Upload multiple files transactionally in Firebase Storage

firebase_storage_multi_file_upload.js

// set it up
firebase.storage().ref().constructor.prototype.putFiles = function(files) { 
  var ref = this;
  return Promise.all(files.map(function(file) {
    return ref.child(file.name).put(file);
  }));
}

// use it!
firebase.storage().ref().putFiles(files).then(function(metadatas) {
  // Get an array of file metadata
}).catch(function(error) {
  // If any task fails, handle this
});

hello, how can i retrieve the downloadUrl of the uploaded files ?
thank you

With "transactionally" , do you mean that if we upload two file and the second file upload fail for some reason, then if the first file upload was completed correctly, it will be canceled from the storage without any effort from me?