Xiaobai talks about the understanding of JS prototype chain_javascript skills

The prototype chain is a bit confusing to understand, and there is a lot of information on the Internet. Every time I can’t sleep at night, I always like to find some articles on prototype chains and closures on the Internet, and the effect is excellent.

Don’t get hung up on that bunch of terminology, it really won’t do anything for you except twisting your mind. Let’s look at the prototype chain simply and crudely, and think about things that have nothing to do with code, such as humans, monsters, and shemales.

1) Humans are born from human beings, monsters are born from monsters. Humans and demons are both object instances, and humans and demons are prototypes. Prototypes are also objects, called prototype objects.

2) Having sex with a human mother and a human father can give birth to a bunch of human babies, and having sex with a demon mother and a demon dad can give birth to a bunch of demon babies. Having sex is the constructor, commonly known as creating humans.

3) Renfu will record the sex information, so you can find the sex information through Renfu, which means you can find the constructor through the prototype object.

4) Humans can give birth to many babies, but these babies only have one mother. This is the uniqueness of the prototype.

5) Human beings are also born from human beings. Through human beings, we can find other human beings, and then through human beings, we can find other human beings. This relationship is called a prototype chain. .

6) The prototype chain is not infinite. When you keep looking up through people, you will finally find that none of the people... are not people, which is where the prototype chain ultimately points. null.

7) The person born from a human will look like a human, and the monster born from a monster will be ugly like a monster. This is called inheritance.

8) You inherited your mother’s skin color, your mother inherited your mother’s skin color, your mother’s skin color, your mother’s..., this is the inheritance of the prototype chain.

9) If you don’t have a home, then your home refers to your mother’s home; if your mother doesn’t have a home, then your home refers to your mother’s home… This is the upward search of the prototype chain.

10) You will inherit your mother's look, but you can also dye your hair, wash it, cut it and blow it, which means that the properties of the object can be customized and will override the inherited properties.

11) Although your hair has been dyed yellow after washing, cutting, and blowing, you cannot change the appearance of your mother. The younger brothers and sisters born to your mother have nothing to do with your yellow hair, which means that the object instance cannot be changed. Prototype properties.

12) But if your house is burned down by you playing with fire, it means that your house, your mother’s house, and your brothers’ houses have all been burned down. This is the sharing of prototype attributes.

13) Your mother’s nickname is A-Zhen, and the aunts next door call you A-Zhen’er, but your mother’s hair has been changed from Rejoice to a Golden Retriever Lion Queen, and the aunts next door have changed their name to call you Golden Retriever Lion Prince. This is called the prototype dynamic. sex.

14) Your mother loves beauty, so she went to Korea to have plastic surgery, so that your mother can’t even recognize her. Even though your mother’s hair has changed back to rejoicing, the neighbor next door still calls you the Golden Retriever Prince. Because no one recognized your mother, your mother has been remade after plastic surgery. This is a complete rewrite of the prototype.

Nah! You have enough! Don't BB! Show me the code!

