code: function _appendMappedString(formatString, mapping, beginIndex, currentIndex, resultString) {
    let tempString;
  
    if (beginIndex !== -1) {
      tempString = formatString.substring(beginIndex, currentIndex);
      // check if the temporary string has a known mapping
      if (mapping[tempString]) {
        tempString = mapping[tempString];
      }
      resultString = resultString.concat(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}  beginIndex      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