socket.io/lib/transports/index.ts

import { Polling as XHR } from "./polling";
import { JSONP } from "./polling-jsonp";
import { WebSocket } from "./websocket";
import { WebTransport } from "./webtransport";

export default {
  polling: polling,
  websocket: WebSocket,
  webtransport: WebTransport,
};

/**
 * Polling polymorphic constructor.
 */

function polling(req) {
  if ("string" === typeof req._query.j) {
    return new JSONP(req);
  } else {
    return new XHR(req);
  }
}

polling.upgradesTo = ["websocket", "webtransport"];