code: function _charIsPrintable(charCode) {
      // C0 control characters
      if((charCode >=0 && charCode <= 0x1F) || 0x7F === charCode) {
        return false;
      }
      // C1 control characters
      if(charCode >= 0x80 && charCode <= 0x9F) {
        return false;
      }
      if(-1 !== _downKeys.indexOf(this.CTRL)) {
        return false;
      }
      return true;
    }
docs: Private functions Return the char corresponding to the key if any