Piping/streaming file from memory to bucket in Firebase/GCS - node.js

Question

I'm trying to generate multiple documents and pack them together to a zip and upload it to Google Cloud Storage via Firebase (using default bucket).
So far:
generating multiple documents works
packing them together doesn't work (I get empty zip and I'm using ArchiverJS)
uploading to the bucket (it's just empty) works
Seems like to upload a file I need to save it to temporary folder within Firebase Function, but I can't find any solution how to empty it after using. So I wanted to use streams, but then I was warned that it's a bad idea because checksum and such.
On the other hand .save() function let's save arbitrary data. It was specifically requested here, but really doesn't seem to work (at least for me).
Also, ArchiverJS seems to also let use of streams.
So, theoretically, it should all work nicely. But it's not so I hope someone else knows better.
const express = require('express')
var router = express.Router()
var archiver = require('archiver')
var admin = require("firebase-admin");
var serviceAccount = require("../servicekey.json")
admin.initializeApp({
credential: admin.credential.cert(serviceAccount),
databaseURL: "https://myName.firebaseio.com",
storageBucket: "myName.appspot.com"
})
var bucket = admin.storage().bucket()
const {
convertCSVtoJSON,
generateDocuments,
generateDocx,
isCorrectTemplateFileType
} = require('./generateServices')
router.post('/', async (req, res) => {
try {
if(!isCorrectTemplateFileType(req.files.template))
return res.status(403).send({
message: 'Wrong file type. Please provide .docx file.'
})
const template = req.files.template.data
const data = await convertCSVtoJSON(req.files.data1)
let zip = archiver('zip')
zip.on('warning', function(err) {
console.log(err)
});
zip.on('error', function(err) {
res.status(500).send({error: err.message})
});
zip.on('entry', function(ars) {
// console.log(ars)
});
zip.on('end', function() {
console.log('Archive wrote %d bytes', zip.pointer())
});
// res.attachment('archive-name.zip')
// zip.pipe(output)
// zip.pipe(res)
data.forEach((docData, index) => {
let buff = generateDocx(template, docData, 'title')
zip.append(buff, { name: `${index}.docx` })
})
zip.finalize()
console.log(zip)
const file = bucket.file("pliki.zip") // nazwa do zmiany
file.save(zip, (err) => {
if (!err) {
console.log("cool");
} else {
console.log("error " + err);
}
});
res.sendStatus(201)
} catch (error) {
console.log(error)
res.send(error)
}
})
module.exports = router