code: function authVectorTileLoader(fetchOptions) {
    return function(tile, url) {
      const loader = () => {
        fetch(url, fetchOptions)
          .then(r => r.arrayBuffer())
          .then((source) => {
            const format = tile.getFormat();
            tile.setProjection(format.readProjection(source));
            tile.setFeatures(format.readFeatures(source));
            tile.setExtent(format.getLastExtent());
          })
          .catch((err) => {
            tile.onError();
          });
      };
  
      tile.setLoader(loader);
    };
  }
docs: Load a vector tile with full fetchOptions
  This is only used when the Authorization header has been set
  in the fetchOptions.
  
  @param {Object} fetchOptions Options to use for fetch calls.
  
  @returns {Function} Tile loading function which uses fetch.