Express backend to upload user-files to Google Cloud Storage

index.js

const express = require('express');
const app = express();
const functions = require('firebase-functions');
const cors = require('cors')({ origin: true });
app.use(cors);

// upload account image
const API_LINK_BASE_URL = 'http://storage.googleapis.com';
const PROJECT_ID = ;// {YOUR_PROJECT_ID, USUALLY A SINGLE WORD};
const BUCKET_ID = ;// {YOUR_BUCKET_ID, E.G. PROJECT_ID.appspot.com};

const Busboy = require('busboy');
const shortid = require('shortid');

app.post('/upload', (req, res) => {
    const { Storage } = require('@google-cloud/storage');
    const storage = new Storage({
        projectId: PROJECT_ID
    });
    const bucket = storage.bucket(BUCKET_ID);

    const busboy = new Busboy({ headers: req.headers });
    const fields = {};
    const fileUrls = [];

    busboy.on('field', (fieldname, val) => {
      console.log(`Processed field ${fieldname}: ${val}.`);
      fields[fieldname] = val;
    });

    busboy.on('file', (fieldname, uploadedFile, filename) => {
      console.log(`Processed file ${filename}`);

      const gcsname = `img_${shortid.generate()}${filename.substr(filename.lastIndexOf('.'))}`;
      fileUrls.push(`${API_LINK_BASE_URL}/${BUCKET_ID}/${gcsname}`);
      const gcsfile = bucket.file(gcsname);
      const writeStream = gcsfile.createWriteStream({
          resumable: false
      });
      uploadedFile.pipe(writeStream);

      return new Promise((resolve, reject) => {
          uploadedFile.on('end', () => {
              writeStream.end();
          });
          writeStream.on('finish', () => {
              console.log('file finished');
              return resolve(); 
          });
          writeStream.on('error', reject);
      }).then(() => {
          console.log(gcsname);
          return gcsfile.makePublic();
      }).catch(e => res.status(400).send({ error: e }));
    });

    busboy.on('finish', () => {
        console.log('busboy finished');
        return res.status(200).send(fileUrls);
    });

    busboy.end(req.rawBody);
});

exports.api = functions.https.onRequest(app);