function Person (name) { this.name = name; }
function Mother () { }
Mother.prototype = { //Mother的原型
age: 18,
home: ['Beijing', 'Shanghai']
};
Person.prototype = new Mother(); //Person的原型为Mother
//用chrome调试工具查看，提供了__proto__接口查看原型
var p1 = new Person('Jack'); //p1:'Jack'; __proto__:18,['Beijing','Shanghai']
var p2 = new Person('Mark'); //p2:'Mark'; __proto__:18,['Beijing','Shanghai']
p1.age = 20; 
/* 实例不能改变原型的基本值属性，正如你洗剪吹染黄毛跟你妈无关
* 在p1实例下增加一个age属性的普通操作，与原型无关。跟var o{}; o.age=20一样。
* p1：下面多了个属性age，而__proto__跟 Mother.prototype一样，age=18。
* p2：只有属性name，__proto__跟 Mother.prototype一样
*/
p1.home[0] = 'Shenzhen'; 
/* 原型中引用类型属性的共享，正如你烧了你家，就是烧了你全家的家
* 这个先过，下文再仔细唠叨一下可好？
* p1：'Jack',20; __proto__:18,['Shenzhen','Shanghai']
* p2：'Mark'; __proto__:18,['Shenzhen','Shanghai']
*/
p1.home = ['Hangzhou', 'Guangzhou']; 
/* 其实跟p1.age=20一样的操作。换成这个理解: var o{}; o.house=['big','house']
* p1：'Jack',20,['Hangzhou','Guangzhou']; __proto__:18,['Shenzhen','Shanghai']
* p2：'Mark'; __proto__:18,['Shenzhen','Shanghai']
*/
delete p1.age; 
/* 删除自定义的属性之后，原本被覆盖的原型值就重见天日了。这里就是向上搜索机制，所以才有下面的动态性
* p1：'Jack',['Hangzhou','Guangzhou']; __proto__:18,['Shenzhen','Shanghai']
* p2：'Mark'; __proto__:18,['Shenzhen','Shanghai']
*/
Person.prototype.lastName = 'Jin'; 
/* 改写原型，动态反应到实例中。正如你妈变新潮了，邻居提起你都说是潮妇的儿子
* 注意，这里我们改写的是Person的原型，就是往Mother里加一个lastName属性，等同于Mother.lastName='Jin'
* 这里并不是改Mother.prototype，改动不同的层次，效果往往会有很大的差异。
* p1：'Jack',['Hangzhou','Guangzhou']; __proto__:'jin';__proto__:18,['Shenzhen','Shanghai']
* p2：'Mark'; __proto__:'jin';__proto__:18,['Shenzhen','Shanghai']
*/
Person.prototype = { 
age: 28, 
address: { country: 'USA', city: 'Washington' }
};
var p3 = new Person('Obama'); 
/* 重写原型！这个时候Person的原型已经完全变成一个新的对象了，也就是说Person换了个娘。
* 换成这样理解：var a=10; b=a; a=20; c=a。所以b不变，变得是c，所以p3变化了，与Mother无关。
* p1：'Jack',['Hangzhou','Guangzhou']; __proto__:'jin';__proto__:18,['Shenzhen','Shanghai']
* p2：'Mark'; __proto__:'jin';__proto__:18,['Shenzhen','Shanghai']
* p3:'Obama';__proto__: 28 {country: 'USA', city: 'Washington'}
*/
Mother.prototype.no = 9527;
/* 改写原型的原型，动态反应到实例中。正如你妈他妈变新潮了，邻居提起你都说你丫外婆真潮
* 注意，这里我们改写的是Mother.prototype，p1p2会变，但上面p3跟Mother已经了无瓜葛了，不影响他。
* p1：'Jack',['Hangzhou','Guangzhou']; __proto__:'jin';__proto__:18,['Shenzhen','Shanghai'],9527
* p2：'Mark'; __proto__:'jin';__proto__:18,['Shenzhen','Shanghai'],9527
* p3:'Obama';__proto__: 28 {country: 'USA', city: 'Washington'}
*/
Mother.prototype = { 
car: 2, 
hobby: ['run','walk']
};
var p4 = new Person('Tony');
/* 重写原型的原型！这个时候Mother的原型已经完全变成一个新的对象了！
* 由于上面Person与Mother已经断开联系了，这时候Mother怎么变已经不影响Person了。
* p4:'Tony';__proto__: 28 {country: 'USA', city: 'Washington'}
*/
Person.prototype = new Mother(); //再次绑定
var p5 = new Person('Luffy');
// 这个时候如果需要应用这些改动的话，那就要重新将Person的原型绑到mother上了
// p5:'Luffy';__proto__: 2, ['run','walk']
p1.__proto__.__proto__.__proto__.__proto__ //null，你说原型链的终点不是null？
Mother.__proto__.__proto__.__proto__ //null，你说原型链的终点不是null？ 

Can you basically understand it after reading it?

Now let’s talk about the difference between p1.age = 20, p1.home = ['Hangzhou', 'Guangzhou'] and p1.home[0] = 'Shenzhen'. p1.home[0] = 'Shenzhen'; To sum up, it is in the form of p1.object.method, p1.object.property.

p1.age = 20; p1.home = ['Hangzhou', 'Guangzhou']; These two sentences are relatively easy to understand. Forget about the prototype first, think about how we add attributes to an ordinary object:

var obj = new Object();
obj.name='xxx'; 
obj.num = [100, 200]; 

Does this make sense? Same thing.

