code: function objectSubset (obj, names) {
    let result = {}
    for (let i = 0; i < names.length; i++) {
      if (obj[names[i]] !== undefined) {
        result[names[i]] = obj[names[i]]
      }
    }
    return result
  }
docs: 获取 obj 的子集
  
  @param {Object} obj obj
  @param {Array.<string>} names property names
  @return {Object} 子集