This is a boilerplate for your project if you wish to host an express server on firebase and use firestore as a database.
Set up npm, firebase-tools on your machine. Clone this repository. Change .firebasesrc.example to .firebasesrc and add your firebase project ID to this file in the placeholder. Change directory into /functions and run npm install. Make changes to /functions/routes/index.js as per your needs. You can use some of the Utils defined in fireStoreUtils.js. Deploy your project.