transit-on-booking-dot-com.user.js

// ==UserScript==
// @name         Show Transit on Booking.com
// @namespace    https://easrng.net
// @match        https://www.booking.com/*
// @grant        none
// @version      1.0
// @author       -
// @description  Shows the Google Maps transit layer on Booking.com maps.
// @run-at       document-start
// ==/UserScript==

(function () {
  "use strict";

  function defineTrap(parent, prop, callback) {
    let value;
    Object.defineProperty(parent, prop, {
      configurable: true,
      enumerable: true,
      get: () => value,
      set: (newValue) => {
        value = newValue;
        callback(value);
      },
    });
  }

  defineTrap(window, "google", (google) => {
    defineTrap(google, "maps", (maps) => {
      defineTrap(maps, "Map", (OriginalMap) => {
        if (OriginalMap.__isProxy) return;

        const MapProxy = function (div, options) {
          options.mapId = null;
          const instance = new OriginalMap(div, options);
          const transitLayer = new google.maps.TransitLayer();
          transitLayer.setMap(instance);
          return instance;
        };

        MapProxy.__isProxy = true;
        Object.setPrototypeOf(MapProxy, OriginalMap);
        MapProxy.prototype = OriginalMap.prototype;

        maps.Map = MapProxy;
      });
    });
  });
})();