node-http-proxy/lib/http-proxy/passes/web-outgoing.js

const url    = require('url'),
      common = require('../common');


const redirectRegex = /^201|30(1|2|7|8)$/;

/*!
 * Array of passes.
 *
 * A `pass` is just a function that is executed on `req, res, options`
 * so that you can easily add new checks while still keeping the base
 * flexible.
 */

module.exports = { // <--

  /**
   * If is a HTTP 1.0 request, remove chunk headers
   *
   * @param {ClientRequest} Req Request object
   * @param {IncomingMessage} Res Response object
   * @param {proxyResponse} Res Response object from the proxy request
   *
   * @api private
   */
  removeChunked: function removeChunked(req, res, proxyRes) {
    if (req.httpVersion === '1.0') {
      delete proxyRes.headers['transfer-encoding'];
    }
  },

  /**
   * If is a HTTP 1.0 request, set the correct connection header
   * or if connection header not present, then use `keep-alive`
   *
   * @param {ClientRequest} Req Request object
   * @param {IncomingMessage} Res Response object
   * @param {proxyResponse} Res Response object from the proxy request
   *
   * @api private
   */
  setConnection: function setConnection(req, res, proxyRes) {
    if (req.httpVersion === '1.0') {
      proxyRes.headers.connection = req.headers.connection || 'close';
    } else if (req.httpVersion !== '2.0' && !proxyRes.headers.connection) {
      proxyRes.headers.connection = req.headers.connection || 'keep-alive';
    }
  },

  setRedirectHostRewrite: function setRedirectHostRewrite(req, res, proxyRes, options) {
    if ((options.hostRewrite || options.autoRewrite || options.protocolRewrite)
        && proxyRes.headers['location']
        && redirectRegex.test(proxyRes.statusCode)) {

      const u = new URL(proxyRes.headers['location']);

      // make sure the redirected host matches the target host before rewriting
      if (options.target.host != u.host) {
        return;
      }

      if (options.hostRewrite) {
        u.host = options.hostRewrite;
      } else if (options.autoRewrite) {
        u.host = req.headers['host'];
      }
      if (options.protocolRewrite) {
        u.protocol = options.protocolRewrite;
      }

      proxyRes.headers['location'] = u.toString();
    }
  },
  /**
   * Copy headers from proxyResponse to response
   * set each header in response object.
   *
   * @param {ClientRequest} Req Request object
   * @param {IncomingMessage} Res Response object
   * @param {proxyResponse} Res Response object from the proxy request
   * @param {Object} Options options.cookieDomainRewrite: Config to rewrite cookie domain
   *
   * @api private
   */
  writeHeaders: function writeHeaders(req, res, proxyRes, options) {
    const preserveHeaderKeyCase = options.preserveHeaderKeyCase;
    let rewriteCookieDomainConfig = options.cookieDomainRewrite,
        rewriteCookiePathConfig = options.cookiePathRewrite,
        rawHeaderKeyMap;

    const setHeader = function(key, header) {
      if (header == undefined) return;
      if (rewriteCookieDomainConfig && key.toLowerCase() === 'set-cookie') {
        header = common.rewriteCookieProperty(header, rewriteCookieDomainConfig, 'domain');
      }
      if (rewriteCookiePathConfig && key.toLowerCase() === 'set-cookie') {
        header = common.rewriteCookieProperty(header, rewriteCookiePathConfig, 'path');
      }
      res.setHeader(String(key).trim(), header);
    };

    if (typeof rewriteCookieDomainConfig === 'string') { //also test for ''
      rewriteCookieDomainConfig = { '*': rewriteCookieDomainConfig };
    }

    if (typeof rewriteCookiePathConfig === 'string') { //also test for ''
      rewriteCookiePathConfig = { '*': rewriteCookiePathConfig };
    }

    // message.rawHeaders is added in: v0.11.6
    // https://nodejs.org/api/http.html#http_message_rawheaders
    if (preserveHeaderKeyCase && proxyRes.rawHeaders != undefined) {
      rawHeaderKeyMap = {};
      for (let i = 0; i < proxyRes.rawHeaders.length; i += 2) {
        let key = proxyRes.rawHeaders[i];
        rawHeaderKeyMap[key.toLowerCase()] = key;
      }
    }

    Object.keys(proxyRes.headers).forEach(function(key) {
      const header = proxyRes.headers[key];
      if (preserveHeaderKeyCase && rawHeaderKeyMap) {
        key = rawHeaderKeyMap[key] || key;
      }
      setHeader(key, header);
    });
  },

  /**
   * Set the statusCode from the proxyResponse
   *
   * @param {ClientRequest} Req Request object
   * @param {IncomingMessage} Res Response object
   * @param {proxyResponse} Res Response object from the proxy request
   *
   * @api private
   */
  writeStatusCode: function writeStatusCode(req, res, proxyRes) {
    // From Node.js docs: response.writeHead(statusCode[, statusMessage][, headers])
    if(proxyRes.statusMessage) {
      res.statusCode = proxyRes.statusCode;
      res.statusMessage = proxyRes.statusMessage;
    } else {
      res.statusCode = proxyRes.statusCode;
    }
  }

};