preserve_advanced_search/js/preserve-advanced-search.js

(function (Drupal, drupalSettings) {
  'use strict';

  Drupal.behaviors.preserveAdvancedSearchParams = {
    attach: function (context, settings) {

      // Capture the a[] advanced search params from the current URL
      const rawSearch = window.location.search;
      const aParams = rawSearch
        .substring(1)
        .split('&')
        .filter(part => part.startsWith('a%5B') || part.startsWith('a['))
        .join('&');

      if (!aParams) return;

      const sliders = settings.facets && settings.facets.sliders;
      if (!sliders) return;

      // Replace any a[] params in a URL with the current page's a[] params.
      // The current URL is always the source of truth — the server-built
      // slider URL may contain stale or partial a[] params.
      function rebuildUrl(url) {
        if (!url) return url;
        const parts = url.split('?');
        const basePath = parts[0];
        const query = parts[1] || '';

        // Keep everything that is NOT an a[] param (i.e. the f[] facet params)
        const keptParams = query
          .split('&')
          .filter(p => p.length > 0)
          .filter(p => !p.startsWith('a%5B') && !p.startsWith('a['))
          .join('&');

        const finalQuery = [keptParams, aParams].filter(Boolean).join('&');
        return basePath + '?' + finalQuery;
      }

      // Range sliders expose a single `url`; single-handle sliders expose a
      // `urls` map keyed by value. Handle both — which one is present depends
      // on the widget the facet is configured with.
      Object.keys(sliders).forEach(function (key) {
        const sliderSettings = sliders[key];

        if (sliderSettings.url) {
          sliderSettings.url = rebuildUrl(sliderSettings.url);
        }

        if (sliderSettings.urls) {
          Object.keys(sliderSettings.urls).forEach(function (urlKey) {
            sliderSettings.urls[urlKey] = rebuildUrl(sliderSettings.urls[urlKey]);
          });
        }
      });
    }
  };

})(Drupal, drupalSettings);