var obj = {
  name: 'John',
  age: 23,
  sex: '男',
  school: '清北'
};

//重点
function extend(obj) {
 ​var o = {},
 ​attr = Array.prototype.slice.call(arguments).slice(1);
 ​attr.forEach(function(val, index) {
 ​ ​if (val in obj) {
 ​ ​ ​o[val] = obj[val];
 ​ ​}
 ​});
 ​return o;
}
console.log(extend(obj, 'name', 'school')); // { "name": "John", "school": "清北"}