function getTypeName(featurePrefix, featureType) {
  featurePrefix = featurePrefix ? featurePrefix : FEATURE_PREFIX;
  const prefix = featurePrefix + ':';
  // The featureType already contains the prefix.
  if (featureType.startsWith(prefix)) {
    return featureType;
  }
  return prefix + featureType;
}
@param {string|undefined} featurePrefix The prefix of the feature.
@param {string} featureType The type of the feature.
@return {string} The value of the typeName property.