Firebase Cloud Functions + Hosting JavaScript

// functions/index.js - and in firebase.json:
//   "hosting": { "rewrites": [{ "source": "/pixel.gif", "function": { "functionId": "pixel" } }] }
import { onRequest } from 'firebase-functions/https';

const GIF = Buffer.from('R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==', 'base64');

export const pixel = onRequest((req, res) => {
  res.set('Cache-Control', 'no-store').type('gif').send(GIF);
});