js面向对象2--原型

一、原型和原型对象

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

二、原型的创建方式

js原型链是什么？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); //falsealert(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); //falsealert(person1.sayName === person2.sayName); //true

javascript面向对象编程、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);

三、 原型对象属性为引用类型

    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