CORS Proxy server for Google Cloud Run functions

index.js

const functions = require('@google-cloud/functions-framework');
const compression = require('compression');
const express = require('express');

const app = express();
app.use(compression());
app.disable('x-powered-by');

app.all('/', async (req, res) => {
  try {
    if (!req.query.url) {
      return res.status(400).send('Missing "url" query parameter.');
    }

    let targetUrl;
    try {
      targetUrl = new URL(req.query.url);
    } catch {
      return res.status(400).send('Invalid "url" query parameter.');
    }

    res.set('Access-Control-Allow-Origin', req.headers['origin'] || '*');

    if (req.method === 'OPTIONS') {
      res.set('Access-Control-Allow-Methods', 'GET, HEAD, PUT, PATCH, POST, DELETE, OPTIONS');
      res.set('Access-Control-Allow-Headers', req.headers['access-control-request-headers'] || '*');
      res.set('Access-Control-Allow-Credentials', 'true');
      res.set('Access-Control-Max-Age', '3600');
      return res.sendStatus(204);
    }

    const targetHeaders = { ...req.headers };
    delete targetHeaders['connection'];
    delete targetHeaders['content-length'];
    delete targetHeaders['origin'];
    delete targetHeaders['proxy-connection'];
    delete targetHeaders['referer'];
    delete targetHeaders['transfer-encoding'];
    targetHeaders['host'] = targetUrl.host;

    const fetchOptions = {
      method: req.method,
      headers: targetHeaders,
    };

    if (req.method !== 'GET' && req.method !== 'HEAD') {
      fetchOptions.body = req.rawBody;
    }

    const targetResponse = await fetch(targetUrl, fetchOptions);

    for (const [header, value] of targetResponse.headers.entries()) {
      const lowerHeaderName = header.toLowerCase();
      if (
        !lowerHeaderName.startsWith('access-control-') &&
        lowerHeaderName !== 'connection' &&
        lowerHeaderName !== 'content-encoding' &&
        lowerHeaderName !== 'content-length' &&
        lowerHeaderName !== 'proxy-connection' &&
        lowerHeaderName !== 'transfer-encoding'
      ) {
        res.set(header, value);
      }
    }

    const rawBody = Buffer.from(await targetResponse.arrayBuffer());
    res.status(targetResponse.status).send(rawBody);
  } catch (error) {
    res.status(500).send(`Proxy error: ${error.message}`);
  }
});

functions.http('corsProxy', app);