index.js

const functions = require('firebase-functions')
const path = require('path')

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

module.exports = functions.storage.object().onChange(event => {
  const object = event.data
  const bucket = gcs.bucket(object.bucket)

  // Exit if this is triggered on a file that is not an image.
  if (!object.contentType.startsWith('image/')) {
    console.log('This is not an image.')
    return
  }

  // Exit if this is a move or deletion event.
  if (object.resourceState === 'not_exists') {
    console.log('This is a deletion event.')
    return
  }

  // Skip if this is the output of a copy op.
  if (path.parse(object.name).name.endsWith('-copy')) {
    console.log('This is an image that is already copied')
    return
  }

  const writeOpts = {
    public: true,
    resumable: false,
    metadata: {
      contentType: object.contentType,
      cacheControl: 'public, max-age=31557600'
    }
  }

  const source = bucket.file(object.name).createReadStream()
  const destFilename = createDestFilename(object.name)
  const dest = bucket.file(destFilename).createWriteStream(writeOpts)

  var start = Date.now()

  source.pipe(dest).on('finish', () => {
    console.log('copy complete %s ms', Date.now() - start)
  })
})

function createDestFilename (filename) {
  const parsed = path.parse(filename)
  return path.join(parsed.dir, parsed.name + '-copy' + parsed.ext)
}