code: function _getLogger(level) {
          if (isNaN(level)) return level;
  
          let str = '';
          switch (Number(level)) {
              case 0:
                  str = 'error';
                  break;
              case 1:
                  str = 'warn';
                  break;
              case 2:
                  str = 'info';
                  break;
              case 3:
                  str = 'debug';
                  break;
          }
  
          return str;
      }
docs: 内部方法，计算 logger 对应的字符串