Upgrade Firebase Functions to Node 10

To upgrade Firebase to Node 10, open package.json:

terminal
  "engines": {
    "node": "10"
  }

It's recommended to update to the latest version of Firebase Functions as well:

terminal
npm i --save firebase-functions@latest

Be aware of the conditions to use Node 10; you'll need to upgrade your Firebase projects to Blaze (a shame by the way).