code: function (formatString, mapping, startIndex, currentIndex, resultString) {
      if (startIndex !== -1) {
        var tempString = formatString.substring(startIndex, currentIndex);
  
        // check if the temporary string has a known mapping
        if (mapping[tempString]) {
          tempString = mapping[tempString];
        }
  
        resultString += tempString;
      }
  
      return resultString;
    }
docs: Checks if the substring is a mapped date format pattern and adds it to the result format String.
  
  @function _appendMappedString
  @param {String}  formatString    The unmodified format String.
  @param {Object}  mapping         The date format mapping Object.
  @param {Number}  startIndex      The begin index of the continuous format characters.
  @param {Number}  currentIndex    The last index of the continuous format characters.
  @param {String}  resultString    The result format String.
  @returns {String}
  @private