code: function (update) {
          var waiting = this.waiting,
              length = waiting.length,
              index,
              progress;
  
          for (index = 0; index < length; index += 1) {
              progress = waiting[index].progress;
              if (progress) {
                  progress(update);
              }
          }
          return this;
      }
docs: Call progress on every waiting function to notify that the promise was updated
  
  @param {*} update
  @return this