code: function makeComputeMeanDistance(fr) {
    return function(descriptors, inputDescriptor) {
      return round(
        descriptors
          .map(d => fr.distance(d, inputDescriptor))
          .reduce((d1, d2) => d1 + d2, 0)
            / (descriptors.length || 1)
        )
    }
  }
docs: /* compute the mean value of the euclidean distances of the input descriptor /* to each of the face descriptors, which the recognizer has been trained on, /* this is used as the metric to judge how similar the faces are to the training data