stripe payment method, so it reaches my javascript function at cloud functions. I'm actually able to see the payments I realizes within my stripe account but I couldn't manage to save it into our firestore database.
this is my setup at Google side:
const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);
const express = require('express');
const cors = require('cors')({origin: true});
const app = express();
const stripe = require('stripe')(functions.config().stripe.token);
//I've preset this data to firebase functions middleware
function charge(req, res) {
const body = (req.body);
const userId = body.userId;
const token = body.token;
const amount = body.amount;
const currency = body.currency;
// Charge card
stripe.charges.create({
amount,
currency,
description: 'Firebase Example',
source: token,
}).then(charge => {
send(res, 200, {
// I WANNA RECORD DATA INTO MY DATABASE HERE!!
message: 'Success',
charge,
});
}).catch(err => {
console.log(err);
send(res, 500, {
error: err.message,
});
});
}
function send(res, code, body) {
res.send({
statusCode: code,
body: JSON.stringify(body),
});
}
app.use(cors);
app.post('/', (req, res) => {
// Catch any unexpected errors to prevent crashing
try {
charge(req, res);
} catch(e) {
console.log(e);
send(res, 500, {
error: `The server received an unexpected error. Please
try again and contact the site admin if the error persists.`,
});
}
});
exports.charge = functions.https.onRequest(app);
And this is our database setup where I want to save like this:
- into payments > userId... I'll save each transaction this userId does with the fields: "token", "currency" and "amount".
Note: I already have all this values in my function and also have userId.
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