Rocket.Chat/app/utils/client/lib/baseuri.js

import { Meteor } from 'meteor/meteor';

export const baseURI = (() => {
	if (document.baseURI) { return document.baseURI; }

	// Should be exactly one tag:
	//   https://developer.mozilla.org/en-US/docs/Web/HTML/Element/base
	const base = document.getElementsByTagName('base');

	// Return location from BASE tag.
	if (base.length > 0) { return base[0].href; }

	// Else use implementation of documentURI:
	//   http://www.w3.org/TR/DOM-Level-3-Core/core.html#Node3-baseURI
	return document.URL;
})();

Meteor.absoluteUrl.defaultOptions = { ...Meteor.absoluteUrl.defaultOptions, rootUrl: baseURI };