function foo(){}
foo.prototype.z = 3;
var obj = new foo();
obj.x=1;
obj.y=2;
obj.x //1
obj.y //2
obj.z //3
typeof obj.toString; //function
obj.valueOf(); // foo {x: 1, y: 2, z: 3}
obj.hasOwnProperty('z'); //false

var obj2 = Object.create(null);
obj2.valueOf(); //undefined

//声明一个构造函数Person
function Person(name,age){
  this.name = name;
  this.age = age;
}
Person.prototype.hi = function (){
  console.log('Hi,my name is ' + this.name +',my age is '+this.age);
};
Person.prototype.LEGS_NUM=2;
Person.prototype.ARMS_NUM=2;
Person.prototype.walk = function (){
  console.log(this.name+' is walking !');
};
function Student(name,age,classNum){
  Person.call(this,name,age);
  this.classNum = classNum;
}
//创建一个空对象
Student.prototype = Object.create(Person.prototype);
//constructor指定创建一个对象的函数。
Student.prototype.constructor = Student;
Student.prototype.hi = function (){
  console.log('Hi,my name is ' + this.name +',my age is '+this.age+' and my class is '+this.classNum);
};
Student.prototype.learns = function (sub){
  console.log(this.name+' is learning '+sub);
};
//实例化一个对象Bosn
var Bosn = new Student('bosn',27,'Class 3');
Bosn.hi(); //Hi,my name is bosn,my age is 27 and my class is Class 3
Bosn.LEGS_NUM; //2
Bosn.walk(); //bosn is walking !
Bosn.learns('Math'); //bosn is learning Math

function Person(name,age){
  this.name = name;
  this.age = age;
}
function Student(){

}
Student.prototype = Person.prototype; //1
Student.prototype = Object.create(Person.prototype); //2
Student.prototype = new Person(); //3