code: function withKeys(obj, keyList) {
      var retObject = {};
      var index, ptr;
  
      if (typeof obj != "object") {
          return retObject;
      }
  
      if (typeof keyList != "object"
          || !keyList.hasOwnProperty("length")
          || keyList.length <= 0) {
          return obj;
      }
  
      for (index in obj) {
          if (obj.hasOwnProperty(index)) {
              ptr = keyList.indexOf(index);
              if (ptr >= 0) {
                  retObject[index] = obj[index];
              }
          }
      }
  
      return retObject;
  }
docs: 保留指定字段
  @param {Object} obj
  @param {Array} keyList
  @returns {*}