docs: function Preference(key, defaultValue, allowedValues, namespace ) {
  
      this.preferences = storage.localNamespace(namespace || DEFAULT_NS_NAME);
      if (key === undefined) {
          throw new Error('Cannot create preference. Preference "key" name must be specified.');
      }
      this.key = key;
      let value = this.preferences.get(key);
      if (!value && defaultValue) {
          this.preferences.set(key, defaultValue);
          value = defaultValue;
      }
      this.value = value;
      this.allowedValues = allowedValues;
  }