code: function keys(obj) {
      var keyArray = [];
      var key;
  
      for (key in obj) {
          if (obj.hasOwnProperty(key)) {
              keyArray.push(key);
          }
      }
  
      return keyArray;
  }
docs: Return a key-list(array) of a given object
  @param {object} obj - Object from which a key-list will be extracted
  @returns {Array} A key-list(array)
  @memberof tui.util