Then why does p1.home[0] = 'Shenzhen' not create a home array attribute under p1 and then set its first position to 'Shenzhen'? Let's forget about this first. Think about the obj object above. If it is written like this: var obj.name = 'xxx', obj.num = [100, 200], can you get the result you want? Obviously, you get nothing but an error. Because obj has not been defined yet, how can we add things to it? In the same way, home in p1.home[0] is not defined under p1, so home[0] cannot be directly defined in one step. If you want to create a home array under p1, of course you would write it like this:

p1.home = []; 
p1.home[0] = 'Shenzhen'; 

这不就是我们最常用的办法吗？

而之所以 p1.home[0] = 'Shenzhen' 不直接报错，是因为在原型链中有一个搜索机制。当我们输入 p1.object 的时候，原型链的搜索机制是先在实例中搜索相应的值，找不到就在原型中找，还找不到就再往上一级原型中搜索……一直到了原型链的终点，就是到null还没找到的话，就返回一个 undefined。当我们输入 p1.home[0] 的时候，也是同样的搜索机制，先搜索 p1 看有没有名为 home 的属性和方法，然后逐级向上查找。最后我们在Mother的原型里面找到了，所以修改他就相当于修改了 Mother 的原型啊。

一句话概括：p1.home[0] = 'Shenzhen' 等同于 Mother.prototype.home[0] = 'Shenzhen'。

由上面的分析可以知道，原型链继承的主要问题在于属性的共享，很多时候我们只想共享方法而并不想要共享属性，理想中每个实例应该有独立的属性。因此，原型继承就有了下面的两种改良方式：

1）组合继承

function Mother (age) {
this.age = age;
this.hobby = ['running','football']
}
Mother.prototype.showAge = function () {
console.log(this.age); 
};
function Person (name, age) { 
Mother.call(this, age);　　//第二次执行
this.name = name; 
}
Person.prototype = new Mother(); 　//第一次执行
Person.prototype.constructor = Person;
Person.prototype.showName = function () {
console.log(this.name);
}
var p1 = new Person('Jack', 20); 
p1.hobby.push('basketball'); //p1:'Jack'; __proto__:20,['running','football']
var p2 = new Person('Mark', 18); //p2:'Mark'; __proto__:18,['running','football'] 

结果是酱紫的：

 

这里第一次执行的时候，得到 Person.prototype.age = undefined, Person.prototype.hobby = ['running','football']，第二次执行也就是 var p1 = new Person('Jack', 20) 的时候，得到 p1.age =20, p1.hobby = ['running','football']，push后就变成了 p1.hobby = ['running','football', 'basketball']。其实分辨好 this 的变化，理解起来也是比较简单的，把 this 简单替换一下就能得到这个结果了。 如果感觉理解起来比较绕的话，试着把脑子里面的概念扔掉吧，把自己当浏览器从上到下执行一遍代码，结果是不是就出来了呢？

通过第二次执行原型的构造函数 Mother()，我们在对象实例中复制了一份原型的属性，这样就做到了与原型属性的分离独立。细心的你会发现，我们第一次调用 Mother()，好像什么用都没有呢，能不调用他吗？可以，就有了下面的寄生组合式继承。

2）寄生组合式继承

function object(o){
function F(){}
F.prototype = o;
return new F();
}
function inheritPrototype(Person, Mother){
var prototype = object(Mother.prototype); 
prototype.constructor = Person; 
Person.prototype = prototype; 
}
function Mother (age) {
this.age = age;
this.hobby = ['running','football']
}
Mother.prototype.showAge = function () {
console.log(this.age); 
};
function Person (name, age) { 
Mother.call(this, age);
this.name = name; 
}
inheritPrototype(Person, Mother);
Person.prototype.showName = function () {
console.log(this.name);
}
var p1 = new Person('Jack', 20); 
p1.hobby.push('basketball');//p1:'Jack'; __proto__:20,['running','football']
var p2 = new Person('Mark', 18); //p2:'Mark'; __proto__:18,['running','football'] 

结果是酱紫的：

 

