Cloudflare worker for rewriting a custom domain to work with Firebase SDK

app.js

import { initializeFirestore } from 'firebase/firestore'

const db = initializeFirestore(app, { host: 'firestore-rewrite.xxx.workers.dev' })

Worker.js

addEventListener('fetch', (event) => {
  event.respondWith(handleRequest(event.request))
})

/**
 * Intercept requests and redirect to Firestore API
 * @param {Request} request
 */
async function handleRequest(request) {
  const url = new URL(request.url)
  // Modify the URL to point to Firestore API
  url.protocol = 'https'
  url.host = 'firestore.googleapis.com'

  // Construct a new request object
  const newRequest = new Request(url, {
    method: request.method,
    headers: request.headers,
    body: request.body,
  })

  // Fetch the Firestore API and get the response
  let response = await fetch(newRequest)
  
  // Clone response so you can modify headers
  const clonedResponse = new Response(response.body, {
    status: response.status,
    statusText: response.statusText,
    headers: new Headers(response.headers),
  })

  // Remove any existing CORS headers to avoid conflicts
  clonedResponse.headers.delete('Access-Control-Allow-Origin')
  clonedResponse.headers.delete('Access-Control-Allow-Methods')
  clonedResponse.headers.delete('Access-Control-Allow-Headers')

  // Add our own CORS headers
  // We need to be specific because the request sets Access-Control-Allow-Credentials which 
  // doesn't allow a wildcard for Access-Control-Allow-Origin
  const origin = request.headers.get('Origin') || 'http://localhost:3000';
  clonedResponse.headers.set('Access-Control-Allow-Origin', origin)
  clonedResponse.headers.set('Access-Control-Allow-Methods', 'GET,HEAD,POST,OPTIONS')
  clonedResponse.headers.set('Access-Control-Allow-Headers', 'Content-Type')
  clonedResponse.headers.set('Access-Control-Allow-Credentials', 'true')

  return clonedResponse
}