Firebase Cloud Functions Code for CORS proxy.

index.js

const admin = require('firebase-admin');
const cors = require('cors')({ origin: true });
const fetch = require('node-fetch');
const functions = require('firebase-functions');

exports.cors = functions.https.onRequest((req, res) => {
  // Reject requests that don't have a URL.
  if (!req.query.url) return res.json({ error: 'No URL provided!' });

  // Extract necessary information from the request.
  const { body, headers, method } = req;

  // deleting these headers prevents the following error:
  // "Hostname/IP does not match certificate's altnames"
  //
  // See this GitHub issue answer for more
  // https://github.com/node-fetch/node-fetch/discussions/1678#discussioncomment-4191424
  delete headers.host;
  delete headers.referer;

  // Create the "options" object for the fetch request.
  const options = { headers, method };

  // Add the body to "options", if it exists and if this is not a GET request.
  if (body && method !== 'GET') options.body = body;

  // Send the request to the URL provided and return the response.
  // Wrapping fetch in cors() sets the necessary CORS headers.
  return cors(req, res, () =>
    // decoding the URI is important if in case queries were pre-encoded.
    fetch(decodeURI(req.query.url), options)
      .then((r) =>
        r.headers.get('content-type') === 'application/json'
          ? r.json()
          : r.text()
      )
      .then((r) => res.send(r))
      .catch((e) => res.json({ error: e.message }))
  );
});