About JavaScript's object-oriented and inheritance, it is helpful for novices to learn_javascript skills

This is an article about object-oriented and inheritance in JavaScript. It was written 1 year ago. The author takes it step by step. It is very helpful for students who want to learn object-oriented in JavaScript, so I try to translate it. Please correct me if I am wrong. Original link Objects and Inheritance in Javascript

While some Javascript users may never need to know about prototypes or the nature of object-oriented languages, developers who come from traditional object-oriented languages ​​will find inheritance in JavaScript when they use it. The model is very strange. Different JS frameworks provide their own methods to write class-like code, which makes JS object-oriented even more difficult to understand. The results of this are:
1. There is no standard way to implement object-oriented.
2. The underlying concept of object-oriented JS is not well known by people

Prototypal inheritance
Prototypal inheritance is a very simple concept, and its essence is:
1. Object a inherits from object b, which means b is the prototype of a.
2. a inherits all the attributes of b, that is, if the value of b. >Let's see the effect in concrete code, assuming there is a John Smith and a Jane that inherits from him.

Copy code The code is as follows: var john = {firstName: 'John', lastName: 'Smith'};

var jane = {firstName: 'Jane'};

jane.__proto__ = john;


Now, we call john the prototype of jane, jane inherits all the attributes of john

Copy code The code is as follows: jane.lastName

"Smith"//This attribute is inherited from john

jane's own attributes have higher priority, as follows

Copy code The code is as follows: jane.firstName;//This attribute overrides the firstName attribute in john

"Jane"

If you add an attribute to john after this, jane will also inherit the attribute dynamically, as shown below

Copy code The code is as follows: john.hair = 'brown'; //Add a new attribute to john

jane.hair;

"brown"//The result shows that jane inherits the new addition Properties of


Now, let’s assume that jane gets married and therefore has a new last name (last name)

Copy code The code is as follows: jane.lastName = 'Doe'

This attribute overrides the attribute with the same name (lastName) in john

Copy code The code is as follows: jane.lastName

"Doe"

However, if we now delete jane’s lastName

Copy the code The code is as follows: delete jane.lastName

The value of this attribute will be restored to john's value

[code]
jane.lastName
"Smith"


Now, jane can also Inherited from other objects. There can be any number of inheritances in this chain. We call it the prototype chain. In fact, john also has a prototype attribute

Copy code The code is as follows: john.__proto__;

Object { }

In the Firebug console, the value of john.__proto__ is set to Object{}, but Object{} represents the object Object.prototype - the parent class of all objects.

This is a brief description of prototypal inheritance. Looks pretty good, right?
However, in fact, we cannot use __proto__. . .

Let me tell you some bad news...
IE does not support the __proto__ attribute. In fact, __proto__ is not an attribute in the ECMAScript specification, and Mozilla also plans to add it to Firefox This attribute will be removed in future versions of the browser.

However, this does not mean that the __proto__ attribute does not exist. Although the __proto__ attribute cannot be directly accessed in some browsers, it still exists in some form, and we still have to deal with it, but it is not so direct.

Classes and Inheritance
Therefore, we can say that JavaScript does not have classes
Please remember: there are no classes in JavaScript
In this case, what happens with methods and inheritance? What was achieved?
By prototype. In traditional object-oriented languages, methods depend on classes, but in JavaScript, methods depend on the prototype of the object, and the prototype is bound to the constructor of the object.

In JavaScript, functions play the role of constructors. By using the new operator, you can use a function as a constructor. The code below shows us creating a Cat function:

Copy the code The code is as follows:

function Cat(name){ // this.name = name // this points to the new object
}

The above code will be automatically created A Cat.prototype object

Copy code The code is as follows:

Cat.prototype
Cat { }

We can use the new operator to create an instance of Cat

Copy code The code is as follows :

var garfield = new Cat('Garfield') // Create an instance - the Cat function acts as the constructor

Now, the Cat.prototype object becomes all The prototype of the object created by new Cat(), for example:

Copy code The code is as follows:

garfield.__proto__ === Cat.prototype
true //See? `Cat.prototype` is now the prototype of the garfield object

Now, we add a Cat.prototype Method, after adding, the method can be accessed by garfield

Copy code The code is as follows:

Cat.prototype.greet = function(){
console.log('Meow, I am ' this.name)
}
garfield.greet()
"Meow, I am Garfield"

Other Cat instances can also access

Copy the code The code is as follows:

var felix = new Cat('Felix')
felix.greet()
"Meow, I am Felix"

So, in JavaScript, the method is Depends on the object's prototype (prototype).

In fact, we can also add a method to garfield, which will override the method of the same name in Cat.prototype, as shown below:

Copy code The code is as follows:

garfield.greet = function(){
console.log("What's new?");
};
garfield.greet();
"What's new?"

But this will not affect other objects

Copy Code The code is as follows:

felix.greet();
"Meow, I am Felix"

Therefore, in JavaScript, a method can be directly associated with an object, with the prototype of the object, or with any parent object of the object, that is, with any link in the prototype chain. This is how inheritance is implemented.

To create a secondary prototype chain, we first need to create another constructor, how about calling it Animal?

Copy code The code is as follows:

function Animal(){
}

Next, we need to point the prototype of Cat.prototype to an Animal object, so that the Cat instance will inherit all Animal methods. Therefore, we set the value of Cat.prototype to an instance of Animal as follows:

Copy code The code is as follows:

Cat.prototype = new Animal();

In addition, we also need to tell the new Cat.prototype that it is actually an instance of Cat:

Copy code The code is as follows:

Cat.prototype.constructor = Cat // Let `Cat. prototype` knows that it is an instance of Cat

Although the purpose of doing this is mainly for the hierarchical relationship between classes, it is usually necessary to do this.

Now, since the objects inherited from Animal.prototype and Cat.prototype belong to the animal class, all instances of Cat also indirectly inherit from Animal.prototype. If we add a new method to Animal.prototype, then all instances of Cat can also access this method.

Copy code The code is as follows:

Animal.prototype.breed = function(){
console.log('Making a new animal!');
return new this.constructor();
};
var kitty = garfield.breed();
Making a new animal!

Through the above code we have achieved inheritance, it’s simple.

Conclusion
Although prototype-based inheritance in JavaScript is weird and takes some time to get used to, its core idea is very simple. As long as you truly understand these essential concepts, you will have the confidence to control JavaScript OO in these mixed codes. (End)^_^