*For this example we are using express

upload_image_with_functions.js

'use strict'

const utils = require('utils')

//...

app.post('/upload/image', (req, res) {
  utils.uploadFile(req, 'your bucket', 'your folder',
    (downloadUrl) => {
      return res.status(201).json({ success: true, downloadUrl: downloadUrl })
    },
    (err) => {
      return res.status(500).json({ success: false, error: err })
    }
  )
})

utils.js

'use strict'

const Busboy = require('busboy')
const path = require('path')
const os = require('os')
const fs = require('fs')
const mkdirp = require('mkdirp-promise')

const Storage = require('@google-cloud/storage')

exports.uploadFile = function (req, bucket, folderToSave, onSuccess, onError) {
    const busboy = new Busboy({ headers: req.headers })

    busboy.on('file', async (fieldname, file, filename) => {
        const tempLocalFile = path.join(os.tmpdir(), '/outputFiles/' + filename)
        const tempLocalDir = path.dirname(tempLocalFile)

        await mkdirp(tempLocalDir)

        const writeStream = fs.createWriteStream(tempLocalFile)

        file.pipe(writeStream)
        file.on('end', () => writeStream.end())

        writeStream.on('finish', async () => {
            var data = await new Storage()
                .bucket(bucket)
                .upload(tempLocalFile, { destination: folderToSave + '/' + filename })

            var file = data[0]
            var downloadUrl = "https://firebasestorage.googleapis.com/v0/b/" + file.bucket.name + "/o/" + encodeURIComponent(file.name) + "?alt=media"

            onSuccess(downloadUrl)
        })
        writeStream.on('error', (err) => onError(err))
    })
    busboy.end(req.rawBody)
}