This article will help you understand the prototype and prototype chain in JavaScript

Prototype and prototype chain are difficult and key points in JavaScript. The following article will help you understand prototype and prototype chain. I hope it will be helpful to you!

If your understanding of prototypes and prototype chains is still at a very shallow and ambiguous stage, you may wish to read this article of mine, it should be able to You can be helpful. If it helps you at all, please like, comment and forward. If you have any questions or doubts, you can leave a message in the comment area. I will reply to you as soon as possible. If you think there is any knowledge error in my article, please let me know and understand the wrong things into right ones, which is beneficial to our industry. It's fatal.

Although I often answered interview questions about prototypes before, I always stayed at a very shallow stage of vague knowledge points, and often forgot (I believe this is the same for everyone, hahaha). On the last day of the New Year's Day holiday (I finally touched the keyboard), I followed a video from Station B to gain some relevant knowledge, and I finally had an overall understanding of it. Here they are organized and summarized.

Wow, I swear here, no matter how busy I am in the next week, I need to review this article,
otherwise
otherwise
The Nuggets will always have bugs.

Know the corresponding relationship first

prototype：Prototype
__proto__：Prototype chain (link point)

• Affiliation relationship

  • prototype: An attribute of the function-> Don’t think too complicated, it is actually an ordinary object{}
  • __proto__: An attribute on the object-> Don’t think too complicated, it is actually an ordinary object{}

• object The __proto__ holds the constructor of the object. The prototype

• ## function is a special object, so

  __proto__ is also used on the function. It exists, and it is a function

##One thing that people often overlook and forget:

Object

is a method (constructor), new Object is an instance object! ! ! <pre class='brush:php;toolbar:false;'>console.log(Object) //typeof Object ===&amp;#39;function&amp;#39; console.log(new Object) //typeof new Object ===&amp;#39;object&amp;#39;</pre>constructor

constructor

is the constructor of the instantiated object <pre class='brush:php;toolbar:false;'>//test.constructor -&gt; 实例化test对象的构造函数 Test console.log(test.constructor===Test) //true //这里个人理解为永无止境的自身调用自身，求解，没找到相关文章。 console.log(test.constructor.prototype.constructor===Test) //true console.log(test.constructor.prototype.constructor.prototype.constructor===Test) //true //constructor允许更改 function Test2() { this.a=123 } test.constructor=Test2 console.log(test)</pre>Prototype

function Test(){}
let test=new Test() //new完之后 test是个实例对象了
console.log(test.__proto__===Test.prototype) //根据上面的对应关系表 可以知道结果为true
//Test.prototype也是一个对象，所以他必须也得有__proto__
//Test.prototype.__proto__已经到达终点了,终点是什么，终点就是Object构造函数，所以下面结果为ture
console.log(Test.prototype.__proto__.constructor===Object)
//且 按照上面对应关系中的规则和上条的结果，下条结果也是ture
console.log(Test.prototype.__proto__===Object.prototype) // 
//终点为null
console.log(Object.prototype.__proto__) //null

Can you describe the prototype chain

The

__proto__

of the object holds the prototype of the constructor of the object, prototype is also an object, so It also has its own __proto__, which goes back and forth to the end point Object.__proto__, thus forming a link point with __proto__ (which is key ) value is a chain of prototype objects of the constructor method, which is the prototype chain. <pre class='brush:php;toolbar:false;'>//__proto__ test{ b:333, a:1, __proto__:Test.prototype={ c:222, b:2, __proto__:Object.prototype={ c:3, __proto__:null } } }</pre>

Special function object

Key point: In JS, a function is a special object! ! !

Remember the correspondence table at the beginning of the article

//函数是特殊对象 所以__proto__是存在的，且是个function
console.log(Test.__proto__) //function
console.log(Test.prototype) //object

Test

Since it is a function, the bottom layer must also be implemented by new Function, then<pre class='brush:php;toolbar:false;'>//对象的__proto__保存着对象的构造函数的prototype console.log(Test.__proto__===Function.prototype) //true 这里是不是和关系表对应上了，能正常理解 const obj={} const obj2=new Object() console.log(Object) //function console.log(typeof Object) //&amp;#39;function&amp;#39;</pre>

Function

Since it is a constructor, should it also have __proto__ and prototype, yes, but there is a special point here Need to remember. The underlying rules stipulate that:

Function.__proto__===Function.prototype

is equal, and both return a function. My understanding is Function Constructed itself. <pre class='brush:php;toolbar:false;'>//正常来说函数的Test.prototype应该是个object， //Function.prototype是个function,这也是一个特殊的点 typeof Test.prototype===&amp;#39;object&amp;#39; //true console.log(Function.__proto__) // 一个function console.log(Function.prototype) // 一个function //Function既然是函数对象_，那么他的_proto__就指向他的构造函数的prototype，也就是 //Function.__proto__===Function.prototype，自己调自己，这样理解是不是也没毛病。 console.log(Function.__proto__===Function.prototype) //true //Object既然是个构造方法，那么底层也是new Function console.log(Object.__proto__===Function.prototype) //true // 因为Function.__proto__===Function.prototype 所以下面代码是成立的 (Object.__proto__===Function.__proto__)===true</pre>hasOwnProperty and in

hasOwnProperty##hasOwnProperty

are used to determine whether it is the object itself Properties (inherited from non-prototype chain)

let test={
    a:1,
    b:2
}
Object.prototype.c=3
console.log(test.hasOwnProperty('a')) //true
console.log(test.hasOwnProperty('b')) //true
console.log(test.hasOwnProperty('c')) //false

in

##in is used to check whether the object contains a certain attributes (including attributes on the prototype chain)

console.log('a' in test) //true
console.log('b' in test) //true
console.log('c' in test) //true
console.log('toString' in test) //true
console.log('d' in test) //false

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