discourse/app/assets/javascripts/discourse-common/addon/helpers/base-url.js

import deprecated from "discourse-common/lib/deprecated";
import getUrl from "discourse-common/lib/get-url";
import { registerRawHelper } from "discourse-common/lib/helpers";

registerRawHelper("base-url", baseUrl);

export default function baseUrl() {
  deprecated("Use `{{base-path}}` instead of `{{base-url}}`");
  return getUrl("");
}