原型中不再有 age 和 hobby 属性了，只有两个方法，正是我们想要的结果！

关键点在于 object(o) 里面，这里借用了一个临时对象来巧妙避免了调用new Mother()，然后将原型为 o 的新对象实例返回，从而完成了原型链的设置。很绕，对吧，那是因为我们不能直接设置 Person.prototype = Mother.prototype 啊。

小结

-------------------------------------------------------------------------------

说了这么多，其实核心只有一个：属性共享和独立的控制，当你的对象实例需要独立的属性，所有做法的本质都是在对象实例里面创建属性。若不考虑太多，你大可以在Person里面直接定义你所需要独立的属性来覆盖掉原型的属性。总之，使用原型继承的时候，要对于原型中的属性要特别注意，因为他们都是牵一发而动全身的存在。

下面简单罗列下js中创建对象的各种方法，现在最常用的方法是组合模式，熟悉的同学可以跳过到文章末尾点赞了。

1）原始模式

//1.原始模式，对象字面量方式
var person = { 
name: 'Jack',
age: 18,
sayName: function () { alert(this.name); }
};
//1.原始模式，Object构造函数方式
var person = new Object();
person.name = 'Jack';
person.age = 18;
person.sayName = function () {
alert(this.name);
}; 

显然，当我们要创建批量的person1、person2……时，每次都要敲很多代码，资深copypaster都吃不消！然后就有了批量生产的工厂模式。

2）工厂模式

//2.工厂模式，定义一个函数创建对象
function creatPerson (name, age) {
var temp = new Object(); 
person.name = name;
person.age = age;
person.sayName = function () {
alert(this.name);
};
return temp; 
}

工厂模式就是批量化生产，简单调用就可以进入造人模式（啪啪啪……）。指定姓名年龄就可以造一堆小宝宝啦，解放双手。但是由于是工厂暗箱操作的，所以你不能识别这个对象到底是什么类型、是人还是狗傻傻分不清（instanceof 测试为 Object），另外每次造人时都要创建一个独立的temp对象，代码臃肿，雅蠛蝶啊。

3）构造函数

//3.构造函数模式，为对象定义一个构造函数
function Person (name, age) {
this.name = name;
this.age = age;
this.sayName = function () {
alert(this.name);
}; 
}
var p1 = new Person('Jack', 18); //创建一个p1对象
Person('Jack', 18); //属性方法都给window对象，window.name='Jack'，window.sayName()会输出Jack 

构造函数与C++、JAVA中类的构造函数类似，易于理解，另外Person可以作为类型识别（instanceof 测试为 Person 、Object）。但是所有实例依然是独立的，不同实例的方法其实是不同的函数。这里把函数两个字忘了吧，把sayName当做一个对象就好理解了，就是说张三的 sayName 和李四的 sayName是不同的存在，但显然我们期望的是共用一个 sayName 以节省内存。

4）原型模式

//4.原型模式，直接定义prototype属性
function Person () {}
Person.prototype.name = 'Jack';
Person.prototype.age = 18;
Person.prototype.sayName = function () { alert(this.name); };
//4.原型模式，字面量定义方式
function Person () {}
Person.prototype = {
name: 'Jack',
age: 18,
sayName: function () { alert(this.name); }
};
var p1 = new Person(); //name='Jack'
var p2 = new Person(); //name='Jack' 

这里需要注意的是原型属性和方法的共享，即所有实例中都只是引用原型中的属性方法，任何一个地方产生的改动会引起其他实例的变化。

5）混合模式（构造+原型）

//5. 原型构造组合模式，
function Person (name, age) {
this.name = name;
this.age = age;
}
Person.prototype = {
hobby: ['running','football'];
sayName: function () { alert(this.name); },
sayAge: function () { alert(this.age); }
};
var p1 = new Person('Jack', 20); 
//p1:'Jack',20; __proto__: ['running','football'],sayName,sayAge
var p2 = new Person('Mark', 18); 
//p1:'Mark',18;__proto__: ['running','football'],sayName,sayAge 

做法是将需要独立的属性方法放入构造函数中，而可以共享的部分则放入原型中，这样做可以最大限度节省内存而又保留对象实例的独立性。