Upload-Fortschritt Wert auf Status ändern, in FB, aber es wird zurückkehren "Nan" wenn ich drucken progress-Wert.

Also wie kann man die Zahl anstelle von "NAN'

function getfile() {

  const ref = firebase.storage().ref('images/');
  const file = $('#file-upload').get(0).files[0];
  const name = (+new Date()) + '-' + file.name;
  const metadata = {
    contentType: file.type
  };
  const task = ref.child(name).put(file, metadata);

  task.on('state_changed',

      function progress(snapshot) {
        var percentage =
          (snapshot.byteTransferred / snapshot.totalBytes) * 100;

        console.log(percentage);
      },

      function error(err) {

      },

      function complete() {
        console.log('Submited Sucessfully');

      }