Vercel Function to Echo HTTP Headers

echo-http-headers-with-real-client-ip-from-cloudflare.js

// "cf-connecting-ip" should contain true client IP when Cloudflare managed 
// transform rule ("Add visitor location headers") is enabled.
// See: https://share.marketkarma.com/cloudflare-managed-transform-add-geoip-http-headers.png

const HTTP_OK = 200;
const HTTP_INTERNAL_SERVER_ERROR = 500;

/**
 * Filters out headers that are not allowed, sorts headers alphabetically,
 * and converts headers to JSON.
 * @param {Object} headers - The headers object.
 * @returns {Object} - The filtered and sorted headers object.
 */
const filterAndSortHeaders = (headers) => {
  return Object.keys(headers)
    .sort((a, b) => a.localeCompare(b))
    .filter(header => allowedHeadersSet.has(header))
    .reduce((obj, header) => {
      obj[header] = headers[header];
      return obj;
    }, {});
};

/**
 * Sets CORS headers for the response.
 * @param {Object} res - The response object.
 */
const setCORSHeaders = (res) => {
  res.setHeader('Access-Control-Allow-Origin', '*');
  res.setHeader('Access-Control-Allow-Methods', 'GET,OPTIONS');
  res.setHeader('Access-Control-Allow-Headers', 'Content-Type');
};

/**
 * Returns the HTTP headers of the request.
 * @param {Object} req - The request object.
 * @param {Object} res - The response object.
 * @returns {Promise<void>} - A promise that resolves with the response.
 */
module.exports = async (req, res) => {
  try {
    const prettyPrintedJSON = JSON.stringify({
      headers: filterAndSortHeaders(req.headers)
    }, null, 2);

    setCORSHeaders(res);
    res.setHeader('Content-Type', 'application/json');
    res.status(HTTP_OK).send(prettyPrintedJSON);
  } catch (error) {
    console.error(`Error processing request: ${error.message}`);
    const errorMessage = process.env.NODE_ENV === 'development' ? `Internal server error: ${error.message}` : 'Internal server error';
    res.status(HTTP_INTERNAL_SERVER_ERROR).send(errorMessage);
  }
};

/**
 * An array of allowed headers.
 * @type {string[]}
 */
const allowedHeadersSet = new Set([
  "accept",
  "accept-encoding",
  "accept-language",
  "cache-control",
  "cdn-loop",
  "cf-connecting-ip",
  "cf-ipcity",
  "cf-ipcontinent",
  "cf-ipcountry",
  "cf-iplatitude",
  "cf-iplongitude",
  "cf-metro-code",
  "cf-postal-code",
  "cf-ray",
  "cf-region",
  "cf-region-code",
  "cf-timezone",
  "cf-visitor",
  "connection",
  "forwarded",
  "host",
  "priority",
  "sec-ch-ua",
  "sec-ch-ua-mobile",
  "sec-ch-ua-platform",
  "sec-fetch-dest",
  "sec-fetch-mode",
  "sec-fetch-site",
  "sec-fetch-user",
  "upgrade-insecure-requests",
  "user-agent",
  "x-forwarded-for",
  "x-forwarded-host",
  "x-forwarded-proto",
  "x-real-ip",
  "x-vercel-deployment-url",
  "x-vercel-forwarded-for",
  "x-vercel-id",
  "x-vercel-ip-city",
  "x-vercel-ip-country",
  "x-vercel-ip-country-region",
  "x-vercel-ip-latitude",
  "x-vercel-ip-longitude",
  "x-vercel-ip-timezone",
  "x-vercel-proxied-for",
  "x-vercel-proxy-signature",
  "x-vercel-proxy-signature-ts"
]);