types

// Create custom basemap to be added to the array of portal basemaps
const basemapGallery = new BasemapGallery({
   view: view,
   source: {
     query: {
       title: "United States Basemaps",
       owner: "Esri_cy_US"
     },
     updateBasemapsCallback: (items) => {
       // create custom basemap to be added to the array of portal basemaps
       let bm = new Basemap({
         portalItem: {
           id: "8dda0e7b5e2d4fafa80132d59122268c"  // WGS84 Streets Vector webmap
         }
       });
       // add basemap to the array
       items.push(bm);
       // return the array of basemaps
       return items;
     }
   }
});

// Add current basemap if not already present
const currentBasemap = map.basemap;
const basemapGallery = new BasemapGallery({
  view,
  source: {
    portal: "https://arcgis.com",
    async updateBasemapsCallback(items) {
      for (const item of items) {
        // load basemap for proper comparison
        await item.load();
        if (basemapGallery.viewModel.basemapEquals(currentBasemap, item))
          return items;
      }
      // add the current basemap if not found in the array of basemap items
      return [currentBasemap, ...items];
    }
  }
});