A more difficult concept to understand in JavaScript-prototype

Prototype is a difficult concept to understand in JavaScript. There are many attributes related to prototype. Objects have "prototype" attributes, function objects have "prototype" attributes, and prototype objects have "constructor" attributes. .

1. First introduction to prototype
In JavaScript, the prototype is also an object. The property inheritance of the object can be realized through the prototype. JavaScript objects all contain a "[[Prototype]]" internal property. This attribute corresponds to the prototype of the object.
"[[Prototype]]" is an internal property of the object and cannot be accessed directly. So in order to conveniently view the prototype of an object, Firefox and Chrome provide the non-standard (not supported by all browsers) __proto__ accessor (ECMA introduced the standard object prototype accessor "Object.getPrototype(object)") . The prototype object of JavaScript also contains a "constructor" attribute, which corresponds to the constructor that creates all instances pointing to the prototype.

2. Rules
In JavaScript, each function has a prototype attribute. When a function is used as a constructor to create an instance, the prototype attribute value of this function will be assigned to all object instances as the prototype (that is, the `__proto__` attribute of the instance is set), that is, the value of all instances The prototype refers to the prototype attribute of the function. (****`Only function objects will have this attribute!`****)

The process of new is divided into three steps

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var p = new Person('Zhang San',20);

1. var p={}; Initialize an object p.
2. p._proto_=Person.prototype;, set the __proto__ attribute of object p to Person.prototype
3. Person.call(p,"Zhang San",20);Call the constructor Person to Initialize p. Regarding the use of call/apply

For example:

There are the following scenarios

Man extends Person extends Object

var a = new Man();

a’s prototype is Man.prototype

Man.prototype is also an object, so it also has a pointer pointing to its own prototype, which is Person.prototype

Similarly, Person.prototype The prototype is Object.prototype

Object.prototype is also an object. In the same way, except that its prototype is null, then it has reached the end of the prototype chain

a’s prototype link is as shown below

# Remember one thing: the prototype of an object is the prototype of its constructor. Taking the a object in the picture above as an example, that is: a.__proto__ === Man.prototype

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