Configure custom DNS lookuper for NodeJS' superagent package

demo.js

'use strict';

const dns = require('dns');
const superagent = require('superagent');

const hostIndexes = new Map();
require('./superagent-custom-lookup')(superagent, (addresses, host) => {
  let index = hostIndexes.get(host) || 0;
  const result = addresses[index];
  index = (index + 1) % addresses.length;
  hostIndexes.set(host, index);
  return result;
});

(function test() {
  superagent.get('http://test/')
    .then(res => {
      console.log(res.text.substr(0, 100));
      setTimeout(test, 1000);
    })
    .catch((error) => {
      console.error(error);
      setTimeout(test, 1000);
    });
})();

superagent-custom-lookup.js

'use strict';

const dns = require('dns');

module.exports = function superagentCustomLookup(superagent, handler) {
  handler = handler || ((addresses) => addresses[(Math.random() * addresses.length) | 0]);

  function lookup(host, options, callback) {
    if (options.all) {
      return dns.lookup(host, options, callback);
    }
    options = Object.assign({}, options, { all: true });

    return dns.lookup(host, options, (err, addresses, family) => {
      if (err) {
        return callback(err, addresses, family);
      }

      const addr = handler(addresses, host);
      return callback(err, addr.address, addr.family || family);
    });
  }

  for (const protocol_key of ['http:', 'https:']) {
    const protocol = superagent.protocols[protocol_key];
    const request = protocol.request;

    superagent.protocols[protocol_key] = Object.assign({}, protocol, {
      request: function (options, cb) {
        if (!options.lookup) {
          options = Object.assign({}, options, {
            lookup,
          })
        }
        return request(options, cb);
      },
    });
  }
};