3 methods and code examples to implement inheritance in Javascript_javascript skills

Inheritance is a very important means when we implement object-oriented programming. Although we say that we should not over-inherit and use composition instead of inheritance, inheritance is always inevitable. What we want to discuss here is the inheritance mechanism in Javascript.

There is actually no concept of inheritance in Javascript, but we can imitate it through some means. This kind of inheritance actually copies one object into another object. You need to note that all local classes and host classes cannot be inherited as base classes, mainly for security reasons.

There are roughly three types of inheritance in Javascript: 1. Object impersonation; 2. Prototypal inheritance; 3. A mixture of the two.

1. Object impersonation

In fact, object impersonation is closely related to the this keyword (so it is very important to fully understand the this keyword in Javascript :P). The constructor uses this to assign values ​​to properties and methods, and the constructor can also be regarded as an ordinary function, so we can make the constructor of our base class the constructor of the subclass, and then call this inside the subclass function, then the subclass will get the attributes and methods of the parent class.

The principle is very simple, so how do we implement it? Below is a code example for actual operation.

Object impersonation implementation method 1, our most commonly used method of creating new objects:

Copy code The code is as follows:

var classA = function(name){
this.name = name;
this.alertName = function(){
alert(this.name);
}
}

var classB = function(name,age){
this.myConstructor = classA;
this.myConstructor(name);
delete this.myConstructor;

this.age = age;
this.alertAge = function(){
alert(this.age);
}
}

In order to verify whether the above method is correct, you can test it yourself. I will write the test code below:

Copy code The code is as follows:

var objA = new classA('DK');
objA.alertName();//DK

var objB = new classB('DS',20);

objB.alertName();//DS
objB.alertAge();//20

This is the so-called object impersonation. In addition, there are two other ways to implement object impersonation. Although their implementation methods are different, their principles are the same.

Object impersonation implementation method 2, use the call method:

Copy code The code is as follows:

var classA = function(name){
this.name = name;
this.alertName = function(){
alert(this.name);
}
}

var classB = function(name,age){
classA.call(this,name);

this.age = age;
this.alertAge = function(){
alert(this.age);
}
}

It can also be seen from the code. In the first method, we create a new function pointer pointing to the parent class, call the function, and then delete the pointer. Here we use the call method to run the constructor of the parent class under this object to achieve the same purpose. In addition, the opposite of the call method is the apply method.

Object impersonation implementation method three , use the apply method:

Copy code The code is as follows:

var classA = function(name){
this.name = name;
this.alertName = function(){
alert(this.name);
}
}

var classB = function(name,age){
classA.apply(this,new Array(name));

this.age = age;
this.alertAge = function(){
alert(this.age);
}
}

In fact, you can see that the apply method is very similar to the call method, except that the parameters passed are slightly different.

2. Prototypal inheritance

Everyone should know something about the prototype object. All properties and methods on the prototype object will be passed to all instances of the class. So when we pay all the properties and methods of the parent class to the prototype object of the subclass, that is It is equivalent to realizing our inheritance.

If a subclass wants to obtain all the attributes and methods of the parent class, then we pay an instance of the parent class directly to the prototype object of the subclass. Then our subclass is equivalent to obtaining all the objects and methods of the parent class. ?

Code sample service:

Copy code The code is as follows:

var classA = function(){
this.name = 'DK';
this.alertName = function(){
alert(this.name);
}
}

var classB = function(name,age){
this.name = name;
this.age = age;
}

classB.prototype = new classA();

classB.prototype.alertAge = function(){
alert(this.age);
}

Note that the constructor of the parent class here needs to ensure that there are no parameters. Because even if there are construction parameters, you cannot pass them =.=!

when implementing prototypal inheritance

3. Mixed inheritance

As the name suggests, mixed inheritance is a mixture of the first two methods.

Copy code The code is as follows:

var classA = function(name){
this.name = name;
}

classA.prototype.alertName = function(){
alert(this.name);
}

var classB = function(name,age){
classA.call(this,name);
this.age = age;
}

classB.prototype = new classA();

classB.prototype.alertAge = function(){
alert(this.age);
}

Use object impersonation to pass parameters to the parent class, and use prototypal inheritance to inherit public methods.

Now that we have talked about these three inheritance methods, it is time to talk about the problem.

You may wonder why there is object impersonation and prototypal inheritance, but there is still some hybrid inheritance. Yes, the most important thing is this problem.

1. If you actually test it, you will find that through inheritance through object impersonation, the subclass cannot access the methods on the prototype chain of the parent class.

2. Using prototypal inheritance will turn all properties into shared properties. If you implement two instances of the same subclass, you will find that all your instances share all properties.

3. But this is definitely inappropriate. So there is hybrid inheritance, which allows properties to remain private while allowing subclasses to access the methods of the parent class's prototype chain.

You can try it yourself. When an object pretends to be inherited, the subclass cannot access the prototype chain method of the parent class. All instances of the prototype chain inheritance subclass share all parent class attributes. I won't write examples here.