[sofixa]

Firebase Cloud Functions: https://firebase.google.com/docs/functions/get-started

You drop a file in the functions dir of your repository, edit the firebase.js(on? it's been a while) config file to say that function X maps to path Y (so you can have api.site.com or site.com/api or whatever you want as a redirect to your function), and run firebase deploy.

[gfodor]

That's FaaS, not middleware for the static contents of your site.