1. 函数的原型prototype：函数才有prototype，prototype是一个对象，指向了当前构造函数的引用地址。

2. 所有对象都有__proto__属性， 所有的__proto__ 指向改对象的原型对象（注意：proto前面是两个__）

关系示意图：

虚线代表：指针

实线：prototype原型对象

实例

    function A() {
        this.a = 1;
    }
    var a = new A();
    var obj = {};

    console.log(a.constructor == A); //实例中无constructor属性，在原型中存在，因为指针链接到a.__proto，输出原型的constructor属性A

    console.log(a.__proto__ === A.prototype);  //ture 实例的__proto__指向构造函数的原型prototype

    console.log(A.__proto__ === Function.prototype);//true 构造函数的__proto__指向 Function的原型prototype
    console.log(A.prototype.constructor === A); //true 构造函数原型prototype的constructor属性，指向构造函数

    console.log(A.prototype.__proto__ === Object.prototype); //所有的prototype都是Object的实例

    console.log(Object.prototype.__proto__ === null); //Object.prototype.__proto__ 是原型链的唯一出口
    console.log(typeof null); //"object"  null是一个空对象指针

2.1 单独添加属性方法

    function Person(){
    }

    Person.prototype.name = "Nicholas";
    Person.prototype.age = 29;
    Person.prototype.job = "Software Engineer";
    Person.prototype.sayName = function(){
        console.log(this.name);
    };

    var person1 = new Person();
    person1.sayName(); //"Nicholas"

    var person2 = new Person();
    person2.sayName(); //"Nicholas"
    //true  访问的都是构造函数原型prototype的方法
    console.log(person1.sayName == person2.sayName);

2.2 对象字面量方法

 a. 采用{}方式

    function Person(name, age, job) {
        this.name = name;
        this.age = age;
        this.job = job;
        this.friends = ["Shelby", "Court"]; //引用类型属性放在构造函数中
    }

    Person.prototype = {
        constructor: Person,
        sayName: function () {
            alert(this.name);
        }
    };

    var person1 = new Person("Nicholas", 29, "Software Engineer");
    var person2 = new Person("Greg", 27, "Doctor");

    person1.friends.push("Van");

    alert(person1.friends); //"Shelby,Court,Van"
    alert(person2.friends); //"Shelby,Court"
    alert(person1.friends === person2.friends); //false
    alert(person1.sayName === person2.sayName); //true

 b. 构造函数和原型结合

  构造函数定义属性，原型模式定义方法和共享的属性

 function Person(name, age, job){
        this.name = name;
        this.age = age;
        this.job = job;
        this.friends = ["Shelby", "Court"];  //引用类型属性放在构造函数中
    }

    Person.prototype = {
        constructor: Person,
        sayName : function () {
            alert(this.name);
        }
    };

    var person1 = new Person("Nicholas", 29, "Software Engineer");
    var person2 = new Person("Greg", 27, "Doctor");

    person1.friends.push("Van");

    alert(person1.friends); //"Shelby,Court,Van"
    alert(person2.friends); //"Shelby,Court"
    alert(person1.friends === person2.friends); //false
    alert(person1.sayName === person2.sayName); //true

2.3 class类

    class A {
        constructor(name) {
            //构造函数
            this.name = name; //给新的对象添加一个name属性
        }

        // sayName相当于 A.prototype.sayName = function(){return this.name}
        sayName() {
            return this.name;
        }
    }

    var a = new A('zhangsan');
    console.log(a);

    function Person() {}

    Person.prototype = {
        constructor: Person,
        name: "Nicholas",
        age: 29,
        job: "Software Engineer",
        friends: ["Shelby", "Court"],

        sayName: function () {
            console.log(this.name);
        }
    };

    var person1 = new Person();
    var person2 = new Person();

    person1.friends.push("Van");

    console.log(person1.friends); //"Shelby,Court,Van"
    console.log(person2.friends); //"Shelby,Court,Van"
    console.log(person1.friends === person2.friends); //true