Firebase Cloud Function: Thumbnail Generator (Serverless)

index.js

const { join, dirname } = require('path')
const sharp = require('sharp')
const { Storage } = require('@google-cloud/storage')
const { promisify } = require('util')
const pump = promisify(require('pump'))

const gcs = new Storage()
const SIZE = 128
const PREFIX = 'thumb@'

exports.generateThumbs = event => {
    const filePath = event.name
    const fileName = filePath.split('/').pop()
    if (fileName.startsWith(PREFIX) || !event.contentType.includes('image')) {
        return
    }
    const bucket = gcs.bucket(event.bucket)
    const imgIn = bucket.file(filePath).createReadStream()
    const resizer = sharp().resize(SIZE, SIZE)
    const thumbOut = bucket.file(join(dirname(filePath), `${PREFIX}${fileName}`)).createWriteStream({
        contentType: event.contentType,
        resumable: false
    })
    return pump(imgIn, resizer, thumbOut)
}

package.json

{
  "name": "thumberator",
  "version": "0.1.0",
  "main": "index.js",
  "scripts": {
    "deploy": "serverless deploy",
    "deploy:prod": "serverless deploy --stage prod"
  },
  "devDependencies": {
    "serverless": "^1.41.1",
    "serverless-google-cloudfunctions": "^2.3.2"
  },
  "dependencies": {
    "@google-cloud/storage": "^2.5.0",
    "pump": "^3.0.0",
    "sharp": "^0.22.1"
  }
}

serverless.yml

service: thumberator

custom:
  stages:
    - dev
    - prod
  buckets:
    dev: <dev-bucket>
    prod: <prod-bucket>

provider:
  name: google
  runtime: nodejs8
  region: <region>
  stage: ${opt:stage,'dev'}
  project: <firebase-project>
  credentials: <path-to-service-json>

plugins:
  - serverless-google-cloudfunctions

package:
  exclude:
    - node_modules/**
    - .gitignore
    - .git/**

functions:
  generateThumbs:
    handler: generateThumbs
    events:
      - event:
          eventType: google.storage.object.finalize
          resource: projects/<firebase-project>/buckets/${self:custom.buckets.${self:provider.stage}}