docs: function getIndex(index) {
          if (this[_privateKey]._options.keysIgnoreCase && typeof index === 'string') {
              const indexLowerCase = index.toLowerCase();
              for (const key in this[_privateKey]._schema) {
                  if (typeof key === 'string' && key.toLowerCase() === indexLowerCase) {
                      return key;
                  }
              }
          }
  
          return index;
      }