getBufferFullness() {
    if (this.video_) {
      const bufferedLength = this.video_.buffered.length;
      const bufferedEnd =
          bufferedLength ? this.video_.buffered.end(bufferedLength - 1) : 0;
      const bufferingGoal = this.getConfiguration().streaming.bufferingGoal;
      const lengthToBeBuffered = Math.min(this.video_.currentTime +
          bufferingGoal, this.seekRange().end);

      if (bufferedEnd >= lengthToBeBuffered) {
        return 1;
      } else if (bufferedEnd <= this.video_.currentTime) {
        return 0;
      } else if (bufferedEnd < lengthToBeBuffered) {
        return ((bufferedEnd - this.video_.currentTime) /
            (lengthToBeBuffered - this.video_.currentTime));
      }
    }
    return 0;
  }
Returns the ratio of video length buffered compared to buffering Goal
@return {number}
@export