code: function loadSequenceGroup(files) {
        const fetchSequence = [];
  
        for (let i = 0; i < files.length; i++) {
          fetchSequence.push(
            new Promise((resolve, reject) => {
              const myReader = new FileReader();
              // should handle errors too...
              myReader.addEventListener('load', function(e) {
                resolve(e.target.result);
              });
              myReader.readAsArrayBuffer(files[i].file);
            }).then(function(buffer) {
              return { url: files[i].file.name, buffer };
            })
          );
        }
  
        return Promise.all(fetchSequence)
          .then(rawdata => {
            return loader.parse(rawdata);
          })
          .then(function(series) {
            seriesContainer.push(series);
          })
          .catch(function(error) {
            window.console.log('oops... something went wrong...');
            window.console.log(error);
          });
      }
docs: Load group sequence